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particles they never touch
BY EMILY CONOVER
If you’re superstitious, a black cat in your
path is bad luck, even if you keep your
distance. Likewise, in quantum physics,
particles can feel the influence of mag-
netic fields that they never come into
direct contact with. Now scientists have
shown that this eerie quantum effect
holds not just for magnetic fields, but
for gravity too — and it’s no superstition.
Usually, to feel the influence of a mag-
netic field, a particle would have to pass
through it. But in 1959, physicists Yakir
Aharonov and David Bohm predicted
that, in a specific scenario, the conven-
tional wisdom would fail. A magnetic
field contained within a cylindrical
region can affect particles — electrons,
in their example — that never enter the
cylinder. In this scenario, the electrons
don’t have well-defined locations but
are in “superpositions,” quantum states
described by the odds of a particle mate-
rializing in one of two different places.
Each fractured particle simultane-
ously takes two different paths around
the magnetic cylinder. Despite never
touching the electrons, and hence exert-
ing no force on them, the magnetic field
shifts the pattern of where the particles
are found at the end of this journey, as
various experiments have confirmed
(SN: 3/1/86, p. 135).
In the new experiment, the same
uncanny physics is at play for gravita-
tional fields, physicists report in the
Jan. 14 Science. “Every time I look at this
experiment, I’m like, ‘It’s amazing that
nature is that way,’ ” says physicist Mark
Kasevich of Stanford University.
Kasevich and colleagues launched
rubidium atoms inside a 10-meter-tall
vacuum chamber, hit them with lasers
to put them in quantum superpositions
tracing two different paths, and watched
how the atoms fell. Notably, the atoms
weren’t in a gravitational field–free zone.
Instead, the experiment was designed so
that the researchers could filter out the
effects of gravitational forces, laying bare
the eerie Aharonov-Bohm influence.
The study not only reveals a famed
physics effect in a new context, but
also showcases the potential to study
subtle effects in gravitational systems.
For example, researchers aim to use
this type of technique to better mea-
sure Newton’s gravitational constant,
G, which reveals the strength of gravity
and is currently known less precisely
than other fundamental constants of
nature (SN: 9/29/18, p. 8).
A phenomenon called interference is
key to the new study. In quantum phys-
ics, atoms and other particles behave
like waves that can add and subtract,
just as two swells merging in the ocean
make a larger wave. At the end of the
atoms’ flight, the scientists recombined
the atoms’ two paths so their waves
would interfere, then measured where
the atoms arrived. The arrival locations
interfered, both with and without the
tungsten mass, teased out a phase shift
that was not due to the gravitational
force. Instead, that tweak was from time
dilation, a feature of Einstein’s theory of
gravity, general relativity, which causes
time to pass more slowly close to a mas-
sive object.
The two theories that underlie this
experiment, general relativity and
quantum mechanics, don’t work well
together. Scientists don’t know how to
combine them to describe reality. So,
for physicists, says Guglielmo Tino of
the University of Florence, who was not
involved with the new study, “probing
gravity with a quantum sensor, I think
it’s really one of … the most important
challenges at the moment.”
s
are highly sensitive to tweaks that alter
where the peaks and troughs of the
waves land, known as phase shifts.
At the top of the vacuum chamber,
the researchers placed a hunk of tung-
sten with a mass of 1.25 kilograms. To
isolate the Aharonov-Bohm effect, the
scientists performed the same experi-
ment with and without this mass, and
for two different sets of launched atoms,
one which flew close to the mass and
the other lower. Atoms in each of those
two sets were split into superpositions,
with one path traveling closer to the
mass than the other, separated by about
25 centimeters. Other sets of atoms,
with superpositions split across smaller
distances, rounded out the crew. Com-
paring how the various sets of atoms
According to the general theory
of relativity, gravity results from
massive objects warping space-
time (illustrated). A quantum
effect reveals that subatomic
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