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WHEN the two most important figures in your life don’t get along, there will always be trouble. Just ask physicists: the two most totemic theories in their field are fundamentally incompatible, and generations of researchers have failed to reconcile them.

Quantum theory describes matter at its smallest scales, tracing three of the four basic forces of nature – the electromagnetic force and the strong and weak nuclear forces – to the subatomic particles that carry them. Einstein’s general relativity, meanwhile, makes sense of the cosmos at its grandest scales, revealing the force of gravity as the product of matter warping space-time.

Perhaps the biggest hint that they should be unified is that when you try to apply general relativity to the extreme conditions at the centre of a black hole, say, its equations go haywire. “That is the theory itself saying that we are stretching it beyond its regime of validity,” says at the University of Southern Denmark.

It makes sense to think that a more fundamental theory of gravity should emerge from quantum mechanics, because quantum mechanics best describes the world at the tiny scales and high energies where general relativity breaks down. But what that quantum theory of gravity looks like has proved a uniquely devilish question to answer.

One knotty problem arises from the way we calculate observable properties of subatomic particles with quantum theory. When you try to calculate an electron’s mass, say, the number of terms in the equations explode to infinities. This “non-renormalisability” has long been an insurmountable barrier, but just recently an idea called scale symmetry has suggested that, once you reach sufficiently high energies, things become more tractable again. The effect really kicks in at energies too high to probe with experiments, but it leaves an imprint at scales we can observe, meaning we can look at how a given idea works at low energies to see what happens at the highest energies, where gravity would be a quantum force.

Mikhail Shaposhnikov at the Swiss Federal Institute of Technology in Lausanne and Christof Wetterich at Heidelberg University in Germany have already used this approach to predict the masses of particles, including the and the top quark. Eichhorn and her colleagues are also using it to predict other particle properties, including interaction strength, and they are finding promising matches with existing measurements.

But what if they are barking up the wrong tree? While the overwhelming majority of physicists assume gravity is a quantum force, there is actually zero evidence to back that up. Sougato Bose at University College London has an idea of how to change that. He has to probe if the quantum-mechanical spins within two microscopic diamonds can become quantum entangled with one another through gravitational interaction, something that would happen only if gravity is a quantum force. “These will be groundbreaking experiments,” says Bose – but it’s too early to tell which way they will fall.