vacuum birefringence

Light from the hottest stage of the universe has been traveling toward us for nearly 14 billion years. This is the cosmic microwave background — a faint glow filling the entire sky.

Light from the very beginning of the cosmos has been traveling toward us for 13.8 billion years. During that time, its polarization, the direction of its oscillations, might have been twisted, like a compass needle that someone quietly nudged. Physicists call this phenomenon cosmic birefringence.

A thousand years ago, Vikings took Iceland spar to sea. The translucent crystal split light into two rays. By their brightness, they could find the sun even in fog. Birefringence — the physical term for this phenomenon — now helps hunt for the most elusive force in the cosmos.

A black hole is not just a dark silhouette against the stars. Its shadow is surrounded by a thin glowing ring of photons that loop around the event horizon before escaping to an observer.

Light loves symmetry: in an open channel, it flows equally both ways. To steer it one way, physicists often use magnets. A new approach does without, using rotation — just the speed of a vinyl record — and interference from two atoms. Rotation changes the “pitch” of light for different directions, just as a siren’s tone shifts when a car approaches or recedes.

Many black holes are surrounded by magnetic fields. As it spins, the black hole drags curved spacetime along with it, like a machine's flywheel, and sets the cosmic factory in motion.

Neutron stars are the ultra-dense remnants of stars, where matter is compressed to the limit described by Subrahmanyan Chandrasekhar. Magnetars are a type of neutron star with a magnetic field a quadrillion times stronger than Earth's.

The universe is not emptiness, but a vast crystalline sphere, threaded with invisible boundaries — domain walls frozen after phase transitions.

In superfluid helium, friction disappears — the liquid flows like a mountain stream, but without a single eddy. A short laser pulse rippled this smooth surface, and scientists using spectroscopy captured the spreading ripples — quantum waves with many speeds.

Light from a distant star is not just a steady stream. It oscillates like a wave and can vibrate in different directions. Usually these oscillations are indistinguishable, but a magnetar can sort them out.

In a strong magnetic field, electrons trapped in a thin layer sometimes arrange into a regular pattern, like guests at a crowded party who find a spot where no one jostles them. This order is called a crystal, although it is not ice or salt, but the electrons themselves, frozen in place.

In the crystal HfTe5, electrons behave almost like light: their mass is so small that they obey the laws of relativity. If you place such a crystal in a strong magnetic field, the electrons start to circle in tiny orbits. Their energy then changes not smoothly but in steps, like floors in a building.

Light is a stream of photons, and ordinarily a photon has no magnetic moment: it doesn't deflect in a magnetic field like iron filings.

Neutron stars are ultra-dense remnants of supernovae the size of a city. In the 1930s, Fritz Zwicky hypothesized their existence, and later Jocelyn Bell Burnell discovered pulsars — rapidly rotating neutron stars.

In 1936, two physicists, Werner Heisenberg and Hans Euler, poring over the equations of quantum electrodynamics, found something strange. In a sufficiently strong magnetic field, vacuum should cease to be empty.

The ACT telescope in Chile's Atacama Desert stared at the sky from 2007 to 2022. Its target: the cosmic microwave background, light that has been traveling toward us for 13.8 billion years. This light is polarized: its waves oscillate in specific planes, like strings stretched in different directions.