Goldfish Learn To Drive

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 
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February 12, 2022 
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GES PLUS
MATTER & ENERGY 
Eerie quantum 
effect confirmed
Gravitational fields can affect