Washington:
For the first time, scientists have observed antimatter particles – the mysterious twins of visible matter all around us – falling due to gravity, the European physics laboratory CERN announced on Wednesday.
The experiment was hailed as a “huge milestone”, although most physicists expected the result, and it had been predicted by Einstein’s 1915 theory of relativity.
It definitively rules out the possibility that gravity repels antimatter upwards – a finding that would have turned our fundamental understanding of the universe upside down.
The Big Bang about 13.8 billion years ago is thought to have produced an equal amount of matter – which is what everything you can see is made of – and antimatter, its equal but opposite counterpart.
However, there is virtually no antimatter in the universe, giving rise to one of physics’ greatest mysteries: what happened to all the antimatter?
“Half the universe is missing,” says Jeffrey Hangst, a member of CERN’s ALPHA collaboration in Geneva, who conducted the new experiment.
“In principle, we could build a universe – everything we know – using just antimatter, and it would work in exactly the same way,” he told AFP.
Physicists believe that matter and antimatter met and almost completely destroyed each other after the Big Bang.
Yet matter now makes up almost five percent of the universe – the rest consists of even lesser-known dark matter and dark energy – while antimatter has disappeared.
Newton’s apple flying up?
One of the main outstanding questions about antimatter was whether gravity caused antimatter to fall the same way as normal matter.
While most physicists believed so, some had speculated otherwise.
A falling apple famously inspired Isaac Newton’s work on gravity – but if that apple had been made of antimatter, would it have shot into the air?
And if gravity did indeed repel antimatter, this could have meant that impossibilities like a perpetual motion machine were possible.
“So why don’t we drop a few and see what happens?” Hangst said.
He compared the experiment to Galileo’s famous – though probably apocryphal – 16th-century demonstration that two balls of different masses falling from the Leaning Tower of Pisa would fall at the same speed.
But this experiment — the result of thirty years of work on antimatter at CERN — was “a little more complicated” than Galileo’s, Hangst said.
One problem was that antimatter barely exists outside of the rare, short-lived particles in space.
However, in 1996, scientists at CERN produced the first atoms of antimatter: antihydrogen.
Another challenge was that because matter and antimatter have opposite electrical charges, the moment they meet they annihilate each other in a violent flash of energy that scientists call annihilation.
A magnetic trap
To study the effect of gravity on antimatter, the ALPHA team constructed a 25 centimeter long bottle, placed on its end, with magnets at the top and bottom.
Late last year, scientists placed about 100 very cold antihydrogen atoms in this ‘magnetic trap’, called ALPHA-g.
When they reduced the force of both magnets, the anti-hydrogen particles – which bounced around at a speed of 100 meters per second – were able to escape from both ends of the bottle.
The scientists then simply counted how much antimatter had been destroyed at each end of the bottle.
About 80 percent of the antihydrogen disappeared from the bottom, which is similar to how regularly bouncing hydrogen atoms would behave if they were in the bottle.
This result, published in the journal Nature, shows that gravity causes antimatter to fall, as predicted by Einstein’s 1915 theory of relativity.
In more than a dozen experiments, the CERN scientists varied the strength of the magnets, observing the effect of gravity on antimatter at different speeds.
Although the experiment rules out that gravity causes antihydrogen to rise upwards, Hangst emphasized that it did not prove that antimatter behaves exactly the same as normal matter.
“That’s our next job,” he said.
Marco Gersabeck, a physicist who works at CERN but was not involved in the ALPHA research, said it was “a huge milestone”.
But it marks “just the beginning of an era” of more precise measurements of gravity’s effect on antimatter, he told AFP.
Other efforts to better understand antimatter include using CERN’s Large Hadron Collider to investigate strange particles called beauty quarks.
And there’s an experiment aboard the International Space Station to capture antimatter in cosmic rays.
But exactly why the universe is awash in matter but devoid of antimatter “remains a mystery,” says physicist Harry Cliff.
Since both should have completely destroyed each other in the early universe, “the fact that we exist suggests that there is something going on that we don’t understand,” he added.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)