How we could discover quantum gravity without rebuilding space-time

In the search for a theory that unites general relativity and quantum physics, many have tried to rethink space-time. But what if space-time emerges naturally, like a hologram?

MODERN physics has two stories to tell about our universe. The first says it is fundamentally made of space-time: a continuous, stretchy fabric that has ballooned since the dawn of time. The other says it is fundamentally made of indivisible things that can’t decide where they are, or even when.

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Both stories are compelling, describing what we observe with incredible accuracy. The big difference, though, is the scale at which they apply. Albert Einstein’s theory of general relativity, which describes gravity, space and time, rules over very massive objects and cosmic distances. Quantum physics, meanwhile, governs tiny, sprightly atoms and subatomic particles.

Ultimately, both stories can’t be true. Nowhere is this more apparent than at the big bang, where everything in the universe was compacted into an infinitesimally small point. Here, you need a single theory that encompasses gravity and the quantum realm. “Why we’re here is the big question,” says Toby Wiseman, a theorist at Imperial College London. “It seems that quantum gravity is the only answer.”

Alas, it is an answer we are yet to find, despite many decades of searching. Quantum gravity means a reconciliation of the continuous and the indivisible, the predictable and the random. There are many ideas, but none can totally incorporate everything. “We’re still no better off at understanding the beginning of space and time,” says Wiseman.

Most physicists attempting this begin with quantum physics, the workhorse of which is quantum field theory. This describes three of the four forces of nature – electromagnetism, the strong nuclear force and the weak nuclear force – by “quantising” them as force-carrying elementary particles. It is only a matter of time, you might think, until gravity is shown to be the effect of another quantum force-carrier, known as the graviton.

In practice, however, gravity has proved nigh-on impossible to quantise. In the calculations required, it throws up senseless infinities that, unlike those for the other forces, can’t be tamed.

The holographic universe

Worse, Einstein’s gravity is unique in that it equates to the geometry of space-time, the very stage on which the other forces act. If gravity becomes just another force, what happens to this stage? This is why most quantum field theories start off with space-time as a given. “You sort of have to assume something,” says Nick Huggett, a philosopher at the University of Illinois in Chicago.

One alternative is to build space-time itself out of indivisible units, such as the loops and foams proposed by a theory called loop quantum gravity. Another option, and the one that is currently more popular, was inspired by a mathematical conjecture called the AdS/CFT correspondence, which suggests that gravity can pop out of a quantum field theory at some cosmic boundary, like the hologram on the back of your credit card.

This holographic correspondence only works in an abstract “toy model” of space-time very different to our own. Still, it has convinced many in the field that rebuilding space-time from the bottom up may be unnecessary. Instead, space-time may emerge naturally from more fundamental mathematical relationships. 

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