Lasers have been used for decades to cool atoms to near absolute zero, the lowest possible temperature. These of course have been atoms of matter, like ourselves and everything around us. Now the same technique has managed a much harder feat, to cool anti-hydrogen to similarly low temperatures. The achievement opens the door to testing some of the most fundamental questions about the nature of the universe.
Antimatter is often referred to as matter's mirror image – identical in some ways, the exact opposite in others. Since encounters between matter and antimatter lead to both being annihilated in a burst of energy, it's extremely hard to contain even tiny amounts of it, let alone study it. That gets harder still if the antimatter you do have won't stay still.
Since temperature is a measure of how fast an atom or molecule is vibrating, getting it to slow enough for great control means removing its heat. That's what a team at Antihydrogen Laser Physics Apparatus (ALPHA) Canada has done. They adjusted ultraviolet lasers so that when a cloud of anti-hydrogen atoms moved towards the laser its light pushed back, stilling them. On the other hand, when the atoms moved in the opposite direction, their frequencies were such that the Doppler shift prevented the lasers from giving them an extra push, as would occur if the light had a broader spectrum. With the energy canceled as they moved one way, and not restored in the other, movement slowed to almost nothing.
One of the great puzzles of modern physics is why there is more matter in the universe than antimatter. It's just as well for us this is true, otherwise, we would constantly be in danger of annihilation from a planet made of antimatter, perhaps even a whole antimatter Sun. Nevertheless, our models of the Big Bang indicate it should have produced the two in equal quantities. The universe's undeniable (and enormous) skew is arguably the largest surviving hole in our understanding of the laws of physics, one that has remained a challenge and embarrassment to physicists for decades.
The announcement in Nature of success in cooling atoms of antihydrogen in this way opens the door to exploring these questions further. "My next dream is to make a "fountain" of anti-atoms by tossing the laser-cooled antimatter into free space. If realized, it would enable an entirely new class of quantum measurements that were previously unthinkable," said co-author Dr Makoto Fujiwara of Canada's TRIUMF.
“We are one step closer to being able to manufacture the world's first antimatter molecules by joining anti-atoms together using our laser manipulation technology," team member Professor Takamasa Momose of the University of British Columbia said.
The imbalance of matter and antimatter in the universe implies that in some way we have identified the two are not perfect reflections of each other. Laser cooling could allow us to test some options that are considered unlikely, but still need to be ruled out, such as unequal responses to the pull of gravity.