50 Scientific American, April 2019
1797 Henry CavendisH, one of Great Britain’s leadinG
scientists, built a contraption to weigh the world.
At the time, Earth’s mass was unknown, as was its
composition. Was it mostly solid rock? Did it vary with
depth? Astronomer Edmond Halley even suggested that
Earth might be hollow. Isaac Newton had compared
Earth’s mass with that of other bodies in the solar system
and knew, for example, that Earth was more massive
than the moon. He had even suggested a way to deter-
mine Earth’s absolute mass: measure the gravitational
attraction between two small spherical masses with great accuracy, then
extrapolate Earth’s own mass from the result. But Newton summarily dis-
missed his own idea—he thought the attraction between the spheres would be
too small to detect, even with impractically large masses. “Nay, whole moun-
tains will not be sufficient to produce any sensible effect,” he wrote in his mas-
terpiece, the Principia, which laid out his laws of motion and gravitation.On an August day more than a century later Cav-
endish proved Newton wrong. The device he had built
in a shed on his estate in southwest London consisted
of two 1.6-pound lead balls attached to opposite ends
of a six-foot-long wood rod, which hung from a wire
fastened to an overhead beam. Two much heavier lead
spheres, each weighing nearly 350 pounds, were sus-
pended separately about nine inches away from the
lighter balls. Cavendish expected that the gravitation-
al pull of the heavy spheres on the smaller ones would
make the wood rod rotate ever so slightly, and he was
right—it moved just over a tenth of an inch.
This al lowed him to directly measure the gravita-
tional force exerted by each of the larger spheres on
the smaller ones. Because he already knew that Earth
exerted a gravitational force of 1.6 pounds on each of
the small spheres (in the English system of units, a
pound is by definition a measure of force), Cavendish
could set up a simple ratio: the gravitational force be -
tween the small sphere and the large sphere com-
pared with the gravitational force between the small
sphere and Earth. Because the gravitational force is
directly proportional to the masses being measured,
he could use that ratio to solve for Earth’s unknown
mass. Over the course of nine months he repeated the
experiment 17 times and found that Earth weighed
13 million billion billion pounds, a result essentially
identical to the best modern estimates.
“It’s an incredible story,” says Markus Aspelmeyer,
who has been recounting the Cavendish experiment
during a Skype call. “It was the first precision tabletop
experiment [with gravity].” Cavendish’s 220-year-old
tour de force, though not actually conducted on a
tabletop, is a source of inspiration for Aspelmeyer, a
physicist at the University of Vienna in Austria. Like
Cavendish, he has plans for an ambitious, seeminglyimpossible ex periment, one that might transform our
understanding of gravity: he wants to use a small-scale
setup—literally on a tabletop in his lab—to find evi-
dence that gravity might be a quantum phenomenon.
Of the four fundamental forces in the universe,
gravity is the only one that cannot be described by the
laws of quantum mechanics, the theory that applies to
all other forces and particles known to physics. Elec-
tromagnetism; the “strong” nuclear force that binds
atomic nuclei; and the “weak” nuclear force that
causes radioactive decay—they are all quantum to the
core, leaving gravity as a sole, mysterious outlier.
This exception has vexed physicists since Albert
Einstein’s heyday. Einstein never managed to unify
his own theory of gravity—the general theory of rela-
tivity—with quantum mechanics. Most physicists
who now work on the problem believe that the unifi-
cation occurs when we zoom in on the cosmos to
what is called the Planck scale, after Max Planck, one
of the founders of quantum theory. Distances on the
Planck scale are so tiny—100 trillion trillion times as
small as a hydrogen atom—that spacetime itself is
thought to assume quantum characteristics. A quan-
tum spacetime would no longer be the smooth con-
tinuum de scrib ed by general relativity; it would be
coarse-grained, like a digital photograph that be -
comes pixelated when magnified. That graininess is
a hallmark of quantum theory, which confines the
energy, momentum and other properties of particles
to discrete bits, or quanta. But what exactly is a
quantum of spacetime? How could time or distance
be measured if space and time themselves are frac-
tured like broken rulers?
“All our theories of physics either explicitly or
implicitly require the existence of rods and clocks:
something occurred [here] at this time and then didTim Folger is a freelance
journalist who writes
for National Geographic,
Discover and other
national publications.
He is also the series editor
for The Best American
Science and Nature Writ-
ing, an annual anthology
pub lished by Houghton
Mifflin Harcourt.
IN