April 2019, ScientificAmerican.com 552of microscopic particles without having to measure
their gravitational fields.
In the proposed experiment, pairs of micron-wide
diamond spheres would be put into superpositions
and allowed to fall in a vacuum for a couple of sec-
onds in Earth’s gravitational field. If the spheres were
close enough together—about 100 microns apart, ac -
cording to Bose’s estimates—their gravitational fields
should cause the particles to become entangled.
When that happens, the properties of the entangled
particles will instantaneously correlate in ways that
are not possible in classical physics. One particle’s
spin, for example—whether it points up or down in a
magnetic field—will flip in the opposite direction as
soon as the spin of its entangled partner is measured.
By tracking how often such correlations occur—
Bose says that 10,000 trials should yield an answer—
he, Marletto and Vedral could determine whether the
falling diamonds had in deed be come entangled. Once
again, entanglement would suggest that gravity itself
must have quantum properties. “Our work will prove
that gravity is quantum in the sense that it obeys the
superposition principle,” Bose says.
The experiment faces many of the same
challenges that Aspelmeyer’s does: the
need for large superpositions that last
for seconds at a time and stay close
enough together so that gravity can
entangle them. “That makes the thing
very difficult,” Bose says. “But I’m sure
I’ll see it in my lifetime.”
Both experiments, if they pan out,
would give physicists their first indi-
rect evidence that gravity—and there-
fore space time itself—must be quan-
tized on the Planck scale. And that is
an exciting prospect for Rovelli and
other quantum-gravity theorists, who
have spent years working on theories
without any experimental feedback. “I
think it’s a game changer, this idea, the
attempt to see quantum gravity in the
lab,” Rovelli says. “As far as we know
[gravity’s quantum nature] should def-
initely be real, otherwise we haven’t
learned a thing about the world.”
A century after its birth quantum
mechanics remains the most baffling
of scientific theories. Some physicists,
most famously Einstein, doubted that it
could be the final word on the nature of
reality. Yet countless experiments have
confirmed the theory’s predictions, typ-
ically with multidecimal-point ac curacy.
In some sense, the question of whether
gravity is quantum or classical repre-
sents a last refuge for those who feel
that there must be something wrong
with quantum mechanics. If these ta-
bletop experiments succeed, that refuge will crumble.
“Quantum theory teaches us a completely different
way of de scribing what we can say about nature,”
Aspelmeyer says. “The rule book that we have found
through quantum theory is a fundamental one and
has to apply in general to all the theories we have.”MORE TO EXPLORE
A Micromechanical Proof-of-Principle Experiment for Measuring the Gravitational Force of Milligram
Masses. Jonas Schmöle et al. in Classical and Quantum Gravity, Vol. 33, No. 12, Article No. 125031;
June 23, 2016. https://iopscience.iop.org/article/10.1088/0264-9381/33/12/125031
Gravitationally Induced Entanglement between Two Massive Particles Is Sufficient Evidence of Quantum
Effects in Gravity. C. Marletto and V. Vedral in Physical Review Letters, Vol. 119, No. 24, Article No. 240402;
December 15, 2017. Preprint available at https://arxiv.org/abs/1707.06036
Spin Entanglement Witness for Quantum Gravity. Sougato Bose et al. in Physical Review Letters,
Vol. 119, No. 24, Article No. 240401; December 15, 2017. Preprint available at https://arxiv.org/
abs/1707.06050
FROM OUR ARCHIVES
Tangled Up in Spacetime. Clara Moskowitz; January 2017.
Crossing the Quantum Divide. Tim Folger; July 2018.
scientificamerican.com/magazine/saVACUUM CHAMBERS isolate small masses from the outside
world to measure their gravitational fields with minute precision.