Physics is full of paradoxes, but one of the most famous is the contradiction between quantum mechanics and Einstein's theory of general relativity. At last, a new set of incredible experiments may reconcile the two systems.

*Image via **Patrick Edwin Moran*

Over at Quanta, Natalie Wolchover introduces the problem:

It starts like a textbook physics experiment, with a ball attached to a spring. If a photon strikes the ball, the impact sets it oscillating very gently. But there’s a catch. Before reaching the ball, the photon encounters a half-silvered mirror, which reflects half of the light that strikes it and allows the other half to pass through.

What happens next depends on which of two extremely well-tested but conflicting theories is correct: quantum mechanics or Einstein’s theory of general relativity; these describe the small- and large-scale properties of the universe, respectively.

In a strange quantum mechanical effect called “superposition,” the photon simultaneously passes through and reflects backward off the mirror; it then both strikes and doesn’t strike the ball. If quantum mechanics works at the macroscopic level, then the ball will both begin oscillating and stay still, entering a superposition of the two states. Because the ball has mass, its gravitational field will also split into a superposition.

But according to general relativity, gravity warps space and time around the ball. The theory cannot tolerate space and time warping in two different ways, which could destabilize the superposition, forcing the ball to adopt one state or the other.

Knowing what happens to the ball could help physicists resolve the conflict between quantum mechanics and general relativity. But such experiments have long been considered infeasible: Only photon-size entities can be put in quantum superpositions, and only ball-size objects have detectable gravitational fields. Quantum mechanics and general relativity dominate in disparate domains, and they seem to converge only in enormously dense, quantum-size black holes. In the laboratory, as the physicistFreeman Dyson wrote in 2004, “any differences between their predictions are physically undetectable.”

In the past two years, that widely held view has begun to change. With the help of new precision instruments and clever approaches for indirectly probing imperceptible effects, experimentalists are now taking steps toward investigating the interface between quantum mechanics and general relativity in tests like the one with the photon and the ball. The new experimental possibilities are revitalizing the 80-year-old quest for a theory of quantum gravity.

“The biggest single problem of all of physics is how to reconcile gravity and quantum mechanics,” said Philip Stamp, a theoretical physicist at the University of British Columbia. “All of a sudden, it’s clear there is a target.”

Theorists are thinking through how the experiments might play out, and what each outcome would mean for a more complete theory merging quantum mechanics and general relativity. “Neither of them has ever failed,” Stamp said. “They’re incompatible. If experiments can get to grips with that conflict, that’s a big deal.”

Want to find out about the experiments — some of which will be in space — that could result in a merging of quantum mechanics and gravity? **Read the whole essay over at Quanta**.