Anti-matter. You've probably heard of it, and you probably have no idea what it is. But don't worry: the brightest minds in the world are only beginning to understand what it is, and how it works. And once they do? Just... wow.

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Anti-matter is pretty much what it sounds like: matter's alter ego. Every particle that makes up matter as we know it, like electrons and protons, has a twin with exactly the same mass but the opposite electric charge. These are anti-particles. A positron, for example, is just like an electron but it has positive charge. And an anti-proton is just like a proton but has a negative charge. And when an electron and a positron, or a proton and an anti-proton meet, they annihilate. That doesn't mean that they just disappear: nothing in the universe ever disappears, that's the law of conservation of mass. They just turn into something else. In the case of the electron and the positron, for example, they form two photons, massless virtual particles that convey the electromagnetic force.

So you can see maybe why physicists are so fascinated with anti-matter, because the laws of physics are exactly the same for anti-matter as they are for matter. And theories about the origin of the universe, you know, big bang stuff, predicted matter and anti-matter should have been produced in equal quantities. So this leads to some big questions like: Why is everything we see only made of matter? Wouldn't equal amounts of matter and anti-matter just annihilate each other? And why do we even exist?

To get some answers, physicists look at neutrinos, fundamental particles emitted during nuclear reactions. They're tiny, way tinier than electrons, with barely any mass and no electromagnetic charge, so they hardly interact with anything. But they can be observed, even if very briefly, and observing how they are like and unlike their anti-particles could help us understand the very nature of the universe.

See, there are three different kinds, or flavors, of neutrinos: electron neutrinos, tau neutrinos, and muon neutrinos. They're all electrically neutral and have their own anti-neutrino alter egos. Now if you've seen me talk about the four fundamental forces of physics, and you should, you know that 'flavors' are just a cute way physicists have of describing the various states that a particle can exist in. In this case, flavors are basically different states of oscillation. But these states are not stable, no no: neutrinos can oscillate into different flavors over and over again as they fly through the universe. So what physicists are trying to learn is how fast neutrinos and their anti-neutrinos change flavor. It's just one way of understanding how particles of matter may be similar to and different from their anti-particles. If it turns out that they're not just mere images of each other, but instead differ in some fundamental properties, that could explain why the observable universe seems so weirdly asymmetrical. Why matter is so totally dominant while anti-matter is almost non-existent.

So where do we find these neutrinos and their anti-neutrinos? Streaming out of a nuclear reactor, of course! At the Daya Bay nuclear reactor in China's Guangdong province, physicists are tracking electron-flavored neutrinos to see how quickly they change flavors. Since anti-neutrinos have no charge and only a minuscule mass, they normally pass right through everything, like light speed ghosts. So to have any hope of interacting with one physicists put enormous tanks of mineral oil, spiked with a heavy element called gadolinium, in a cave under a mountain near Daya Bay.

Thousands of anti-neutrinos of different flavors pass through the cave every day, and every so often an electron anti-neutrino passes through and hits a proton in the oil, which starts a chain reaction. The collision first forms a neutron, and then a positron. The positron then immediately finds an electron, the two annihilate making a tiny flash of light: a photon. The mineral oil magnifies the flash, which is picked up by photodetectors. At the same time physicists a couple of kilometers away are measuring the amount of electron flavored anti-neutrinos that are passing through their detectors.

What they found is that fewer of these anti-neutrinos are being detected farther away, then are found right beneath the reactor. The difference between these two amounts tells us exactly how fast the anti-neutrinos are oscillating into different flavors.

So what in the name of Paul Dirac's ghost does this have to do with why we exist? Well, with a little more data, physicists will be able to compare how fast all kinds of neutrinos and anti-neutrinos oscillate between flavors, and if those rates are different it'll show that neutrinos and anti-neutrinos aren't exact mirror images of each other, and understanding how they differ will give us a clue as to why the universe seems to favor matter over anti-matter and, why we exist.

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