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Is the speed of light constant?

  1. Sep 7, 2007 #1
    I suppose the speed of light is determined by the refractive index of the medium it is travelling through. I wonder whether the energy density and the refractive index of 'space' are constant in an expanding universe. Suppose the refractive index of space / vacuum used to be bigger or smaller than it is now, would this mean the speed of light would be different too? I wonder whether the speed of light is measured regularly and what time interval should be taken to be able to notice a deviation beyond the uncertainty.
  2. jcsd
  3. Sep 7, 2007 #2


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    The speed of light is constant. Though its passage thorugh a medium may cause it ro propogate slower.

    If subway trains all moved at exactly 35mph, but you chose to jump off at every stop and take the next one, your propogation along the subway line would be much slower than your constant velocity of 35mph while aboard the trains.
  4. Sep 7, 2007 #3


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    That wasn't quite the question. The question was whether there is evidence that the speed of light has been the same constant throughout the history of the universe. I think we can only say that as long as we have been measuring the speed of light, it has remained constant, at least within measurment error.
  5. Sep 8, 2007 #4
    Most standards of physics rely directly or indirectly on a constant speed of light. Until physics is observed to deviate substantially in some way from its current bases, constant light speed will maintain its central and pervasive role. If photons were shown to possess mass (likely <10-66 gm, a limit from the de Broglie wavelength of a photon traversing the observable universe), it also would vary in velocity and thus (perhaps circularly) wavelength.
  6. Sep 8, 2007 #5
    How will one actually come to the conclusion that speed of light has been different some time in the past? If I'm not mistaken, the speed of light has been defined to be 299,792,458 m/s for the past couple of decades or so, and any change observed would lead to the refinement of the metre, right?
  7. Sep 8, 2007 #6


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    Right, but there are astronomical measurements of the fine structure constant that seem to suggest that it has changed over the past few billion years. So it is possible.
    Also, compared to some of the other problems with the SI system this is just a minor problem; the changes we are talking about are so small that they would not have any practical effects.
  8. Sep 9, 2007 #7

    Claude Bile

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    The refractive index of a vacuum is 1 by definition! The quantity I think you are looking for is the permittivity and permeability of free space, and yes, if those quantities were to change then the speed of light in a vacuum would also change.

    There is no evidence to suggest that the speed of light has been any different to the currently known value in the history of the universe. There is some speculation that the speed of light has been different in the past, but to my knowledge this is just a hypothesis and no testable theories have emerged from this.

  9. Sep 14, 2007 #8
    299,792,458 m/s hummm?
    so how long is a second? doesn't this vary with gravity?
    Is it possible that a photon would continue to move at its constant but the length of the second could slow with a stronger gravitational field thus it would then appear (depending on where the observer was) that the photon changed speed.

    I have wondered about this for a long time (pun intended)
  10. Sep 14, 2007 #9
    1 second = 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
  11. Sep 14, 2007 #10
    I was really asking about the effects on time caused by gravity.
    can you help me out with this?
    Let me as a different question,
    if time slows in a higher gravity, how strong does the gravity have to be to stop time?
  12. Sep 14, 2007 #11
    It is not possible to stop the flow of time, in any configuration, the flow of time is constant when the observation and the measuring positions are the same.
    If you mean the flow of time as seen by a far away oberver not experiencing the gravitational field, the gravity must be infinite, else that RELATIVE flow of time wont be null.
    Even then, the observer must be infinitely far from the measuring point in order not to feel the gravity.
  13. Sep 14, 2007 #12
    I think this question maybe slightly off-topic in this thread. I'm no expert in General Relativity, but you could find an answer if you look through the threads (or post one, if you don't find an answer) at the Special & General Relativity forum.
  14. Sep 14, 2007 #13
    Actually I did mean both and you answered both. Thank you.
    This leads me to wonder if an event horizon only has a fixed diameter to a given observers location.
  15. Sep 14, 2007 #14


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    Einstein claimed that the constancy of the speed of light in a vacuum was confined to a special case (the Special Theory of Relativity) and was invalidated when the gravitational effects of embedded masses needed to be considered. He regarded gravitational lensing as an example of classical refraction, and spent much of the rest of his life trying to determine what properties of space could be modified by embedded matter, and how the variations in these properties affected the propagation of EM through the vacuum.

    What if he was right?
  16. Sep 14, 2007 #15
    I think we limit our ability to forge new concepts when we constrain our parameters to present day measurments - There is nothing in GR or SR that cannot be accommodated by a discovery that revealed the universe to be different in other epochs. In my opinion, it is unlikely that the properties of space would be the same when the universe were the size of a basketball as it is when it has a scale factor approximately 10^26 meters
  17. Sep 14, 2007 #16


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    As I pointed out above, there ARE astronomical measurements that seem to suggest that the speed of light HAS changed, not by much but by a measurable amount. You can find quite few papers on this using Google Schoolar. There was a nice PRL on this about a year ago where did measure a change (unfortunately, I don't remember the name of the authors), that got quite a lot of attention when it was published.
    However, there have also been other measurements that have tried to detect if alpha is changing today and they have all -as far as I know- come up negative, Hence, it could be that the speed of light was changing relatively rapidly just after big bang but that it later stabilized and is now changing so slowly that it can't be detected.

    The nice thing about measurements of the fine structure constant (and therefore c) is that it can be done with extremely high precision meaning even very small changes can be detected.
  18. Sep 16, 2007 #17


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    if [itex]\alpha[/itex] changes sufficiently to be measure, that means something. something tangible in physical reality actually changed. no so for any dimensionful constant such as c or G or similar.

    [tex] \alpha = \frac{e^2}{4 \pi \epsilon_0 \hbar c} [/tex]

    how would we know that a change in [itex]\alpha[/itex] is due to c? why not [itex]\hbar[/itex]? or e? or even [itex]\epsilon_0[/itex] (never mind that it is a defined constant, so is c and that doesn't seem to bother anyone)? no one is yet suggesting that the value of 4 or [itex]\pi[/itex] has changed.

    my question is that if somehow you were omniscient and knew that the change of [itex]\alpha[/itex] was due to c, how would the physical world be different, from the POV of its own measurement devices or experience of reality, if it was due to a change in [itex]\hbar[/itex]?

    if the fine-structure constant changes, that's salient but that is all that there is, [itex]\alpha[/itex] changed. it doesn't matter which of the constituent dimensionful components to [itex]\alpha[/itex] had changed since that is a reflection of which system of (natural) units you decided to use to measure and express physical quantity. Mother Nature doesn't give a rat's ass which system of units we or the aliens on the planet Zog choose to use to measure things.

    like measuring a length with a ruler or tape measure and counting the tick marks on the measurement standard, that is how we measure, or even perceive, all of physical reality. we really only measure or perceive dimensionless values when we measure anything. if we think (by our anthropometric measurement) that the speed of light changed, what really changed is the number of Planck lengths per meter or the number of Planck times per second or maybe both. but both of those values are dimensionless and are the salient values.

    the change of a dimensionful constant is just not meaningful. if some dimensionless constant changes that's a big deal and that is what changed, the dimensionless value. we do not know what to attribute that change to and, what dimensionless value we choose to attribute a change of [itex]\alpha[/itex] to is essentially a consequence of the system of units we choose to use to express physical quantity.
  19. Nov 23, 2007 #18
    measurements of light speed have been found to be decreasing. be it this is from very early measurments but despite innacurcys a trend has been observed that measurements are getting slower. it could be an increddible coincidence that all the results where innacurate in a way that produced an exponential curve of c slowing. infact i read a thing recently that suggested that 2000 years ago light could have been 20-30% faseter. our knowledge of the age of the universe is based on a constant value of c and if it was varying by this much the universe could be as young as 15000 years. i was astonished that i couldnt find anything writen disproving this. i mean we have evidence that earth has been around longer than that right. wrong, the our knowledge of the age of the earth is based on radioactive decay dating and the rate of radioactive decay is proportional the the speed of light. i'll admit im sceptical about the universe being 15000 years old but still astounded that the idea holds water.
  20. Nov 23, 2007 #19


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    Let's start with the evidence in favour.

    Do you have any references for this claim?
  21. Nov 23, 2007 #20


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    [EDITED so as not to look like a loonie]

    Lacking any credible references, it's safely outside the realm of "holding water". Is there any reason this is more astounding to you than whether or not Britney really lost her virginity at 14?
    Last edited: Nov 23, 2007
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