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Author Topic: Does Lorentz contraction affect a stationary object that you pass at high speed?  (Read 33305 times)

Offline DoctorBeaver

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Lorentz contraction has been mentioned a few time in threads just lately. I understand that an oblect travelling at very high speed will contract along its length in the direction of travel.

Now, GR states that if 2 objects pass each other with nothing to reference against, it is impossible for a person on 1 of them to know whether it is the object he is on or the other 1 that is moving.

So, my question is, if I was in a spaceship travelling at relativistic speed and we passed a stationary object, would it appear to me as being contracted? Surely it must to fit in with GR. But if it is velocity that causes Lorentz contraction then it wouldn't affect a stationary ship, would it?


 

Offline Vern

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It is relative velocity that causes the length contraction. So the answer is yes. You would not be able to determine whether it was you or the object that was moving without some other reference. All motion is relative.

You get the same answer whether you calculate using the SR, GR, or Lorentz conventions. I like the Lorentz version because it doesn't require that we think of space and time as being distorted. All of the distortion can be attributed to the matter that moves. However, the Lorentz version is more complicated and requires a special frame of reference fixed in space.
« Last Edit: 20/03/2009 21:52:10 by Vern »
 

Offline DoctorBeaver

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Thanks, Vern. I guessed it would look contracted but I wasn't sure.
 

Offline yor_on

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I think I can see where your thoughts went DB. Is the contraction real? And if it is a real one, and I know that I've been accelerating my ship and therefore have 'objective evidence' of me traveling very fast and then passes that other ship that, according to what we earlier agreed on, just would be standing still relative what frame of reference we both shared before this thought experiment. Will that ship be contracted 'for real' although we knew before whom it is moving faster, as we both had the same frame of reference originally (Earth).
 

Offline DoctorBeaver

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yor_on:

I wasn't sure whether it was due to velocity per se or relative velocity. I guessed it was relative velocity as otherwise it would contradict GR.
 

Offline lightarrow

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Just to precise the things a little bit: the Lorentz contraction doesn't mean that the material is "compressed" as a string giving an internal tension; it's an effect only due to relativity of simultaneity:
by definition, an object's lenght is the difference of the positions of its extremes "measured simultaneously". It's for this simultaneity in the definition, that an object's lenght is frame-dependent.
 

Offline DoctorBeaver

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Just to precise the things a little bit: the Lorentz contraction doesn't mean that the material is "compressed" as a string giving an internal tension; it's an effect only due to relativity of simultaneity:
by definition, an object's lenght is the difference of the positions of its extremes "measured simultaneously". It's for this simultaneity in the definition, that an object's lenght is frame-dependent.

You're trying to confuse me again. And succeeding.
 

Offline lightarrow

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Just to precise the things a little bit: the Lorentz contraction doesn't mean that the material is "compressed" as a string giving an internal tension; it's an effect only due to relativity of simultaneity:
by definition, an object's lenght is the difference of the positions of its extremes "measured simultaneously". It's for this simultaneity in the definition, that an object's lenght is frame-dependent.

You're trying to confuse me again. And succeeding.

??? Think that I bieleved to clear things up!

What is unclear?
« Last Edit: 21/03/2009 18:48:50 by lightarrow »
 

Offline DoctorBeaver

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Alberto - It's OK. I undertood it.
 

Offline LeeE

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There's a fun relativistic length-contraction thought experiment concerning a ladder and a two-doored shed.

In the thought experiment, a twelve foot long ladder is approaching an eleven foot long shed, which has a door in each end, at relativistic speed.  Because of the apparent length contraction, as observed by someone standing in the shed, the observer should be able to close both shed doors while the twelve foot ladder is entirely inside the eleven foot shed.  However, from the ladder's point of view, it is the shed that is contracted and it is therefore impossible for both doors to be closed while it is inside it  ;D
 

Offline yor_on

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I got to admit that the question if the Lorentz contraction is a real process have been on my mind for quite some time. Those two links sees it as being as real as we see time dilation to be. http://arxiv.org/pdf/physics/0606171v1 as well as http://renshaw.teleinc.com/papers/simiee2/simiee2.stm

If it is so then I made this thought experiment wherein we have a predefined common nominator (Earth) that we start both ships from. One of them we will place being at rest with Earth, the other one will pass it accelerating to a near 'c'. They will measure each other (lasers) and will both find the other one shorter. Although we now already know that one of the ships are being at rest relative Earth and we also know that the other ship is accelerating through its creation of a 'gravity well' situated behind it we still will observe this phenomena? And it will be real?? If so,that implies that this goes for Earth to the same degree too, right?

Think now of the same system (two ships & Earth) with the exception that the formerly accelerating ship now have stopped its acceleration and are now in what we call 'uniform motion'. This mean that there is no longer any real proof of what speed or motion this ship might have, and even when looking out, there is no proof that it isn't the rest of the universe that is moving relative them instead of the other way around. But when they meet this ship again the same phenomena (Lorentz contraction) will be seen. What does this say about length? If we can't guarantee any motion as being any better than a 'preconception' based on arbitrary choices, isn't that the same as saying that our universe, depending on our choice of frame will have different sizes, also that this frame do not act only on you (accelerating ship) but also on the frame you compare it too (ship at rest versus Earth). It is a intriguing concept if it is not a optical illusion.

----

Expressed as Energy one could say that the ship accelerating are collecting a lot of 'relative energy' distorting spacetime. But the ship being at rest with Earth? They will see the same effect, yet, haven't collected or received any relative energy as I understands it. To see the strangeness here you must understand that, according to those ideas, both ships length contraction is real.

----
http://adsabs.harvard.edu/abs/2001APS..APR.C9001R
« Last Edit: 21/03/2009 21:55:09 by yor_on »
 

Offline DoctorBeaver

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There's a fun relativistic length-contraction thought experiment concerning a ladder and a two-doored shed.

In the thought experiment, a twelve foot long ladder is approaching an eleven foot long shed, which has a door in each end, at relativistic speed.  Because of the apparent length contraction, as observed by someone standing in the shed, the observer should be able to close both shed doors while the twelve foot ladder is entirely inside the eleven foot shed.  However, from the ladder's point of view, it is the shed that is contracted and it is therefore impossible for both doors to be closed while it is inside it  ;D

Sometimes I really hate you!
 

Offline lightarrow

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There's a fun relativistic length-contraction thought experiment concerning a ladder and a two-doored shed.

In the thought experiment, a twelve foot long ladder is approaching an eleven foot long shed, which has a door in each end, at relativistic speed.  Because of the apparent length contraction, as observed by someone standing in the shed, the observer should be able to close both shed doors while the twelve foot ladder is entirely inside the eleven foot shed.  However, from the ladder's point of view, it is the shed that is contracted and it is therefore impossible for both doors to be closed while it is inside it  ;D

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Noo! If you think about what I wrote, you should grasp that the paradox comes from the wrong assumption that when you measure the lenght of the ladder (or the shed) the simultaneity is absolute, while instead is frame-dependent.
Another name for this paradox is "The barn and the Pole paradox":
http://math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html
 

Offline DoctorBeaver

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There's a fun relativistic length-contraction thought experiment concerning a ladder and a two-doored shed.

In the thought experiment, a twelve foot long ladder is approaching an eleven foot long shed, which has a door in each end, at relativistic speed.  Because of the apparent length contraction, as observed by someone standing in the shed, the observer should be able to close both shed doors while the twelve foot ladder is entirely inside the eleven foot shed.  However, from the ladder's point of view, it is the shed that is contracted and it is therefore impossible for both doors to be closed while it is inside it  ;D

Sometimes I really hate you!
Noo! If you think about what I wrote, you should grasp that the paradox comes from the wrong assumption that when you measure the lenght of the ladder (or the shed) the simultaneity is absolute, while instead is frame-dependent.
Another name for this paradox is "The barn and the Pole paradox":
http://math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html

ohhh, let me hit him anyway.
 

Offline yor_on

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Yeah, the barn and the pole is a nice example too:)
So moving/accelerating begets a shrinking universe from the frame of the one moving and for the frame 'not moving' the other ship will be the 'thing' shrinking. the first ships revelation could be explained by spacetime distortion, and what the other ships sees? Also a spacetime distortion? But if what the moving ship experience as 'shrinking' is a real effect, Then it seems to have no proportion to the energy being spent creating it. As, in fact, the whole universe reacts to this ships motion in time, and all for real. If this is true I can't help but wonder about how 'energy' transforms into 'work'.

------

If work is done on an object when you transfer energy to that object, then there has been no transfer of energy as far as I can see, still the universe have shrunk as has all objects in it, the thing bugging me is that it is 'real'?

So where is this concept defining how this can be? It's not any work done on the objects outside the ships frame of reference, it's only work done on 'space' and the ship while accelerating it, and that 'work' seems to go a long way :)
« Last Edit: 22/03/2009 11:38:15 by yor_on »
 

Offline LeeE

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Ouch, ouch, ouch, ouch!
 

Offline DoctorBeaver

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If 2 objects are following each other at relativistic speed then the distance between the front and back of each object appears to shrink. So what about the distance between the back of the first object and the front of the second? Does that also appear to shrink so that they seem closer together?

 ???
 

Offline yor_on

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If 2 objects are following each other at relativistic speed then the distance between the front and back of each object appears to shrink. So what about the distance between the back of the first object and the front of the second? Does that also appear to shrink so that they seem closer together?

 ???

If they are traveling at a uniform velocity and being 'at rest' when compared to each other they will belong to the same 'frame of reference' and there will be no Lorentz contraction seen between them. But if you are thinking of them accelerating at the same exact velocity? I guess they still could be seen as being 'at rest'? I don't really know, that's seems a tricky one DB.
 

Offline Vern

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If you were considering them as a system from another frame they should seem to close the distance between them. Edit: and as yor_on indicates, from the same frame they would see no change.

This brings to mind something that just occurred to me. As most of you now know, I prefer the Lorentz description of relativity phenomena to the Einstein version. With Lorentz, length contraction of objects in motion is a physical distortion of the objects, and not a distortion of space and time as with Einstein. So far, there has never been an experiment that could test a difference in the two concepts. So, Occam's razor cuts out Lorentz.

Now, lets apply the Lorentz version to the scenario in the OP. My near-light-speed ship is passing a stationary ship. What is my perception of the length of that stationary ship? Einstein says I see it length contracted. What about Lorentz?

It seems now as I think about it, the Lorentz version where my measuring devices suffer the contractions, and the stationary ship does not, Lorentz should say the stationary ship seems expanded, not contracted.

There is a proposed test of this by a satellite containing a very precise measuring device that can measure the angle between two stars. The test proposal is that the speed of the earth in orbit is enough to change the apparent angle between two stars from one earth season to the next. If my thinking is right, the Einstein version would see the contraction. The Lorentz version should see no contraction.

The reason why is: Half of the effect is due to aberration, the other half, contraction due to earth's motion relative to the stars. Aberration and contraction are additive so produce double the calculated contraction with Einstein. Aberration and Lorentz expansion would cancel, so that no effect would be seen.

Edit: That last sentence should read: Aberration and apparent expansion due to Lorentz contraction of local measuring devices would cancel ....

Here's a link to the article about the test.
« Last Edit: 22/03/2009 15:56:47 by Vern »
 

Offline DoctorBeaver

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If 2 objects are following each other at relativistic speed then the distance between the front and back of each object appears to shrink. So what about the distance between the back of the first object and the front of the second? Does that also appear to shrink so that they seem closer together?

 ???

If they are traveling at a uniform velocity and being 'at rest' when compared to each other they will belong to the same 'frame of reference' and there will be no Lorentz contraction seen between them. But if you are thinking of them accelerating at the same exact velocity? I guess they still could be seen as being 'at rest'? I don't really know, that's seems a tricky one DB.

I meant to an outside observer.
 

Offline yor_on

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Vern, when I read him I got the impression that he accepted length contraction as real, reading him again I see that he seem to doubt it by his conclusive words? But there is still the muon test wherein he writes "In the muon'sí frame of reference, the situation is quite different. The only way this can happen in the muon'sí reference frame is if the actual physical distance that must be traveled by them is shortened as in (8). This is not a visual effect for the muon. If the distance traveled by the muon is not physically shorter, the muon simply does not remain in existence long enough to make the trip, even at speeds greater than .9c. To the muon, length contraction is clearly not merely a visual effect, as the muon is not "seeing" anything. The distance to be traveled by the muon from the upper atmosphere to sea level is physically shorter than the same distance measured by a slower moving particle. The high speed muon performs Einsteinís train embankment experiment first hand."

I don't really know what to think here, in what way do you mean that this aberration cancels out Lorentz contraction? It seems to go both ways depending on the observer? ". The maximum amount of the aberrational displacement of a star is approximately 20 arcseconds in right ascension or declination. ".It seems to depend on the movement relative the observer? Or am I thinking wrong here?? http://en.wikipedia.org/wiki/Aberration_of_light#Apparent_and_true_positions
« Last Edit: 22/03/2009 19:51:35 by yor_on »
 

Offline yor_on

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Ok DB, got it.
And again, I don't know :)
The difficulty here seems to be to decide what the 'system' observed consists of. Is it only the material objects that we will observe this phenomena on or should space between them be included. If one consider the 'muon description' it definitely seem to consider 'distance' in itself so considering this I would expect it to have an effect on the space surrounding those two objects. Even if space is 'empty' of matter it still can contain a 'distance', but is that space moving? Shouldn't it be seen the same way as the observer? as relative the moving ship containing non moving 'static points' in spacetime? This one is sooo strange to me :) Lightarrow et al, give it a shot. There is definitely at least two 'camps' of reasoning here, if not more?? http://www.physicsforums.com/showthread.php?t=106526 and no one seems totally 'wrong'. The muon example seems like a reasonable proof to me as we have proofs for time dilation and this one, according to what I understands (not much:), is similar..
 

Offline Vern

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Hi yor_on; I agree that Curt Renshaw, in the link, has some notions about the cause of the Lorentz contraction that is at odds with what most of us suspect. But he does describe the proposed experiment that would see a difference between the Lorentz version of space-time and the Einstein version IMHO.

Yes; the muon must see the distance shortened as well as the time dilated in order to reach the earth's surface as a muon. I haven't thought about this enough to understand if it rules out the Lorentz version.

The referenced experiment was supposed to take place in 2005. I wonder what the results indicated. A positive result would support the Einstein version, a negative result would support the Lorentz version -- but it wouldn't rule out Renshaw either.
 

Offline yor_on

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I have a link saying that the experiment should start 2008.
http://adsabs.harvard.edu/abs/2001APS..APR.C9001R but I've found no conclusions drawn from it on the 'net yet' njet :)) :  (<--- Lorentz contracted smiley:)

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Offline Vern

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Thanks for the link yor_on. If it did start in 2008, someone should have results by now. But this in one of those cases where a positive result would be announced right away. A negative result may never be reported :)

 
 

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