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Does light experience time and space simultaneously?

  1. Jul 16, 2012 #1
    Does light experience the time of the entire universe simultaneously or just from the beginning to end of its own existence simultaneously?

    I assume that light experiences the past, present and future simultaneously of its beginning to the end. If this is true than does light experiences all the space it moves through as existing simultaneously? If this is true is there any way that you could view light in the middle of its existence and predict the future of its path and find out all its information, including the past?

    What if the light gets divided, what happens to the information?

    Could you create a conscious creature that only runs on light? What is space-time?

    What is space made up of?
  2. jcsd
  3. Jul 16, 2012 #2


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  4. Jul 16, 2012 #3


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    You can't make anything out of just light, not even a clock, and without a clock, you cannot measure time, and since time is what a clock measures, according to Einstein, light cannot experience time.

    So now that you know that, are you still interested in answers to the only two questions you have left:
  5. Jul 16, 2012 #4
    Sure answers these questions.

    What is space-time?

    What is space made up of?
  6. Jul 16, 2012 #5


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    Best description I've heard recently right here on this forum (paraphrased):

    Space is not a thing in and of itself; space is the geometry of things (stars, atoms). When we say space expands or space warps, what we're describing is simply the distances of things changing.
  7. Jul 16, 2012 #6


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    Well, since you asked on the Relativity Forum, I will point you to Einstein's answers in his 1905 paper introducing Special Relativity. Please read article 1, it's just a little over a page long, and notice how he defines so many things. How many can you count? Does it help you understand what space-time is and how space is measured? That's what counts--if we can't measure it, it's not physics.
  8. Jul 16, 2012 #7
  9. Jul 16, 2012 #8
    Your questions are a little off-the-wall for us disciplined physicists here. We would typicallly dismiss you out of hand. But, on the other hand, sometimes it's a little invigorating to have someone shake things up a little. The best I could do to help you consider some special relativity in the general context of your question is to show you a sequence of space-tlime diagrams. You really should google the space-time diagram topic and try to learn how to visualize the universe in the context of 4-dimensional geometry. It doesn't matter if you would rather not take such a description of the universe as actual physical reality (the earlier posts here have cautioned you against that).

    Once you have a grasp of this 4-dimensional world you see how to represent 4-dimensional objects with their worldlines that extend along the 4th dimension (typically for distances of the order of 10^13 miles). The sketches below depict rest frames in which you see slanted time axis lines (they are shown here as the X4 dimension)--their slopes correspond to the speeds of the observers moving along their world lines (every observer, regardless of his inertial frame of reference, moves along his worldline at the speed of light. A worldline oriented at a 45-degree angle with respect to the rest frame corresponds to a photon of light.

    Note in the sketches that as the time axis slants more and more in the sequence of sketches, the moving observer is going faster and faster. A very curious thing about nature (as revealed with special relativity) is that as the observer's worldline rotates clockwise more and more (for example) his spatial X1 axis rotates in the opposite direction so that the photon worldline always bisects the angle between the time axis and the spatial axis (of course this results in a constant speed of light for all observers, and it highlights the even more curious characteristic of nature that observers experience different cross-section views of 4-dimensional space-time; this explains things like time dilation and length contraction as well as some other intriguing phenomena).

    Now, the X1 axis represents the space that an observer lives in at any instant along the time axis (the X2 and X3 axes are suppressed for simplicity).

    Finally, to your question about a photon experiencing all past and future at once (not a ligitimate idea for us physcists), the closest I can come to responding to something you seem to be driving at is to recognize that in the limit, as our traveling observer approaches the speed of light, his time axis and spatial axis would converge. This seems to set up the context for your question. So, to your question as we look at the sequence of the ever faster observer, what would be the meaning of the time axis and the spatial axis converging? You probably know that no material object can accelerate to the speed of light, so that's why you asked about what a photon would experience. If you ever catch one you should ask him. In the meantime, ask yourself if the photon's time axis and spatial axis are colinear.
    Last edited: Jul 16, 2012
  10. Jul 21, 2012 #9
    Superb spacetime diagram introduction, bob. But I would say: X1 represents 1 dimension of 3D space the observer lives in. We take the X1 dimension in the direction of relative movement. Because here the peculiar aspects of SR show up. Sorry to be so critical ;)
  11. Jul 21, 2012 #10
    That's exactly what I had in mind, but my description was sloppy. Yours is the correct way of stating it. Thanks.
  12. Jul 21, 2012 #11


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    The limiting process you illustrate here doesn't really show that the photon's "time axis" and "space axis" are collinear. What it really shows is that a photon can't have a "time axis" and a "space axis" in the ordinary sense, because the Lorentz transformation becomes singular when the relative velocity v goes to c. "Singular" means "not valid".

    It is possible to set up a coordinate chart in which one coordinate axis is a photon's worldline; but it won't be like the ordinary coordinate chart you're used to. See, for example, here:

  13. Jul 21, 2012 #12


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    The Lorentz Interval between any two points of a photon's worldline is zero, it's a null interval. The geometry of the photon's worldline is well described by an affine geometry.


    You can mark out equal intervals along a photon's worldline through an affine parameterization (for a physical example, think of marking a point on the worldline every time the E-field of a classic EM wave goes to zero). But these equal intervals are not like time, nor are they like space - they are null intervals.
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