Is space-time quantum? Six experiments that unpick the fabric of the universe

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IF YOU could empty the universe, what would be left over? The underlying structure of the cosmos is called space-time, and it is often likened to a fabric. But “space-time fabric is a science-fiction term”, says Jonathan Oppenheim, a physicist at University College London. There is no consensus about what it really means.

In classical physics, namely in Albert Einstein’s general theory of relativity, the fabric of space-time doesn’t exist on its own. Instead, space-time is intertwined with – and shaped by – mass and energy, giving rise to gravity. Most importantly, Einstein’s equations are continuous, so, in the classical view, the fabric must be smooth.

But today, most physicists think that space-time must abide by the rules of quantum mechanics, which govern the behaviour of subatomic particles and fields. In which case, it can be broken down into discrete chunks, or quantised. This would mean that, although space-time appears as a smooth background against which everything in the universe plays out, if you could zoom in sufficiently closely, you would see that it is actually made of something, just like everything else.

The problem is, we still have no evidence that space-time is quantised. It is difficult to prove it one way or another, because what you might imagine as the “pixels” of space-time – its most fundamental constituents – would be so vanishingly small that directly observing them would be impossible.

That leaves us with indirect observations. The good news is that physicists have devised a range of ingenious experiments that could finally settle the question of what space-time is made of, if anything, once and for all.