# What would happen if the speed of sound were equal to the speed of light?

I was wondering what would happen if sound could travel at the speed of light. Some obvious things come to my mind: No supersonic flight, no way to calculate the distance of a storm counting the time between lightning and thunder… but what else would be different? Would it have any really big implications on our daily life?

Hell of a lot more noise pollution, as sound would travel a lot further. That rock concert in the stadium on the other side of town will now be in your bedroom for practical intents and purposes.

The problem is that sound is a medium-dependent effect, whereas light is not.

This is one of those questions that requires magic as an answer. If sound isn’t something that needs a medium to propagate then what is it? What are its physical properties? How could you know anything about whether you could hear it farther if it doesn’t obey any laws of physics?

You may get some answers, but remember to take them with a grain of salt because they probably are not going to use the same flavor of magic as one another.

You’ll kill the person you’re shouting at.

Actually, the speed of light is medium-dependent…it’s just still really, really, really fast. The speed of light in a vacuum is what we generally think of as the “speed of light”, but the speed of light in any other medium is something less than c. For example, the speed of light through water is only about 0.75c, while through air it’s .9997c, which is why the straw in your glass of water looks “bent”.

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

/hijack

But light does not require a medium. It is not dependent on a medium in that it can exist just fine without one. Sound’s existence and propagation require a medium. That is the manner of dependence that Expano Mapcase was referring to, I believe.

Isn’t that how prisms work?

They’d have to rewrite Don’t Stop Me Now.

Yeah, that’s a misnomer. Light doesn’t propagate as part of a medium and it never travels slower than c. When it goes through a medium, it will often be absorbed by the matter and reemitted, which causes a delay and the illusion of slowing down, but for all of the time that the light is actually traveling, it is going the speed of c. It can also do some fancy things involving transforming the phase of the light wave, but even if the phase changes speed, the signal speed is always c.

Concorde would be a time-machine.

Absorption hasn’t anything to do with it. Watch the videos here.

No question that one of things that most mess people up is this confusion. In previous threads I’ve suggested that Einstein’s Constant be represented as C while the local speed of light in a medium be represented as c.

I’ve got to assume that the OP is asking something like “what if sound moved the same speed in air as sound does in air,” for which C and c are so close as to make no practical difference.

No Doppler effect.

If sound were fundamentally different, I’m having a hard time thinking of what else would change.

If it were still a qualia of atomic collisions through a medium, the whole universe would be nuts. I’m not sure one could still make sense of the question.

It’s not a misnomer. It’s equally valid to say “light is a wave whose speed depends on the medium it’s propagating through”.

By the way, the speed of sound is higher in a denser medium. And the speed of sound in a neutron star can be very close to the speed of light.

I think if the speed of sound through air were close to c, it would mean that air is so dense that we couldn’t walk through it.

I’m thinking this would screw up a whole load of things.

Assuming this happened not just by magic, but because the properties of matter changed so that the propagation speed of mechanical vibration was c, everything would work differently - because everything would essentially become the equivalent of an almost perfectly rigid, incompressible material.

Anything that requires flexibility stops being flexible (i.e. almost all forms of life die) - and many kinds of musical instruments are no longer possible (strings don’t vibrate any more, air cannot resonate in a wind or brass instrument) - so the funeral of all life would be a silent affair, even if there was someone to play or attend it.

Well, within an order of magnitude, anyway. But even in a photon gas, the speed of sound is still less than the speed of light by a constant factor (IIRC, 1/sqrt(3)). To have the speed of sound be equal to c, you’d need to redefine what’s meant by “sound”, since it inherently can’t happen in any medium whatsoever. At best, you might be able to contrive some medium where the bulk speed of some wavelength of light is slower than sound, but that’d be more about changing the bulk speed of light, not of sound, and it’d still only apply over a limited wavelength range.

All of my rifles would be even more quiet than they are.

You’d still have 1/r^2 fall-off with distance, and reflections and scattering from houses and trees and whatever between you and the sound source. The sound would get there faster, but I’m not seeing why it would go farther.
If sound was really fast, but not so fast that it gives people heartburn to even consider it, say 1/4 the speed of light, you still wouldn’t have supersonic planes, or noticeable sound lag versus what you see. Whips wouldn’t crack. You’d hear the ducati’s high-powered rifle go off before you took the shot to the head, although without any sonic boom.

Faster passenger planes would be the biggest difference, I would think. The Concorde wouldn’t have a sonic boom, so it could fly cross-country, not just over oceans.

Everything would catch fire because of the friction of air (and everything else capable of transmitting sound) molecules accelerated to light speed. I have to imagine that even if we were vewy, vewy qwiet the planet would vaporize in short order.