We May Soon Know Why Antimatter Lets the Universe Exist

Why is antimatter so scarce in the universe when it could be annihilating your face right now? It wouldn’t take very much antimatter to do it. A collision of matter and antimatter is a perfect release of energy, while most of the things we do to release energy from matter are very far from perfect. That matter-antimatter annihilation should give up energy with 100-percent efficiency — disregarding the painful and energy-consuming process of making antimatter — whereas the best nuclear reaction out there gives up about a third of that. So it would actually take a pretty small scrap of antimatter to annihilate most of the faces in your city. But it doesn’t actually happen because antimatter occurring naturally is very rare and, because antimatter annihlates when it touches matter, always, you can really only contain it within magnetic fields, and only then after you produce it via high-energy collisions in a particle accelerator. To produce and contain enough of it in such a way that it could be used as a weapon against faces or anything else would take lifetimes. Consider that when it’s made at CERN, it’s done particle by particle. More interesting is its existence in nature, or its mostly non-existence in nature, which is a big mystery and one that researchers at Chicago’s Fermilab have gotten us closer to solving, according to new results released this week.

Scientists at CERN and Fermilab are interested in antimatter for reasons less having to do with sci-fi weaponery and spacecraft propulsion. Understanding antimatter is understanding why the universe exists. It shouldn’t, is the thing. Or it should have existed only very, very briefly before annihlating. Antimatter is a natural product of matter creation. When you create matter through high energy particle collisions (and some particle decays), like those that happen CERN and in our atmosphere and after the big bang, you also get antimatter in exactly equal amounts. That bit of antimatter, if it’s not trapped and contained, goes off and finds a particle of regular matter and they annihilate together into gamma rays.

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After the big bang, the universe existed as a super-high energy miasma. It was pure energy, no matter. The universe’s matter came into being slightly later, formed from energy via E=mc^2. An equal amount of antimatter should have formed, obliviating the universe-of-matter before it could do much of anything, much less give way to things like planets and human beings to study antimatter. It didn’t, of course, and those human beings have found that antimatter is just like matter in every way, except for charge and spin (the direction in which its particles spin). You could build up a human being and all of the usual laws of physics would apply, but electrons would have a positive charge, protons would have a negative charge, and so forth. A positively charged electron is called a positron and it’s actually a pretty normal thing used in medical PET scans.

The why of antimatter is kind of a tough concept. One way it’s explained is via “hole theory.” Simply, when a negatively charged, postive energy electron is formed, it leaves an indentation — in the sea of negative energy electron states waiting for an energy boost via a photon — that functions exactly as a positively charged electron. Like, if you can imagine digging a hole, you will have not just a hole as a result, but a pile of dirt mirroring the hole. The hole would be the positron, and the pile the electron. You can’t get dirt without leaving a hole.

Digging metaphor aside, antimatter started as a mathematical construct in 1928. It’s an implication of another very important and famous piece of math called the Dirac equation. It implies the potential negative energy states in its solutions, and thus how positively-charged electrons could come from them. Something like that. Not long after the time Paul Dirac was working all this out mathematically, Carl Anderson found something strange happening to particles moving through his cloud chamber. There was a particle leaving a trail that should be an electron, but it curved in relation to a magnetic field as if it was positively charged. This was the positron right there visible to the naked human eye.

There should be some slight difference in matter and antimatter explaining the reason matter currently dominates the universe. In some way to-be-determined, the universe likes matter more than antimatter. What physicists are looking for is called CP violation, some sort of assymetry between anti- and “normal” matter. What we’re looking for is differences in rates of particle decay — in which particles lose energy and become other sorts of particles. We want to see one form of matter favoring a certain end-state. This has been found a few times actually but only in very small amounts, not enough to explain the universe we see. The asymmetry needs to be higher.

The CDF detector at Fermilab looks at D0 meson particles, very short-lived particles made up of a quark and antiquark, which quickly decay into pairs of other particles. By looking at these new particle pairs, physicists can discern the rates of decay for each, and how those rates might differ. These new results point to a difference in decay rates of about six-percent, which, coupled with similar results from last fall, give us a three sigma result. In other words, there’s only about a one-in-10,000 chance the result is a fluke. An actual discovery requires a five sigma result, with a less than a one-in-a-million chance of being a fluke.