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Is the speed of light a constant

  1. Jul 9, 2008 #1
    Is the speed of light a constant or can it be slowed down or sped up?
  2. jcsd
  3. Jul 10, 2008 #2


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    It can't be faster than it is in a vacuum (about 3x10^8 ms^-1) but it is slower in different mediums. I think there is a FAQ section on it.
  4. Jul 10, 2008 #3
    Such as water.

    Anyways, this been talked about before already. In fact you can just scroll down and see the related threads to find your answer: https://www.physicsforums.com/showthread.php?t=183408
    Last edited: Jul 10, 2008
  5. Jul 10, 2008 #4
    Rhg. The speed of light is not slower in different mediums. The photons just propagate slower.

    There was a purported physicist, I think in Maryland, who at one time believed that he could make a time machine by slowing down the speed of light in the medium he was using, believing that this slower value of the speed of light would reduce the energy requirements given by the Einstein field equations. Thankfully he realized he was out of his mind.

    'c' is always a constant, and if ever shows up in an equation, it cannot ever change as far as we know. Don't confuse that with photons propagating more slowly in a medium.
  6. Jul 10, 2008 #5
    Are you sure? I have always thought individual photons always travel at the speed of light in vaccuum, no more, no less. In a medium, the group of photons that form a lightpulse interact in such a way that the lightpulse itself moves slower (not the individual photons). Am I wrong?
  7. Jul 10, 2008 #6
    Hello smufcatcher.

    c is the speed of light in a vacuum. It is constant. Light cannot exceed this speed in any medium but can be slowed down in denser media. When it is slowed down its speed is not c as c is the speed of light in a vacuum.

  8. Jul 10, 2008 #7
    the phase velocity of light can be less that c. the group velocity can be greater than or less that c.
  9. Jul 10, 2008 #8
    What you mean with lightpulse?

    Well, Photons move in vacuum at c, in some medium the photons just interact with electrons, exciting then. When they go back to ground state, emits another photon. That's why, according to QED, speed light is slower than c in a medium.(Now, between interactions, yes, they move at c, it comes from invariance of electromagnetic waves and particle-wave duality i suppose).
  10. Jul 10, 2008 #9
    Yeah I don't know what is meant by "lightpulse" either.

    There is a certain amount of time when the photon in a medium is exciting the electrons and they bounce around a little bit and emit new photons. The group velocity winds up being less than c but if you isolate any one photon, it will be moving at c.
  11. Jul 11, 2008 #10
    But you said 'photons propagate slower in a medium'... They don't? The individual photons still move at c... Right?
  12. Jul 11, 2008 #11
    As said, it depends on the medium.

    However, /c/ in a vacuum will always be the same.
  13. Jul 11, 2008 #12


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    They do propagate slower in different mediums. The 'speed of a photon' in dimond is about 0.5c. The speed of light in a vacume is always c. Saying can the speed of light be changed is wrong because the speed of light normaly means 'the speed of light in a vacume'.
  14. Jul 11, 2008 #13
    But I thought a massless particle like a photon cannot move at any speed other than c, the speed of light in a vaccuum? I thought it was the interaction with the medium that made the speed of light in a medium slower, but it does not make the individual photons slower. Note that with 'the speed of light in a medium', I mean the speed at which a pulse of light travels through the medium, NOT the speed of the photons.

    Am I really wrong? I'm so sure that massless particles always move at c... Do you have any source for your statement?
  15. Jul 11, 2008 #14


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  16. Jul 11, 2008 #15
    Yes, I know... I'm not talking about the speed of light, I'm talking about the speed of the individual photons. They are not the same...! (they are only the same in a vaccuum)

    I know the speed of light is lower in a medium, but the speed of the individual photons take make up the light is still c, the speed of light in a vaccuum.
  17. Jul 11, 2008 #16


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    The photons must go slower because that is what light is. If you fired one photon under water it would have to move at the speed of light in water.
  18. Jul 11, 2008 #17
    As far as I know, no!

    There was a thread about this some time ago... The individual photons travel at the speed of light in a vaccuum, ALWAYS. However, in a medium, they interact with the atoms which makes the (average?) speed of a pulse of light slower.

    It's not exactly what happens, but you can imagine it as if you are driving your car through a city. Your car resembles the photon, and you can only move at say 50 mph. However, you must sometimes stop for traffic lights or pedestrians or things like that (resembling the atoms).
    Now imagine the exact same car driving in a straight road outside of the city (resembling a vacuum). While the distance traveled through the city and over the straight road is equal, and obviously the speed of the car (50 mph) is equal, the car driving through the city will arrive at the destination later, making it's average speed less then 50 mph (or c).

    Someone said above in this thread that the photons are absorbed and re-emited by the atoms, but I believe it was established in the other thread that this was not what happened. I don't know what exactly happens (probably some advanced quantum mechanical stuff) but I am pretty sure about my point that the individual photons can NOT move slower (or faster) than c, the speed of light in a vacuum.
  19. Jul 11, 2008 #18


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    Yea there was and there is also a FAQ explaining it. They would still move at c sometimes because inside a atom between the electron orbits and nucleus is a vacume.
  20. Jul 11, 2008 #19
    No, not sometimes, always... If you say that between the electron orbits and the nucleus is a vacuum, then everything is a vacuum for the photon. (As far as I know it does not make sense to say that a photon is inside an electron for example)...

    Where is the FAQ you mention?
  21. Jul 11, 2008 #20

    George Jones

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    https://www.physicsforums.com/showpost.php?p=899393&postcount=4 [Broken]
    Last edited by a moderator: May 3, 2017
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