Question About Speed of Light

Discussion in 'Physics & Math' started by OilIsMastery, Nov 30, 2008.

Page 1 of 3
  1. OilIsMastery

    OilIsMastery Banned Banned

    Messages:
    3,288
    If, according to special relativity, the speed of light is a constant "c," how is it possible that scientists have slowed the speed of light to < c?
     
    OilIsMastery, Nov 30, 2008
    #1

  2. Google AdSense Guest Advertisement



    to hide all adverts.
  3. BenTheMan

    BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

    Messages:
    8,967
    The speed of light in vacuum is defined to be 300,000,000 m/s. There is a simple relationship, known to Maxwell, relating the speed of light in vacuum to the speed of light in matter:

    v = c/n

    where n is the optical index of the new medium. n=1 for a vacuum.
     
    BenTheMan, Nov 30, 2008
    #2

  4. Google AdSense Guest Advertisement



    to hide all adverts.
  5. OilIsMastery

    OilIsMastery Banned Banned

    Messages:
    3,288
    Thx. I consider that to be a real answer unlike the other thread.

    So if space isn't a vacuum but contains plasma then the speed of light isn't constant.
     
    OilIsMastery, Nov 30, 2008
    #3

  6. Google AdSense Guest Advertisement



    to hide all adverts.
  7. BenTheMan

    BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

    Messages:
    8,967
    Space isn't a vacuum anyway.
     
    BenTheMan, Nov 30, 2008
    #4
  8. OilIsMastery

    OilIsMastery Banned Banned

    Messages:
    3,288
    I didn't think so.

    So, if space isn't a vacuum and the speed of light can be slowed, is special relativity preserved?
     
    OilIsMastery, Nov 30, 2008
    #5
  9. prometheus

    prometheus viva voce! Registered Senior Member

    Messages:
    2,045
    Quite often people have said that you can have light moving in a vacuum at lower speeds that c as well.

    The group velocity depends on the Fourier composition of the wave, so does not have to be c. This is consistent with special relativity. The [url="http://en.wikipedia.org/wiki/Phase_velocity]phase velocity[/url] of light is always c.
     
    prometheus, Nov 30, 2008
    #6
  10. BenTheMan

    BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

    Messages:
    8,967
    Yes.
     
    BenTheMan, Nov 30, 2008
    #7
  11. OilIsMastery

    OilIsMastery Banned Banned

    Messages:
    3,288
    Can you say more please? Why? How so? Etc.
     
    OilIsMastery, Nov 30, 2008
    #8
  12. Trippy

    Trippy ALEA IACTA EST Staff Member

    Messages:
    10,890
    You understand, of course, that c is defined as being the speed of light in vaccou and that Special Relativity posits that c, being the speed of light in vaccou is a constant?

    So questions about what speed light travels at in space, which we have all agreed isn't a perfect vacuum, and how that affects the basic applicability of SR and GR are kind of like asking how the colour Blue affects the flavour of Potatoes.

    According to Wikipedia, the refractive index of air is about 1.0003, which means (assuming that it's a meaningful result) even in the earth's atmosphere light is still travelling at approximately 0.9997c.

    And before you mention it, the Wikipedia source is in agreement with 5 other sources, including Microsoft Encarta, and three Chemistry and Physics textbooks, all of which give it precise values of between 1.00027 - 1.00032, depending on things such as the precise wavelength of light used.
     
    Trippy, Dec 1, 2008
    #9
  13. James R

    James R Just this guy, you know? Staff Member

    Messages:
    39,426
    Those slower speeds of light refer to group velocities and not phase velocities.

    The group velocity of light can change in different media, yes.

    Yes.
     
    James R, Dec 1, 2008
    #10
  14. OilIsMastery

    OilIsMastery Banned Banned

    Messages:
    3,288
    If space isn't a vacuum, then c is impossible?
     
    OilIsMastery, Dec 1, 2008
    #11
  15. CheskiChips

    CheskiChips Banned Banned

    Messages:
    3,538
    Until you create a vacuum.
    Actually a true vacuum doesn't exist, I think they just get very close.

    Maybe this will interest you.
    http://au.arxiv.org/abs/hep-ph/9807278
     
    CheskiChips, Dec 1, 2008
    #12
  16. James R

    James R Just this guy, you know? Staff Member

    Messages:
    39,426
    No.

    c is the speed of light in a vacuum, not the speed of light in space.

    Get it?
     
    James R, Dec 1, 2008
    #13
  17. CheskiChips

    CheskiChips Banned Banned

    Messages:
    3,538
    James R; can you give a working example of a true vacuum?
     
    CheskiChips, Dec 1, 2008
    #14
  18. OilIsMastery

    OilIsMastery Banned Banned

    Messages:
    3,288
    Careful. You might get banned for saying something like that.
     
    OilIsMastery, Dec 1, 2008
    #15
  19. OilIsMastery

    OilIsMastery Banned Banned

    Messages:
    3,288
    This ought to be good.
     
    OilIsMastery, Dec 1, 2008
    #16
  20. CheskiChips

    CheskiChips Banned Banned

    Messages:
    3,538
    If I am wrong he'll prove my wrong.
     
    CheskiChips, Dec 1, 2008
    #17
  21. OilIsMastery

    OilIsMastery Banned Banned

    Messages:
    3,288
    OilIsMastery, Dec 1, 2008
    #18
  22. D H

    D H Some other guy Valued Senior Member

    Messages:
    2,257
    The two are experimentally indistinguishable, with experiments now stretching to halfway across the universe.
     
    D H, Dec 1, 2008
    #19
  23. CheskiChips

    CheskiChips Banned Banned

    Messages:
    3,538
    \(T=\left(\frac{n}{\zeta(3/2)}\right)^{2/3}\frac{h^2}{2\pi m k_B} \)

    \( k_B = boltzmans \)
    \( \zeta = Riemann zeta \)
    \( h = plank \)
    \( n = particle density \)

    This is used to calculate critical temperature for B.E.C., if it can also be used (I need verification) to calculate temperature regularly then it's evidence that 6 Kelvin and 3 Kelvin are not the same physical composition.
     
    CheskiChips, Dec 1, 2008
    #20
Page 1 of 3

Share This Page