The stuff we deal with every day is made of matter. It is inescapable. However, it wasn’t always this way. When the universe was vey young and very hot, there were large amounts of both matter and antimatter present. Particles of antimatter have the same mass as their matter counterparts, but the charges are reverses. So, for example, the electron encountered in ordinary matter has a negative charge, while the anti-electron (or positron) has a positive charge. When such counterparts meet, they annihilate each other releasing high-energy radiation in the form of gamma-rays. In experiments, matter and antimatter particles are always made in equal amounts, satisfying sensible rules such as charge balance. (if you start off with no net electrical charge before making any particles, after you’ve made some new particles, their charges must still add up to zero.)
Given these rules and these observations, you would think there would be the same amount of matter and antimatter in the universe, but in fact, that’s not correct. With rare and fleeting exception, everything we observe in the universe is made of matter. There is no net antimatter left from the early days of the universe. That means that unequal amounts of matter and antimatter was made in the early universe, and only the dominant matter survived to the present day. This suggests that, despite the apparent identical behavior of matter and antimatter particles (other than charge), there must be some kind of deep, fundamental difference between the two. For this kind of imbalance to have occurred, physicists believe that there must have been a violation in two of what they consider to be the three fundamental symmetries of the universe.
What constitutes a fundamental symmetry? These are conditions whereby you obtain the same experimental results when you reverse conditions, and most of these are kind of obvious. The three consist of time (T), charge(C), and parity (P). Taking them in reverse order, parity symmetry asks, if you set up your experiment as the exact mirror image of what you did, would you get the same result? That should be a clear yes. Charge symmetry asks, if you reverse the charges of the particles in an experiment, would you get the same result? Coulomb’s law governing electrical forces gives the same result if you reverse charges, so again, we expect the answer to be yes. Finally, what would happen if we reverse time, and run the experiment backward? It turns out that the kinematic laws of physics run backwards the same way they run forward, so the answer again would be that you would get the same result.
But in order to explain the imbalance of matter and antimatter in the early universe—resulting in a present-day universe consisting of matter only, two of these symmetries, C and P, must be violated. So, there has been a decades-long search for experimental evidence of CP symmetry violation. In the analysis of a decade’s-worth of data from experiments observing neutrinos, those ghostly near-massless particles that rarely interact with anything, scientists in Japan believe they have observed a hint of just such a violation strong enough to explain why the universe is made up of just matter.
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