| 1 liam@ork.net | Speed of Gravity | maandag 3 september 2001 5:24 | |
| 2 dlzc@aol.com (formerly) |
Re: Speed of Gravity | maandag 3 september 2001 7:40 | |
| 3 Martin Hogbin |
Re: Speed of Gravity | maandag 3 september 2001 10:27 | |
| 4 Tom Van Flandern |
Re: Speed of Gravity | dinsdag 4 september 2001 19:07 | |
| 5 Martin Hogbin |
Re: Speed of Gravity | dinsdag 4 september 2001 20:41 | |
| 6 fred b mcgalliard |
Re: Speed of Gravity | dinsdag 4 september 2001 21:09 | |
| 7 Nemesis |
Re: Speed of Gravity | woensdag 5 september 2001 10:51 | |
| 8 Tom Van Flandern |
Re: Speed of Gravity | donderdag 6 september 2001 22:55 | |
| 9 Steve Carlip |
Re: Speed of Gravity | vrijdag 7 september 2001 1:57 | |
| 10 Nemesis |
Re: Speed of Gravity | vrijdag 7 september 2001 9:45 | |
| 11 Steve Carlip |
Re: Speed of Gravity | zaterdag 8 september 2001 3:42 | |
| 12 Nemesis |
Re: Speed of Gravity | zaterdag 8 september 2001 6:13 | |
| 13 Sam Wormley | Re: Speed of Gravity | zaterdag 8 september 2001 6:29 | |
| 14 G=EMC^2 Glazier |
Re: Speed of Gravity | zaterdag 8 september 2001 15:16 | |
| 15 tj Frazir |
Re: Speed of Gravity | zondag 9 september 2001 4:25 | |
| 16 Tom Van Flandern |
Re: Speed of Gravity | maandag 10 september 2001 19:41 | |
| 17 Nemesis |
Re: Speed of Gravity | dinsdag 11 september 2001 7:22 | |
| 18 Sam Wormley |
Re: Speed of Gravity | dinsdag 11 september 2001 7:39 | |
| 19 Sam Wormley |
Re: Speed of Gravity | dinsdag 11 september 2001 7:46 | |
| 20 Nemesis |
Re: Speed of Gravity | dinsdag 11 september 2001 8:51 | |
| 21 Charles Francis |
Re: Speed of Gravity | dinsdag 11 september 2001 9:34 | |
| 22 Charles Francis |
Re: Speed of Gravity | dinsdag 11 september 2001 9:41 | |
| 23 Charles Francis |
Re: Speed of Gravity | dinsdag 11 september 2001 10:05 | |
| 24 Nemesis |
Re: Speed of Gravity | dinsdag 11 september 2001 10:52 | |
| 25 Nemesis |
Re: Speed of Gravity | dinsdag 11 september 2001 11:01 | |
| 26 Charles Francis |
Re: Speed of Gravity | dinsdag 11 september 2001 13:27 | |
| 27 Charles Francis |
Re: Speed of Gravity | dinsdag 11 september 2001 13:44 | |
| 28 G=EMC^2 Glazier |
Re: Speed of Gravity | dinsdag 11 september 2001 14:47 | |
| 29 Steve Carlip |
Re: Speed of Gravity | woensdag 12 september 2001 1:04 | |
| 30 Steve Carlip |
Re: Speed of Gravity | woensdag 12 september 2001 22:10 | |
| 31 Charles Francis |
Re: Speed of Gravity | donderdag 13 september 2001 8:45 | |
| 32 Nemesis |
Re: Speed of Gravity | zaterdag 15 september 2001 7:33 | |
| 33 Charles Francis |
Re: Speed of Gravity | zaterdag 15 september 2001 8:42 | |
| 34 Nemesis |
Re: Speed of Gravity | zaterdag 15 september 2001 21:04 | |
| 35 Nicolaas Vroom |
Re: Speed of Gravity | zaterdag 15 september 2001 21:41 | |
| 36 Tom Van Flandern |
Re: Speed of Gravity | maandag 17 september 2001 5:08 | |
| 37 tj Frazir | Re: Speed of Gravity | maandag 17 september 2001 17:02 | |
| 38 Jim Carr | Re: Speed of Gravity | zondag 23 september 2001 5:17 | |
| 39 Steve Carlip | Re: Speed of Gravity | maandag 24 september 2001 1:33 | |
| 40 Aleksandr Timofeev | Re: Speed of Gravity | maandag 24 september 2001 15:28 | |
| 41 Steve Carlip | Re: Speed of Gravity | maandag 24 september 2001 19:20 |
Where are we today on this?
First we read that it is speed of light, then not...
We are very consistently at c for propagation of changes in gravity, say due to sudden loss of mass.
Anything else is speculation. It is surprising that the Sun and Jupiter experience curvature centered exactly where they are located *now* and not offset by 2 hours.
| > |
Where are we today on this?
First we read that it is speed of light, then not... |
Do you have a citation?
David A. Smith
--
Martin Hogbin
First we read that it is speed of light, then not...
http://hepweb.rl.ac.uk/ppUK/PhysFAQ/grav_speed.html
Martin Hogbin
John Smith
That is because widespread confusion exists between changes in
gravitational fields and gravitational radiation (also called "gravitational
waves"). These are two different, essentially unrelated phenomena.
Gravitational radiation is an ultra-weak "spacetime" disturbance that
has never yet been directly detected in the solar system, although it
apparently has been seen indirectly in distant binary pulsars. Analogous to
the electromagnetic radiation (e.g., light) that is emitted when a charge is
accelerated, gravitational radiation occurs when a mass is accelerated, as
in supernova explosion or when binary pulsars orbit one another. Like any
wave propagating through the "spacetime medium", gravitational radiation
travels at the speed of light.
Changes in gravitational fields are major effects in the solar system,
where the planets all perturb one another. Sensitive gravimeters can easily
"see" the gravitational field of a person walking around a laboratory (but
are not thereby seeing gravitational waves). Six experiments are sensitive
to the speed of changes in gravitational fields, and have set a lower limit
to that speed that is orders of magnitude faster than light. No experiment
in existence has ever measured a speed as slow as c, the speed of light.
Indeed, computer experiments show that, if gravitational field changes are
updated as slowly as the speed of light, dynamical systems become unstable
and fly apart because angular momentum is not conserved.
For more information about the six experiments sensitive to the speed of
changes in gravitational fields, see "The speed of gravity - What the
experiments say", Phys.Lett.A, v. 250, #1-3, pp. 1-11 (1998/12/21); also
available on the web at http://metaresearch.org , "cosmology" tab,
"gravity" sub-tab.
For more information on the distinction between gravitational waves and
changes in gravitational fields, and on other consequences of gravitational
fields propagating faster than light, see "The speed of gravity - Repeal of
the speed limit" at the same web site location.
and David A. Smith
This, as you see from the preceding descriptions, is a statement about
the speed of gravitational radiation, which is probably not what was meant
by the question about the "speed of gravity".
It might be speculative to guess what the speed of gravity actually is,
but it is a firm result of all existing experiments that it cannot be as
slow as lightspeed. This statement about the curvature of the Sun-Jupiter
binary pair is correct, and is only "surprising" if one were expecting
changes in gravitational fields to suffer the same retardation that light
fields experience. They do not.
As for the supposed causality violations of ftl propagation, those arise
only in special relativity (SR). But it is now widely recognized that the
mathematically equivalent Lorentzian Relativity (LR) has no such
difficulties because, in it, "time dilation" is really "clock slowing".
Speed produces no effects on time itself, so travel backwards in time does
not occur.
and Martin Hogbin
However, the current FAQ was written before the first paper cited above
was published, and makes the same confusion between gravitational waves and
changes in gravitational fields described above. Until the FAQ is updated to
reflect the current state of the ongoing debate about this, it will merely
add to the widespread confusion. -|Tom|-
Tom Van Flandern - Washington, DC - see our web site on replacement
astronomy research at http://metaresearch.org
--
Martin Hogbin
"Tom Van Flandern"
Where are we today on this [the speed of gravity]?
First we read that it is speed of light, then not...
That is because widespread confusion exists between changes in
gravitational fields and gravitational radiation (also called "gravitational
waves"). These are two different, essentially unrelated phenomena.
Tom has a unique point of view on this subject.
Martin Hogbin
"Tom Van Flandern"
Where are we today on this [the speed of gravity]?
First we read that it is speed of light, then not...
That is because widespread confusion exists between changes in
gravitational fields and gravitational radiation
Tom. Thanks for the interesting reference. It is worth a read and a few
minutes thought. However, I should point out that as far as I can come in a
quick noon time read, this may be little more than a heavy handed modeling
of the situation. Good for college students to test their teath on, but not
of much value otherwise. Whatever gravity is, it is not as simple to deal
with as EM fields. The fact that you can't shield it suggests that the
change is not mediated by radiation in the same way an EM field change is. A
good clue is probably provided by analysing the problem your web site
presents, but I am way not convinced that this shows gravity propagates
faster than C. In fact, if we could show this in a more rigid study, it
would not just overthrow all our existing physics, it would demand a lot of
stuff that is shockingly absent. I suggest taking two steps, not one.
Consider what the universe must look like if gravity has an infinite top
speed. How would this work if we send gravity wave info a light day to a
space ship pushing 3/4C, and then send it back. How does this look if we are
the ones moving and the ship and gravity sources are stationary in the ships
view? Every way I look at this I come to the conclusion that the two
"frames" physics can't have exactly the same form and constants for this to
be true, but the form and constants we get do not seem to be so frame
dependant?
In article <9n31op$arc$1@bob.news.rcn.net>, "Tom Van Flandern"
Anything else is speculation. It is surprising that the Sun and Jupiter
experience curvature centered exactly where they are located *now* and
not offset by 2 hours.
It might be speculative to guess what the speed of gravity actually is,
but it is a firm result of all existing experiments that it cannot be as
slow as lightspeed. This statement about the curvature of the Sun-Jupiter
binary pair is correct, and is only "surprising" if one were expecting
changes in gravitational fields to suffer the same retardation that light
fields experience. They do not.
This is interesting. I would like to make a few comments and ask some
questions:
Newton and Signal Delay
If gravitational changes occur much faster than light speed, then
Newtonian gravity is more or less correct since it assumes that
changes are instantaneous. However, since Newton and the physicists of
his day did not know the speed of light, they probably assumed that
the observed positions of the various planets and the moon were their
actual positions at the time of observation. In other words they did
not compensate for the signal delay which can be many seconds to
several minutes. Does anyone know if the delayed signal measurements
were within the margin of error that would be expected in those days?
Physics Curriculum
According to the essay on your site, you were taught at Yale that the
correct astronomical answers are obtained only if gravitational
changes induced by the movements of the sun and the planets are
assumed to be felt instantly by all bodies. I find this amazing since
all relativists insist that changes in gravity propagates at c. Is
this still being taught the same way in physics classes around the
world?
Action at a Distance
You quote Newton thus: "That one body may act upon another at a
distance through a vacuum, without the mediation of any thing else, by
and through which their action and force may be conveyed from one to
the other, is to me so great an absurdity, that I believe no man who
has in philosophical matters a competent faculty of thinking, can ever
fall into it."
You also write in the introduction: "Indeed, far from upsetting much
of current physics, the main changes induced by this new perspective
are beneficial to areas where physics has been struggling, such as
explaining experimental evidence for non-locality in quantum
physics..."
I am not sure how instant gravity would explain non-locality since
they are both types of "action at a distance" in need of a common
explanation. Newton is obviously right, action at a distance is out of
the question. Neither instant gravity not entangled particles can be
explained by action at a distance. This leaves us with a serious
dilemma: how can the action of one body instantly affect the behavior
of another millions of miles away? There is only one solution.
The Illusion of Distance
In my opinion (and this is something I've been saying for a long time)
distance (or space) is an illusion. In my model of reality, there
exist only particles, their properties and their interactions. The
entire collection of particles comprise the universe. Nature keeps
everything working through the law of conservation of energy.
Why is there no space? Because the concept of space is both
self-referential and redundant. If space exists, where is it? And if
particles already have positional properties, what is the purpose of
space?
If there is no space, position can no longer be viewed as a property
of space but as an intrinsic property of the particles themselves,
just like mass, charge or spin. To keep a long story short, I'll
conclude by saying that, in my model, non-local phenomena (entangled
particles) and instant gravitational actions (which I have recently
come to accept as necessary) are due to the non-local principle of
conservation of energy which acts almost instantaneously (in Planck
time) to maintain a balance of energy in the universe. There is no
need for action at a distance because there really is no distance.
Nemesis
Nasty Little Truth About Spacetime Physics:
http://home1.gte.net/res02khr/crackpots/notorious.htm
"Martin Hogbin"
Uniqueness aside, I explained exactly why I made the above statement.
See my complete exposition on this in section 3 of "The speed of gravity -
Repeal of the speed limit" at http://metaresearch.org , "Cosmology" tab,
"Gravity" sub-tab. If you find fault with my reasoning, please elaborate.
For example, how can it be that we detect gravitational field variations in
the solar system and in the laboratory, but have yet to detect a
gravitational wave in either locale?
and "fred b mcgalliard"
My conclusion was just the opposite. Fast gravity propagation is
consistent with existing physics because nothing in the mathematical
theories needs to change, just the interpretations thereof. And it helps
physics in several areas where dilemmas now exist, such as the non-locality
problems in quantum physics. All we really need to do is replace Einstein's
interpretation of the relativity of motion with Lorentz's, with no change in
the math. Both interpretations are consistent with all eleven independent
experiments that test various aspects of the relativity of motion. But with
Lorentz's interpretation, the proof that nothing can go faster than the
speed of light just goes away.
Allow me to simplify and localize your example. Mars is several
light-minutes away at all times. Suppose we send a signal to Mars that
arrives one second later. Wouldn't all observers agree that was an ftl
signal in forward time? If you think not, then let a Martian transponder
send the ftl signal back to Earth, which takes another second. The signal
then makes a round trip to Mars and back in two seconds, way faster than
light (ftl).
In Einstein's interpretation of the relativity of motion, the signal
would be traveling backwards in time. But in Lorentz's interpretation, the
signal is simply ftl in forward time. So no causality violations or
complications arise. If you haven't previously encountered Lorentzian
relativity, this may be a fresh way of looking at physics. But it is also
simple and rewarding, and most importantly it is consistent with all
experimental evidence.
The essential point is that everybody agrees it takes light 8.3 minutes
to get from the Sun to Earth. Something that makes the same trip in half the
time is traveling ftl, but in forward time, and produces no causality
violations (e.g., able to return before it was sent).
In Lorentzian relativity, the dominant local gravity field is the
"preferred frame". In the example I just gave, that would be the Sun's
field. We synchronize all clocks in that frame. Then clocks moving relative
to that frame slow down, but nothing happens to time itself.
This implies the (to some) surprising fact that all clocks with a
uniform speed in the local gravity field, regardless of what that speed is,
can be simultaneously synchronized (epoch-synched) and syntonized
(rate-synched), and will then remain in agreement indefinitely. But this was
actually achieved in the Global Positioning System (GPS), where all ground
clocks on the rotating Earth and all orbiting clocks in various
near-circular orbits in different planes, even those at different distances
(Glonass), can all be synchronized and syntonized in this way. See a fuller
discussion of this in "What the GPS tells us about relativity" in "Open
Questions in Relativistic Physics", F. Selleri, ed., Apeiron, Montreal, pp.
81-90 (1998), also available at the web site cited above.
and Nemesis Nemesis@nospam.com) writes:
Yes, Newtonian gravity is an excellent approximation of reality, failing
only for effects of order (v/c)^2 (typically 10^-8 in the solar system) due
to "spacetime curvature" or varying field density effects.
Roemer was the first to notice the effect of a finite speed of light
because the eclipse times of Jupiter's moons were delayed when Jupiter was
farther away as compared with the eclipse times when Jupiter was closer.
Roemer was a 17-th century contemporary of Newton. But it was not until 1728
that Bradley discovered the aberration of light.
In short, yes, in those days the light-signal delays were within the
errors of position measurements until Roemer's discovery.
Anyone can do a simple computer experiment with an orbit computation
program and verify that the gravitational interactions must be nearly
instantaneous compared to light-speed to get reasonable orbits. So yes,
celestial mechanics is still taught as it was, because that is what works.
The disagreement with relativists is more semantic than substantive.
Gravitational waves propagate at the speed of light, and many relativists
aren't careful to distinguish those from changes in gravitational fields,
which provably propagate faster than light in reality (binary pulsars) and
in GR equations of motion (consider the field of any binary star at a
distance of more than one light-period).
Of course, the speed of gravity is not instantaneous. That was Newton's
point. It is simply too fast to measure at present. That is why it readily
explains non-locality experiments. However, the difference between any
finite speed, however large, and infinite speed is still infinite.
Consider the varying times required by laser or radar beams to travel
between points in space. These are proportional to distance. Moreover, we
can measure distance by triangulating when we use two observers. I agree
that, philosophically, the idea of empty space is absurd. But space is
probably not really empty because we can detect such things as "zero-point
energy" (the Casimir effect). It is probably filled densely with entities
too small for us to detect at present. These would provide meaning to
distance.
You point out a legitimate philosophical problem. But I think your
proposed solution is both impractical and inconsistent with experiment. Best
wishes. -|Tom|-
Tom Van Flandern - Washington, DC - see our web site on replacement
astronomy research at http://metaresearch.org
Tom Van Flandern
This, of course, is complete nonsense. The field equations of
general relativity rigorously and unambiguously imply that no
gravitational effect can propagate faster than light. See, for
example, R. Low, Class.Quant.Grav.16 (1999) 543.
(Tom knows about this paper. His first reaction was to claim
that it said something different than it did. The author responded
that Tom was wrong. Tom has subsequently simply ignored this
and similar proofs, and persists in making statements about the
``mathematical theories'' that are demonstrably wrong.)
What Tom means, of course, is ``Anyone can do a simple experiment
with an orbit computation program using Newtonian gravity and
verify that in a Newtonian model, gravitational interactions must
be nearly instantaneous.'' No one is arguing about this. But it's
not the point, unless you think Newtonian gravity is right.
The same is true if you consider two oppositely charged particles
held in orbit by their electromagnetic interactions. If you try to
use Coulomb's law to describe the interaction, you'll find that
the electric field must propagate much faster than light. But that's
the wrong thing to do, of course---to get a correct description,
you need to use the full electromagnetic interaction, including
the various velocity-dependent terms. If you do that, you find
stable orbits even though the field propagates at the speed of
light. The same is true in general relativity.
Tom believes, for whatever reason (I'll let him explain), that
gravity must propagate much faster than light. But he also knows
that general relativity is an extremely successful theory. So he
tries to have it both ways, pretending that he can continue to use
general relativity and just ``reinterpret'' it. He can't, but rather
than learning enough general relativity to understand this, he
evidently prefers to repeat claims about general relativity that
are simply wrong.
Steve Carlip
In article <9n92gu$ssj$1@woodrow.ucdavis.edu>, Steve Carlip
My conclusion was just the opposite. Fast gravity propagation is
consistent with existing physics because nothing in the mathematical
theories needs to change, just the interpretations thereof.
This, of course, is complete nonsense. The field equations of
general relativity rigorously and unambiguously imply that no
gravitational effect can propagate faster than light. See, for
example, R. Low, Class.Quant.Grav.16 (1999) 543.
(Tom knows about this paper. His first reaction was to claim
that it said something different than it did. The author responded
that Tom was wrong. Tom has subsequently simply ignored this
and similar proofs, and persists in making statements about the
``mathematical theories'' that are demonstrably wrong.)
Maybe so but it seems to me that the equivalence principle assumes
instantaneous gravitational effects. Using Einstein's thought
experiment of a body in an elevator or a rocket, it's easy to see that
any change in the acceleration of the elevator's floor instantly
affects all bodies anywhere in the elevator. I may be overlooking
something here but I'm not one to hold on to my erroneous views in the
face of strong evidence to the contrary. If I am wrong, I'll stand
corrected.
What Tom means, of course, is ``Anyone can do a simple experiment
with an orbit computation program using Newtonian gravity and
verify that in a Newtonian model, gravitational interactions must
be nearly instantaneous.'' No one is arguing about this. But it's
not the point, unless you think Newtonian gravity is right.
I think it's a pretty good point. It seems to work pretty damn good by
assuming instantaneous gravitational effects. In fact, some bodies in
the solar system that were unknown in Newton's time were accurately
predicted later using Newtonian gravity.
The only problem is that electrons are not in orbit around the
nucleus. This model is hopelessly flawed. The most plausible model is
that the electrons go right through the nucleus and oscillate back and
forth.
Which specific terms are you talking about? Even if you assumed that
electrons are really orbiting the nucleus, the distances are so much
smaller than astronomical distances as to make the analogy ludicrous.
He explains the reason on his site. He says that it leads to unstable
orbits because, if one assumes a gravitational propagation of c,
planetary bodies in orbit would be reacting to the delayed positions
of other moving bodies as opposed to their actual positions.
You are attacking the man rather than his arguments.
Nemesis
Nasty Little Truth About Spacetime Physics:
http://home1.gte.net/res02khr/crackpots/notorious.htm
Nemesis
Only with a somewhat odd definition of ``affect.'' No body in the
elevator acts any differently when the floor starts accelerating.
No body even knows that the floor is accelerating until, at least,
lighht from the floor reaches it. If my lab bench is freely falling
inside a (very large) elevator and the floor starts moving, no
measurement I can make on the bench will tell me that the floor
has started until the light-travel time from the floor to my bench.
I suggest that you look at Low's paper, which does this all very
carefully.
Sure. Just as Coulomb's law works pretty damn good in describing
interactions between charged bodies, provided that nothing is
moving very fast. But we know that Newtonian gravity is just
an approximation to general relativity. If we want to know how
fast gravity propagates, it's not a very good idea to start with such
an approximation, unless you know exactly how the approximation
was made.
That's important here. In Newtonian gravity, light-speed propagation
would introduce aberration, and would give new velocity-dependent
accelerations that aren't seen. But general relativity is *not* just
``Newtonian gravity with light-speed propagation''---it has added
velocity-dependent effects that just aren't there in the Newtonian
theory. And when you actually follow through the math that gives
you the Newtonian approximation, you find that these extra velocity-
dependent terms almost exactly cancel the effects of aberration.
Now, I'm not trying to argue that general relativity is The Truth,
and must be believed. If Tom wants to say that he thinks general
relativity is wrong, and to develop a replacement, well, good luck
to him. But he shouldn't go around claiming that ``nothing in the
mathematical theories needs to change,'' and then provide an
``interpretation'' that is directly contradicted by ``the mathematical
theories'' that he says don't need to be changed.
I'm not talking about electrons in atoms, but about what Maxwell's
theory says about large, classical charged bodies. The specific terms
I'm talking about are described in Volume II, Chapter 21 of the Feynman
Lectures---see the discussion in section 21-1.
That argument does, indeed, show that if gravity were described
by Newton's theory, it would have to propagate much faster than
light. It says nothing per se about general relativity.
I apologize. It comes from the frustration of having explained Tom's
errors over and over again, only to have him repeat them.
Once again, I have no objection to Tom's not liking general relativity,
and trying to come up with an alternative in which gravity is more
like Newton's gravity and propagates much faster than light. But I
*do* object to his making false claims about what ``the mathematical
theories'' do and do not say, and ignoring corrections from people
who do, in fact, know much more about the details of general relativity
than he does.
Steve Carlip
In article <9nbt1e$n3r$1@woodrow.ucdavis.edu>, Steve Carlip
The field equations of
general relativity rigorously and unambiguously imply that no
gravitational effect can propagate faster than light. See, for
example, R. Low, Class.Quant.Grav.16 (1999) 543.
Maybe so but it seems to me that the equivalence principle assumes
instantaneous gravitational effects. Using Einstein's thought
experiment of a body in an elevator or a rocket, it's easy to see that
any change in the acceleration of the elevator's floor instantly
affects all bodies anywhere in the elevator.
Only with a somewhat odd definition of ``affect.'' No body in the
elevator acts any differently when the floor starts accelerating.
No body even knows that the floor is accelerating until, at least,
lighht from the floor reaches it. If my lab bench is freely falling
inside a (very large) elevator and the floor starts moving, no
measurement I can make on the bench will tell me that the floor
has started until the light-travel time from the floor to my bench.
I agree that "affects" was a poor choice of words on my part given
that relativists believe literally in the PE and assume that falling
bodies are not accelerating. What I meant to say (I am surprised you
did not get my meaning) was that, if the floor suddenly accelerates,
it does so *instantly*, relative to all bodies in the elevator
regardless of the floor's distance from any given body.
IOW, there is no delay dictated by the speed of light that I can see.
It seems that if there really is an equivalence (I have no reason to
doubt that there is other than that I view it as an inverse
equivalence) between acceleration and gravity, it is obvious that
gravitational changes are "felt" instantly everywhere.
To PE or not to PE, that is the question. :-)
If it treats the subject the way you do in your online essay "Does
Gravity Travel at the Speed of Light?" I am afraid I'll have to pass.
I found your essay unconvincing. One is left with the impression that
GR magically cancels gravitational propagation delays. You do not
explain the physical mechanism. One does not cancel a propagation
delay with math.
I think it's a pretty good point. It seems to work pretty damn good
by assuming instantaneous gravitational effects.
Sure. Just as Coulomb's law works pretty damn good in describing
interactions between charged bodies, provided that nothing is
moving very fast. But we know that Newtonian gravity is just
an approximation to general relativity. If we want to know how
fast gravity propagates, it's not a very good idea to start with such
an approximation, unless you know exactly how the approximation
was made.
It's just an inverse square approximation that works really well by
assuming instant gravitational changes everywhere in the system. The
only thing that seems to be missing is that it does not take time
dilation into consideration, an "oversight" that Newton can be
forgiven for.
Apparently not since adding light speed propagation to Newtonian
gravity destabilizes the system, which what Van Flandern correctly
showed. But, as I mentioned above, I believe that adding "time
dilation" to Newtonian gravity would probably correct any deviation
from GR.
Please don't tell me to follow the math because that would make your
stance highly suspect. If it cannot be explained it in plain everyday
language, it is snake oil from my point of view. As I explained above,
the PE assumes instant gravity. If you feel that I am wrong about the
PE, I am ready to listen to your counterargument.
[cut]
He explains the reason on his site. He says that it leads to unstable
orbits because, if one assumes a gravitational propagation of c,
planetary bodies in orbit would be reacting to the delayed positions
of other moving bodies as opposed to their actual positions.
That argument does, indeed, show that if gravity were described
by Newton's theory, it would have to propagate much faster than
light. It says nothing per se about general relativity.
I don't see how Newtonian gravity can be so accurate by assuming
instant propagation while GR assumes a propagation of c that magically
gets canceled out. How else can one cancel a propagation delay? Sorry,
mathematical magic is not an explanation. Nothing less than a physical
explanation will do.
You are attacking the man rather than his arguments.
I apologize. It comes from the frustration of having explained Tom's
errors over and over again, only to have him repeat them.
Once again, I have no objection to Tom's not liking general relativity,
and trying to come up with an alternative in which gravity is more
like Newton's gravity and propagates much faster than light. But I
*do* object to his making false claims about what ``the mathematical
theories'' do and do not say, and ignoring corrections from people
who do, in fact, know much more about the details of general relativity
than he does.
IMO, there is no propagation at all and this why the equivalence
principle works so well. I am convinced that gravity is a nonlocal
effect mitigated by the principle of energy conservation, which is a
nonlocal principle.
Nemesis
Nasty Little Truth About Spacetime Physics:
http://home1.gte.net/res02khr/crackpots/notorious.htm
Nemesis wrote:
IMO, there is no propagation at all and this why the equivalence
principle works so well. I am convinced that gravity is a nonlocal
effect mitigated by the principle of energy conservation, which is a
nonlocal principle.
Nemesis, Read:
http://www.cnde.iastate.edu/staff/swormley/eo/bkr/bkr.95.12
Starts down about 1/5 of the page.
To All Just a thought. Could gravity be what QM refers to as the
fabric of space?Could we think of this like a spider web with all the
strands of the web connecting all of the universe's energies and
particles. This would answer "action at a distance".Wave function". In
every book it tells us all is connected. A spider knows instantly where
his prey has hit the web by the vibrations of the strings. This could
play well in the "string theory" Best regards to all herb
Speed of G is time . Time dont chang space dose. Time is the only
answer you have .
I did not read any of the post on this but Im quite shure they are just
stoopid .
Steve Carlip (carlip@dirac.ucdavis.edu) writes:
Allow me to provide a simple counterexample that exemplifies this whole
discussion. When we look at a binary star in the sky, we see the components
where they were in their mutual orbits when their light left the stars, not
where they are now. If we could detect their gravity, would that agree with
where we see the light from the stars, or with where the stars are now? The
former would indicate a speed of gravity equal to the speed of light, and
the latter would indicate an instantaneous speed of gravity. Agreed?
Here's the setup to answer this question. Consider the gravitational
field of a binary star with orbital period P, as measured at a field point
in the orbital plane of the binary and a distance R away from the binary's
center of mass. Consider only large distances R > cP, where c is the speed
of light. Then an observer at distance R will see light from the retarded
positions of the binary stars as they were one light-time (R/c) ago. Those
orbital positions will differ from the actual instantaneous position of the
stars by more than one complete revolution, so the retarded and
instantaneous orbital phases of the stars will generally be quite different.
At the observer, the strength of the gravitational field oscillates up
and down as the binary stars orbit one another. The combined field strength
has a maximum when the binary stars are in-line with the observer, and a
minimum when they are at 90 degrees to the observer. Let's ask the GR
equations of motion (e.g., those on p. 1095 of MTW) whether the gravitationa
l field maxima at the observer are in synch with the instantaneous positions
of the binary stars, or with their retarded positions. Surely, if the
gravitational field propagates at lightspeed, the gravitational field maxima
and minima will be in synch with the observed, retarded positions of the
stars. But if and only if the field changes propagate with infinite speed
(in GR) will the field for an arbitrarily distant observer remain in synch
with the instantaneous positions of the stars. Are we still agreed so far?
Now we turn to the answer given by the GR equations of motion. MTW
equation (39.64) has a Newtonian part factored by the bracket [1 - (expr)],
with two additional terms added on at the end. However, (expr) is an
expression containing non-cumulative terms that are very small compared with
unity for ordinary binary stars, and therefore can be neglected here because
we seek only the phase of the main field, and don't care about small
perturbations thereof that cannot appreciably affect phase. To show that
these terms are small compared to unity, note that every term contains
either a potential or a velocity squared, with an implied c^2 in the
denominator that makes the units correct. However, potential is generally of
the same order as v^2, and (v/c)^2 is negligible compared with unity for our
binary star and observer. In any case, nothing in the bracket can build up
to produce an arbitrarily large phase displacement.
Next, we examine the last two add-on terms. The first of these is the
Newtonian acceleration multiplied by (v/c)^2, and the second is the
Newtonian acceleration multiplied by a potential of the same order as
(v/c)^2. So both of these are negligible by the same reasoning as before,
and cannot appreciably affect orbital phase. Indeed, it was obvious at the
outset that nothing in the equations of motion is related to the speed of
light to the first power or the light-time from the binary stars. We are
then left with only the Newtonian acceleration to represent the
gravitational field of the binaries at a great distance. This is reasonable
and meets other known constraints because GR is supposed to approach
Newtonian gravity in the weak-field, low-velocity limit, which this example
certainly represents.
But then we have the result that the distant field produces
accelerations in synch with the instantaneous positions of the binaries, not
their retarded positions-- in agreement with the prediction of Newtonian
gravity. Therefore, in these GR equations of motion, changes in
gravitational fields are propagated to great distances instantly.
So is there an error in my analysis? Is something important missing from
the GR equations of motion even for this weak-field, low-velocity case? Does
GR not really approach the Newtonian approximation in the limit? . Or does
this really illustrate gravitational field changes arriving at a distant
target instantaneously in GR?
This is the most important point I would like to address to you in this
message. The rest of this deals with details of lesser importance.
Then why are you having such a difficult time demonstrating these
errors, both here and in our private exchanges? In my last private message
to you, I reaffirmed my openness to any reasonable argument showing an error
I have made, with neutral parties as the judges, and asked you to make the
same commitment to finding the truth, wherever it may lie. I have not yet
heard that you are open to this suggestion.
I was speaking of computer experiments using the GR equations of motion.
I have corrected you on this misrepresentation before. This whole discussion
is about the speed of gravity in GR and in reality, not its speed in
Newtonian gravity, over which there is no controversy. Everybody agrees it
is infinite.
Now that you know what I meant, that anyone can do simple computer
experiments using GR equations of motion with an orbit computation program
and verify that the gravitational interactions must be nearly instantaneous
compared to lightspeed to get reasonable orbits, do you agree or disagree?
Assuming you agree (many people have done this), how do you explain this
with your interpretation of GR?
If we consider that (1) all six existing experiments indicate gravity
propagates faster than light (Phys.Lett.A, v. 250, pp. 1-11, 1998) while no
existing experiment sets this speed as slow as c; (2) the speed of
gravitational waves (which is c) must not be mixed up with that of changes
in gravitational fields; and (3) Lorentzian relativity is experimentally
viable and allows faster-than-light propagation in forward time (no
causality violations); it then seems reasonable to conclude that gravity
propagating at speed c is the "belief" unbacked by experiment, and gravity
propagating at speeds faster than c is the reasoned deduction from the
experimental evidence.
If we start with Newtonian gravity propagating at infinite speed, and
add a refractive medium to produce the light-bending, redshift, radar
time-delay, and pericenter advance effects, we get the same equations of
motion as GR gives (except for a few small terms of no relevance here). Many
authors (cited in my papers) have previously shown this. How, exactly, do
you propose to make this model mathematically different? And if it is
essentially the same mathematically, wouldn't if be unfair to represent it
as a completely new theory with no credit to Einstein?
Are you suggesting that I (or anyone) should accept a point just because
an expert says it is so? The "appeal to authority" is a tactic of the
"unscientific method", as described in Chapter 20 of my book. And the
history of science is filled with examples of where all the "experts" were
wrong. So if you intend to defend a position under challenge, you must do
the work of reasoning to that position, just as I must do the work of
reasoning both that the standard interpretation has errors and that a
replacement interpretation does not.
Here is a description of the precise point where the two of us differ.
Steve notes correctly that the potential field around a mass is treated as a
"retarded potential" in GR. In essence, potential is a scalar equal to M/r
at a field point a distance r from a source mass M. A "retarded potential"
is M/|r-v(r/c)|, where bold characters are vectors, so the denominator
represents the distance from the source mass one light-time ago. Then the
scalar value of this retarded potential field is evaluated at every fixed
field point in the space around the source mass; and those potential values
are then used to calculate the gradient of the field (effectively, the slope
of the potential) that gives rise to gravitational force.
However, this picture contains a serious error of physics that is hidden
by using Minkowski diagrams, 4-space, and the geometric interpretation of
GR. If the "field point" is not fixed, but rather is a moving body such as
an orbiting test particle, its motion changes the direction of the gradient
of the potential field, and therefore the direction of the gravitational
force applied. This is for the same reason that the direction of a light
source is different as seen from a moving body than from a fixed one, the
same reason that radiation pressure slightly retards the forward motion of a
particle in a circular orbit around a star, and the same reason that an
arrow always moving radially outward from the Sun will cross the cabin of a
passing train diagonally rather than radially. This effect is called
"aberration", and simply means that the apparent direction of the source and
of any force it transmits are altered toward the direction of the particle's
relative motion by the angle (v/V), where v is the particle's speed relative
to the source and V is the arrow/photon/wave/field change's propagation
speed. Minkowski diagrams hide this effect because the effect is visible
only by comparing the perspectives of two different frames of reference at
once -- something the "curved space-time" interpretation and 4-space are
ill-suited to doing.
In physics, aberration necessarily exists in reality for any entity
propagating linearly from a monopole source to a relatively moving target at
a finite speed. Indeed, it is essentially just (v/V) in radians, which
obviously exists for all v > 0 and 0 < V < infinity. Steve cites convenient
cancellation of aberration by a deus ex machina in the math of GR that has
no other need or purpose to exist than to cancel aberration. But as my
binary star and computer experiment examples both show, this mechanism
cannot avoid the common sense meaning of "gravity propagates faster than
light" because changes in gravitational fields arrive at the target long
before photons that show a picture of what is happening back at the source
arrive.
and "Nemesis"
You made some very good points in your messages, and this is an
admirable scientific stance. I would go one step further. Being shown the
errors in our present thinking is one of the best ways by which can we hope
to expand our understanding and grow intellectually. It is something to look
forward to, even if error should not be conceded lightly. -|Tom|-
Tom Van Flandern - Washington, DC - see our web site on replacement
astronomy research at http://metaresearch.org
In article <9niu04$gnp$1@bob.news.rcn.net>, "Tom Van Flandern"
I may be overlooking something here but I'm not one to hold on to my
erroneous views in the face of strong evidence to the contrary. If I am
wrong, I'll stand corrected.
You made some very good points in your messages, and this is an
admirable scientific stance. I would go one step further. Being shown the
errors in our present thinking is one of the best ways by which can we hope
to expand our understanding and grow intellectually. It is something to look
forward to, even if error should not be conceded lightly. -|Tom|-
I appreciate your posts and your steadfastness in sticking with what
you think is right. All too often we encounter a condescending group
of people who think that only they are the legitimate oracle of truth
and that the rest of the world has neither the right nor the
capability to think for itself. I am a firm believer in both freedom
of thought and speech.
You seem to favor the hypothesis that gravitational changes propagate
at a fixed speed, albeit one that is much faster than c. This is where
you and I disagree. Both the principle of equivalence and Newton's
gravity equation assume that there is no propagation taking place.
Changes in gravity are felt instantly by all bodies regardless of
distance. It is for this reason that I am convinced that gravity is a
nonlocal phenomenon. I believe that Newtonian gravity is essentially
right and needs only minor corrections having to do with "time
dilation."
Having said that, I don't believe for a second that there is a curved
spacetime in which bodies are following their geodesics. The obvious
reason is that nothing can move in spacetime. Therefore the answer to
what causes gravity must be found somewhere else.
The locality of the spacetime of relativity is in direct contradiction
with the nonlocal nature of gravity. Ironically, even though classical
physics denies nonlocality, it turns out that nonlocality was an
essential part of it from day one, witness Newton's instant
gravitational effects. IMO, the only way to explain all this magic is
by questioning our most cherished assumptions. I choose to question
our concept of space or distance. You, OTOH, choose to believe that
things can move much faster than c. I disagree for various reasons
that I am not going to bore you with.
Although I belive that gravity effects are instantaneous, I do not
make the mistake of thinking that everything about gravity is
nonlocal. There is an aspect of it that can be explained only by
assuming some form of radiation that emanates from bodies and
propagates at c. I am talking about the inverse square nature of the
field strength. I could go further but I think my post is already
longer than it should be.
Nemesis
Nasty Little Truth About Spacetime Physics:
http://home1.gte.net/res02khr/crackpots/notorious.htm
Nemesis wrote:
Although I belive that gravity effects are instantaneous, I do not
make the mistake of thinking that everything about gravity is
nonlocal. There is an aspect of it that can be explained only by
assuming some form of radiation that emanates from bodies and
propagates at c. I am talking about the inverse square nature of the
field strength. I could go further but I think my post is already
longer than it should be.
Ref: http://hepweb.rl.ac.uk/ppUK/PhysFAQ/grav_speed.html
updated 29-Apr-1998 by Steve Carlip, Matthew Wiener and Geoffrey Landis
original by Steve Carlip
Does Gravity Travel at the Speed of Light?
To begin with, the speed of gravity has not been measured directly in the
laboratory---the gravitational interaction is too weak, and such an
experiment is beyond present technological capabilities. The "speed of gravity"
must therefore be deduced from astronomical observations, and the answer
depends on what model of gravity one uses to describe those observations.
In the simple Newtonian model, gravity propagates instantaneously: the force
exerted by a massive object points directly toward that object's present
position. For example, even though the Sun is 500 light seconds from the Earth,
Newtonian gravity describes a force on Earth directed towards the Sun's
position "now," not its position 500 seconds ago. Putting a "light travel delay"
(technically called "retardation") into Newtonian gravity would make orbits
unstable, leading to predictions that clearly contradict Solar System
observations.
In general relativity, on the other hand, gravity propagates at the speed of
light; that is, the motion of a massive object creates a distortion in the curvature
of spacetime that moves outward at light speed. This might seem to contradict
the Solar System observations described above, but remember that general
relativity is conceptually very different from Newtonian gravity, so a direct
comparison is not so simple. Strictly speaking, gravity is not a "force" in
general relativity, and a description in terms of speed and direction can be
tricky. For weak fields, though, one can describe the theory in a sort of
Newtonian language. In that case, one finds that the "force" in GR is not quite
central -- it does not point directly towards the source of the gravitational field
-- and that it depends on velocity as well as position. The net result is that the
effect of propagation delay is almost exactly cancelled, and general relativity
very nearly reproduces the Newtonian result.
This cancellation may seem less strange if one notes that a similar effect occurs
in electromagnetism. If a charged particle is moving at a constant velocity, it
exerts a force that points toward its present position, not its retarded position,
even though electromagnetic interactions certainly move at the speed of light.
Here, as in general relativity, subtleties in the nature of the interaction
"conspire" to disguise the effect of propagation delay. It should be emphasized
that in both electromagnetism and general relativity, this effect is not put in ad
hoc but comes out of the equations. Also, the cancellation is nearly exact only
for constant velocities. If a charged particle or a gravitating mass suddenly
accelerates, the change in the electric or gravitational field propagates
outward at the speed of light.
Since this point can be confusing, it's worth exploring a little further, in a
slightly more technical manner. Consider two bodies -- call them A and B --
held in orbit by either electrical or gravitational attraction. As long as the force
on A points directly towards B and vice versa, a stable orbit is possible. If the
force on A points instead towards the retarded (propagation-time-delayed)
position of B, on the other hand, the effect is to add a new component of force
in the direction of A's motion, causing instability of the orbit. This instability, in
turn, leads to a change in the mechanical angular momentum of the A-B
system. But total angular momentum is conserved, so this change can only
occur if some of the angular momentum of the A-B system is carried away by
electromagnetic or gravitational radiation.
Now, in electrodynamics, a charge moving at a constant velocity does not
radiate. (Technically, the lowest order radiation is dipole radiation, which
depends on the acceleration.) So to the extent that that A's motion can be
approximated as motion at a constant velocity, A cannot lose angular
momentum. For the theory to be consistent, there must therefore be
compensating terms that partially cancel the instability of the orbit caused by
retardation. This is exactly what happens; a calculation shows that the force on
A points not towards B's retarded position, but towards B's "linearly
extrapolated" retarded position. Similarly, in general relativity, a mass moving
at a constant acceleration does not radiate (the lowest order radiation is
quadrupole), so for consistency, an even more complete cancellation of the
effect of retardation must occur. This is exactly what one finds when one
solves the equations of motion in general relativity.
While current observations do not yet provide a direct model-independent
measurement of the speed of gravity, a test within the framework of general
relativity can be made by observing the binary pulsar PSR 1913+16. The orbit
of this binary system is gradually decaying, and this behavior is attributed to
the loss of energy due to escaping gravitational radiation. But in any field
theory, radiation is intimately related to the finite velocity of field propagation,
and the orbital changes due to gravitational radiation can equivalently be
viewed as damping caused by the finite propagation speed. (In the discussion
above, this damping represents a failure of the "retardation" and "noncentral,
velocity-dependent" effects to completely cancel.)
The rate of this damping can be computed, and one finds that it depends
sensitively on the speed of gravity. The fact that gravitational damping is
measured at all is a strong indication that the propagation speed of gravity is
not infinite. If the calculational framework of general relativity is accepted, the
damping can be used to calculate the speed, and the actual measurement
confirms that the speed of gravity is equal to the speed of light to within 1%.
(Measurements of at least one other binary pulsar system, PSR B1534+12,
confirm this result, although so far with less precision.)
Are there future prospects for a direct measurement of the speed of gravity?
One possibility would involve detection of gravitational waves from a
supernova. The detection of gravitational radiation in the same time frame as
a neutrino burst, followed by a later visual identification of a supernova, would
be considered strong experimental evidence for the speed of gravity being
equal to the speed of light. However, unless a very nearby supernova occurs
soon, it will be some time before gravitational wave detectors are expected to
be sensitive enough to perform such a test.
See also the section on gravitational radiation
References
There seems to be no nontechnical reference on this subject. For a technical
reference, see
T. Damour, in Three Hundred Years of Gravitation, S.W. Hawking and W.
Israel, editors (Cambridge Univ. Press, 1987)
For a good reference to the electromagnetic case, see
R.P. Feynman, R.B. Leighton, and M. Sands, The Feynman Lectures on
Physics, chapter II-21 (Addison-Wesley, 1989)
G=EMC^2 Glazier wrote:
A spider knows instantly where
his prey has hit the web by the vibrations of the strings. This could
play well in the "string theory" Best regards to all herb
No, the spider knows only after the mechanical wave has propagated along
the web between the trapped prey and the spider--a finite time interval.
In article <3B9DA380.EEDF0945@cnde.iastate.edu>, Sam Wormley
Although I belive that gravity effects are instantaneous, I do not
make the mistake of thinking that everything about gravity is
nonlocal. There is an aspect of it that can be explained only by
assuming some form of radiation that emanates from bodies and
propagates at c. I am talking about the inverse square nature of the
field strength. I could go further but I think my post is already
longer than it should be.
updated 29-Apr-1998 by Steve Carlip, Matthew Wiener and Geoffrey Landis
original by Steve Carlip
I've already read this article and I disagree with it. I don't care
what the GR experts believe in. I like to do my own thinking, thank
you very much. The principle of equivalence squarely and irrefutably
contradicts the notion that gravity propagates at the speed of light.
Besides, if gravity effects were not instantaneous, Newtonian gravity
would be completely useless. As it is, it's almost perfect. Its only
shortfall is that it does not take time dilation into consideration,
something for which Sir Isaac can be forgiven.
Nemesis
Nasty Little Truth About Spacetime Physics:
http://home1.gte.net/res02khr/crackpots/notorious.htm
In article
Nemesis wrote:
Although I belive that gravity effects are instantaneous, I do not
make the mistake of thinking that everything about gravity is
nonlocal. There is an aspect of it that can be explained only by
assuming some form of radiation that emanates from bodies and
propagates at c. I am talking about the inverse square nature of the
field strength. I could go further but I think my post is already
longer than it should be.
Ref: http://math.ucr.edu/home/baez/physics/Relativity/SR/grav_speed.html
updated 29-Apr-1998 by Steve Carlip, Matthew Wiener and Geoffrey Landis
original by Steve Carlip
I've already read this article and I disagree with it. I don't care
what the GR experts believe in. I like to do my own thinking, thank
you very much. The principle of equivalence squarely and irrefutably
contradicts the notion that gravity propagates at the speed of light.
Besides, if gravity effects were not instantaneous, Newtonian gravity
would be completely useless. As it is, it's almost perfect. Its only
shortfall is that it does not take time dilation into consideration,
something for which Sir Isaac can be forgiven.
"That one body may act upon another at a
distance through a vacuum, without the mediation of any thing else, by
and through which their action and force may be conveyed from one to
the other, is to me so great an absurdity, that I believe no man who
has in philosophical matters a competent faculty of thinking, can ever
fall into it."
So, if you think gravity propagates instantaneously, take comfort in the
knowledge that Sir Isaac has reached down through the centuries simply
to call you a complete jerk.
Regards
--
Charles Francis
In article <22394-3B9A1A42-130@storefull-131.iap.bryant.webtv.net>,
G=EMC^2 Glazier
I have not heard that QM refers to the "fabric of space", but this is
basically true. Gravity is simply the rate of change of velocity of an
inertial body whose position is being monitored by another inertial
body. It arises from measurement of position, and measurement of
position could be described as your "spiders web". The speed of the
effect being transmitted with the messages required to measure position,
i.e. light speed.
No. String theory is quite different, and takes for granted the things
it supposedly sets out to explain.
Regards
--
Charles Francis
In article <6v3jptom0onje5sh7bmjt5nj7buioheart@4ax.com>, Nemesis
Nemesis
The field equations of
general relativity rigorously and unambiguously imply that no
gravitational effect can propagate faster than light.
Steve, What we want are intuitive reasons to see that this is true, not
obtuse equations. I believe that there is an intuitive reason for it.
Namely that the metric is determined by two way photon exchange, and so
changes in the metric are transmitted by photons, at light speed.
Actually it is easy to see that it doesn't. The change in acceleration
of the elevator's floor is not detectable by an inertial body at the
centre of the elevator until light passes between the two. Fairly
conclusively illustrating the speed of the gravitational effect.
I think we got that, but it is simply not true. the acceleration is
change in position, and position is just a number determined by photon
exchange, so position cannot change until the photons can return.
IOW, there is no delay dictated by the speed of light that I can see.
Can you see it now?
If it treats the subject the way you do in your online essay "Does
Gravity Travel at the Speed of Light?" I am afraid I'll have to pass.
I found your essay unconvincing. One is left with the impression that
GR magically cancels gravitational propagation delays. You do not
explain the physical mechanism. One does not cancel a propagation
delay with math.
The way gr is normally done does not explain physical mechanisms. The
cancellation does take place in the math, its just that math does not
explain physics, it only tells us what is true of the answer. Someone
following the math of gr will definitely know the right answer, but they
will not definitely understand the physical reason for it.
Yes. But there is more to time dilation than we find in special
relativity. When we use time dilation to adjust Newton's laws for flat
space then we find the accelerations due to gravity, without needing to
postulate anything else. That is basically what gr says.
As I say, adding time dilation to Newtonian flat space exactly
reproduces GR and explains Newtonian gravity.
The fundamental point is that gravity just arises from measurement of
position, and measurement of position requires two way messaging.
Messaging is a non-local thing to do, by definition. So you are right,
gravity is a non-local effect. As is position itself.
Regards
--
Charles Francis
In article
Nemesis
Actually it is easy to see that it doesn't. The change in acceleration
of the elevator's floor is not detectable by an inertial body at the
centre of the elevator until light passes between the two. Fairly
conclusively illustrating the speed of the gravitational effect.
The point I wanted to make is that all bodies start accelerating
toward the elevator floor immediately when the floor begins
accelerating. In the case of gravity, for the equivalence to hold, all
bodies must feel a change in a gravitational field instantly.
I think we got that,
Others may have gotten it but apparently you didn't.
So say you but it's obviously true.
Position is a number determined by photon exchange *only* according to
your silly theory. When something accelerates, it does so relative to
all objects in the universe,instantly, your opinion to the contrary
notwithstanding.
Can you see it now?
How can I see something that is a figment of your imagination?
The way gr is normally done does not explain physical mechanisms. The
cancellation does take place in the math, its just that math does not
explain physics, it only tells us what is true of the answer. Someone
following the math of gr will definitely know the right answer, but they
will not definitely understand the physical reason for it.
There is no physics behind it only because gravitational effects are
instantaneous and are mediated by the nonlocal application of
conservation principles.
Yes. But there is more to time dilation than we find in special
relativity.
I was referring to gravitational time dilation in strong fields *and*
SR-type dilation of moving bodies.
Not according to Carlip et al since they keep insisting that gravity
propagates at c. Adjusting Newton's laws to include time dilation does
not change the fact that gravitational effects are instantaneous.
As I say, adding time dilation to Newtonian flat space exactly
reproduces GR and explains Newtonian gravity.
What the point of repeating what I wrote?
The fundamental point is that gravity just arises from measurement of
position, and measurement of position requires two way messaging.
Gravity does not arise from measurement of position since particles do
not measure one another's position.
Messaging is nonlocal? Since when? Nonlocal phenomena is specifically
not about messaging since messaging must happen at the speed of light.
Entangled particles do not communicate their states via messages. That
is the whole point of nonlocality. You are a truly confused man,
Francis.
Nemesis
Nasty Little Truth About Spacetime Physics:
http://home1.gte.net/res02khr/crackpots/notorious.htm
In article
Nemesis wrote:
Although I belive that gravity effects are instantaneous, I do not
make the mistake of thinking that everything about gravity is
nonlocal. There is an aspect of it that can be explained only by
assuming some form of radiation that emanates from bodies and
propagates at c. I am talking about the inverse square nature of the
field strength. I could go further but I think my post is already
longer than it should be.
http://hepweb.rl.ac.uk/ppUK/PhysFAQ/grav_speed.html
updated 29-Apr-1998 by Steve Carlip, Matthew Wiener and Geoffrey Landis
original by Steve Carlip
I've already read this article and I disagree with it. I don't care
what the GR experts believe in. I like to do my own thinking, thank
you very much. The principle of equivalence squarely and irrefutably
contradicts the notion that gravity propagates at the speed of light.
Besides, if gravity effects were not instantaneous, Newtonian gravity
would be completely useless. As it is, it's almost perfect. Its only
shortfall is that it does not take time dilation into consideration,
something for which Sir Isaac can be forgiven.
Sir Isaac obviously thought its shortfalls were much more serious:
"That one body may act upon another at a
distance through a vacuum, without the mediation of any thing else, by
and through which their action and force may be conveyed from one to
the other, is to me so great an absurdity, that I believe no man who
has in philosophical matters a competent faculty of thinking, can ever
fall into it."
So, if you think gravity propagates instantaneously, take comfort in the
knowledge that Sir Isaac has reached down through the centuries simply
to call you a complete jerk.
I am sure he would say that I was wrong but I doubt that he would call
me a complete jerk since his own equation assumes instantaneous action
at a distance. However, once shown the evidence for nonlocality
(really nonspatiality), he would have to agree with me that
distance/space is an illusion and that therefore, there is no
contradiction in saying that a principle can be applied instantly even
over what you think are vast distances.
The "complete jerk" part is your wanting to ascribe to Newton your
personal emotional frustration at not being regarded as the great
physicist that you want others to think you are. Oh well!
Nemesis
Nasty Little Truth About Spacetime Physics:
http://home1.gte.net/res02khr/crackpots/notorious.htm
In article
Sir Isaac obviously thought its shortfalls were much more serious:
"That one body may act upon another at a
distance through a vacuum, without the mediation of any thing else, by
and through which their action and force may be conveyed from one to
the other, is to me so great an absurdity, that I believe no man who
has in philosophical matters a competent faculty of thinking, can ever
fall into it."
So, if you think gravity propagates instantaneously, take comfort in the
knowledge that Sir Isaac has reached down through the centuries simply
to call you a complete jerk.
I am sure he would say that I was wrong but I doubt that he would call
me a complete jerk since his own equation assumes instantaneous action
at a distance.
And as he pointed out anyone who assumes that the equation is perfect,
or that it really implies instantaneous action at a distance is a
complete jerk.
As this is one of the big things he argued with Leibniz about, it is
quite certain that he would not have agreed with you. Leibniz or
Descartes adopted the view that distance/space is an illusion, Newton
seems to have thought there really was something like it, even if not
exactly.
Regards
--
Charles Francis
In article <8dirptgdlpt77pekb1av9e97sc1i49ke5d@4ax.com>, Nemesis
Actually it is easy to see that it doesn't. The change in acceleration
of the elevator's floor is not detectable by an inertial body at the
centre of the elevator until light passes between the two. Fairly
conclusively illustrating the speed of the gravitational effect.
The point I wanted to make is that all bodies start accelerating
toward the elevator floor immediately when the floor begins
accelerating. In the case of gravity, for the equivalence to hold, all
bodies must feel a change in a gravitational field instantly.
Yes I know. I was explaining why that is not true. Think of it from the
point of view of the inertial body. The distance to the floor is half
the time taken for light to travel to the floor and back to the body.
When the floor starts to accelerate the inertial body cannot know
anything about it until the light arrives back from the floor. So it
cannot be aware of a change acceleration of the floor, and it cannot
therefore be affected by it.
The way gr is normally done does not explain physical mechanisms. The
cancellation does take place in the math, its just that math does not
explain physics, it only tells us what is true of the answer. Someone
following the math of gr will definitely know the right answer, but they
will not definitely understand the physical reason for it.
There is no physics behind it only because gravitational effects are
instantaneous and are mediated by the nonlocal application of
conservation principles.
You're just making that up. The physics of measurement is behind it,
just as the physics of measurement is behind the existence of
conservation principles. But you do have to look into the maths to prove
it.
Yes. But there is more to time dilation than we find in special
relativity.
I was referring to gravitational time dilation in strong fields *and*
SR-type dilation of moving bodies.
Good. Just making sure.
When we use time dilation to adjust Newton's laws for flat
space then we find the accelerations due to gravity, without needing to
postulate anything else. That is basically what gr says.
Not according to Carlip et al since they keep insisting that gravity
propagates at c. Adjusting Newton's laws to include time dilation does
not change the fact that gravitational effects are instantaneous.
In fact it does. But you have to do the maths to see the possible ways
in which time dilation can work, and how its effects propagate.
But, as I mentioned above, I believe that adding "time
dilation" to Newtonian gravity would probably correct any deviation
from GR.
As I say, adding time dilation to Newtonian flat space exactly
reproduces GR and explains Newtonian gravity.
What the point of repeating what I wrote?
You wrote that you should add time dilation to Newtonian gravity. All
you have to do is add time dilation to Newtonian flat space without
gravity. The time dilation itself is what is described in gtr as "curved
space-time" (a grossly misleading phrase btw, it is better just to think
of it as a "time dilation field" or something like that). Time dilation
causes what we percieve as gravity.
The fundamental point is that gravity just arises from measurement of
position, and measurement of position requires two way messaging.
Gravity does not arise from measurement of position since particles do
not measure one another's position.
But they do. Are you not composed of particles, and are you not able to
measure the position of other particles?
Messaging is nonlocal? Since when?
Since a message passed from one location to another, obviously.
Particles cannot be entangled. Only the information we have about the
particles is entangled. Entanglement appears precisely when there is no
communication of state via messages. Without the messages there is not
enough information to say much about the particles, and they seem
entangled.
Regards
--
Charles Francis
Hi Sam I know there is some time lapse with the spider and the web
vibrations. My point was to show gravity as a lattice of space. Much
like space is used as a rubber sheet to show how gravity works. If
everything in the universe is connected my thoughts show how it is
connected. Now if you want my thoughts that there is a time lapse my
answer is no. Nature has made the universe to big for gravity to move at
the speed of light. Gravity has to move the way Newton saw it
"instantaneously" It would make no sense to have gravity move at light
speed. Instantaneous action I'm sure would be well received by QM. Like
lots of stuff in the QM theory we in our macro realm can't relate.
Hard to find a frame of reference to instantaneous action. That is the
way nature keeps its best kept secret from us. Best regards herb
Tom Van Flandern
Neither. The direction of each star's gravity is determined by
starting with the position ``where we see the light from the stars''
and extrapolating it forward, in a somewhat complicated way
that depends on the star's velocity and acceleration at its
propagation-delayed position. The result is generally very
close to ``where the stars are now,'' though there can be small
differences.
This isn't just a quibble. Consider the electric field of a moving
charge, which has roughly the same behavior (details differ) and
has the advantage that we can actually run experiments. Suppose
first that the charge is moving at a constant velocity, and that at
time t=0 it is one light-second away from us. Then at t=1 second,
we will indeed see the field pointing to ``where the charge is.''
Here's an attempt at a picture:
a --> b --> c
o
Wrong. Suppose that at t=.5, the charge suddenly turns around,
so that at t=1 it's back where it was at t=0.
a --> b --> c X
d <--
t=1
o
This is the prediction of Maxwell's equations. But it's more than
a prediction -- it's tested every time you turn on a radio. The
sudden swing of the electric field in a delayed response to the
``extrapolation error'' *is* an electromagnetic wave. I don't
just mean this figuratively; this picture, combined with Gauss's
law, is enough to actually calculate the strength and direction of
the radiation field, and the power radiated. You'll find a nice
derivation in Appendix B of Purcell's undergraduate textbook,
_Electricity and Magnetism_.
MTW equation (39.64) is not ``the GR equations of motion.''
It is an approximation in which the extrapolation I described
above has already been accounted for.
Tom, I note that you once again refused to address Low's paper.
If you think it's wrong, please let me know exactly where.
Exact reference, please. Preferavbly to a paper published in a
real journal.
If the issue is what a particular theory (in this case, GR) says,
and if the experts understand the details of the theory and you
don't, then, yes, that's what I'm suggesting. Or alternatively,
you should learn the theory well enough to be an ``expert''
yourself. For example, if you think Low's paper is wrong,
find a mistake, and publish. But if the ``experts'' unanimously
say, ``GR predicts X,'' you really shouldn't go around saying
``GR predicts Y'' without at least acknowledging that people
who know much more about GR than you all disagree.
Steve Carlip
Following up my own post:
The behavior of the gravitational (or electric) field of a
moving body is somewhat unintuitive, as is apparent from
my last post. Here's one way to think about it that might
help. For simplicity I'll talk about electromagnetism rather
than gravity; this will let lots of readers check my claims,
since Maxwell's equations are a lot easier than the Einstein
field equations. Note: the speed of light is approximately
one foot per nanosecond (ns).
It's often said (by me, among others) that if a charge moves
at a constant velocity, its electric field points directly toward
its ``instantaneous'' position. Strictly speaking, this is only
true of the charge has been moving at a constant velocity
for an infinite amount of time. It's helpful to think about
what happens when the charge first *starts* to move.
Suppose you have a charge that is at rest (and has been at
rest for a long time) at point x. Say that at time t=0, it
starts to move at a constant velocity. At t=1 ns, the field
within a foot of the origin will point toward the instantaneous
position of the charge, but the field 1.1 feet away will still
point toward the origin. At t=2 ns, the field within two feet
of the origin will point toward the instantaneous position,
but the field 2.1 feet away will still point toward the origin.
The region of transition between ``pointing toward the
instantaneous position'' and ``pointing toward the origin''
will move outward at the speed of light, and will have a
shape that depends on the details of the charge's initial
acceleration. It is what we call an electromagnetic wave
---specifically, ``the electromagnetic wave emitted by an
accelerated charge.''
Now picture the electric field itself. It does *not* respond
instantly to the charge's acceleration. But once it has begun
to move, it will continue to move along with the charge. So
after t=1 ns, for instance, the field within a foot of the charge
will continue to point to the charge, because, roughly speaking,
it's moving at the same velocity that the charge is.
This isn't so difficult to picture, I think. It won't make sense
if you don't really believe the field exists as an independent
phenomenon, or if you think (as Tom does) that it has to be
continuously ``renewed.'' But the field doesn't move because
it's somehow just an outgrowth of the charge. It moves because
it has started moving, and nothing has stopped it. The charge
couples to the field, and can *start* it moving, but that's all
it has to do.
Hence, for example, if the charge suddenly stops (at t=100 ns,
say), the field won't instantly stop moving, either. At t=101 ns,
the field within a foot of the charge will have stopped moving,
and will point to the charge. But the field 1.1 feet away will
continue to move, and thus will point to the ``extrapolated''
position, the place where the charge would have been if it
hadn't stopped. At t=102 ns, the field within two feet of the
charge will have stopped moving, and will point to the charge,
but the field 2.1 feet away will still be moving. Once again, the
transition region between ``pointing toward the instantaneous
position'' and ``pointing toward the extrapolated position''
will move out from the charge at the speed of light. This is
what we call ``the electomagnetic wave emitted by a decelerated
charge.''
Let me emphasize again that this picture leads to quantitative
predictions about the electromagnetic radiation emitted by
an accelerated charge, and that these predictions are verified
extremely well. If you believe instead, as Tom does, that
the electric field propagates (almost) instantaneously, then
you have to postulate some other, separate cause of radiation,
which just by coincidence also couples to charge, and by some
*enormous* coincidence happens to agree exactly with the
predictions of the picture I've just described.
The situation for gravity is more complicated, of course. The
main difference is that you can start or stop a charge by using
uncharged matter, so you only have to worry about the electric
field of the charge itself. To start or stop a mass, you have to
use some other mass (or energy), and the gravitational field
depends on that as well. But the basic principle is the same.
Steve Carlip
In article <9noff8$9k6$1@woodrow.ucdavis.edu>, Steve Carlip
It's often said (by me, among others) that if a charge moves
at a constant velocity, its electric field points directly toward
its ``instantaneous'' position. Strictly speaking, this is only
true of the charge has been moving at a constant velocity
for an infinite amount of time. It's helpful to think about
what happens when the charge first *starts* to move.
Hi, Steve,
I have found these two posts fascinating, thanks. I have been working on
an intuitive approach to gtr [1], and I think it is possible to see from
what you said about e.m. how this works for gravitation without getting
too heavily into the field equations. Please comment on the accuracy of
these intuitive notions.
The starting point is the radar method of measuring space-time
coordinates. The position I take is that the manifold consists only of
the measurements, what is actually measured and what would be measured
if we were to do a measurement - I take this last phrase from a Dirac-
Von Neumann type interpretation of qm. The overall philosophy is that
the equations of physics apply to the information we have about what is
and what would be, not what actually is, and that this applies as much
to curvature as it does to the quantum domain.
I will consider the conditions in turn:
A Static Gravitating Body.
A Uniformly Moving Gravitating Body
A Inertially Moving Gravitating Body
Corollary: Precession of the Perihelion. Considered from a reference
frame on a planet using a z-axis radial to the sun, the centre of
gravity of the sun is always just ahead of the point where it is seen in
the sky.
An Instantaneously Accelerated Gravitating Body
An Accelerated Gravitating Body
[1] Is there any chance I could get you to read it before I send it to
journals? Unlike my previous attempt, which I now feel looks pretty lame
in comparison, I do actually get the equations out of it instead of just
a hand wavy account, which to my chagrin, no one recognised as gtr. I
feel I cover 75% of Dirac's nice short account in about 1/3rd the space
and with much easier ideas (about 1st year undergraduate), so I really
hope it will have pedagogical value as well as insight.
Regards
--
Charles Francis
In article
Maybe so but it seems to me that the equivalence principle assumes
instantaneous gravitational effects. Using Einstein's thought
experiment of a body in an elevator or a rocket, it's easy to see that
any change in the acceleration of the elevator's floor instantly
affects all bodies anywhere in the elevator.
Actually it is easy to see that it doesn't. The change in acceleration
of the elevator's floor is not detectable by an inertial body at the
centre of the elevator until light passes between the two. Fairly
conclusively illustrating the speed of the gravitational effect.
The point I wanted to make is that all bodies start accelerating
toward the elevator floor immediately when the floor begins
accelerating. In the case of gravity, for the equivalence to hold, all
bodies must feel a change in a gravitational field instantly.
Yes I know. I was explaining why that is not true. Think of it from the
point of view of the inertial body. The distance to the floor is half
the time taken for light to travel to the floor and back to the body.
When the floor starts to accelerate the inertial body cannot know
anything about it until the light arrives back from the floor. So it
cannot be aware of a change acceleration of the floor, and it cannot
therefore be affected by it.
I once called you a moron Francis. I now realize I was mistaken. You
are not a moron. You are a *fucking* moron. And you're wasting my
time. Goodbye.
Nemesis
Nasty Little Truth About Spacetime Physics:
http://home1.gte.net/res02khr/crackpots/notorious.htm
In article
I once called you a moron Francis. I now realize I was mistaken. You
are not a moron. You are a *fucking* moron. And you're wasting my
time. Goodbye.
Well you are obviously totally fucking pissed off that I have used your
own argument to show that you were wrong, and you are not talking about
it because, many days later, you still have no answer to it.
Regards
--
Charles Francis
In article
I once called you a moron Francis. I now realize I was mistaken. You
are not a moron. You are a *fucking* moron. And you're wasting my
time. Goodbye.
Well you are obviously totally fucking pissed off that I have used your
own argument to show that you were wrong, and you are not talking about
it because, many days later, you still have no answer to it.
What I have written is enough and I don't need to defend it against
your stupidity.
Nemesis
Nasty Little Truth About Spacetime Physics:
http://home1.gte.net/res02khr/crackpots/notorious.htm
Steve Carlip
In this situation, at t=1, we will *still* see the field pointing toward
the ``extrapolated'' position, point X in this picture, even though
the charge never got there and is actually back at d. Then very
soon after t=1 -- the exact time depends on exactly where the
charge turned around -- the field will swing around toward the
actual location of the charge. (If the charge has stopped at d, the
field will actually overshoot, and then swing around again to point
toward d.)
This is the prediction of Maxwell's equations. But it's more than
a prediction -- it's tested every time you turn on a radio.
I do not understand when I turn on my radio that that proves that
the electric(magnetic) field points towards its "extrapolated" position.
IMO my radio is not accurate enough to demonstrates any effect that
depents on c
Consider
two observers O and M which are a distance 3 light seconds a part.
Observer O is positive charged and observer M negative charge.
Both observers have a clock which are synchronised.
The time at both's clocks is 12.00
1. IMO when O's clock shows 12.00 he will see
that M's Clock shows 11.57
because it takes 3 secs for the image of M's clock to reach O.
2. IMO the same for M.
At 12.00 at M's clock he will see that O's clock shows 11.57
At 12.00 M starts to move with a lineair speed around O.
O will see this (change in position) at 12.03 at his clock
i.e. when M's clock shows 12.00
At 12.03 M has moved forward.
For the electric field generated by M the same is true.
Before 12.00
The question is how do we test (with an experiment) if statement 6
is right or wrong.
IMO this is difficult and may be imposible.
If it is impossible than which is the best experiment that we can perform
that most closely describes this situation.
SNIP
My apologies for a slow response in this discussion, but I live in
Washington, DC, and have family living in Manhattan and Brooklyn. So I have
had to think about matters more pressing than the "speed of gravity" during
the past week.
"Steve Carlip"
Do we agree on the meaning of "speed"? For a gravitational signal from a
binary star, do we agree the speed of the signal is equal to the distance
traveled divided by the time interval elapsed during its travel? You agree
the gravity field oscillations of a binary star would be "very close to" in
phase with "where the stars are now", as opposed to in phase with their
retarded positions. So if you agree that a gravitational signal (phase
oscillations for a binary star) reaches an arbitrarily large distance in
much less than the light time, doesn't that mean the signal is traveling
much faster than light by definition?
This binary star example does not depend on aberration, so the invisible
"velocity-dependent terms" you claim cancel aberration are irrelevant here.
But your response here seems atypically vague and irrelevant. Is that
because you are still thinking about the example or because you have no
better response? I don't ask that you work out the equations here for what
you call "a somewhat complicated way"; but you should be able to explain the
concept behind it, if one exists.
BTW, notice how easy the explanations become if one simply recognizes
that gravitational signals propagate at infinite speed in GR.
This claim is in contradiction to the results of the Sherwin-Rawcliffe
experiment (previously cited), which attempted to look for this hypothetical
"overshoot" effect. In the experiment, a pair of like charges a fixed
distance apart are simultaneously and continually accelerated from rest.
According to your description, the field of the forward charge should lag,
producing an increased repulsion on the rearward charge. The experiment
showed that did not occur. It follows that, if the experiment was conducted
competently, something is wrong with the picture you describe.
That is an *interpretation*, not a fact. I don't wish to confuse the
issue here by introducing any other model. But just to illustrate a
different interpretation (without mentioning any model to support it), the
electric field could exist apart from a "universal aether" (defined as the
wave-carrying medium for light), and act upon that aether. Then a sudden
acceleration of the source charge would then set off a wave in the aether
while the electric field itself imitated the source charge motions nearly
instantly.
As you know, part of our problem in reaching a meeting of the minds is
due to some confusion between what is established fact and what is still
open to interpretation, and your apparent insistence that any interpretation
of GR other than the one you favor is not GR, but some other theory. I
remind you that Feynman, no slough in such matters, said in Feynman Lectures
on Gravitation [Addison-Wesley, New York (1995). Section 8.4, p. 113]: "It
is one of the peculiar aspects of the theory of gravitation, that it has
both a field interpretation and a geometrical interpretation. ... The
geometrical interpretation is not really necessary or essential to physics."
Those equations are more than accurate enough for our purposes here, as
we have discussed earlier and in my last message. They show the distant
field of a binary star being in-phase with the instantaneous positions of
the stars, no matter how many complete revolutions the two stars have made
during the light transit time to the distant field point (which can be as
far away as we please). No extrapolation based on position, velocity, and
acceleration at a retarded emission time can be good for multiple
revolutions. Please address yourself to explaining that fact if, as you
claim, the speed of gravity is the speed of light. My binary star example
obviously cannot be explained by nature extrapolating the velocity and
acceleration of the sources based on their values when the gravitational
signal leaves the stars. The gravitational signal arriving nearly in-phase
with the instantaneous positions of the stars, even over arbitrarily large
distances, meets the ordinary definition of a field propagation speed
traveling much faster than light.
There is no aberration to cancel, and no "velocity-dependent terms" can
cancel an effect that lasts for multiple revolutions. This example is a
simple, direct demonstration of the point of this discussion. The
oscillating gravitational signals from a binary star pair are nearly in
phase with the instantaneous positions of those stars, and badly out of
phase with the light-time-retarded optical positions of those stars, over
arbitrarily large distances. And this happens right in the GR equations
describing the motion of just such systems, even though no important or
relevant terms are truncated from those equations.
I answered you in our previous discussion last year, which you can look
up as well as I can. I'm not fresh on Low's paper now, since it seemed at
the time I read it to have little relevance to this discussion. In fact,
that author seemed not to understand why aberration is necessary, or why
propagation speeds of c for the distance factor (denominator) in a retarded
potential have no observational consequences at present - which is why they
are irrelevant here.
Here's a mainstream journal article with a good bibliography to other
similar articles that shows how an optical medium can produce general
relativistic effects: Gen.Rel.&Grav. 2 #4, 347-357 (1971) by Fernando de
Felice, titled "On the gravitational field acting as an optical medium". He
notes that Einstein himself first suggested the idea that gravitation is
equivalent to an optical medium. From the abstract: ". Maxwell's equations
may be written as if they were valid in a flat space-time in which there is
an optical medium . this medium turns out to be equivalent to the
gravitational field. . we find that the language of classical optics for the
'equivalent medium' is as suitable as that of Riemannian geometry."
Tom Van Flandern - Washington, DC - see our web site on replacement
astronomy research at http://metaresearch.org
Gravity. All mater regardless of its atomic mass falls at the same
rate.
In article <9niu04$gnp$1@bob.news.rcn.net>
"Tom Van Flandern"
Aren't you aware of the fact that posting about relativity in
sci.physics without setting followups to sci.physics.relativity
is an indicator of crankdom? That Carlip failed to notice
that your comments were off-topic is no excuse.
--
James Carr
Tom Van Flandern
Yes. Do we agree that ``speed'' means a different thing from
``direction''?
Ah, yes. The famous experiment that shook the foundations of modern
physics, showing that special relativity was wrong, that signals could
be sent faster than light, and that Maxwell's equations had to be dumped
... and that somehow never got published. Very convincing.
It is an unambiguous prediction of Maxwell's equations. It's easy enough
to write down the (unique) exact solution for this situation, graph out
the field lines, and see how they behave.
Of course, this is an ``interpretation'' of the observational results, in the
sense that there could conceivably be a different theory that explained
electromagnetic waves in a different way. In this, it's no different from
any of the rest of physics. But Maxwell's equations don't just say, ``This
is what an electromagnetic wave is.'' They give exact, quantitative
predictions for the behavior of such waves---their amplitude, their
direction, their dependence on the acceleration of the charge, their
polarization---that are all quantitatively confirmed by experiment.
It bewilkders me that you are prepared to call this an ``interpretation''
and yet don't understand that your arguments about aberration and
speed are also an ``interpretation,'' based on a particular model (that
*hasn't* had the successes of Maxwell's theory).
My goodness! You think this somehow supports your bizarre idea that
gravitational fields propagate instantaneously? I suggest that you go
back to the book and try to understand the remark after eqn. (3.2.9).
The ``field theory'' approch Feynman is writing about makes it, if
anything, much easier to see that the propagation speed is c.
No, they're not.
You're being silly. Low demonstrates an exact, rigorous consequence of
the Einstein field equations. If you think the conclusion does not
describe nature, then you necessarily believe that the Einstein field
equations are wrong. Fine...but stop pretending otherwise.
You haven't actually read this paper, have you? I have; it has nothing to do
with your claims quoted above. It doesn't ``start with Newtonian gravity
propagating at infinite speed''; it applies only to stationary fields; and it
has nothing to do with pericenter advances. It does show that the effects
of a stationary gravitational field on light can be mimicked by a suitably
complicated optical medium. But it gives no ``equations of motion'' that
would determine the properties of that medium from the distribution
of surrounding masses---to get those, it has to go back to GR, solve the
Einstein field equations, plug the resulting metric into Maxwell's equations
as a background, and then reinterpret the results. In particular, all of the
properties of the ``optical medium'' are determined by the standard Einstein
field equations, and Low's proof that they propagate at the speed of light
stands exactly as it does in conventional GR.
Steve Carlip
Steve Carlip
I shall try to attract your attention in the
possible alternate approach to a problem of
speed of propagation of interaction considered
in this thread.
The presented below example should be considered
as allegorical.
Let's consider own oscillations of a string with
the fixed ends. In this case oscillations of a
string represent standing waves. All points of
a string make oscillations in the same phase
(analogy - in a problem of two bodies, the bodies
make motion in the same phase too).
Here there is no aberration during motion of parts
of a system and we _have _illusion of an _infinite
_transfer _rate of _interaction_ between parts of
a system - a string or two bodies.
But we perfectly know, that the speed of propagation
of elastic interaction between parts of a string is
limited.
You can ask what analogy can be between own oscillations
of a string and problem of two gravitating bodies?
In case of own oscillations of a string or own oscillations
of a rectangular elastic thin plate we deal with a class of
physical systems located in a _stationary_ _state_.
_stationary_ _state_ !!!
For this reason indicated by Tom Van Flandern the absence
of aberration in gravitational systems can be connected with
a stationary state of gravitational systems.
The given approach is the alternate approach to a problem of
speed of propagation of gravitational interaction considered
in this thread, but the given point of view empirically is
justified by existence of the empirical data for the benefit
of a stationarity of the Solar system and so on.
The paradox of a problem of speed of propagation
of gravitational interaction consists of impossibility to determine
speed of propagation of interaction by motion of parts of a system
located in a stationary state.
The Stationary gravitational system is a system located in a state
of own free oscillations, when all bodies of a system make motion
in the same allegorical "phase".
Comments.
[snip]
Tom Van Flandern
Having reread Tom's illustration, I realized that I had initially
misunderstood, and had given an incorrect answer. Here's the
right answer.
The question, again, was this:
In my hasty reading, I thought the question was where each star
saw the other one's gravity. I now see that the question is where
we, far away from the system, see the gravity. This is a nice
question. Let's first look at the answer in Newtonian gravity.
The first, and crucial, point, is that we can only see the combined
gravitational fields of the two stars. Forces don't come with little
name tags (``Hi, I'm the gravitational field of star #1''). While
each star has its own field, we can only observe the sum.
Now, let the two stars have masses m_1 and m_2 and position
vectors r_1 and r_2. The center of mass position R is defined as
(m_1 = m_2)R = m_1 r_1 + m_2 r_2. The distance between the
stars is d = |r_1 - r_2|, and by assumption d<< |R|.
Then to a very good approximation, the gravitational field of the
system has magnitude G(m_1 + m_2)/|R|^2, and points to the
center of mass. Since the center of mass doesn't change position
as the stars orbit each other, the field doesn't change direction.
It looks just the same whether it comes from ``where we see the
light from the stars'' or from ``where the stars are now.''
This isn't quite the whole story, of course. There will be small
corrections to this force that do change with time. The easiest
way to describe them, in Newtonian gravity, is to expand the
total force in powers of the small quantity d/|R|. This is just the
multipole expansion, and should be familiar to anyone who's
taken an introductory course in electromagnetism.
The first correction, the dipole contribution, turns out to be
exactly zero for gravity. This is a straightforward computation:
the dipole force comes out to be proportional to the mass dipole
moment m_1(r_1 - R) + m_2(r_2 - R), which vanishes by the
definition of the center of mass position R.
The next correction, the quadrupole term, *does* depend on
time. The Newtonian prediction is that it depends on the value
of the quadrupole moments ``where the stars are now.'' The
prediction of general relativity is, completely unambiguously,
that it depends on the value of the quadrupole moments ``where
we see the light from the stars.'' The quadrupole approximation
is exactly the point in GR at which the cancellations due to
velocity-dependent interactions stop, and propagation delay
becomes obvious.
Note that the quadrupole term is smaller than the leading center-
of-mass term by a factor on the order of (d/|R|)^2. This is very
small, and this difference between Newtonian gravity and GR
has not been tested. For electromagnetism, on the other hand,
exactly the same type of analysis holds (although there the dipole
contribution is nonzero as well), and the time dependence is very
well tested; it is the basis of antenna theory.
MTW equation (39.64) is, among other things, a multipole
expansion that stops before the quadrupole term. It is exactly
the wrong approximation for your question.
Steve Carlip
Back to my home page Contents of This Document
Have a look at the FAQ:
>
Where are we today on this?
4 Speed of Gravity
Van: Tom Van Flandern
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 4 september 2001 19:07
>
Where are we today on this [the speed of gravity]?
First we read that it is speed of light, then not...
>
We are very consistently at c for propagation of changes
in gravity, say due to sudden loss of mass.
>
Anything else is speculation. It is surprising that the Sun and Jupiter
experience curvature centered exactly where they are located *now* and
not offset by 2 hours.
>
Have a look at the FAQ:
5 Speed of Gravity
Van: Martin Hogbin
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 4 september 2001 20:41
>
John Smith
> >
>
6 Speed of Gravity
Van: fred b mcgalliard
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 4 september 2001 21:09
>
John Smith
> >
>
7 Speed of Gravity
Van: Nemesis
Onderwerp: Re: Speed of Gravity
Datum: woensdag 5 september 2001 10:51
[cut]
>
John Smith
>
>>
>
8 Speed of Gravity
Van: Tom Van Flandern
Onderwerp: Re: Speed of Gravity
Datum: donderdag 6 september 2001 22:55
> >
[tvf]: That is because widespread confusion exists between changes in gravitational fields and
gravitational radiation (also called "gravitational waves"). These are two different, essentially
unrelated phenomena.
>
[mh]: Tom has a unique point of view on this subject.
>
A good clue is probably provided by analysing the problem your web site presents, but I am way
not convinced that this shows gravity propagates faster than C. In fact, if we could show this in a
more rigid study, it would not just overthrow all our existing physics, it would demand a lot of
stuff that is shockingly absent.
>
Consider what the universe must look like if gravity has an infinite top speed. How would this
work if we send gravity wave info a light day to a space ship pushing 3/4C, and then send it back.
How does this look if we are the ones moving and the ship and gravity sources are stationary in
the ships view?
>
Every way I look at this I come to the conclusion that the two "frames" physics can't have exactly
the same form and constants for this to be true, but the form and constants we get do not seem to
be so frame dependant?
>
If gravitational changes occur much faster than light speed, then Newtonian gravity is more or
less correct since it assumes that changes are instantaneous.
>
However, since Newton and the physicists of his day did not know the speed of light, they
probably assumed that the observed positions of the various planets and the moon were their
actual positions at the time of observation. In other words they did not compensate for the signal
delay which can be many seconds to several minutes. Does anyone know if the delayed signal
measurements were within the margin of error that would be expected in those days?
>
According to the essay on your site, you were taught at Yale that the correct astronomical
answers are obtained only if gravitational changes induced by the movements of the sun and the
planets are assumed to be felt instantly by all bodies. I find this amazing since all relativists insist
that changes in gravity propagates at c. Is this still being taught the same way in physics classes
around the world?
>
You quote Newton thus: [action at a distance is logically absurd] You also write in the
introduction: "Indeed, far from upsetting much of current physics, the main changes induced by
this new perspective are beneficial to areas where physics has been struggling, such as explaining
experimental evidence for non-locality in quantum physics..." I am not sure how instant gravity
would explain non-locality since they are both types of "action at a distance" in need of a
common explanation. Newton is obviously right, action at a distance is out of the question.
Neither instant gravity not entangled particles can be explained by action at a distance. This
leaves us with a serious dilemma: how can the action of one body instantly affect the behavior
of another millions of miles away?
>
In my opinion . distance (or space) is an illusion.
9 Speed of Gravity
Van: Steve Carlip
Onderwerp: Re: Speed of Gravity
Datum: vrijdag 7 september 2001 1:57
>
My conclusion was just the opposite. Fast gravity propagation is
consistent with existing physics because nothing in the mathematical
theories needs to change, just the interpretations thereof.
>
Anyone can do a simple computer experiment with an orbit
computation program and verify that the gravitational interactions
must be nearly instantaneous compared to light-speed to get
reasonable orbits.
10 Speed of Gravity
Van: Nemesis
Onderwerp: Re: Speed of Gravity
Datum: vrijdag 7 september 2001 9:45
>
Tom Van Flandern
>>
>
>>
Anyone can do a simple computer experiment with an orbit
computation program and verify that the gravitational interactions
must be nearly instantaneous compared to light-speed to get
reasonable orbits.
>
>
The same is true if you consider two oppositely charged particles
held in orbit by their electromagnetic interactions.
>
If you try to
use Coulomb's law to describe the interaction, you'll find that
the electric field must propagate much faster than light. But that's
the wrong thing to do, of course---to get a correct description,
you need to use the full electromagnetic interaction, including
the various velocity-dependent terms.
>
If you do that, you find
stable orbits even though the field propagates at the speed of
light. The same is true in general relativity.
>
Tom believes, for whatever reason (I'll let him explain), that
gravity must propagate much faster than light.
>
But he also knows
that general relativity is an extremely successful theory. So he
tries to have it both ways, pretending that he can continue to use
general relativity and just ``reinterpret'' it. He can't, but rather
than learning enough general relativity to understand this, he
evidently prefers to repeat claims about general relativity that
are simply wrong.
11 Speed of Gravity
Van: Steve Carlip
Onderwerp: Re: Speed of Gravity
Datum: zaterdag 8 september 2001 3:42
>
In article <9n92gu$ssj$1@woodrow.ucdavis.edu>, Steve Carlip
>>
The field equations of
general relativity rigorously and unambiguously imply that no
gravitational effect can propagate faster than light. See, for
example, R. Low, Class.Quant.Grav.16 (1999) 543.
>
Maybe so but it seems to me that the equivalence principle assumes
instantaneous gravitational effects. Using Einstein's thought
experiment of a body in an elevator or a rocket, it's easy to see that
any change in the acceleration of the elevator's floor instantly
affects all bodies anywhere in the elevator.
>>
What Tom means, of course, is ``Anyone can do a simple experiment
with an orbit computation program using Newtonian gravity and
verify that in a Newtonian model, gravitational interactions must
be nearly instantaneous.'' No one is arguing about this. But it's
not the point, unless you think Newtonian gravity is right.
>
I think it's a pretty good point. It seems to work pretty damn good
by assuming instantaneous gravitational effects.
>>
If you try to
use Coulomb's law to describe the interaction, you'll find that
the electric field must propagate much faster than light. But that's
the wrong thing to do, of course---to get a correct description,
you need to use the full electromagnetic interaction, including
the various velocity-dependent terms.
>
Which specific terms are you talking about? Even if you assumed that
electrons are really orbiting the nucleus, the distances are so much
smaller than astronomical distances as to make the analogy ludicrous.
>>
Tom believes, for whatever reason (I'll let him explain), that
gravity must propagate much faster than light.
>
He explains the reason on his site. He says that it leads to unstable
orbits because, if one assumes a gravitational propagation of c,
planetary bodies in orbit would be reacting to the delayed positions
of other moving bodies as opposed to their actual positions.
>>
But he also knows
that general relativity is an extremely successful theory. So he
tries to have it both ways, pretending that he can continue to use
general relativity and just ``reinterpret'' it. He can't, but rather
than learning enough general relativity to understand this, he
evidently prefers to repeat claims about general relativity that
are simply wrong.
>
You are attacking the man rather than his arguments.
12 Speed of Gravity
Van: Nemesis
Onderwerp: Re: Speed of Gravity
Datum: zaterdag 8 september 2001 6:13
>
Nemesis
>>
In article <9n92gu$ssj$1@woodrow.ucdavis.edu>, Steve Carlip
>
>>>
>
>>
>
>
I suggest that you look at Low's paper, which does this all very
carefully.
>>>
What Tom means, of course, is ``Anyone can do a simple experiment
with an orbit computation program using Newtonian gravity and
verify that in a Newtonian model, gravitational interactions must
be nearly instantaneous.'' No one is arguing about this. But it's
not the point, unless you think Newtonian gravity is right.
>
>>
>
>
That's important here. In Newtonian gravity, light-speed propagation
would introduce aberration, and would give new velocity-dependent
accelerations that aren't seen. But general relativity is *not* just
``Newtonian gravity with light-speed propagation''
>
---it has added
velocity-dependent effects that just aren't there in the Newtonian
theory. And when you actually follow through the math that gives
you the Newtonian approximation, you find that these extra velocity-
dependent terms almost exactly cancel the effects of aberration.
>
Now, I'm not trying to argue that general relativity is The Truth,
and must be believed. If Tom wants to say that he thinks general
relativity is wrong, and to develop a replacement, well, good luck
to him. But he shouldn't go around claiming that ``nothing in the
mathematical theories needs to change,'' and then provide an
``interpretation'' that is directly contradicted by ``the mathematical
theories'' that he says don't need to be changed.
>>>
Tom believes, for whatever reason (I'll let him explain), that
gravity must propagate much faster than light.
>
>>
>
>>>
But he also knows
that general relativity is an extremely successful theory. So he
tries to have it both ways, pretending that he can continue to use
general relativity and just ``reinterpret'' it. He can't, but rather
than learning enough general relativity to understand this, he
evidently prefers to repeat claims about general relativity that
are simply wrong.
>
>>
>
13 Speed of Gravity
Van: Sam Wormley
Onderwerp: Re: Speed of Gravity
Datum: zaterdag 8 september 2001 6:29
>
14 Speed of Gravity
Van: G=EMC^2 Glazier
Onderwerp: Re: Speed of Gravity
Datum: zaterdag 8 september 2001 15:16
15 Speed of Gravity
Van: tj Frazir
Onderwerp: Re: Speed of Gravity
Datum: zondag 9 september 2001 4:25
16 Speed of Gravity
Van: Tom Van Flandern
Onderwerp: Re: Speed of Gravity
Datum: maandag 10 september 2001 19:41
>>
[tvf]: Fast gravity propagation is consistent with existing physics
because nothing in the mathematical theories needs to change, just
the interpretations thereof.
>
[sc]: This, of course, is complete nonsense. The field equations of
general relativity rigorously and unambiguously imply that no
gravitational effect can propagate faster than light. See, for
example, R. Low, Class.Quant.Grav.16 (1999) 543.
>
[sc]: Tom ... persists in making statements about the ``mathematical
theories'' that are demonstrably wrong.
>>
[tvf]: Anyone can do a simple computer experiment with an orbit
computation program and verify that the gravitational interactions
must be nearly instantaneous compared to light-speed to get
reasonable orbits.
>
[sc]: What Tom means, of course, is ``Anyone can do a simple experiment
with an orbit computation program using Newtonian gravity and verify that
in a Newtonian model, gravitational interactions must be nearly instantaneous.''
No one is arguing about this. But it's not the point, unless you think
Newtonian gravity is right.
>
[sc]: Tom believes, for whatever reason (I'll let him explain), that gravity
must propagate much faster than light.
>
[sc]: Now, I'm not trying to argue that general relativity is The Truth, and
must be believed. If Tom wants to say that he thinks general relativity is
wrong, and to develop a replacement, well, good luck to him. But he
shouldn't go around claiming that ``nothing in the mathematical theories
needs to change,'' and then provide an ``interpretation'' that is directly
contradicted by ``the mathematical theories'' that he says don't need to be
changed.
>
[sc]: I *do* object to [Tom's] making false claims about what ``the mathematical
theories'' do and do not say, and ignoring corrections from people who do, in fact,
know much more about the details of general relativity than he does.
>
I may be overlooking something here but I'm not one to hold on to my
erroneous views in the face of strong evidence to the contrary. If I am
wrong, I'll stand corrected.
17 Speed of Gravity
Van: Nemesis
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 7:22
>
and "Nemesis"
>>
>
18 Speed of Gravity
Van: Sam Wormley
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 7:39
>
19 Speed of Gravity
Van: Sam Wormley
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 7:46
>
20 Speed of Gravity
Van: Nemesis
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 8:51
>
Nemesis wrote:
>>
>
>
21 Speed of Gravity
Van: Charles Francis
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 9:34
>
In article <3B9DA380.EEDF0945@cnde.iastate.edu>, Sam Wormley
>>
>>>
>>
Sir Isaac obviously thought its shortfalls were much more serious:
>
22 Speed of Gravity
Van: Charles Francis
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 9:41
>
To All Just a thought. Could gravity be what QM refers to as the
fabric of space?Could we think of this like a spider web with all the
strands of the web connecting all of the universe's energies and
particles. This would answer "action at a distance".Wave function". In
every book it tells us all is connected. A spider knows instantly where
his prey has hit the web by the vibrations of the strings.
>
This could
play well in the "string theory" Best regards to all herb
23 Speed of Gravity
Van: Charles Francis
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 10:05
>
In article <9nbt1e$n3r$1@woodrow.ucdavis.edu>, Steve Carlip
>>
>>>
In article <9n92gu$ssj$1@woodrow.ucdavis.edu>, Steve Carlip
>>
>>>>
>>>
Maybe so but it seems to me that the equivalence principle assumes
instantaneous gravitational effects. Using Einstein's thought
experiment of a body in an elevator or a rocket, it's easy to see that
any change in the acceleration of the elevator's floor instantly
affects all bodies anywhere in the elevator.
>
I agree that "affects" was a poor choice of words on my part given
that relativists believe literally in the PE and assume that falling
bodies are not accelerating. What I meant to say (I am surprised you
did not get my meaning) was that, if the floor suddenly accelerates,
it does so *instantly*, relative to all bodies in the elevator
regardless of the floor's distance from any given body.
>
>
>
It's just an inverse square approximation that works really well by
assuming instant gravitational changes everywhere in the system. The
only thing that seems to be missing is that it does not take time
dilation into consideration, an "oversight" that Newton can be
forgiven for.
>
But, as I mentioned above, I believe that adding "time
dilation" to Newtonian gravity would probably correct any deviation
from GR.
>
IMO, there is no propagation at all and this why the equivalence
principle works so well. I am convinced that gravity is a nonlocal
effect .
24 Speed of Gravity
Van: Nemesis
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 10:52
>
In article <6v3jptom0onje5sh7bmjt5nj7buioheart@4ax.com>, Nemesis
>>
In article <9nbt1e$n3r$1@woodrow.ucdavis.edu>, Steve Carlip
>>>
>>>>
In article <9n92gu$ssj$1@woodrow.ucdavis.edu>, Steve Carlip
>>>
[cut]
>>>>
Maybe so but it seems to me that the equivalence principle assumes
instantaneous gravitational effects. Using Einstein's thought
experiment of a body in an elevator or a rocket, it's easy to see that
any change in the acceleration of the elevator's floor instantly
affects all bodies anywhere in the elevator.
>
>>
I agree that "affects" was a poor choice of words on my part given
that relativists believe literally in the PE and assume that falling
bodies are not accelerating. What I meant to say (I am surprised you
did not get my meaning) was that, if the floor suddenly accelerates,
it does so *instantly*, relative to all bodies in the elevator
regardless of the floor's distance from any given body.
>
>
but it is simply not true.
>
the acceleration is
change in position, and position is just a number determined by photon
exchange, so position cannot change until the photons can return.
>>
IOW, there is no delay dictated by the speed of light that I can see.
>
>>
If it treats the subject the way you do in your online essay "Does
Gravity Travel at the Speed of Light?" I am afraid I'll have to pass.
I found your essay unconvincing. One is left with the impression that
GR magically cancels gravitational propagation delays. You do not
explain the physical mechanism. One does not cancel a propagation
delay with math.
>
>>
It's just an inverse square approximation that works really well by
assuming instant gravitational changes everywhere in the system. The
only thing that seems to be missing is that it does not take time
dilation into consideration, an "oversight" that Newton can be
forgiven for.
>
>
When we use time dilation to adjust Newton's laws for flat
space then we find the accelerations due to gravity, without needing to
postulate anything else. That is basically what gr says.
>>
But, as I mentioned above, I believe that adding "time
dilation" to Newtonian gravity would probably correct any deviation
from GR.
>
>>
IMO, there is no propagation at all and this why the equivalence
principle works so well. I am convinced that gravity is a nonlocal
effect .
>
>
Messaging is a non-local thing to do, by definition. So you are right,
gravity is a non-local effect. As is position itself.
25 Speed of Gravity
Van: Nemesis
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 11:01
>
In article
>>
In article <3B9DA380.EEDF0945@cnde.iastate.edu>, Sam Wormley
>>>
>>>>
>>>
>>
>
26 Speed of Gravity
Van: Charles Francis
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 13:27
>
In article
>>>
I've already read this article and I disagree with it. I don't care
what the GR experts believe in. I like to do my own thinking, thank
you very much. The principle of equivalence squarely and irrefutably
contradicts the notion that gravity propagates at the speed of light.
Besides, if gravity effects were not instantaneous, Newtonian gravity
would be completely useless. As it is, it's almost perfect. Its only
shortfall is that it does not take time dilation into consideration,
something for which Sir Isaac can be forgiven.
>>
>
>
However, once shown the evidence for nonlocality
(really nonspatiality), he would have to agree with me that
distance/space is an illusion and that therefore, there is no
contradiction in saying that a principle can be applied instantly even
over what you think are vast distances.
27 Speed of Gravity
Van: Charles Francis
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 13:44
>
In article
>>>>>
Maybe so but it seems to me that the equivalence principle assumes
instantaneous gravitational effects. Using Einstein's thought
experiment of a body in an elevator or a rocket, it's easy to see that
any change in the acceleration of the elevator's floor instantly
affects all bodies anywhere in the elevator.
>>
>
>>>
If it treats the subject the way you do in your online essay "Does
Gravity Travel at the Speed of Light?" I am afraid I'll have to pass.
I found your essay unconvincing. One is left with the impression that
GR magically cancels gravitational propagation delays. You do not
explain the physical mechanism. One does not cancel a propagation
delay with math.
>>
>
>>>
It's just an inverse square approximation that works really well by
assuming instant gravitational changes everywhere in the system. The
only thing that seems to be missing is that it does not take time
dilation into consideration, an "oversight" that Newton can be
forgiven for.
>>
>
>
>>
>
>
>>>
>>
>
>>>
IMO, there is no propagation at all and this why the equivalence
principle works so well. I am convinced that gravity is a nonlocal
effect .
>>
>
>>
Messaging is a non-local thing to do, by definition. So you are right,
gravity is a non-local effect. As is position itself.
>
>
Nonlocal phenomena is specifically
not about messaging since messaging must happen at the speed of light.
Entangled particles do not communicate their states via messages. That
is the whole point of nonlocality.
28 Speed of Gravity
Van: G=EMC^2 Glazier
Onderwerp: Re: Speed of Gravity
Datum: dinsdag 11 september 2001 14:47
29 Speed of Gravity
Van: Steve Carlip
Onderwerp: Re: Speed of Gravity
Datum: woensdag 12 september 2001 1:04
>
When we look at a binary star in the sky, we see the components
where they were in their mutual orbits when their light left the
stars, not where they are now. If we could detect their gravity,
would that agree with where we see the light from the stars, or
with where the stars are now?
t=-1 t=0 t=1
We are at o. At t=1, the observed field will point toward point c,
not point b. ``Instantaneous,'' right?
t=-1 t=0 t=.5
In this situation, at t=1, we will *still* see the field pointing toward
the ``extrapolated'' position, point X in this picture, even though
the charge never got there and is actually back at d. Then very
soon after t=1 -- the exact time depends on exactly where the
charge turned around -- the field will swing around toward the
actual location of the charge. (If the charge has stopped at d, the
field will actually overshoot, and then swing around again to point
toward d.)
>
Now we turn to the answer given by the GR equations of
motion. MTW equation (39.64) has a Newtonian part
factored by the bracket [...]
>
If we start with Newtonian gravity propagating at infinite speed,
and add a refractive medium to produce the light-bending,
redshift, radar time-delay, and pericenter advance effects, we
get the same equations of motion as GR gives (except for a few
small terms of no relevance here).
>>
[sc]: I *do* object to [Tom's] making false claims about what
``the mathematical theories'' do and do not say, and ignoring
corrections from people who do, in fact, know much more
about the details of general relativity than he does.
>
Are you suggesting that I (or anyone) should accept a point
just because an expert says it is so?
30 Speed of Gravity
Van: Steve Carlip
Onderwerp: Re: Speed of Gravity
Datum: woensdag 12 september 2001 22:10
31 Speed of Gravity
Van: Charles Francis
Onderwerp: Re: Speed of Gravity
Datum: donderdag 13 september 2001 8:45
>
Following up my own post:
The behavior of the gravitational (or electric) field of a
moving body is somewhat unintuitive, as is apparent from
my last post. Here's one way to think about it that might
help. For simplicity I'll talk about electromagnetism rather
than gravity; this will let lots of readers check my claims,
since Maxwell's equations are a lot easier than the Einstein
field equations. Note: the speed of light is approximately
one foot per nanosecond (ns).
-------------------------
There is no issue. The measurement tells us where the body was at the
time of radar reflection. It is still in the same place, and hence that
is where curvature is centred.
-----------------------------------
Lorentz transform to the case of a static gravitating body. The centre
of curvature is always where the body is, so when we transform back to
the moving case, that is where the centre of curvature has to be. I.e.
the centre of curvature is where the body is now, not where it was at
the time of measurement.
-----------------------------------
This is really a correction to the uniformly moving case. To do it
properly we have to think in terms of inertial reference frames, and
uniform movement is not necessarily inertial. So the centre of curvature
is where the body is now as a result of inertial motion, not uniform
motion.
-----------------------------------------------
Consider a body following an inertial path, instantaneously accelerated,
and then following an inertial path again. Up until the time when we
receive light from the body at the moment of acceleration the situation
is exactly as for the inertially moving gravitating body. There being no
other information, the prediction must be identical. The centre of
gravity is where the body would have been had the body continued
inertial motion, and starts to move away from that point after we
recieve information about the acceleration.
-------------------------------
Future accelerations cannot be predicted, so the centre of gravity
should always be at the point where we predict the body would be based
on inertial motion following the last information we had about it.
32 Speed of Gravity
Van: Nemesis
Onderwerp: Re: Speed of Gravity
Datum: zaterdag 15 september 2001 7:33
>
In article <8dirptgdlpt77pekb1av9e97sc1i49ke5d@4ax.com>, Nemesis
>>
In article
>
>>>>>>
>>>
>>
>
33 Speed of Gravity
Van: Charles Francis
Onderwerp: Re: Speed of Gravity
Datum: zaterdag 15 september 2001 8:42
>
Yes I know. I was explaining why that is not true. Think of it from the
>>
point of view of the inertial body. The distance to the floor is half
the time taken for light to travel to the floor and back to the body.
When the floor starts to accelerate the inertial body cannot know
anything about it until the light arrives back from the floor. So it
cannot be aware of a change acceleration of the floor, and it cannot
therefore be affected by it.
>
34 Speed of Gravity
Van: Nemesis
Onderwerp: Re: Speed of Gravity
Datum: zaterdag 15 september 2001 21:04
>
In article
>>
Yes I know. I was explaining why that is not true. Think of it from the
>>>
point of view of the inertial body. The distance to the floor is half
the time taken for light to travel to the floor and back to the body.
When the floor starts to accelerate the inertial body cannot know
anything about it until the light arrives back from the floor. So it
cannot be aware of a change acceleration of the floor, and it cannot
therefore be affected by it.
>>
>
35 Speed of Gravity
Van: Nicolaas Vroom
Onderwerp: Re: Speed of Gravity
Datum: zaterdag 15 september 2001 21:41
>
SNIP
3. That means O sees M at a retarded position.
4. O will agree that the electric field generated by M points towards
the instantaneous position of M.
5. For M the same is true.
O will also agree that the electric field points to that same position
untill 12.03.
At 12.03 O will detect a change in the electric field.
6. That means (IMO) that O does not agree that the electric field points
to its "extrapolated" position.
Because then already at 12.00 O should detect a change.
(For M this is different).
>
The
sudden swing of the electric field in a delayed response to the
``extrapolation error'' *is* an electromagnetic wave. I don't
just mean this figuratively; this picture, combined with Gauss's
law, is enough to actually calculate the strength and direction of
the radiation field, and the power radiated. You'll find a nice
derivation in Appendix B of Purcell's undergraduate textbook,
_Electricity and Magnetism_.
>
Steve Carlip
36 Speed of Gravity
Van: Tom Van Flandern
Onderwerp: Re: Speed of Gravity
Datum: maandag 17 september 2001 5:08
components where they were in their mutual orbits when their light left the
stars, not where they are now. If we could detect their gravity, would that
agree with where we see the light from the stars, or with where the stars
are now? The former would indicate a speed of gravity equal to the speed of
light, and the latter would indicate an instantaneous speed of gravity. .
Surely, if the gravitational field propagates at light speed, the
gravitational field maxima and minima will remain in synch with the
observed, retarded positions of the stars. But if and only if the field
changes propagate with near-infinite speed will the field for an arbitrarily
distant observer remain in synch with the instantaneous positions of the
stars.
>>
[tvf]: When we look at a binary star in the sky, we see the two
starting with the position ``where we see the light from the stars'' and
extrapolating it forward, in a somewhat complicated way that depends on the
star's velocity and acceleration at its propagation-delayed position. The
result is generally very close to ``where the stars are now,'' though there
can be small differences.
>
[sc]: Neither. The direction of each star's gravity is determined by
charge . we will *still* see the field pointing toward the ``extrapolated''
position . even though the charge never got there . If the charge has
stopped at d, the field will actually overshoot, and then swing around again
to point toward d.
>
[sc]: This isn't just a quibble. Consider the electric field of a moving
``extrapolation error'' *is* an electromagnetic wave.
>
[sc]: The sudden swing of the electric field in a delayed response to the
equation (39.64) has a Newtonian part factored by the bracket [...]
>>
[tvf]: Now we turn to the answer given by the GR equations of motion. MTW
approximation in which the extrapolation I described above has already been
accounted for.
>
[sc]: MTW equation (39.64) is not ``the GR equations of motion.'' It is an
you think it's wrong, please let me know exactly where.
>
[sc]: Tom, I note that you once again refused to address Low's paper. If
and add a refractive medium to produce the light-bending, redshift, radar
time-delay, and pericenter advance effects, we get the same equations of
motion as GR gives (except for a few small terms of no relevance here).
>>
[tvf]: If we start with Newtonian gravity propagating at infinite speed,
journal.
>
[sc]: Exact reference, please. Preferably to a paper published in a real
37 Speed of Gravity
Van: tj Frazir
Onderwerp: Re: mater is allways proportional
Datum: maandag 17 september 2001 17:02
1 Before anything can fall more force must be one one side of each
atom then the other AND it must allways be displaced twards less energy.
2 it dose not mater the atomic sise . It wil be displaced twards less
at the sae rate. SO all mater is proportional to the rate energy
expands in space. Because space is bent the energy rate becomes less
near mater. Mater dose not expand so is allways proportional to
expanding space ( expanding energy) .
Our universe is still a big bang expanding . The presure of energy
filling space still pushes outward . Mater is still condenced energy .
38 Speed of Gravity
Van: Jim Carr
Onderwerp: Re: Speed of Gravity
Datum: zondag 23 september 2001 5:17
... about general relativity in sci.physics ...
>
39 Speed of Gravity
Van: Steve Carlip
Onderwerp: Re: Speed of Gravity
Datum: maandag 24 september 2001 1:33
>
"Steve Carlip"
>
Do we agree on the meaning of "speed"? For a gravitational
signal from a binary star, do we agree the speed of the signal
is equal to the distance traveled divided by the time interval
elapsed during its travel?
>>
[sc]: This isn't just a quibble. Consider the electric field of a moving
>
charge . we will *still* see the field pointing toward the ``extrapolated''
position . even though the charge never got there . If the charge has
stopped at d, the field will actually overshoot, and then swing around
again to point toward d.
>
This claim is in contradiction to the results of the Sherwin-Rawcliffe
experiment (previously cited), which attempted to look for this
hypothetical "overshoot" effect.
>>
[sc]: The sudden swing of the electric field in a delayed response to
>
the ``extrapolation error'' *is* an electromagnetic wave.
>
That is an *interpretation*, not a fact.
>
I remind you that Feynman, no slough in such matters, said in Feynman
Lectures on Gravitation [Addison-Wesley, New York (1995). Section 8.4,
p. 113]: "It is one of the peculiar aspects of the theory of gravitation,
that it has both a field interpretation and a geometrical interpretation.
... The geometrical interpretation is not really necessary or essential to
physics."
>>
[sc]: MTW equation (39.64) is not ``the GR equations of motion.''
>
It is an approximation in which the extrapolation I described above
has already been accounted for.
>
Those equations are more than accurate enough for our purposes here
>>
[sc]: Tom, I note that you once again refused to address Low's paper. If
>
you think it's wrong, please let me know exactly where.
>
I answered you in our previous discussion last year, which you can look
up as well as I can. I'm not fresh on Low's paper now, since it seemed at
the time I read it to have little relevance to this discussion. In fact,
that author seemed not to understand why aberration is necessary, or why
propagation speeds of c for the distance factor (denominator) in a retarded
potential have no observational consequences at present
>>>
[tvf]: If we start with Newtonian gravity propagating at infinite speed,
>
and add a refractive medium to produce the light-bending, redshift, radar
time-delay, and pericenter advance effects, we get the same equations of
motion as GR gives (except for a few small terms of no relevance here).
>>
[sc]: Exact reference, please. Preferably to a paper published in a real
>
journal.
>
Here's a mainstream journal article with a good bibliography to other
similar articles that shows how an optical medium can produce general
relativistic effects: Gen.Rel.&Grav. 2 #4, 347-357 (1971) by Fernando de
Felice, titled "On the gravitational field acting as an optical medium".
40 Speed of Gravity
Van: Aleksandr Timofeev
Onderwerp: Re: Speed of Gravity
Datum: maandag 24 september 2001 15:28
>
My goodness! You think this somehow supports your bizarre idea that
gravitational fields propagate instantaneously? I suggest that you go
back to the book and try to understand the remark after eqn. (3.2.9).
The ``field theory'' approch Feynman is writing about makes it, if
anything, much easier to see that the propagation speed is c.
>
---
Aleksandr Timofeev
http://groups.google.com/groups?ic=1&q=msgid:3B372CA5%40MailAndNews.com
41 Speed of Gravity
Van: Steve Carlip
Onderwerp: Re: Speed of Gravity
Datum: maandag 24 september 2001 19:20
>
When we look at a binary star in the sky, we see the components
where they were in their mutual orbits when their light left the
stars, not where they are now. If we could detect their gravity,
would that agree with where we see the light from the stars, or
with where the stars are now?
>
Now we turn to the answer given by the GR equations of motion.
MTW equation (39.64)
Created: 24 September 2001