The exact opposite of matter, antimatter, has been known to exist for decades now, although how it behaves in the presence of a gravitational field remains mostly unknown even though countless experiments have been conducted over the years. Each particle described in the Standard Model has its own antiparticle counterpart, having similar mass and spin, albeit opposite charge. For example, the electron has as antiparticle the positron, which is a lepton with mass similar to that of the electron, a spin of 1/2 and an electrical charge of +1.
Several physicists now say they want to measure the effects of gravitational fields on antimatter, by creating a horizontal beam of antiparticles and determine how it is deflected by the gravitational force. Most physicists believe that antimatter behaves in the same way as matter in gravitational fields, although, if proven otherwise, antimatter may provide us with a way to produce gravitational forces and even antigravity.
"If antimatter fell down faster, it would mean the discovery of at least one new force, probably two. If it fell up, it would mean our understanding of general relativity is incorrect", says Thomas Phillips from Duke University.
In general relativity, gravity would interact similarly both with matter and antimatter. But theories trying to unify general relativity and quantum mechanics show that there could in fact be two additional distinct forces that could resemble gravity. Ordinary matter would not feel the effect of either of these forces, but in antimatter, the two could add up in order to double the strength of the force of attraction.
Meanwhile, antigravitational forces, although never observed, could allow antimatter particles to be repelled in the presence of a gravitational field and may also provide an explanation to why antimatter is so scarce in the universe today, when in fact soon after the Big Bang event antimatter could have been as abundant as ordinary matter.
One of the experiments suggested by Phillips involves the test of antihydrogen in the Fermilab particle accelerator, while another two experiments could be carried out by CERN. All require the use of antihydrogen atoms that are structures containing antiprotons instead of protons and positrons instead of electrons. Antiprotons are relatively easy to produce by colliding protons into a target, but controlling the antihydrogen atom is another story, because when matter interacts with antimatter, they annihilate each other.
Additionally, finding the newly created antihydrogen atom can prove rather difficult. "That experiment is hard, and I applaud them for having the guts to try it. This is not like most particle physics experiments, where you know what the beam is, you know how to handle it, and you know the detector will work. It's no cup of tea", said Michael Nieto of the Los Alamos National Laboratory.
One of the possible experiments with antimatter that could be conducted at CERN, called AEGIS, proposes the use of an antihydrogen beam that would be passed through a diffraction grating. When this happens, part of the antihydrogen atoms would annihilate with the diffraction device, thus producing a light-dark pattern that would indicate the location of the collision with an accuracy of 1 percent.
"If they got 10%, I would just be aghast and cheering wildly", said Nieto. There is the risk however, that antimatter and matter behave identically in gravitational fields, or that the difference is so small that it cannot be detected. Nevertheless, its worth trying.
"If I had to bet a case of champagne, I would bet that antihydrogen and hydrogen fall exactly the same. And that's a case of champagne I'd love to lose we're dreaming to see something unexpected", said Michael Doser, AEGIS project member.