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Space and the velocity of light

  1. Oct 15, 2013 #1
    Does space limit the velocity of light or is it the formulas of relativity that limit the velocity of light?
    Isn't a math formula only a description or is it a controlling mechanism?
     
  2. jcsd
  3. Oct 15, 2013 #2

    Dale

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    Obviously math is descriptive, not controlling. If I write a different formula it doesn't control reality into a different state.

    Did you really need to ask this question?
     
  4. Oct 15, 2013 #3

    ghwellsjr

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    Are you asking about the measureable round-trip velocity of light or the one-way propagation speed of light that is postulated to have the same value in Special Relativity?
     
  5. Oct 15, 2013 #4

    marcus

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    What I understand you to be wondering about is what determines the speed of light. Could it be special rel? In a sense no, it is the other way around! Maxwell 1864 discovered the equations for electromagnetic waves, and those equations determine the speed of light. And a curious fact about Maxwell equations (that the speed is the same for another observer) puzzled people and caused Einstein 1905 to figure out special rel!

    One way to answer is to look here:
    http://en.wikipedia.org/wiki/Maxwell's_equations#Conventional_formulation_in_SI_units
    You can see in the lower right corner of the first box the ∇xB equation with two parameters that can be measured in the Lab: called epsilon-naught and mu-naught. That's the key to the speed.
    Then jump down to the part about the speed:
    http://en.wikipedia.org/wiki/Maxwel...s.2C_electromagnetic_waves_and_speed_of_light

    Curiously these two quantities were actually already measured by experimenters by Maxwell's time! Two guys did it with a kind of big chemical capacitor called a "Leyden jar". in the 1850s!
    So they actually measured physical properties of space which determine the speed of light.

    See footnote 2 of that same article:
    ==quote==
    2. The quantity we would now call 1/sqrt(μoεo), with units of velocity, was directly measured before Maxwell's equations, in an 1855 experiment by Wilhelm Eduard Weber and Rudolf Kohlrausch. They charged a leyden jar (a kind of capacitor), and measured the electrostatic force associated with the potential; then, they discharged it while measuring the magnetic force from the current in the discharge wire. Their result was 3.107×108 m/s, remarkably close to the speed of light. See The story of electrical and magnetic measurements: from 500 B.C. to the 1940s, by Joseph F. Keithley, p115
    ==endquote==
    So Maxwell equations (1864) describe how EM waves propagate and actually determine the speed!
    And the equations tell us that the speed will appear the same from perspective of different moving observers (they all have Leyden jars and can measure epsilon and mu in their moving Labs). So this seeming paradox was what forced Einstein (1905) to come up with the early "flat" version of Relativity.
     
    Last edited: Oct 15, 2013
  6. Oct 15, 2013 #5

    D H

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    Of course it's a description. Physicists use mathematics as a way to describe the workings of the universe. The mathematics is our map of the territory. "The map is not the territory" (Alfred Korzybski).
     
  7. Oct 15, 2013 #6
    Just trying to figure out the mechanism of speed control.
     
  8. Oct 15, 2013 #7

    ghwellsjr

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    And I'm just trying to figure out what you are asking. Please respond to my question in post #3.
     
  9. Oct 16, 2013 #8
    I was asking about the one way propagation of light in SR. thanks for your reply.
     
  10. Oct 16, 2013 #9

    D H

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    The immediate answer is geometry. What's the mechanism for that? Nobody knows. That's way, way beyond the standard model material.

    There's nothing wrong in science with an answer of "We don't know. For now, that is." Science is first and foremost about modeling observations. Explaining those observations is a secondary concern. Explaining those explanations? Ultimately that's an unachievable task. You can always ask "How?" "Why?" to some explanation of some phenomenon, or to an explanation of an explanation of some phenomenon.


    Word of advice: You do not want to go that route. You really, really do not want to go that route. My crackpot spidey sense gets all tingly when someone does go that route.
     
  11. Oct 16, 2013 #10

    Dale

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    Any explanation requires a set of fundamental assumptions which are outside the scope of the explanation. The speed control, as you put it, is the mechanism which is used to explain other things in our current theories. It is fundamental, meaning that it is not explained by current theories. It is simply an assumption that has been found to enable the correct prediction of a lot of experimental results.
     
  12. Oct 16, 2013 #11

    ghwellsjr

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    I kind of gave away the answer in my response to you in post #3. It is not space that is limiting the propagation of light nor is it the formulas of relativity but rather it is Einstein's second postulate that starts with the assumption (as DaleSpam indicated twice in his previous response) or declaration or stipulation that the one-way propagation of light in all directions is the same as the measurable two-way speed of light which always comes out the same for any inertial observer. So when someone sets up an experiment to measure the speed of light using a single clock and a mirror some measured distance away, he can assume that the light took the same amount of time to get to the mirror as it took for the reflected light to get back to him and this allows him to define the time that the light arrived at the mirror.
     
  13. Oct 16, 2013 #12
    The formulas are a description of reality, or at least we think they are. In the reality they describe, spacetime has a certain geometric structure that precludes anything from traveling faster than a certain speed, usually denoted "c". If you keep accelerating at some rate - steadily firing a rocket engine, for example - your speed will increase steadily for a while, but as it gets close to c your speed will increase slower and slower, so you'll get closer and closer to c but never get there (even though your rocket is thrusting with the same force, and no other forces are acting on you). The faster you accelerate over any given period of time, the closer you get to c. It's a strange thing, but that's what the math says (and experiments back it up).

    Light as it turns out is massless (i.e. it has zero inertia). If you recall Newton's second law, it says a=F/m. So according to that formula the acceleration of light is infinite, and so it can travel at speed c (that's not a very accurate explanation, since Newton's laws don't apply in relativity, but it more or less gets you there).
     
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