mach number

The churning cauldron of turbulence burns energy in two distinct ways. Vortices, like whirlpools in a maelstrom, twist the fluid and rub layers against each other. Shock waves, like a sudden clap, abruptly compress the gas and turn motion into heat.

If you push water in a narrow channel too fast, a steep wave forms and breaks into foam. In the world of ultracold atoms, cooled to near absolute zero, quantum laws prevent such a wave from breaking. Physicists took a cloud of rubidium atoms stretched into a thin thread and sharply compressed it from one end with a movable wall of light.

In 2017, the LIGO and Virgo detectors recorded the spacetime tremor from the collision of two neutron star merger — event GW170817. After the initial burst came an after-ring: a kilohertz oscillation of the newly formed superdense remnant.

A laser is like a tuning fork that rings at a single note. But even in complete silence, its sound trembles slightly. Previously, it was thought that this tremor, random kicks from spontaneous emission of light by atoms inside the laser, was the main limit to precision.

A cylinder slams into the air at enormous speed, and a shock wave is born around it — a sharp boundary where the air is instantly compressed and heated. Ernst Mach was the first to photograph these jumps, and since then we've thought of them as thin, almost invisible walls.

A boiling pot, a gust of wind, a wisp of smoke from a campfire — all obey equations that mathematicians call 'nonlinear and dissipative.' In plain terms, energy is lost to friction, and parts of the flow influence each other so strongly that predicting the motion ahead of time is a task on the edge of feasibility.

When a person breathes, air doesn't flow in a steady stream but in waves. Previously, doctors and scientists only looked at overall numbers: how much air entered per breath, what the peak flow was. But within a single breath, there are nuances — small variations that speak to the workings of nerves and muscles. The new approach breaks a breath into parts, much like a musician breaks a chord into individual notes.

A frictionless liquid that flows forever: that's superfluidity. Helium-3 only achieves it at thousandths of a degree above absolute zero. The theory of this state was developed by Lev Landau and Richard Feynman.