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

Light Question

  1. Oct 18, 2007 #1
    Right please bear with me on this, as I have only had 8th grade science for this =].

    Right so I understand that no matter how fast you are moving light always travels at the same speed relative to you. So for example if you (A) were in a race with someone (B) and B was running at the speed of light relative to an observer (C) he would also be running at the speed of light relative to A. So B would look like light to A and C (as the converse would also be true I believe) and A would look like a blur to C (he is a fast runner =])

    A->B.... light
    A->C.... Blur
    B->A.... light
    B->C.... light
    C->A.... Blur
    C->B.... light (I'm not sure if relative rest to light would look like light to light, so correct me on this please)

    So when something looks like light relative to to another thing it seems to shoot ahead of the non-light thing at the speed of light yea?

    So my question is that what would something at the speed of light look like relative to another thing traveling at the speed of light. So if both A and B were running at the speed of light they would both look like light relative to each other? So would this mean the A would look to B as if it were shooting ahead at the speed of light and B would look to A as if it were shooting ahead at the speed of light?

    Thanks, and if my writing is to convolutely written please let me know and I'll fix it =]
  2. jcsd
  3. Oct 18, 2007 #2

    Shooting Star

    User Avatar
    Homework Helper

    If A and C are "non-light" and B is light, then B will have the same speed with respect to A and C. B can be other things, like neutrinos, which also move at the speed of light.

    Ordinary matter cannot travel at the speed of light, so the answer to your second question actually does not exist. There are many contrived scenarios, but no definitive answer can be given, because light is not at rest in any inertial frame.

    (But you can think of it this way: accoring to us non-light observers, time is not passing at all in the "light frame", and the whole universe has become a 2-d plane for the light with no thickness in the direction in which it is travelling. So, in its frame, light takes no time to traverse any distance. As I said, this is one of the contrived scenarios, and you can ignore it. Persisting with paridigms like these ultimately leads to paradoxes.)
  4. Oct 18, 2007 #3
    I understand that it is impossible for a person to move at the speed of light, but I was using the person idea as more of an analogy, and I understand the time dilation bit. I suppose what I am asking is that does light still have the same relative speed to light as it does to a non-light object? And if it does, does that mean from both points of view from the different lights, the other light seems to shoot ahead.

    A. point of view(if both are light)


    B. point of view (if both are light).


    Thanks =]
    Last edited: Oct 18, 2007
  5. Oct 19, 2007 #4

    Shooting Star

    User Avatar
    Homework Helper

    Your question cannot be answered within the context of relativity. It actually loses meaning.

    That doesn't mean that it was bad question.
  6. Oct 19, 2007 #5


    User Avatar
    Science Advisor

    When we talk about the speed of A relative to B, what we really mean is A's speed in the inertial rest frame of B. And light simply doesn't have its own inertial rest frame in relativity, the Lorentz transformation gives infinities if you try to plug in v=c.
  7. Oct 19, 2007 #6
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Light Question
  1. Light question (Replies: 1)