Does potential energy really exist or is it just a concept?

Does energy have mass?

Does a photon have mass? I seem to remember that it doesn't but that doesn't really make sense given that they exist...

A while ago I saw an article on some grad student who had created a laser that cooled things down, how did he do that?

does potential energy really exist or is it just a concept?

does energy have mass?

does a photon have mass? I seem to remember that it doesn't but that doesn't really make sense given that they exist....

a while ago I saw an article on some grad student who had created a laser that cooled things down, how did he do that?
A) Yes, Potential Energy does exist. Just like air, you can't see it, but you can prove that it exists. Classic example of it is the pendulum experiment, where amplitude of the motion is estimated using PE.

B) You can't define mass of an energy. Energy is a capacity to do work, it's a number which has different units, but definitely not kg/lbs/N. Its like you asking for a mass of your affection towards your wife/gf/kids etc. In classical MetaPhysics this is called as "Conceptual Inferences", you can find more information about this in Eastern Physics, especially Vedantas, an enceinte Indian manuscripts.

C) Don't have enough expertise to answer it.

D)Please refer Cyclic Refrigeration in any Thermo book. I think the basic concept is use of a heat at one subspace to remove heat from another. But, I may need more details, may be an article itself.

Hope this helps.