All energy has mass. When folks talk about the energy released from a nuclear reaction, they are talking about energy that was beforehand able to be measured on a scale because it was confined to the nucleus of an atom; that energy was in a sense holding the nucleus together (but don’t take it all that literally). When the nucleus changes, the resulting particles appear to have less mass, but if you were somehow able to contain all the products in a closed system the system’s mass would not change; the mass would be less confined, but still there.
Einstein’s famous equation in its basic form tells you how much mass energy has. The amount of energy a single photon has can be transformed to zero*: by traveling nearly as fast as the photon directly away from it. In doing so the photon’s wavelength increases due to (relativistic) Doppler effect and thus loses energy as a photon’s energy is inversely proportional to its wavelength. If you have two photons going in exact opposite directions though, there would be a corresponding increase in energy in the other photon. I don’t know how the mathematics behind it works out, just the vague description, but in such a system there is, in some sense, a well-defined positive invariant mass even though it is composed of particles that taken separately have zero invariant mass.
It is probably drastically oversimplified, but it’s how I’ve come to understand the nature of it.
*To be precise, can be transformed to be less than any given positive number. When that happens, we can pretend it actually can go to zero even though such a situation would require moving at the speed of light.
Both Pasta and Chronos are honest to God physicists. Believe me, they know of which they speak. Although Pasta has some trouble defining mass. (Just kidding)
Some of the new battery chemistries like the lithium-air battery should actually get lighter when you charge them, since the chemical reaction that generates electricity involves combining lithium with atmospheric oxygen, which would mean the battery would get heavier as it discharged.
Wouldn’t this then also apply to standard chemical reactions, exothermic reducing mass and endothermic increasing?
Or for that matter, any material changing temperature?
I think the problem for us who aren’t as deep into physics, we understand the conversion of mass to energy in a nuclear reaction due to the change in atomic weight. But this idea that additional energy adds mass without a change to the atomic structure is foreign. It means the energy has mass without being converted to a thing (like a particle). That’s something I wasn’t taught in basic physics class. Or it was and I forgot.
The energy isn’t converted into new particles (or at least, not real particles) in a nuclear reaction, either. The number of protons and neutrons is the same before and after.
