For a long time, the community believed it was logically impossible for gravity to be classical because coupling a quantum system with a classical system would lead to inconsistencies. In the 1950s, Richard Feynman imagined a situation that illuminated the problem: He began with a massive particle that is in a superposition of two different locations. These locations could be two holes in a metal sheet, as in the famous double-slit experiment. Here, the particle also behaves like a wave. It creates an interference pattern of light and dark stripes on the other side of the slits, which makes it impossible to know which slit it went through. In popular accounts, the particle is sometimes described as going through both slits at once.<\/p>\n

But since the particle has mass, it creates a gravitational field that we can measure. And that gravitational field tells us its location. If the gravitational field is classical, we can measure it to infinite precision, infer the particle\u2019s location, and determine which slit it went through. So we then have a paradoxical situation \u2014 the interference pattern tells us that we can\u2019t determine which slit the particle went through, but the classical gravitational field lets us do just that.<\/p>\n

But if the gravitational field is quantum, there is no paradox \u2014 uncertainty creeps in when measuring the gravitational field, and so we still have uncertainty in determining the particle\u2019s location.<\/p>\n