winding number

Toss a handful of sand onto a table — the grains scatter equally in all directions. But if a magnet is hidden under the table and the sand is iron, the trajectories get trickier: an invisible structure overlays the chaos. That's roughly how molecular motion works inside a living cell.

A superfluid gas is like a crowd of people marching in step: each atom knows where to go and doesn't fall out of the common rhythm. But this crowd has three 'valleys'—three different directions it can take. In a new experiment, physicists made one valley slightly more 'favorable' than the others and watched how the crowd darted between them.

Cut a spin chain, and particles without an energy gap appear at the edges. They are called zero modes, and they live only at the boundary. Previously, they were counted using complex topological invariants, but now there is a simpler way.

An ocean current hundreds of kilometers wide lives its own life, fed by energy from tiny eddies. Something similar happens in the atmosphere: small swirls pass their power on to huge air currents. This process, the inverse energy cascade, has been known for a long time.

In the microscopic world, noise, random fluctuations, usually destroys fragile quantum states, much like ripples on water break the reflection in a puddle. However, in some systems, collective, coordinated noise from many particles produces the opposite effect: a stable unidirectional flow emerges.

Some materials are internally like a Möbius strip: their electronic states are twisted in a special way. Such topological properties are usually manipulated with a powerful magnetic field, as if trying to flatten the strip with a sledgehammer. A group of researchers has found a more delicate tool.

Neutrinos are the true ghosts of the microcosm. Nearly massless and chargeless, they pass through planets as if through empty space. But inside each neutrino hides not one particle but three — three 'flavors' (electron, muon, and tau) that constantly morph into one another. This dance is called oscillation.

Liquid helium at ultra-low temperatures enters a state of superfluidity: it flows without friction, like a perfect lubricant.

A pot of water heated from below is a simple example of turbulence. Hot fluid rises, cold fluid sinks, and the flow spontaneously breaks into rotating cells that look like horizontal rolls. In a new study, physicists took a wide rectangular container (ten times longer than its height) and tracked how many such rolls appear.

A thin plate stands across a flow of water or air. Behind it, a wake forms — a region where the flow becomes tangled and mixed. Usually, such a wake becomes turbulent when the flow speed is high enough, quantified by the Reynolds number.

In a chain of cold spins stretched along a single line, order and disorder can intertwine into a pattern that defies the usual rules. Two states of matter, two kinds of "order," are separated by a crossroads where the roads do not converge but diverge.