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Why does time depend on light?

  1. Oct 8, 2011 #1
    I'm not sure if this question is valid or not. I'm in Modern Physics now, and we just did relativity. I understand lorentz transformations, how maxwells equations can give the speed of light, and so on. What I think I'm asking is a philosophical question (maybe it can be phrased as something like "what is time without light?" - or "why does time dilation DEPEND on light".)
    I understand how/what happens, and the math (at least at the undergraduate level) behind it, but its still bugging me.

    Note: this question arose when we started thinking about time dilation.
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  3. Oct 8, 2011 #2


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    Our current standard of time comes from the cesium atom. This is related strongly to electromagnetism, of course, since it's the electromagnetic forces that cause the energy levels, and the current standard definition is:


    Philosphically, though time is more general than just electromagnetism - it just happens to be convenient to use it for our standards definitio.

    But we don't see any discrepancies between the sort of time kept by muon decay, for instance, which isn't controlled by electromagnetism, and our cesium-atom based idea of time. So it would be a mistake to think that time is only applicable to electromagnetism or to light.

    Note that the speed of other force-carrying bosons is expected to be 'c'. Photons/light just happen to be ubiquitous and easy to work with.
  4. Oct 8, 2011 #3


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    Time dilation does not depend on light. There have been experiments on muons showing time dilation. Muons decay via the weak interaction, not EM.
  5. Oct 8, 2011 #4


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    as to "why does time DILATION depend on light", I don't think that it DOES depend on light, but it might seem that it does because the constant speed of light provides the simplest way to EXPLAIN time dilation. In fact I'm not sure I've ever read an explanation of time dilation that did NOT use light to explain it, but it would be a mistake to extrapolate the excellence of light as an explanatory mechanism over to the belief that it CAUSES the thing it helps explain.
  6. Oct 8, 2011 #5

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    Time dilation doesn't depend on light; it depends on the speed of light.


    As far as the initial development of special relativity is concerned, the answer is "Because". That the speed of light is the same to all observers is axiomatic in Einstein's 1905 paper. Einstein chose this axiom because that is what theory and experiment had been telling physicists for 40 years, if only they had truly listened. Because this conflicted with Newtonian physics, everyone else was trying to find the aether or to come up with theories that attributed time dilation as an illusion (and kept Newtonian physics intact). Einstein's insight was to accept that apparent oddity as fact instead of illusion.

    If you want a deeper answer, you need to go beyond special relativity, and to do that you need math that is well beyond the high school / freshman mathematics. If you don't know the math, the answer will still be a "because I told you" kind of answer, and it will still have postulates of some sort.
  7. Oct 8, 2011 #6
    I understand this much, but I guess what bugs me is that the equations have the speed of light in them (btw I did mean to ask why it depends on the speed of, not light itself. I apologize for being unlclear). As for the math, I am a math major as well, have taken real analysis, metric spaces and topology (among other upper level courses), and I do understand their connection to and use in physics (again maybe in a naive undergraduate way).
    Typically, to me, when it is part of the equation, it must have some meaning (on some deeper level).
    If it is indeed supposed to be taken axiomatically, then I guess my question is moot...
  8. Oct 8, 2011 #7

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    What's axiomatic in special relativity is that the speed of light is the same to all observers, and that the laws of physics are the same in all inertial frames. The Lorentz contraction derives from these assumptions. In other competing formulations, it was the Lorentz contraction itself that was axiomatic. That along with the assumption of an undetectable absolute inertial frame made these competing formulations less attractive than Einstein's formulation.
  9. Oct 9, 2011 #8


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    Looking at the mathematics of this, relativity is modelled using a 4 dimensional space where 3 of the dimensions are measured in metres and 1 of the dimensions is measured in seconds. So you can think of c as being the conversion factor between seconds and metres in order to give all 4 dimensions the same units.

    The reason we use the speed of light rather than some other speed is down the the experimental observation that the speed of light is invariant.

    I don't know how much relativity you have studied, but you can also take one-dimensional energy and three-dimensional momentum to form a four-dimensional vector whose length is mass (="rest mass"). The equation that links these three[tex]
    mc^2 = \sqrt{E^2 - |\textbf{p}|^2 c^2}
    [/tex]has lots of cs in it to convert between energy-units, momentum-units and mass-units.

    You may find https://www.physicsforums.com/showthread.php?t=534862 [Broken], by forum member bcrowell, helpful.
    Last edited by a moderator: May 5, 2017
  10. Oct 14, 2011 #9
    Well, while I realize your point here, the fundamental point at hand here is that a cosmological speed limit is assumed to exist. Einstein did assume that light's speed defines that limit. Gravity waves too. Add the weak interaction. Time dilation exists because a cosmological speed limit exists. In regards to the OP's question, I'd figure that the relative rate of time depends upon the cosmological speed limit, which is the speed at which light travels. Yes? The relativistic effects must arise if both postulates of the special theory are true, but the 2nd postulate is more truely about the cosmological speed limit.

    Last edited: Oct 14, 2011
  11. Oct 14, 2011 #10


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    That isn't how I understood the OP's question at first. I thought he was asking about light, not c. The OP clarified later.
  12. Oct 14, 2011 #11


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    The name "speed of light" is a historical accident. Physicists first encountered relativistic effects in studying the propagation of light, so this fundamental constant c was called the speed of light because that's what it was equal to. Later on, physicists came to learn that this constant stems from the more general fact that our universe has a universal speed limit. Light happens to travel at this speed because photons are massless, but so will any other massless particle, like gluons and (hypothetical) gravitons. There's nothing special about light in that regard, but referring to c as the speed of light stuck. It's certainly more pleasing to the ear to say "speed of light" than to say "universal speed limit" (at least to me).
    Last edited: Oct 14, 2011
  13. Oct 15, 2011 #12
    Time does not depend on light. Space and time depends on substance whose parameters can be light speed c in vacuum(theory of relativity), c/n in media (Wiki_Abraham–Minkowski controversy), Fermi velocity of condensed matter physics(graphene, "Einstein's Relativity Theory Proven With The 'Lead' Of A Pencil"), sonic speed and that of a neutrino("Graphene,neutrino and oscillation"),etc.
  14. Oct 15, 2011 #13
    The law of nature is such that there is a special speed which is the same in all inertial frames. An effect of having an universal speed is time-dilation, but it is not apparent until you examine phenomena that moves near this speed.

    Electromagnetic wave is the first phenomena that we studied that traveled at this speed. There is additional reason such that light can ONLY travel at this speed. So studying light necessarily leads to conclusion such as time dilation. But light is NOT the reason for time dilation.

    Suppose we observed muon decay before we understood light and relativity, we would have been puzzled to why a traveling muon has a longer half-life. And we can still have backtracked relativity's rule without referring to electromagnetism.
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