54 Scientific American, April 20191To realize Feynman’s thought experi-
ment, Aspelmeyer and his colleagues will
need to work with objects considerably
smaller than paper clips. They are now
developing a prototype experiment to de-
tect the gravitational fields of millimeter-
wide gold spheres (gold was chosen for its
density and purity) weighing just a few
tens of milligrams. “That’s a factor of tens
or hundreds less heavy than anything else
that has been measured so far,” Aspelmey-
er says. In the experiment, the researchers
will place two gold spheres a few millime-
ters apart, with one attached to a small,
spring-mounted magnet and the other
fixed to the end of a micromechanical can-
tilever. When the electromagnet is turned
on, the sphere on the spring will start vi-
brating, creating a changing gravitational
field that in turn makes the mass on the
cantilever bounce up and down like a div-
er on a board. The cantilever’s motion—tracked by la-
sers—essentially amplifies the gravitational force of
the sphere attached to the spring, making it easier to
detect against the background of Earth’s field.
After honing their gravitational-measuring skills
with ordinary, nonquantum masses, Aspelmeyer’s
team would then tackle superpositions. If he could
put two small spheres into superpositions, Aspel-
meyer could test how their gravitational fields inter-
acted. The results could suggest that the particleswere entangled, supporting Feynman’s intuition
about gravity’s quantum nature.
What will it take to pull all this off? To have a real-
istic shot at creating a quantum superposition, Aspel-
meyer will need to shrink his millimeter-size gravi-
tational test masses down to fractions of a micron—
a 1,000-fold reduction. At the same time, he will
need superpositions of objects that are massive
enough to have detectable gravitational fields. For
that he will likely draw on the talents of a colleague
at Vienna, Markus Arndt, who holds the record for
the largest object ever placed in a superposition: a
behemoth of a molecule containing more than 800
atoms. And instead of being stuck to springs and
cantilevers, the masses would be suspended in spacewith “optical tweezers”—tightly focused laser beams.
“If I can detect the gravitational field of an object
over which I can obtain quantum control, then I am
in business,” Aspelmeyer says. “This would be the
long-term dream—not tomorrow, not in five years.
Both from the top down and bottom up—from mak-
ing [the gravitational] masses smaller and making
the [superposition] masses larger—we think we
know how to get there and bring those two domains
together. Now we just need to work hard.”
Arndt, Aspelmeyer’s likely collaborator,
says the experiment presents a host of
challenges: the small, spherical masses
will be difficult to isolate gravitationally
and prone to interacting with any nearby
surface. “There are so many effects that
are hard to suppress,” he says. “Still, it has
to be tried, by all means. If we don’t start
now, it won’t be done in 10 years.” Arndt
compares the effort that will be required
with the search for gravitational waves, a
phenomenon predicted by Einstein’s gen-
eral theory of relativity. More than three
years ago the giant Laser Interferometer Gravitation-
al-wave Observatory (LIGO) finally detected the first
gravitational wave, but the discovery was a long time
coming. “It was a 40-year effort to get the gravitation-
al-wave detector going,” Arndt says.THE LAST REFUGE OF QUANTUM HOLDOUTS
aspelmeyer is not tHe only pHysiCist working on the
problem. In December 2017 two independent groups
simultaneously published their own very similar
takes on Feynman’s thought experiment. Sougato
Bose, a physicist at University College London, and
his colleagues and Marletto and her Oxford col-
league Vlatko Vedral de scribed a way to test for the
gravitational entanglement be tween superpositionsA quantum spacetime
would no longer be the
smooth continuum described
by general relativity; it
would be coarse-grained.