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52 Scientific American, April 2019 Illustration by Ben Gilliland

small things happening in the Planckian realm

might similarly influence phenomena accessible to

tabletop experiments. And although particle acceler-

ators cannot be upgraded by orders of magnitude—

we are unlikely to see accelerators with 1,000-kilo-

meter circumferences—the precision of tabletop

experiments may well improve by a few orders of

magnitude in the decades ahead.

Such gains might allow Aspelmeyer to test a key

assumption shared by all theories of quantum gravity:

that gravity itself should display some profoundly

strange quantum properties. “If that is really true,

there should be some consequences for phenomena

at an energy scale that is much much smaller [than

the high energies that correspond with the Planck

scale]”—that is, at roughly the scale we inhabit, Aspel-

meyer says. “The question is: Can we come up with

experiments that possibly test those consequence?”

What Aspelmeyer has in mind is an experiment

that would measure the gravitational attraction

between two spherical masses. Unlike Cavendish,

though, Aspelmeyer will not be weighing Earth, and

his milligram masses are orders of magnitude small-

er than Cavendish’s lead balls. He wants to test

whether gravity interacts at all with the quantum

properties of small masses. Specifically, he intends to

look at what kind of gravitational effects might be

generated by an object placed in a “Schrödinger’s

cat”-like state of being both here and there at once.

In the quantum world, particles have the uncanny

ability to be in two places simultaneously—a super-

position, as physicists call it. Scientists have observed

quantum superpositions many times in laboratories,

but they are delicate states. Interactions with any

nearby particles quickly cause objects in superposi-

tion to “collapse” into a single position. But while the

superpositions last, Aspelmeyer wonders what prop-

erties these particles have. Do they create their own

minuscule gravitational fields, for instance? “Imag-

ine you place an object in a superposition,” he says,

“and now you ask a question: How does it gravitate?

That is the question we want to answer.”

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

PRELIMINARY EXPERIMENT #1

One experiment, proposed by physicist Markus Aspelmeyer, will ultimately

attempt to put a mass into a superposition state of being in two locations

simultaneously and then try to see if the gravitational field of the mass

splits into two as well. A preliminary version of this trial will develop the

technology to detect gravitational fields of smaller objects than ever

before—in this case, two tiny gold spheres. An electromagnet attached

to a spring will cause one ball to vibrate, and the other, at the end of a

cantilever, should oscillate in response to the changing gravitational pull.

ULTIMATE EXPERIMENT #1

Eventually the team will aim to put one of these spheres into a state

of superposition. If this ball’s gravitational field goes into superposition,

too, and exists in two places, then the other mass should feel the pull

of both fields and become entangled, entering superposition as well.

Quantum Gravity

Experiments

To understand whether gravity fits into quantum theory, physicists

are designing experiments to measure gravitational fields with

extreme precision to search for signs of quantum behavior. Such

behavior might include “superposition”—the ability of quantum

particles to occupy two places simultaneously—and “entangle­

ment”—a kind of connection between quantum objects where their

fates become intertwined. If researchers can find evidence of gravi­

tational fields displaying superposition or entanglement, they will

know that gravity has quantum properties.