In general what happens to the atomic mass in each period?

What happens to the mass of the object which emits energy in the form of mass-less particles such as photons?

• Where does the mass go? What does this mean for the total mass of the universe, and the eventual state of all matter in the universe? Does the mass remain in the form of a sub-atomic particle, or is it converted to energy? Can photons convert into particles with mass? Is the universe tending towards a featureless field of massless photons? If this is the case then what is happening to all the mass in the universe, is the universe losing mass? When a photon interacts with a massive object (as in the case of gravitational lensing), is there an equal and opposite reaction by the object toward the photon, or is only the photon affected by the mass of the object and not vice-versa? If photons do have a gravitational effect, what is the significance of the total effect of all photons emitted since the big bang?

• Answer:

  There are a lot of questions here. I'll try to address all of them. What  happens to the mass of the object which emits energy in the form of  mass-less particles such as photons? Where does the mass go? Mass isn't conserved, in general. Energy is conserved. The particle that emits the photon loses energy, and the energy of the photon produced is precisely this missing amount. Does the mass remain in the form of a sub-atomic particle, or is it converted to energy? The mass remains. When an electron emits a photon, it's still an electron. Every electron in the universe has the same mass, 0.511 MeV/c^2. If the electron appears to be "lighter", that's because it's actually energy that gravitates, not mass. The mass hasn't changed, but the energy has. Can photons convert into particles with mass? Sure, two photons can collide and form a massive particle-antiparticle pair. The kinetic energy of the photons is converted into the rest energy of the massive particles (i.e., the energy required to create their masses). Whatever is left over, the particles carry away as kinetic energy. Is the universe tending towards a featureless field of massless photons? Maybe. The ultimate fate of the universe is not known. But a particle can't just keep emitting photons until it has no energy left and ceases to exist---it can only keep emitting photons until it has no kinetic energy left. A single electron can't be converted into photons, because electrons have charge, and photons are neutral. When an electron collides with a positron, both particles may be annihilated and photons produced. However, unless proton decay is possible, there won't be enough positrons around to annihilate all the electrons, and there are too many protons around to be annihilated by antiprotons. So unless proton decay occurs, the universe will never consist entirely of photons. If this is the case then what is happening to all the mass in the universe, is the universe losing mass? When a massive particle and its antiparticle annihilate to photons, mass is lost. Mass is also lost when stars fuse hydrogen into helium; the helium nucleus is lighter than the two hydrogen nuclei combined, and energy is released. Over a long period of time, the mass of a closed system tends to decrease if it can, rather than increase. This is purely for reasons of entropy. When mass decreases, kinetic energy increases, and a state with more kinetic energy has more entropy, because there is more momentum phase space for it to occupy. When  a photon interacts with a massive object (as in the case of  gravitational lensing), is there an equal and opposite reaction by the  object toward the photon, or is only the photon affected by the mass of  the object and not vice-versa? The Newtonian "equal and opposite reaction" follows from the more general principle that momentum is conserved. In every experiment in which we have been able to measure momentum, we have found that it is conserved, and physics would be in great theoretical difficulty if it were not. So, unless we eventually figure out how to measure this reaction (which would be extremely minuscule), it makes sense to assume that, yes, it's there, and yes, it's equal and opposite. If photons do have a gravitational effect, what is the significance of  the total effect of all photons emitted since the big bang? See, e.g, http://hyperphysics.phy-astr.gsu.edu/hbase/astro/expand.html