1 Speed of light in vacuum, what definition?

Your disagreement is merely YOUR problem, and does not change the FACT that what I said is correct. This _IS_ what these words mean (in this context, i.e. physics; in casual speech the meaning is not so precise).

NONSENSE! You are VERY confused and have missed a basic point of SR.

>	Otherwise we couldn't ascribe an absolute value zero and consequently not a constant value to it.

NONSENSE! You are VERY confused and have missed a basic point of SR.

We CANNOT ascribe any sort of "absolute zero" to ANY speed/velocity, just like there is no "absolute rest".

Newton got it wrong, and we have known this for over a century. Remarkably, the equations Newton derived assuming his absolute space work equally well using any inertial frame, without any reference to "absolute space". (The very same thing happened to Maxwell's theory of electrodynamics.)

For some phenomena there happens to be SET OF COORDINATES to which we normally refer its speed:
* a wave in a medium is normally referenced to the inertial frame in which the medium is at rest.
* an automobile is normally referenced to the inertial frame in which the LOCAL surface of the earth is at rest.

It has become ingrained in our psyche to use inertial frames. It is also ingrained to use those normal frames for these specific phenomena. Indeed if one uses anything else it is incumbent on the author to explicitly mention it.

[#] I.e. Nature is neither involved nor consulted, and does not care which choice is made.

[@] And many experiments do: the inertial frame in which the laboratory is instantaneously at rest.

Note that in all cases there is NOTHING to prevent us from using some other coordinates, it's just that in such cases as mentioned above there is a specific coordinate system relative to which the calculations are easier.

>	The movement of a body, e.g. a car has not such absolute zero. We may refer it's zero to the ground. However, this is clearly an arbitrary choice.

Yes. ALL such choices are arbitrary, for any phenomenon.

Being humans, and therefore lazy, we generally use this arbitrariness to select coordinates that make the calculations easier. Nature has nothing to do with this.

The exceptions you claim above are not really exceptions, it's just that you don't understand this.

>	In case of acoustic waves in air, we needn't construct coordinates in which the air is at rest.

Right! See above.

In another post to Poutnik you said:

>	You have to admit that you and Roberts are wrong when we consider a stone that is thrown from one train to another one moving relative to each other.

Nope. It is YOU who is confused, not Poutnik or me.

>	In this case one must arbitrarily decide whether the emitting or the hit train are the reference.

Yes. That arbitrariness is INHERENT in the physical situation. One could also choose the inertial frame of the tracks. Or any other frame....

>	However, the speed of sound doesn't depend on any chosen point of view. It is not earth-centered, not sun-centered, and not galaxy-centered but does only refer to the medium.

One must CHOOSE to which coordinates one references any speed. For sound it is so ingrained in our psyche that one must explicitly mention using any coordinates other than the inertial frame in which the medium is at rest. Since you did not mention any such frame, then "speed of sound" of course refers to the inertial frame in which the medium is at rest.

You confuse a human convention of speech with physical law. That's outrageous and merely demonstrates how confused you are.

>>	Car speed wrt Earth is convenient and natural, but arbitrary. Sound speed wrt medium is convenient and natural, but arbitrary.

>	Natural in the sense of non-arbitrary is the opposite of arbitrary.

There is NOTHING "natural" about this, it's just that it is EASIER to use those coordinates. The choice is indeed ARBITRARY.

Tom Roberts Click here to Reply

2 Speed of light in vacuum, what definition?

I disagree with you in all essential points. Having addressed many of them within the many equally-named topics "Re: Speed of light in vacuum, what definition?", I would like you to ask you for your opinion concerning Cramer's explantion of twin paradox. Is it the only correct one? Are you aware of a better one? Is there a commonly agreed one. Did NPA declare their petition a success?

Eckard B

3 Speed of light in vacuum, what definition?

Yes. YOUR problem, not mine. I merely discuss how modern physics addresses these issues.

>	I would like you to ask you for your opinion concerning Cramer's explantion of twin paradox.

(repeating...)

4 Speed of light in vacuum, what definition?

>

NONSENSE! You are VERY confused and have missed a basic point of SR. We CANNOT ascribe any sort of "absolute zero" to ANY speed/velocity, just like there is no "absolute rest". Newton got it wrong, and we have known this for over a century. Remarkably, the equations Newton derived assuming his absolute space work equally well using any inertial frame, without any reference to "absolute space". (The very same thing happened to Maxwell's theory of electrodynamics.) For some phenomena there happens to be SET OF COORDINATES to which we normally refer its speed: * a wave in a medium is normally referenced to the inertial frame in which the medium is at rest. * an automobile is normally referenced to the inertial frame in which the LOCAL surface of the earth is at rest. It has become ingrained in our psyche to use inertial frames. It is also ingrained to use those normal frames for these specific phenomena. Indeed if one uses anything else it is incumbent on the author to explicitly mention it. There is NOTHING "natural" here [#], except humans' natural desire to make thecalculations easier. [#] I.e. Nature is neither involved nor consulted, and does not care which choice is made. But for most phenomena, including light and all massive and massless particles,there is no such "normal" coordinate system, unless the physical situationprovides one [@]. For instance, we can refer the speed of photons to theinertial frame in which their source is at rest, or we can reference to theinertial frame in which the detector is at rest -- the choice is OURS, and wegenerally select the coordinates which make the desired calculations easier. [@] And many experiments do: the inertial frame in which the laboratory is instantaneously at rest. Nature does not care which coordinates we use to describe her behavior, becauseshe clearly does not use any coordinates -- that imposes the constraint on all physical theories to be independent of coordinates; indeed all of our modern fundamental theories of physics are so independent (aka "invariant"). Note that in all cases there is NOTHING to prevent us from using some other coordinates, it's just that in such cases as mentioned above there is a specific coordinate system relative to which the calculations are easier.

I agree Nature has nothing to do with coordinates, however still there is an issue if you want to describe certain physical phenomena.

Consider a super nova in star A of Galaxy A. The light of this SN will propagate in a sphere through space.

Consider a different super nova (almost "simultaneous") but now in star B of Galaxy B. Light of this SN will also propagate in a sphere through space.

At a certain moment both spheres will meet each other. If "you" are at that moment at that specific position "you" will observe both SN's'.

Starting from that moment the points were you can observe both SN's will propagate as a circle through space.

Suppose that the result of the super nova A event is a neutron star A. After the event the neutron star will move in a large circle around the center of galaxy A (like all the other stars in its surroundings). The same for super nova B, which becomes neutron star B and which will move in a large circle through galaxy B.

My question is will neutron star A be positioned (and stay) at the center of its sphere. The same for neutron star B.

IMO both will not stay at the center of their respectivily spheres. They are only at the center a short time after the SN.

This raises an issue related to SR (if I understand correctly) SR claims that the speed of light is the same in both directions. That may be true for a certain reference frame, but IMO that same reference frame cannot be used to describe both SN.

If this is correct then it is very important to select a one and only reference frame to describe this whole "experiment".

Nicolaas Vroom.

5 Speed of light in vacuum, what definition?

>	My question is will neutron star A be positioned (and stay) at the center of its sphere. The same for neutron star B. IMO both will not stay at the center of their respectivily spheres. They are only at the center a short time after the SN.

Yes, as an electrical engineer, I support your idea that the far field of a wave isn't bound to the emitter but to space in the sense of actual mutual distances.

6 Speed of light in vacuum, what definition?

I'am also an electrical engineer (measurement and control) but that has nothing to do with this subject. But maybe it has.

The point is that measurements (observations) are highly subjective to human activities. The control part (in a broader sense) tries to unravel the laws of nature

completely independent of human activities and opinions. The laws of nature are highly mathematical. It should be possible to verify its predictions by observations and it should be realised that

mathematics (a description) in itself explains nothing.

SR belongs in the measurement part. Its basic concept or tool is light to make observations.

The control part starts from the concept that all the positions and speeds of the objects we try to study are known (measured). Newton's Law and GR belong in the control part. Light (photons) only have a very limited use in the control part. The biggest problem with the evolution of the universe is that the present is influenced (often invisible) by the past. To describe these influences mathematically, build around what is physical happening, is tricky. The basic concept to explain the forces that 'control' the objects of the universe are gravitons. Newton's Law is incomplete in the sense that it assumes that these forces act instantaneous. This is not the case.

We only use one basic clock for time keeping. The clock is used for both

measurements and control. Its primary purpose is for save keeping of all our measurements. All the events (measurements) happening with the same clock reading are called simultaneous. The behaviour of a clock (a mechanical process) is in itself a control issue. The moving "picture" I painted in my previous posting with the two SN's belongs to the control part.

Just some thoughts.

7 Speed of light in vacuum, what definition?

>

The point is that measurements (observations) are highly subjective to human activities. The control part (in a broader sense) tries to unravel the laws of nature completely independent of human activities and opinions. The laws of nature are highly mathematical. It should be possible to verify its predictions by observations and it should be realised that mathematics (a description) in itself explains nothing.

Wishful thinking. You should realize it yourself. The reality is a logical loop: you observe/ measure what your theory told you, because you believe that theory is right and different results would be erroneous. And the purpose of science is not explaining you The Great Mystical Essence. The purpose of science is putting into your head some info giving you new, strange abilities... i.e programming you.

8 Speed of light in vacuum, what definition?

Why is that electrical engineers consider themselves above physicists? I'm an electrical engineer, with a PhD in electrical engineering but I do not presume to have any knowledge above of what physicists and/or mathematicians have.

Even more, some of my colleagues at the Department of Electrical Engineering all of them electrical engineers) do collaborate with astrophysicists and physicists from the Faculty of Physics. We are doing research in adaptive optics for large telescopes (such as the ELT and GMT) and also collaborating in the Linear Collider Collaboration. A good number of our electrical engineering graduates go into getting MSc and PhD degrees in physics.

To work in those projects, we electrical engineers do have to study physics to a level we can talk with our fellow physicists. There is no free lunch in this topic.

9 Speed of light in vacuum, what definition?

>	Why is that electrical engineers consider themselves above physicists? I'm an electrical engineer, with a PhD in electrical engineering but I do not presume to have any knowledge above of what physicists and/or mathematicians have.

Bullshit, we have no picture of your diploma to prove that.

10 Speed of light in vacuum, what definition?

Sorry I do not consider myself above physicists. I try to understand them, mainly by raising questions.

>	I'm an electrical engineer, with a PhD in electrical engineering but I do not presume to have any knowledge above of what physicists and/or mathematicians have.

The only thing I silently do is, I sometimes assume that certain scientists have it wrong (this can be both ways) or don't express themself clearly.

>	To work in those projects, we electrical engineers do have to study physics to a level we can talk with our fellow physicists. There is no free lunch in this topic.

When you study my home page: maybe its a free lunch. Good luck

Nicolaas Vroom

11 Speed of light in vacuum, what definition?

> >	Yes, as an electrical engineer, I support your idea that the far field of a wave isn't bound to the emitter but to space in the sense of actual mutual distances.

>

>	The point is that measurements (observations) are highly subjective to human activities.

Mathematicians and physicists need philosophy more directly than do engineers. I felt forced to question some philosophy and interpretation-related issues when experts admitted being unable to answer my questions.

From FQXi I got the impression that there is still no clear meaning for what is an "observer". What does the speed of light wrt an observer mean? According to my definition, c relates to the two only involved locations, emitter at the moment of emission and receiver at the moment of arrival.

I interprete the Sagnac effect as due to changed length of path, not changed timescale.

12 Speed of light in vacuum, what definition?

On 6/27/17 6/27/17 10:41 AM, Nicolaas Vroom wrote:

>

I agree Nature has nothing to do with coordinates, however still there is an issue if you want to describe certain physical phenomena. [Two supernovas, each orbiting a different galaxy, and discussion of their "light spheres"] My question is will neutron star A be positioned (and stay) at the center of its sphere. The same for neutron star B IMO both will not stay at the center of their respectivily spheres. They are only at the center a short time after the SN.

This is really too complicated to discuss, as it inherently requires GR, and ambiguities arise due to the presence of gravitation. For example, nether source actually has a light SPHERE, because the light paths are distorted by the gravitation.

Let me simplify the scenario so it can be analyzed in SR.

Consider two pulsed omnidirectional light sources, A and B, each mounted on the rim of a circular table. The two tables are identical, and they rotate in the X-Y plane of an inertial frame S, with centers at rest in S. They are separated by more than their diameter, and are counter-rotating at identical speeds relative to S; they are synchronized such that A and B cross the line between their centers simultaneously in S, and that is when they both emit a light pulse (once per rotation). At the instant of each emission, A and B are both at rest in inertial frame S' -- let us only discuss one such emission and use it to define Minkowski coordinates in S and S', with the x and x' axes along their velocity relative to S, and with the emission happening at (x,t)=(0,0) and (x',t')=(0,0). Everything is in vacuum; spacetime is flat throughout the region of interest. Ignore shadows from the apparatus.

This is a more precise description than yours, but I think it captures the situation you had in mind.

Relative to S', A and B each have a light sphere that expands at c, and all observations of either see no redshift or blueshift.

Note that after the light pulses are emitted, neither A nor B remains at the center of its light sphere, relative to either S or S'. As expected -- once emitted, light has no connection to its source.

>	This raises an issue related to SR (if I understand correctly) SR claims that the speed of light is the same in both directions. That may be true for a certain reference frame, but IMO that same reference frame cannot be used to describe both SN.

But either S or S' can be used to describe light sources A and B. The speed of light is isotropically c in S, and isotropically c in S'. You could choose any other inertial frame as well, and the light from each source would expand in a sphere relative to it.

The same would apply to your supernovas, ignoring the effects of gravity.

>	If this is correct then it is very important to select a one and only reference frame to describe this whole "experiment".

To perform an analysis, one must indeed select coordinates. But for the situation I described one can select ANY inertial frame. It's just that S and S' will make the analysis simpler than any of the others, because these are directly related to the physical situation.

>	The point is that measurements (observations) are highly subjective to human activities.

Some are, some aren't. But to test a physical theory one must ensure that external actions/activities, human or otherwise, do not affect the result.

>	The laws of nature are highly mathematical.

What God whispered in your ear and told you this?

Our MODELS of nature are indeed highly mathematical. But it seems HIGHLY doubtful that nature actually uses mathematics, she just does her thing (whatever that is).

See: Wigner, "The Unreasonable Effectiveness of Mathematics in the Natural Sciences", Communications in Pure and Applied Mathematics, vol. 13, No. I (February 1960)

>	[excessively simplistic attempt to discuss "measurement and control"]

Tom Roberts

13 Speed of light in vacuum, what definition?

>	On 6/27/17 6/27/17 10:41 AM, Nicolaas Vroom wrote:

> >	IMO both will not stay at the center of their respectivily spheres.

>	This is really too complicated to discuss, as it inherently requires GR, and ambiguities arise due to the presence of gravitation. For example, nether source actually has a light SPHERE, because the light paths are distorted by the gravitation.

I agree with you.

>	Let me simplify the scenario so it can be analyzed in SR.

>	This is a more precise description than yours, but I think it captures the situation you had in mind.

It is much more precise, but maybe too.

In some sense I have in mind a binary star system (at large distance) which both explode and become a neutron star.

>	Note that after the light pulses are emitted, neither A nor B remains at the center of its light sphere, relative to either S or S'. As expected -- once emitted, light has no connection to its source.

That is correct. What is even more there is IMO no way that "nsA" can establish its position within his sphere nor can "nsB". "nsA" can not establish where the center is of the sphere nor in which direction "nsA" moves relative to his sphere.

> >	This raises an issue related to SR (if I understand correctly) SR claims that the speed of light is the same in both directions. That may be true for a certain reference frame, but IMO that same reference frame cannot be used to describe both SN.

>	But either S or S' can be used to describe light sources A and B. The speed of light is isotropically c in S, and isotropically c in S'.

The problem IMO is that it is impossible, assuming we are speaking of one sphere for each SN what the speed of light is.

>	You could choose any other inertial frame as well, and the light from each source would expand in a sphere relative to it.

IMO we can only speak of one inertial frame for each sphere with the origin in the center of the sphere. However the neutron star (ns) will not stay in the origin and move to a different location.

>	The same would apply to your supernovas, ignoring the effects of gravity.

> >	If this is correct then it is very important to select a one and only reference frame to describe this whole "experiment".

>	To perform an analysis, one must indeed select coordinates. But for the situation I described one can select ANY inertial frame. It's just that S and S' will make the analysis simpler than any of the others, because these are directly related to the physical situation.

The problem IMO is in principle identical as when I turn a light ON and OFF outside in the open air. This signal will also be propageted in a sphere around me. IMO there is only one frame which origin coincides with the center of this sphere. When I turn on the light my position will be at the origin of this frame, there after I move to a different position.

It is important to state that the speed of light is the same in all directions. The problem is that it is not possible to establish (with as a reference point the ns) where this sphere is at any one moment, nor to establish what the speed of this sphere is or to establish what my own speed (the ns) is relatif to speed of this sphere.

>	Tom Roberts

Thanks.

Nicolaas Vroom

14 Speed of light in vacuum, what definition?

Hmmm. You seem to think that each source has "one" light sphere. This is incorrect.

Note that whenever I mentioned a "light sphere" I ALSO mentioned to which inertial frame it was referenced. This is because EACH inertial frame sees a DIFFERENT light sphere for a given source. Relative to any inertial frame the pulsed omindirectional source's light expands outward in a sphere; because simultaneity is different in the different frames, and the sphere is a locus that is SIMULTANEOUS in the frame, these spheres are DIFFERENT in different frames; each expands at c relative to their frame.

>	The problem IMO is that it is impossible, assuming we are speaking of one sphere for each SN what the speed of light is.

YOU discussed "SN", while I discussed pulsed omindirectional light sources. I'll ignore the difference, still using SR.

First, your assumption is WRONG, and there is not "one sphere" -- each inertial frame sees a DIFFERENT light sphere for a given source.

Second, if you want to measure the speed of light, you must have TWO observations of the light (along with a known distance between the measurements, and synchronized clocks to measure the time of flight). So there is nothing special here, if all you have is the pulsed light source (SN), you cannot measure the speed of its light.

>

The problem IMO is in principle identical as when I turn a light ON and OFF outside in the open air. This signal will also be propageted in a sphere around me. IMO there is only one frame which origin coincides with the center of this sphere. When I turn on the light my position will be at the origin of this frame, there after I move to a different position.

EVERY inertial frame will observe a light sphere, and the source AT THE TIME OF EMISSION is at the center of each frame's sphere. Of course after the emission the source can move away from the center; a non-inertial source will move away from the center of every sphere; an inertial source will move away from the center of every sphere except one:

There is one unique inertial frame in which there is no Doppler shift anywhere in the light sphere -- this is the inertial frame in which the source was at rest at the moment of emission.

>	It is important to state that the speed of light is the same in all directions.

OK. But it is important to qualify this, as it is only valid relative to an inertial frame, and in vacuum.

>	The problem is that it is not possible to establish (with as a reference point the ns) where this sphere is at any one moment, nor to establish what the speed of this sphere is or to establish what my own speed (the ns) is relatif to speed of this sphere.

Given the requisite measuring apparatus, it would be easy to measure where the sphere is and how fast it is moving. This apparatus must be distributed throughout spacetime and communicate results to a single observer for analysis. And of course, given sufficient knowledge of physics, one can calculate where it is....

Tom Roberts

15 Speed of light in vacuum, what definition?

"Proper time" is not a translation from German, it is a technical term in English, specific to physics. I believe the corresponding "Fachwort" in German is indeed "die Eigenzeit". Note that "proper time" does mean "a clock's (person's) own time", which is a not unreasonable translation of "die Eigenzeit".

>	I don't see questions of mine answered.

Because you have not phrased them well enough to have answers.

>>	This is a direct analogy to a triangle ABC in the Euclidean plane: The triangle has two paths from A to B (AB and ACB) that clearly differ in length. The twin paradox, with instantaneous accelerations for the traveling twin, is just a triangle in a space-time plane.

>	Hmm. Isn't AC + CB shorter than AB?

Yes ("shorter" = "less elapsed proper time") -- this is an ANALOGY to a triangle in the Euclidean plane. The geometry in a space-time plane is hyperbolic, not Euclidean. That is:

d\tau^2 = dt^2 - dx^2

Tom Roberts

16 Speed of light in vacuum, what definition?

>	Note that whenever I mentioned a "light sphere" I ALSO mentioned to which inertial frame it was referenced. This is because EACH inertial frame sees a DIFFERENT light sphere for a given source.

Note also, that walking on a street EACH observer sees buildings and trees running around him. A relativistic moron imagined! So whoever doesn't see them is a stupid relativity denying crank.

17 Speed of light in vacuum, what definition?

>	"Proper time" is not a translation from German, it is a technical term in English, specific to physics.

And "best system" is a tehnical term specific to communist's newspeak.

18 Speed of light in vacuum, what definition?

>>	The translation of "Eigenzeit" as "proper time" is a bit misleading because proper means real and satisfactory rather than inadequate.

EigenZeit is merely Own Time, not proper time. Go to school, young man.

>	"Proper time" is not a translation from German, it is a technical term in English, specific to physics. I believe the corresponding "Fachwort" in German is indeed "die Eigenzeit". Note that "proper time" does mean

No, you don't know germ, I'm disappointed.

>	"a clock's (person's) own time", which is a not unreasonable translation of "die Eigenzeit".

Eigen -> Own
Zeit -> Time.

What on Earth is so difficult.

19 Speed of light in vacuum, what definition?

Hmmmm. This depends strongly on one's theoretical context. In QM the answer is MUCH different than in SR and GR.

In SR and GR the meaning of "observer" can usually be determined from context, at least in a well-written description. Normally it means a pointlike, timelike object that can record observations of its immediate vicinity; often the object is given human qualities and can also perform calculations, write reports, etc.

Describing "observer" in QM would take a book....

>	What does the speed of light wrt an observer mean?

Nothing, by itself. This is a verbal shortcut, and means the speed of light relative to the coordinates in which the observer is at rest (invariably a locally-inertial frame).

>	According to my definition, c relates to the two only involved locations, emitter at the moment of emission and receiver at the moment of arrival.

That is incomplete, and utterly unable to measure a speed. To actually make such a measurement, the "two locations" need to have a known distance between them (at the instants of their measurements), and clocks that are SYNCHRONIZED -- this is equivalent to constructing coordinates.

As I keep saying and you keep ignoring, "velocity" and "speed" are ONLY defined relative to a given set of coordinates. THIS IS WHAT THESE WORDS MEAN. Both are inherently coordinate dependent. Moreover, unless explicitly mentioned, the coordinates are presumed to be a (locally) inertial frame.

Note that "4-velocity", while superficially similar, is quite different; it is a geometrical quantity that is independent of coordinates.

Except, of course, in a Sagnac interferometer the light path DOES NOT CHANGE. So your "interpretation" is contrary to observed facts.

The Sagnac effect is simply due to the fact that the vacuum speed of light is isotropically c ONLY relative to a locally inertial frame, and the rotating interferometer is not at rest in such a frame. From this basic fact one can quantitatively calculate the fringe shift, and such predictions are in agreement with observations. So this is an excellent example of the importance of COORDINATES in measuring speed, and especially the importance of (locally) inertial frames. You have failed to understand this, and need to go STUDY -- nobody ever learned anything by writing nonsense to the 'net....

Tom Roberts

20 Speed of light in vacuum, what definition?

Am Dienstag, 4. Juli 2017 16:46:54 UTC+2 schrieb tjrob137:

>	On 6/30/17 6/30/17 4:27 AM, Eckard B wrote:

> >	I got the impression that there is still no clear meaning for what is an "observer".

>

Hmmmm. This depends strongly on one's theoretical context. In QM the answer is MUCH different than in SR and GR. In SR and GR the meaning of "observer" can usually be determined from context, at least in a well-written description. Normally it means a pointlike, timelike object that can record observations of its immediate vicinity;

Isn't anything timelike (r

>	often the object is given human qualities and can also perform calculations, write reports, etc.

Well, the so called bird's (or God's) view combines a lot.

>	Describing "observer" in QM would take a book....

Yes, beginning with so called observables.

>

> >	What does the speed of light wrt an observer mean?

>	Nothing, by itself. This is a verbal shortcut, and means the speed of light relative to the coordinates in which the observer is at rest (invariably a locally-inertial frame).

My one-way definition doesn't refer to a coordinate system, not to coordinates of the emiiter, not to those of receiver, not to any third object. It refers to a difference between two locations at different points of time.

>

> >	According to my definition, c relates to the two only involved locations, emitter at the moment of emission and receiver at the moment of arrival.

>	That is incomplete,

Well you might miss the absolute reference point.

>	and utterly unable to measure a speed.

I see it tho only reasonable definition.

>	To actually make such a measurement, the "two locations" need to have a known distance between them (at the instants of their measurements),

Yes. This is not impossible in case the distance doesn't change or the change is of known size.

>	and clocks that are SYNCHRONIZED -- this is equivalent to constructing coordinates.

I several times explained how clocks can be synchronized even if they are moving relative to each other.

>	As I keep saying and you keep ignoring, "velocity" and "speed" are ONLY defined relative to a given set of coordinates. THIS IS WHAT THESE WORDS MEAN. Both are inherently coordinate dependent.

Coordinates imply the origin of a scale. My definition doesn't need it because it refers to a difference. Hence, the origin doen't matter.

>

> >	I interprete the Sagnac effect as due to changed length of path, not changed timescale.

>	Except, of course, in a Sagnac interferometer the light path DOES NOT CHANGE. > So your "interpretation" is contrary to observed facts.

Really? I consider space as mutual distances. Rotation may change the length of path.

>

The Sagnac effect is simply due to the fact that the vacuum speed of light is isotropically c ONLY relative to a locally inertial frame, and the rotating interferometer is not at rest in such a frame. From this basic fact one can quantitatively calculate the fringe shift, and such predictions are in agreement with observations.

While the effects of rotation compensate each other in case of Michelson's 1881 and 1887 experiments, this is not the case in case of Sagnac effect and Michelson's 1923 experiment with Gale. I am not sure whether or not one needs coordinates in this case as to understand the effect. The center of rotation may serve as absolute reference point.

21 Speed of light in vacuum, what definition?

> >	Of course. Minkowski applied hyperbolic geometry which is formally correct while one more reason to distrust SR.

>	Minkowski hyperbolic geometry is embedded in the LT. Didn't you know that?

A serious reason for me to distrust SR was already that LT was based on length contraction which was fabricated as to rescue the untenable aether hypothesis. I would like to better understand the electromagnetic mechanism behind what Fuller seems to cryptically illustrate. Lorentz' formula seems to explain what was observed with accelerators. I don't deny this. However, I distrust Poincaré who iIrc is characterized like a butterfly jumping from one flower to the next one instead of consequently thinking to the end.

22 Speed of light in vacuum, what definition?

This is just plain not true. "Length contraction" is a CONCLUSION of the Lorentz transform, not any basis of it.

Yes, Lorentz and Fitzgerald each considered "length contraction" before 1904. But go READ Lorentz's 1904 paper that derived the LT and you'll see it is not used in the derivation.

Einstein historically intermixed two quite different theories which we now keep separate: SR as a theory of geometry (only), and electrodynamics (both classical and quantum versions). A primary reason to separate them is so the transition SR -> GR remains purely geometrical, independent of electrodynamics.

>	I would like to better understand the electromagnetic mechanism [...]

A modern derivation of the LT uses no electromagnetism at all:

From the basic definition of inertial frames, elementary group theory shows that there can be only three transformation groups among them: Euclid, Galilei, and Lorentz. Only the last survives confrontation with experiment (which also determines its free parameter, c, to be equal to the vacuum speed of light).

Tom Roberts

23 Speed of light in vacuum, what definition?

Am Mittwoch, 5. Juli 2017 19:08:40 UTC+2 schrieb tjrob137:

>	On 7/5/17 7/5/17 - 2:27 AM, Eckard B wrote:

> >	A serious reason for me to distrust SR was already that LT was based on length contraction which was fabricated as to rescue the untenable aether hypothesis.

>

This is just plain not true. "Length contraction" is a CONCLUSION of the Lorentz transform, not any basis of it. Yes, Lorentz and Fitzgerald each considered "length contraction" before 1904. But go READ Lorentz's 1904 paper that derived the LT and you'll see it is not used in the derivation. TODAY we derive the equations of SR, including the Lorentz transform, from basic geometrical principles, without any reference to light at all.

When FitzGerald and Lorentz independent from each other fabricated the idea of length contraction, they intended to explain Michelson's failure to explain his unexpected null result. This led much later to what Poincaré called LT.

The history of G. Cantor's untenable but up to now not yet abandoned naive definition of an infinite set also exhibits numerous maneuvers to rescue something even by means of complicated systems of highly abstract axioms without any reference to numbers at all.

>

Einstein historically intermixed two quite different theories which we now keep separate: SR as a theory of geometry (only), and electrodynamics (both classical and quantum versions). A primary reason to separate them is so the transition SR -> GR remains purely geometrical, independent of electrodynamics.

Cantor claimed: The essence of mathematics is its freedom. I am not aware of a mathematical reason to prefer e.g. Riemannian geometry in physics. On the other hand, there are often unseen possibilities of interpretation.

>

> >	I would like to better understand the electromagnetic mechanism [...]

>

A modern derivation of the LT uses no electromagnetism at all: From the basic definition of inertial frames, elementary group theory shows that there can be only three transformation groups among them: Euclid, Galilei, and Lorentz. Only the last survives confrontation with experiment (which also determines its free parameter, c, to be equal to the vacuum speed of light).

Which basic definition of inertial frames do you refer to? Does group theory obey reality? Does it distinguish between past and future?

24 Speed of light in vacuum, what definition?

No. An observer has a worldline and so can observe over time along that worldline. But it does require zero DISTANCE: An observer can only observe right where he/she/it is located -- for distant happenings it requires SIGNALS from the happening to the observer, who observes the SIGNALS, not the happening itself. (All too many people around here forget this, and omit analyzing effects on those signals.)

I use "happening" rather than "event", because the latter is a technical word with a quite different meaning. A "happening" is some physical phenomenon that happens, while an event is a point in the spacetime manifold.

That actually requires a set of observers distributed throughout the region of interest, so there is one co-located with any happening of interest. They must all communicate to some single location where the results can be analyzed.

>>>	What does the speed of light wrt an observer mean?

>>	Nothing, by itself. This is a verbal shortcut, and means the speed of light relative to the coordinates in which the observer is at rest (invariably a locally-inertial frame).

>	My one-way definition doesn't refer to a coordinate system, not to coordinates of the emiiter, not to those of receiver, not to any third object. It refers to a difference between two locations at different points of time.

But you need to SYNCHRONIZE THE CLOCKS, and that is effectively constructing a coordinate system. You just don't call it that. What is in a name?

>	I several times explained how clocks can be synchronized even if they are moving relative to each other.

Hopeless, in general. Because "synchronization" is eternal -- once synchronized, clocks must REMAIN in synch, or they are not really synchronized. Except under very special circumstances, clocks moving relative to each other do not remain in sync, and thus cannot be synchronized.

>>	As I keep saying and you keep ignoring, "velocity" and "speed" are ONLY defined relative to a given set of coordinates. THIS IS WHAT THESE WORDS MEAN. Both are inherently coordinate dependent.

>	Coordinates imply the origin of a scale.

Not just that. You need to LEARN what coordinates actually are. And how they are used.

>>>	I interprete the Sagnac effect as due to changed length of path, not changed timescale.

>>	Except, of course, in a Sagnac interferometer the light path DOES NOT CHANGE. So your "interpretation" is contrary to observed facts.

>	Really? I consider space as mutual distances. Rotation may change the length of path.

Just LOOK at the Sagnac interferometer. Except for the rotation nothing moves, and it rotates rigidly. So the light paths remain unchanged relative to the instrument, which is what matters.

Tom Roberts

25 Speed of light in vacuum, what definition?

Physicists also considered caloric, phlogiston, and aether -- that does NOT mean that modern physics is "based upon" them.

Similarly, in deriving the Lorentz group, Poincare' considered the invariance group of the Maxwell's equations, not "length contraction" or "time dilation" -- GO READ HIS PAPERS, and you will find no mention of them in the derivations. Just like Lorentz and Einstein.

"Post hoc ergo propter hoc" is a fallacy that you must learn to avoid.

>	[... further baseless ranting and raving unrelated to the subject]

Tom Roberts

26 Speed of light in vacuum, what definition?

If a source generates a sequence of flashes each flash generates a light sphere.

>

Note that whenever I mentioned a "light sphere" I ALSO mentioned to which inertial frame it was referenced. This is because EACH inertial frame sees a DIFFERENT light sphere for a given source. Relative to any inertial frame the pulsed omindirectional source's light expands outward in a sphere; because simultaneity is different in the different frames, and the sphere is a locus that is SIMULTANEOUS in the frame, these spheres are DIFFERENT in different frames; each expands at c relative to their frame.

This is a tricky discussion. As far as I understand all the points on one physical sphere represent the same time and as such are simultaneous events. All the points on one sphere at each moment are at the same distance from the center of the sphere. The interesting part starts if you want to draw light spheres of two sources in the same coordination system (CS), Our first assumption can be that the origin of source 1 is at the origin of the CS. The next assumption is that the source 1 is at rest in the CS. In that case the LS stays at the origin if the source moves in a straight line.
The next step is to draw the position of source 2 in this CS. For simplicity draw these points somewhere in a straight line with the indications t0, t1 and t2. Suppose there is a light flash of source 2 at t0. Where do we draw the light sphere at t1? centered around t1 or not? Suppose there also is a light flash of source 1 at t0. Where do we draw the light sphere at t1? around the origin at t1 or not?

Suppose there is a third light source. This light source is at the origin at t0 but moves away from the origin at t1 and t2 in a straight line. Where do we draw the light sphere at t1? around the origin at t1 or not?

The reason why I write this text is because it is important to study the movement of more than one object. To be more specific it is wrong to consider one object at rest and forget the others. You need one approach.

> >	The problem IMO is that it is impossible, assuming we are speaking of one sphere for each SN what the speed of light is.

>	YOU discussed "SN", while I discussed pulsed omindirectional light sources. I'll ignore the difference, still using SR. First, your assumption is WRONG, and there is not "one sphere" -- each inertial frame sees a DIFFERENT light sphere for a given source.

What is the meaning of the word sees? An observer at the center can not see the light sphere.

>	There is one unique inertial frame in which there is no Doppler shift anywherein the light sphere -- this is the inertial frame in which the source was at rest at the moment of emission.

As I said above "we" should study the movement of all objects together in one Coordinate System. Above I used source 1. But why source 1, it also could be source 2 or source 3. IMO the preferred CS is the one in which the center of each Light sphere always stays in the center. (As if it is fixed). The Question is, if such a CS "exists".

Nicolaas Vroom

27 Speed of light in vacuum, what definition?

It was meant in context of the single flash.

For each such a flash, there is as many different spheres as there are different, mutually moving inertial frames.

As for each such a frame, there is a light sphere centre that is in rest wrt it.

And these centres in different frames move wrt each other as well as the frames do.

It is the direct consequence of invariant vacuum light speed.

28 Speed of light in vacuum, what definition?

>	As for each such a frame, there is a light sphere centre that is in rest wrt it.

The only thing I understand is that a light sphere has a center. The only question to answer is does the original source stays at this center. I have my doubts.
When the source generates many flashes and many spheres the question is: Does each sphere has the same center (which stays at rest within these spheres). I have my doubts (in general).

Maybe you can create a coordinate system in which all these flashes are generated in the origin of that CS. In such a CS all light spheres have the same center and if I'am correct are concentric.

The problem is how can I use that CS to describe (the spheres of) other sources? I think, it is very difficult. You need, I think, a different approach.

>	And these centres in different frames move wrt each other as well as the frames do. It is the direct consequence of invariant vacuum light speed.

?

Nicolaas Vroom

29 Speed of light in vacuum, what definition?

>

Okay

>>	For each such a flash, there is as many different spheres as there are different, mutually moving inertial frames.

>	No. For each flash at any moment in time there exists only one sphere.

At the moment of the flash emission, all the spheres share the same centre.

Since that moment, the depart each other, as the frames depart each other.

>	This sphere, in time, grows in size.

They do.

>	Such a flash has a source and you can assign a Coordinate system with source at its origin. You can also assign to this CS a speed.

Yes. but the sphere centres share their positions with the source ONLY in the frame where the source is in rest.

As no matter. what is the velocity of the source, the pulse propagates isotropically by the same speed from the position where the source was and pulse emission, not where it is.

>>	As for each such a frame, there is a light sphere centre that is in rest wrt it.

>	The only thing I understand is that a light sphere has a center.

that is not much...

>	The only question to answer is does the original source stays at this center.

Yes, if the source is in rest with given inertial frame. No, if the source is not in rest with given inertial frame.

>	I have my doubts. When the source generates many flashes and many spheres the question is: Does each sphere has the same center (which stays at rest within these spheres). I have my doubts (in general).

Yes, if the source is in rest with given inertial frame. No, if the source is not in rest with given inertial frame.

>	Maybe you can create a coordinate system in which all these flashes are generated in the origin of that CS. In such a CS all light spheres have the same center and if I'am correct are concentric.

Yes, if the frame is inertial.

>	The problem is how can I use that CS to describe (the spheres of) other sources? I think, it is very difficult. You need, I think, a different approach.

why difficult ?
CS does not belong to a single source. All sources are in all frames.
And centres of all spheres are in rest wrt all inertial frames, regardless of motion of the sources.

>

>>	And these centres in different frames move wrt each other as well as the frames do. It is the direct consequence of invariant vacuum light speed.

>

Any light has the same speed wrt any inertial system. Therefore within any inertial frame, all sphere centres are static. even if these frames mover each wrt other.

30 Speed of light in vacuum, what definition?

> >	The only question to answer is does the original source stays at this center.

>	Yes, if the source is in rest with given inertial frame. No, if the source is not in rest with given inertial frame.

> >	I have my doubts. When the source generates many flashes and many spheres the question is: Does each sphere has the same center (which stays at rest within these spheres). I have my doubts (in general).

>	Yes, if the source is in rest with given inertial frame. No, if the source is not in rest with given inertial frame.

> >	Maybe you can create a coordinate system in which all these flashes are generated in the origin of that CS. In such a CS all light spheres have the same center and if I'am correct are concentric.

>	Yes, if the frame is inertial.

> >	The problem is how can I use that CS to describe (the spheres of) other sources? I think, it is very difficult. You need, I think, a different approach.

>	why difficult ?

I think it is difficult.

Let us try to find an answer using an example.

Consider a source which explodes and at that moment creates a light sphere. The explosion creates 3 identical objects which move away:
1 in the x direction with a speed vx,
2 in the y direction with a speed vy and
3 in the z direction with a speed vz.
IMO think you can draw 3 different inertial frames:
one #1 using object 1 which moves along the x axis with speed vx.
one #2 using object 2 which moves along the y axis with speed vy and
one #3 using object 3 which moves along the z axis with speed vz.

The problem is how do describe the light sphere in each inertial frame The problem is all the inertial frames are almost "identical". The difference is the speed: vx, vy and vz. That means there is no preference. Either the source is at rest in all inertial frames
or the source is not at rest in any inertial frame.

You can also consider that the source before the explosion is already moving in the vx direction. Now you can claim that the source was and is at rest in inertial frame #1 of object 1. However this does not physical makes a difference with the inertial frames #2 and #3 because in all cases the light sphere is the same.

Nicolaas Vroom

31 Speed of light in vacuum, what definition?

Dne 11.7.2017 v 18:23 Nicolaas Vroom napsal(a):

>

I think it is difficult. Let us try to find an answer using an example. Consider a source which explodes and at that moment creates a light sphere. The explosion creates 3 identical objects which move away: 1 in the x direction with a speed vx, 2 in the y direction with a speed vy and 3 in the z direction with a speed vz. IMO think you can draw 3 different inertial frames: one #1 using object 1 which moves along the x axis with speed vx. one #2 using object 2 which moves along the y axis with speed vy and one #3 using object 3 which moves along the z axis with speed vz.

OK..

>

The problem is how do describe the light sphere in each inertial frame The problem is all the inertial frames are almost "identical". The difference is the speed: vx, vy and vz. That means there is no preference. Either the source is at rest in all inertial frames or the source is not at rest in any inertial frame.

The source was not in rest in any (future) inertial frame.
The centre of the light sphere is in rest in all inertial frames.

>	You can also consider that the source before the explosion is already moving in the vx direction.

In all vx, vy,vz directions in respective frames.

>	Now you can claim that the source was and is at rest in inertial frame #1 of object 1.

It was in rest in no frame.

>	However this does not physical makes a difference with the inertial frames #2 and #3 because in all cases the light sphere is the same.

It is not. in all frames, the centre is statics, yet the frames move wrt each other.

32 Speed of light in vacuum, what definition?

Irrelevant. A single omnidirectional flash produces a DIFFERENT light sphere in each inertial frame. This is what has confused you. See below.

>>

Note that whenever I mentioned a "light sphere" I ALSO mentioned to which inertial frame it was referenced. This is because EACH inertial frame sees a DIFFERENT light sphere for a given source. Relative to any inertial frame the pulsed omindirectional source's light expands outward in a sphere; because simultaneity is different in the different frames, and the sphere is a locus that is SIMULTANEOUS in the frame, these spheres are DIFFERENT in different frames; each expands at c relative to their frame.

>	This is a tricky discussion.

Not really. You just have to keep things straight, and understand how SR works. But that seems unfamiliar to you.

>	As far as I understand all the points on one physical sphere represent the same time and as such are simultaneous events. All the points on one sphere at each moment are at the same distance from the center of the sphere.

Yes. Since simultaneity is DIFFERENT in different inertial frames, so are their light spheres. That is, not only do observers in different inertial frames see different redshifts of the light (at different places), they also see spheres that are different loci.

We can take the "external analyst" point of view, in that we are not part of the gedanken, we are looking at it and can see and measure everything of interest. In particular, "time" to us is COMPLETELY DIFFERENT from "time in the gedanken", as is space [#]. You can think of the gedanken as happening in this small region of space inside this breadbox -- WE can look inside the breadbox, at any time (to the gedanken). So we can see the entire light sphere from a single flash, and can project it onto multiple inertial frames and thus conclude what observers in those frames would see.

[#] Compare to a simple plot of position vs. time for a falling object. WE can examine any value of time IN THE PLOT. The spacetime manifold of SR is analogous to the PLOT, not to the object (or its falling). The plot is a MODEL of the falling object, in the same sense that spacetime is a model of some potential world in which the gedanken happens.

Later you say:

>	For each flash at any moment in time there exists only one sphere.

This is wrong. Every inertial frame will see a DIFFERENT light sphere (as analyzed by an external analyst). Because the sphere is a locus simultaneous in the frame, and "simultaneous" is DIFFERENT for different inertial frames.

Think of it this way: to an external analyst, the light sphere from a single flash is not a sphere in spacetime, it is a hyper-tube that expands over time. An external analyst can see it all, for all times in the manifold, and there is just one of these expanding hyper-tubes. A hyper-tube in 4 dimensions is hard to visualize, so suppress one spatial dimension giving {x,y,t} that form a volume (inside the breadbox). The light from a single omnidirectional flash is now a CONE with axis along t, apex at the flash, expanding at the speed of light in x and y as time advances [@]. We external analysts can see the entire cone, as we are divorced from time in the manifold. In the manifold, a given inertial frame takes a slice of the cone at each value of its time coordinate, but different frames take slices that are TILTED differently, so they see different circles (spheres with z restored).

NOTE: The geometry of this is Euclidean, and those slices are ellipses, while the actual geometry should be Minkowskian in which those slices are circles (spheres with z restored).

[@] Implicitly I have described the volume using coordinates of the inertial frame in which the light source is at rest.

33 Speed of light in vacuum, what definition?

> >	My one-way definition doesn't refer to a coordinate system, not to coordinates of the emitter, not to those of receiver, not to any third object. It refers to a difference between two locations at different points of time.

>	But you need to SYNCHRONIZE THE CLOCKS, and that is effectively constructing a coordinate system. You just don't call it that. What is in a name?

OK, I accept that the good old conjecture of common time is something like a coordinate that provides synchrony by definition even in excess of the border between the order of all what happened in the past and what may be expected to happen in the future. The only measurable basis of this coordinate is elapsed time with its natural reference zero.

However, those who are using the word coordinate system usually refer to space, and there is no preferred point of reference in space. This doesn't imply that there is no order in space. The good old imagination of space is mutual distances in three orthogonal to each other dimensions. Any Cartesian representation of it is arbitrarily chosen. If we are dealing with the basic definition of velocity, we may restrict to just one staight line. A corresponding coordinate x implies already a chosen point of reference.

The decisive argument of mine is that the definition of speed doesn't require this choice but only a difference between two locations that belong to two subsequent points at the time coordinate.

On this basis, we need no x-coordinate and no observer for measurements of elapsed time delta t and of delta x between two locations that are at rest to each other. In case of using signals, we have to take into account Doppler transformation, not Woldmear Vogt's formulas, in order to compensate for delays.

34 Speed of light in vacuum, what definition?

I agree with what you describe about each inertial frame.

> >>

Note that whenever I mentioned a "light sphere" I ALSO mentioned to which inertial frame it was referenced. This is because EACH inertial frame sees a DIFFERENT light sphere for a given source. Relative to any inertial frame the pulsed omindirectional source's light expands outward in a sphere; because simultaneity is different in the different frames, and the sphere is a locus that is SIMULTANEOUS in the frame, these spheres are DIFFERENT in different frames; each expands at c relative to their frame.

> >	This is a tricky discussion.

>	Not really. You just have to keep things straight, and understand how SR

>	works. But that seems unfamiliar to you.

Here we have a problem? I also want to understand how SR works but I also want to understand how the physical world works i.e. for example how the planets move around the Sun.

> >	As far as I understand all the points on one physical sphere represent the same time and as such are simultaneous events. All the points on one sphere at each moment are at the same distance from the center of the sphere.

>	Yes. Since simultaneity is DIFFERENT in different inertial frames, so aretheir light spheres. That is, not only do observers in different inertial frames see different redshifts of the light (at different places), they also see spheres that are different loci.

I have no problem with this. But why do we need different observers?

>

We can take the "external analyst" point of view, in that we are not part of the gedanken, we are looking at it and can see and measure everything of interest. In particular, "time" to us is COMPLETELY DIFFERENT from "time in the gedanken", as is space [#]. You can think of the gedanken as happening in this small region of space inside this breadbox -- WE can look inside the breadbox, at any time (to the gedanken). So we can see the entire light sphere from a single flash, and can project it onto multiple inertial frames and thus conclude what observers in those frames would see.

My problem is why do you need observers. Each observer only sees a distorted image of the physical reality. An observer only sees a small part of a light sphere when the light sphere comes very close at his location (in time)

You need a grid of observation points to get an image of each light sphere.

Of course you can think about an external analyst who can observe all the light spheres at once at a certain instance or look. IMO you need many looks to observe the evolution of all the light sources.

The question is do the individual positions of the spheres change while looking,

>

[#] Compare to a simple plot of position vs. time for a falling object. WE can examine any value of time IN THE PLOT. The spacetime manifold of SR is analogous to the PLOT, not to the object (or its falling). The plot is a MODEL of the falling object, in the same sense that spacetime is a model of some potential world in which the gedanken happens. Later you say:

> >	For each flash at any moment in time there exists only one sphere.

>	This is wrong. Every inertial frame will see a DIFFERENT light sphere (as analyzed by an external analyst). Because the sphere is a locus simultaneous in the frame, and "simultaneous" is DIFFERENT for different inertial frames.

Generally speaking a simulataneous event observed by one observer is not a simultaneous event for an other observer except if they are at the same location independent if they are moving or not. For an external analyst this is different because he observes all what is happening simultaneous at each observation, look. (?)

>

Think of it this way: to an external analyst, the light sphere from a single flash is not a sphere in spacetime, it is a hyper-tube that expands over time. An external analyst can see it all, for all times in the manifold, and there is just one of these expanding hyper-tubes. A hyper-tube in 4 dimensions is hard to visualize, so suppress one spatial dimension giving {x,y,t} that form a volume (inside the breadbox). The light from a single omnidirectional flash is now a CONE with axis along t, apex at the flash, expanding at the speed of light in x and y as time advances [@]. We external analysts can see the entire cone, as we are divorced from time in the manifold. In the manifold, a given inertial frame takes a slice of the cone at each value of its time coordinate, but different frames take slices that are TILTED differently, so they see different circles (spheres with z restored).

I understand the concept of your lightcone. It is like the letter X rotating in the verical axis. (in the horizontal plane it is space and in the vertical direction time?) The question what happens when there are more light sources? Do you get many X's scattered through spacetime? Does it make a difference if all the light sources have a different speed? In case of many X's uses each X the same time dimension t?

Thanks.

Nicolaas Vroom

35 Speed of light in vacuum, what definition?

>	Here we have a problem? I also want to understand how SR works but I also want to understand how the physical world works i.e. for example how the planets move around the Sun.

Too bad for You - neither SR nor physical world works.

36 Speed of light in vacuum, what definition?

If you want to describe how light behaves, you need to observe it. If you want to describe its behavior over large regions, you need many different observers, or detectors.

>	Tom Roberts wrote:

>>

We can take the "external analyst" point of view, in that we are not part of the gedanken, we are looking at it and can see and measure everything of interest. In particular, "time" to us is COMPLETELY DIFFERENT from "time in the gedanken", as is space [#]. You can think of the gedanken as happening in this small region of space inside this breadbox -- WE can look inside the breadbox, at any time (to the gedanken). So we can see the entire light sphere from a single flash, and can project it onto multiple inertial frames and thus conclude what observers in those frames would see.

>	My problem is why do you need observers.

Because you cannot possibly describe how some physical phenomenon behaves without observing it.

>	Each observer only sees a distorted image of the physical reality. An observer only sees a small part of a light sphere when the light sphere comes very close at his location (in time) You need a grid of observation points to get an image of each light sphere.

That's why the "external analyst" approach is so fruitful -- we analysts can examine the manifold at any point and know what is happening there and then. We can do this for however many points we wish. including sampling the entire region of interest -- that's how we know light propagates as a sphere -- no finite number of observation points could determine that, but an external analyst can.

>	The question is do the individual positions of the spheres change while looking,

Just LOOK -- if they change you'll see it, because you are EXTERNAL to the manifold.

It ought to be clear that the external analyst cannot change anything in the manifold.

>

Generally speaking a simulataneous event observed by one observer is not a simultaneous event for an other observer except if they are at the same location independent if they are moving or not. For an external analyst this is different because he observes all what is happening simultaneous at each observation, look. (?)

No. The external analyst must PROJECT the manifold onto a given inertial frame in order to determine what is simultaneous in that frame. Using different frames gives different answers.

>	The question what happens when there are more light sources?

Each generates its own light cone. This ought to be obvious.

Tom Roberts

37 Speed of light in vacuum, what definition?

You also need a coordinate system common for them; but, of course, poor idiot Einstein couldn't know, as he was laboratory oriented.

38 Speed of light in vacuum, what definition?

Because you are busy spitting poison, you have not noticed that.

39 Speed of light in vacuum, what definition?

As he was a physicist, he was laboratory oriented. That's why he imagined his noronic local models will be sufficient. Poor, insane idiot.

40 Speed of light in vacuum, what definition?

Good question

>	If you want to describe how light behaves, you need to observe it. If you want to describe its behavior over large regions, you need many different ... or detectors.

I would rather need a model that is based on experience and logic.

>

> >	Tom Roberts wrote:

> >>

We can take the "external analyst" point of view, in that we are not part of the gedanken, we are looking at it and can see and measure everything of interest. In particular, "time" to us is COMPLETELY DIFFERENT from "time in the gedanken", as is space [#]. You can think of the gedanken as happening in this small region of space inside this breadbox -- WE can look inside the breadbox, at any time (to the gedanken). So we can see the entire light sphere from a single flash, and can project it onto multiple inertial frames and thus conclude what observers in those frames would see.

Hmm. From my mathematical model, I rather imagine photons like pairs, and light waves in contrast to acoustic spherical waves like dipoles. Detectors primarily only respond to light that is incoming on their location.

> >	My problem is why do you need observers.

>	Because you cannot possibly describe how some physical phenomenon behaves without observing it.

I disagree.

41 Speed of light in vacuum, what definition?

You have them, but you seem to be oblivious.

>	.... Hmm. From my mathematical model, I rather imagine photons like pairs, and light waves in contrast to acoustic spherical waves like dipoles.

So what "experience and logic" leads to this speculation?

>	Detectors primarily only respond to light that is incoming on their location.

Do you expect to sense photons that DON'T come?

> > >

My problem is why do you need observers.

> >	Because you cannot possibly describe how some physical phenomenon behaves without observing it.

>	I disagree.

That directly contradicts your desire to base a theory on "experience and logic."

42 Speed of light in vacuum, what definition?

I fully agree. However I want to describe the laws of nature. To do that you first need observations. The more the better. The longer period the better. Each of these observations define a small part of the universe at a specific moment. Secondly you develop/define the laws i.e. the laws that describe the physical evolution of the universe. Thirdly you again need observations to test the predictions of step 2.

> >	Tom Roberts wrote:

> >>

We can take the "external analyst" point of view, in that we are not part of the gedanken, we are looking at it and can see and measure everything of interest. In particular, "time" to us is COMPLETELY DIFFERENT from "time in the gedanken", as is space [#]. You can think of the gedanken as happening in this small region of space inside this breadbox -- WE can look inside the breadbox, at any time (to the gedanken). So we can see the entire light sphere from a single flash, and can project it onto multiple inertial frames and thus conclude what observers in those frames would see.

> >	My problem is why do you need observers.

>	Because you cannot possibly describe how some physical phenomenon behaves without observing it.

I agree with your concept of "external analyst" except I don't agree with the name. I would call it an "omni-present being" who has an image of the total universe at present in his mind with his eyes closed. As such this "external analyst" can travel instantaneous to all the corners of the universe at present,but that does not mean he knows what has happened in the past nor does he know the future. The issue is what this "external analyst" knows (not sees) about the present or the future state of the universe depents the laws of step 2 and these laws IMO have nothing to do with the speed of light but more about the speed of gravitational radiation. Only in step 3 when you want to translate the calculated state of the universe in what individual observers see you need the speed of light.

> >	Each observer only sees a distorted image of the physical reality. An observer only sees a small part of a light sphere when the light sphere cwhatomes very close at his location (in time) You need a grid of observation points to get an image of each light sphere.

>

That's why the "external analyst" approach is so fruitful -- we analysts can examine the manifold at any point and know what is happening there and then. We can do this for however many points we wish. including sampling the entire region of interest -- that's how we know light propagates as a sphere -- no finite number of observation points could determine that, but an external analyst can.

IMO gravitational radiation propagates in a sphere. A light flash in general does not propagate in a sphere as it is influenced by gravity.

IMO the simplest approach, if you want to unravel and or use the laws, you should start from one coordinate system and one clock (at rest). This raises two problems: The first problem is in step 1. In that step you have to translate the measurements of each observer with his own clock to the parameters of the coordinate system. This translation depents on the the behaviour of photons (speed of light) which is influenced by the laws of step 2.(#1)

The second problem is in step 3. In that step you have to translate the parameters of the coordinate with the clock at rest into the coordinates of each observer with his own clock. Also in this case you need the laws as described in step 2 including the laws that describe the behaviour of light, in order to perform these calculations.

The same situation also exists when you use Newton's Law in step 2. If you want to use Newton's Law you need the masses of all the objects studied. As part of step 1 based on observations (and using Newton's law) you need to calculate these parameters. This is difficult but relatif speaking simple compared with, if you want to use GR.(#2)

> >	The question is do the individual positions of the spheres change while looking,

>	Just LOOK -- if they change you'll see it, because you are EXTERNAL to the manifold.

I think this is easily written but very difficult to perform in reality As I say you need first to calculate this manifold in step 1 before you can use it in step 2 or step 3

>	It ought to be clear that the external analyst cannot change anything in the manifold.

> >

Generally speaking a simulataneous event observed by one observer is not a simultaneous event for an other observer except if they are at the same location independent if they are moving or not. For an external analyst this is different because he observes all what is happening simultaneous at each observation, look. (?)

>	No. The external analyst must PROJECT the manifold onto a given inertial frame in order to determine what is simultaneous in that frame. Using different frames gives different answers.

IMO the whole concept (sorry to say) of simultaneity of relativity is not very helpfull to study the laws of nature.
Ofcourse when I see two events simultaneous (generally speaking) a different observer will not see the same events simultaneous. Is that important: No. The only thing that is important are the two events simultaneous in some coordinating system. Or more specific are the two events happening when the clock strikes twelve in that coordinate system. What is important are the momemts of the events in a coordinate system, not when an observer sees these events.

As I indicated above in the coordinate system the speed of light is not important. In this coordinate system there are no observers. There is no Quantum Mechanical issue like related to Schrodingers Cat if the cat is both alive or dead. Immediate when there is a radioactive decay, the poison is released and the cat is dead.

(#1). This problem is agrevated because the clock in the coordinate is considered at rest or moving at a constant rate. In any case the time between each tick of the clock is the same. For a clock from any observer this is not the case because generally speaking the speed is not constant (relatif to the coordinate system), but accelerating. In that sense all these clocks are bad clocks. This makes it is difficult to calculate the initial conditions and different for each observer.

(#2). Newton's Law also assumes one coordinate system. In such a system you have the same problems as when you want to translate observations by humans into that systems. The clocks used by humans are different in the sense that there tickings (rate) are different compared with the rate of the coordinate system. The biggest problem using Newton's law is that the forces assumed do not act instantaneous. That is the reason of the advancement of the planet Mercury. That means you should make a clear difference between the speed of light (photons) and the speed of gravity (gravitons) which are physical completely different concepts (objects).

>	Tom Roberts

Thanks

43 Speed of light in vacuum, what definition?

That is wishfull thinking. You need to be more specific. Newton's Law is one solution but not accurate to describe all observations. GR should be more accurate but IMO to complex. I try to unravel a simpler solution.

> >

> > >

Tom Roberts wrote:

> > >>

We can take the "external analyst" point of view, in that we are not part of the gedanken, we are looking at it and can see and measure everything of interest. In particular, "time" to us is COMPLETELY DIFFERENT from "time in the gedanken", as is space [#]. You can think of the gedanken as happening in this small region of space inside this breadbox -- WE can look inside the breadbox, at any time (to the gedanken). So we can see the entire light sphere from a single flash, and can project it onto multiple inertial frames and thus conclude what observers in those frames would see.

>	Hmm. From my mathematical model, I rather imagine photons like pairs, and light waves in contrast to acoustic spherical waves like dipoles. Detectors primarily only respond to light that is incoming on their location.

I do not understand. You need more clarity. Any way to prove your mathematical model you need observations.

> > >

My problem is why do you need observers.

> >	Because you cannot possibly describe how some physical phenomenon behaves without observing it.

>	I disagree.

How do you know about certain physical phenomenon without observing them? The most important word Robert uses is "some". When you want to study the movement of objects (stars) you definite need observations. When you study the inner workings of stars or black holes direct observations are more difficult.

Nicolaas Vroom

44 Speed of light in vacuum, what definition?

>	I fully agree. However I want to describe the laws of nature.

You have to start from fabricating them.

45 Speed of light in vacuum, what definition?

Then you are not doing science. Goodbye.

Tom Roberts

46 Speed of light in vacuum, what definition?

Not really. "external analyst" includes the notion that she is looking at a MODEL. "Omni-present being" does not do that, even though it is essential; it has other unsavory connotations as well....

IOW: your "being" is an unphysical and unreasonable inhabitant of the MODEL; the analyst is NOT AN INHABITANT OF THE MODEL (she is EXTERNAL to it).

>	As such this "external analyst" can travel instantaneous to all the corners of the universe at present,but that does not mean he knows what has happened in the past nor does he know the future.

This is just plain wrong. The external analyst is completely independent of "past and future" IN THE MODEL -- she can "look" anywhere and anywhen and "observe" what is happening there and then. No "travel" is involved, merely ANALYSIS of what the model says happens there and then (this is ANALYSIS, not "looking" or "observing" in the usual sense).

>	The issue is what this "external analyst" knows (not sees) about the present or the future state of the universe depents the laws of step 2

Those laws apply to the MODEL, and describe how it evolves. The external analyst is COMPLETELY independent of all that.

That is, we are NOT attempting to model the universe in which the analyst herself lives -- that would involve a nasty recursion.... But in the model we do use the "laws of physics" as we know them to apply to the world we inhabit, so the model is as accurate a model as we know how to make, of some aspects of our world.

>	IMO gravitational radiation propagates in a sphere. A light flash in general does not propagate in a sphere as it is influenced by gravity.

Two problems with your GUESS:
1. In vacuum, light and gravitational radiation both propagate at the LOCAL speed of light, everywhere and everywhen. So if one is a sphere, both are.
2. In the presence of gravity, the manifold is curved, and "sphere" simply does not apply.

>	IMO the whole concept (sorry to say) of simultaneity of relativity is not very helpfull to study the laws of nature.

Those laws are LOCAL, and the issue of simultaneity NEVER COMES UP. That is, you need no definition of simultaneity to apply and solve the differential equations we use to specify those laws.

But you do need to use generally covariant (invariant) laws. So you cannot use Maxwell's equations or QED. But you can use the Maxwell/Einstein equations within their domain (these are the covariant extension of classical electrodynamics to curved manifolds). (Attempting to similarly extend QED fails badly.)

Tom Roberts

47 Speed of light in vacuum, what definition?

That's were we disagree if you specific mean a mathematical model. Starting point is that the external analyst is studying the total physical reality with his eyes closed. He has an image in his mind based on previous observations and calculations. Using a set of mathematical tools he tries to describe how this image changes in time.

>

This is just plain wrong. The external analyst is completely independent of "past and future" IN THE MODEL -- she can "look" anywhere and anywhen and "observe" what is happening there and then. No "travel" is involved, merely ANALYSIS of what the model says happens there and then (this is ANALYSIS, not"looking" or "observing" in the usual sense).

Okay

> >	The issue is what this "external analyst" knows (not sees) about the present or the future state of the universe depents the laws of step 2

>	Those laws apply to the MODEL, and describe how it evolves. The external analyst is COMPLETELY independent of all that.

Okay

> >	IMO gravitational radiation propagates in a sphere. A light flash in general does not propagate in a sphere as it is influenced by gravity.

>	Two problems with your GUESS: 1. In vacuum, light and gravitational radiation both propagate at the LOCAL speed of light, everywhere and everywhen. So if one is a sphere, both are.

IMO there is a big difference between light and gravitational radiation specific in the sense when you want to study the behaviour of the stars in our galaxy. As such when you study the cause why objects behave the way they do you do not need light and or the concept of photons. That does not mean you don't need light at all. Light is important to make your observations and the behaviour of light is important to calculate your image of the objects in the Universe.

The Book GRavitation at page 399 reads:
Many different mathematical entities are associated with gravitation: the metric, the Riemann curvature tensor, the Ricci curvature tensor, the curvature scalar, the covariant derivative, the connection coefficients etc. etc is the "geometry of spacetime"

My question is do we really need all if we want to understand the movement of stars and galaxies. When the concept of light is excluded (which is related to our human point of view) only gravitation is required and this is a much simpler concept. You can postulate (much "easier" than light) that its speed is constant and that it (gravitons) moves in straight lines. No light bending is involved.

>	2. In the presence of gravity, the manifold is curved, and "sphere" simply does not apply.

Insofar a manifold is based on the speed of light it is not used. A sphere is only used when applied to gravitational waves (radiation?).

> >	IMO the whole concept (sorry to say) of simultaneity of relativity is not very helpfull to study the laws of nature.

>	Those laws are LOCAL, and the issue of simultaneity NEVER COMES UP. That is, you need no definition of simultaneity to apply and solve the differential equations we use to specify those laws.

But you need a good definition about the concept of time. In fact you need only one clock within your MODEL or what I call image.

>

But you do need to use generally covariant (invariant) laws. So you cannot use Maxwell's equations or QED. But you can use the Maxwell/Einstein equations within their domain (these are the covariant extension of classical electrodynamics to curved manifolds). (Attempting to similarly extend QED fails badly.)

Also here I expect when the speed of light is not used mathematics becomes simpler. Maxwell's equations are an intertwinned concept of both electricity and magnetism (?), Gravitation I expect is physical a much simpler concept.

When you try to study nature completely without human intervention, as something completely physical, the laws of nature become simpler. (No Black holes? Dark matter? The Schrödinger cat in QM disappears?)

>	Tom Roberts

Thanks

Nicolaas Vroom