To detect the quantum friction of empty
space, scientists are going for a spin.
A twirling nanoparticle, suspended in a
laser beam inside of a vacuum, can measure tiny twisting forces, making it the most sensitive detector of torque yet created. Researchers say the device could one day detect an
elusive quantum effect called vacuum friction.
The suspended nanoparticle can spin more
than 300 billion times a minute. “This is the fastest human-made rotor in the
world,” says physicist Tongcang Li of Purdue University in West Lafayette, Ind.
To measure torque
with the device, Li and colleagues hit the nanoparticle with a second laser,
which they switched on and off at regular intervals. The laser was circularly
polarized, meaning that the light’s electromagnetic waves rotated over time,
and this twist imparted a torque on the nanoparticle. The researchers estimated
the amount of torque by measuring how the particle’s speed changed as that
second laser switched on and off.
When operated for 100 seconds, the sensor
could measure torques as small as about 0.4 trillionths of a quadrillionth of a
newton-meter. For comparison, one newton-meter is the approximate amount of
torque needed to twist a cap off a soda bottle. The device is around 700 times as
sensitive as the previous best torque sensor, the researchers report January 13
in Nature Nanotechnology.
The device is so sensitive that it could
be used to observe the minuscule impact of vacuum friction, a counterintuitive
quantum effect in which an object spinning rapidly in empty space feels drag — despite
being surrounded by nothingness. The never-before-seen effect is predicted to arise
from interactions of the spinning object with electromagnetic fields that,
according to quantum mechanics, appear and disappear constantly, even in empty space (SN: 11/13/16).