Antimatter Is Starting to Yield Its Secrets

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Dec 22, 2016
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Is antimatter just like matter, but, well, opposite?

This question has bedeviled physicists for decades. Antimatter is very similar to matter —it’s made up of subatomic particles juts like matter is, but they have an opposite electric charge. So an antimatter electron has a positive charge and is called a positron (a “normal” matter* electron has a negative charge). An antimatter proton has a negative charge, and though I really wish they were called negatrons instead they’re just called antiprotons. Which is still cool.

Other than that, antimatter should behave just like normal matter. You can take an antielectron and an antiproton, put them together, and they’ll form an antihydrogen atom, the antimatter version of a normal hydrogen atom.

But does it behave like a normal matter of hydrogen? We know a lot about how hydrogen goes about its business, to extremely high precision. But antihydrogen is tougher to figure out for two reasons: Making it is hard, and studying it is harder.

Antimatter has an irritating quality: If it touches its normal matter counter part, they annihilate, turning into pure energy. A lot of it. That is, after all, why the Federation uses it as a power source for their starships. Duh.

Anyway, it takes extremely high energies to make antimatter, the kind of energies you get in a particle accelerator, and we see positrons from them all the time (they occur in small quantities naturally, too, when some radioactive elements decay, and in fact are even used for medical imaging in PET scans; the P stands for positron).

High energies means high speeds, so you get these antiparticles whizzing around, and you have to slow (cool) them. But when you do that, then what? If you try to put them in a jar or a vault or a net, they react with the matter, and kaboom!

To overcome this, scientists use magnets. Charged particles are affected by magnetic fields, so if you’re clever —and we humans are— you can create a magnetic bottle to trap the particles. This has actually been done for years.

The tricky part is catching lots of them. Then the next tricky part is catching both antiprotons and positrons. Then the next tricky part after that is merging the two so they can naturally form antihydrogen. Then the subsequent next tricky part after that is to keep that antiatom trapped.

That last part is pretty hard; once a positron hooks up with an antiproton, they form a neutral antihydrogen atom, which is no longer affected by the magnetic bottle. It then falls away, hits the side of your metal container in your lab, and produces a teeny weeny flash of light.

Now comes the next clever bit. An antihydrogen (or even normal hydrogen) atom looks neutral when you’re far from it, but up close it’s still made of a pair of particles, one positively and the other negatively charged. This means it’ll react very weakly to much more sophisticated magnetic fields (an octopole field, for those taking notes at home), which can then be used to trap the antihydrogen before it goes poof.

Still with me? Because now comes the very, very cool part: Once you have an antihydrogen atom trapped, you can start poking at it with science and see what makes it tick.

And now (finally!) we come to why I’m writing this article: Scientists at CERN have, for the very first time, been able to take an optical spectrum of an antihydrogen atom. That’s pretty big news! It shows that to the accuracy of their measurements, antihydrogen obeys the same laws of physics as good ol’ normal hydrogen.

In a normal hydrogen atom, the electron whizzes around the proton in the nucleus in a very specific way (if you want a more detailed explanation, my episode about light for Crash Course Astronomy will cure what ails ya). If you give the electron a little more energy, it jumps to a new level, whizzing around in a very specific but different way. It takes a very precise amount of energy to do that, which we call a quantum, and is where the whole “quantum mechanics” thing comes from.

Anyway, it’s like walking up a flight of steps; you can climb up one step at a time, or two, or three, but not one-and-a-half; if you don’t have enough energy to get up to the higher step, you stay where you are.

For electrons, that energy can be given to it in the form of light. What we call “color” is really the energy of the light; red is lower energy and blue higher. So if you can tune a laser’s color to be the exact right energy, you can use it to zap an atom and ping the electron to the next level, then see if you had any effect on it.

This is precisely what the scientists did. But there’s one final problem: How do you measure what it is you did? How do you know that you had an effect on the antihydrogen when you zap it with the laser?

The actual physics involved is quite complicated, but in the end, one thing that can happen is the laser can excite the positron enough to make it leave the antihydrogen atom. If that happens, then the positron and the antiproton from the atom fall their separate way, react with the normal matter in the chamber, and give off little flashes of light. Those flashes can be carefully measured to make sure they actually did come from the positron and antiproton, confirming the laser zapping worked.

This procedure only works if the laser is tuned to precisely the right energy. What the scientists found (by running the experiment with different laser energies) is that this only works if you use the same exact laser energy as you would on normal matter hydrogen atoms. In other words, antihydrogen and hydrogen obey the same laws of physics!

Well, within experimental error that is, which is a few parts in ten billion. We can do far better with normal hydrogen, which is easier to study; we know the energy we need to a few parts in a thousand trillion. Still, this new result is exciting, and is one of the first steps to understanding antimatter far better. Here’s a short video with Jeffrey Hangst, the spokesperson for the ALPHA collaboration that did the experiments:

We still don’t know everything about antimatter. We have very complex physical models built up over the past century or so on how it should all work, but we really want experimental results to make sure the theories are right. The good news here is that, so far, they are.

But there are still questions. One funny one I like in particular: Does antimatter fall up or down? If you had a tennis ball made of antimatter (warning: Do not try this at home or on any habitable planet) and dropped it, would gravity pull it up or down? Most physicists think it would fall down just like a normal tennis ball, but there are some ideas that gravity would work on it backwards. I doubt very strongly that’s the case, but now that we can actually make antihydrogen atoms we can start testing ideas like that directly.

I know it’s small scale and a little bit weird, but This. Is. So. Amazing. We are probing the fundamental forces and energies that the Universe itself is made of, testing it to see how it works, and what we can make of it.

And I mean that literally. I may have joked about warp drives earlier, but antimatter would make a dandy energy source for rockets even as we use them now, and who knows what else we’ll learn, what science fictional technologies await as we uncover more of what antimatter is telling us.

Remember: The computer (or phone or tablet or whatever) on which you are reading these very words is possible because we figured out how electrons work, how atoms work, even that they exist at all. What will we be able to do as we find out even more about the quantum Universe?

* Calling the kind of matter we see all around us and from what we’re made “normal” sounds vaguely racist to me. Or maybe fermionist. But in this case it’s OK; the vast majority of stuff we can see in the Universe is normal matter, with very little antimatter. The reason for the extreme lack of antimatter is still a mystery.