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Michelson-Morley vs Tennis Balls

  1. Oct 2, 2013 #1
    Hello! I'm hoping someone or group of someones can help me with something I'm stuck on, having to do with the Michelson Morley Experiment (MME for short if that's okay). I want to say that I'm learning Relativity and I'm getting it, as long as I accept the Given that the speed of light will always be measured to be c, regardless of what reference frame the observer measuring it is in. I get all that and it's totally cool.

    What I'm stuck on is how exactly the MME proves that. I get the idea and understand it (not 100% obviously, as I wouldn't be asking this question I suppose if I did), but to explain what I'm stuck on, I have to talk about the Tennis Balls now.

    Say I'm in a space ship cargo hold. I'm hanging out between galaxies in empty space and the thrusters of my ship are off and everything is quiet and off to save energy (I've gone green!). The cargo hold has had all the air pumped out. It's a vacuum. It's exactly 10m by 10m, or 100 m2. There are no gravitational effects acting on it and the ship isn't accelerating in any way.

    In the middle of this cargo hold is a device bolted to the floor that shoots tennis balls. One "gun" shoots tennis balls in direction x and the other "gun" shoots them in direction y, perpendicular to the gun shooting in the x direction.

    I activate the machine and it shoots tennis ball X (tbX for short) in the x direction and tennis ball Y (tbY for short) in the y direction. It shoots them both at the exact same time and at exactly 10m/sec. After 2 seconds, both balls return to their firing point. That's 1 second to hit the cargo hold wall, and 1 second to come back after bouncing off the wall.

    So, there it is... I fired two tennis balls in different directions and they came back at the exact same time. How is this not the same as the MME and light? Why could I not conclude, whilst hanging out in my spaceship eating a sandwich, playing with tennis balls, that the speed of a tennis ball is the same for all observers regardless of their reference frame?

    This is what my brain says the MME is saying, but I'm sure it's saying something else. What is different about the MME that I'm missing? Any thoughts and/or help is appreciated!

    - Noyhcat

    P.S.: I do get that the MME was attempting to prove the existence of the aether, but when reading about Relativity, it always references this experiment and how because the returning light waves did NOT interfere destructively, it means there was no variation in speed, and therefore, the speed of light is the same in all reference frames.
    Last edited: Oct 2, 2013
  2. jcsd
  3. Oct 2, 2013 #2
    You're right. The MME does not prove that c is constant for all observers.
  4. Oct 2, 2013 #3


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    Even if MME could be explained ballistically like you want to do, there are other experiments that show that the light emitted from two relatively moving sources at the time they are colocated arrives at a distant destination at the same time. That cannot be explained ballistically or by analogy to tennis balls.

    Have a look at this thread and see if it helps you understand MME:

    "www.physicsforums.com/showthread.php?t=626807" [Broken]
    Last edited by a moderator: May 6, 2017
  5. Oct 2, 2013 #4
    Wow, that was very helpful! I think you've nudged my brain free.

    Right, this helped a click go off because the MME only proved that there was no aether, and it caused folks to alter their thinking which helped lead to SR, but did not prove it directly itself!

    That was awesome, and think cleared what I was missing, with both MME and the tennis balls. I hear you in that using a ballistic analogy is perhaps not prudent, but it still helps me:

    If in my cargo hold, I was moving in the x direction at 5 m/s, and then measured the speed of tennis ball tbX moving at 10 m/s from the gun's point of view, I would say the ball was traveling at 5 m/s, when someone standing still at the gun would say it was traveling at 10 m/s. We disagree on the speed.

    With MME, if I was traveling in the direction towards one of the reflecting mirrors, I would say that light is traveling at c, as would someone who was standing still at the light emitter. We would agree on the speed!

    Ergo, the tennis balls lose because their speed is not constant across reference frames whereas light is, taking into account length contraction, time dilation and relativity of simultaneity, and proven by other non-MME related experiments, which I will now find and read up on.

    Thanks a lot! This has been bothering me for a couple days, and you've helped get me going again!

  6. Oct 3, 2013 #5


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    MME did not prove that there was no aether--just that there was no detectable aether wind. Scientists continued to believe in an aether, culminating in Lorentz Ether Theory (LET), but this aether was different than they expected as it caused Length Contraction and Time Dilation for objects moving through it.

    Note also that MME compared the round trip speed of light along two perpendicular paths. All measurements of the value of the speed of light, c, must of necessity use a round trip for the light so that a single timing device can be used. So MME supported the idea that c was a constant value but it did not say anything about the one-way propagation time of light and it was almost two decades before Einstein realized that his second postulate would be consistent with his first postulate.

    That's because you and the other person have something, light, with which to observe the tennis ball while it is traveling away from the both of you because the light is traveling millions of times faster than the tennis ball. And I would say that you both agree on the relative speeds of the gun, the tennis ball, the walls of the cargo hold, and each other. Of course you both would never say that the tennis ball is traveling at the same speed with respect to each of you.

    Well, since no one has anything faster than light with which to observe the propagation of the light as it is traveling on its way to the reflecting mirror, no one can say that you are wrong if you claim that it is traveling at c with respect to you, but then neither can anyone say that the observer standing still is wrong when he makes the claim that the light is traveling at c with respect to him. But we can say that both claims cannot be true at the same time so in this regard I would say that the two of you would disagree about the speed. In other words, you can make the claim that the light is traveling at c with respect to yourself or with respect to the observer standing still (or any other inertial observer) but you cannot say that light is traveling at multiple different speeds all at the same time. You have to pick one Inertial Reference Frame (IRF) according to which light propagates at c and analyze everything according to it and then you can transform to another IRF moving with respect to the first IRF and in which light travels at c and see how everything appears in that frame.

    Just remember, the tennis balls lose because the idea that each observer can claim that their relative speed is the same leads to inconsistencies whereas that same claim with regard to light does not lead to inconsistencies. But also realize that any single IRF is just like what an ether frame in LET could be like. Both approaches are consistent with all other observations but SR wins, not because it is proven by any experiments over LET, but because, as Einstein said, it leads to "a simple and consistent theory".

    You're welcome.
    Last edited by a moderator: May 6, 2017
  7. Oct 3, 2013 #6
    Right, I do recall reading that. Sloppy writing on my part!

    I just like reading that. :)
  8. Oct 3, 2013 #7


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  9. Oct 3, 2013 #8
    Whoa... there I went, and saw I did. Binary stars!

    They rotate around each other at constant speed as seen from very far away. If c was not the same in all reference frames, we would see changes in velocity as the stars approached and receded away from us.

    de Sitter's double star argument:

    Thanks!! And as proof that I mean it, I shall not compare this to tennis balls in any way.
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