A particle can be in two places at once – even if it is made of antimatter. Researchers have just performed an antimatter twist on a classic experiment used to show one of the foundational tenets of quantum mechanics: that all particles are also waves.
In the most basic version of the double-slit experiment, first performed in 1801, a beam of light illuminates a plate with two parallel slits cut into it. The light that passes through the slits hits a screen, creating stripes of light and darkness as a result of the interference between the two light waves that went through the slits.
This interference is proof that light is not just a classical particle, as Isaac Newton had argued, but also a wave.
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Classic experiment
In the centuries since, variations of the double-slit experiment have been repeated with many other types of particles, from electrons up to molecules, showing that they are all waves as well. The experiment even shows that an individual particle can in effect go through both slits at once and interfere with itself.
These demonstrations firm up quantum mechanics as an integral part of the universe, but until now, it had never been done with a beam of antimatter.
“Antimatter is precious, it’s hard to produce, and it’s even harder to produce in a setup where you can make a beam out of it,” says Michael Peskin at the SLAC National Accelerator Laboratory in California.
Akitaka Ariga at the University of Bern in Switzerland and his colleagues have performed the double-slit experiment and observed the interference between positrons, the antimatter equivalent of electrons.
Antimatter beam
The setup starts with radioactive sodium, which sheds positrons at a rate of about 5000 per second. The positrons pass through a pair of circular openings, which focuses them into a beam.
The remaining positrons are aimed towards two silicon nitride crystals, each of which acts as a set of many slits. Those that pass through the slits hit a screen that acts as a photographic plate, recording where every positron hits.
Only about 100 positrons per second hit the screen, so the experiment had to run for up to 200 hours to build up a strong enough signal. That final image revealed stripes of light and darkness, showing that positron waves do interfere with one another.
This isn’t the first time that we’ve shown that antimatter particles also behave like waves. For example, exotic molecules that are a mixture of matter and antimatter particles have been made to interfere with one another as waves, says Peskin, but it is satisfying to achieve the classic double-slit. “It’s a big technical challenge to actually do this particular experiment with antimatter, but the conclusions are very much expected.”
This experiment was a first step for the researchers’ ongoing efforts to study the effect of gravity on antimatter. A big unanswered question is whether antimatter behaves like normal matter under gravity, or does something different – it could even float upwards. Their eventual goal is to determine how the interference pattern changes when the positrons are under a gravitational force of varying strength.
“It’s possible that gravity doesn’t work exactly the same for antiparticles and particles, and this could be part of the reason the universe doesn’t appear to be made of the same amount of particles and antiparticles,” says David Christian at the Fermi National Accelerator Laboratory in Illinois.
If gravity does act differently on matter and antimatter, it is only a very slight difference, so experiments will need to be extremely precise to reveal the change. “The effect of gravity on an individual particle is really tiny, even if it’s the gravity of the whole Earth,” says Christian. “This is an interesting experimental technique, but it’s still a real long shot.”
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