Experimental Confirmation Length Contraction Velocity Addition

[jeremyfiennes]

TL;DR Summary

What, if any, is there?

What if any is the experimental confirmation for length contraction and the velocity addition formula?

 

[Vanadium 50]

What, if any, research have you done on this? I sure don't want to write a multi-page response only to hear "oh, I know all that".

 

[hutchphd]

Try googling " experimental confirmation for length contraction ".

 

[jeremyfiennes]

Ok.

 

[FactChecker]

S

Summary:: What, if any, is there?

What if any is the experimental confirmation for length contraction and the velocity addition formula?

You should start with the understanding that this is the last thing that scientists of that era wanted to believe. There was so much experimental proof, so well done, that even the most stubborn of them had to finally admit these facts.

 

[jeremyfiennes]

I had a new look. What I found is best summarized by this quote from one of them: "Despite the vast majority of experiments that have been done, no firm conclusion can be made as to whether length contraction really exists." And much the same with regard to the addition formula. E.g. "I've looked through pages that list the tests of SR, and read papers on it, but no answer appears."
Would you agree?

 

[hutchphd]

Supply a reference for thee quote please.

 

[jeremyfiennes]

Sorry. I googled and went quickly through a whole load. I copied these two comments simply as representative. I can't remember which site they came from.

 

[hutchphd]

Then try again.

 

[PeterDonis]

Sorry. I googled and went quickly through a whole load. I copied these two comments simply as representative. I can't remember which site they came from.

This is not an acceptable way to reference anything. Either provide valid references or this thread will be closed.

 

[Vanadium 50]

What I found is best summarized by this quote from one of them: "Despite the vast majority of experiments that have been done, no firm conclusion can be made as to whether length contraction really exists."

Interestingly, Google found not a single site using that exact quote. Removing the quotes produces many hits, the vast majority of which are crackpottery.

 

[jeremyfiennes]

Ok. So forget the crackpottery. I asked you exactly because I couldn't find a satisfactory answer on the web.

 

[hutchphd]

I was rapidly conveyed to the Mt Washington muon experiment (no quotes on search). Seems pretty definitive to me, although I wasn't riding on the muon. I suppose you also need Michelson Morley to tie up the logical bundle.

 

[jeremyfiennes]

I know of the muon experiment. For me it convincingly demonstrates time dilation. But to hold that it also demonstrates length contraction seems to imply that the two are one and the same thing, different sides of the same coin. I don't see any theoretical reason for this.

 

[hutchphd]

No if the laws of physics are invariant then there is length contraction. Take your pick.

 

[jeremyfiennes]

Sorry, not with you.

 

[hutchphd]

muons decay in a fixed time (on average) in their rest frame. Either they travel a shorter distance or the laws of physics they are subject to differ.

 

[jeremyfiennes]

Ok. Relative to an observer on Earth, high speed muon time delates and their decay slows, enabling more to arrive. I get that. For the same observer their length also contracts. But this is not measurable, so how does that demonstrate length contraction?

 

[atyy]

https://en.m.wikipedia.org/wiki/Fizeau_experiment
See the section "Lorentz's Interpretation"

More generally, length contraction is one of the implications of the Lorentz transformations between inertial frames. The Lorentz transformations represent a symmetry of the laws of physics, which has been tested many times, as summarized in https://arxiv.org/abs/gr-qc/0502097.

 

[jeremyfiennes]

Ok, I'll look them up. Thanks.

 

[Dale]

The Michelson Morley experiment demonstrates length contraction.

I don’t know about an experiment specifically about velocity addition, but along with the rest of SR it is implied by the combination of Michelson Morley, Ives Stillwell, and Kennedy Thorndike.

 

[jeremyfiennes]

Thanks.

 

[Nugatory]

But to hold that it also demonstrates length contraction seems to imply that the two are one and the same thing, different sides of the same coin. I don't see any theoretical reason for this.

The theoretical justification is clear if you start with the Lorentz transformations. Consider an artillery shell with a timed fuse fired at event A and detonated by the timer at event B, moving with speed $v$ using coordinates in which the gun is at rest. Calculate the coordinates of B using coordinates in which the gun is at rest and again using coordinates in which the shell is at rest. It’s the same shell doing the same thing in both frames, but in one the result is due to time dilation and in the other it is due to length contraction.

But you also wanted experimental evidence of velocity addition and length contraction.

For length contraction, google for “magnetism Lorentz contraction Purcell”.

For velocity addition, google for “Fizeau speed of light water”. This one is especially interesting because Fizeau observed relativistic velocity addition more than fifty years before Einstein discovered relativity; his results were a perplexing mystery until 1905.

 

[Dale]

For velocity addition, google for “Fizeau speed of light water”.

Oh, yes, that is a perfect demonstration of velocity addition.

 

[PeterDonis]

Relative to an observer on Earth, high speed muon time delates and their decay slows, enabling more to arrive. I get that. For the same observer their length also contracts.

To an observer riding along with the muon, time dilates and length contracts, relative to an observer on Earth.

The observer on Earth says the muons travel a longer distance and take a longer time.

The observer riding along with the muon says the Earth (in this frame the Earth is moving, not the muons) travels a shorter distance and takes a shorter time to do it.

The laws of physics for muon decay say that the time that elapses in the muon's rest frame is the time that plugs into the decay law. So the observer on Earth, even though they say the muons travel a longer distance and take a longer time, also says that fewer muons decay, because the law of muon decay doesn't use the time in the Earth frame, it uses the time in the muon frame.

(Strictly speaking, the muon decay law uses an invariant, the muon's proper time, since laws of physics must involve invariants, not coordinate-dependent quantities. But the muon's proper time is the same as the coordinate time in the muon's rest frame.)

 

[pervect]

I'd suggest starting with our FAQ:

https://www.physicsforums.com/threads/faq-experimental-basis-of-special-relativity.229034/

That currently refer to another FAQ: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html, plus a GR FAQ that isn't relevant here. There are some other FAQ's stickied that I haven't looked at that you might check out.

Looking at the above, the existence of the magnetic field has been suggested as indirect evidence of length contraction. There isn't anything listed for velocity addition, though there are listed experiments for the speed of light from moving sources, and there are tests that "c" is a limiting velocity. Both are relevant to the velocity addition question - they don't directly validate the textbook expression for the velocity addition formula, but they do indicate that velocities cannot add linearly.

I would say that the experimental results, of which there are many, don't specifically test the two effects you have singled out, from what I've read. But the effects you ask about can be deduced from the theory, which has been tested via other observations.

The theory itself is directly based on the constancy of the speed of light for all observers, plus the principle of relativity.

Both of these have been tested, the first having IMO been tested more thoroughly.

 

[Sagittarius A-Star]

the existence of the magnetic field has been suggested as indirect evidence of length contraction.

Yes. A related experiment would be to measure the Lorentz-force on a cat, charged with $q$:
$$\vec F = q (\vec E +\vec v \times \vec B)$$
If the charged cat is moving parallel to a wire at rest with an electric current in it, then it feels in transversal direction only the magnetic part of the Lorentz-force, because the wire is electrically neutral.

In the rest frame of the charged cat, the same force is only the electric part of the Lorentz-force. In this frame, the wire is not electrically neutral because of it's length contraction.

Source:

 

[jeremyfiennes]

Thanks all. I thik I've got it now.