Pressure from individual particles measured for the first time

The pressure produced by a single particle can now be measured for the first time, thanks to a device that uses a tiny bead held in place by a laser. It is so sensitive that researchers hope that it could help find elusive new particles, such as those that could make up dark matter.

Pressure is caused by particles hitting an object and collectively exerting a force across its area. Researchers typically think of it as an average effect rather than zooming in on each particle, but when pressure is extremely low, such as in experiments conducted in near-perfect vacuum, tracking every particle is needed to properly account for its effects.

Yu-Han Tseng at Yale University and his colleagues have now built the first device capable of making such measurements. The central component is a tiny silica sphere, half the size of some viruses, held in place with a laser beam thanks to electromagnetic interactions between the two. Whenever a particle hits the sphere, it reflects light which the researchers can then detect.

To test this setup, the team placed the device into an ultra-high vacuum then systematically sent in particles of three different gases. They measured the device’s motion when hit by these particles, then calculated pressure from those measurements, compared it to mathematical predictions and found good agreement between the two – the device was doing exactly what they designed it for.

“You need to get everything right to get this measurement working,” says Tseng. “When we did everything carefully enough, the measurement turned out to be beautiful.”

Team member Clarke Hardy, also at Yale University, says that the new device could be used to establish a new definition for what counts as an extremely high vacuum where standard pressure sensors would simply read zero. “You could just count the number of collisions, and that would be good enough to give you an estimate of the pressure in these extreme high-vacuum regimes,” he says.

“Individual molecular collisions are rarely observed in real time. Traditionally, their effects are only seen on average, like how a fast-moving object appears blurred in a long-exposure photograph,” says Joseph Kelly at King’s College London.

Animesh Datta at the University of Warwick in the UK says that similar device design, including some that his own team has been developing, could be used in astronomy, for example helping us understand the low pressure spaces between stars better by detecting gas particles that reside there but may have been missed by other sensors.

But the team have another goal in mind – using the device to detect hypothetical so-called sterile neutrino particles, which could resolve decades-old anomalies in particle-physics experiments, explain why particles with incredibly tiny masses exist in our universe and even be a convincing candidate for what dark matter is made of.