# What kind of energy gravitates, and why?

When listing energies for the purposes of keeping track of conservation, or when writing down a Laplacian for a given system, we blithely intermix mass-energy, kinetic energy and potential energy; they are all forms of energy, they all have the same units, and so this looks OK. For example, in the LHC, turning kinetic energy into new particles of mass-energy is routine. We just converted "energy which does not gravitate" (kinetic energy) into "energy which does gravitate". Isn't it a bit peculiar that this same thing called energy can manifest into two different kinds of forms - those forms which gravitate, and those which do not?

How about potential energy? It would be of course ridiculous to calculate your potential in relation to the galactic centre and expect that huge (negative, by convention) quantity of energy to gravitate; and yet if we allow its conversion into kinetic energy, and thence into particle creation, lo and behold we end up with something that does gravitate.

We know that the massless photon gravitates, because it can be "bent" around a star, per GR. A photon also expresses energy in the form E = p c. So clearly finite rest mass is not a requirement for certain forms of energy to gravitate.

So what's the rule here? When does energy gravitate, and why? Isn't it all supposed to be "just energy"?

Then there's the flip side of the equivalence principle - inertia. Do fields have inertia? - they do gravitate, so if they possess no inertia, doesn't that break EEP?

-

This is a topic many get confused on. Energy(more specifically, the energy-momentum tensor, but in non-relativistic cases, the momentum is negligible compared to the energy aka rest energy + kinetic energy) is what gravitates, NOT mass, a common misconception. If fields carry energy(such as electromagnetic fields), then they gravitate.

ALL types of energy(the non-potential forms, at least) gravitate. According to General Relativity, the current widely accepted theory of gravitation, gravitation is coupled to the energy momentum tensor, which basically includes all forms of energy including contributions from momentum, pressure, rest energy, stress, and kinetic energy. However potential energy is not included in the energy momentum tensor so is non gravitating. So take your example, in LHC, the 'kinetic energy' does gravitate and the mass also gravitates, so there's no change in 'degree of gravitationess'. Note that on such small scales, gravitation is so small that its highly negligible.

Now, I don't think it would be meaningful or is aware of any general definition of inertia for fields, so I suppose it doesn't exist. Answering your question: an equivalent formulation of the EEP is that on local scales, acceleration is indistinguishable from gravitation. As long as the field behaves this way, then it doesn't break the EEP.

-
Hi, can you refer to some paper or book which proves (or at least mentions) that potential energy doesn't contribute to curvature of stacetime. – user10001 Dec 4 '12 at 13:47
Will you accept physics.stackexchange.com/questions/45145/… ? – namehere Dec 4 '12 at 14:21
I can't see any proof or reference for a proof in your link. I am not saying that your statement is wrong. But if its true then, given that relativity is now a well established subject, there must be some good references where its proved. I actually do not understand how potential energy is a different thing from usual energy. I always thought that energy momentum tensor includes it all. – user10001 Dec 4 '12 at 14:47
Well, as I mentioned, the energy momentum tensor includes momentum, pressure, rest energy, stress, and kinetic energy. However, it does not include what we call potential energy. So this fact is inherent in General Relativity. Look up the energy momentum tensor on wiki. You won't find potential energy as one of its components. – namehere Dec 4 '12 at 14:54
00 th component of energy momentum tensor is Hamiltonian which includes both kinetic and potential energy. – user10001 Dec 4 '12 at 15:08

I'm guessing you're asking about what is included in the $T_{00}$ element of the stress-energy tensor. If so, potential energy in the form of pressure or shear stress go into the $T_{11}$ to $T_{33}$ entries. Kinetic energy is a bit more complicated. It does go into $T_{00}$ because that contains the particle energy:

$$E = \sqrt{p^2c^2 + m_0^2 c^4}$$

but it also goes into the other entries because associated with the kinetic energy is a momentum flux. The Wikipedia article on the stress-energy tensor explains how to calculate it for an isolated (moving) particle.

Re your last question: massless particles can carry momentum (photons do) if this is what you mean by inertia. You obviously can't assign them a mass using Newton's first law because they always travel at $c$ so you can't accelerate them.

-
So I'm getting a discrepant description of the stress-energy tensor, although you both seem to agree that gravitational potential energy does not in and of itself gravitate, which would be a form of self-action I suppose. I found the commment about the efficacy of inertia in the case of fields/photons moving at c very useful. EEP clearly cannot be applied here. – Andrew Palfreyman Dec 4 '12 at 12:03
@AndrewPalfreyman I don't think our descriptions are really that different. I just included the different forms of energies in the form of a list. – namehere Dec 4 '12 at 12:14
@JohnRennie I thought kinetic energy went into $T_{00}$!? – namehere Dec 4 '12 at 12:15
@namehere - If kinetic energy went into $T_{00}$ we would see a heavy object that was travelling fast turn into a black hole. – John Rennie Dec 4 '12 at 14:56
@JohnRennie Really, if it went into momentum the same thing happens. – namehere Dec 4 '12 at 14:59

The generator of the gravitational field is the "stress-energy-momentum" tensor $T^{\mu\nu}$, whose components are energy $T^{00}$, co-momentum $T^{0j}$ and co-stress $T^{ij}$ [*]

By symmetry considerations, anything with a non-zero $T^{\mu\nu}$ will feel gravity. In the non-relativistic limit co-momentum and co-stress vanish and energy reduces to $mc^2$, which explains why masses appear in a non-relativistic description of gravity.

All energies contribute to gravitation. I do not know why you believe that kinetic energy "does not gravitate". In fact, the kinetic energy of a photon $E = pc$ is responsible for the light bending effect.

Inertia is an ill-defined term and I cannot answer to your last question without knowing what you mean by inertia.

[*] Many references do not care about dimensions and refer to "momentum and "stress".

-