PARIS: For the first time, scientists have observed antimatter particles — the mysterious twins of the visible matter all around us — falling downwards due to the effect of gravity, Europe’s physics lab CERN announced on Wednesday.

The experiment was hailed as “huge milestone”, though most physicists anticipated the result, and it had been predicted by Einstein’s 1915 theory of relativity.

It definitively rules out that gravity repels antimatter upwards — a finding that would have upended our fundamental understanding of the universe. Around 13.8 billion years ago, the Big Bang is believed to have produced an equal amount of matter — what everything you can see is made out of — and antimatter, its equal yet opposite counterpart.

However, there is virtually no antimatter in the universe, which prompted one of the greatest mysteries of physics: what happened to all the antimatter? “Half the universe is missing,” said Jeffrey Hangst, a member of CERN’s Antihydrogen Laser Physics Apparatus (ALPHA) collaboration in Geneva which conducted the new experiment.

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“In principle, we could build a universe — everything that we know about — with only antimatter, and it would work in exactly the same way,” he said.

Physicists believe that matter and antimatter did meet and almost entirely destroyed each other after the Big Bang. Yet matter now makes up nearly five per cent of the universe — the rest is even less understood dark matter and dark energy — while antimatter vanished.

Newton’s apple flying up?

One of the key outstanding questions about antimatter was whether gravity caused it to fall in the same way as normal matter. While most physicists believed that it did, a few had speculated otherwise.

A falling apple famously inspired Isaac Newton’s work on gravity — but if that apple was made of antimatter, would it have shot up into the sky? And if gravity did in fact repel antimatter, it could have meant that impossibilities such as a perpetual motion machine were possible. “So why not drop some and see what happens?” Hangst said.

He compared the experiment to Galileo’s famous — though likely apocryphal — 16th-century demonstration that two balls of different mass dropped from the Leaning Tower of Pisa would fall at the same rate.

But this experiment — the result of 30 years of work on antimatter at CERN — was “a little bit more involved” than Galileo’s, Hangst said.

One problem was that antimatter barely exists outside of rare, short-lived particles in outer space.

However, in 1996, CERN scientists produced the first atoms of antimatter — antihydrogen. Another challenge was that, because matter and antimatter have an opposite electrical charge, the moment they meet they destroy each other in a violent flash of energy scientists call annihilation.

Published in Dawn, September 28th, 2023