Weight of Hot Body: Balance in Thermal Equilibrium?

[bernhard.rothenstein]

Consider a spherical hot body having a temperature T, located on one of the pans of a very sensitive balance. It is equilibrated puting a given mass on the other pan of the balance. The temperature T is lower then the ambiental one. The balance is in mechanical equilibrium but out of thermal equilibrium. Does the balance remain in a state of mechanical equilibrium when its temperature equates the ambiental one? The transmission of energy is isotropic.

 

[pervect]

The balance does not remain in a state of mechanical equilibrium - this follows from argments presented in

http://arxiv.org/abs/gr-qc/9909014

According to the general theory of relativity, kinetic energy contributes to gravitational mass. Surprisingly, the observational evidence for this prediction does not seem to be discussed in the literature. I reanalyze existing experimental data to test the equivalence principle for the kinetic energy of atomic electrons, and show that fairly strong limits on possible violations can be obtained.

It's also been discussed a lot here on the board, you can probably find the past discussions if you look.

Carlip's formula for "gravitational mass", the intergal of density + 3*pressure, is basically the flat-space version of the more general Komar mass formula which applies to any static metric.