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

## Main Question or Discussion Point

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?

• AlyzsaM

Related Other Physics Topics News on Phys.org
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.

Staff Emeritus
2019 Award
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.

phinds
Gold Member
2019 Award
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 • billy_joule and rolotomassi

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

http://en.wikipedia.org/wiki/Planck_particle

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.

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.

Staff Emeritus
2019 Award
I think we are discussing real materials. Otherwise this will quickly turn into "How about kryptonite"?

Dale
Mentor
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?
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.

DaveC426913
Gold Member

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.

I think we are discussing real materials. Otherwise this will quickly turn into "How about kryptonite"?
Unlike kryptonite, Planck particles are at least theoretically possible. My comment was a bit snarky, though.

A traveling wave tube is one way to slow radio frequency light. By how much, I don't know.

anorlunda
Staff Emeritus

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.
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

• AlyzsaM
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.

phinds
Gold Member
2019 Award
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.
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)

• davenn
tech99
Gold Member

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.
But can a sound wave propagate in a dense plasma? If so, then molecules are not involved and the speed can be very great.

DaveC426913
Gold Member
I think you mean the speed of light in a vacuum; c. The OP was about light in a medium.
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.

But can a sound wave propagate in a dense plasma? If so, then molecules are not involved and the speed can be very great.
Molecules, atoms, subatomic particles, whatever.

Someone should calculate the speed of sound through a neutron star.*

DaveC426913
Gold Member
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...)

anorlunda
Staff Emeritus
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 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:
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.

• anorlunda
Staff Emeritus
2019 Award
Keith, is your position that taking more time to cover the same distance isn't "technically changing speed"?

Staff Emeritus
2019 Award
It's the 2nd item in the FAQ.

Khashishi
You can get some mixing of light waves and sounds waves into something called phonon polaritons. The speeds of sound and light are wavelength dependent, so you get a plot of frequency against wavelength which is called a dispersion curve.
The phase speed of the wave is given by the slope of the dispersion curve. The red lines which represent light and sound waves when they don't interact with each other, and the light wave has a higher slope and speed. But when they interact, you get the black polariton curves. The upper polariton curve is "light-like" on the right side and "sound-like" on the left side, and vice versa for the lower polariton curve. But they don't cross each other in the center; rather, they change identities.

I'm not sure about this, but I think something similar or more complicated will happen if the sound speed and light speed are close together in a material. The two waves will mix together and the question no longer has a simple answer.

 "Phonon polaritons" by Mathieu Perrin - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Phonon_polaritons.svg#/media/File:Phonon_polaritons.svg

rude man
Homework Helper
Gold Member
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?
Certainly. Harvard physicists have succeeded in bringing down the speed of light to well below that of sound.
http://news.harvard.edu/gazette/1999/02.18/light.html