This post is about an experiment in the news this week, which might have taken the first step towards elucidating one of the greatest mysteries in physics: the observable universe seems to be dominated by matter rather than anti-matter. This is a mystery because the laws of physics make no distinction between matter and anti-matter. So where is all the anti-matter hiding?
Before we describe this experiment, let us describe what anti-matter is.
History
In 1928, Paul Dirac successfully combined the theories of quantum mechanics and special relativity by proposing a new equation, which was named after him. When he solved the equation, he found electrons could be described as having positive as well as negative energies. He predicted that the negative energies corresponded to anti-electrons, which were later called positrons. Thus was born the concept of anti-matter.
The positron was discovered in 1932. Discovery of other anti-particles, such as the anti-proton in 1955 and the antineutron in 1956, followed. Generally, if a particle and its antiparticle meet, they annihilate: all mass disappears, and is replaced by the equivalent energy. In an electron-positron collision, both particles disappear and photons are released.
Note: Antimatter has percolated into the public consciousness. For example, in Dan Brown's Novel Angels and Demons a stolen canister of antimatter plays a central role. Also, since annihilation is a source of energy, matter-anti-matter engines have been seriously discussed for space travel applications.
Availability
Antimatter seems to be rare in the universe, though. A few antiparticles come to Earth from outer space, in the form of cosmic ray showers. Some positrons are emitted in radioactivity (this is the basis of PET scans in medicine). But if we want a regular supply, we have to make our own, such as at the LEP (Large Electron-Positron Collider) which has been dismantled, and the CEPC (Circular...) which has been proposed. Thus, matter seems to dominate in abundance over antimatter.
The laws of physics do not offer any clear hint about why this imbalance exists. Experimentally, therefore, it would be interesting to find if antimatter shows any different characteristics from matter. This might provide a clue to the difference in their perceived presence in the universe.
The idea behind the experiment I will now discuss is to see how antimatter responds to gravity. Do antiparticles fall to the Earth under gravity? Are they indifferent to it? Or as some people have suggested, do they display 'anti-gravity'?
The idea is simply to first trap a number of antiparticles and then let them out of the trap to see what happens in the presence of gravity. There are several things to be kept in mind here.
First, the trap has to be 'contactless', that is, it cannot be built out of any regular material because the antiparticles would annihilate in contact with that material (since it would be made of matter). In the experiment, the trap was made from a magnetic field.
Second, the antiparticles cannot carry net electrical charge, as that would make them very sensitive to stray electric and magnetic fields, which would mask the effects of gravity. This is the reason the experiment could not be done with positively charged positrons or negatively charged anti-protons, which have been available for some time now. The two had to be combined into electrically neutral anti-hydrogen first, which recently became possible to do in large quantities (hundreds, which gives a large enough detection signal) and for long times (hours, required to accumulate enough anti-particles).
Finally, since the trapped anti-hydrogen is not at zero temperature, the atoms are actually moving around in all directions inside the trap. So when the trap is switched off, some atoms go up, some to the side and some fall down below.
The outcome
Roughly speaking, the test was carried out by switching off the trap and detecting how many antihydrogen atoms left from the top and how many from the bottom. If gravity has the expected effect, then more should leave from the bottom. They do.
The data are consistent with a theory that includes gravity and do not agree with theories that neglect gravity or include antigravity (Fig. 5 in the paper, which shows this, is quite fun to look at).
The next step
I tried to choose my words carefully above. The agreement of the data with regular gravity is at the level of consistency, not agreement. The authors say the next step is to explore this aspect further. In other words, the present study shows that antiparticles see gravity, much like regular particles (i.e. the sign of the acceleration is the same for both). The aim of the future study would be to find out exactly how much gravity is seen by antiparticles (i.e. whether the magnitude is the same as that for regular particles).
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