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Author Topic: Evidence for large scale length contraction?  (Read 56952 times)

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #100 on: 13/09/2012 21:28:33 »
Just a quickie:
This part of the thread is intended to be a sincere discussion of a (future) practical application of length contraction with each person listening to the other and replying to specific points.
As I told David Cooper, the ‘rules of engagement’ (as I understand) for a thought experiment dictate that the specific situation is taken as the author presents  it.

yor_on:

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If you think of it OG. To retrieve that probe it will have to be in its so called 'rest frame'.

As I already said, the shuttle must match velocities with the probe ("come alongside") to capture it, as we well know from all “docking” procedures in space. It is still going at near ‘c’ but now the shuttle is at rest with it and therefore sees it as it is, much longer that 10 meters, not as it was observed from earth, going very fast relative to earth... i.e., “contracted.”

yor-on:
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Assuming that the cargo bay is built to the dimensions we found the probe to have before joining up with it,

 I originally said that our shuttle has a 10 meter bay, as it was built. We saw the probe, from earth, as 10 meters. Will it fit in the bay? Simple. Clear. No.

yor_on
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becoming 'still' relative it, it will be found not to fit.

Exactly my point. It was not ‘actually’ 10 meters, as it was seen from earth. That “actually” means “really” is demonstrated by the fact that it is actually, really way too long to fit into the 10 meter bay.
yor_on:
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You still don't get it OG? We measure, and from there construct what is 'real'.

You are not listening. We measured the probe to be 10 meters. However, it was not “really” 10 meters, as the example has abundantly demonstrated.

I now put this experiment to rest. I do hope to hear answers to my last reply.

 

Offline JP

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Re: Evidence for large scale length contraction?
« Reply #101 on: 13/09/2012 21:42:43 »
Old Guy, if you define "really" to mean in the rest frame then your logic holds.  This isn't an accepted meaning of the term "really" in science, which is why you're getting push-back from others.

Here's a question for you: would you agree that the result of the measurement of the probe from the earth's frame (where the probe is moving very rapidly) is 10 meters?
« Last Edit: 13/09/2012 21:46:54 by JP »
 

Offline damocles

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Re: Evidence for large scale length contraction?
« Reply #102 on: 13/09/2012 22:44:59 »
From JP, a few posts ago in this busy thread
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However, Lightarrow's example of light certainly causes a bit of an issue for this definition of rest properties=intrinsic.  Light has no rest frame, yet a photon clearly has invariant mass which seems to be intrinsic.  It also has spin, which certainly meets the criteria most folks would set for intrinsic (and indeed, it is technically termed an intrinsic property in relativistic QM).

Yes I clearly goofed up here. Clearly a photon has (at least? only?) three properties that should be regarded as intrinsic, in spite of the fact that it has no "rest frame": a zero spin, a zero "rest mass", and a "vacuum speed" of c0. But the "colour as emitted" of a photon must be seen as an intrinsic property of the emitting body rather than the photon, because it is only by translation to the rest frame of the emitting body that different observers can agree on this sort of property, and it is only in this frame that the "character" of the photon can be assessed (e.g. a sodium emission line).

It seems to me (a propos of lightarrow response 87) that whether or not a particular observer finds this translation practically feasible is not particularly relevant to how the property should be regarded.
 

Offline imatfaal

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Re: Evidence for large scale length contraction?
« Reply #103 on: 13/09/2012 23:00:25 »
And - furthermore DavidC introduced a really nice variation on your theme which demonstrates the almost paradoxical nature.  If the space shuttle and probe share a rest frame then no, of course it doesn't fit; but if you think of a shuttle bay with and open back end that the probe could enter at high relative velocity then the probe would fit (before destroying itself and the space shuttle when it hit the other end of the bay)! 

For those of you who don't mind adapting a thought-experiment... relativity is of course reciprocal; just as we can define the rest frame as the frame in which the shuttle has zero momentum, we can also define the rest frame as the frame in which the probe has zero momentum and the shuttle is approaching it at high velocity (in the opposite direction).  From the rest frame of the probe we now see the shuttles cargo bay as length contracted, and the probe is no longer contracted ...and the probe would still all fit in just before destroying itself and the shuttle!  The timings are very tough to reconcile, events seem to be switched, and with relativistic speeds you cannot be sure of any simultaneities without the maths; but in the end the same set of events happens - it must be the case because it is the exact same scenario.
 

Offline imatfaal

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Re: Evidence for large scale length contraction?
« Reply #104 on: 13/09/2012 23:08:24 »
From JP, a few posts ago in this busy thread
Quote
However, Lightarrow's example of light certainly causes a bit of an issue for this definition of rest properties=intrinsic.  Light has no rest frame, yet a photon clearly has invariant mass which seems to be intrinsic.  It also has spin, which certainly meets the criteria most folks would set for intrinsic (and indeed, it is technically termed an intrinsic property in relativistic QM).

Yes I clearly goofed up here. Clearly a photon has (at least? only?) three properties that should be regarded as intrinsic, in spite of the fact that it has no "rest frame": a zero spin, a zero "rest mass", and a "vacuum speed" of c0. But the "colour as emitted" of a photon must be seen as an intrinsic property of the emitting body rather than the photon, because it is only by translation to the rest frame of the emitting body that different observers can agree on this sort of property, and it is only in this frame that the "character" of the photon can be assessed (e.g. a sodium emission line).

It seems to me (a propos of lightarrow response 87) that whether or not a particular observer finds this translation practically feasible is not particularly relevant to how the property should be regarded.

Polarization and charge (or lack of same) are also intrinsic for a photon, and photons are spin 1 btw not spin 0 (that would be the higgs) .  I would argue that these (spin, invariant mass, charge, polarization, speed) are intrinsic because they are valid from any frame of measurement - ie no frame can be postulated that can change circular polarization to linear - whereas a frame can be postulate that would red-shift/blue-shift.
« Last Edit: 13/09/2012 23:12:37 by imatfaal »
 

Offline JP

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Re: Evidence for large scale length contraction?
« Reply #105 on: 13/09/2012 23:47:35 »
Polarization stater* are the spin states by the way.

*states, even.  I love autocorrect
« Last Edit: 14/09/2012 00:59:23 by JP »
 

Offline damocles

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Re: Evidence for large scale length contraction?
« Reply #106 on: 14/09/2012 00:14:20 »
Please! We on this forum are not The Mythbusters! We do not need to go about blowing up perfectly good and horrendously expensive spacecraft and shuttle bays with our thought experiments! old guy's original formulation was in terms of a fly-by and compare rather than an actual attempt at docking at relativistic speed.

I am going to outline a thought experiment that I think is a genuine proxy for old guy's original proposal, that will highlight some of the issues involved, and that might be critical in that it could give a different result depending whether or not OG's term "real" applies to a Lorentz contraction.

1.) Prepare a metal disk that is 20% larger in diameter than the mirror of the Hubble telescope.
2.) Tow the Hubble telescope to a location in "empty" space outside the solar system.
3.) Focus the telescope on Sirius (or another conveniently bright star), and monitor the light reaching the focal point -- our observer.
4.) Launch the disk, frisbee-wise, on a trajectory that "should" completely block off the telescope to light from Sirius, at a near-light speed.

I believe that the Lorentz contraction is "real" and that the observer should not observe an absence of Sirius light at any instant during this exercise. If, on the other hand, the Lorentz contraction is "not real", then there should be a brief period of time when Sirius is in complete eclipse.

I therefore ask OG -- is this a fair proxy for your thought experiment? Would you agree with my interpretation of the possible results of such an experiment?

And I ask the several SR experts here -- do you think that I have correctly interpreted the SR mathematical model in my belief about the conclusions?
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #107 on: 14/09/2012 00:49:12 »
a spinning disk Damocles? near 'c' :)

As the contraction is in the direction of spin it will 'crack', and yes, that will let light through ::))
Sorry, couldn't help that one. It's a classical example that might be feasible to test for with spinning black holes? Maybe that is? If we can 'see' them good enough that is :)

But, I do get your idea.

You think the same way as me. A frame of reference is where you are. What measurements you make in that frame will be true for you. And it doesn't matter if someone else far away have a different opinion on lengths or 'time'. That doesn't change a thing for you, inside your 'frame', if we say so.
==

And sorry OG, instead of trying to see the points made you use your opinion as some club, and on us no less :) The contraction is no illusion :)
« Last Edit: 14/09/2012 00:51:37 by yor_on »
 

Offline damocles

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Re: Evidence for large scale length contraction?
« Reply #108 on: 14/09/2012 00:55:36 »
I am having a bad run with ambiguous expression. I was meaning a relatively slow rotation, just to achieve a stable flight, and a fast translational velocity such that the centre of the disk ran over the centre of the telescope. Maybe a rotation is not even needed to provide stable flight in an airless environment,
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #109 on: 14/09/2012 01:02:08 »
That's one of the trickiest things there is Imatfaal. Discussing relative (uniform) motion . There is one thing though, that blue-shift you will see, locally, after a acceleration will only belong to the accelerating frame, not to both (not to Earth for example). So even though you can labor with changing perspectives, the probe 'knows' its speed, just as the universe at large must have a definition.
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #110 on: 14/09/2012 01:04:17 »
It's cool D :)
I make lot of those myself ::))
Where's the fun if we're not allowed to make some eh, faus paux(?) whatever :) at times?
 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #111 on: 14/09/2012 01:05:18 »
Old Guy, if you define "really" to mean in the rest frame then your logic holds.  This isn't an accepted meaning of the term "really" in science, which is why you're getting push-back from others.


Yes, I do. I am advocating realism here as apposed to the idealism that reality is defined by observation and measurement. The question of how best to measure something is a technical question. I think it is reasonable to be "in the rest frame" with whatever science is measuring, rather than flying by the object being measured at relativistic speeds.

Quote
Here's a question for you: would you agree that the result of the measurement of the probe from the earth's frame (where the probe is moving very rapidly) is 10 meters?

Yes. But I already said several times that the probe as measured from earth appeared to be 10 meters long. I wonder why I think no one is paying attention to what I actually say.
« Last Edit: 14/09/2012 01:25:49 by old guy »
 

Offline JP

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Re: Evidence for large scale length contraction?
« Reply #112 on: 14/09/2012 01:12:05 »
Yes, as I understand special relativity that's a valid thought experiment, Damocles.  I don't know if it will satisfy Old Guy or not, but it's a clever way of getting at the problem. 

I won't spoil the outcome just yet, though.  :)
 

Offline JP

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Re: Evidence for large scale length contraction?
« Reply #113 on: 14/09/2012 01:15:39 »
Old Guy, if you define "really" to mean in the rest frame then your logic holds.  This isn't an accepted meaning of the term "really" in science, which is why you're getting push-back from others.


Yes, I do. I am advocating realism here as apposed to the idealism that reality is defined by observation and measurement. The question of how best to measure something is a technical question. I think it is reasonable to be "in the rest frame" with whatever science is measuring, rather than flying by the object being measured at relativistic speeds.

Quote
Here's a question for you: would you agree that the result of the measurement of the probe from the earth's frame (where the probe is moving very rapidly) is 10 meters?

Yes.


Ok, that's fair enough.  It sounds like you don't disagree with the predictions of special relativity.  You just disagree with the idea that we call our measurements "real" lengths.  I guess that's fair, but it's why no one in special relativity uses the term "real."  You can go into philosophy and argue about the definitions, but as far as science is concerned, which deals with measurements, we simply have to admit that we measure one length in the rest frame and another in the moving frame and leave it up to philosophers to argue over what's "real" or not.
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #114 on: 14/09/2012 01:22:00 »
Thinking of a 'stationary' example. Take the inside of a event horizon, there you will have a 'gravity' so strong it will 'bend' all 'lightpaths' so that they all point one way. To the center of that black hole (in some 'centrifugal' path). Now, will the light inside be blue shifted too? And for whom, what frame of reference, if so?

This is assuming a equivalence with a constant uniform acceleration relative a 'gravity', and then stretch the definition into uniform motion. The other way to see it is that a uniform motion, no matter its speed, always must present us with the same 'energy' and that only a acceleration ('gravity') will present a blue shift. But that can't be right.
==

This one is tricky on so many levels though. First of all, is gravity uniform accelerations? If it is, then mass always must be moving in some 'degree of freedom' that we can't measure. The other is using blue shifts as a definer for both uniform motion and accelerations. But assuming that blue shift must be there in a uniform (relative) motion, after your acceleration to near light speed, we do have one coherent description locally. And by 'locally' we can just as easily exchange to a 'rest frame', where you are 'at rest', meaning that you follow a geodesic.
« Last Edit: 14/09/2012 01:30:51 by yor_on »
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #115 on: 14/09/2012 01:36:36 »
Assuming that mass is a constant uniform acceleration what is then a geodesic?
'ripples'?
I better put this in New Theories' huh :)

 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #116 on: 14/09/2012 01:39:47 »
damocles:
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old guy's original formulation was in terms of a fly-by and compare rather than an actual attempt at docking at relativistic speed.

To what original formulation are you referring here? The experiment in discussion here is the alien probe retrieval project with no “fly-by” involved... except the shuttle flying by the probe before it turns around and catches up.

damocles:
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I therefore ask OG -- is this a fair proxy for your thought experiment? Would you agree with my interpretation of the possible results of such an experiment?

No, and no.
 But why complicate my experiment with a spinning, flying disk, asking whether it is “really” big enough to cover the Hubble lense? My scenario was an attempt to illustrate the “real” length of a probe by the simplest possible means. Maybe the more complicated you make it the more it will obscure the fact that the probe will not “really” fit in the bay.
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #117 on: 14/09/2012 01:46:01 »
Assuming that mass is a constant uniform acceleration what is then a geodesic?
'ripples'?
I better put this in New Theories' huh :)

The point I'm struggling with here is that 'c' is a constant, but 'energy' is relative. But to define a 'speed' close to 'c' there must be a blue shift, no matter if it is measured in a acceleration or in its subsequent uniform motion. There is no other way I can come up with to define a 'speed', and, the universe needs one if 'motion' exist.
 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #118 on: 14/09/2012 01:50:36 »
JP:
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Ok, that's fair enough.  It sounds like you don't disagree with the predictions of special relativity.  You just disagree with the idea that we call our measurements "real" lengths.  I guess that's fair, but it's why no one in special relativity uses the term "real."  You can go into philosophy and argue about the definitions, but as far as science is concerned, which deals with measurements, we simply have to admit that we measure one length in the rest frame and another in the moving frame and leave it up to philosophers to argue over what's "real" or not.

I think I see a glimmer of mutual understanding emerging here. I do not, in fact, disagree with the predictions of SR.
As to what is "real," what was seen from earth to be 10 meters in length ("contracted") turned out to be however much longer than 10 meters, depending on its velocity relative to earth, causing the apparent contraction.
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #119 on: 14/09/2012 01:55:34 »
OG give it a rest. It's perfectly okay to define it from rest frames, I do it too. But to then lift forward those as covering all possibilities between frames of reference, including relative motion and accelerations, must by definition find its way to new theories, and then you need to explain what the muon experience, using mathematics.
 

Offline imatfaal

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Re: Evidence for large scale length contraction?
« Reply #120 on: 14/09/2012 11:17:55 »
Polarization stater* are the spin states by the way.

*states, even.  I love autocorrect

I wish my autocorrect had caught my classical/quantum blunder. Skitt's Law struck again
 

Offline imatfaal

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Re: Evidence for large scale length contraction?
« Reply #121 on: 14/09/2012 11:59:51 »
Quote
Here is another “reality check” against large scale length contraction:
Say an alien probe is discovered heading toward Earth at a significant fraction of light speed. From Earth’s frame it is measured to be ten meters long, length contracted because of its velocity relative to earth. It is decided to go out and intercept/capture it in one of our very high speed space shuttles (of the future.)
Our shuttle has a ten meter cargo bay. Will the probe fit into the bay?... A very practical test of “actual length” vs “contracted length.”
The answer is “no” because the probe’s “actual length” must be longer than its “contracted length” for it to appear as ten meters long from earth’s frame in this case.
  This was the formulation - and I am pretty sure that no one has argued about the answer as it is uninteresting.  The question doesn't make it clear that you are placing the shuttle also at rest within the rest frame of the probe - but with that condition then obviously the probe will not fit  - there is no need for special relativity here.

it is your insistence that the contraction is only apparent, and the concomitant implication that this is an artefact, an illusion that better observation greater understanding would dispel which has lead other to postulate refined experiments that have interesting consequences.  DavidC's concept of the cargo bay with open end(s) makes the experiment doable with the shuttle in a frame of high relative motion to the frame of probe.  I have been told off for wrecking space shuttles (it's not as if we are using them anymore) - and Damocles managed to take it to another level by leaving the shuttles unharmed yet encompassing the objects in high relative motion.

You presented another thought-experiment regarding alpha centauri
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I will give an illustration. The Alpha Centauri complex (AC) is 4.37 light years away from Earth. It takes  light from AC 4.37 (edit) years to reach Earth, and no “thing” with mass can travel that fast.
A ship traveling at near ‘c’ velocity from here to there must, therefore take longer than 4.37 years to get there, even though the onboard clock will have slowed down and recorded much less than 4 years passing. Yet Earth will have orbited the Sun (the "year" standard) much more than 4 times during the ship’s journey to AC. So, even though “for the ship” much less than 4 years will have passed, the distance between Earth and AC will not have contracted to way less than 4 light years. The ship’s journey obviously will not make Earth and AC move closer together.

I posted a link and quotes to a very interesting blog which I believe correctly showed the principles and unsettling consequences of Special relativity.  Skulls in the Stars 
Quote
...For a spacecraft traveling at 90% of the speed of light, the distance will only be 1.90 light-years, less than half the distance that an Earth-bound observer would measure!  ...

I am completely confused as to what you want from us.  If you want an admission that length contraction is illusory and in some sense not "real" - then you just won't get it (from me at least); mainly because SR deals in maths and not semantics, and secondly because I can fit the probe (very briefly) entirely within the shuttle's cargo bay.
 

Offline lightarrow

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Re: Evidence for large scale length contraction?
« Reply #122 on: 14/09/2012 13:31:56 »
Ok. So, which is the intrinsic property of a monocromatic light (or X-ray, or gamma ray) beam coming from a very distant star? If you say "the frequency", I ask you "in which frame?" And if you reply "in the source frame" I can reply that the star which have emitted it, now can be non existing anylonger, and its ancient position can be  non identifiable.

Interesting point, but I have a quick question.
Let's assume there is a distant source (like our Sun) that emits harmless visible light for an Earth observer.
The observer take a  relativistic ship (0.9999c)  and travel toward the source. His ship screen out all radiation except those wavelengths that for the Earth frame were harmless visible light. Due to Doppler Effect the light that penetrates the ship will be now gamma-ray.
My question is: Will the observer on ship get hurt by gamma rays? Or in his ship those penetrating gamma rays (that on Earth were visible light) have the same impact as the visible light on Earth frame, i.e. they will be harmless?
No, the ship will be hit by gamma rays and it will become radiactive and, furthermore, the intensity of the gamma rays will be much greater than the intensity of the visible light hitting the Earth. Apart from interstellar powder, the mere presence of the CMBR (cosmic background microwave radiation) puts a serious limit to relativistic travels for this reason.
 

Offline JP

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Re: Evidence for large scale length contraction?
« Reply #123 on: 14/09/2012 14:37:17 »
There's a nice quote from Einstein on the wiki about "real" and length contraction:
http://en.wikipedia.org/wiki/Length_contraction#Reality_of_Lorentz_contraction
Quote
The question as to whether the Lorentz contraction really exists or not is misleading. It doesn't "really" exist, in so far as it doesn't exist for a comoving observer; though it "really" exists, i.e. in such a way that it could be demonstrated in principle by physical means by a non-comoving observer.
—Albert Einstein, 1911

It also goes on to point out what most of us have been stating, that argument over what is "real" only depends on terminology.  If you define "real" to mean "in the rest frame" then your argument is right.  If you define "real" to mean "what is measured" then your argument is wrong.  In actual science, we don't use muddled terms like "real," and constrain ourselves to describing the predicted measurements, for which there is no ambiguity.

In essence, "real" matches your definition of "real" because you defined it to do so from the start.
« Last Edit: 14/09/2012 14:39:27 by JP »
 

lean bean

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Re: Evidence for large scale length contraction?
« Reply #124 on: 14/09/2012 16:20:25 »
For those of you who don't mind adapting a thought-experiment... relativity is of course reciprocal; just as we can define the rest frame as the frame in which the shuttle has zero momentum, we can also define the rest frame as the frame in which the probe has zero momentum and the shuttle is approaching it at high velocity (in the opposite direction).  From the rest frame of the probe we now see the shuttles cargo bay as length contracted, and the probe is no longer contracted ...and the probe would still all fit in just before destroying itself and the shuttle!  The timings are very tough to reconcile, events seem to be switched, and with relativistic speeds you cannot be sure of any simultaneities without the maths; but in the end the same set of events happens - it must be the case because it is the exact same scenario.

made me think of the old pole and barn
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/polebarn.html

 

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Re: Evidence for large scale length contraction?
« Reply #124 on: 14/09/2012 16:20:25 »

 

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