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

[Dr Cadaver]

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.

 

[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.

 

[microsansfil]

Hello,

In Wiki "Mass in general relativity" we can read

The concept of mass in general relativity (GR) is more complex than the concept of mass in special relativity. In fact, general relativity does not offer a single definition for the term mass, but offers several different definitions which are applicable under different circumstances. Under some circumstances, the mass of a system in general relativity may not even be defined.

Generalizing this definition to general relativity, however, is problematic; in fact, it turns out to be impossible to find a general definition for a system's total mass (or energy). The main reason for this is that "gravitational field energy" is not a part of the energy–momentum tensor; instead, what might be identified as the contribution of the gravitational field to a total energy is part of the Einstein tensor on the other side of Einstein's equation (and, as such, a consequence of these equations' non-linearity). While in certain situation it is possible to rewrite the equations so that part of the "gravitational energy" now stands alongside the other source terms in the form of the stress–energy–momentum pseudotensor, this separation is not true for all observers, and there is no general definition for obtaining it

This does not seem to be so easy.

Patrick

 

[Dale]

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.

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.

 

[jtbell]

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

 

[mfb]

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.

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

 

[Dale]

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