Elizabeth Gibney in Nature:
If the cat in Erwin Schrödinger's famous thought-experiment behaved according to quantum theory, it would be able to exist in multiple states at once: both dead and alive. Physicists' common explanation for why we don’t see such quantum superpositions — in cats or any other aspect of the everyday world — is interference from the environment. As soon as a quantum object interacts with a stray particle or a passing field, it picks just one state, collapsing into our classical, everyday view.
But even if physicists could completely isolate a large object in a quantum superposition, according to researchers at the University of Vienna, it would still collapse into one state — on Earth's surface, at least. “Somewhere in interstellar space it could be that the cat has a chance to preserve quantum coherence, but on Earth, or near any planet, there's little hope of that,” says Igor Pikovski. The reason, he asserts, is gravity.
Pikovski and his colleagues’ idea, laid out in a paper published in Nature Physics on 15 June, is at present only a mathematical argument. But experimenters hope to test whether gravity really does collapse quantum superpositions, says Hendrik Ulbricht, an experimental physicist at the University of Southampton, UK. “This is a cool, new idea, and I’m up for trying to see it in experiments,” he says. Assembling the technology to do so, however, may take as long as a decade, he says.
More here.