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Speed of Light

  1. Dec 2, 2011 #1
    Hey, this is my first post of many as I have hundreds of questions!

    As speed of light is a constant, does it travel from the eye of the observer ?

    e.g is a stationary person observing light travel away at ≈ 3x108m/s
    but a person travelling at 1/2 of, c, witnessing it travel 1/2 that speed or an additional c ?

    Mixed up between newton & Einstein !

    please help

    c= speed of light
  2. jcsd
  3. Dec 2, 2011 #2
    Hi, welcome to the forums...go slow and read carefully.....maybe save descriptions you like as notes and gradually add to them as you learn more....

    Light travels into, towards, the eye of the observer, not "from". That's how we "see" distant images.

    In flat space, no gravity, all inertial observers measure the speed of light as "c". Both near and far. Makes no difference how fast the observer is traveling!!!! It's Weird!!!! But at high
    speeds, space (distance) and time vary, not the speed of light!!!!!

    In curved spacetime, with gravity, only the LOCAL speed of light is measured as c. That means the observer is measuring light right where she is (locally) and any curvature is eliminated.

    In curved spacetime, gravity present, distant observations are impacted by the curvature of spacetime so different measures will be observed.

    Hence, as an example, a stationary remote observer will see time slow to a crawl near the horizon of a distant black hole. But a free falling observer headed directly into the horizon records nothing unusual...his time passes normally.

    If you SEARCH (top of this page when signed in) SPEED OF LIGHT you'll find many discussions and explanations....
  4. Dec 2, 2011 #3
    To answer your question simply, the speed of light(or how fast it appears to be travelling) is always uniform in a vacuum.

    The only instance that I can recall when "something" can travel greater than the speed of light (ignoring CERN) is by comparing two polar-opposite sides of the universe: each is expanding at the speed of light. IIRC, this is called "grey space" or something of the sort.
  5. Dec 3, 2011 #4
    I don't know what "polar opposite sides of the universe" means, because it is not defined. but it makes little difference...If you look in any given direction in the heavens, we believe the most distant galaxies ARE moving away from us at greater than the speed of light...but that is because new space is being created in the intergalactic voids...not because the intrinsic speed of those distant galaxies.

    There are a number of phenomena that move faster than the speed of light, but the key is that none can be used to transmit information.

    see "phase velocity" as an example:

  6. Dec 3, 2011 #5
    Thank you for your detailed reply,
    but how come people in motion observing a light source see it travel at different speeds to those stationary ?

    e.g in Brian Greene's 'The elegant universe' the analogy of people on a train with a light and those at a station , those who were stationary observed light reach the person with his back to the trains motion - towards the front of the train.

    how is this so ?
  7. Dec 3, 2011 #6


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    The people on the ground are not observing the speed of light travelling at different speeds. They are observing light travelling in all directions at the same speed. This is why the train can "catch up" to the light. (Light moves forward and back at 300,000,000m/s, so 60m/s train encounters it at 300,000,060 in one direction and 299,999,940 in the other.)
  8. Dec 3, 2011 #7


    Staff: Mentor

    The eye is a light detector, so light travels to the eye of the observer, not from the eye of the observer.*

    Both observers would measure the same beam of light to be travelling at c.

    *technically reflected light from the eye itself does travel from the eye of the observer, but that is kind of incidental.
  9. Dec 3, 2011 #8
    Mod note: thread moved to relativity forum
  10. Dec 4, 2011 #9


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    Actually, it is impossible to see how light travels. We cannot observe how it moves. All we can do is have it reflect off of something and wait for the reflected light to get back to us. So when we want to measure the speed that light travels at, we have to time how long it takes to get from us, over to a mirror and back to us, and then we can calculate the total round-trip speed. It turns out that every observer, no matter how fast they are moving will get the same answer. This is a situation that is covered by Einstein's first postulate.

    His second postulate covers the situation that we cannot observe and that is: what time did the light hit the mirror? Einstein says that it is exactly half way between when it started out from us and when its reflection got back to us. This defines the unmeasurable one-way speed of light to be equal to the measurable two-way speed of light and allows us to synchronize remote clocks at rest to the same time as a local clock. This becomes the basis for Einstein's concept of a Frame of Reference with its spacetime coordinates referred to as events. You can read Einstein's original account of these ideas in his famous 1905 paper.

    With this background, we can see the answers to your questions. If we select a Frame of Reference in which the station is at rest and the train is moving, then light travels at c with respect to the station and in that frame, the light going from the observer in the middle of the train reaches the front and rear of the train at different times because that is how we have defined time for that Frame of Reference. Note that neither the observer on the moving train nor the stationary observer can actually see when the light hits the ends of the train, they have to depend on a definition and if they are using different definitions, then they will come to a different conclusion about when the light hits the ends of the train.

    So now if we look at the same situation using a Frame of Reference in which the train is at rest, the light will hit both ends of the train at the same time, and that is merely because of our different definition in this different Frame of Reference.
  11. Dec 4, 2011 #10
    Heisenberg Uncertainty Principle
  12. Dec 4, 2011 #11


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    This has nothing to do with relativity.
  13. Dec 4, 2011 #12
    I wasn't referring to relativity, and neither was he if I read his post correctly.

    For reference:
  14. Dec 4, 2011 #13


    Staff: Mentor

    The HUP has nothing to do with why people in different states of motion see a light source as traveling at different speeds.
  15. Dec 4, 2011 #14
    I thought he was referring to observing objects moving at the speed of light (in this case, electrons around an atom) and why they can appear to have different speeds (the difference between measured and theoretical). I then stated that this can be explained using the uncertainty principle.

    Forgive me, I misunderstood.
  16. Dec 4, 2011 #15


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    Objects, such as electrons, cannot move at the speed of light nor will they be observed or measured as moving at the speed of light, nor is there any theory that claims that they can do so.
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