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

Question on Time Dilation

  1. Jan 31, 2012 #1
    I watched a documentary that introduced SR.

    Ok, I don't know the theory very mathematically. But what I know of and the documentary says is that time itself slows down to prevent the maximum speed, c, to be broken. It said that if time doesn't slows down, when we ride a rocket moving close to c,lets say 99%, and if we'd move at a some other speed on the rocket , lets say 2% of c, the total speed of us is 101% which means time has to slow down so that c may never be reach. (Enlighten me on this if it is wrong.)

    Anyways, my question is if light travels at the speed of light, how would time (in the perspective of light) behave?
    Last edited: Jan 31, 2012
  2. jcsd
  3. Jan 31, 2012 #2


    User Avatar
    Science Advisor
    Gold Member

    When we say that time slows down in a rocket traveling at 99% of c, we mean that the clocks traveling in that rocket are running at a low rate. It takes a clock to measure time. Clocks are made out of matter which means they can never travel at c. Since light travels at c, it cannot have a clock traveling along with it in order to measure time, correct? That means it cannot have a perspective on time. Does that make sense to you?
  4. Jan 31, 2012 #3
    So light doesn't have a perspective on time?
  5. Jan 31, 2012 #4
    Light doesn't have a "perspective" on anything. It has no rest frame.
  6. Jan 31, 2012 #5


    User Avatar
    Science Advisor
    Gold Member

    Yes, elfmotat, is correct.
  7. Jan 31, 2012 #6
    Another version of an answer:
    For a photon, which travels on a lightlike geodesic, the space-time interval and thus the proper time interval of two events is zero. This kind of geodesic is called null geodesic therefore.
    So, one could argue from the "perspective" of a photon that the time stands still. On the other side, one should be careful with the wording "perspective" in this very case, because as was already said one can't define a restframe for a photon.
  8. Feb 1, 2012 #7
    Ok then. Then why does two photons moving in different direction have a center of mass frame but a single photon doesn't
  9. Feb 1, 2012 #8
    Because the single photon moves with c, the center of mass for photons moving in different directions however doesn't.
    Last edited: Feb 1, 2012
  10. Feb 1, 2012 #9
    good posts above...takes time and some reflection to understand what they mean....

    Time slowing for a fast moving observer can be thought of in many different ways.....there
    are a lot of fascinating and not obvious effects. That's why it took an 'Einstein' [a smart fellow] to discover them....and he developed his insights before most of the math, somehow understanding things were different than almost all other scientists believed at the time.

    When George posts

    he's referring to the ticking of the clock [passing of time] in the rocket as seen by an outside observer; inside the rocket an observer sees her local time in the rocket tick by at the normal rate, but observes the outside observer's clock ticking slow! In this example from special relativity, different speed observers usually don't agree on the passage of time.

    I guess that's ok as a start, but misses a lot of what's really happening. Keep in mind we think we understand WHAT is happening[confirmed by many experimental observations] and that's what we model mathematically. We don't fully understand WHY it happens.

    For unknown reasons, so far, everyone observes light when measured in flat spacetime as 'c'. [It gets a bit more complicated if gravity is present.] Einstein realized this means that instead of space and time being fixed and immutatable, they are not: it is the speed of light that is constant [fixed and immutable].

    In other words, when you travel faster and faster your measures of distant space and time change, those measures locally [right where you are, in your own frame], are not affected: it's called space contraction and time dilation in relativity. Two things affect the passage of time, that is the tick rate of a clock when viewed from a distance: relative speed and gravitational potential.

    Our mathematics doesn't, strickly speaking, cover that as posted by others. That particular question, as noted above, has no 'real' meaning in relativity. It's kind of like asking "Is red a happy color?" But a rough, crude, answer is that "a photon doesn't age". This
    means that when viewed from an outside inertial frame, IF a clock could be carried along at lightspeed, it is believed it would be observed to stop ticking. We know this slowing occurs as velocity inscreases because some fast moving decay particles have been observed to last longer than expected..if they were stationary.....their 'half lives' [life times] are extended beyond what is expected as we observe them whizzing by.
  11. Feb 1, 2012 #10
    To elaborate: such a frame is a theoretical construct, however it represents imagined rulers, generally with clocks added to them. No clocks and rulers can move at c but if they could, they would measure zero volume and zero time. That's not useful. :tongue2:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Question on Time Dilation
  1. Time Dilation question (Replies: 12)