law of conservation of momentum

Water in a tank spins at nearly four revolutions per second, and, as if by magic, a column the size of a dinner table emerges. The vortex lives for dozens of rotations, even though everything around it churns and trembles. Where does such stability come from? The secret is that the vortex is not isolated.

In the first moments after the Big Bang, the universe resembled a boiling cauldron. According to one hypothesis, dark matter, the invisible substance that holds galaxies together, could have been born there.

A pendulum in a noisy room is constantly jostled by people passing by. Its motion is a mix of regular swings and random jerks. Something similar happens with weak gravitational waves against the backdrop of an expanding universe. In Einstein's general relativity, these waves are tiny ripples in spacetime.

On September 26, 2022, the DART probe slammed into the natural satellite at a speed of 6,6 km/s. A fountain of debris burst into space. Rocks, dust, sand grains — everything erupted in a silent flash. Two years later, a numerical simulation of 20 million trajectories revealed the fate of each particle.

Ordinary electric current heats up wires — anyone who has touched a phone charger knows that. But there's another kind of current: instead of electrons rushing in a crowd in one direction, they spin in sync, like figure skaters in a pairs routine. This coordinated rotation is called a spin current, and it produces almost no heat.

In a world where Reynolds numbers are minuscule, water turns into thick syrup, and the swimming tricks we know stop working. Here reigns the insidious scallop theorem: reciprocal motions produce no net displacement. Microorganisms found a way out: they grow polymer tails.

After the Big Bang, the Universe went through a series of phase transitions, like water turning into ice, but with unpredictable patterns.

Inside planets and stars, in rotating oceans of plasma, turbulent flows, Coriolis forces, and magnetic tension collide. The central question is how the energy of this chaos crystallizes into a stable magnetic framework that can last billions of years.

Eight hundred participants, firms and workers, enter the market. Each holds a strict preference list in mind. The challenge: pair up so that no one wants a do-over. In mathematics, this is called a game theory with a ban on repeat alliances, and its roots trace back to the work of John von Neumann and Oskar Morgenstern.

A billiard ball strikes another, and the sum of their velocities before and after the collision obeys strict rules. The same happens in the microscopic world: energy, momentum, and charge cannot simply vanish.

Active galactic nuclei (AGN) are the universe’s powerhouses: their X-ray coronae blaze millions of times hotter than stellar atmospheres. But what strikes the spark? A ten-day X-ray campaign by the XRISM and XMM-Newton satellites, aimed at NGC 3783, provided the answer: it’s actual thunderstorms.

At the heart of a warp drive lies Einstein's idea that massive bodies curve spacetime — the very stage on which all cosmic events unfold. Picture a bubble of such curved spacetime. Inside it, the ship floats without any G-forces, like a passenger in an airplane cabin who doesn't feel the headwind.

In an orchestra, one musician plays so quietly that their sound is almost inaudible in the overall chorus. In the quantum world, such 'quiet' states are called dark: their energy barely escapes, like light in a darkened room. But if there are many musicians, their mutual influence can unexpectedly awaken even the most silent one.

A plastic heart rests on the water and doesn't tip over, no matter how you turn it. It freezes in any position, as if the water were holding it by invisible strings. This trick is possible thanks to the Zindler curve — a shape in which all lines that divide the area in half have the same length. The Zindler curve is a closed curve where any chord that bisects the area has the same length.

Atoms settled along a chain of traps, like a line of coastal forts. Nearby, a quantum sea: a Bose-Einstein condensate (a substance with zero resistance, where particles move as a single wave).

Plasma in a fusion reactor is like a swarm of fireflies that you need to keep inside an invisible jar. It used to be thought that the jar's walls had to be perfectly smooth, or the fireflies would find a crack and escape. New work shows that the jar can be glued together from pieces with noticeable seams, and the swarm will still stay inside.

Astronomers, including Vera Rubin, noticed that stars on the outskirts of galaxies rotate faster than visible mass allows. To explain this, they invented dark matter — an invisible ballast. But there's another possibility: gravity isn't always the same.

Two pendulums on the same wall, swinging out of step, will be in sync within an hour. Christiaan Huygens noticed this synchronization back in 1665, watching the clocks in his workshop. Today, researchers repeat his observation in the lab, but with a twist: the pendulums are not wooden but quantum.

A single photon passes through two slits and leaves an interference pattern on a screen. Quantum mechanics describes this process with a wave function, but what happens to the particle itself?