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Light Beam Problem

  1. May 13, 2005 #1
    Hello, I'm new, and I have a problem I can't seem to imagine to solve. Any help?

    You have 5 light beams, moving from the same line, 1m apart from eachother, all moving to the same target exactly 10 light seconds away. If each light beam sees the others move at light speed, will each light beam see the others hit first? Do they all hit first?

    Simplified: What will each light beam see? What will an outside observer see?

    Kinda confusing...
  2. jcsd
  3. May 13, 2005 #2
    It's tough to say what a photon would see. Presumably time is irrelevant to a photon; it's at all of its destinations simultaneously. It seems that you are taking the postulate that all observers measures light's velocity at the speed of light, and applying that to light itself. But presumably that does not apply. The "observers" are assumed to be material (not photons).
  4. May 13, 2005 #3


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    Light beams do not see. I mean, even pretend-like. Objects (photons) moving at the speed of light do not experience time at all, thus one thing arriving before another is meaningless.

    However, if an observer were flying at sublight speeds next to any one of the beams, she will see the beam closest to her hit first, followed by beams father away hitting later.

    Remember, seeing any event at a distance is seeing it in the past. An event happening 1m away is 1/300,000,000ths of a second old. An event 2m away is 2/300,000,000ths of a second old.

    One of the major assertions of Einstein's theory is that we can no longer have an objective definition of simultaniety (i.e.: it's all relative).
  5. May 13, 2005 #4
    The photon's null geodesics in our environment essentially say that there can exist no time dimension for photons, i.e., their physical environment is ours minus 1 dimension. This results in parallel moving photons standing still with respect to each other if observed from this 1-less-dimensional environment. Photons likewise can have relative velocities ranging from zero (parallel moving) to c (orthogonal paths) between each other. The dimension in the direction of motion may actually play the role of "time" dimension from the perspective of the "photonic observer".
  6. May 14, 2005 #5
    You can not do such "see" when you imagine that you are riding on a light. I think it is because if you measure velocity of anything, you should use a period of time. But from the view point of a person outside the light, say, standing on the earth, he thinks the period of time takes infinity length of time.
  7. May 14, 2005 #6


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    Light does not have its own inertial reference frame, since the principle that the laws of physics should work the same way in all inertial frames would be violated by a coordinate system where a light wave was at rest. You can consider a sort of limiting case as v approaches c, but certain quantities don't converge to a well-defined limit as v approaches c, and the velocity of other light beams is one of them. See this thread for more discussion of this.
  8. Jun 2, 2005 #7
    Light beams can't see. The observer is the one who needs light to see. This proves that all physical motion is absolute, not relative. Time is an illusion deduced from the motion of http://www.groupsrv.com/science/about96836-0-asc-0.html [Broken].
    Last edited by a moderator: May 2, 2017
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