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A question about the relationship between light and time.

  1. Jul 16, 2011 #1
    I don’t’ have any background in physics and SR/GR is something I became very interested in just over the last few years. Most of the concepts I seem to get, but I have not engaged with the math to any great extent.

    The one thing that keeps cropping up for me is the relationship between light and time. I don’t know what the correct way is to put this, but as I understand it, photons do not travel through time.

    I found one of the most useful ways for me to conceptualise this was a post from DaveC426913 here:


    So the question for me relates to the relationship between a mass-less particle and space time.

    Is the effect mentioned due to the speed a photon moves through space time or is it because a photon has no mass and therefore does not interact with space time? (And the speed is irrelevant in that respect?)
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Jul 16, 2011 #2


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    rede96, Photons certainly do travel through time. What's confusing you perhaps is trying to imagine what it's like to ride on a photon. "What time does the photon experience" is meaningless. Time is measured in a rest frame, and is different for every rest frame. Never mind "the rest frame of the photon" - there isn't any. Photons do not carry pocket watches.
  4. Jul 16, 2011 #3
    Hi Bill, Yep, that is confusing me because I read so many contradictory things. It would nice to clear that one up!

    Yes I understand that too. So to make it clear I am not trying to look at time or anything else from a photons point of view. That is not my question.

    However, I can see that asking questions like mine can be very confusing, so perhaps I can ask it in a different way.

    If you move relative to me, I can measure time dilation and length contraction.

    My question would be:

    Is it the fact that we have mass and are moving relative to each other through space time that cause this effect? Or is it simply the speed that we are moving relative to each other in space time that causes the effect? And mass, for the purposes of time dilation etc, is irrelevant?

    How I would see it is:

    You are in a rocket ship moving at speed x relative to me and your ship has double the rest mass of mine. I measure the time dilation.

    I see someone else in their rocket ship moving at the same speed x relative to me and I know that their ship has the same rest mass as mine. So I assume that I would measure the same time dilation as I did for your ship.

    So mass may limit our ability to ‘move’ particularly at c, but it is not directly responsible for the effects mentioned.


    It would be the speed that a photon travels at through space time that would cause any of the differences we may observe relative to us (Can’t think of a better way to put that.) Not the fact that it was mass-less.
  5. Jul 16, 2011 #4


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    rede96, Yes you've got it. The effects we talk about in special relativity, time dilation and length contraction among them, are only geometrical effects caused by the relative speed between us. People often mistakenly try to attribute them to some physical property such as the mass of the ship, or try to imagine what would cause an atom to really contract, or a clock to really slow down. But those effects are a feature of the space we live in, not the objects themselves.
  6. Jul 17, 2011 #5
    Yes, it's the relative motion that matters, as already pointed out. However, here's the thing ...

    Relative motion only has meaning from the POV of material entity. The ability to perceive the passage of time has much to do with the ability to be held at a state of rest, which of course requires rest mass (not effective mass). We assign the origin of a coordinate system as the reference for measurements, including for motion. Coordinate system origins exist at a specific point in space at a specific time, because it's the nature of rest mass to exist as such ... and it matters not if the body is imagined or real, because imagined bodies assume the rest mass exists. So while it requires only relative motion (under the context of invariant light speed) to produce relativistic effects, relative motion only has meaning (in the first place) per the POV of rest mass. They are mutually linked concepts.

  7. Jul 17, 2011 #6
    Thanks Bill. I wouldn't say I've completely got it yet :) but maybe I'm on the right track.

    I still find it difficult grasp that it is purely geometrical effects caused by the relative speed between two frames and not the result of geometrical effects moving relative to space-time, and when we move relative to another frame, it is the net result of those two movements that we measure.

    However, I know that would suggest that there is an absolute rest frame.

    Anyway, more for me to think about. :)
  8. Jul 17, 2011 #7
    Thanks for the reply GrayGhost.

    I'd always found it a strange coincidence that if rest mass does not affect time dilation (As long as you have rest mass that is) and it is purely a geometrical effect, then why is it that the speed where time dilation no longer applies is the speed c, the speed of a particle with no mass.

    Even though I don't fully understand the relationship, I guess from what you are saying is that just had to be, as they are mutually linked concepts. (Forgive the poor terminology.)
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