In 1935,
when both quantum mechanics and Albert Einstein’s general theory of relativity
were young, a little-known Soviet physicist named Matvei Bronstein, just 28
himself, made the first detailed study of the problem of reconciling the two in
a quantum theory of gravity.

Two micro-diamonds would be used to test the quantum nature of gravity.

This
“possible theory of the world as a whole,” as Bronstein called it, would
supplant Einstein’s classical description of gravity, which casts it as curves
in the space-time continuum, and rewrite it in the same quantum language as the
rest of physics.

Bronstein
figured out how to describe gravity in terms of quantized particles, now called
gravitons, but only when the force of gravity is weak — that is (in general
relativity), when the space-time fabric is so weakly curved that it can be
approximated as flat. When gravity is strong, “the situation is quite
different,” he wrote. “Without a deep revision of classical notions, it seems
hardly possible to extend the quantum theory of gravity also to this domain.”

His words
were prophetic. Eighty-three years later, physicists are still trying to
understand how space-time curvature emerges on macroscopic scales from a more
fundamental, presumably quantum picture of gravity; it’s arguably the deepest question in physics. Perhaps, given the chance, the whip-smart Bronstein might
have helped to speed things along. Aside from quantum gravity, he contributed
to astrophysics and cosmology, semiconductor theory, and quantum
electrodynamics, and he also wrote several science books for children, before
being caught up in Stalin’s Great Purge and executed in 1938, at the age of 31.

The problem
is gravity’s extreme weakness. Whereas the quantized particles that convey the
strong, weak and electromagnetic forces are so powerful that they tightly bind
matter into atoms, and can be studied in tabletop experiments, gravitons are
individually so weak that laboratories have no hope of detecting them. To
detect a graviton with high probability, a particle detector would have to be
so huge and massive that it would collapse into a black hole. This weakness is
why it takes an astronomical accumulation of mass to gravitationally influence
other massive bodies, and why we only see gravity writ large.

Not only
that, but the universe appears to be governed by a kind of cosmic censorship:
Regions of extreme gravity — where space-time curves so sharply that Einstein’s
equations malfunction and the true, quantum nature of gravity and space-time
must be revealed — always hide behind the horizons of black holes.

“Even a few
years ago it was a generic consensus that, most likely, it’s not even
conceivably possible to measure quantization of the gravitational field in any
way,” said Igor Pikovski, a theoretical physicist at Harvard University.

Now, a pair
of papers recently published in Physical Review Letters has changed the
calculus.

The papers contend that it’s possible to access quantum gravity after all — while learning nothing about it. The papers, written by Sougato Bose at University College London and nine collaborators and by Chiara Marletto and Vlatko Vedral at the University of Oxford, propose a technically challenging, but feasible, tabletop experiment that could confirm that gravity is a quantum force like all the rest, without ever detecting a graviton. Miles Blencowe, a quantum physicist at Dartmouth College who was not involved in the work, said the experiment would detect a sure sign of otherwise invisible quantum gravity — the “grin of the Cheshire cat.”

The papers contend that it’s possible to access quantum gravity after all — while learning nothing about it. The papers, written by Sougato Bose at University College London and nine collaborators and by Chiara Marletto and Vlatko Vedral at the University of Oxford, propose a technically challenging, but feasible, tabletop experiment that could confirm that gravity is a quantum force like all the rest, without ever detecting a graviton. Miles Blencowe, a quantum physicist at Dartmouth College who was not involved in the work, said the experiment would detect a sure sign of otherwise invisible quantum gravity — the “grin of the Cheshire cat.”

A levitating
microdiamond ( green dot ) in Gavin Morley’s lab at the University of Warwick, in
front of the lens used to trap the diamond with light.

The proposed
experiment will determine whether two objects — Bose’s group plans to use a
pair of microdiamonds — can become quantum-mechanically entangled with each
other through their mutual gravitational attraction. Entanglement is a quantum
phenomenon in which particles become inseparably entwined, sharing a single
physical description that specifies their possible combined states. (The
coexistence of different possible states, called a “superposition,” is the
hallmark of quantum systems. ) For example, an entangled pair of particles might
exist in a superposition in which there’s a 50 percent chance that the “spin”
of particle A points upward and B’s points downward, and a 50 percent chance of
the reverse. There’s no telling in advance which outcome you’ll get when you
measure the particles’ spin directions, but you can be sure they’ll point
opposite ways.

The authors
argue that the two objects in their proposed experiment can become entangled
with each other in this way only if the force that acts between them — in this
case, gravity — is a quantum interaction, mediated by gravitons that can
maintain quantum superpositions. “If you can do the experiment and you get
entanglement, then according to those papers, you have to conclude that gravity
is quantized,” Blencowe explained.

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