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Equations or reality

  1. Oct 22, 2011 #1
    Equations or reality....

    Hello all,

    I am new to physics forum and very stupidly I don't have a background or formal education in science. I have one simple question:

    Time dilation, length contraction, curvature of space-time, are they all mathematically deducable through equations or can they be physically verified by some experiments?

    Thank you.
     
  2. jcsd
  3. Oct 22, 2011 #2
    Re: Equations or reality....

    Global positioning system (GPS) is the everyday experimental confirmation.
     
  4. Oct 22, 2011 #3

    ghwellsjr

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    Re: Equations or reality....

    Why don't you back up in this forum and click on the link called "FAQ: Experimental Basis of Special Relativity"?

    There're probably more experiments listed there than you will want to investigate. After all, it was the famous Michelson-Morley Experiment that got this whole thing started.
     
  5. Oct 22, 2011 #4
    Re: Equations or reality....

    Time dilation is experimentally verified, for instance GPS would not work if we would not account for time dilation. Length contraction is not and it is probably not even possible to experimentally verify length contraction.

    The curvature of spacetime is a bit tricky. If you consider gravitational time dilation curvature of spacetime then the answer is yes, it has been experimentally verified by measuring the effect the Sun has on light rays from far away stars or the red- and blueshift between signals on Earth at different heights. However if you only consider the strong gravitational effects (e.g. the deviation from Euclidean spacetime) curvature of spacetime then it has not, at least not directly, as our instrumentation is not accurate enough to measure that.
     
  6. Oct 22, 2011 #5

    russ_watters

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    Re: Equations or reality....

    That is clearly wrong as it is self-contradictory: you can't have one without the other.
     
  7. Oct 22, 2011 #6
    Re: Equations or reality....

    If you think it is wrong, please provide an experiment, date and place, that shows length contraction.
     
  8. Oct 22, 2011 #7

    Doc Al

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    Re: Equations or reality....

    I don't think Passionflower meant that length contraction isn't true, but that there is no direct experiment confirming it. (Of course, for consistency it must be true.)
     
  9. Oct 22, 2011 #8
    Re: Equations or reality....

    Exactly.
     
  10. Oct 23, 2011 #9
  11. Oct 23, 2011 #10

    russ_watters

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    What does "direct" mean? Is not a muon decaying after traveling a shorter than expected distance an example of it?

    Or are you saying that since time is measured by counting events, but length is calculated from time, that makes time direct and length indirect?
     
  12. Oct 23, 2011 #11

    Doc Al

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    Re: Equations or reality....

    Good question. Naively, measure the length of something moving to be contracted by the predicted amount.
    I would say no.
     
  13. Oct 23, 2011 #12

    russ_watters

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    Re: Equations or reality....

    So....because it is "distance", not "length" we're not seeing length contraction? How are they not the same thing?!

    We measure the "length" of the atmosphere in a host of different ways and the muon sees that "length" to be vastly different, no?

    This whole line of discussion seems like hairsplitting to me. In any case, there are references that disagree:
    It's titled "High Energy Astrophysics" and looks like a textbook to me.

    http://books.google.com/books?id=KG...ay direct evidence length contraction&f=false
     
  14. Oct 23, 2011 #13
    Re: Equations or reality....

    I have heard that you couldn't measure length contraction because your ruler would also become smaller, but you can't measure time dialation moveing with the frame of reference either. So length contraction could possibly be measured, but it would have to be done from another frame of reference that wasn't moveing with the object in question.
     
  15. Oct 23, 2011 #14
    Re: Equations or reality....

    Hello All,

    Than you very much for all your valuable replies. What I can understand is that some of the result of Relativity might be verified by experiments, while others are not. Say, Hawking radiation through black hole or the black hole rule of thermodynamics. Can they be seen through our physical eyes? Or they are just results of equations that lead to the entropic principle following with the mass of black hole multiplied by Boltzmann-constant? As black holes cannot be seen by eyes are the radiations just numerical figures or can they be seen during the emission process?
     
  16. Oct 24, 2011 #15

    ghwellsjr

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    Re: Equations or reality....

    Russ_watters is exactly correct here. If you are going to accept the result of experiments comparing the accumulated times on a moving clock to a stationary clock as direct verification of time dilation, then you must also accept the result of those experiments as direct verification of length contraction.

    The reason is that length contraction only applies along the direction of motion and if it were not happening, time dilation would change with the orientation of the clock and would not follow the theoretical prediction.

    Think about this: a light clock in motion is usually depicted as having a beam of light that is bouncing at right angles to the direction of motion between a pair of mirrors. In this case there is no length contraction.

    But now rotate the light clock so that the beam of light is bouncing along the direction of motion. Now if the mirrors stay the same distance apart, the light will take longer to bounce between them and our light clock will be sensitive to its orientation with respect to the direction of motion and will no longer follow the theoretical prediction and we will have proved that time dilation is in affect but length contraction is not.

    So in order for us to get a consistent result from our experimental evidence of direct verification of time dilatation, we must also accept that it is a direct verification of length contraction.
     
  17. Oct 24, 2011 #16

    A.T.

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    Re: Equations or reality....

    The question is if you could show length contraction experimentally without invoking time dilation in the interpretation of the results.

    It works the other way around: The twins will have different times on their clocks on reunion. That is a direct experimental proof that doesn't require the assumption of length contraction, and is not based on conventions of how to synchronize spatially separated clocks.
     
    Last edited: Oct 24, 2011
  18. Oct 24, 2011 #17
    Re: Equations or reality....

    It is not a matter of "must accept" things, I am not questioning that length contraction should happen conform relativity theory.

    But that is not what we are talking about, we are talking about experiments. You can have all your faith in a theory but that does not replace experimental verification.

    I can see experiments about the elapsed time of two clocks and notice it is different. Can you name the date and place of an experiment that compares the lengths and notice they are different?
     
  19. Oct 24, 2011 #18

    ghwellsjr

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    Re: Equations or reality....

    The experiments you are talking about that you say confirm time dilation, by comparing accumulated time ticks of a moving clock to a stationary clock, also confirm length contraction, unless, of course, the clocks are designed to tolerate length contraction along the direction of motion without affecting their tick rate and the moving clock was always oriented along the direction of motion to be immune to the effects of length contraction.

    Please note that I started my post by saying if you are going to accept these experiments as direct evidence of time dilation, then you also have to accept them as direct evidence of length contraction. The reason I say "if" is because these experiments are not directly observing time dilation which has to do with the tick rate of a clock, in other words, of a metronome. I could say to you that in order to be a direct confirmation of time dilation, you need to use metronomes, not a clocks, because you are demanding that I use rods, not odometers, to directly confirm length contraction.
     
    Last edited: Oct 24, 2011
  20. Oct 24, 2011 #19
    Re: Equations or reality....

    Looks like I am talking to a wall. You do know the difference between experiment and theory right? There are several experiments in the literature that compare the elapsed times on clocks that travel at a different average velocity between two shared events. At the end we simply look and record the elapsed times. Now name me one experiment where lengths are compared?

    I know relativity predicts that lengths must also contract but there is no experiment that demonstrates that. Does not mean it is false, does not mean relativity is denied, something you apparently are so worried about on this forum, no, it just means that there is no experiment that demonstrates that lengths contract.

    Yes I noted it and it is completely wrong. You make inferences, and inferences I happen to agree with, about a theory, but that is not the same as measuring it by setting up an experiment.

    So you claim clocks, for instance, atomic clocks, cannot be used to verify time dilation? If so you are completely wrong, there are many experiments in the literature that show time dilation.
     
  21. Oct 24, 2011 #20

    russ_watters

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    What of my muon reference?
     
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