Speed of Light

  1. Hi All,

    It is Experimentally very evident that the speed of light in any any frame of reference is constant. But is there any explanation for why this happens.

    Velocity of light seems to be like zero in mathematics. Any thing multiplied my zero results in zero irrespective of the number you are using for multiplication.

    If there is any explanation for the consistancy of speed of light,can anyone please explain me?

    Thanks,
    Kannan
     
  2. jcsd
  3. wolram

    wolram 3,781
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  4. mathman

    mathman 6,618
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    The constancy of the speed of light is a given in special relativity. The theory is in excellent agreement with all experiments. "Why" is impossible to answer. I would say this (can't answer "why") is probably true for any basic theory in physics.
     
  5. Because physical perspective requires a constant speed of light at some level?

    Because a constant speed of light helps define spherical (highly symmetric) macroscopic and microscopic horizons?

    Because the tautology of constant c, h, e, kappa and G provides the illusion of a complete physics?
     
  6. Thanks for the replies,

    But if we say E=hu, or F=GMm/r2 ,the constants here is not making any kind of nonsense to us.

    But the consistancy of the velocity of light is TRUE, but still it remains to be a nonsense.
    There is something in the universe which make this happens, else i can say there is some predefined property which make this, and most probable it was set when the universe set out in big bang.

    And i positively think all the constants are related to each other, else i can say it is related to one basic constant, which we can say the "primary constant". Hope the TOE will come up with one.
     
  7. My inkling too ...

    May be it's related to the initial amount of energy, or energy "release" that kicked it all off.

    Whatever "energy" means.'

    Whatever "kicked it all off" means.

    ;-)
     
  8. Integral

    Integral 7,351
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    Yet again.

    Einstein did not choose to make the speed of light constant. He was able to postulate its constancy because of Maxwells work. According to Electoromagnetic Theory

    [tex] c = \frac {1} {\sqrt {\epsilon_0 \mu_0}}[/tex]

    Where [tex]\epsilon_0 [/tex] is the permitivity of free space and [tex]\mu_0[/tex] is the permeability of free space. So the speed of light is related to the fundamental electrical properties of space itself.
     
    Last edited: Dec 12, 2003
  9. The Measured speed of light appears to be constant in the earth centered reference frame. It is postulated to be constant in SR. Does this mean "constant for all time" ... or is it maybe a variable that could change as the universe ages - or perhaps it could vary if the experiments were accurate enough to measure the effect of gravitation on the speed of light - or if experiments were made between a source and receiver moving with respect to one another in a free space (non gravitationally centered) environment. Many experiments have been made and so far SR has survived - but the search for an underlying physical dynamic is still unrevealed.
     
  10. selfAdjoint

    selfAdjoint 8,147
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    In special relativity, constant means "measured the same by all inertial observers". From this this is deduced "Constant in all physical interactions" - but that tacitly means short time (less than a million years?) interactions. Also variations that are too small to detect would not be covered.
     
  11. Do these laws also apply near the event horizon of a black hole.
    If light on the outside of the event horizon can escape the gravity of a black hole and light on the inside is destined for the singularity then can't we assume there is a balance point somewhere inbetween where we would observe the speed of light to be 0?

    Go easy, I'm new to physics and still learning.
     
  12. jcsd

    jcsd 2,226
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    A remote (infintely seperated) observer would view the velocity of light at the evnt horizon as zero.
     
  13. russ_watters

    Staff: Mentor

    There are people looking at that possibility right now. It appears that IF is isn't constant in time, it doesn't change by very much.
    Already been done. Gravitational lensing allows us to study that.
     
  14. Russ Waters - question - I have never been able to find an unequivocal proof as to whether gravity simply bends light or slows it - seems it depends a lot on the interpreter - Shipero for example seems to take the view that the effect associated with his name is merely a bending whereas others have regarded the delay as due to gravitational slowing - same as to gravitational lensing - do you have a good reference?
     
  15. Kannan, it has not been experimentally verified that the speed of light in any frame of reference is constant. Furthermore, it has not been experimentally verified that the speed of light in any inertial reference frame is a constant (which was Einstein's postulate).


    The fact of the matter is, that the speed of light in the reference frames we have measured it results in approximately 299792458 meters per second, with some degree of experimental uncertainty. In fact, the speed of light is now defined as exactly 299792458 meters per second with zero uncertainty, and the uncertainty has been pushed onto the length of the meter itself, rather than the speed of light.

    In maxwellian electrodynamics, one of the speeds of light is related to the permittivity of free space constant, and the permeability of free space constant, which both showed up in magnetic and electric theory before maxwell came along. The relationship is:

    [tex] c = \frac {1} {\sqrt {\epsilon_0 \mu_0}} = 299792458 m/s[/tex]

    Maxwell keenly noticed that this was roughly Fizeau's measurment on the speed of light relative to that which emits light, and so he knew this was more than coincidence. Now, there is no question whatsoever, that Maxwell learned that there is some connection between the three constants. When Einstein came along, he just wondered what c was measured relative to? In other words he was asking himself, "The speed of light is c relative to what?"

    There is no clear answer for this in Maxwellian electrodynamics, the constant just sort of shows up out of the blue, when the equations of electrodynamics are written as second order partial differential equations. But, because the solutions of the equation are mathematical expressions of wave phenomena, his conclusion was that light is a wave. A ripple in the electromagnetic field. Now, empty space was assumed to be a vacuum, and so there was nothing that could ripple, as light moves outwards from a source, so a luminiferous ether was postulated. That idea necessarily dictates that the speed of light would depend upon ones relative motion toward or away from the source (if you are moving with respect to the medium you come up with one speed, and if you are at rest in the medium you measure a different speed). Experiments were done by Albert Michelson using a refined device similar to the one Fizeau used to measure the speed of light. The Michelson experiments did not measure different speeds, but rather always resulted in 299792458 meters per second (approximately). The real conclusion then, is that relative motion of earth and ether was not detected. Hence, there is no ether, space is a vaccum, and light is a particle.

    So at this point, Einstein then came along with his postulate:

    Postulate of SR: The speed of light is 299792458 meters per second, in any inertial reference frame.

    The previous postulate did explain the results of the Michelson experiment, but it also says more, which was not verified by experiment. It is the 'more' part which is still in question. This needs to be explained.


    The Einstein postulate is ludicrous, because it requires that simultaneity be relative, rather than absolute. In the other thread, I am trying to show that if you assume the Lorentz formula is true, then you can reach a contradiction mathematically, at which point you have absolute knowledge that the Lorentz formula is false. It will then immediately follow that the time dilation formula is also false, and that simultaneity is absolute and not relative, and it will also follow that the speed of light is relative, rather than absolute. Note that c=299792458 is defined to be a constant, hence dc/dt=0 even if the speed of light is a variable.

    Now granted that the speed of light is relative, that still leaves us with the question as to what the constant c really represents. The simplest answer of all is contained in a new postulate:

    Postulate I: The speed of a photon is 299792458 meters per second, relative to that which emits the photon.

    So then, this would still mean that the constant c is intriguing, because we would then wonder why all bodies must emit photons at the same speed relative to their center of mass. The answer would almost definitely have to do with the initial conditions of the universe. At any rate, this is really where all the attention should be focused.

    If the postulate above is right, then the speed of light is only c in some reference frames, specifically reference frames at rest with respect to the center of mass of the emitter, regardless of whether the emitter is in an inertial or non inertial frame when the photon is emitted.

    To show you clearly why Einstein's postulate is ludicrous consider the following:

    A body is about to emit a photon in the direction of two observers. One observer is at rest with respect to the emitting body, and the other observer is moving away from the emitting body at 3/4 c.

    Now, by my postulate, once the body emits the photon, that photon must be moving away from the body at 299792458 meters per second. Thus, the observer at rest with respect to the emitting body must measure this speed. So let his ruler tell him that he is a distance d away from the emitting body, and let his clock tell him that the time of travel of the photon from the emitting body to him is t. He must get the following result:

    299792458 m/s = d/t

    Now consider the other observer who was initially moving away from the emitting body at the constant speed 3/4 c. Since he was moving away from the emitting body, it follows that the speed of the photon would be different from the other guy's measurement. In fact, he should measure the speed of the photon as being slower than 299792458 meters per second. In fact, he should come up with c/4 as the photon's speed as it moves by him. However, according to the theory of relativity, he must come up with the same value as the other guy, simply because he is in an inertial reference frame.

    Now, suppose that initially he was moving away at a speed of 29979458 meters per second from the emitting body. Common sense tells you that the photon should never reach him, that the distance between him and the photon should remain constant. Again relativity says that not only will the photon pass him, but it will pass him at a speed of 299792458 meters per second, simply because he is in an inertial reference frame.

    The above conclusions show why the assumption that the speed of light is the same in ALL inertial reference frames is nonsensical.

    Anyways, my point is that the speed of light has not been proven to be 299792458 meters per second in all frames, but only some. If you investigate experiments designed to test my postulate, you will see that there is too much experimental uncertainty in them to be conclusive. Hence, neither SR nor state theory have been experimentally verified yet.

    In order to experimentally verify that state theory is correct, one would simply have to fire a photon at a mirror which is moving very fast towards the emitter. If state theory is correct, the experimental results will show a speed of light in excess of c. In order to detect this, the mirror has to be moving towards the source extremely fast. The point is, we cannot construct an experiment this delicate. How can we get a mirror to approach a photon emitter at say c/2, and know precisly when the photon is emitted, and when it strikes the mirror? (Hence it is easier to logically verify state theory).

    The customary way to measure the speed of light is to use a stationary mirror and measure the time it takes light to move to the mirror and back. Thus, the time of travel can be measured by a single clock. However, this kind of experiment will not validate relativity, since the same result will be obtained under my postulate I.

    You may find experimenter's who claim that they have ruled out my postulate I, but if you investigate the uncertainties in their experimental setup, not only uncertainties in 'amounts of time' but uncertainties in 'distance traveled by photon' as well, you will see that state theory has not been invalidated by experiment, from which it follows that relativity theory has not been validated by experiment.
     
    Last edited by a moderator: Dec 14, 2003
  16. Hurkyl

    Hurkyl 16,089
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    True, though you appear to understate the accuracy of such experiments. Was that intentional?


    You meant one's relative motion through the medium, right? One's motion WRT the source would only affect frequency.


    You verify Maxwell's 4 laws, and you have a verification of this postulate. *shrug*


    Why does that make the Einstein postulate ludicrous?


    I would like to point out that time dilation has been experimentally verified quite well.


    Or, have a particle in a particle accelerator emit a photon... wait, I think they've done that before.
     
  17. Time dilation works in such a way as to equalize inertial reference frames - it may seem counterinitve that all observers measure the velocity of light as having the same value - but if this were not the case, we could not explain how GPS satellite signals always arise "on time" irrespective of the direction of the signal with respect to the earth's motion vis-a-vis space. As I have said - there are still tests that need to be perform to validate SR in all cases - but this does not mean that its postulates are flawed per se.
     
  18. jcsd,

    How can a remote observer view light travelling at c=0? That light would get nowhere, and never reach her.
     
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    Last edited by a moderator: Dec 15, 2003
  20. russ_watters

    Staff: Mentor

    That isn't how science works. Since there is no such thing as a perfectly accurate experiment, you can only verify a calculation to within the sensitivity of the instruments used to measure the effect. This is part of the reason no theory can ever be said to be 100% correct.
    There have been many, many, many, many experiments. Clocks on towers, clocks on planes, clocks in space, etc. I'll give my favorite SR/GR evidence example:HERE is what the GPS system tells us about Relativity. A few snippets:
    Basically, the point of that is the confirmation given is what you can assume just by the fact that GPS works and without doing a more in depth study.
    The GPS link also talks about the speed of light. If it slowed down or sped up due to gravity, it would be noticeable in the GPS system.
     
  21. quote:
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    Originally posted by TheAtheistKing
    Something is verified, or not verified. There is no such thing as 'verified quite well'.
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    That isn't how science works. Since there is no such thing as a perfectly accurate experiment, you can only verify a calculation to within the sensitivity of the instruments used to measure the effect. This is part of the reason no theory can ever be said to be 100% correct.


    Law Of The Excluded Middle (LEM):
    For any statement X, X or not X

    Let S denote something that we can say has been verified. It follows from the law of the excluded middle that either S has been verified or not (S has been verified).

    Let S denote something that we can say has been verified quite well. It follows from LEM that either S has been verified quite well or not (S has been verified quite well).

    So now the issue is what is the meaning of 'verify'. To say that something has been verified, is to say that its truth value has been determined.(As a side note, veritas is the latin word for truth, of which verify is an English derivative).

    Please note I do not wish to get into semantical games, but semantics is an issue here.

    Only statements can be true, and only statements can be false. Let us set aside the issue of what a statement is for now, and focus on the word 'verify'. We have some statement S, whose truth value we in particular do not know. We wish to know the truth value of S. Presuming the truth value of S can be known by us, it now follows that at the moment we know the truth value of S, we can say that S has been verified by us. I see no room whatsoever to say that a statement has been verified quite well. To understand me consider this:

    At least one person knows the truth value of S.

    Suppose the previous statement is true. Now even if you don't happen to know the truth value of S, if you know that someone else does, then you can honestly say that S has been verified. You wouldn't need to say that S has been verified quite well. But, if you cannot personally verify the truth value of S, how can you know that someone else has?

    So, to say that something has been verified quite well, is semantically equivalent to saying that something hasn't been verified.

    Now, you don't use the word 'verify' as an operator on the set of statements, rather you talk about 'calculations' as being verifiable. Perhaps you know what you mean here, but I do not. I am interested in the idea you are trying to express here though, perhaps you can make yourself clearer?

    quote:
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    Choose the exact experiment, and give me the day on which is was performed, that proved conclusively that the time dilation formula is true.
    --------------------------------------------------------------------------------

    There have been many, many, many, many experiments. Clocks on towers, clocks on planes, clocks in space, etc. I'll give my favorite SR/GR evidence example:HERE is what the GPS system tells us about Relativity. A few snippets:


    Let me state my position clearly. Simultaneity must be absolute for logical reasons alone, and therefore it is impossible for time to dilate. Therefore, no experiment has ever confirmed time dilation.


    One final word on the speed of light. To even say "THE speed of light" you have already biased yourself. In point of fact, there are many speeds of light, because the speed of a photon in an inertial reference frame is different from the speed of the same photon in a non inertial reference frame.

    And even if you modify your phrase to "THE speed of light in an inertial reference frame" you are still biased, because you have already assumed there is only one such speed for ALL inertial reference frames, and I am arguing that the speed of a photon relative to that which emitted it, is not the same in a system which is moving with respect to the source.
     
    Last edited by a moderator: Dec 15, 2003
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