One of physics’ greatest mysteries remains unsolved, with scientists reporting that antimatter falls under gravity — just like ordinary matter.
Key points:
- Physicists have been trying to understand why there is much more matter than antimatter in the Universe
- Some people thought antimatter might be repulsed by gravity, driving it away from the known Universe
- A new study shows this is not the case, and that antimatter behaves the same as ordinary matter under gravity
In the first direct measurement of antimatter’s behaviour under Earth’s gravity, physicists at CERN’s Antimatter Factory made, corralled and dropped the antimatter version of hydrogen atoms in a tube.
Turns out they fall a lot like plain old ordinary hydrogen atoms.
Antiparticles are almost identical to their ordinary particle “twins”. They have the same mass, but carry the opposite charge.
For instance, an electron has a negative charge, while its antimatter sibling — called a positron — is positively charged.
The CERN experiment, called ALPHA-g, is one of many ways physicists are probing antimatter’s properties and searching for any deviations from ordinary matter.
The reason? To discover the fate of a whole lot of missing antimatter.
Why missing antimatter matters
Pretty much all the matter in the observable universe — the stuff that makes us, cities, planets, stars — is ordinary matter, made from electrons, protons, neutrons and their more obscure kin.
The Standard Model of particle physics outlines all these particles and their interactions that make up the Universe.
And according to this, equal amounts of matter and antimatter should have been made during the Big Bang, Curtin University theoretical physicist Igor Bray, who was not involved in the new study, said.
“But we have only a little bit of antimatter, and many, many orders of magnitude more ordinary matter.”
Yet despite years of scanning the observable Universe, we simply don’t see any sign of huge quantities of antimatter out there, Professor Bray said.
“It’s a huge puzzle … as to why there are different amounts of matter and antimatter.”
One possible solution to this “missing antimatter” mystery is that gravity actually repulses antimatter. If Newton’s apple fell from the tree towards the ground, an anti-apple would fling up into the sky.
If such “repulsive antigravity” was the case, some physicists theorise, antimatter made in the Big Bang might have been propelled into an anti-Universe populated by antiparticles, and therefore be missing in the Universe we observe.
But the new finding that antimatter and ordinary matter act the same under gravity, reported in the journal Nature, all but rules out this potential explanation, Professor Bray said.
And the mystery of antimatter’s scarcity stands.
Antimatter wranglers
According to Einstein’s theory of general relativity, antimatter should be subject to the same forces as ordinary matter, including the force of gravity.
But until now, no-one knew for sure if that was the case. At CERN, a few groups have spent years trying to find out.
The Antihydrogen Laser Physics Apparatus or ALPHA team, which is behind the new findings, didn’t start out doing gravity experiments.
They originally set out to study the internal structure of hydrogen’s antimatter counterpart antihydrogen, said Jeffrey Hangst, an experimental physicist with the ALPHA collaboration.
“Once we realised we were getting pretty good at trapping, accumulating and manipulating antihydrogen, we thought we should build a gravity machine.
“It was kind of an afterthought.”
Antihydrogen, which comprises a negatively charged antiproton orbited by a positron, carries no overall electric charge, making it ideal for gravity experiments.
That’s because charged particles are affected by the magnetic field generated by the Earth, which can override any gravitational effects.
Professor Hangst and his crew created a cloud of antihydrogen atoms in a “trap” inside their tall, pipe-like ALPHA-g device.
Magnetic fields held the antihydrogen atoms inside the trap, stopping them from colliding with the sides and annihilating.
Those magnetic fields at the top and bottom of the trap were then slowly removed, releasing the antihydrogen cloud.
As the anti-atoms dropped through the bottom or jiggled their way out the top, they hit ordinary matter and annihilated, producing flashes of gamma rays which were counted by sensitive detectors.
Around 80 per cent of the antihydrogen atoms fell downwards — the same as ordinary hydrogen in the same situation.
The team also adjusted the magnetic field strength at either end of the trap to counteract or boost the force of gravity, then watched what the antihydrogen atoms did.
In every iteration, antihydrogen atoms behaved like ordinary hydrogen.
Is that the end for ‘repulsive antigravity’?
Professor Hangst said that while the study pointed to antimatter and matter acting identically under gravity, “there’s still wiggle room”.
The behaviour of antihydrogen matched very closely to simulations, “but there’s a lot of room for improvement”.
While most antihydrogen atoms did fall down from the trap under normal gravity, one in five ended up emerging from the top.
That’s because the antihydrogen atoms inside the trap weren’t stationery. They jiggled around a little — not enough to escape the trap’s magnetic clutches, but just enough to shimmy up and out when released.
Professor Hangst said from next year, the team planned to use lasers to cool the antihydrogen atoms and stop — or at least reduce — that jiggle.
“The colder they are, the more of them should go out the bottom, like a liquid,” he said.
This will give he and his colleagues much more precise and sensitive measurements, and might help explain the mystery of the missing antimatter.
Or not, whatever the case may be.
“My standing joke is, if it falls up, you win a Nobel Prize. If it falls down, people tell you ‘I told you so,'” Professor Hangst said.
“But we’ve been talking about this for many, many years and speculating about what would happen.
“And you never know in physics until you actually make the observation.”
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