Is the speed of sound always less than the speed of light?

[halfeye]

I mean, the speed of sound increases as the density or hardness or somesuch of materials increases, while the speed of light decreases iunder more or less the same conditions, do they ever cross over, or does the cross-over only theoretically happen after a black hole forms?

[Bucky]

It's possible for the speed of sound to exceed the speed of light in a metamaterial that has been engineered for that purpose.

Imagine, for example, a fiber optic cable many miles long, with a collector arrangement gathering/dispersing light at each end, but rolled into a coil only a few inches thick. It's "transparent" in that light shone on one end will transit the full length of the coil, and emerge from the other end. Sound, meanwhile, can pass through the coil by transiting its full width, entering at one collector but passing sideways between loops until it emerges at the other end.

Clearly, if the loop is long enough, the speed of light through that material will be slower than the speed of sound.

[georgie_leech]

It's possible for the speed of sound to exceed the speed of light in a metamaterial that has been engineered for that purpose.

Imagine, for example, a fiber optic cable many miles long, with a collector arrangement gathering/dispersing light at each end, but rolled into a coil only a few inches thick. It's "transparent" in that light shone on one end will transit the full length of the coil, and emerge from the other end. Sound, meanwhile, can pass through the coil by transiting its full width, entering at one collector but passing sideways between loops until it emerges at the other end.

Clearly, if the loop is long enough, the speed of light through that material will be slower than the speed of sound.

Mind you, that's not to say that the speed of sound along the same pathway as the light takes will be faster, just that the distance the light travels will be much greater than that of the sound.

[crayzz]

Intuitively, I want to say no. The speed of light is also the maximum speed of information propagation. When the speed of light is slow, it's because there are a lot of interactions happening; the light isn't so much slower as much as it's being interfered with. To have sound move faster than light, you need to be in a situation where entire atoms moving around experience less interference than photons, which seems implausible.

I think the slowest speed of light achieved happens in Bose-Einstein Condensates at around 10-20 meters per second. This paper seems to suggest that the speed of sound in BECs sits around somewhere on the order of millimetres per second. Of course BECs form at very cold temperatures, so maybe if you found a way to slow light down in a high temperature medium (plasma maybe? idk I'm not a physicist) where sound tends to be faster it could be done.

EDIT

That said, you could say that the speed of sound in rock is on the order of kilometers per second while the speed of light is effectively 0.

[halfeye]

Intuitively, I want to say no. The speed of light is also the maximum speed of information propagation. When the speed of light is slow, it's because there are a lot of interactions happening; the light isn't so much slower as much as it's being interfered with. To have sound move faster than light, you need to be in a situation where entire atoms moving around experience less interference than photons, which seems implausible.

I suspect you are overgeneralising here. "c" is the speed of causality, the speed of light in diamond is much slower, but the light shines through diamond as brightly (or more so), than it does through glass, i.e. the light isn't interfered with and thus scattered.

I think the slowest speed of light achieved happens in Bose-Einstein Condensates at around 10-20 meters per second. This paper seems to suggest that the speed of sound in BECs sits around somewhere on the order of millimetres per second. Of course BECs form at very cold temperatures, so maybe if you found a way to slow light down in a high temperature medium (plasma maybe? idk I'm not a physicist) where sound tends to be faster it could be done.

EDIT

That said, you could say that the speed of sound in rock is on the order of kilometers per second while the speed of light is effectively 0.

I should have written electro-magnetic radiation, gamma rays go through rock fine. I'm guessing that the boundary condition would be in neutron star material, partly because they are hot, and as in your example hot typically makes for faster sounds. Sounds causing Cherenkov radiation would be interesting.

[Rydiro]

Intuitively, I want to say no. The speed of light is also the maximum speed of information propagation. When the speed of light is slow, it's because there are a lot of interactions happening; the light isn't so much slower as much as it's being interfered with. To have sound move faster than light, you need to be in a situation where entire atoms moving around experience less interference than photons, which seems implausible.

Doesn't Sound use the electromagnetic field, i.e. electric charges, to propagate? The molecules push each other because their charges get in each others way. This is afaik moderated by (virtual) photons. So it should be impossible. Did i get that right? Don't know enough physics.

[Chronos]

Yes, but the electromagnetic properties between one atom and the next are not the same as the electromagnetic properties in the bulk material as a whole. One simple model for why the speed of light is slower in a material is that photons still travel at c from one atom to the next, but at each atom they're briefly (unstably) absorbed, and then re-emitted after a short delay. You can then decrease the overall speed of light by increasing that delay between absorption and re-emission.

And there's no inherent problem with transmitting information through a medium faster than the bulk speed of light in that medium. Any highly-energetic particle passing through will work. This is a well-known phenomenon, and is what leads to Cherenkov radiation.

For the OP's question, I don't know for sure, but I suspect it might be true for neutron stars. Sound waves are quite relevant in the interior of a neutron star, and can travel at a speed of up to 1/3 c. The bulk speed of light within a neutron star is much less relevant, because they're extremely opaque, but I would not be surprised at all to learn that it's less than that.

[crayzz]

Doesn't Sound use the electromagnetic field, i.e. electric charges, to propagate? The molecules push each other because their charges get in each others way. This is afaik moderated by (virtual) photons. So it should be impossible. Did i get that right? Don't know enough physics.

Right, which is what I was getting at. Looking around, there doesn't seem to be a principled reason why it can't happen, it would just be very strange to be in a scenario where the EM waves of light are severely messed with while the EM field of atoms are unaffected.

And there's no inherent problem with transmitting information through a medium faster than the bulk speed of light in that medium. Any highly-energetic particle passing through will work. This is a well-known phenomenon, and is what leads to Cherenkov radiation.

This is true! But sound is the propagation of a pressure wave through the medium itself. I can certainly e.g. fire up a particle accelerator and launch protons through a BEC at much faster than 17m/s, you just can't reasonably describe that as sound.

[LibraryOgre]

Arguably, the speed of light through a truly opaque barrier is 0m/s. If said barrier allowed the transmission of sound, however, then sound could travel more rapidly through that medium than light.

[Lord Torath]

Do different frequencies of light travel at different speeds through a given media? Visible light bends when it goes through a glass prism, because it moves more slowly through the glass than through air (or vacuum). Would, say, gamma rays do the same?

And after thinking about it for all of 5 seconds, I have my answer: different frequencies obviously travel at different velocities through a given medium.

If all frequencies of light have the same speed through a particular material, we wouldn't get the rainbow by passing light through a prism, we'd just get a bent beam of white light.

So we could have gamma rays moving much more quickly through a given BEC than radiowaves. Sound could conceivably travel faster than some EM rays, but not others.

[halfeye]

Do different frequencies of light travel at different speeds through a given media? Visible light bends when it goes through a glass prism, because it moves more slowly through the glass than through air (or vacuum). Would, say, gamma rays do the same?

And after thinking about it for all of 5 seconds, I have my answer: different frequencies obviously travel at different velocities through a given medium.

If all frequencies of light have the same speed through a particular material, we wouldn't get the rainbow by passing light through a prism, we'd just get a bent beam of white light.

So we could have gamma rays moving much more quickly through a given BEC than radiowaves. Sound could conceivably travel faster than some EM rays, but not others.

You are half correct, if your diagram is correct (which I certainly don't remember) then it's the lower frequencies which are least bent (which I have a vague suspicion is incorrect). <edit> Strange, Wikipedia have a photo which agrees with the diagram, so it is almost certainly correct.

[Rockphed]

You are half correct, if your diagram is correct (which I certainly don't remember) then it's the lower frequencies which are least bent (which I have a vague suspicion is incorrect). <edit> Strange, Wikipedia have a photo which agrees with the diagram, so it is almost certainly correct.

Dispersive media (i.e. anything where the speed of propagation is not constant over all frequencies) can have just about any dependence on frequency. You can have things where higher frequencies get bent more. You can have things where lower frequencies get bent more. You can have things where red and blue get bent more (or less) than the middle of the visible spectrum.

[Radar]

Dispersive media (i.e. anything where the speed of propagation is not constant over all frequencies) can have just about any dependence on frequency. You can have things where higher frequencies get bent more. You can have things where lower frequencies get bent more. You can have things where red and blue get bent more (or less) than the middle of the visible spectrum.

Which also leads us to another detail of speed of light (or sound) in a given medium as there are two key ways of defining it:
1. Phase speed, which can be understood as the speed of a travelling maximum of a single frequency, constant amplitude wave.
2. Group speed, which is a speed of a peak of a finite pulse which can be described as an interference pattern of multiple waves with various frequencies (typically from a continuous band).

You can easily get phase speed of light above c in waveguides for example and - as I was very surprised to find out - group velocity can be arbitrarily greater than c or even negative, which was achieved not only for EM pulses but for sound as well. As I understand, what cannot be done is for the front of a pulse to propagate faster than c so causality is preserved.

Bottom line is, speed of sound or light can be defined in many different ways with different, often very surprising and mind-boggling results. That being said, in specific materials you can have a sound pulse travelling faster than an EM pulse, but I am not sure, if this answers the question or just starts many more questions.

[Lord Torath]

You are half correct, if your diagram is correct (which I certainly don't remember) then it's the lower frequencies which are least bent (which I have a vague suspicion is incorrect). <edit> Strange, Wikipedia have a photo which agrees with the diagram, so it is almost certainly correct.

It ought to, since Wikipedia is where I got the image from.