Einstein was right: antimatter does what you’d expect

It’s easy to assume that antimatter is the opposite of matter – it’s in the name, right?

Mundanely, the ‘opposite’ merely refers to a particle’s electric charge. The ‘anti’ of a negatively charged electron is a positively charged positron, for instance. This is where differences between antimatter start and finish. They otherwise share the same properties, such as mass, and if you smash matter and antimatter particles together, they’ll annihilate each other in a process that converts all their mass to energy.

Antimatter remains elusive, and speculation as to where it’s gone, given that matter and antimatter are created together, has led to interesting ideas, including that antimatter might levitate, pushing in the opposite direction to gravity.

An artist's conceptual rendering of antihydrogen atoms falling out the bottom of the magnetic trap of the alpha-g apparatus. As the antihydrogen atoms escape, they touch the chamber walls and annihilate. Most of the annihilations occur beneath the chamber, showing that gravity is pulling the antihydrogen down.
An artist’s conceptual rendering of antihydrogen atoms falling out the bottom of the magnetic trap of the ALPHA-g apparatus. Credit: Keyi “Onyx” Li/U.S. National Science Foundation

But an experiment at CERN has dashed that idea, and in the process proved Einstein right, yet again.

A collaborating team of researchers tested antimatter against Einstein’s general theory of relativity, publishing their study in Nature, and confirming it is attracted towards the Earth at the same rate as normal matter – about 9.8 m/s.

That means any notion of “repulsive antigravity” has been ruled out.

“It [antimatter] surely accelerates downwards, and it’s within about one standard deviation of accelerating at the normal rate,” says Professor Joel Fajans, a physicist at UC Berkeley.

“The bottom line is that there’s no free lunch, and we’re not going to be able to levitate using antimatter.”

He along with theoretical physicist Professor Jonathan Wurtele proposed this experiment more than 10 years ago. In lay terms, gravitational attraction to the Earth is the idea that objects with mass are pulled towards each other by gravity – if a made-of-matter person jumps towards the sky, the mass of the Earth pulls them back. An antimatter person would do the same.

Fajans, Wurtele and their colleagues have demonstrated this effect using the Antihydrogen Laser Physics Apparatus – or ALPHA – at CERN in Europe. 

But it’s not so simple as dropping a tiny antimatter particle and seeing if it hits the ground.

Careful manipulation

“Gravitational force is incredibly weak compared to electrical forces,” says Fajans.

That means a ‘drop’ experiment would likely see surrounding electrical forces contaminating the research. For context, Fajans says a 1 V/m electrical field – weaker than the field you would experience standing beneath a light bulb – would have 40 trillion times more influence on an antiproton than the weak tug of Earth’s gravity.

“We have to manipulate the antimatter very carefully or we will lose it,” said Fajans.

So, careful to avoid destroying their experimental antimatter via a collision with its normal counterpart, the ALPHA team enclosed 100 antihydrogen atoms in a 25cm tall magnetic bottle. Inside, the antihydrogen atoms are being thrown around the magnetic fields generated at either end of the bottle at speeds of 100m/s. When the bottle is tipped vertically, the antihydrogen moving in the downward direction began accelerating. Those moving upward slowed.

Because of the sudden boost in energy, the down-moving antihydrogen would, according to the study, be capable of escaping the magnetic fields, bumping into the bottle’s surface and triggering a matter-antimatter collision that ends in their destruction.

This annihilation would result in the release of pions – tiny subatomic particles consisting of quark and antiquark – amid a burst of light. Measuring this release calculated which way the antihydrogen had moved.

Finally, the magnetic fields were reduced to see where all the matter eventually exited the bottle.

“If you walk down the halls of this department [at Berkeley] and ask the physicists, they would all say that this result is not the least bit surprising. That’s the reality,” says Wurtele.

“But most of them will also say that the experiment had to be done because you never can be sure.”

Still, demonstrating both matter and antimatter obey the rules of gravitational attraction is a tiny step forward in understanding this elusive material.

“Physics is an experimental science,” says Wurtele.

“You don’t want to be the kind of stupid that you don’t do an experiment that explores possibly new physics because you thought you knew the answer, and then it ends up being something different.”

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