Matter vs antimatter1/6/2023 ![]() This created a small surplus of matter, and as the universe cooled, all the antimatter was destroyed, or annihilated, by an equal amount of matter, leaving a tiny surplus of matter. Scientists believe that in the very hot and dense state shortly after the Big Bang, there must have been processes that gave preference to matter over antimatter. ![]() Over the next few decades physicists found that all matter particles have antimatter partners. But in 1932 Carl Anderson discovered an antimatter partner to the electron – the positron – while studying cosmic rays that rain down on Earth from space. At first, it was not clear if this was just a mathematical quirk or a description of a real particle. The existence of antimatter was predicted by physicist Paul Dirac’s equation describing the motion of electrons in 1928. So what happened to it? Using the LHCb experiment at CERN to study the difference between matter and antimatter, we have discovered a new way that this difference can appear. But today, there’s nearly no antimatter left in the universe – it appears only in some radioactive decays and in a small fraction of cosmic rays. The problem is that would have made it all annihilate. If antimatter and matter are truly identical but mirrored copies of each other, they should have been produced in equal amounts in the Big Bang. When an antimatter and a matter particle meet, they annihilate in a flash of energy. All the particles that make up the matter around us, such electrons and protons, have antimatter versions which are nearly identical, but with mirrored properties such as the opposite electric charge. The velocity eigenstates of the Dirac equation are known as the zitterbewegung theory of the electron.It’s one of the greatest puzzles in physics. You then have a 50% chance of being right, which is better than if you assign particle / anti particle labels to all the elementary particles at random. If you can expand the Dirac equation so that it contains solutions for more particles, you can look at the velocity eigenstates to divide the elementary particles into two sets, and then arbitrarily call one set particles, and the other set anti particles. #Matter vs antimatter how toHowever, the fact that the Dirac equation solutions can be put into eigenstates of velocity provides a clue for how to distinguish between particles and antiparticles in a non arbitrary (well, less arbitrary) way. This was cause for much debate back when it was a new idea. One of the mysteries of the Dirac equation was that it is natural to put together eigenstates of velocity. ![]() If we could distinguish between particles that travel forwards in time from particles that travel backwards, we could distinguish between particles and antiparticles. In Feynman's interpretation, antiparticles are particles traveling backwards in time. On the other hand, protons are very common in our neck of the woods, antiprotons are not, so that's what we call them. And the pi+ and pi- are equally common, so we can't decide whether to call them matter or antimatter. These are composites, and they are made up of partly what we call quarks and partly what we call anti quarks. And there are things that are on the line, such as the pions. What we commonly see in our chunk of spacetime is called "matter", rare stuff we call "antimatter". (but pardon me if it has been, and I just missed it.) I think this should be a very elementary question, but I haven't seen it addressed anywhere before. Then the other leptons (including neutrinos) have to fall in on the electron into their proper matter/anti-matter formation. Defining the electrons as matter makes sense. Everything around us is pretty much made up of protons and neutrons (with quarks we defined as matter) and electrons. But the universe certainly gives us a hint. ![]() I'm not sure if defining the quarks that way absolutely forces us to define the leptons as we do. ![]() You can't have an up and a down quark in a meson, only the up and the anti-down (pion +) or the anti-up and the down (pion -). Mesons have one quark and one anti-quark. Baryons are comprised of three quarks or of three anti-quarks. Once we make that choice, the other quarks fall into their place. We choose that the +2/3e charged quark is the up quark and the negative one is the antiquark. Once we have a set definition of positive, we can choose how we want to define anti-matter and matter (and how we define right-handed and left-handed). It is the charge carried by the lepton preferentially produced in the decay of the long-lived neutral K meson. Griffiths chapter on CP violation (4.8) gives a convention-free definition of positive charge. It's the weak interactions (flavordynamics) not the strong (chromodynamics) that violate CP, thereby allowing us to distinguish anti-matter from matter. ![]()
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