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Does relativity imply alternate realities?

  1. Nov 18, 2003 #1
    To anyone smarter than myself(which should be a lot of people here):

    It's clear that relativity neccessitates that time, distance, or both vary according to one's perspective, or relative position. I ask what would happen in the following situation:

    Relative to a third body, a body is moving through space at one half the speed of light(of course, relative to it is is not moving at all). Also relative to this third body, another body is moving at half the speed of light in the opposite direction, in fact, on a collision with the other body. Relative to each of the bodies, the other has obtained the speed of light and thus carries an infinite mass. When they collide, this will cause a different collision from the perspective of the third body than it wil from the perspective of either of the bodies. In my mind, this suggests alternate realities. Is this logical, or am I missing something here(I am 100% unversed in relativity, so this is the likely possibility)?

    Also, if we were to add the fact the the third body, along the same direction of motion, is moving at half the speed of light relative to a fourth body, has one of the first bodies broken the speed of light(albeit only relative to the fourth body)?

    I suspect that these questions arise out of my lack of knowledge, and I would appreciate being shown what concepts I am misunderstanding.

  2. jcsd
  3. Nov 18, 2003 #2
    No, one body approaches the other at four-fifths of the speed of light, by the relativistic velocity addition law:


    (Use the law v = (w - u)/(1 - wu/c2 with w = -u = 0.5c.)

    I don't even know what an "alternate reality" is, let alone why this has anything to do with one.
  4. Nov 18, 2003 #3
    thanks! that shows wherein lay mine error. On a related question, I have always wondered where einstein got all these equations from. Did he merely induce them from data sets, or did he have a solid mathmematical basis for them? I realise that some of them can be extrapolated/derived from others, but from whence came his first one?

  5. Nov 19, 2003 #4
    Einstein was a big fan of Maxwell's theory of electromagnetism. Maxwell's theory predicted that the speed of light had a certain constant value. But that theory was inconsistent with Newton's mechanics, which did not allow for the speed of something to be the same according to all inertial observers. Thus, most people thought that Maxwell's equations only worked in one frame (the "aether rest frame"), and that in other frames, the speed of light would be different from what Maxwell's equations predicted. Einstein preferred to toss out Newton's mechanics rather than Maxwell's electromagnetism, so he decided to look for a new mechanics in which Maxwell's equations (and thus the speed of light) would be the same in all inertial frames.
  6. Nov 19, 2003 #5


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    Alternate realities are generally the realm of quantum mechanics, and not general relativity. However, you are on the right track in your thinking that different observations define the different realities. If you look for material on the "Everett interpretation" or "Many-Worlds View", you'll see where the whole idea of parallel universes got started. Or, if you want to trace it back to its very earliest origins, you could look up the "Schrodinger's Cat" thought-experiment.
  7. Nov 19, 2003 #6
    Re: Re: does relativity imply alternate realities?

    thanks! I am familiar with the schrodinger's cat thought-experiment and so forth, but it was my understanding that these considerations hypothesized rather than required 'alternate realities'. By contrast, my hypothetical situation would have neccessitated an alternate reality, though of course it was fatally flawed as I was not adding the velocities relativistically.

  8. Nov 20, 2003 #7
    My curiousity would be as to if we have ever located or produced a situation in which bodies are moving fast enough(that is, relative to each other) for some of these implications to be observed. To my knowledge, at typical speeds the effects of many of einstein's equations are minute beyond the capability of our measuring systems to detect, or small enough at least to question any experimental conclusion. I read somewhere that the tilt of mercury's axis was a real-world verification of relativistic equations, but I have never had the nuances of that situation explained to me.

  9. Nov 20, 2003 #8
    You don't have to have really fast-moving bodies, if you have sensitive enough instruments. (The twin paradox was tested in the Hafele-Keating experiment using an ordinary jet airplane, and an atomic clock.)

    There are plenty of tests of both special and general relativity. See:


    Not tilt, but precession. In general relativity, gravity effectively does not obey an inverse-square law; there is a small inverse-cube correction that makes orbits precess. This has been observed (and not just for Mercury).
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