# Why interactions between fundamental particles are reversible processes?

I read that all fundamental interactions between fundamental particles are reversible. Why?

P.S. Don't use any heavy Mathematics. I am a 12th grader.

• The question is usually asked in the other sense: "Given that near all microscopic processes are observed to be reversible how does irreversibility arise at the macroscopic scale?"  Some weak processes seem to violate this rule. – dmckee Oct 8 '16 at 2:59
• Yeah, that's Loschmidt's paradox. But I don't see why would the fundamental interactions be reversible? – Mockingbird Oct 8 '16 at 3:58

## 1 Answer

The answer to this is fundamentally the same as the answer to this question posed to Richard Feynman on magnets. Fundamentally, it is an observed fact that the laws of physics, at the smallest scale, are reversible - the same process can be run backwards as forwards with the same probability, in principle. It didn't have to be that way, but it is something we observe.

The technical name for the principle, so you can look for papers that test how well the quantity is conserved, is CPT symmetry - for charge conjugation (flipping to opposite charge), parity (flipping space), and time reversal. As one commenter noted, it is already known that the weak force violates CP, but CPT is still thought to be a good symmetry, so a violation of CP implies a balancing violation of T. There is a mathematical theorem called the CPT theorem, but all that that proves is that CPT has to be conserved if other features of reality hold, and if CPT isn't a good symmetry then at least one of those other features has to not be present, too.

In practice, when a process creates more particles than are put in then arranging the circumstances needed to see the reverse happen become harder, and therefore intrinsically less likely. Consider neutron decay - it produces a proton, an electron, a neutrino, and (ultimately) some photons. In practice we never see that exact process reversed because it is very unlikely for a proton, electron, etc to all be in the same place with enough energy for the reverse to happen. So we observe partial reversal in radioactive decays like electron capture or positron production. There are circumstances where the time reverse of the neutron decay are equally likely to happen as the forward version, but they involve a very large number of the products of the decay zipping about in a high pressure state, a large enough number to balance out the improbability of the three things coming together in the first place. Such a situation, where the reverse of all processes happens as often as the forward, is called detailed balance, and it is a condition needed for a thermodynamic equilibrium where, on average, nothing happens.