DrZoidberg said:
Then why does one O16 atom have a smaller mass then 8 deuterium atoms?
15.999 vs 16.12
It's the same number of protons and neutrons. The only difference is the binding energy.
With chemical binding energy the same effect happens, it's just much smaller.
The same goes for thermal or any other form of energy. In fact matter only has mass because it is a form of energy.
So yes, energy has mass. Only energy has mass.
Someone might claim that light is a form of energy and it has no mass.
Light has no rest mass, because it can never be at rest. But it does possesses gravity.
Take a box that consists out of perfect mirrors and put some light inside. The mass of the box will increase due to the presence of the light inside.
Agreed, but that's just because when you break apart an oxygen atom, the particles fly apart. Measure the moving products in your rest frame and you'll find that the total rest mass before and after is the same.
As you said, light has no rest mass. However, the common misconception is with the past definition of mass which led to the photon being described with relativistic mass (or mass equivalent), which is just another way of saying energy. This in no way means that if you manage to grab a photon and weigh it on a scale, it has mass.
Photons also do not have their own gravitational field. If you say they do due to being able to be bent around stars and planets, that is due to the curvature of space-time due to the massive object. The path of light is straight in the photon's frame, it just appears deflected to us. (General Relativity) You don't have any situations where photons are attracted to each other. (Which should happen if there were any form of attraction between the photons.)
Regarding your box, I believe that is due to the pressure of electromagnetic radiation on the box. Yes, the mass of the box will increase, but it is not saying that light has mass, just that light with a box in total has a greater mass than just a box. This is rather wonky, I know.
Anyway, if photons did have mass, people wouldn't be spending so much money on the LHC just to find the Higgs Boson, the particle supposedly explaining the mass between photons and matter.
Also... Not all energy is in the form of photons. I can raise a book above a table, which results in the increase in potential energy. You would agree with me that the mass doesn't increase in any frame of the scenario. (Doesn't relate to the topic, but relates to your energy has mass statement.)