Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light doesn't travel through time?

  1. Jan 20, 2013 #1
    According to the special theory of relativity: The combined speed of any object’s motion through space and its motion through time is always precisely equal to the speed of light.

    But light travels through space precisely at the speed of light. Doesn't that imply that light doesn't travel through time? What does that even mean?
     
  2. jcsd
  3. Jan 20, 2013 #2
    I'm no expert, but I'll take a stab here.

    If you were able to completely stand still in space and did not move at all, you'd not be moving through space - only through time.

    Now, imagine you're in a spaceship with a speedometer, and you slowly accelerate. As you speed up, you move less through space and more through time. You can easily imagine time dilation taking effect when you're at speeds much greater than normal - you'd be experiencing more time for every unit time an outside observer is looking at you. One year for you could be a second for everybody else.

    The closer your speedometer reading gets to c, the slower the time goes for you. When you eventually do reach c, time will be so slow, it has stopped moving completely. An infinite amount of time can pass by for you, but not a millisecond has passed for the rest of the universe.

    At that point, you'd be moving through space completely, and not through time. Exactly what a photon does.
     
  4. Jan 20, 2013 #3

    Dale

    Staff: Mentor

    We have had a rash of threads on this exact topic recently (must be infectious). Please spend a few minutes looking. I am getting tired of this same discussion.
     
  5. Jan 20, 2013 #4

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    This is one possible geometrical interpretation of special relativity, where proper time is a dimension, and aging corresponds to movement along that dimension. Since light doesn't age, it is indeed only traveling through space, in this particular model.

    See:
    http://www.adamtoons.de/physics/relativity.swf
    When you pull the speed slider towards 1c, the rocket becomes light, and the proper time goes towards zero.
     
  6. Jan 20, 2013 #5

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I posted a detailed reply in one of the other threads. Link.
     
  7. Jan 20, 2013 #6

    jtbell

    User Avatar

    Staff: Mentor

    More likely, some TV network has been re-running Brian Greene's programs again. :rolleyes:
     
  8. Jan 20, 2013 #7

    Dale

    Staff: Mentor

    That didn't occur to me, but it seems likely now that you mention it.
     
  9. Jan 23, 2013 #8
    Light is special, unlike other matters.If it doesn't travel in time how do you find it moving. Light is only object which has constant velocity in all frames.Special Relativity doesn't apply to light. Can a single photon see another photon moving at velocity of c? Light is the basis of Special Relativity but it can't predict light.
     
    Last edited: Jan 23, 2013
  10. Jan 23, 2013 #9

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There's no inertial coordinate system that's comoving with light, but that doesn't mean that SR doesn't apply to light. It certainly does.
     
  11. Jan 23, 2013 #10

    Dale

    Staff: Mentor

    It is true that light is not predicted by SR, it is predicted by Maxwell's equations. However, strictly speaking, light is only the basis of SR in a historical sense. Logically, all that is necessary for SR is for there to be a finite speed which is invariant.
     
  12. Jan 23, 2013 #11
    note that the time travel or spatial travel is found only by other inertial coordinate system. We find it to be moving at c. Then, how can you say that light obeys SR's second law.
    SR doesn't define coordinate system moving at velocity c. These corrections were made so that light travels at the same speed in all coordinate systems.
     
  13. Jan 23, 2013 #12

    Dale

    Staff: Mentor

    The second postulate says it moves at c in all inertial frames. We find it to be moving at c in all inertial frames. Therefore it obeys the second postulate.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Light doesn't travel through time?
  1. Time traveling Light? (Replies: 22)

Loading...