quantum classical transition

A black hole that swallows matter might not just evaporate over billions of years: it could pop like a soap bubble and turn into its opposite: a white hole. This scenario emerged from numerical experiments in semiclassical gravity.

Drop by drop from a faucet, or a steady stream without a single splash. That's roughly what the two regimes of current in a superconducting contact look like. Only here, the role of water is played by Cooper pairs — bound electrons that move without resistance.

In a vast hall, billiard balls are scattered about. At first they lie sparsely, each on its own. Then someone adds more, and suddenly the balls compress into dense clusters. This is a phase transition — the moment when matter abruptly changes its state. Physicists have long described such a picture with the hard-sphere model: particles are identical balls that cannot overlap.

Water turns to ice at zero degrees — sharply, with no intermediate states. Something similar happens to complexity when we take a random quantum state of an entire system and look only at a part of it. As long as that part occupies less than half the system, its complexity grows smoothly. But as soon as the fraction crosses 50%, a jump occurs.

A magnetic field holds a tiny particle in mid-air, like an invisible dance partner. A special tuning of the field creates a trap with two "positions." In the ordinary world, a dancer occupies only one, but in the quantum world — both at once. Physicists have learned to achieve this for dust grains weighing trillionths of a gram.

A swing can rock strongly or barely move. Usually, if left alone, it stays in its chosen mode. But in the microworld, things are different: a tiny membrane, oscillating under the action of a laser beam, can spontaneously jump from one amplitude to another.

A lightning strike splits the sky in a fraction of a second, leaving a branched trail. At the same moment, in a tiny superconducting crystal cooled to near absolute zero, an avalanche of magnetic field rushes through — thin fingers of magnetization piercing the material.

When hot soup cools, clumps form on the surface — similarly, as the early universe cooled, it gave birth to threadlike clumps of energy: cosmic strings. Some of them are like a stretched rubber band: as long as it's intact, it's stable. But if you nick the band, it snaps.

Two accomplices are interrogated separately: betray or stay silent? If neither knows what the other will do, their choice breaks the classical rule of probability. It’s like riding an old bicycle over bumps — the hard seat jolts you off. The textbook rule, the classical model, fails. The authors of a new study found a simple fix: the cyclist needs a soft saddle.

A snowdrift grows by strict rules: falling snowflakes form bumps, those attract more snow, and the bumps amplify. The same mathematics describes city growth, patterns on bark, ripples on water, and clusters of cosmic dust. Physicists call this uniformity universality.

A spring compressed to the limit breaks. Likewise, classical gravity theory leads to a singularity — a point where all laws break down. Quantum mechanics turns this end into an elastic bounce.

Light trapped in a tiny cavity is like a pendulum that gets pushed harder and harder. At first, it swings smoothly and predictably, like an ordinary clock. But if you push it hard enough, the motion becomes strange: the pendulum seems to be in two extreme positions at once.

The world of physics works like a music box: there's a speed knob. Turn it slowly, and the melody is strict — balls roll along trajectories (classical mechanics). Speed it up — the notes blend into a chord: a particle is in many places at once, all probabilities (quantum regime). The secret of switching is in the parameter κ.

The early universe was an extremely hot and dense medium. In this medium, like bubbles of steam in boiling water, bubbles of a new state of space spontaneously emerged — regions where physical properties changed abruptly.

A quantum particle can be in two places at once, but large objects never are. Is it noise that creates chaos and increases entropy? Or does nature itself forbid quantum tricks in the macroworld? One hypothesis suggests that quantum 'fuzziness' disappears on its own, and the heavier the object, the faster it goes.

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.