transverse momentum

A billiard ball hangs in midair. Tiny peas shoot at it from a blowgun. Each hit is barely perceptible, but if the ball is very light and barely trembles from heat, you can make out individual nudges. That's essentially the setup: a glass sphere the size of a bacterium levitates in a vacuum while a narrow beam of xenon atoms flies at it.

Sugar dissolves in tea, and then with a single motion gathers back into the spoon. In an ordinary kitchen, you won't see that — viscosity and disorder get in the way. But in the world of ultra-slow motions, where inertia vanishes, such tricks become reality.

A tiny glass bead hangs in a laser beam like a speck of dust in sunlight. Around it is a rarefied gas: atoms of krypton, xenon, and heavy molecules of sulfur hexafluoride. They collide with the bead, and each hit transfers a tiny impulse, like a pea thrown at a taut sheet makes it tremble. Impulse is a measure of motion: mass times velocity.

Light usually pushes an atom straight ahead, like wind pushing a sail. But if the beam is twisted into a corkscrew, the picture changes: the atom can fly off to the side, as if caught by a spinning wheel. In the new work, physicists calculated that such a vortex photon can transfer almost all of its rotation to the atom — up to 100 percent.

In an insulating crystal, heat usually flows from the hot end to the cold end. But when you switch on a magnetic field, part of the heat flow veers sideways, like water running down a slope that drifts to the side when the wind pushes it. This behavior is called the thermal Hall effect.

At the Large Hadron Collider, protons are smashed into lead nuclei. At the moment of impact, hundreds of new particles are born, and some of them fly out in narrow beams — jets. Inside such a jet, like in a dense stream, protons and neutrons can stick together to form larger nuclei, for example a deuteron — the nucleus of heavy hydrogen.

In a plasma accelerator, particles are accelerated like surfers riding a wave created in the plasma. If the leading bunch and the trailing bunch are not aligned, an instability arises that shakes the trailing bunch. It's as if a surfer on a wave starts wobbling from side to side: their trajectory gets messed up, and they might fall off the wave.

An ordinary sensor is like a scale that always shakes a little on its own. Even in total silence and at zero temperature, every particle jitters — this is quantum jitter, and it limits the precision of any measurement.

Water in a kettle heats to 100 degrees, and stops being water, turning into steam. Something similar happens to atomic nuclei when they are smashed together at enormous speeds in the Large Hadron Collider.

All bodies in vacuum fall with the same acceleration: this principle, formulated by Einstein in general relativity, underpins our understanding of gravity. Yet so far it has been experimentally confirmed only for particles of ordinary matter.

A quantum particle quantum particle can exhibit backward motion even if its overall direction is forward. Imagine a river: the main current rushes toward the mouth, but individual currents suddenly swirl and momentarily dart against the flow, creating local disorder.