Physicists investigate why matter and antimatter are not mirror images

AS MISMATCHES go, it’s a big one. When physicists bring the Standard Model of particle physics and Einstein’s general theory of relativity together they get a clear prediction. In the very early universe, equal amounts of matter and antimatter should have come into being. Since the one famously annihilates the other, the result should be a universe full of radiation, but without the stars, planets and nebulae that make up galaxies. Yet stars, planets and nebulae do exist. The inference is that matter and antimatter are not quite as equal and opposite as the models predict.

This problem has troubled physics for the past half-century, but it may now be approaching resolution. At CERN, a particle-physics laboratory near Geneva, three teams of researchers are applying different methods to answer the same question: does antimatter fall down, or up? Relativity predicts “down”, just like matter. If it falls up, that could hint at a difference between the two that allowed a matter-dominated universe to form.

Testing this idea is tricky. Antimatter needed for experiments is usually made in accelerators, in the form of particles travelling at nearly the speed of light. To use it in a gravitational test, it must be slowed down, isolated from all other forces and kept intact for long enough to track its fall (or rise). All without allowing any matter to touch it, and thus annihilate it.

This is possible only at CERN, which has dedicated one of its smaller accelerators to creating a stream of low-energy antiprotons. It does this by smashing a high-energy beam of protons into a block of iridium, and then feeding the resulting antiprotons into a ring 180 metres in circumference. This ring acts as a particle accelerator in reverse, slowing the antiprotons to a mere tenth of the speed of light.

Since antiprotons are electrically charged (they are negative; normal protons are positive), gravity’s effect on them is swamped by any electromagnetic forces around, which will be much larger. All three experiments deal with this by combining their antiprotons with positrons (antielectrons) to create antihydrogen atoms. These are electrically neutral, and thus immune to such disturbances.

One team, AEGIS (Antihydrogen Experiment: Gravity, Interferometry, Spectroscopy), guides the resulting beam of antihydrogen into something called a Moiré deflectometer. This splits the beam up and then recombines it to create an interference pattern. Gravitational effects should show up as small shifts in the pattern.

A second group, GBAR (Gravitational Behaviour of Antihydrogen at Rest), runs two more stages of deceleration, then adds two positrons to each antiproton, to create a positively charged antihydrogen ion. Such ions, precisely because they are charged, can be slowed down even further using an electric field. This brings them to something approaching human walking pace. The field then guides them towards a target point where they are pinned in place by a second field and the extra positron is shot off by pulses of laser light. At this point, the ions become electrically neutral, so gravity is the only remaining force acting on them. The direction of their fall can thus be observed directly.

The third team, ALPHA, may be the closest to success. This group has spent the past 18 years measuring every possible property of antihydrogen, and has become adept at trapping it and chilling it to within half a degree of absolute zero. (A cold atom is a slow-moving atom.) ALPHA’s best attempt so far gathered antihydrogen atoms in a trap and turned off the magnetic field to watch which way they drifted. But even at this temperature, there was still plenty of noise in the system. The team could conclude only that antimatter might fall downwards anywhere up to 110 times faster than normal matter does, or fall upwards anywhere up to 65 times faster than normal matter falls down.

Now, ALPHA’s researchers may be able to do better. As a consequence of work they reported in August on a phenomenon called the Lyman-alpha transition, they think they can bring a technique called laser cooling to bear on antihydrogen. This should bring the antiatoms to within a few hundredths of a degree of absolute zero. With antihydrogen that cold, a similar drift test would give a much more precise measurement of gravitational behaviour.

All three teams are now in a race to perfect their apparatus in time to make observations before CERN’s accelerators come to the end of their current run and take a two-year maintenance break. That is scheduled for November. There appears to be a good chance of at least a “sign test” by then (ie, one that provides an answer to the question of whether antimatter falls up or down), even if it does not show exactly how fast the atoms are falling or rising.

Few really believe that antimatter falls up, because so much existing theory predicts it will not. But in 1887 existing theory predicted that the apparent speed of light would vary with the speed at which the observer was moving. When Albert Michelson and Edward Morley showed that it does not, they tore up the rule book of physics. It could happen again.