April 2019, ScientificAmerican.com 53The experiment Aspelmeyer hopes to carry out was
first proposed as a Gedankenexperiment —a thought
experiment—by the legendary physicist Richard Feyn-
man at a conference in 1957. Feynman argued that if
gravity is indeed a quantum phenomenon, a superpo-
sition of a particle in two places at once would create
two separate gravitational fields. According to the
general theory of relativity, gravitational fields are
distortions of space and time. Thus, in the case of a
small mass in a quantum superposition, two differ-
ent spacetimes would coexist side by side, almost
like two separate mini universes, a state of affairs
that should not exist in Einstein’s theory.
If that spacetime superposition arose, how would
another object—a test mass—interact with it? Would
the motion of the test mass indicate that it had felt
the pull of two different gravitational fields? Or
would the interaction cause the superposition to col-
lapse, as some physicists believe, resulting in normal
gravitational dynamics? If the superposition persist-
ed and if the test mass did interact with the superpo-sition’s gravitational fields, it would be
strong evidence that the test mass and
the superposition had become “en -
tangled”—a telltale feature of quan-
tum mechanics where the properties
of two separate particles become inex-
tricably linked. Feynman argued that
because only quantum phenomena
can become entangled, the experi-
ment would show that gravity, like all
other known forces in the universe, is
fundamentally quantum.
Such an outcome would not in
it self validate any particular theory of
quantum gravity, but it would be indi-
rect evidence that gravity is quantized
on the Planck scale. Even more broad-
ly, the experiment would provide com-
pelling evidence that the laws of quan-
tum mechanics hold at all scales, not
just in the realm of photons, atoms
and other fundamental particles.
Some physicists have clung to the idea
that quantum mechanics might break
down when it comes to describing the
macroscopic world. Roger Penrose, for
example, a physicist at the University
of Oxford, and Diósi have suggested
that gravity causes superpositions
above a certain size to collapse, effec-
tively dividing the quantum world
from the so-called classical one.
“One of the areas where quantum
theory is supposed to fail is when it
comes to describing gravity,” says Chi-
ara Marletto, a theoretical physicist at
Oxford. “There have been a number of
eminent scientists who maintain that
gravity will be exactly the place where quantum theo-
ry breaks down. So, instead of having a quantized
[theory of ] gravity, we should actually make quantum
theory classical for it to describe gravity.” In this way of
thinking, quantum theory might need to be modified
to make it consistent with general relativity, rather
than trying to fit gravity into quantum theory as it is.TURNING THOUGHT INTO REALITY
tHe teCHnoloGy and expertise needed to decide the
issue did not exist when Feynman came up with his
idea, and even now the project remains daunting. For
several years now Aspelmeyer’s lab has been pushing
to measure the gravitational fields of ever smaller
masses. It is a tricky undertaking: Earth’s enormous
gravity swamps the fields of even relatively large
objects. The smallest mass for which a gravitational
field has been measured so far is a 700-milligram
tungsten sphere. That is about the mass of a paper
clip or a raisin—a gargantuan object compared to
quantum particles.Laser
beam< 1 μmFalling diamond spheres in superpositionSpheres become entangledBoth masses in superpositionTest mass Superposition massGold spheres1 mmGravitational
field
Cantilever
SpringElectromagnet1 μmEXPERIMENT #2
A second experimental concept, proposed by two groups (by Sougato
Bose and his colleagues and, independently, by Chiara Marletto and Vlatko
Vedral), would drop two diamond spheres side by side for a couple of
seconds. If the spheres are just 100 microns apart, the proximity of their
gravitational fields should cause the spheres to become entangled, the
physicists reason. If that happens, the experimenters will detect a
correlation between the direction of their spins after the drop. If the
particles do not become entangled—presumably because gravity does not
experience this quantum phenomenon—then the spins should be random.