Physicists have explained yet another quirk of the quantum world: why, if you swing a pendulum through a quantum fluid, it speeds up rather than slowing down. Tiny “quasiparticles” ricocheting around in the fluid are to blame, Finnish researchers report in an upcoming issue of Physical Review Letters.

The effect is the opposite of that experienced in the ordinary world. Immerse the pendulum of a grandfather clock in water, for instance, and it will slow down.

It takes a special kind of fluid to pull off this quantum trick. Physicists Timo Virtanen and Erkki Thuneberg of the University of Oulu have been studying helium-3 atoms, which at very low temperatures form a substance known as a Fermi liquid. In such a liquid, the atoms stop interacting with each other as they ordinarily do and instead start behaving in strange quantum ways.

Researchers have studied Fermi liquids for decades to better understand phenomena that kick in at cold temperatures, such as superconductivity. “It’s a very profound theory — one of the most basic things to understand,” says Thuneberg.

So he was intrigued when, in the early 2000s, researchers in Helsinki reported experiments in which a pendulum sped up when dunked in a Fermi liquid mixture. He decided to see if he could figure out why. In a series of calculations, Thuneberg and his student Virtanen worked out the mathematics of how the pendulum interacts with the fluid.

When chilled down into a Fermi liquid, particles no longer interact strongly with one another as they do at higher temperatures. Instead there appear quasiparticles, which are the combination of a particle itself along with how it affects the environment around it. Like the original particle, each quasiparticle carries spin, charge and momentum.

The researchers calculated that the quasiparticles ricochet around in the liquid like bullets, increasing the force on the pendulum. They do not, as ordinary particles would, interact with each other strongly enough to create resistance to the pendulum moving through them. “That’s why the behavior is different,” says Thuneberg.

The scientists dub the newfound effect the “Landau force” and plan to calculate how it might work in other systems, such as oscillating walls.

George Pickett, a physicist at Lancaster University in England and a member of the team originally reporting the effect, says the new study is an interesting and direct demonstration of the importance of Fermi liquids.

Image: Flickr/Dave-F

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