[thedoc (OP)]

Candice asked the Naked Scientists:
   
Hi, my name is Candice and I have a quick question.

Sound travels slower than light, and light is the speed limit of the universe because it doesn't have mass. Does this mean that sound has mass?

Thank you

What do you think?

[namaan]

If it weren't for us sensory beings, sound might might not be a "thing" at all. Sound as we know it is an interpretation by our nervous system via our sensory organs (well, our ears) of what are essentially vibrations in the air. It has wave-like properties similar to actual things such as light/electromagnetic radiation. But really, sound isn't a thing onto itself outside of what we make of it in our minds, so the question of mass doesn't come into play. Incidentally, the speed of sound is simply how fast vibrations are able to travel through a medium, such as air.

[wolfekeeper]

The answer is yes, sound has energy, and E=mc^2.

But it doesn't have rest mass. These days physicists mostly refer to rest mass when they use the term 'mass'.

If you have a box with sound in it, it weighs more than the same box with no sound in it.

[namaan]

The answer is yes, sound has energy, and E=mc^2.

But it doesn't have rest mass. These days physicists mostly refer to rest mass when they use the term 'mass'.

If you have a box with sound in it, it weighs more than the same box with no sound in it.

Interesting, I hadn't thought of it that way, but doesn't it depend on the definition? I mean, rather than saying sound has energy, wouldn't it be more accurate to say the medium, namely air, has energy, since it's the medium that has heat? Sound travels through the medium, it doesn't seem to make sense making it analogous to the medium.

[yor_on]

That was a nice deduction, and definition Candice , of what mass could mean. Can we from that then assume that everything massless travel at 'c'?

So, if 'energy' exist on its own, what speed have it?

[lightarrow]

If you have a box with sound in it, it weighs more than the same box with no sound in it.

hmmm... I'm not sure this is always true.
Take a box, completely isolated from the environment, which have inside a battery connected to an electronic circuit and a speaker. The circuit activates authomatically after some time and generate sound inside the box. The total energy of the box, and so its mass, never varies, even if you have sound after a while.

But if sound comes from the out of the box, I think your statement is true.

[JP]

If you have a box with sound in it, it weighs more than the same box with no sound in it.

hmmm... I'm not sure this is always true.
Take a box, completely isolated from the environment, which have inside a battery connected to an electronic circuit and a speaker. The circuit activates authomatically after some time and generate sound inside the box. The total energy of the box, and so its mass, never varies, even if you have sound after a while.

But if sound comes from the out of the box, I think your statement is true.

It's not quite the same box after it emits a sound, though.  The battery discharged a bit.  I think if you analyze it from an E=mc² (center of mass rest frame) side of things, sound has energy, so taking two identical boxes and putting sound energy into one increases its mass.  Draining an internal battery to make sound will leave the total energy of the box unchanged, so you won't gain mass.

[lightarrow]

It's not quite the same box after it emits a sound, though. 

I intended that the sound remains inside of the box. Of course if the sound escapes (or enters, in this sense I wrote "comes from the out of the box") then of course the box's mass varies.