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.
Cinema-goers who saw the film Interstellar are already familiar with the basic principle behind the Vienna team’s work. Einstein’s theory of general relativity states that an extremely massive object causes clocks near it to run more slowly because its strong gravitational field stretches the fabric of space-time (which is why a character in the film aged only an hour near a black hole, while seven years passed on Earth). On a subtler scale, a molecule placed nearer the Earth’s surface experiences a slightly slower clock than one placed slightly further away.
Because of gravity’s effect on space-time, Pikovski’s team realised that variance in a molecule’s position will also influence its internal energy—the vibrations of particles within the molecule, which evolve over time. If a molecule were put in a quantum superposition of two places, the correlation between position and internal energy would soon cause the duality to 'decohere' to the molecule taking just one path, they suggest. “In most situations decoherence is due to something external; here it’s as though the internal jiggling is interacting with the motion of the molecule itself,” adds Pikovski.
Scientific American: Gravity Kills Schrödinger's Cat, Elizabeth Gibney and Nature magazine