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

Light speed and space-time distortion

  1. Oct 25, 2004 #1
    what causes space-time to become distorted by traveling at light speed or close to it at a quantum level
  2. jcsd
  3. Oct 26, 2004 #2


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    I think this question mixes up intuitions about special relativity with intuitions about general relativity. The basic fact about light in special relativity is that its speed is the same for all unaccelerated observers. So if you had a space station in space, and a rocket travelling fast by it, and the rocket turned on a headlight, the people on the space station and the people on the rocket would see the SAME speed of light; the rocket's own speed wouldn't be added. Once you accept this, then the relative physics between separated observers with a relative speed between them becomes clear. And it is that relative physics, not distortion of spacetime, that makes the contraction and dilation of lengths and times.

    Now in General Relativity, Einstein's theory of gravitation, spacetime is warped, or at least the geometry varies from point to point and time to time. It is momentum and energy (including mass) that warps spacetime and causes orbits and black holes and all.
  4. Oct 26, 2004 #3
    so what if i was traveling at 99% of the speed of light and i pass a space station and shine the light would it get to them the same speed if i was going slower?
  5. Oct 26, 2004 #4


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Yes it would. Light speed is independent of the speed of its source.
  6. Oct 26, 2004 #5
    alright thanx
  7. Nov 3, 2004 #6
    Why exactly is it that light travels at the same speed for all observers? I mean, I understand that observers moving at 99% the speed of light would observe light propogating at the same speed as those who are standing still, but only because this has been drilled into me - not intuitivily or even fully. In fact, now that I think about you, why would you not see light traveling slower?

    - Alisa :bugeye:
  8. Nov 3, 2004 #7


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    It's an empirical fact. The first hint of it was the Michelson-Morley experiment. Voigt saw right away this could be explained if the speed of light was independent of its source. Then Lorentz, trying to clean up the Maxwell theory of electromagnetism developed his Transformations that mixed time with space. Finally Einstein deduced the transformaions from the fact that light speed is independent of its source. Later it was found that the postulate that every inertial observer measures the same speed of light was equivalent and made easier derivations.
  9. Nov 4, 2004 #8
    I completely udnerstand how light travels at one speed and all, since they are waves. Another example would be sound. They both travel in waves, and sound emitting from a moving source reaches the observer at the same time despite the speed of the source. Like if your traveling 5 times the speed of sound, the people you pass won't hear you coming untill after you have passed.

    but i'm still confused on how traveling the speed of light, or faster, or close to it can affect time.
  10. Nov 4, 2004 #9
    Okay, well, that one I can help with: Einstein postulated that we are all moving at the speed of light through time. However, when we start to move in any spatial dimension, we take away from our speed through time in order to supply the desired speed in the spatial dimension. This is sort of like taking a straight path between two LINE SEGMENTS (The geodesic, if you can call it that with lines... Like, picture two straight parallel lines, and you're just travelling from the dead-center of the first to the dead-center of the second.), or taking a trajectory. If we move in a diagonal (the trajectory) (from one endpoint of one of the segments to the opposite endpoint of the other, while still facing straight - like, sort of side-ways moving), it will obviously take us longer to get there, though we are basically covering the same "vertical" distance (you can call this the "h" of the triangle that forms between the path and the second line). This is because some of the time must be wasted for our movement in the second dimension - the diagonal. Before, we were only moving in one spatial dimension - straight. We had to take away from our "moving straight" time to add to our "moving diagonally time".

    Likewise, we are taking away from our "moving through time" time (as retarded as that sounds), to add to our "moving forward in a spatial dimension" time. This is unnoticable at average Earthly speeds, but becomes very evident if propagating at nearly the speed of light. Now do you see why it can influence time?

    I hope I don't have any misconceptions. If I do, please, someone correct me! Thanks = ) Hope I was able to help! Oh, and thank you, hispanic panic, for your analogy with sound waves. Is this true, selfAdjoint? I have never heard of this before... By the way you put it, we don't exactly know WHY light speed is always constant, but we just know that it is.

    - Alisa :bugeye:
  11. Nov 5, 2004 #10
    The way you put it, it makes it seem as if we as human beings, can only direct ourselves in one direction. weather it be straight, in our own, or sideways traveling to another. Like you said, we have to take away from our own movement to add to another.

    Well.... what if we didn't have to? What if, we can make the same movements, still keeping our attention forward as if nothing was happening, but at the same time, moving to this other "line segment." think of it like this.... You have to cross this river but the water is traveling at a 20 knots. The boat you have will only carry you 50 feet (for some reason). but 50 feet just happens to be the exact width of the river. So instead of taking a diagonal rout, you decide to take a straight rout across the river, but as you are crossing the river, you notice you don't go straight, you drift sidways due to the current so you actually end up somewhere else, but you've still traveled the same distance horizontally (across the riveR). And again, the force of the river moved you (vertically) to another point across the river.

    What i'm getting at, is maybe traveling near the speed of light has nothing to do with time, and that it is something else. Or maybe nothing at all...

    I guess i just don't understand :redface:
  12. Nov 5, 2004 #11


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    The velocity between one observer an another can be in any direction, and any magnitude less than c. Depending on what the velocity is you can find a Lorentz transformation to tell you what the lengths and times of one observer are for the other. In simple cases it's convenient to line up your coordinate axes (you can tilt them any way you like) so that "x" points along the velocity. This make the math simpler. But you don't have to do that if you don't want to, the Lorentz transformations will work however you tilt your axes.
  13. Nov 5, 2004 #12
    You're right! Exactly! Now read over what I said before! "Same vertical distance"

    hispanic panic, that is exactly what I was talking about! You travel the same distance in the boat (across the river), but it WILL take you a longer time! I'm not saying we as human beings can only travel in one direction, I'm saying that the more directions we choose to travel in, like travelling straight across the river is just one, while crossing it diagonally is two dimensions, the more time it will take you! You can't argue with that since any straight path is basically the geodesic. Like I said before, we are supposedly travelling through time at speed c. But since nothing can travel faster than c, not even time, we have to take AWAY from our speed of time (c) the speed that we need to "borrow" to travel in as many spatial dimensions as we want. When doing this, we have to consider the speed at which we want to travel. This is why you will be a lot younger than someone left on Earth if you travelled somewhere at speeds nearing c. Do you see why? Since you cannot go at a speed greater than c totaling from all of the FOUR directions you can possibly move in, you can't just ADD onto your speed of moving through time - that would be > c! So you "borrow" from your "moving through time" speed.

    Now, the faster you are going in any spatial dimensions, the more speed you take away from your "time speed", and thus the slower your PERSONAL time will run.

    Does that make more sense?

    - Alisa :bugeye:
    Last edited: Nov 5, 2004
  14. Nov 9, 2004 #13
    yes i think so

    But the point i was getting at in my boat analogy was that we were traveling across the river, but the thing that made the boat move from one point of the river to another, was an additional vector; the water current. So with or without that additional vector, we would have reached the other side of the river at the same time. Only difference is that with the additional vector, we would have reached a different spot on the river bank.

    But yes i think i'm starting to understand.
  15. Nov 9, 2004 #14
    I'm not sure being analytical will solve this case!
  16. Nov 9, 2004 #15

    I think that space/time distortion and increase of effective inertial masss due to velocity are all the result, at least at the elementary particle level, of distorted field symmetries in the neighborhood of the particle field convergence point. (the center of the particle)

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Light speed and space-time distortion
  1. Light Speed and Space (Replies: 13)