Scientists have embarked on one of the strangest experiments ever - to see whether atoms of exotic antimatter fall up instead of down.
Antimatter is weird stuff, being a kind of mirror image of ordinary matter with an opposite electric charge.
When an atom of normal matter meets its antimatter counterpart the two annihilate each other in a flash of light.
That much scientists already know. What is much less certain is whether antimatter has fundamental properties that are different from normal matter - especially the way it is affected by gravity.
A few experts have proposed the outlandish idea that antimatter might be repelled by the gravity - in other words, that it falls up.
One reason for the mystery is that antimatter is extremely rare in nature and to date has only been made in tiny quantities in laboratories. Artificially-created atoms of antimatter are suspended in magnetic traps, and nobody has ever looked to see what happens when it is "dropped".
Now scientists have taken the first steps towards answering this question. A paper reported in the journal Nature Communications describes the first direct measurement of gravity's effect on antimatter. Unfortunately, the results are still too uncertain to resolve the riddle of what happens to antimatter in free-fall.
Lead scientist Professor Joel Fajans, from the University of California at Berkeley, US, said: "This is the first word, not the last. We've taken the first steps toward a direct experimental test of questions physicists and non-physicists have been wondering about for more than 50 years.
"Is there such a thing as anti-gravity? Based on free-fall tests so far, we can't say yes or no. We certainly expect antimatter to fall down, but just maybe we will be surprised."
The work was conducted at Cern, the European Centre for Nuclear Research in Geneva, Switzerland, home of the Alpha (Antihydrogen Laser Physics Apparatus) experiment.