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Why does light not reach any distance instantly?

  1. Jan 17, 2014 #1
    After learning about special relativity, and the behavior of light, I do not understand why light is not able to reach any distance instantly. Doesn't time stop at the speed of light? So if no time is passing, yet light is moving, wouldn't that mean that light is traveling infinitely fast? Obviously light doesn't travel that fast, but I can't see what stops it from going that fast if it is able to move without the passing of time.
     
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  3. Jan 17, 2014 #2

    PeterDonis

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    No. A better way of describing what SR says about lightlike objects (things that move at the speed of light) is that the concept of "time" (more precisely, "proper time") does not apply to them. That is, the question "how much time passes for a light ray?" is not well-defined.
     
  4. Jan 17, 2014 #3
    Oh I see... The more you learn about physics, the more you realize you don't understand the half of it!
     
  5. Jan 17, 2014 #4

    ghwellsjr

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    How would you know if it did or it didn't? Have you ever thought about how you would determine how long it took for light to get somewhere? You can't watch it like you can watch a baseball being thrown, or even with the aid of high-speed photography, you can't capture the motion of light, can you? For other things, we illuminate them with light so we can tell where they are, but what are you going to do with light? Have you considered that as light travels from the sun to the moon, you can't see it? Even the light that is reflected off the moon coming back to the earth you can't see while it is in transit. All you can see is when it finally gets to your eyes. And that's the problem. We cannot determine how fast light is traveling but we can measure how long it takes to make a round trip from your laser to a remote target and back to you. In fact, that's how a laser rangefinder works that you can buy at a hardware store; it measures how long it takes for the light to make a round trip between you and a target and then using the defined value for the speed of light, it can calculate the distance. So what we do in Special Relativity, is we say that the light took the same amount of time to get to the target as it did to get back. That's Einstein's second postulate.

    No, this is a common misconception. Einstein said that time is what a clock measures and since a clock has to be made out of some material substances and since material substances cannot travel at the speed of light, we cannot talk about time slowing down for light like we can for material objects.

    Well, since your assumptions are wrong, then there is no quandary for you, is there?
     
    Last edited: Jan 17, 2014
  6. Jan 17, 2014 #5

    berkeman

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    Of course we can. The velocity of a foot per nanosecond is not hard to measure in the lab...
     
  7. Jan 17, 2014 #6

    WannabeNewton

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    There's a difference between measuring the one-way speed of light and measuring the two-way speed of light. The latter only requires one clock so there's no issue but the former requires at least two synchronized distant clocks; in other words you need a notion of distant simultaneity. But how do you go about synchronizing the two clocks in the first place? If you use light signals to synchronize the clocks, as is usually done, then you need to know something about the one-way speed of light so we're back to square one. This is why in Einstein's original paper he established by definition that the one-way speed of light is isotropic in inertial frames. Using this we can then synchronize distant clocks. So it's important to distinguish between the two-way speed of light (which can be measured using a single clock) and the one-way speed of light (which requires a synchronization convention such as Einstein's).
     
  8. Jan 17, 2014 #7

    ghwellsjr

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    I don't know how, please tell me.
     
  9. Jan 18, 2014 #8

    berkeman

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  10. Jan 18, 2014 #9

    ghwellsjr

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    I read the article. I saw where the OTDR can measure fiber length, similar to what I said about a laser rangefinder, but I didn't see any claim or mention that it can measure how long it takes for a signal to get from one end of the fiber optic cable to the other.

    So I still don't know how. Can you please point me to the place in the article where I misunderstood or overlooked or whatever your understanding is that I don't have.
     
    Last edited: Jan 18, 2014
  11. Jan 18, 2014 #10

    berkeman

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    "How long" is just the horizontal axis on the oscilloscope.
     
  12. Jan 18, 2014 #11

    ghwellsjr

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    The article says the horizontal axis on the oscilloscope is length, not time:
     
  13. Jan 18, 2014 #12

    berkeman

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    That is for the cable tester. Length is time -- about a foot per nanosecond in free space, less in a dielectric like a cable. It is definitely a "time of flight" measurement, which is what you were asking about. It can be done with multiple photodetectors along the path of flight of a short laser pulse in air as well.
     
  14. Jan 18, 2014 #13

    ghwellsjr

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    Are you saying that if the clock in the instrument measures 10 nanoseconds from the time the pulse was emitted to the time when it was detected, then the length is about 10 feet?

    Can we deal with this more complicated case later?
     
  15. Jan 18, 2014 #14

    AlephZero

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  16. Jan 18, 2014 #15

    ghwellsjr

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    That's measuring how long it takes for light to get there and back. Do you have a way to measure how long it takes for the light to get there?

    You are again seeing light that has gone from a source to different places on the objects and back to the camera. This does not measure how long it takes for the light to get just from the source to the objects, does it?
     
  17. Jan 18, 2014 #16

    berkeman

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    Yes. Are you really not understanding what Aleph and I are saying? The only practical issue for electrical detection is to use a fast laser pulse as the source, and have high bandwidth detectors that can see nanosecond light pulses. The rest is pretty trivial.
     
  18. Jan 18, 2014 #17

    ghwellsjr

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    Are you sure? I would have thought that if the OTDR measured 10 nsecs, it would have reported 5 feet for the length of the fiber. Please help me understand your answer.

    I really don't understand how what you are saying is related to measuring how long it takes for light to reach a distance, which is the subject of this thread.
    Can we please finish with your first answer regarding the OTDR before going on to other issues?
     
  19. Jan 19, 2014 #18
    Not to diminish your point in the slightest, but there was an example of just this on UK TV recently (using a state of the art high speed video camera), I think Brian Cox introduced it in one of his pop science programmes but I haven't managed to find it online yet. If anyone can provide a link I'd be interested in seeing it again ;)

    EDIT: here's the link
    http://www.ted.com/talks/ramesh_raskar_a_camera_that_takes_one_trillion_frames_per_second.html
     
    Last edited: Jan 19, 2014
  20. Jan 19, 2014 #19

    DrGreg

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    To ghwellsjr and berkeman

    It seems that neither of you is understanding what the other is saying.

    I think ghwellsjr's point is that you can't directly measure the 1-way speed of light; all you can do is measure the 2-way speed. The 1-way speed is set by convention rather than by experiment.

    I think berkeman is using the convention that the 1-way and 2-way speeds are equal to each other so there's no need to make any distinction between them. I think all the methods proposed are 2-way measurements.

    Have I summarised each of your postions correctly, and does that help you understand each other?
     
  21. Jan 19, 2014 #20

    berkeman

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    Yes, 10 feet in free space, and more like 5 feet in fiber.

    If you space out high-speed photodetectors (with beam splitters) and use equal length coax cables to get the detected signals to a high-speed oscilloscope, you can see the optical pulse as it hits each photodetector, with the pulses spaced out in time. That's the way I read the OP's question.

     
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