# Can sound use light as a medium to pass through?

1. Sep 2, 2008

### winchy

ok, ill try to explain myself as best as i can. you have a box with all walls with "perfect mirrors", remove all air, then force light into the box. assuming no light escapes would the "density" become great enough for sound to use the light as a medium to travell through. i hope explained myself well enough and hope someone might help me with this problem.
Thanks

2. Sep 2, 2008

### mgb_phys

No, to carry sound a medium needs to be able to pass along vibrations from one atom/molecule to the next.
Photons do not affect other nearby photons (except in some self focussing effects of very high power lasers).

3. Sep 2, 2008

### atyy

I'm not sure that this is impossible. I'm going to speculate along these lines. In the casimir effect, there is a force between uncharged plates in the "vacuum". The casimir force is due to the fact that the available states of an electromagnetic field between the plates is discrete due to boundary conditions, compared to the field outside the plates. So that would be a photon field between the plates. Since there is a force between the plates, then moving one plate mechanically (counting any mechanical movement as sound) would be transmitted to the other plate. In this way, perhaps photons could transmit sound?

4. Sep 2, 2008

### atyy

There's another way, but it's pretty stupid. If I blast a high intensity laser at your ear drum, I could probably pop it. And you might hear the popping.

5. Sep 2, 2008

### schroder

What you are describing amounts to modulating the light by vibrating the source (mirror). This is quite another matter! Of course, modulated light waves can carry sound information. However, the OP was asking about using light as a medium for the transmission of sound based upon the density of the light. That is not possible as sound requires a medium which can pass a mechanical wave, which means the medium must contain something mechanical such as atoms or molecules. Light consists of massless photons only, no atoms or molecules so it cannot pass a mechanical wave regardless of how “dense” or intense the light.

6. Sep 2, 2008

### schroder

Well, I agree with the first sentence, at least.

7. Sep 2, 2008

### Topher925

Your not hearing anything from the light itself, just the sound of your eardrum popping.

In order for a median to carry sound, it needs to have mass and be affected by electromagnetic atomic forces. I'm pretty sure light doesn't have either of those qualities not to mention its always traveling a lot faster than the speed of sound.

8. Sep 2, 2008

### Staff: Mentor

Simply put, light is not a medium.

9. Sep 2, 2008

### atyy

I think the casimir effect would be a bit different from the usual way of changing sound into electricity. The most common way is to use a microphone, in which sound essentially vibrates the charged plates of a capacitor. The casimir effect is a bit different since it involves uncharged plates. Of course the effect is extremely delicate, and requires our best precision to detect it, so it's not anywhere close to being technologically useful for a microphone now (though the nanotech guys may find it important).

But I think it's mostly semantic. Like whether a current can pass through a capacitor. In classical electromagnetism, no, by the *definition* of current - we say it changes into a "displacement current".

It's interesting that there is an accepted way of saying that a current passes through a capacitor - within the approximation of circuit theory, for alternating current.

10. Sep 2, 2008

### atyy

OK, I get how this is not just semantic. If I put my ear in the box of photons, I would not hear anything, no matter how hard the mirror was vibrated. In principle it is possible if an artificial ear was used, but because of the limits of sensitivity on my hearing, we will need to up the photon density to the point where I'd probably vaporise my ear first!

So it is semantic! There are two definitions of sound - one is mechanical and physicky. The other is biological and perceptually. In the first, sound is just another type of energy, and the ability to convert between different forms of energy is a fundamental idea in physics - there is no well-defined sub-discipline of physics called "mechanics" that is distinct from "electromagnetism". To make a clear distinction between sound and light, we need biology.

11. Sep 5, 2008

### winchy

but light has a mass, 2kg of light falls on the earth everyday. and isnt light and mass the same thing, thats why E=mc2. mass energy..?

12. Sep 5, 2008

### atyy

Yes. The very strictly correct answer is yes. Sound is defined as a mechanical oscillation. Light is an electromagnetic oscillation. But strictly speaking, we don't know where mechanics ends and electromagnetism begins.

In everyday physics, we can distinguish between mechanics and electromagnetism, so in that sense, the answer is no.

13. Sep 5, 2008

### winchy

ok, so in a perfect and toally therical world it could happen?

14. Sep 5, 2008

### atyy

Yes!

The difference in opinion between schroder and me comes down to a matter of definitions. That sound can be turned into light is not contested - it's called radio broadcasting. The one technicality is whether we allow the walls of the box to be charged or not. In radio, and all technologically common ways to turn sound into light, the walls have to be charged. There is an effect called the casimir effect ("the friction of vacuum"), where the walls don't have to be charged. It is an extremely delicate effect, long predicted, but only recently measured.

The way you are approaching it, from E=mc2 is a useful heuristic. But even without that, just from classical physics, we can say that light has momentum. That is where the division between mechanics and electromagnetism begins to disappear. The equations of classical physics that say this are Maxwell's equations, and they led logically (with lots of help from experiments!) to the relativistic notion of mass-energy equivalence you started from.

Last edited: Sep 5, 2008
15. Sep 5, 2008

### Staff: Mentor

No, no, no. Light does not have mass like ordinary objects do. When someone says that mass is lost as light by the sun, it is converted from matter to energy. That's what the equation is talking about. It doesn't mean that energy and mass are the same thing, just that you can transform one into the other.

And the casimir effect isn't the same thing as using the light as a medium - and that's even assuming it would work at all. The casimir effect is an extremely weak force and acts over a very short distance.

The wording of the op is pretty specific: "Can sound use light as a medium to pass through?" The answer is quite simply no. Light is not a medium and it cannot transmit sound the way a medium (ie, air, water, steel) transmits sound.

16. Sep 8, 2008