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How does light travel so fast?

  1. Oct 1, 2008 #1
    What makes light travel so fast? Is light its own energy?
  2. jcsd
  3. Oct 1, 2008 #2
    same thing that makes all waves move at the speed they do. if you tighten a spring it will carry waves faster. space itself has a certain 'stretchiness'. in fact it has 2. one for the electric field and one for the magnetic field. I think its the ratio of these 2 factors that determines the speed of light in any medium.
  4. Oct 1, 2008 #3


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    It doesn't check baggage.

    Anything with no mass has no choice but to travel at the speed of light, the actaul value is set by the nature of our universe. If you want to travel around the universe the speed of light is annoyingly slow.
  5. Oct 1, 2008 #4
    ...for those who waits on Earth (not for the one who travels).
  6. Oct 1, 2008 #5
    Hey, you are the first one in ages who is not annoyed by the fact that light's speed is too low! You should win a prize! :smile:
  7. Oct 1, 2008 #6
    Do You know that some theories predict possible velocities for undiscovered particles as much as 1000 000 of the velocity of light?

    So the velocity of light is about the velocity of hand keystroking.

    As Einstein wrote all is relative. Even the speed of light.

    What does God think about it and about absolute restrictions?
    The only real power comes out of a long rifle. Joseph Stalin.
  8. Oct 1, 2008 #7


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    Can you provide a reference?
  9. Oct 1, 2008 #8
    Not quite- it's the reciprocal of their product.

    Actually, that's not true. His point was that the speed of light was always, always constant, and he didn't like the name "relativity" for this reason; he's said to have wished it was called the principle of invariance. (The term "relativity theory" is said to have been coined by Max Planck).
  10. Oct 1, 2008 #9
    Last edited by a moderator: Apr 23, 2017
  11. Oct 1, 2008 #10
    I agree. It is very usefull unit.

    So does the speed of sound in the air. And the Mach number for supersonic velocities.

    Some physisists believe that there is the physical vacuum for quantized electromagnetic fields. Can such physical vacuum flow? How can we measure the velocity of vacuum flow and how can we control or change it's velocity?
    Last edited: Oct 1, 2008
  12. Oct 1, 2008 #11


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    You're missing the point, this isn't about 'convenient units'. Muppet's point was that the speed of light in vacuum is invariant, i.e. it is not relative as you seem to think.
  13. Oct 1, 2008 #12
    There's a fundmental difference between the constancy of the speed of sound and that of the speed of light though (which I assume is what you meant by "so does the speed of sound"?). An observer moving at 100 miles an hour away from a source of sound will measure the speed of sound to be about 240 miles an hour (that constant speed measured relative to the source, less the speed of the observer). But an observer moving away from a light source at 0.9c will still measure light from that source to be travelling at c.

    EDIT: Short and sweet, Hoot.
  14. Oct 2, 2008 #13
    Excuse me, I see that You misunderstand me.
    I want to say that in acoustic we can define speed of sound that will be invariant.
    We also can have several definitions of the speed of sound invariant or uninvariant under some transformations of the coordinate systems. There are such varieties of gas dynamics description (lagrangian, euleur,...).
    We know experimentally that the speed of light depends (ie it is not a constant) on gravitational field.
    Is it possible to consider, that the speed of light depends on nuclear forces? Were there experiments defining speed of light in nuclear matter?
  15. Oct 2, 2008 #14
    That line interests me profoundly. How can you define a speed of sound that will be invariant?

    If I measure the speed of sound in the source's rest frame, I might get a figure of 343 m/s. If I then travel away from the source at 300 m/s, won't I always measure it at 43 m/s? If I travel faster than the speed of sound, I will never hear it. I can't therefore measure it. And so it can't be invariant under velocity transformations. Which is the whole point of special relativity. The invariance of the speed of light under velocity transformations. This leads to all the lorentz transformations as the true velocity transformations. Or have I misunderstood?
  16. Oct 2, 2008 #15


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    No it doesn't! The physical speed (i.e. experimentally measured speed) is always c. However, the coordinate speed (which is not a physical quantity) may be greater than c in a gravitational field. To repeat, if one correctly measures the speed of light (even in a gravitational field) the speed will always be c.
  17. Oct 2, 2008 #16
    Yes, it is textbook explanation for beginners. We don't need even experiment to verify obvious conclusions.
    About invariant speed of sound i already gave the answer.

    My supervizor in laser project Oleg Kolosovski liked to question:

    What is the difference between physisist and engineer?
    Engineer never measure quantity, if the result is supposed to be obvious.
    It is obvious, that there (our laser apparatus) we must have 30000 volts. Let us take voltmeter and measure the voltage now...
  18. Oct 2, 2008 #17


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    If this thread doesn't get back to addressing the original question, then it will be locked. If you want to discuss SR beyond what is contained in standard textbooks that is of no help to the OP, then please do it elsewhere.

  19. Oct 2, 2008 #18
    Is it? I was not interested in gravitation for many years. In student time we derived equation for light propagating in gravitational field. It was analogous to propagating in ordinary matter with varing effective n(x,y,z)>1, so the speed of light couldn't be equal to c.

    Gravitational lensing in geometrical optics approximation.

    And there were the results of Venera laser location, that showed the retardation of signal when it passes near the Sun.

    Can You give the reference if i am wrong?
    Last edited: Oct 2, 2008
  20. Oct 2, 2008 #19
    If spacetime were flat, then you could say that light's speed varies in a gravitational field; but spacetime is not flat in GR; it's for this reason that a light beam is never bent and never changes its speed.
  21. Oct 2, 2008 #20
    Light is not its own energy, light is energy, though that isn't the reason why it travels fast. It travels "fast" because given the laws of our universe it need travel fast, do it's mass-less-ness.
  22. Oct 2, 2008 #21

    George Jones

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    You're talking about coordinate speeds, which aren't necessarily physical speeds, while Hootenanny refers to physical speed. All observers that physically measure the speed of light measure the same speed, even if, for example, they're inside a black hole.
  23. Oct 2, 2008 #22
    time slows down in a gravitational field so light does slow down but so does everything and everybody else. so no observer wall measure the speed of light in a vacuum at has own location to be less than c.
  24. Oct 7, 2008 #23
    In conclusion, how we feel about the speed of light is relative to the frame of reference.
  25. Oct 7, 2008 #24
    Sorry, I can't grasp what you mean; light's speed is not relative to the frame of reference.
  26. Oct 7, 2008 #25
    Yes, but the keyword is "how we feel about the speed of light". :wink:
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