When the universe Big Banged itself into existence, about 13.7 billion years ago, equal amounts of matter and antimatter were created, according to physicists. But when we look around today, everything is made of matter, and there's no antimatter anywhere—luckily for us. Matter and antimatter annihilate on contact, leaving only pure energy.
So where did the antimatter go?
The answer has huge implications: it's another way of asking why the universe exists. Although matter and antimatter are thought to be perfectly equivalent, there must be some tiny difference that led the universe to prefer matter over its twin.
Scientists have been trying to characterize that difference for decades, because it would explain why we're all here, as well as the planets and stars and trees and dogs and everything that we know of.
In a new paper, published Monday in Nature, they've taken a big step closer. The ALPHA experiment at CERN in Geneva, Switzerland (where the Higgs boson particle was famously discovered) has used lasers to make the first-ever spectroscopic measurement of an antimatter atom—in this case, antihydrogen. Taking a detailed laser measurement of antimatter has been a goal for decades.
"Antimatter is kind of problem zero. We can't explain why there's a universe," lead author Jeffrey Hangst, who's based at Aarhus University in Denmark, told me over the phone. "We see only normal matter in any quantity."
ALPHA experiment: new results on antimatter. Video: CERN/YouTube
First the ALPHA team created antihydrogen, then enclosed it in a small magnetic trap, where it was cryogenically cooled. (The cooling cryostat used was made at TRIUMF, a particle and nuclear physics lab in Vancouver, and at the University of Calgary.)
Then ALPHA beamed a laser into the chamber to excite the anti-atoms.
Spectroscopic measurements are some of the most precise that scientists can do, and tell them in great detail about an atom's characteristics. Because hydrogen has been well-studied and characterized, it makes a great jumping off point for comparison—and so far the two look really similar. Antihydrogen's spectrum was consistent to that of hydrogen to a relative precision of two parts in ten billion, according to TRIUMF.
"This is really the first measurement anyone has ever done with a laser of an antimatter atom," said Makoto Fujiwara, senior scientist at TRIUMF and leader of ALPHA's Canadian contingent, and a co-author of the paper. "This is the beginning of an exciting season."
It opens the door to all kinds of new research. The ALPHA team is trying to build a new kind of apparatus called ALPHA-g (the "g" stands for gravity) which is designed to test how antimatter falls—whether gravity treats it the same as regular matter.
"Matter and antimatter are so fundamental to the laws of physics," Fujiwara said. "If we find any significant difference, we'll really have to rewrite the history of the universe."
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