| 1 "ripzone23" |
Length Contraction is WRONG | vrijdag 12 april 2002 21:06 |
| 2 "Eric Prebys" |
Re: Length Contraction is WRONG | zaterdag 13 april 2002 0:55 |
| 3 "Russell Blackadar" |
Re: Length Contraction is WRONG | zaterdag 13 april 2002 1:33 |
| 4 "Stephen Speicher" |
Re: Length Contraction is WRONG | zaterdag 13 april 2002 7:57 |
| 5 "John" |
Re: Length Contraction is WRONG | zaterdag 13 april 2002 13:10 |
| 6 "Oriel36" |
Re: Length Contraction is WRONG | zaterdag 13 april 2002 21:11 |
| 7 "David Hartley" |
Re: Length Contraction is WRONG | zaterdag 13 april 2002 23:09 |
| 8 "Bilge" |
Re: Length Contraction is WRONG | zondag 14 april 2002 11:25 |
| 9 "Oriel36" |
Re: Length Contraction is WRONG | zondag 14 april 2002 19:43 |
| 10 "Bilge" |
Re: Length Contraction is WRONG | zondag 14 april 2002 23:52 |
| 11 "Nicolaas Vroom" |
Re: Length Contraction is WRONG | maandag 15 april 2002 17:59 |
| 12 "ripzone23" |
Re: Length Contraction is WRONG | dinsdag 16 april 2002 22:16 |
WRONG? No, just wanted someone's attention. My other post on the subject was ignored, rightly so, since the FAQ explains this "paradox" somewhat.
I still had one question concerning the barn & pole "paradox". What would happen if the runner took a "snapshot" of the barn and pole when he was at the exact center of the barn (no closing doors involved) and the barn viewer did the same? Now, regardless of whether or not they take their respective pictures at the same time, what will the pictures show? Will the runners picture show a pole much longer than the barn? Will the barn-viewer's picture show a pole much shorter and able to fit inside the barn?
The difficulty I have is that it seems to me that the center of the barn is THE center of the barn regardless of viewer.
Also, if this question is answered by the simultaneity clause, let me put a twist in. What if the barn viewer is standing at the center of the barn and the runner crashes into him, and at that exact moment of impact, a picture is taken by barn viewer and runner, what will the picture show? Will the runners pole be sticking out past the barn door, or will the pole fit exactly in the barn? How would you be able to tell who was moving and who was standing still?
Obviously, the impact will create a simultaneous event. But is seems like it would be important to define who was moving because the energy from the moving person will push on the stationary person, right? Would this fact not disallow the moving person from calling his frame of reference as being at rest?
ripzone23 wrote:
| > |
WRONG? No, just wanted someone's attention. My other post on the subject was ignored, rightly so, since the FAQ explains this "paradox" somewhat. I still had one question concerning the barn & pole "paradox". What would happen if the runner took a "snapshot" of the barn and pole when he was at the exact center of the barn (no closing doors involved) and the barn viewer did the same? Now, regardless of whether or not they take their respective pictures at the same time, what will the pictures show? Will the runners picture show a pole much longer than the barn? Will the barn-viewer's picture show a pole much shorter and able to fit inside the barn? The difficulty I have is that it seems to me that the center of the barn is THE center of the barn regardless of viewer. Also, if this question is answered by the simultaneity clause, let me put a twist in. What if the barn viewer is standing at the center of the barn and the runner crashes into him, and at that exact moment of impact, a picture is taken by barn viewer and runner, what will the picture show? Will the runners pole be sticking out past the barn door, or will the pole fit exactly in the barn? How would you be able to tell who was moving and who was standing still? Obviously, the impact will create a simultaneous event. But is seems like it would be important to define who was moving because the energy from the moving person will push on the stationary person, right? Would this fact not disallow the moving person from calling his frame of reference as being at rest? |
You're implicitly assuming that a camera would correctly show the instantaneous state of the barn and the pole in *either* frame. In fact, it shows the state of things when the light left the source, and while both observers would have to agree on what the camera image showed, they would *disagree* on what that meant in terms of when the light left the source. Try the problem again, only this time factor in the propagation time of the light, and you'll find there's no "paradox".
It's a common misunderstanding that a Lorentz transformation tells you what things "look" like to different observers (partly because of sloppy wording in textbooks). Lorentz transformations tell you the positions of things assuming you've already appropriately corrected for signal propagation delays. In general, the problem of what things really "look" like (to your eye, or to a camera) is more complicated.
--
-------------------------------------------------------------------
Eric Prebys, Fermi National Accelerator Laboratory
Office: 630-840-8369, Email: prebys@fnal.gov
WWW: http://home.fnal.gov/~prebys
-------------------------------------------------------------------
ripzone23 wrote:
| > |
WRONG? No, just wanted someone's attention. My other post on the subject was ignored, rightly so, since the FAQ explains this "paradox" somewhat. I still had one question concerning the barn & pole "paradox". What would happen if the runner took a "snapshot" of the barn and pole when he was at the exact center of the barn (no closing doors involved) and the barn viewer did the same? |
Well, real pictures are tricky because they are always views of the *past* -- it takes finite time for light to travel to the film from the events being photographed. And the image on film does not even record a single (simultaneous) instant of time, since in general the events being photographed are not all at the same distance from the film and hence have different light-travel times before they are recorded.
Also, I am not sure how you'd propose to measure the length of the pole (or the barn) from a snapshot taken by someone at its center. The camera would have to shoot both directions simultaneously -- which could be arranged, I suppose -- but the orientation would be wrong. It would be like looking end-on at a 2x4 and trying to guess its length.
We can minimize these problems by placing the barn-frame's photographer not the at the center of the barn, but rather, at a distance of (say) 1000m away on a perpendicular line drawn through the barn's center. This photographer must click his shutter at time t=1000/c after the event which we want to record, namely the passing of the pole's center and the barn's center. At a distance of 1km, parallax is very small and so, at least in the vertical plane containing the pole, we can consider all the events in the image as essentially simultaneous. Also, we have the advantage of seeing the pole and barn side-on from this vantage point. (Perhaps we should put a big window in the barn so we can see the pole when it is inside.)
Similarly, we can have an accomplice of the runner, moving parallel to the runner and at the same speed, but separated by 1000m, take a picture, also along the perpendicular in his frame so that the barn and pole are imaged side-on. He too will click his shutter at t'=1000/c (in his frame; note the prime) after the event.
Now, regardless of whether or
| > | not they take their respective pictures at the same time, what will the pictures show? |
In my changed statement of the problem, they don't actually get taken at the same time -- but they do both record the same event, namely, the event at which the center of the pole is at the center of the barn. However, the recorded events at the ends of the pole will not be the same in the two images, due to the relativity of simultaneity, and this accounts for the differences that I describe below.
Will the runners picture show a
| > | pole much longer than the barn? |
Yes, or rather that's what his accomplice's picture will show.
(As an aside for more advanced readers, yes, I'm aware of Penrose-Terrell rotation. It's true that the image will present the illusion that the barn is rotated rather than contracted; however, in the present case it will be clear that the barn is not *really* rotated since the pole will manifestly be sticking out of both doors in the image.)
Will the
| > | barn-viewer's picture show a pole much shorter and able to fit inside the barn? |
Yes.
| > |
The difficulty I have is that it seems to me that the center of the barn is THE center of the barn regardless of viewer. |
Of course, but that's not the relevant issue here. The center of the pole being at the center of the barn is of course a single event and is perceived as such by both frames. But the interesting thing is when you consider the events that are simultaneous to this center event, but at the respective ends of the pole. Since the distance to the ends is nonzero, SR predicts that simultaneity will depend on frame. The simultaneous front and back events in the barn frame (which as we know happen to be inside the barn) will not be simultanous in the runner's frame, but rather, *different* events will be simultaneous in that frame. Ones that are outside the barn.
| > |
Also, if this question is answered by the simultaneity clause, |
R of S is the answer, but as we've seen, *not* applied at the center of the pole but rather at its ends.
let me put a twist in.
| > | What if the barn viewer is standing at the center of the barn and the runner crashes into him, and at that exact moment of impact, a picture is taken by barn viewer and runner, what will the picture show? |
If you mean at the first instant of the crash, before either photographer has accelerated due to the crash, then the answer is of course the same as above.
But if you mean after the crash, the answer is complicated and you really have to give us more information before an answer can be given. For one thing, since the runner cannot stop in zero time (infinite acceleration) he won't be at the middle of the barn at the end of the crash. And surely the "stationary" observer will no longer be stationary in the barn frame after the crash. Also the pole may well have been distorted physically by the huge forces generated by the crash. But if it survives without distortion, and if it comes to rest in the barn frame, it will be longer than the barn.
Will the runners
| > | pole be sticking out past the barn door, or will the pole fit exactly in the barn? How would you be able to tell who was moving and who was standing still? |
You can't. That conjecture -- or perhaps we can say that fact since nobody yet has found a way to do what you propose -- is the very foundation of relativity.
| > |
Obviously, the impact will create a simultaneous event. But is seems like it would be important to define who was moving because the energy from the moving person will push on the stationary person, right? |
And the "stationary" person will push on the moving person. It's precisely symmetrical; their roles are perfectly reversed when you change frames.
(Yes, the stationary person has zero kinetic energy in his own frame, but in the runner's frame he has an enormous KE. Energy is frame dependent, and so is momentum, which I think is even more relevant to your question. The upshot is that nobody can say which observer's point of view is the "right" one.)
Would this fact not disallow the
| > | moving person from calling his frame of reference as being at rest? |
It would if it were a "fact". But it isn't.
I suppose you could force it to be true by fiat, but there would be no physical basis for doing so. Besides, a barn in the U.S. is not in the same frame as a barn in India, so one must ask which barn would you designate as "stationary"? Whichever you picked would be damned inconvenient, so physicists would probably just ignore your fiat and do things the easier way, which turns out to be SR.
On Fri, 12 Apr 2002, Russell Blackadar wrote:
| > | ripzone23 wrote: |
| > > |
I still had one question concerning the barn & pole "paradox". |
| > |
That was a really great explanation, Russell.
Stephen
sjs@compbio.caltech.edu
Welcome to California. Bring your own batteries.
Printed using 100% recycled electrons.
--------------------------------------------------------
Very cool, I'm going to save that one and go over it for a while.
Many thanks,
John
Russell Blackadar
WRONG? No, just wanted someone's attention.
My other post on the subject was ignored,
rightly so, since the FAQ explains this
"paradox" somewhat.
I still had one question concerning the
barn & pole "paradox". What would happen
if the runner took a "snapshot" of the
barn and pole when he was at the exact
center of the barn (no closing doors
involved) and the barn viewer did the
same?
Well, real pictures are tricky because they
are always views of the *past* -- it takes
finite time for light to travel to the film
from the events being photographed. And the
image on film does not even record a single
(simultaneous) instant of time, since in
general the events being photographed are
not all at the same distance from the film
and hence have different light-travel times
before they are recorded.
Wonderful!,now take the next step and apply it to cosmic expansion in
all directions and figure out why nature prohibits a true discernment
of its actual structure and motion insofar as we are surrounded by
'past' observance in all directions.Reducing acceleration to 'distance
covered' by light is exactly the same as distance covered at different
locations on the planet either side of the Equitorial line,seconds of
a clock being a proportion of varying distance rather than a measure
of a seperate quantity that SR covered calls time.
Looking down from space at the axis of rotation (North or South pole)
we notice that the distance covered is at the poles is at a min while
at the greatest circumference (Equator) it is at a max.Nobody mistakes
the notion of acceleration as anything but rotation in this
instance,distance covered for a given interval changes but nothing
else does, making SR concepts superfluous otherwise cosmic
acceleration would be incorrectly misinterpreted.
'Every point is the valid center' is a really poor misinterpretation
of the true structure of the cosmos insofar it takes our position as a
'center' at face value and works on from there,quite profound in its
infantile perspective actually.At a fundamental level SR cannot
distinguish acceleration from rotation for it locks in time with space
whereas a more subtle approach would discern that
seconds,minutes,hours of a clock relate to proportions of distance
covered.
Where for the last 100 years,SRists may have thought that
clocks,seconds, minutes,etc were measuring a seperate quantity (time)
,they were instead toying around with proportions and ratios of
distance covered.It does'nt take a genius to figure out why you cannot
translate acceleration into rotation through SR and especially with
the ambiguous if poorly objective "Clocks measure time".
In article <3CB76EB8.5C0BFDEF@mdli.com>, Russell Blackadar
WRONG? No, just wanted someone's attention.
My other post on the subject was ignored,
rightly so, since the FAQ explains this
"paradox" somewhat.
I still had one question concerning the
barn & pole "paradox". What would happen
if the runner took a "snapshot" of the
barn and pole when he was at the exact
center of the barn (no closing doors
involved) and the barn viewer did the
same?
The barn viewer - if he is next to the centre of the pole, at the centre
of the barn - sees the front of the pole half way between him and the
exit, and the back end still outside, well short of the entrance. But
when he sees the doors close, a little while later, he sees the pole
completely inside the barn.
The runner sees the same light as the barn observer, as he is at the
same event, so he sees the same pattern: the front of the pole appears
to be about half way to the exit door, the back of the pole has not yet
reached the entrance. The actual distances are different but their
ratios are the same. Using the figures in the FAQ (barn 40m, pole 80m,
gamma 2), the exit door of the barn is 10m in front of the runner, but
appears to be about 77m away. The door has already closed and opened
again - the front end of the pole has already exited - but the runner
hasn't seen the door shut yet and sees the front end as still within the
barn. The entrance door is 10m behind but seems to be only a little over
5m away. The runner will have left the barn before he sees the back of
the pole go through this door and then sees the door close.
Also, I am not sure how you'd propose to
measure the length of the pole (or the barn)
from a snapshot taken by someone at its center.
The camera would have to shoot both directions
simultaneously -- which could be arranged, I
suppose -- but the orientation would be wrong.
It would be like looking end-on at a 2x4 and
trying to guess its length.
We can minimize these problems by placing
the barn-frame's photographer not the at the
center of the barn, but rather, at a distance
of (say) 1000m away on a perpendicular line
drawn through the barn's center. This
photographer must click his shutter at time
t=1000/c after the event which we want to
record, namely the passing of the pole's
center and the barn's center. At a distance
of 1km, parallax is very small and so, at
least in the vertical plane containing the
pole, we can consider all the events in the
image as essentially simultaneous. Also, we
have the advantage of seeing the pole and barn
side-on from this vantage point. (Perhaps we
should put a big window in the barn so we can
see the pole when it is inside.)
Similarly, we can have an accomplice of the
runner, moving parallel to the runner and at
the same speed, but separated by 1000m, take
a picture, also along the perpendicular in
his frame so that the barn and pole are imaged
side-on. He too will click his shutter at
t'=1000/c (in his frame; note the prime) after
the event.
It is interesting to consider another accomplice, also moving parallel
to the runner and 1000m to the side, but lagging behind so that he takes
his photo when he coincides with the barn-frame photographer. He sees
the same light as the barn photographer, so his photo will show the pole
inside the barn. But the pole is fixed in his frame, ahead and to the
side, so he has to angle his camera forward. This shows the
Penrose-Terrell rotation, he sees the same thing but at a different
angle.
In my changed statement of the problem, they
don't actually get taken at the same time --
but they do both record the same event, namely,
the event at which the center of the pole
is at the center of the barn. However, the
recorded events at the ends of the pole will
not be the same in the two images, due to the
relativity of simultaneity, and this accounts
for the differences that I describe below.
Will the runners picture show a
Yes, or rather that's what his accomplice's
picture will show.
(As an aside for more advanced readers, yes, I'm
aware of Penrose-Terrell rotation. It's true
that the image will present the illusion that
the barn is rotated rather than contracted;
however, in the present case it will be clear
that the barn is not *really* rotated since the
pole will manifestly be sticking out of both
doors in the image.)
Will the
Yes.
--
Oriel36 left bird droppings:
Wonderful!,now take the next step and apply it to cosmic expansion in
OK.
root@radioactivex.lebesque-al.net (Bilge) wrote in message news:
>
>Wonderful!,now take the next step and apply it to cosmic expansion in
OK.
Simple,is'nt it !,so simple in fact that with one stroke you can do
away with time dilation and at the same time infer rotation from
acceleration.
Imagine if the SRist existed back at the time of Copernicus,he would
have got nowhere with a heliocentric system as the SRist is concerned
only with what the observer 'sees',likewise if the SRist sees cosmic
expansion and it is accelerating then that is what it actually is
doing.Numbskulls !,you don't have the intellectual strenght to infer
indirectly that acceleration is not what it 'appears' nor that in a
rotating system the distance covered increases from center to outer
rim and therefore looks like acceleration.
Science used to be great at figuring out indirectly what was actually
happening,Copernicus is one,Ole Roemer is another and his discerment
that light covers a finite distance for each second that elapses
through observing the trajectory of Io .Is'nt it amazing that dark
matter is used in an attempt to explain away acceleration or that the
missing mass is derived from a superficial observation of what the
cosmos looks like,truly you are hopeless cases.
Mental midgets who are truly happy in ignorance is such a waste of a
life,despite the sanitised image of science (at least in this area )
presented to the mainstream public behind it are small minds and even
though true contention is enjoyable and work does get done by this
means,the shelf life of SR expired long ago with nothing fresh to take
its place nor will grovelling minds like yours allow it.
You knew what I was saying in regards to a second and varying
distances through rotation yet you do not give a damn,even your
opponents are mute on the topic where rotation can be infered from
acceleration by reducing speed to 'distance covered' and SR cannot
operate without 'speed'.
Oriel36 said some stuff about
Re: Length Contraction is WRONG to usenet:
>
>Wonderful!,now take the next step and apply it to cosmic expansion in
OK.
Simple,is'nt it !,so simple in fact that with one stroke you can do
away with time dilation and at the same time infer rotation from
acceleration.
Incorrect.
"ripzone23"
I still had one question concerning the
barn & pole "paradox". What would happen
if the runner took a "snapshot" of the
barn and pole when he was at the exact
center of the barn (no closing doors
involved) and the barn viewer did the
same? Now, regardless of whether or
not they take their respective pictures
at the same time, what will the pictures
show? Will the runners picture show a
pole much longer than the barn? Will the
barn-viewer's picture show a pole much
shorter and able to fit inside the barn?
The difficulty I have is that it seems to
me that the center of the barn is THE center
of the barn regardless of viewer.
Also, if this question is answered by the
simultaneity clause, let me put a twist in.
What if the barn viewer is standing at the
center of the barn and the runner crashes into
him, and at that exact moment of impact, a
picture is taken by barn viewer and runner,
what will the picture show? Will the runners
pole be sticking out past the barn door, or
will the pole fit exactly in the barn?
One rod (observer) is fixed (at rest) = Barn
When the moving observer MO meets
at meeting point MP
the fixed observer FO then
the MO makes a picture
and the FO makes a picture.
IMO if they both make the picture
in the same direction the pictures (no delay)
will be identical because the pictures will
be taken at the same place and moment.
On both pictures you will see the ends
of each rod (the points A or B) but you
can not say were they are in radial direction.
To get more about the pictures you should
place clocks at each end
(and one at MO and FO)
ie the time for the fixed rod is identical.
For the moving rod the result is "identical".
Please study:
https://www.nicvroom.be/calc1.htm
In order to understand this result it is important
to take clock synchronisation into account.
https://www.nicvroom.be/length3.htm#clsync
I hope this helps
Nick
Thank you to everyone who answered my
questions. I sort of figured out on
my own that there is no real way to
tell who is moving based on my
examples; I just didn't have to time
to follow up and let you all know.
One final question: I would assume
that if the runner collided with
the barn viewer, at the moment of
impact, the two should agree on
measurement and KE would be exchanged.
(I know, I'm being overly simplistic,
bear with me.) Would this be approx.
accurate?
Created: 16 April 2002
Back to my home page Contents of This Document
>
ripzone23 wrote:
> >
>
7 Length Contraction is WRONG
Van: "David Hartley"
Onderwerp: Re: Length Contraction is WRONG
Datum: zaterdag 13 april 2002 23:09
>
ripzone23 wrote:
>>
I partially answered this in my post on "Visual appearance of moving
rod" which I sent before seeing this one. The time-lag distorts things
considerably: "what you see is not what you observe".
>
>
Well, real pictures are tricky because they
are always views of the *past* -- it takes
finite time for light to travel to the film
from the events being photographed. And the
image on film does not even record a single
(simultaneous) instant of time, since in
general the events being photographed are
not all at the same distance from the film
and hence have different light-travel times
before they are recorded.
>
Now, regardless of whether or
>>
not they take their respective pictures
at the same time, what will the pictures
show?
>
>>
pole much longer than the barn?
>
>>
barn-viewer's picture show a pole much
shorter and able to fit inside the barn?
>
David Hartley
8 Length Contraction is WRONG
Van: "Bilge"
Onderwerp: Re: Length Contraction is WRONG
Datum: zondag 14 april 2002 11:25
>
9 Length Contraction is WRONG
Van: "Oriel36"
Onderwerp: Re: Length Contraction is WRONG
Datum: zondag 14 april 2002 19:43
>
Oriel36 left bird droppings:
10 Length Contraction is WRONG
Van: "Bilge"
Onderwerp: Re: Length Contraction is WRONG
Datum: zondag 14 april 2002 23:52
>
root@radioactivex.lebesque-al.net (Bilge) wrote in message news:
>>
Oriel36 left bird droppings:
>
11 Length Contraction is WRONG
Van: "Nicolaas Vroom"
Onderwerp: Re: Length Contraction is WRONG
Datum: maandag 15 april 2002 17:59
>
You can not
>
How
would you be able to tell who was moving and
who was standing still?
IMO what you have are two rods
and two observers at the middle of each rod.
>
One rod (observer) is moving
with a speed v in the rest frame.
That means IMO the pictures are independent
of who takes them (at MP)
A-------MO-------B moving ---->
A----------FO---------B fix
MP
In reality
MO should make a picture in direction A
(against direction of movement)
FO should make a picture in direction B
(in direction of movement)
and you should compare the two pictures.
Suppose the length of each rod is
2 light minutes and MO meets FO
at 12:00:00 at FOs fixed watch
than
at the picture in A direction you will see
11:59:00 at the A end of the fixed rod.
at the picture in B direction you will see
11:59:00 at the B end of the fixed rod.
That means
Suppose the length of each rod is
2 light minutes and MO meets FO
at 11:00:00 at MOs moving watch
than
at the picture in A direction you will see
10:59:00 at the A end of the moving rod.
at the picture in B direction you will see
10:59:00 at the B end of the moving rod.
12 Length Contraction is WRONG
Van: "ripzone23"
Onderwerp: Re: Length Contraction is WRONG
Datum: dinsdag 16 april 2002 22:16