This story is part of our Cosmic Perspective special, in which we confront the staggering vastness of the cosmos and our place in it. Read the rest of the series here.
We tend to think of space-time as the underlying structure of the universe. But whether it really is fundamental or emerges from something deeper is a question that keeps physicists up at night. “It’s not just a philosophical question that you discuss over a beer,” says at the University of Birmingham in the UK. “It is actually something that comes into the calculations that people do.”
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The best place to start is quantum mechanics, which describes the behaviour of subatomic particles. Famously counterintuitive, one of the theory’s core tenets is that connections between particles can transcend our usual notions of space and time. This happens via a phenomenon called entanglement, in which particles can affect each other’s properties even when they are half a universe apart.
Cosmologists now generally accept that entanglement is intimately linked to the emergence of space. If we know the degree of entanglement between two quantum particles, we can derive the distance between them. Do that for a network of many particles and you start to form a geometry from which what we call space can emerge. Perhaps, then, space emerges from quantum entanglement.
Entanglement and space-time
What’s more, advances in string theory, a candidate for a theory of everything, say that the goings-on in space can be fully described by data held on the outer surface, or boundary, of that space, a phenomenon known as holographic duality. Put that together with quantum entanglement and you can build a universe that boasts spatial structure: distances and geometry.
, a mathematical physicist based at the California Institute of Technology, has worked with at Johns Hopkins University in Maryland and at Virginia Tech to build an intriguing explanation of space-time’s origins. Entanglement between particles on the boundary of space-time gives rise to a particular “distance”, the idea goes, and the entanglement distances translate into “geodesics”, or the trajectories followed by particles as they move through the universe. These geodesics build a geometry to space-time, something like the curved geometry that Albert Einstein’s general theory of relativity says lies behind gravity. “Entanglement becomes curvature, and that curvature can be thought of as geometry,” says Michalakis.
Yet this doesn’t answer the fundamental question. Saying space and time arise from the boundary surface of space-time is just kicking the question down the road. “We haven’t, in a deep sense, explained why space should exist,” says Taylor.
The answer might be something completely different. That is certainly what at the University of Oxford thinks. She works with her Oxford colleague and others on , which aims to express the laws of physics in terms of which transformations of a physical system are possible, while boiling everything down to quantities of information. Because the universe seems to run as a kind of information processor, constructor theory seeks an information-based take on the origins of space. “We would say that time is not fundamental, and space-time is not fundamental either,” says Marletto. Such ideas remain a work in progress, though.
A deeper understanding will come through the study of cosmological phenomena such as black holes and the singularity at each one’s centre, says Taylor. “The whole notion of space-time breaks down there, and understanding how it breaks down is intimately linked to the question of how it emerged in the first place,” she says. “We know that things are going crazy there. Once we understand that, we can flip it round and see how the three spatial dimensions actually emerge.”
