energy barrier

Two metal plates in a vacuum attract each other without any magnets or charges. This is the Casimir effect: the void is not empty; particles constantly pop in and out of existence, and there are fewer of them between the plates than outside. The pressure difference pushes the plates together.

In the mountains, there is a saddle — a point where the trail from one valley crosses over into another. Standing exactly on it, you take a step and are already descending the new slope. In chemistry, such a place is called the transition state: a brief moment when atoms are frozen on the threshold of transformation.

Hot tea in a freezer doesn't freeze all at once. First, tiny ice needles dart around — forming and melting until the temperature drops enough for the ice to set solid. The early universe behaved similarly as it cooled and changed its internal structure.

A typical photon source can be likened to a club entrance with a single bouncer. He tries to let people in one at a time, but occasionally couples sneak past. Physicists call this the purity problem: photons must arrive strictly solo, otherwise quantum computing breaks down.

Since time immemorial, physicists have dreamed of tunnels piercing the cosmos — wormholes, capable of connecting far-flung corners of the Universe in an instant.

A stone falls into a pond, and ripples spread across the water. But if the stone is very small and the water is not quite ordinary, the ripples pass right through it, as if it weren't there.

An optical tweezer is a laser beam that holds an atom in place, much like fingers holding a pea. Arrays of these tweezers form the basis of future quantum computers. But there's a catch: not every tweezer catches an atom. Typically, only half of the traps get filled, and for molecules, even fewer. Empty spots break the whole setup.

Fusion, the merging of a proton with a boron-11 nucleus, promises a near-perfect energy source: no dangerous neutron flux, just a stream of charged particles that can be directly converted into electric current. But there's a fundamental problem: the Coulomb barrier.

A river flows toward a whirlpool, but a dam with a notch blocks the way. Most water piles up before the dam, but some, gaining height, leaps over it and falls straight into the funnel. So it is with gas in a galaxy: it streams toward the center but hits a ring of stars and gas, like a dam. Usually, everything stops there, swirling into a dense vortex.

Some black holes are remnants from the Big Bang. Usually they fade via a slow radiation leak — predicted by Stephen Hawking. But a quantum switch could invert them into white holes, explosively releasing all energy like popcorn kernels that suddenly pop.

In quantum communication, a secret key is transmitted using single photons. Eavesdropping on such a channel is supposedly impossible: any interception attempt disturbs the particle and leaves a trace. But the idea has a weak spot — the real hardware, which sometimes behaves differently than theory dictates.

An ion channel in a cell membrane is a gate just a few atoms wide. Every second, millions of charged particles, sodium and potassium ions, pass through it. Classical physics said that to get through, an ion must jump over an energy barrier, like a traveler crossing a high mountain pass.

Igniting a miniature sun is a long-held dream of humanity. Typically, scientists try to confine scorching plasma (a gas of charged particles) at temperatures of hundreds of millions of degrees. But plasma tends to escape magnetic traps, and confinement comes at enormous cost.

Dark energy and dark matter are the invisible conductors of the cosmos. Usually considered independent, new research proposes seeing them as dance partners. The lead — dark energy — changes the mass of dark matter particles. Like a dancer who lifts and lowers their partner, dark energy increases or decreases the weight of dark matter.

For two atomic nuclei to fuse, they must overcome their fierce electric repulsion — like trying to bring two angry bulls together. Normally, only monstrous energy does the trick, as in stellar cores, where Hans Bethe explained fusion back in the 1930s.

In the quantum world, particles can walk through walls: this is quantum tunneling. If you place a detector right in the path of such a 'ghostly' passage, the moment of detection acts like a starting pistol: the particle gets a sharp kick and flies out of the barrier with excess energy, turning into a tiny engine.

A tiny tungsten tip with a radius of less than a hundred nanometers is the perfect stage for watching a quantum performance. Apply an electric field of a few volts per nanometer, and electrons start flying out of the metal: that’s field emission.

A crowd in a square. At first, everyone spins to their own rhythm, but suddenly the music changes, and they all start moving in sync, as one. That's roughly how electrons behave in some metals: they either cling to magnetic impurities or break away and form a collective order.