# Why doesn't light, which travels faster than sound, produce a sonic boom?

I know that when an object exceeds the speed of sound ($340$ m/s) a sonic boom is produced.

Light which travels at $300,000,000$ m/s, much more than the speed of sound but doesn't produce a sonic boom, right?

Why?

• – user6760 Apr 16 '15 at 9:37
• At first I also thought "Cherenkov radiation", but this question is asking why light doesn't make a sonic boom, not why there is no equivalent of a sonic boom when it comes to light. – Roman Starkov Apr 16 '15 at 11:38
• Light doesn't make a sound. – OrangeDog Apr 16 '15 at 12:38
• @OrangeDog Even if it's in the woods and there is no-one around to hear it? – Phill Healey Apr 17 '15 at 8:42
• @JaapEldering Lightning is electrons, not photons – Jim Apr 17 '15 at 18:26

I know that when an object exceeds the speed of sound[340 m/s] a asonic boom is produced .Light which travels at 300000000m/s [much more than the speed of sound] doesn't produce a sonic boom right? Why?

The answer is already in your own question: just because light is not an object.

Sound "is a vibration that propagates as a typically audible mechanical wave of pressure and displacement, through a medium such as air or water" and must propagate itself by compressing particles (atoms/ molecules). The sonic boom is, as you rightly say, sound produced by compression of air molecules by an object, and is also propagating through the air. In this animation is represented a:

..sound source traveling at 1.4 times the speed of sound (Mach 1.4). Since the source is moving faster than the sound waves it creates, it leads the advancing wavefront. The sound source will pass by a stationary observer before the observer hears the sound it creates.

the shock wave on the edge

Light is an electromagnetic wave that propagates also in vacuum modifying electric and magnetic fields. These fields do not interact with air enough to compress them and produce sounds.

• I agree with your answer, but a "fun fact" on light interacting with air. A strong laser pulse can actually ionize air, such you expect sound. – mikuszefski Apr 16 '15 at 10:09
• @jbarker2160 not at all; that's why I posted it as a comment on a specific detail of an otherwise correct answer. – mikuszefski Apr 16 '15 at 13:24
• Light is also a stream of photons. It also does interact with air. Not just strong pulses; any light does (it scatters and gets absorbed). – Jan Hudec Apr 16 '15 at 14:09
• @TheBlackCat: It does not. But the answer says it does not interact, without qualifying it. And it needs to be qualified. – Jan Hudec Apr 16 '15 at 16:06
• This is an apple and oranges comparison. Sound is not a thing, it's the effect of air molecules and other particles being moved in patterns, whereas light is an particle, which in the case of @mikuszefski's comment can actually produce sound like anything else under the right conditions. A jet bursting through the air at supersonic speeds moves a lot of air causing a resulting loud noise. The interactions on air caused by photons is much more subtle, typically, which is why there's no boom. – Chris Pratt Apr 16 '15 at 16:26

A sonic boom is produced when a macroscopic object (say, roughly: larger than the average spacing between air molecules, $\approx 3\,\mathrm{nm}$) moves so fast that the air has no time to “get out of its way” in the usual way (linearly responding1 to a pressure buildup, which creates a normal sound wave that disperses rather quickly, more or less uniform in all directions). Instead, the air has to create a sharp shock wave then, which is two-dimensional and therefore can be heard much further.

Now, with small particles like light, this issue doesn't arise, because the air doesn't need to get out of the way in the first place: at least visible photons don't interact with air much at all, so they simply “fly past”. When there is an interaction, it pretty much means just a single air molecule is hit by a photon. This gives it a slight “knock” but nothing dramatic. And in particular, it doesn't happen simultaneously along a whole front, so there's no reason a shock wave would build up.

1Another way to look at this is if you consider the gas on a molecular level. The molecules have a lot of thermal movement – the average speed is in the same order of magnitude as the speed of sound. On this microscopic level, sound propagation is basically a “chain of messengers”: one molecule gets knocked to be slightly faster or slower than usual. This extra momentum information is carried on not so much by the sound-wave movement, but by the random thermal movements – in a “smooth” way. Therefore a slow-moving object, or a sufficiently small object (like an alpha particle) only causes normal sound waves. But it doesn't work like that if you hit the air on a whole front at faster than the speed of sound: in this case, the forward momentum you impart is larger than the usual thermal movement, and you get supersonic behaviour.

• And if we consider small particles, think about why alpha & beta radiation doesn't make sonic booms. – jamesqf Apr 16 '15 at 17:06
• From what I understand, the speed of sound in a medium increases with pressure; an object moving at the ambient speed of sound must increase the pressure in front of it sufficiently that the pressure wave can stay in front of the object; since light can't pressurize air enough to raise the speed of sound to the speed of light, the only way it can go through air is by "missing" all the molecules therein. Any photon that hits a gas molecule is going to get deflected, since it can't push the molecule out of the way. Would that be a fair statement? – supercat Apr 16 '15 at 17:35
• How much light would you need then? – PyRulez Apr 18 '15 at 2:21
• +1. This is a much better answer than the accepted answer. This answer could be made even better by noting that most of the molecules that make up the atmosphere are, on average, moving faster than the speed of sound. – David Hammen Apr 19 '15 at 8:47

There are many differences between light and sound waves noted in other answers, such as the impossibility of any object with nonzero rest mass reaching lightspeed.

However, there is one likeness that I don't think has been noticed yet and that is the following: a sound wave travelling at the speed of sound does not make a sonic boom!

This is because the sound wave, like a light wave in the EM field, is simply the propagation of a fixed amount of energy. There is nothing "adding to" the sound - or light - wave as it travels.

In contrast, an aeroplane flying at the speed of sound is constantly adding energy to the propagating wave through drag mechanisms. That energy can't propagate faster than the object adding energy to the acoustic field, with the result that you get a bunching up of a great deal of acoustic energy in a narrow wavefront. The object is keeping up with the wavefront, continuously adding energy. If the wavefront can outrun the object, the energy gets spread out over a wide volume.

Exactly the same thing happens in the phenonemon of Cherenkov radiation where a particle is constantly adding to the electromagnetic field but the presence of dielectric matter means that the disturbance propagates at less than $c$, so we have the same situation of a body "keeping up" with the wavefront and continuously adding to the latter's energy. Cherenkov radiation indeed is an electromagnetic analogue of the sonic boom.

When comparing light waves and sound waves in this fashion, we need to consider what is waving.

• In a sound wave, the position of air molecules are waving.
• In a light wave, the strength and direction of the electromagnetic field is waving. This does not exert any force on air molecules (actually it does, but that force is so small, and the frequencies are so fast that there might as well be no force occuring).

In order for a sonic boom to happen, an object must move through air faster than the air-molecule-position wave can propagate, so the front edge of the wave builds up behind the object, moving at the speed of sound, with a very high intensity.

Even though light moves a million times faster than sound, its impact on the motions of air molecules is practically 0.

## protected by Qmechanic♦Apr 17 '15 at 23:21

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