By Davide Castelvecchi
September 27, 2023
Physicists have shown that, like everything else experiencing gravity, antimatter falls downwards when dropped.
This outcome is not surprising — a difference in the gravitational behaviour of matter and antimatter would have huge implications for physics — but observing it directly had been a dream for decades, says Clifford Will, a theoretician who specializes in gravity at the University of Florida in Gainesville. “It really is a cool result.”
Because gravity is much weaker than other ubiquitous forces such as electrostatic attraction or magnetism, separating it from other effects in the laboratory is a delicate affair, says Jeffrey Hangst, who leads of the ALPHA-g experiment at CERN, the particle physics laboratory near Geneva, Switzerland. “Gravity is just so bloody weak, you really have to be careful,” says Hangst, who is also a physicist at the University of Aarhus in Denmark. He and his collaborators reported the findings on 27 September in Nature1.
Similar experiments will aim to test whether gravity acts with the same strength on antimatter as it does on matter. Any tiny discrepancies could help to solve one of the biggest problems in physics — how the Universe came to be made almost exclusively of matter, even though equal amounts of matter and antimatter should have arisen from the Big Bang.
Same mass, same gravity
In the topsy-turvy world of antimatter, atomic nuclei are made of negatively charged antiprotons, orbited by positively charged antielectrons, or positrons. According to the standard model of particle physics, however, the opposite charges should be pretty much the only difference: particles and antiparticles should have nearly all the same properties. In particular, experiments have confirmed that positrons and antiprotons have the same masses as their matter counterparts, within the limits of tiny experimental errors.
According to Einstein’s general theory of relativity, all objects of the same mass should weigh the same — in other words, they should experience exactly the same gravitational acceleration.
To put this principle to the test, Hangst and his collaborators wanted to design an experiment that would show what happened when the neutral atom antihydrogen was dropped. “It’s almost impossible to do an experiment with a charged particle, so antihydrogen is the perfect candidate,” says Hangst.
Antimatter particles are routinely created in laboratories. For example, most particles produced by high-energy particle collisions are made in pairs — one particle of matter and its antiparticle. But it is hard to get antiparticles to combine into antiatoms because antimatter particles are typically very short-lived. When an antiparticle meets a particle, they both cease to exist and turn back into energy, in a process called annihilation. In a world made primarily of matter, this makes it hard for antimatter particles to find each other.
CERN is currently the only place in the world where antihydrogen can be made. It has an accelerator that makes antiprotons from high-speed proton collisions, and a ‘decelerator’ called ELENA that slows them down enough to be held for further manipulation. Several different experiments feed off ELENA in CERN’s antimatter research hall. ALPHA-g is one of them, and it combines antiprotons with positrons it collects from a radioactive source.
Antimatter in a can
After making a thin gas of thousands of antihydrogen atoms, researchers pushed it up a 3-metre-tall vertical shaft surrounded by superconducting electromagnetic coils. These can create a kind of magnetic ‘tin can’ to keep the antimatter from coming into contact with matter and annihilating. Next, the researchers let some of the hotter antiatoms escape, so that the gas in the can got colder, down to just 0.5 °C above absolute zero — and the remaining antiatoms were moving slowly.
The researchers then gradually weakened the magnetic fields at the top and bottom of their trap — akin to removing the lid and base of the can — and detected the antiatoms using two sensors as they escaped and annihilated. When opening any gas container, the contents tend to expand in all directions, but in this case the antiatoms’ low velocities meant that gravity had an observable effect: most of them came out of the bottom opening, and only one-quarter out of the top.
To make sure that this asymmetry was due to gravity, the researchers had to control the strength of the magnetic fields to a precision of at least one part in 10,000. This was perhaps their most remarkable feat, says Patrice Pérez, a physicist at the French Alternative Energies and Atomic Energy Commission in Gif-sur-Yvette, and the leader of GBAR, another of CERN’s antihydrogen experiments.
The results were consistent with the antiatoms experiencing the same force of gravity as hydrogen atoms would. The error margins are still rather large, but the experiment can at least conclusively rule out the possibility that antihydrogen falls upwards.
In 2010, Hangst’s team was the first one to succeed at trapping antihydrogen for an extended time, and starting in 2016 they were able to measure how the antiatoms absorb light. But the gravity experiment required a new level of sophistication, he says. “This is by far the most difficult thing that we’ve done.”
Ruggero Caravita, a physicist at the Italian National Institute for Nuclear Physics in Trento, points out that no one would have expected antimatter to fall up, if nothing else, because antiprotons are made of antiquarks, but these only constitute less than 1% of an antiproton’s mass: the rest is the energy that keeps them together. “Insiders have long expected that any violation, if it exists, cannot be over 1%,” says Caravita. Going beyond that would subvert not only the theory of gravitation, but also the standard model of particle physics. Still, the ALPHA-g result was a milestone, he says.
Caravita is leading a third CERN experiment, called AEgIS, which will attempt to measure the gravitational force on a beam of antihydrogen atoms in the absence of any magnetic fields. Perez’s GBAR will aim to reach 1% precision by first making positive antihydrogen ions (antihydrogen with an extra positron), which will help to cool the gas down to a fraction of a degree above absolute zero.
Other efforts aim to measure gravity acting on positronium, a short-lived particle made of one electron and one positron orbiting each other. ALPHA-g itself plans to aim for 1% precision by letting antihydrogen atoms bump up and down and form a quantum superposition with themselves.
* This article was automatically syndicated and expanded from Nature.