# Does gravity acting on a resting object produce any heat?

Let's compare two systems.

System 1: A box is completely isolated. There are no forces acting on that object, and no interactions of any kind with other objects, waves, etc..

System 2: The same box is isolated from other systems. It is only affected by the force of gravity, let's assume with the same acceleration as that of gravity on the surface of Earth, and the normal force of a horizontal surface. The object is at rest and will never accelerate.

I am aware that the force of gravity's effects are negligible at the quantum scale. However, negligible does not necessarily mean non-existent. I was wondering if within the body of the box in the second system there was any more heat energy than in the body of the box in the first system due to the acceleration imposed on the atoms by gravity.

The concept stems from the idea that any contact forces imposed on an object will increase that object's heat energy if nothing else affects it. I could not find anything by a google search that suggested that at-a-distance forces were any different.

I understand that this is rather conceptual as measuring such a difference is as I understand impossible, but I'd like to know what current theory would imply.

• If the object is not a point particle, as you imply, then tidal forces can produce heat. – auxsvr Nov 11 '15 at 23:37
• @auxsvr I edited the original post to say "box" rather than object, which does imply a point particle. Hopefully that satisfies all the criteria for tidal forces. Thank you. – Striker Nov 11 '15 at 23:43
• Oh umm, I think you missed a portion of the statement. I wrote "It is only affected by the force of gravity... and the normal force of a horizontal surface". That's fine, correct? – Striker Nov 11 '15 at 23:52
• Yes, you're right, I missed that line. – auxsvr Nov 12 '15 at 12:33

If the object is at rest it would imply that gravity transfer heat without an increase in potential energy, and there are no other forces that produce work. This would violate the conservation of energy.

Regarding your comment "any contact forces imposed on an object will increase that object's heat energy": this is incorrect, friction only results in heat exchange when there is relative motion between the surfaces, not when they are at rest relative to each other.

The acceleration of gravity is the weight of the object on the surface. If the structure, either of the object or the surface, is deformed by the weight, then part of the gravitational potential energy of the object will turn into kinetic energy of surface moleucles and therefore heat, and this will be shared between the object and the surface. In this sense gravity generates heat as long as the deformation proceeds. When a stable molecular configuration is reached on both surfaces no more heat will be generated. Example: a large weight on a wax surface.

A body at rest in a gravitational field ((as standing on the surface of the earth) is the same as a body being accelerated. A free falling object is equivalent to not accelerating (The equivalency principal). The Accelerated one at rest would heat up.

OK so maybe I didn't articulate that so well please let me try again. The equivalence principle states that gravitational and inertial forces are similar in nature and often indistinguishable. I was attempting to agree with the answers above that a body at rest in a gravitational field would heat up. My point was that of course it would because a body at rest in a gravitational field is equivalent to a body being accelerated as with the man in the elevator being accelerating upward. And of course Accelerated bodies heat up.

• Why do accelerated bodies heat up? – DilithiumMatrix Nov 13 '15 at 19:38
• This is not a case were assuming the general relativistic view helps you (for the reason expressed in @DilithiumMatrix's question). – dmckee --- ex-moderator kitten Nov 14 '15 at 23:52

Does gravity acting on a resting object produce any heat?

The interior of the earth is hot. Only the gravity can be the source of such a heat.

• Ahh, so the box example is simply a rescaled version with less pressure above the box. Thank you. – Striker Nov 11 '15 at 23:54
• No, this is wrong. @KennyDuran. The heat on earth is a combination of residual heat from when it formed (associated collapse), and radioactive heating. See this nice post. – DilithiumMatrix Nov 13 '15 at 19:37
• The source of the heat inside earth is the "gravitational contraction of the Earth's interior" plus the heat generated by unstable isotopes. Gravity generates heat (see the Kelvin–Helmholtz mechanism). – Energizer777 Nov 13 '15 at 21:15
• There certainly is gravitational heating (or more generally Virial heating), but it occurs because of a net motion of mass toward the center, and accordingly doesn't answer the question about an object at rest. – dmckee --- ex-moderator kitten Nov 14 '15 at 23:51
• @DilithiumMatrix Thanks for the shout-out, but a better link is physics.stackexchange.com/a/154514/520. David Hammond actually does this stuff. – dmckee --- ex-moderator kitten Nov 14 '15 at 23:54