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

From Lights Point of Reference Is It Traveling Infinetly Fast?

  1. Dec 16, 2005 #1
    From the lights point of reference is it traveling infinitly fast? is it's travel instantaneous?
  2. jcsd
  3. Dec 16, 2005 #2
    You can't transform into the reference frame of light. No matter how close you "think" you're getting light will always be c. However, you will see everything else you used to see as stationary moving closer to the speed of light as well. The energy of all that will go to infinity, but the energy of the photon you are trying to catch will go to zero.
  4. Dec 16, 2005 #3


    User Avatar
    Science Advisor
    Homework Helper

    From the light's point of view there is no time and there is no space - hence speed is meaningless.
  5. Dec 17, 2005 #4
    I know, i know, any object with mass can not travel at the speed of light, it was meant to be a thought experiment, but what do you mean "The energy of all that will go to infinity" i just can't seem to get my head around it.
  6. Dec 17, 2005 #5
    Say you have a spaceship that is your laboratory, that has a laser pointed in some direction. Your question is basically, what does the spaceship look like from the point of view of the photons in the laser? Well, you can say they are simply going at the speed of light, so I'll plug that into the lorez transformations. This gives infinite energy, infinite time dilation, infinite length contraction, etc for the the spaceship.
    But there is still something funky if you do this.
    The lorenz transform for velocity is:

    {v^`}_x = \frac{v_x - u}{1 - {v_x u}/c^2}

    Setting [tex]v_x = c[/tex], the speed of the photons, we can take the limit of [tex]u\rightarrowc[/tex] for our fictional observer.

    \lim_{\substack{u\rightarrow c}} \frac{c - u}{1 - u/c} = c
    (note: since pluging c directly in gives 0/0, you have to use L'Hospital's rule, taking the derivitive of the top and bottom, giving you c.)

    So you can never catch the photons. This is interesting right, since our observer is going exactly at c relative to the spaceship.
    Last edited: Dec 17, 2005
  7. Dec 17, 2005 #6
    the reason you cant put a reference frame on light is because if you did, everything would be going at the speed of light relative to this reference frame. This is impossible. However, as a reference frame (S') approaches the speed of light relative to reference frame S, everything at rest relative to S appears to slow down to t' = infinity * t, and contract in length to l' = 0 * l. However, were S' to slow down to the be at rest with S, it would find that as that maximum speed approaches the speed of light, the time that has passed in S has approached infinity. So in a way, a photons reference frame is instantaneous.
    Last edited: Dec 17, 2005
  8. Dec 17, 2005 #7
    and why is this in the quantum mechanics section? this is a relativity question.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?