There is a possibility that we live in a universe dominated by antimatter

We first created positrons, electrons' antimatter counterparts, in 1932. But it took decades to create more antiparticles. Now, the newly-discovered antihelium-4 could help us figure out whether there are vast pockets of antimatter in our universe.

Specifically, researchers at the Brookhaven National Laboratory's Relativistic Heavy Ion Collider were able to observe 18 antinuclei of helium-4 - in other words, a nucleus composed of two antiprotons and two antineutrons. Soviet scientists observed antihelium-3 back in 1970, and the first antihydrogen particles were observed in 1995. And although we've known about positrons since 1932, it wasn't until 1955 that it was possible to observe larger subatomic antiparticles like antiprotons and antineutrons.

Why such long lags between discoveries? It's because for each additional antiparticle in the nucleus, it requires about a thousand times more energy. That means antihelium-4 requires a thousand times the amount of energy as antihelium-3, and it requires a trillion times the amount of energy needed to generate the first positron back in the thirties.

Just to see these 18 antinuclei, the Relativistic Heavy Ion Collider had to smash together a billion gold nuclei at energies of 200 giga-electronvolts. And this is all on the cutting edge of what we can even accomplish - the next antimatter atom would be antilithum-6, which would need a million times the amount of energy and is beyond what any particle accelerator can currently create.

Simply managing to see something as impossibly rare as antihelium-4 is a colossal achievement, but there may be a more practical side to all this. Because we're surrounded by regular matter, we make the assumption that the universe as a whole is dominated by matter. But we can't be absolutely sure about that, and this is where antihelium-4 enters the picture.

The eighteen antinuclei created in the collision are precisely what would be predicted by the laws of thermodynamics. Considering the tremendous energies needed to create even such a minute quantity, that means there's next to zero chance of there being any naturally occurring deposits of antihelium-4, unless an entire part of the cosmos was already dominated by antimatter.

It's these improbable pockets of antimatter that we're about to start looking for. The space shuttle Endeavour's final mission will include bringing the Alpha Magnetic Spectrometer to the International Space Station, where it will begin looking for antimatter particles in cosmic rays. If we live in a matter-dominated cosmos, Alpha shouldn't detect any anti-helium at all.

But if - and it's a big if - Alpha detects any antihelium or other heavy antimatter particles, then that would be a game-changer for physicists. As the researchers explain, "any observation of antihelium or even heavier antinuclei in space would indicate the existence of a large amount of antimatter elsewhere in the Universe." That is most definitely not something we would expect to find, but stranger things have happened.

arXiv via Technology Review. Image via Flickr.