More: Seven things that don鈥檛 make sense about gravity
Quantum mechanics and relativity, our two most successful theories of how the world works, both seem strangely at odds with the everyday world as we experience it 鈥� and with each other.
Quantum theory, which describes how things behave at the subatomic level, is certainly weird. Quantum objects can simultaneously be in two locations, or move in different directions.
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Relativity is, if anything, worse. We can use it to describe gently curving space-time, but the equations turn to nonsense in the extreme conditions found at the heart of a black hole or the beginning of the universe.
From a physicist鈥檚 point of view, the big problem is that no one has figured out how quantum theory and relativity fit together to make a quantum theory. There has to be a better theory out there, one that describes how and why everything works, from subatomic to cosmological scales, but it is proving extraordinarily elusive.
Einstein was among the first scientists to try to marry gravity with other theories of physics, yet we may be no closer now than he was when he began his search. The most popular quantum gravity theories today seem to have fundamental problems that no one knows how to solve.
Does that mean we鈥檒l never get there? We shouldn鈥檛 despair, says Lee Smolin at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. 鈥淚 am a big believer in our capacity to understand the universe we find ourselves in,鈥� he says.
According to Roger Penrose of the University of Oxford, the final quantum gravity theory won鈥檛 look like anything that鈥檚 around at the moment. He reckons today鈥檚 theories are not powerful enough to even be considered as candidates, as they ignore important issues such as solving the mysteries of the quantum world鈥檚 strange behaviour. 鈥淭here ought to be such a theory, but I would expect it to represent a major revolution in our picture of the physical world,鈥� he says. 鈥淚t will require a major breakthrough, and a fundamental departure from current thinking.鈥�
Will we get there? Penrose is optimistic. 鈥淲e might destroy ourselves in a nuclear war or by overheating of the Earth,鈥� he says. 鈥淏ut barring that sort of thing, I don鈥檛 see why not.鈥�