Physicists Learn to Superfreeze Antimatter (Hint: Pew Pew!)

The thing about antimatter is that there just really isn’t very much of it at all. No one knows why. And making the stuff from scratch is like trying to win a GBBO showstopper. (The theme is “antiprotons.”) Plus, plain-vanilla matter and oppositely charged antimatter annihilate each other if they touch. Very finicky. So the real thing about antimatter is that physicists don’t know very much about it.They have a good theory, though. Actually it’s the theory, the “standard model” that describes how subatomic particles are supposed to behave. Antimatter is supposed to do everything that matter does, only backwards-and-in-high-heels and looks-the-same-except-with-a-goatee. (More formally this is called “CPT symmetry,” as in charge-parity-time, which basically says that if you swap matter for antimatter and time reversed, the new universe would be the same as the current one.) It’s a theory; it needs testing, which is hard—see above. But it’s about to get a lot easier. A big group of scientists centered at CERN, the Swiss particle physics lab, was already the best in the world at making antihydrogen, the antimatter version of hydrogen. Today they published results in the journal Nature showing that they could freeze that stuff down to just fractions of a degree Kelvin—very, very cold. Cold atoms (and antiatoms) are sloooooooow, which makes them much easier to study. The secret to getting antimatter to chill out? Pew pew.

One well-understood way to get atoms to cool off is to slow them down—by shooting them with a laser . This makes more sense than you’d think. Motion, kinetic energy, is also heat. Lasers are made of light, and light is made of subatomic particles called photons. Photons, the wee-est little packets of electromagnetic energy, have momentum but no mass, juice but no oomph. When a photon with the right amount of energy—or the right wavelength, depending on how you want to think about it—hits an atom, that atom absorbs the photon, gains some energy, and then re-emits it. In the process, the atom literally recoils, bounces back a bit.

Now, those atoms are moving around, like in a cloud of gas. That means the actual wavelength of light that’ll do that trick is a little different for the ones moving toward the laser versus the ones moving away, thanks to the Doppler effect. To an observer, light sources moving away from them look more reddish as their wavelength seems to stretch out. That means you can get sneaky. Tune the laser to only push back the atoms moving at a certain velocity—a high one—and then do that a bunch of times, and you slow everything down. You make it all colder.
That all works with the antihydrogen that the CERN team makes too. But antihydrogen is a bucket of trouble. “If I go and buy some cesium atoms, I can buy a laser off the shelf that will do this for me,” says Jeffrey Hangst, a physicist and spokesperson for the Antihydrogen Laser Physics Apparatus project, “Alpha,” at CERN. “But because hydrogen is so light, that photon I need is in the vacuum ultraviolet. That light doesn’t propagate through air. It’s completely absorbed.” The laser light isn’t the green of a laser pointer; it’s the ultraviolet of … well, invisible things.
This, in physics terms, sucks. But the researchers don’t really have a choice. “We can’t make antimatter rubidium or cesium,” says Makoto Fujiwara, a research scientist at Triumf, the Canadian particle accelerator center, and head of the Alpha-Canada group. “But to drive hydrogen, you have to have a laser in very short wavelengths and high energy.” This chillaxatron 5000 has to make light at 121 nanometers, very ultraviolet, and shine that light into a bottle of magnetically contained antihydrogen completely in vacuum.