# Can Light Create Sound in Air? Exploring Momentum Transfer

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• Sorcerer
In summary, Dale explains that sound is created through the emission of electrons from atoms, which is why thunder might be caused by high voltage or electric fields.

#### Sorcerer

If a pulse of light, which has momentum p = E/c, interacts with particles of air, would it not change their momentum over time, causing mechanical energy (sound)?

As I understand it, sound is mechanical energy moving through particles as they vibrate. Why can't the momentum of a pulse of light excite particles into motion, thereby causing sound?

Your idea is generally correct, but air is mostly transparent so it doesn’t interact with visible light much. So you won’t be able to do this with a normal laser and air.

Sorcerer
Dale said:
Your idea is generally correct, but air is mostly transparent so it doesn’t interact with visible light much. So you won’t be able to do this with a normal laser and air.
What about other particles light interacts with? Say light hitting atoms or electrons? Or maybe through water?

As I understand it the momentum would be absurdly tiny, and thus so would any sound, but really what I'm asking is, theoretically could there be sound caused by light hitting particles and transmitting momentum (causing a change in momentum, and since the particles would then be moving through space, we'd have the integral of dp/dt over dx, i.e. work, and hence energy => sound), even if it so tiny that we couldn't be able to detect it?

Thanks again Dale you are always a great helper on this site.

Dale
Sorcerer said:
What about other particles light interacts with? Say light hitting atoms or electrons? Or maybe through water?
Have you ever seen a radiometer in action?

These things sometimes touted as demonstrating light pressure. In fact, they demonstrate the opposite. They rotate away from the blackened side of the vanes. Light pressure would cause them to rotate away from the silvered side. This suggests that creating sound with light will work much better if you exploit the thermal properties of light (its energy) rather than its mechanical properties (its momentum).

Shine your laser on a photoelectric cell and use it to power a speaker.

Dale said:
Your idea is generally correct, but air is mostly transparent so it doesn’t interact with visible light much. So you won’t be able to do this with a normal laser and air.
I'll add that large military lasers do interact with light enough to matter - heating the air - but the process is too slow to be controllable sound. It isn't like lightning and thunder.

Dale
Sorcerer said:
theoretically could there be sound caused by light hitting particles and transmitting momentum
Theoretically, yes. Lots of engineering challenges, but it would be appropriate for a hard sci-fi book, meaning the laws of physics don’t explicitly prevent it.

I suspect for most available light sources the strongest coupling is through dust. The dust absorbs the momentum and transfers it to the air. This can't be very efficient. If you're allowed to shine the light on a wall (which sounds to me like cheating), that would dominate.

One needs to consider energetics. A 3 mW laser pointer that was fully absorbed could produce at most 3 mW of sound. That's 30 dB softer than typical 3 W computer speakers. And "fully aborbed" would mean it wouldn't work as a laser pointer.

I'm not sure if this is relevant but try googling photo acoustic effect. I think Bell first demonstrated it back in 1880 or thereabouts.

The process would be light is absorbed by charged particles and thus excited electrons move and generate sounds with interactions with atoms arouond.
For an example thunderbolt is caused by high voltage or high electric field. Light might trigger thunder by emitting electrons from atoms around by photoelectric effect. In this sense lights have something to do with loud thunder sounds.

sweet springs said:
The process would be light is absorbed by charged particles and thus excited electrons move and generate sounds with interactions with atoms arouond.
For an example thunderbolt is caused by high voltage or high electric field. Light might trigger thunder by emitting electrons from atoms around by photoelectric effect. In this sense lights have something to do with loud thunder sounds.
Note that this involves the energy in light, not its momentum.

A related phenomenon is the fact that craters on airless moons tend to be circular regardless of the angle of impact. That is because the crater is not produced by the momentum of the impactor scratching out a gouge. It the energy of the impactor that produces an explosion.

russ_watters
I think that on the atomic scale this sort of thing can happen. The interaction of light with matter leads to a number of propagating phenomena, for example, optical phonons, excitons, and plasmons. These types of processes involve the movement of atoms and/or electrons in a material and couple to light. Typically these processes require the wavenumber of the photon to match the wavenumber of the quantum(a) produced, so they depend on the momentum the light is carrying. However, most of these quasiparticles involve transverse oscillations of the matter involved, and so do not directly correspond to sound waves (which are longitudinal).