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

Space-time at the speed of light

  1. Jun 25, 2011 #1
    How does time and space appear to a photon in vacuum (i.e. photon at at the speed of light)?

    Does time stop, when looked, from the frame of reference of a photon? What about space?

    Time-space are in a senses interchangeable/inseparable,

    so how does space-time appear to a photon?

    Does a photon even experience (or is effected by) space-time?
     
  2. jcsd
  3. Jun 25, 2011 #2
    It doesn't "appear" to the photon at all, there is nothing like the frame of reference for a photon. For something moving at c, the concept of the reference frame breaks down.
     
  4. Jun 25, 2011 #3

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    FAQ: What does the world look like in a frame of reference moving at the speed of light?

    This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

    The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

    Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)

    What if a system of interacting, massless particles was conscious, and could make observations? The argument given in the preceding paragraph proves that this isn't possible, but let's be more explicit. There are two possibilities. The velocity V of the system's center of mass either moves at c, or it doesn't. If V=c, then all the particles are moving along parallel lines, and therefore they aren't interacting, can't perform computations, and can't be conscious. (This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.) If V is less than c, then the observer's frame of reference isn't moving at c. Either way, we don't get an observer moving at c.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook