How to test length contraction by experiment?

Discussion in 'Physics & Math' started by PengKuan, Jun 17, 2019.

  1. PengKuan Registered Senior Member

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    136
    Relativistic length contraction is theoretically predicted but not directly tested, which lead to incorrect interpretation of the theory illustrated by Bell's spaceship paradox and Ehrenfest paradox. But these paradoxes can help us designing experiments to test length contraction.
    Ideal direct experimental proof should contain the following steps:
    1. Measure the tested object's length at rest, the value l0.
    2. Put this object in motion.
    3. Measure the object's speed, the value v.
    4. Measure the object's length in motion, the value l.
    5. Check if these 3 values verify length contraction law.
    For doing this experiment, the difference of length l0  l should be in measurable range. If the object is a chunk of matter, l0  l is not measurable. For example, matter objects with the highest speed we can make are satellites, whose speed is generally 7.8 km/s. If a satellite is made of a string of 100 km long, the value of l0  l would be 0.03 mm, which is absolutely not measurable from the ground. This is why contraction of length has never been measured.
    Below I propose two experiments inspired from Bell's spaceship paradox and Ehrenfest paradox.
    Please read the article at
    PDF: How to test length contraction by experiment? https://pengkuanonphysics.blogspot.com/2019/06/how-to-test-length-contraction-by.html
    or
    Word: https://www.academia.edu/39584663/How_to_test_length_contraction_by_experiment
     
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  3. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

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    Don't think so

    Put a summary here please and for my personal benefit small words and no maths unless really really needed

    Please Register or Log in to view the hidden image!

     
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  5. DaveC426913 Valued Senior Member

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    While I applaud your desire to verify relativistic contraction, it's not as simple as you make out.
    How exactly do you measure the length of an object moving at relativistic velocities with respect to your yardstick?

    Hint: the key concept of relativity (some would say the only concept) is relativity of simultaneity. How do you measure the two ends of an object that is passing you at near c? You can't be at a distance, or your measurements will be off, and you can't be in both places at once.
     
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  7. PengKuan Registered Senior Member

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    My idea is:
    1. Measure the distance between 2 electrons at rest, which is the distance between 2 electron guns. l0
    2. Fire the 2 electrons in the way at the same time.
    3. Measure the electron's speed, the value v.
    4. Measure the distance between the 2 electrons in motion, the value l.
    5. Check if these 3 values verify length contraction law.
     
  8. PengKuan Registered Senior Member

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    136
    I make 2 electrons in motion. I shine light on them, I take photograph of the scattered light from the electrons.
     
  9. DaveC426913 Valued Senior Member

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    And what does that get you?
     
  10. DaveC426913 Valued Senior Member

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    I'm sorry, your experiment will not work.

    1] You won't know their velocities until the hit your detectors. Only then can you calculate their velocity. Their velocity will simply be the distance from gun to detector divided by the time it took.

    This time will be the same for both of them, since both detectors are the same distance from their guns. You haven't measured anything except that both electrons had the same velocity.
     
    Last edited: Jun 18, 2019
  11. exchemist Valued Senior Member

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    12,451
    I don't think you can do that. An electron will scatter one photon and its path will be altered by the interaction. You have no hope of determining its position from this interaction, even if you succeed in detecting the scattered photon. You certainly can't take a photograph with one photon.

    By the way, what's the issue with the Ehrenfest paradox? I thought that was resolved by general relativity - in fact I think it was one of the things that gave Einstein the idea for it.
     
  12. Neddy Bate Valued Senior Member

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    2,548
    I don't think there is any issue with the Ehrenfest Paradox or Bell's Spaceship Paradox. Also, I think length contraction is evidenced in particle accelerators, because more particles can fit in the ring when they are moving at high speed, compared with when they are not.
     
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  13. DaveC426913 Valued Senior Member

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    Here's how we do know that relativistic contraction works:

    Muons are short-lived particles that are created when Cosmic rays blast into atoms in the upper atmosphere. The muons created are moving at about.9999c.

    We know how long muons live before they decay into other particles - it's about 2 x 10^-6s or about 2 microseconds. That is not enough time for them to reach the ground. In fact, it's only enough time for the muons to travel about 0.3 miles.

    But they do reach the ground. The only way this is possible is if they are living longer than expected.

    At .9999c the calculated dilation is about 70, so they should be living 70 times longer - enough time to travel the 26 miles to the ground.


    OK, here's the thing: you can look at dilation of time as the same thing as contraction of distance. As far as the muon is concerned, it only lives for 2 microseconds, this means that, in order for it to reach the ground before decaying, the ground must be less than .3 miles distant.

    Let's look at that from the muon's perspective:

    The muon sees the Earth approaching it at .9999c and - since the Earth is length-contracted by about 70 times, the Earth's atmosphere - from the muon's birth to its arrival at the ground - is only .3 miles thick!


    Earth's relativistic velocity has caused it to contract in the direction of travel by 70 times.
     
    Last edited: Jun 18, 2019
  14. DaveC426913 Valued Senior Member

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    Yes, something called the "bunch" length of a cloud of particles contracts along its length.
     
  15. PengKuan Registered Senior Member

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    136
    Can we know their positions with the detectors?
     
  16. PengKuan Registered Senior Member

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    136
    This is the principle of the proposed experiment, not a practical project. I think we can get the position of the electrons from the photons. But how we can do that is another question.

    Yes. Einstein got the idea that a disk whose circumference is not 2*Pi*R is surely curved. Thus curved space-time.
     
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  17. PengKuan Registered Senior Member

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    136
    This is interesting.
     
  18. PengKuan Registered Senior Member

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    136
    The case of Muon in atmosphere, which I used to explain why time on earth is not shortened in the Twins paradox. See: PDF: Twin paradox when Earth is the moving frame .
    https://pengkuanonphysics.blogspot.com/2019/05/twin-paradox-when-earth-is-moving-frame.html
    or Word:
    https://www.academia.edu/39216040/Twin_paradox_when_Earth_is_the_moving_frame

    The case of Muon takes the time dilation value then derives the distance of travel which is contracted. But no length at rest is measured, that is, no standing and moving rods. Scientists are paranoid. They want to see real measurement of every phenomenon. Derivation are not proof for them.

    If an experiment confirms length contraction law, everybody will be happy as for gravitational wave. If not, we will be extremely excited. So, why not do it?
     
  19. DaveC426913 Valued Senior Member

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    18,935
    I'm not sure what your response is intending. Are you agreeing?

    Scientists already know length contraction works.

    It's not derivation. A muon reaches the ground because the relativistically-moving Earth is length contracted - exactly as much as is predicted by SR.

    Because relativity has been tested as much as any theory in history, and has passed every time. It is not in doubt by any serious scientist.
     
  20. PengKuan Registered Senior Member

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    136
    Yes, I agree. Length contracts between Muons and the ground.
    What I'm saying is that the length between Muons and the ground is not measured at rest.
    Let us take a Moun and another Muon. What is the distance between them in the frame of the earth and in their own frame?
     
  21. DaveC426913 Valued Senior Member

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    18,935
    Yes it is.
    We know how high the atmosphere is. It is almost two orders of magnitude more than .3 miles.
     
  22. DaveC426913 Valued Senior Member

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    This is kind of a trivial form of length contraction - it's can't really be called length contraction at all, since the two muons are separate entities that are free to move about with respect to each other.

    What you want to do is measure the length of a solid object, whose length cannot change - except via relativity.
     
  23. exchemist Valued Senior Member

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    12,451
    Agreed. There is plenty of other evidence for Lorentz contraction too, ranging from the cosmic ray muon phenomenon to the existence of magnetism. I think, though, that what our poster wants to do is observe it directly.
     

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