It is not easy. “Hydrogen is really hard to cool with a laser, because of those bloody UV lasers,” Hangst says.
The laser has to be precise for a bunch of different tasks. “You have to control the frequency very precisely so that we can do the Doppler shift,” says Takamasa Momose, a chemist at the University of British Columbia and one of the builders of the laser. In addition, the laser must produce enough energy in its pulses so that cooling does not take forever.
But it is not impossible. The team built it all. And when they shot the antihydrogen, it cooled like hydrogen would, already a good sign.
To be clear, it’s not like you can just stick a thermometer in the magnetic trap. You measure this energy differently. Last year, this same team did spectroscopy on their antihydrogen, by analyzing it by looking at the spectra of light it emits. Slower atoms emit a narrower spectrum, and when the researchers looked at their post-laser atoms, that’s exactly what these cold atoms did. They also tested their new results by checking how long it took for their cooled atoms to bounce out of the group and hit the back wall of their container (where, yes, they annihilate each other). This is called “time of flight”, and cooler atoms should take longer. They did it.
Just like you can’t exactly measure their temperature, you can’t point a radar gun at antihydrogen atoms either. Antihydrogen typically rotates at around 100 meters per second, says Fujiwara, and ultra-cold atoms move at around 10 meters per second. “If you’re fast enough, you could almost grab the atom as you walk by,” he says. (It would wipe out one of your atoms, but you’re tough.)
At this point, it’s reasonable to ask if it’s all worth it. Who needs very slow and very cold antimatter? The answer is, physicists. “Unless something really sucks, this technique will be important, and possibly crucial,” said Clifford Surko, a physicist at UC San Diego who is not on Team Alpha. “The way I look at it as an experimenter is you now have a whole ‘other bag of tricks, another handle on the antihydrogen atom. It’s really important. This opens up new possibilities. “
These possibilities involve determining whether antimatter really echoes the physics of matter. Take gravity: the principle of equivalence in the general theory of relativity says that the gravitational interaction should be independent of whether your matter is anti or not. But no one knows for sure. “We want to know what happens if you have a little antihydrogen and you drop it,” Hangst says.
Is not it? Sure. But this experiment is difficult to do, because gravity is actually a wuss. Hot, fizzy things don’t fall off until they bounce back. The antimatter would hit the walls of the machine and annihilate itself. “The gravity is so low that you might not see anything at all,” Hangst says.
However, slow this antihydrogen down to almost absolute zero and it starts to act more like a liquid than a gas. Down, body, instead of spraying all over the place. “The first thing you want to know is, is the antihydrogen going down? Because there’s a crazy fringe out there who think it’s going up – theorists who say there’s a repulsive gravity between matter and antimatter, ”Hangst says. “That would be pretty cool.”
Physicists don’t really need laser cooling to see if antihydrogen acts like Jules Verne’s cavourite. It would be… dramatic. “But if you now assume, as most theorists do, that antihydrogen is going to fall, then you want to ask yourself, does it really fall the same way?” Hangst asks. Accurately measuring acceleration due to gravity is short game for money here, and laser cooling just might make it achievable.