Antigravity gets first test at Cern's Alpha experiment

By Jason Palmer
Science and technology reporter, BBC News

image captionThe Alpha experiment's antimatter chamber uses magnetic fields to sequester antihydrogen atoms

Researchers at Cern in Switzerland have tested a novel way to find out if antimatter is the source of a force termed "antigravity".

Antimatter particles are the "mirror image" of normal matter, but with opposite electric charge.

How antimatter responds to gravity remains a mystery, however; it may "fall up" rather than down.

Now researchers reporting in Nature Communications have made strides toward finally resolving that notion.

Antimatter presents one of the biggest mysteries in physics, in that equal amounts of matter and antimatter should have been created at the Universe's beginning.

Yet when the two meet, they destroy each other in what is called annihilation, turning into pure light.

Why the Universe we see today is made overwhelmingly of matter, with only tiny amounts of antimatter, has prompted a number of studies to try to find some difference between the two.

Tests at Cern's LHCb experiment and elsewhere, for example, have been looking for evidence that exotic particles decay more often into matter than antimatter.

Last week, the LHCb team reported a slight difference in the decay of particles called Bs mesons - but still not nearly enough to explain the matter mystery.

One significant difference between the two may be the way they interact with gravity - antimatter may be repelled by matter, rather than attracted to it.

But it is a difference that no one has been able to test - until the advent of Cern's Alpha experiment.

Getting annihilated

Alpha is an acronym for Antihydrogen Laser Physics Apparatus - an experiment designed to build and trap antimatter "atoms".

Just as hydrogen is made of a proton and an electron, antihydrogen is an atom made of their antimatter counterparts antiprotons and positrons.

The trick is not just in making it, but in making it hang around long enough to study it - before it bumps into any matter and annihilates.

image captionAn artist's conception of an anti-hydrogen atom being released from the trap after 1,000 seconds

The team has now gone back to their existing data on 434 antihydrogen atoms, with the antigravity question in mind.

"In the course of all the experiments, we release (the antihydrogen atoms) and look for their annihilation," said Jeffrey Hangst, spokesperson for the experiment.

"We've gone through those data to see if we can see any influence of gravity on the positions at which they annihilate - looking for atoms to fall for the short amount of time they exist before they hit the wall," he told BBC News.

The team has made a statistical study of which antihydrogen atoms went where - up or down - and they are able to put a first set of constraints on how the anti-atoms respond to gravity.

The best limits they can suggest is that they are less than 110 times more susceptible to gravity than normal atoms, and less than 65 times that strength, but in the opposite direction: antigravity. In short, the question remains unanswered - so far.

"It's not a very interesting band yet but it's the first time that anyone has even been able to talk about doing this," said Prof Hangst.

"We actually have a machine that can address this question, that's what's exciting for us here, and we know how to get from here to the interesting regime."

The Alpha experiment's main task is to study the energy levels within antihydrogen, to spot any differences between it and the hydrogen that physicists know to extraordinary precision.

Prof Hangst said the antigravity measurement was just an "interesting sideshow" for the experiment.

"We have a lot of options for studying antimatter and this is a new one that has a future."

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