6 MARCH 2015 • VOL 347 ISSUE 6226 1097 SCIENCE sciencemag.org
are the same with a precision 100 times
better than any previous experiment, and
other efforts could go even further.
According to general relativity, the equivalence principle must hold exactly, as acceleration and gravity are essentially the same
thing. But general relativity may not be the
last word on gravity, because so far it cannot be reconciled with quantum mechanics, which governs physics on the smallest
scales. Efforts to bridge that gap often violate the equivalence principle, says Clifford
Will, a theorist at the University of Florida
in Gainesville. Spotting a violation “would
definitely mean that there is some sort of
physics beyond Einstein’s theory,” he says.
IRONICALLY, THE MOST FAMOUS TEST
of the principle, Galileo’s demonstration at
Pisa, probably never happened. “It’s a fiction,” says Alberto Martínez, a historian at
the University of Texas, Austin. The first
account of the event was penned long after Galileo died by his assistant Vincenzo
Viviani, who said the great man wanted to
show that Aristotle was wrong when he contended that heavier objects fall faster than
lighter ones do.
Galileo did write about such tests in 1638
in his Dialogues Concerning Two New Sciences: “[T]he variation of speed in air between balls of gold, lead, copper, porphyry,
and other heavy materials is so slight that …
I came to the conclusion that in a medium
totally devoid of resistance all bodies would
fall with the same speed.” But Galileo likely
inferred the result by timing balls rolling
down ramps, says John Heilbron, a historian emeritus at the University of California
(UC), Berkeley. “He had a clear idea that it
didn’t matter what he made the ball out of,”
Heilbron says. “I think he was too lazy” to
actually drag weights up a tower.
Although Galileo’s analysis jibes with the
equivalence principle, he wouldn’t have
understood it that way, says Domenico
Bertoloni Meli, a historian of science at Indiana University, Bloomington. The concepts of
inertial and gravitational mass were invented
later by Isaac Newton. Newton proved that
the two types of mass were equal by showing
that pendulums of equal lengths but different materials swing at the same rate, as he
described in Philosophiae Naturalis Principia Mathematica in 1687.
The equivalence principle proved key to
Einstein’s invention of general relativity.
Einstein deduced that gravity arises when
energy and mass bend spacetime. In that
warped spacetime, freefalling objects follow
the straightest possible paths, or geodesics,
which to us appear as the parabolic arc of
a thrown ball and the elliptical orbit of a
planet. The change of the object’s speed and
direction is its acceleration, which depends
on the amount of warping of spacetime. If
such warping is all there is to gravity, then in
a given situation all things must accelerate
at the same rate as they fall. That’s because
for any starting position and velocity, there
is only one straightest path in spacetime.
But gravity could be more complicated,
says Thibault Damour, a theorist at the Institute of Advanced Scientific Studies (IHES)
in Bures-sur-Yvette, France. According to
Einstein’s famous equation E = mc2, an object’s inertial mass measures the energy
trapped inside it. So a sliver of an atom’s
mass comes from the electromagnetic force
that binds the electrons to the nucleus. Much
more comes from the energy of the strong
force that binds particles called quarks inside the nucleus’s protons and neutrons. In
general relativity, all energy has the same
effect regardless of its source, Damour says.
However, in some theories that aim to
unify gravity and quantum mechanics, it
matters how such energy arises. For example, string theory posits that every fundamental particle is an infinitesimal string
rippling through a complex 10-dimensional
space. In string theory a “dilaton field” acts
Forty years ago, a pair of stars locked in a cosmological danse macabre gave cosmologists a vivid glimpse of general relativity in action. One key prediction of
the theory is that massive, accelerating objects send out ripples
in spacetime. Scientists haven’t
detected such gravitational waves
directly, but the orbiting stars
showed that they exist.
Astrophysicist Joseph Taylor Jr.
and his doctoral student Russell
Hulse were surveying the galaxy
for pulsars: collapsed stars, or
neutron stars, that sweep the
universe with tight, lighthouse-like beams of energy. Using the
305-meter-wide dish of the Arecibo
Observatory in Puerto Rico, Hulse
and Taylor could see those beams
as regular pulses of radio waves.
One of their finds, known as PSR
B1913+16, raised eyebrows. The
intervals between pulses, about 59
milliseconds, were oddly irregular—
sometimes tens of microseconds
longer than expected, sometimes
shorter. Apparently the pulsar was
orbiting another neutron star, causing its signals to vary as it moved
toward and away from Earth.
More tantalizing was what happened in the following years: The
orbit of the pulsar contracted. It was
shrinking exactly as Albert Einstein’s
equations predicted it should if the
stars were dissipating energy in the
form of gravitational waves. Hulse
and Taylor’s observations won them
the 1993 Nobel Prize in physics.
Since then, several other binary pulsars have told the same story.
In a few hundred million years, PSR
B1913+16 and its companion will collide and merge, emitting a new, more
powerful burst of gravitational waves.
Detectors such as the Advanced
Laser Interferometer Gravitational-Wave Observatory may soon detect
such signals from other pairs of
perishing stars—finally observing the
gravitational waves that physicists
are already sure are there. ■ –E.C.
show effects of
Mark Kasevich of Stanford University
plans to recreate Galileo’s famous tower
experiment with atoms.
GENERAL RELATIVITY SPECIAL SECTION