###### By Don Lincoln, Fermilab

## The Standard Model can explain essentially all of the matter of the universe. It only takes two quarks to explain our universe and two leptons, specifically the electron and one neutrino. The others are unstable and exist for only a short amount of time. So, all of matter is explained. But you might have heard of another substance, which is called antimatter.

### The Carbon Copy of Matter

Antimatter, which is an entirely real thing, sounds more like science fiction than science fact. It is a carbon copy of ordinary matter, meaning that there are antimatter quarks and antimatter leptons. Kicking it up a notch, there could also be antimatter atoms, molecules. it’s better to say that there could be antimatter oranges and cats and people. Such things have never been seen, but under the right circumstances, it’s completely possible.

Antimatter is a destructive substance. If you combine matter and antimatter, they eradicate one another, developing pure energy. And we’re not talking about a little energy. It’s a lot. You can figure that out yourself pretty easily by using the most famous equation of physics: *E* = *mc*^{2}.

If you pick a small amount of matter, say a gram, which is equivalent to a paperclip or so, and combine it with an equal amount of antimatter, you get two grams times that huge 10 to the 17th power. The net result is an incredible amount of energy, about 10 to the 14th joules, which is roughly equivalent to the energy released in the Hiroshima nuclear explosion.

### What’s the Proof?

How is it that we scientists are so sure it exists? Well, the story starts during the heyday of the development of quantum mechanics. Einstein developed his theory of special relativity in 1905 and quantum mechanics in the 1920s. But there was a real problem with the original equations of quantum mechanics, and it is that those equations did not incorporate special relativity. In order for those two seminal advances in physics to coexist, the two had to be merged somehow.

The person who achieved that was a British physicist by the name of Paul Dirac. He merged the two worlds of relativity and quantum mechanics into a single equation, which we now aptly called the Dirac equation.

### Dirac Equation

Now, the Dirac equation is sort of middlingly hard from a mathematics point of view. However, we’ll jump to the final answer, which illuminates the key point. When all is said and done, the equation that encapsulates the solution to Dirac’s equation boils down to something pretty much like the equation squared equals one.

So, that’s not too hard at all. If you want to do the final step of the solution, you take the square root of both sides. Take the square root of equation squared and you get equation. Take the square root of one and you get one. But when you take the square root of a number like one, there is not just one answer. There are two.

Not only does one squared equal one, but negative one also equals one when you square it. So, there are the two answers, which are, perhaps belaboring the point, equation equals plus one and equation equals negative one.

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### Dirac’s Equation and the Discovery of Positron

Dirac was very serious about his equations and he insisted that not one, but both solutions were meaningful. The positive equation described the behavior of the electron which, kind of confusingly, had a negative electrical charge. But what did the negative solution describe? It was possible to determine that the negative solution described a particle with positive electrical charge.

Now, initially, Dirac didn’t quite know what to make of it, but he surmised, completely sensibly, that the negative solution described the proton. But the proton has a different mass and that means that, while the equation was perfectly symmetric, the physical situation wasn’t. The situation resolved itself just a couple of years later when a bright young postdoc at Caltech by the name of Carl Anderson made an interesting discovery.

### The Discovery of Antimatter

Anderson set up a cloud chamber and noticed a positively charged electron. He sent his result to *Physical Review*, which is a prestigious physics journal, and a journalist by the name of Watson Davis proposed the word *positron* to describe the new particle as sort of a portmanteau of the words positive and electron. The positron was exactly what Dirac’s theory had predicted. Antimatter had been discovered.

So, there you have it, antimatter is real. We have found antimatter equivalents for all known subatomic particles, from the familiar proton, neutron, and electron, to all of the exotic ones we have found in particle physics experiments. We’ve recently even made very simple antimatter atoms. What we call hot antihydrogen was first made in the 1990s.

Cold antihydrogen was made in 2002 at CERN, a laboratory just outside Geneva, Switzerland. We’ve also made antihelium, but so far that’s it. Antihelium is just very hard to make in particle accelerators because you need to make two antiprotons and two antineutrons at the same time and at the same place and with exactly the right energy that allows them to assemble into an antihelium nucleus.

It’s technically hard. But that’s just an engineering limitation. In principle, we could make anti-anything, including anti-people. But that at least is just science fiction and will certainly be so for probably forever.

### Common Questions about How Antimatter Was Discovered

**Q: What is antimatter?**Antimatter is the mirror image of ordinary matter. It means that there is antimatter for everything thing from the tiniest particles such as quarks and leptons to the more tangible and visible things such as fruits, animals, and even people.

**Q: What was Dirac equation’s relationship with discovering antimatter?**The story of discovering antimatter started with Paul Dirac’s effort to merge the two equations—relativity and quantum mechanics—to make one single equation now known as the Dirac equation. This equation made one positive and one negative solution in which the positive described the behavior of the electron and the negative, which was resolved many years later, described the behavior of the positron.

**Q: Who was Carl Anderson?**Carl Anderson was a young postdoc who continued and resolved what Einstein and Paul Dirac have started. He played a major role in discovering antimatter by making an interesting discovery using a cloud chamber to notice a particle with a positive electrical charge which was later named positron, short for positive and electron. Positron was tangible proof of the existence of antimatter.