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Hypothesis about speed of light

  1. Mar 6, 2008 #1
    if we assume the light it is a wave that travels in the vacuum to speed c, then since the vacuum is firm regarding whichever reference system the light will travel to c in every reference system, for this reason?
     
    Last edited: Mar 6, 2008
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  3. Mar 6, 2008 #2

    HallsofIvy

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    I hate when a language problem prevents me from understanding a question! Exactly what do you mean by "the vacuum is firm"?
     
  4. Mar 6, 2008 #3
    i mean static, immovable, speed of vacuum is 0, in every reference system.
    sorry, i used an on line dictionary.
     
  5. Mar 6, 2008 #4

    ZapperZ

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    How exactly do you measure the speed of vacuum to know that it is 0 in every frame?

    Zz.
     
  6. Mar 6, 2008 #5

    HallsofIvy

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    "Speed of vacuum"? (And I want no jokes about Hoovers here!)
     
  7. Mar 6, 2008 #6
    I think what he means is that since we can't really measure the speed of vacuum because in a true vacuum there isn't anything to measure. Since there isn't anything, then nothing is moving and so its speed is 0. But if that were through, would it not mean that light moving through other materials would move at different relative speeds?

    I'm quite new to the special relativity, so I apologize if I don't make any sense or said something obviously wrong.
     
  8. Mar 6, 2008 #7

    ZapperZ

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    Be very careful here. "nothing is moving" is different than having speed of 0! An empty set (i.e. {}) is not the same as a set of 0 (i.e. {0}).

    Zz.
     
  9. Mar 6, 2008 #8

    tiny-tim

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    Hi DonnieD!

    The vacuum is not static - it does not have a speed.

    If it had a zero speed (or any veocity), then there would be an "absolute speed", which Einstein showed there isn't. :smile:
     
  10. Mar 6, 2008 #9

    rbj

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    if the vacuum of free space truly is nothing, nothing at all, no aether nor anything else, then there is no meaning to the notion of the vacuum moving. if that is the case, there remains nothing to differentiate between two inertial observers that happen to be in motion relative to each other. there is no way to say that it is Observer A who is moving while Observer B is stationary or the other way around. so then both Observer A and Observer B must have the same laws of physics applying to each of them which means they both must observer (or measure) the speed of propagation of EM (as well as the speed of propagation of gravity or any other fundamental action) to be the same.
     
  11. Mar 6, 2008 #10
    thanks, yes i wanted to say that one, we can consider the vacuum the space-time?

    Hi tiny-tim!
    i'm not sure of this, if vacuum had a speed this would be always 0 in all the systems, therefore all would be in agreement with that Einstein said.
    I recognize that does not have much sense to define the speed of the vacuum, but to say that the speed of the light in the vacuum is c, and so relatively to vacuum, isn't also an affermation that doesn't have sense in equal misure?

    excuse me but I am still using a translator, I could have written mistaken things without know it, and thanks to all!
     
  12. Mar 6, 2008 #11

    tiny-tim

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    Hi DonnieD!

    Speed must be measured relative to something.

    When we say that the speed of the light in the vacuum is c, we mean its speed relative to the observer.

    Perhaps this just a matter of English - we only say "speed in the vacuum" to distinguish it from "in air" or "in water" - we don't mean "speed relative to the vacuum." :smile:
     
  13. Mar 7, 2008 #12
    I don't think we can call the vacuum of space "nothing". In cosmology, distant galaxies are observed to be receding from us at greater than c, but the explanation is that they are actually stationary relative to the local vacuum they are embeddded in and that they are being carried along by the expanding vacuum. Light coming from the distant galaxy that is receding from us at 5c starts out its journey at -4 c relative to us, while maintaining a speed of c relative to the local vacuum. Nothing can move faster than c relative to the local nothing. The nothing around a massive body is curved resulting in the bending of light rays and defining the trajectories of orbiting bodies. This "nothing" is curved, warped, expanded, accelerated, stretched in ways that that have tangible measurable effects. It is a very interesting nothing :P
     
  14. Mar 7, 2008 #13

    tiny-tim

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    No, these galaxies are not regarded as stationary - nor are they - they move around just as our local group of galaxies move relative to each other.

    Light is not regarded as "maintaining a speed of c relative to the local vacuum" , but as maintaining a speed of c relative to any local observer.

    Nothing can move faster than c relative to any local observer.
     
  15. Mar 7, 2008 #14
    Well, that last post was slightly "tongue in cheek" but there are some serious points buried in it. Of course I agree that local groups of galaxies move relative to each other (proper motion) but the relative motion is pretty small compared to the speed of light and superluminal recession velocities and can be aproximated as stationary relative to the local "fabric of space". In the absence of any local observers, what regulates the speed of light? Does light still travel at c relative to the local spacetime when no one is looking? In the FLRW metric objects are swept away from us at superluminal speeds carried along by the expanding fabric of space. The time it takes a photon to reach us from a distant galaxy is determined by the speed of light relative to expanding fabric of space that is acting like a conveyor belt going away from us. The time it takes a photon to reach us from a star 10 billion light years away from us is a lot longer than 10 billion years because the photon is going against this flow outward flow of space. So imagine a star is receding from us at the 6 times the speed of light. A local observer measure the star to be moving at 0.5c relative to him but going away from us. The local observer is then moving at 5.5c relative to us. Now the challenge to you is to explain why the observer can move at 5.5c relative to us without using phrases like "fabric of space", "expansion of space", "local space", "spacetime geometry" or anything that implies that the vacuum of space is anything more than nothing. "Nothing" is here defined as something that can not expand, stretch, curve, move, regulate, carry or have any physical properties.
     
  16. Mar 7, 2008 #15
    I think I have read something about quantum vacuum being invariant under Lorentz transformations. In that sense it would be "immovable", wouldn't it?

    Does anyone have a reference to such an article?
     
  17. Mar 7, 2008 #16

    JesseM

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    The behavior of the quantum vacuum is the same in all inertial frames (which is equivalent to being invariant under the Lorentz transformations), but then so is the behavior of the classical electromagnetic field. I don't understand why you would say this means the quantum vacuum or the classical electromagnetic field are "immovable" though--for something to be moving or non-moving, doesn't it have to have a definite position (or be composed of parts which have definite positions, like particles of aether) which is either changing or not changing as a function of time?
     
  18. Mar 7, 2008 #17

    ZapperZ

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    There's a lot of things you are tripping over here. The speed of something in vacuum is NOT the same as the speed of something RELATIVE to that vacuum. The speed of something is always measured relative to something else. If you see something moving at some speed, it is relative to YOU, not relative to the "medium" that surrounds it.

    There are a lot of confusing premise in this thread. Because of that, I am highly skeptical that you'll get any kind of a rational answer out of this jumbled mess of confusion.

    Zz.
     
  19. Mar 7, 2008 #18
    I had some dim idea about the difference between quantum vacuum and the aether; that you may see that you are moving relative to the aether, but that you can't see that you are moving relative to the quantum vacuum (since it is invariant).

    Btw, is the classic electromagnetic field really invariant under Lorentz transformations? Can't we start with an electrostatic field and then by a Lorentz transformation get magnetic fields?

    (Sorry if I'm a bit unclear. I studied theoretical physics about 25 years ago, but have not thought about it much since then... I'm also not very comfortable with English. :shy: )
     
  20. Mar 7, 2008 #19

    JesseM

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    Yes.
    By switching between frames, you can show that a magnetic force on a moving test charge in one frame can be understood purely as an electric force in the test charge's rest frame (remember that the magnetic force on a charge is a function of its velocity, so a charge at rest experiences no magnetic force)--the diagrams on this page have some nice illustrations of this. But that doesn't mean there's some preferred frame with no magnetic force, it's just a consequence of the fact that the test charge is at rest in the second frame--if you added a new test charge to the second frame which was in motion in that frame, then you'd need to take into account the magnetic force to correctly predict its behavior. The fact that different frames all make the same predictions about the behavior of test charges if they use the classical electromagnetic equations in their frame, despite disagreeing on the relative contributions of the electric and magnetic force on each particular test charge, is a nice way of showing that classical electromagnetism is invariant under the Lorentz transformation.
     
  21. Mar 7, 2008 #20
    Just curious, how does one observe something moving away at v>c?
     
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