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Is there a medium in which sound travels faster than light?

  1. May 23, 2015 #1
    So we all know that the speed of sound increases with greater density of medium. Conversely, the speed of light decreases with greater density of medium.

    Does there exist such a medium that is so dense that the speed of sound overtakes the speed of light?
     
  2. jcsd
  3. May 23, 2015 #2
    We don't know such a thing. The speed of sound is inverse proportional with the square root of density.
    The speed of sound in lead is about 1/3 of the speed in aluminum, for example.
     
  4. May 23, 2015 #3

    Vanadium 50

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    You have a factor of about a million in speed difference. That means you need about a factor of a trillion in density. That's not easy.
     
  5. May 23, 2015 #4

    phinds

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    Well, take solid steel. I don't know what the speed of sound is in it, but the speed of light in it is zero so sound is faster :smile:
     
  6. May 23, 2015 #5
    How about a material made of Planck particles?

    Table 1: Physical dimensions of a Planck particle
    Parameter Dimension Value in SI units
    Mass M 3.85763×10−8 kg
    Radius L 5.72947×10−35 m
    Volume L3 7.87827×10−103 m3
    Density M L−3 4.89655×1094 kg m−3

    http://en.wikipedia.org/wiki/Planck_particle
     
  7. May 24, 2015 #6
    If light cannot pass through a material that mean that absorbs some frequencies, not that the speed of light in this material is zero. Microwaves may can pass through with another speed vg<c.
     
  8. May 24, 2015 #7
    But the initial assumption is wrong. Increasing density makes the sound slower and not faster.
    What is needed is a medium very stiff an with very LOW density. Something like the classical ether. This is the kind of medium required if light were some sort of ellastic wave. Or the medium for which the speed of an ellastic wave will have the same speed as that of light.
     
  9. May 24, 2015 #8

    Vanadium 50

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    I think we are discussing real materials. Otherwise this will quickly turn into "How about kryptonite"?
     
  10. May 24, 2015 #9

    Dale

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    To my knowledge, the media with the slowest speed of light is a Bose Einstein condensate. The speed of light can be on the order of 10's of m/s. The speed of sound is also very slow, on the order of 0.001's of m/s.
     
  11. May 24, 2015 #10

    DaveC426913

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    Kyuutoryuu: The very simple answer:

    The speed of sound in a material is a function of transfer of motion between the molecules of the material.
    No molecule can move - and no energy can be transferred between molecules - at the speed of light (let alone faster), so even in principle, no material can have a speed of sound as fast as light.
     
  12. May 24, 2015 #11
    Unlike kryptonite, Planck particles are at least theoretically possible. My comment was a bit snarky, though.
     
  13. May 24, 2015 #12
    A traveling wave tube is one way to slow radio frequency light. By how much, I don't know.
     
  14. May 24, 2015 #13

    anorlunda

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    I think you mean the speed of light in a vacuum; c. The OP was about light in a medium. As far as I know, there is no lower limit to the speed of light in a medium. However, the defnition is a bit fuzzy. In the glass of a lens for example, I've heard that the effective speed of light is the average considering that the photons are constantly scattered, but that they travel at c in between interactions with particles.

    There are also reports such as the following. I don't even know how to define the speed of light in those circumstances.

    Light stopped completely for a minute inside a crystal: The basis of quantum memory
     
  15. May 27, 2015 #14
    Correct me if I'm wrong but isn't the maximum speed information can possibly propagate through a medium equal to the speed of light? In other words, the wave energy passing through a medium can only move as fast as the particles within the object can transmit that energy from one particle to another. Because light has a fixed speed no matter what medium it goes through, I think technically there is no way for sound to travel faster than light.
     
  16. May 28, 2015 #15

    phinds

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    You are right about information, BUT ... light does NOT have "a fixed speed no matter what medium it goes through". Light has a fixed speed in a vacuum and can travel at lower speeds in materials. Sometimes MUCH lower. Did you not read post #9 or do you think DaleSpam is wrong? (Warning, that probably has happened but it would be unwise to count on it)
     
  17. May 28, 2015 #16
    But can a sound wave propagate in a dense plasma? If so, then molecules are not involved and the speed can be very great.
     
  18. May 28, 2015 #17

    DaveC426913

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    No, I meant speed of sound.

    I interpreted the OP's question to be about a medium so dense that its speed of sound being so fast as to approach that of light in a vacuum.
    It didn't occur to me that he might be asking about the speed of light being so slow as to to approach the speed of sound.

    Molecules, atoms, subatomic particles, whatever.

    Someone should calculate the speed of sound through a neutron star.*
    *someone probably already has.
     
  19. May 28, 2015 #18

    DaveC426913

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    I don't know if it is possible to have the speed of light be slower than the speed of sound but - it if exists - I think it will be called ...

    (...wait for it...)

    Chesnekov Radiation.
     
  20. May 28, 2015 #19

    anorlunda

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    I thought I knew what I was talking about, but after consulting WIkipedia, I'm very confused because of group versus phase versus speed (unspecified group or phase) quotations and statements.

    Wiki Speed of Light says:
    The speed at which light propagates through transparent materials, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels atc / 1.5 ≈ 200000 km/s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about299700 km/s or 90 km/s slower than c.
    Wiki Refractive Index brings in the phase versus group velocity question. It says:
    [refractive index] is defined as
    98f38596714705e32b04edea50f6aa96.png
    where c is the speed of light in vacuum and v is the phase velocity of light in the medium. ... The refractive index measures the phase velocity of light, which does not carry information.

    So, the speed of light in a vacuum is exactly c. Is that group or phase velocity?

    In a medium with a refractive index 2, the phase velocity of light is c/2. What is the group velocity of light in that medium?

    Does information in a transparent medium travel at a speed different than the light? That sounds silly.

    Light refracts as it enters a medium with n>1. I presume that the information carried by the photons refracts also. Correct?

    The Wiki mentions refractive indicies up to 2, but I've never heard of a theoretical upper limit on refractive index. Could it theoretically be arbitrarily large?

    Nobody commented on the link I provided in #13 of this tread. That article said that scientists had slowed light pulses to 16 m/s in 1999, and to zero in 2015. Is that bogus, or is it a matter of semantics and definitions? It was that I had in mind when I tried to answer the OP.
     
    Last edited by a moderator: May 7, 2017
  21. May 28, 2015 #20
    A lot of questions.:)
    In vacuum all these speed are the same and equal to c.

    In a medium, the phase velocity depends on frequency. This is called dispersion. So when they say that index of refraction is 2, they mean some mean value, good for some interval of frequency or maybe the value for a given frequency.
    Now if you have a pulse of light, this is composed of various frequencies which will have different phase velocities. So when you follow the pulse in time, it will move with some sort of "average" or "effective" velocity which is the group velocity. This will depend on how the velocities of various components varies with frequency. Mathematically, the group velocity is a derivative
    vg=dω/dk
    where ω is the angular frequency and k is the wave-number (2Π/wavelength).
    So to find the group velocity you will need the function ω(k) which is the dispersion relation.
    Equivalently, the dispersion can be given as the phase velocity versus wavelength.

    For more you should read about phase and group velocity in a textbook (either optics or electromagnetic waves or even acoustics).

    The information is usually transmitted with the group velocity but there are cases when it is not. Then a "signal velocity" is defined.
    To say that the information diffracts I think it is meaningless.
     
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