# On the origin of gravity. (Mass, or energy density?)

1. Aug 23, 2014

So I have been reading about the nature of gravity, and I have become a little confused as to how gravity is produced. In some videos and books, they state that it is the mass of an object that generates gravity. However, others say that it's not so much as mass, but the density of energy/mass in a given volume of space. I don't know if the two ways of looking at it are one in the same or different. Any help on this would be most appreciated.

2. Aug 23, 2014

### phinds

Density doesn't "produce" gravity in any sense, but it CAN tell you something about the possible gravity on, for example, the surface of a large object. If it's a gas giant it would have one gravity at the "surface" but if it were the same size and all rock, and thus MUCH denser, it would have much higher gravity.

The other way of looking at the same situations shows you that it isn't the density at all. It's just a matter of how much mass there is under you. If you are a the "surface" level of a gas giant, and you change the situation to being exactly as far from the center of ANY equivalent mass smaller than the gas giant, the gravity at that point will be the same so you see that the density doesn't matter.

3. Aug 23, 2014

### microsansfil

Hello,

In Wiki "Mass in general relativity" we can read

This does not seem to be so easy.

Patrick

4. Aug 23, 2014

### Staff: Mentor

The source of gravity in GR is the stress energy tensor. For ordinary gravitating objects, like planets and stars, the dominant term in the stress energy tensor is energy density, and the dominant portion of energy density is mass.

5. Aug 23, 2014

### Staff: Mentor

In GR, the "source" of gravitation is the stress-energy tensor, which has components related to both energy and momentum.

http://en.wikipedia.org/wiki/Stress–energy_tensor

The notion of "mass" does not enter directly into the stress-energy tensor or the Einstein field equations of GR. There are various kinds of mass which can be derived from the stress-energy tensor, depending on how you want to use it.

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

6. Aug 23, 2014

### Staff: Mentor

And the dominant portion of mass in ordinary objects comes from energy density in hadrons not associated to elementary particle masses.

7. Aug 23, 2014

### Staff: Mentor

Hmm, yes, I forgot about that but you are absolutely correct.