Relativity v quantum mechanics – the battle for the universe

Physicists have spent decades trying to reconcile two very different theories. But is a winner about to emerge - and transform our understanding of everything from time to gravity?

It is the biggest of problems, it is the smallest of problems. At present physicists have two separate rulebooks explaining how nature works. There is general relativity, which beautifully accounts for gravity and all of the things it dominates: orbiting planets, colliding galaxies, the dynamics of the expanding universe as a whole. That's big. Then there is quantum mechanics, which handles the other three forces - electromagnetism and the two nuclear forces. Quantum theory is extremely adept at describing what happens when a uranium atom decays, or when individual particles of light hit a solar cell. That's small.

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