Usually, to feel the influence of a magnetic 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 conventional 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 materializing in two different places. Each fractured particle simultaneously takes two different paths around the magnetic cylinder. Despite never touching the electrons, and hence exerting no force on them, the magnetic field shifts the pattern of where particles are found at the end of this journey, as various experiments have confirmed.
In the new
experiment, the
same uncanny physics is at play for gravitational 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 particles 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 measure Newton’s
gravitational constant, G, which reveals the strength
of gravity, and is currently known less precisely than other fundamental
constants of nature.
A phenomenon
called interference is key to this experiment. In quantum physics, 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 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 tungsten with a mass of
1.25 kilograms. To isolate the Aharonov-Bohm effect, the scientists performed
the same experiment with and without this mass, and for two different sets of
launched atoms, one which flew close to the mass, and the other lower. Each of
those two sets of atoms 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. Comparing how the various sets of atoms 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 massive 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.”