We first discovered the laws of gravity, and then those of quantum mechanics. But new work suggests nature might go about it the other way around: space-time, and hence gravity, could emerge from a fundamental quantum mechanical description of the universe.
According to Einstein鈥檚 general relativity, gravity is the curvature of space-time. That this geometry might be related to the minuscule quantum world was first understood in the 1970s when Stephen Hawking and Jacob Bekenstein showed that the entropy of a black hole聽鈥� which depends on the black hole鈥檚 microscopic quantum structure聽鈥� is proportional to its surface area.
While at Harvard University in the late 1990s, Juan Maldacena discovered a connection between a theory of gravity that describes a volume of space and a quantum field theory that describes the volume鈥檚 surface, and doesn鈥檛 include gravity. Since then, others have used Maldacena鈥檚 conjecture to show that the area of certain surfaces within such a volume is related to the amount of quantum entanglement between different regions in the quantum field theory. Two regions are entangled if the state of one cannot be described independently of the other.
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In this model, changing the amount of entanglement between different surface regions can create or destroy space-time in the volume, suggesting that it emerges from entanglement. There is one catch: this space-time is not quite the same as the space-time of our universe.
Entangled points
Now, and at the California Institute of Technology in Pasadena have tried to extract the kind of space-time we would find in the vicinity of our solar system from standard quantum mechanics. This type of space-time is one whose curvature is mostly flat, but with small undulations due to weak gravitational fields.
To see if space-time can emerge from this quantum description, Cao and Carroll used an abstract mathematical concept called Hilbert space that can be split into different tiny parts, such that each one corresponds to a single point in 3D space. 鈥淭here is entanglement between these little parts,鈥� says Carroll. 鈥淎 lot of entanglement between some, and very little between others.鈥�
The researchers relate entanglement to geometry by further assuming that the greater the entanglement between two parts, the closer they are.
The entire system is also assumed to be in some state of equilibrium, such that increasing the entanglement in one region decreases it elsewhere, and vice versa. Given a handful of such assumptions, Cao and Carroll have shown that the equations governing the dynamics of entanglement are similar to Einstein鈥檚 equations of general relativity. In other words, space-time and gravity emerge from entanglement.
at Princeton University, who researches emergent space-time in the context of Maldacena鈥檚 conjecture, says the work is 鈥渆xcellent鈥�. But the assumptions must be validated. 鈥淭o construct an actual system that satisfies these assumptions is going to be difficult, but it鈥檚 also very exciting,鈥� he says.
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Read more: Gravity may be created by strange flashes in the quantum realm
