Can a Planet Made of Glass Generate Free Energy?

[Pony]

As far as I know, if vacuum antigravitates, it makes the light that travels through it redshift, e.g. in a very large empty box with mirror walls, light will just lose energy.

I am interested in the reverse effect, if we had a very large box or planet with mirror walls, that is filled with something that gravitates, will that make the light inside of it blueshift, and generate free energy (negentropy)? ((Thus allowing mankind to exist forever, at least according to the currect state of physics.))

 

[Nugatory]

As far as I know, if vacuum antigravitates...

It doesn't, and without that premise the rest of the question makes no sense. Light can be blueshifted as it moves down in a gravitational well, but there is a corresponding redshift on the way back up so no net energy gain from bouncing the light back and forth through the planet.

As it is based on a mistaken premise this thread is closed.