# Does sound have gravity?

1. Sep 9, 2008

### matpec

I come to these forums asking for help as I have searched up and down google without any success on finding an answer to this question and I frankly lack the knowledge to figure this out on my own...

but do sound waves have gravitational force?

2. Sep 9, 2008

### derek.basler

Not necesarrily. when you think of what sound is, it is just vibrating molecules. A disturbance of molecules. Energy, on the other hand, does have a gravitational force. Hope that helps! If you need anymore clarification just ask, im sure i could help out more.

3. Sep 9, 2008

### matpec

I would imagine that to cause a disturbance within molecules that energy would be required ... making that disturbance have some sort of a gravitational force?

4. Sep 9, 2008

### derek.basler

well yes in a way. but the disturbance at the smallest part is molecules pushing against each other. therefore sound is primarily molecules pushing against each other. Therefore the molecules and the energy in the molecules has gravity, but not sound itself. Its hard to comprehend, this is a good question!

5. Sep 9, 2008

### atyy

Air consists of molecules. Each molecule has a mass. Mass attracts other masses and that attraction is what we call gravity. Sound is a disturbance of the positions of the air molecules - so it is a disturbance in the positions of mass - so it will cause a change in the gravitational field.

6. Sep 9, 2008

### matpec

in a sense like two tall building next to each other and a wind current flowing through the center?

should the molecules that compose sound be ignored, then is sound just a frequency?

7. Sep 9, 2008

### derek.basler

yes thats exactly is. sound is not matter nor energy. sound is an interaction. therefore it wouldnt have gravity. Does this clear things up for you at all?

8. Sep 9, 2008

### matpec

unfortunately yes :\

9. Sep 9, 2008

### matpec

but wait ... I just did a lil more reading and found that since E=hf and E=mc^2 then hf=mc^2

so a frequency would indeed have a mass or am I misunderstanding something here

10. Sep 9, 2008

### derek.basler

i think the E in that equation is different. I just looked up the definition of sound in my dictionary of physics. it gave this equation, c^2= E/p where E= mediums relevant elastic constant, and P being density of the medium. C is the speed of sound. so there maybe a mix up of variables. however, i can ask my physics teacher tomorrow. This is really one to make you think.

11. Sep 9, 2008

### matpec

12. Sep 9, 2008

### derek.basler

the frequency that talks about is the wavelength of a particular photon. the frequency of that wave, not a sound wave. common misconception.

13. Sep 9, 2008

### matpec

aaahh damn it ... I got my hopes up for a slight moment

/facepalm

14. Sep 9, 2008

### derek.basler

hahaha its ok! thats what physics is all about really. It was a good idea and you seemed to give it a good, intellectual approach. It sure seemed like it for a second.

15. Sep 9, 2008

### atyy

Yes, you are misunderstanding something. But you also have a good idea. The frequency in the above formula is (very, very roughly) a vibration of the internal state of one molecule, whereas the frequency of the sound wave involves the displacement of many, many molecules. So our "internal frequency" and the "sound frequency" are not related, except through many intermediate steps (involving many other sorts of frequencies also, but let's avoid those).

Let me sketch out the intermediate steps.
1a) By the formula you quoted, the "internal state" frequency is related to the energy of the molecule.
1b) By E=mc2, we know that energy has mass.
2) Because a sound wave increases the energy of the air molecules, it also increases the mass of the air molecules.
3) Since the mass of the air molecules has increased, the gravity it causes will increase. So in this sense, a sound wave has gravity.

Note that we actually did not need statement 1a to reach statement 3. We could have just started from 1b (E=mc2). We currently have a good theory of how to get from 1b to 3. We do not have a good theory of how to get from 1a to 3, and it is an area of active research.

16. Sep 9, 2008

### rbj

sorry to p00p on this, but isn't the question sorta like "Mommy, do kitten-fairy souls go to heaven when they die?"

at least that's what it seems like to me.