Would an antimatter apple fall up?
New experiments are being proposed to test a big unknown in physics: how antimatter reacts to gravity.
Physicists have studied antimatter, the mirror version of ordinary matter, for decades. They know, for example, that antiparticles have the same mass as ordinary particles, but opposite charge. But no one knows what effect gravity will have on such particles.
Now several groups want to measure exactly how the Earth will pull on antimatter. The tests would create a horizontal beam of the stuff and measure how much gravity deflects it.
The complicated ballistic test may show no difference between the way matter and antimatter fall. But some experimentalists are holding out hope that they may see something completely unexpected, which could point the way to new gravity-like forces, or perhaps even antigravity.
"If antimatter fell down faster, it would mean the discovery of at least one new force, probably two. If it fell up, it would mean our understanding of general relativity is incorrect," says Thomas Phillips, a physicist at Duke University in Durham, North Carolina, US.