Superposition -classical and quantum

The classic way to investigate quantum superposition is to fire an atom at a screen with two slits, also known as an interferometer. The atom can go through either, but experiments show that if no one is measuring which slit it passes through, it will actually go through both. The result is an "interference pattern" that forms in a detector placed behind the slits. This reveals a series of well-defined patches where the atoms appear to hit the detector, alternated with blank spaces where no atom seems to land. The only explanation for such a pattern is that each atom splits in two, with one part going through each slit, then interfering before it reaches the detector.

But general relativity says that mass distorts space and time in a way that causes things around it to feel the attractive force we know but in superposition, is an atom's mass creating two distinct distortions in space-time – and thus exerting a gravitational pull on itself?

Special relativity says that an atom moving through space will have a unique experience of the flow of time. This phenomenon is known as time dilation. But if a moving atom is in superposition, the time dilation must occur along two different paths at once, and will be different on each path. So when the superposition ends and the two become one again, have they aged differently?

More fundamentally, it is questionable whether general relativity even allows superposition. "There's a conflict here," says Penrose. "You can't have a superposition of two gravitational fields: that's illegal."

We have made interference patterns with molecules composed of 800 atoms, but the more massive they get, the shorter-lived the superposition. This has led some to suspect that gravity might be the real reason why massive collection of molecules do not superpose.

Superpositions are impossible for objects composed of more than a certain number of particles because of a phenomenon called spontaneous localisation, which suggests that the distribution of mass – its density – is what matters.