For the past forty years the problem of quantum gravity has attracted many serious research efforts. Some of these have fallen by the wayside, dismissed as incorrect or outlandish. Others have contributed to the toolbox of string theory. Still more have been developed independently. These alternative approaches provide different explanations of reality.
The most popular challenger is loop quantum gravity (LQG), pioneered by Abhay Ashtekar, Carlo Rovelli and Lee Smolin. Inspired by general relativity, this aims to reformulate physics without a fixed spacetime background. Remember that Einstein saw spacetime as an evolving geometry, not a static absolute.
LQG radically changes our view of the universe at small scales. Just as quantum mechanics forces energy into lumps, LQG claims that spacetime itself comes in chunks of a definite minimum size. More precisely the fabric of space becomes a web of interwoven loops known as a spin network (see left). This can evolve in time, providing a model for the quantum foam at the heart of reality.
It is certainly an appealing notion of quantum space and time. But LQG is far less ambitious than string theory; it is not a unified theory of forces and particles. Moreover we don’t yet know whether LQG correctly reproduces Einstein’s gravity on large scales. Finally it suffers from a familiar problem – no experimental verification.
Twistor theory is the brainchild of Roger Penrose. He does away with the idea of spacetime entirely, replacing it with complex quantum mechanical objects. Twistor theory caught on in the 1970s and 1980s as a possible road to quantum gravity, and recently had a dramatic resurgence as a calculational tool in QFT.
Others advocate Alain Connes’ noncommutative geometry, which revolutionises our intuition about distances at the quantum level, or the causal dynamical triangulations of Jan Ambjorn and Renate Loll.
All these areas employ far fewer researchers than string theory, perhaps because they are generally pure theories of quantum gravity. Strings have become useful for a wider range of problems and have succeeded in attracting researchers who would not have regarded themselves as string theorists.
Other byways towards quantum gravity have been explored less thoroughly. Many were invented by aspiring individuals. Isolated pockets of enthusiasts continue to probe these concepts, hoping to secure mainstream funding. Some ideas – like supergravity – have been incorporated into the M-theory framework. A few mavericks continue down their routes alone, occasionally stepping out of the woodwork to proclaim a new breakthrough.
So where does string theory fit into the picture? Perhaps it will be abandoned in favour of a simpler alternative. If we find experimental evidence for another theory, doubtless manpower and funding will be directed away from strings. Maybe some revolutionary principle will uproot all previous attempts to comprehend quantum geometry. We might need to start from a clean slate.
But some hope that the ultimate theory will be a blend of many ideas. A chef concocting the perfect dish selects the best from myriad ingredients. Likewise physicists may succeed by using elements of all the alternative descriptions. Some progress has already been made in this direction.
So much remains mysterious about quantum gravity. Which theory is right? Can we ever fully understand quantum spacetime? Only time will tell.