Gravity

[steve_bank]

I only have an overview of relativity. A few questions.

I lift a rock and work is done against gravity. I let it go and the rock drops to its original position and the energy added by my muscles shows up as heat and deformation in the rock and ground.

Gravity can do work. Considering Laws Of Thermodynamics where does an energy loss show up in gravity?

Gravity is proportional to mass, and mass is made up discrete atoms, so whatever gravity is I would think it originates from individual atoms. I would think that when gravity does work there has to be an energy debit in the atoms.

 

[bilby]

whatever gravity is I would think it originates from individual atoms

Not really, if it's curvature of space-time in response to mass-energy, it's no more (or less) about individual atoms, than it is about whole rocks, or about subatomic particles.

What matters is total mass-energy in whatever you're observing, and the shape of space-time in its vicinity, not how finely you divvy it up.

OTOH, if gravity is a force similar to electromagnetism and nuclear forces, it is carried by "gravitons", which would be a family of gauge bosons, and from a QM perspective differs from the other three forces only in the details. Gravitons would need to interact with any concentration of mass-energy, including rocks, atoms, and sub-atomic particles.

 

[Loren Pechtel]

I believe it's reflected in the same equation as everything else: e=mc^2. In turning the potential energy of the stone into actual work you lose the amount of mass that the liberated energy corresponds to. In normal realms this is like most everything else with Einstein's stuff--either completely undetectable or detectable only in extreme precision environments. The classic example of the latter is GPS. Normal civilian GPS requires precision on the order of a handful of nanoseconds--and at that realm a Newtonian answer will get you lost. However, in truly extreme environments the gravitational energy becomes a big factor. Neutron stars have a significant fraction of their mass in gravity.