91av

PHYSICS HAS A problem. In general relativity, time is entwined with space. It is relative, depending on speed or gravity. In quantum mechanics, however, time is an absolute. It is a smooth background into which we can plug our equations. How can time be these two conflicting things at once? “Solving the time problem is kind of the crucial issue really,” says at the Rotman Institute for Philosophy in Canada. If we want to unite these two incompatible theories into a theory of everything, something’s got to give.

Lots of research aims to tackle this problem. Some treat quantum systems as relative, as in general relativity, while others view space-time as a quantum field, the fundamental entity that everything is made of in quantum mechanics. Now, a new approach is gaining interest. It is inspired by the famous Schrödinger’s cat thought experiment, in which an unlucky feline is trapped in a box appearing to be both dead and alive. We know particles can be in a state of superposition, appearing to occupy many states at the same time until we measure them. What if those particles could act like a clock?

and , both at the University of Oxford, UK, want to run . “It’s like the Schrödinger’s cat experiment,” says Marletto, “but instead of a cat, you’ve got what, loosely speaking, acts like a clock.” Take two masses large enough to interact through gravity and no other forces – say, molecules or tiny diamonds. Put both in a superposition. Each mass location in the experiment corresponds to a different space-time, according to general relativity.

If a clock can be in a superposition of ticking at different rates, it would indicate that time in quantum mechanics isn’t fundamental, but relative to a given clock. The experiment is theoretical, but the pair hope it will be possible in five to 10 years. Since gravity would be the only thing connecting the masses, the pair says, seeing an entanglement would mean gravity , giving clues about theories of quantum gravity that could help link relativity and quantum mechanics.

Others hope that putting an atomic clock in a superposition could test quantum gravity. research institute in Boulder, Colorado, has recently – a millimetre-tall cloud of atoms – ticks faster at the top, where Earth’s gravity is ever so slightly weaker, than at the bottom (see interview). Ye hopes to make the clock around 20 times more precise in the next few years so that if the clock contains entangled atoms, gravity might break that entanglement. “Nobody has tested general relativity effects on the quantum scale,” says Ye. Perhaps Schrödinger’s clock – and not his cat – will be the way to do it, after all.