Skulls in the Stars

Hey folks, as you might have seen from my previous post, things have been a little hectic lately, and I haven’t had an opportunity to write some in-depth blog posts. While I wait for life to settle a bit again, I thought I’d share a bit of fascinating science news that you might have missed: researchers at CERN have figured out which way antimatter goes in a gravitational field!

To quote from the NSF’s press release today:

If you dropped some antimatter, would it fall down or up? Scientists now know the definitive answer: down. That is, if you can somehow prevent it from exploding into pure energy long enough to see where it goes.

A scientific paper describing the landmark experiment behind that conclusion is published today in the journal Nature and comes from the international Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at the European Organization for Nuclear Research (CERN) in Switzerland. The ALPHA collaboration’s unique, painstaking experiment has answered the longstanding fundamental question about whether antimatter is gravitationally attracted or repelled by regular matter by observing the downward path taken by individual atoms of antihydrogen. Their work also provides a key piece in one of the most immense unsolved puzzles in science — why is there so little antimatter in the observable universe?

We’ve known about the existence of antimatter for quite some time. It was first predicted to exist in a Paul Dirac’s relativistic quantum wave equation, now known as the Dirac equation, in 1928. The Dirac equation naturally results in solutions which are particles with negative energy, which Oppenheimer then speculated could represent antiparticles, such as a positively-charged antielectron and a negatively-charged proton. The first antimatter particle, the positron, was definitively detected in 1932 by Carl David Anderson, for which he won the Nobel Prize.

With such a long history, you might expect that we’ve nailed down quite well all the properties of antimatter, and you would be (mostly) correct. Antimatter is produced in the laboratory, or more accurately a particle accelerator, through collisions of high-energy particles. If enough energy is present to account for the mass of a particle/antiparticle pair, there is a chance that they will be created. But if particles and antiparticles are only created in pairs, where is all the antimatter in the universe, which appears to be made primarily of ordinary matter? This is one puzzle that we still do not have a definitive answer for.

Related to this is the question studied by the ALPHA-g experiment: does antimatter get attracted to ordinary matter via the force of gravity, or does it have the “anti-” effect and get repelled? Most theoretical considerations suggest that antimatter must be gravitationally attracted like ordinary matter, but a direct measurement had not yet been achieved to show this conclusively.

A big difficulty with doing such an experiment is getting enough slow-moving antimatter in one place! As I said, antimatter is produced through high-energy collisions, and the antimatter ends up moving really fast, which makes it hard to see how gravity affects it. Antimatter also annihilates with ordinary matter, so it usually doesn’t stick around long enough for experimental uses. Also, antimatter like positrons or antiprotons have electric charge, so they will experience a very strong electrical force, which will tend to overwhelm any gravitational effects.

In the ALPHA experiment, antiprotons are collected from CERN’s particle accelerator, and are brought together with positrons created from a radioactive isotope. In this way, electrically-neutral antihydrogen atoms can be created, and then they are routed into a vertical tube where they are trapped by magnetic fields. Then, the magnetic fields at the top and bottom of the tube are reduced, allowing the antiatoms to fall whichever way gravity takes them! In this case, they found that antimatter does indeed fall downward, attracted to the gravitational pull of the Earth.

This rules out one explanation of why there is so little antimatter in the universe. If antimatter were repelled from ordinary matter, it might have been pushed away in the early moments of the universe. But now that we know it isn’t, that explanation has been largely ruled out, allowing researchers to focus on other possibilities.

Research is ongoing at ALPHA to see if they can find any other differences between matter and antimatter — it is an experiment worth watching!