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April 2019, ScientificAmerican.com 53

The 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 μm

Falling diamond spheres in superposition

Spheres become entangled

Both masses in superposition

Test mass Superposition mass

Gold spheres

1 mm

Gravitational
field

Cantilever

Spring

Electromagnet

1 μm

EXPERIMENT #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.