Re: Can the measured mass of an isolated system change

According to Newton, you can measure the mass of an object by measuring the gravitational force on another object of known mass. Unfortunately, that concept had to be generalized when relativity came along, and now it's not possible to equate a force to a single number we can call "mass". Instead, the generalization of mass is something called the Stress Energy Tensor, and instead of a single number it's a 4x4 matrix of numbers.

In fact, the only mass that's meaningful in general relativity is rest mass. Using this definition, this means that photons have zero mass -- same thing for electric and magnetic fields. But it does not mean that a collection of photons (or fields) will have a stress energy tensor that is equal to a bunch of zeros! The stress energy tensor takes into account energy and momentum, and photons have both.

So the answer to your question is that if there are electric or magnetic fields, the energy in those fields ends up in the Stress Energy Tensor, which will then determine the gravitational forces. I doubt the measureable force will change if you turn mass into energy inside a closed box, but I guess it might if the energy becomes distributed in a different way (the math gets complicated!).

Hope that helps...