# Relativistic parallel rods

Discussion in 'Physics & Math' started by Pete, Apr 30, 2013.

1. ### TachBannedBanned

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I am , I was just explaining how you are trying to rig the experiment in the other thread and why it doesn't work, even after you rigged it.

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3. ### PeteIt's not rocket surgeryRegistered Senior Member

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Take it to the other thread, Tach. Posts continuing that argument in this thread will be deleted.

Any argument you had relevant to this thread appears to be done:

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5. ### TrooperSecular SanityValued Senior Member

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So, Pete, using the rockets and missile as a comparison, in this example that you used, a third observer would think that the missile would hit the rocket, but it doesn't, right?
Are you saying that in the other thread that Tach is implying that it would hit the rocket in this case?

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7. ### PeteIt's not rocket surgeryRegistered Senior Member

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Hi Trooper,
No, the third observer would think that the missile misses. In every reference frame, the missile is fired from the tail of one rocket before the nose of the other rocket arrives.
No, the other thread is completely different.

8. ### mikelizziRegistered Senior Member

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Oh, I thought this thread was about the parallelism (or lack thereof) of the rods (or rod and train floor) after a transformation. I thought what happens during the collision of the rod and train floor was the subject of another thread. The collision is the more interesting part of the problem. Are we all agreed that, from the frame of a ground observer, the rod bends but does not break when it hits the train floor? Then we could discuss why there are no axial or shear stresses in the rod, only compressive stress from the floor impact.

9. ### mikelizziRegistered Senior Member

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I'm only commenting on the falling rod problem. As far as I am concerned, the animation Pete showed of the rod bending as it leaves the roof of the train car, falling non-horizontal, and then bending as it hits the floor is exactly correct. Do we really want to add the discussion of another problem to this thread? Or do I have the subject of the thread wrong again?

10. ### PeteIt's not rocket surgeryRegistered Senior Member

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Hi mikelizzi, this thread is about whether the rods in box-diagonals show that parallelism in general is frame-independent.

The original rod falling under gravity is here: [thread=134380]Relativity paradox[/thread]
The rod moving across the train in zero gravity is here: [thread=134429]Basic Special Relativity Question[/thread]

11. ### TachBannedBanned

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Yes, this is the problem that spawned three different threads , in addition to the original one. Four total, and no resolution, despite the fact that it is very easily solvable if you one doesn't try dumbing it down by gratuitously removing the effects of the gravitational fields.

Who is "all" that agreed? I certainly did not agree. The "bending" is the effect that I dispute, it is simply an artifact of the ground observer assigning differing time stamps to the collision points, nothing physical.

12. ### mikelizziRegistered Senior Member

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Oh, now I'm on board, I think.
And so my first comment on the real subject of this thread is, add me to the list of people who say parallelism of rods in boxes is frame independent. The resolution of the "Meterstick and the Hole Paradox", among others, depends upon it.

Typing Correction: add me to the list of people who say parallelism of rods in boxes is frame dependent.

Last edited: May 15, 2013
13. ### RJBeeryNatural PhilosopherValued Senior Member

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Well done for naming the paradox, mikelizzi, it gave me something to search for. This link is quite analogous to the OP. This issue was actually settled weeks ago, but perhaps reading the answer in a published 25 year-old text book on SR will convince the remaining denier(s).

14. ### brucepValued Senior Member

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If local proper frame measurements we're frame dependent then the physics would be frame dependent. Our existence would be frame dependent. Spacetime events are frame invariant. The theory isn't dependent on any choice of coordinates. Choosing a set of coordinates is a luxury for doing the physics. You can solve all [close to it but all is a big word] using invariants. One of the first thing discussed in relativity theory are invariants. Generally the comment is 'more the merrier' and as you move into the theory it becomes obvious why. There is only one set of rules that describe the relativistic universe. Any remote coordinate frame measurement, or theoretical prediction, has to be conserved in the local proper frame where the measurement was made. So my focus was always to show that the initial claim of a paradox was nonsense. The paper is a good resource for those who might still think remote frame observations by the 'rod' and 'slit' are frame invariant.

15. ### Neddy BateValued Senior Member

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Please tell me you meant to type, "...parallelism of rods in boxes is frame dependent?" Post #1 of this thread shows that very nicely, I think.

16. ### mikelizziRegistered Senior Member

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My bad. I typed just about the worst mistake I could in a thread like this. Thanks for catching it. My positions is that parallelism of rods in boxes is frame dependent.

17. ### mikelizziRegistered Senior Member

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I and my computer program have no problem applying the same set of physics rules to any inertial reference frame without contradictions. My referenced explanation of the "Meterstick and the Hole Paradox" is mainstream physics. Are you saying there is something wrong with either the statement of the problem or its published resolution? I repeat the reference where it was treated in the American Journal of Physics over 50 years ago.

American Journal of Physics
Volume 30, Issue 1, Page 72
Title “Length Contraction Paradox”
By R. Shaw

That doesn't guarantee its right. But it does mean you have a lot of work to do if you want to change the minds of the scientific community.

18. ### UndefinedBannedBanned

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Oh I see. Yes, Earth's gravity field can be ignored. I was thinking of the energy flux other than from gravity which can be high enough to "curve" local space in the LHC tube during experiments due to the enormous electromagnets and currents which activate and accelerate the particle beams. Thankyou for your clarification, brucep.

19. ### brucepValued Senior Member

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In the local proper frame of the box all measurements of line segments, parallels, and angles transform as invariants. For instance the frame dependent measurements made from the remote coordinate frames of the slot and rod are frame dependent. This thread compares measurements made in remote coordinate frames and conclude they're frame dependent. Duh.

20. ### UndefinedBannedBanned

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Can you clarify exactly which measurements you mean and where they are made? Because I am confused what you mean by "has to be conserved in the local proper frame where the measurement was made". First you mention remote frame measurement and then you say that remote measurement "has to be conserved in proper frame where measurement was made"? I think I know what you are saying but I am not sure because it could be naively read in two ways at least, which is confusing to me. Can you clarify please, brucep?

21. ### brucepValued Senior Member

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The paper is right. The authors attempt to convey what measurements would be predicted [they figured it out] in the remote coordinate frames of the spacetime event 'rod fits through the slot'. Both these different coordinate frame measurements recover the invariant measurements made in the local proper frame of the 'rod and slot'.

22. ### brucepValued Senior Member

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4,098
It's really simple but seems confusing because you haven't learned the theory. This is an example of a local proper frame measurement. Keep it simple: You measuring a piece of plywood in your garage. These measurements are invariant direct measurements made in the local proper frame of what's being measured. Remote coordinate frame measurements are made from remote coordinates. For example a spaceship at relativistic relative velocity makes the same measurement as you but from remote coordinates. These measurements are frame dependent. The local proper frame measurements are invariant [make sense?] while the remote coordinate frame measurements can vary from remote coordinate frame to remote coordinate frame based on the relativistic geometry of spacetime. Being able to know or predict the invariants is what the theory does. For the weak field these predictions are very close since all relativistic effects are miniscule. For the strong field these predictions can be very diverse. We can use remote coordinate measurements to predict what the measurements are if conducted in the local proper frame of the spacetime event being measured.

23. ### UndefinedBannedBanned

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1,695
Yes, I already naively understood what the difference is between measuring in proper frame and measuring from another remote frame. That is why your previous "brief" description of what you meant was confusing to me as you wrote it and I read it. Thankyou for confirming that "remote measurements" is a "process" which may differ from the measurement process in proper frame itself. The information used in remote frame must be transformed to "recover" (like you say) the same values for the proper frame, yes? This is done via "transform" algorithm applied to remote frame information about proper frame parameters, yes? So by "conserved" you meant "recovered via transformation theory/algorithm" to agree on the proper frame's parameters, yes?