proper time

The two states of a strontium atom are like twins running down the same track, but one is slightly higher than the other. Usually, the light used to manipulate atoms pushes one twin harder and the other more weakly. Here, they chose a wavelength of 813 nanometers—'magic'—where both get the same kick.

Neutrinos are tiny particles that pass through our bodies by the billions every second without touching a single atom. They have a mysterious property: on their way from the Sun or an accelerator, they can change their type, or 'flavor'—for example, turning from an electron neutrino into a muon neutrino.

A quantum particle moves in a circle, passing through several stations. At each station, something happens to it — for example, its properties change slightly. After completing a full circle, the particle returns, but its state is no longer exactly what it was at the start.

The universe isn't just a place; it's a schedule of events: what happened earlier and what happened later. In Einstein's geometry of spacetime, this schedule sometimes breaks off. For instance, inside a black hole, particle paths run into a singularity — an edge beyond which the equations stop.

The quantum world does not forgive forgetfulness. Throw a stone into a pond — the ripples spread and vanish, the information about the impact dissipates without a trace. In quantum mechanics, everything is different: the evolution of a system is reversible, and the past is never completely erased.

The ticking of atomic clocks is like the beat of a metronome, but instead of a pendulum, it's the oscillation of electrons in an atom. Two such clocks — one using an ytterbium ion, the other using strontium atoms — were made to tick side by side, and their rhythms were compared.

The ticking of atomic clocks is like the beat of a metronome, but instead of a pendulum, it's the oscillation of electrons in an atom. Two such clocks — one using an ytterbium ion, the other using strontium atoms — were made to tick side by side, and their rhythms were compared.

The world's most precise stopwatch still runs with one eye on an old pendulum. Hydrogen masers — the best radio-frequency flywheels (devices that keep the rhythm between comparisons with a reference)—accumulate error because noise between ticks leaks into the final signal.

Clocks in the Universe are tricky things. Usually, in calculations of the birth of the world, a special scalar field plays the role of a clock — something like an invisible fluid filling all of space. It ticks, and physicists watch how the world changes relative to that ticking. But these clocks are man-made, introduced for convenience. The authors of the new work took a different path.

A typical tracking program resembles a navigator steering a ship by the stars. But as soon as the sky clouds over, coordinates drift, and entropy (a measure of uncertainty) builds up in the data. The new algorithm gets a ‘co‑pilot’ — a module that monitors how confident the main navigator is.

A city signal bounces between walls like a ball, and your phone catches dozens of its 'reflections'—echoes. Each microsecond of delay adds 300 meters to the path. The signal's speed is constant, like that of light (speed of light).

Two synchronized swimmers dive into the sea. One hits calm water, the other a whirlpool. Their movements, perfectly coordinated at the start, diverge. Something similar happens to two quantum particles when one moves in flat space and the other in curved space.

In the grand orchestra of the universe, every instrument has its own rhythm. Electrons in an atom make quantum leaps in femtoseconds, neurons in the brain exchange signals in milliseconds, and a human needs tenths of a second to become aware of the 'now.' Physicists have long sought the conductor…

The heart doesn't beat like a metronome: its rhythm dances to the time of day. Each of us has a moment when our pulse hits its peak. This peak is like the hand of an internal clock, and it shifts from person to person. But what moves that hand? Researchers gave people smartwatches and tracked them for nearly a month.

Quantum electrodynamics, or QED, describes how light interacts with charged particles — for example, electrons. It is one of the most precise theories in history: its predictions match experiment to the eleventh decimal place.

A pendulum that swings on its own, without any winding — in the quantum world, such 'pendulums' are called time crystals. In them, rhythmic motion arises spontaneously and does not decay. But until now, controlling this rhythm was nearly impossible: any intervention threatened to destroy the fragile order.

A pendulum that swings almost half a quadrillion times per second. This isn't mechanics: it's the pulse of light trapped by ytterbium atoms. Instead of gears, there's an optical lattice — a standing wave that holds atoms at its nodes like peas in a pod.

Time is a fickle thing. Inside a massive star, it flows calmly, like sand in an old clock on a mantelpiece. Outside, on the scale of the entire universe, it stretches along with space, as if the same clock were in an elevator that speeds up and slows down. For a long time, physicists puzzled over whether these two worlds could be glued together without visible seams.

A pendulum that almost nothing can knock off balance (not shaking, not heat, not magnetic fields) because it is hidden in the very heart of the atom. That is the kind of pendulum physicists used when they first switched on a nuclear clock. Previously, the best clocks, optical ones, relied on electron transitions.

Time in physics is like a conductor setting the tempo for the whole orchestra. But in the quantum world, there's no single baton: each particle can live by its own clock. Usually, we simplify the picture and assume time flows the same for everyone.

Tea in a cup cools quietly on the table. But if you zoom past at near-light speed, the thermometer will read higher — even though no one heated the tea. Calculations confirm: the needle will creep up. A century ago, physicists split into three camps.

You can zoom into a high-resolution photo for a long time — until suddenly a smooth face dissolves into individual pixels. Something similar, according to a new study, happens to spacetime itself. The authors propose that tiny intervals of length are not pre-existing 'parts' of the universe but results of measurements.

An old mechanical alarm clock ticks on the shelf. You hear the steady sound, see the hands moving, and you're certain time is passing. But if you take the clock apart, you won't find a single part labeled "time." Only springs and gears.

A stone falls into a well, and the splash reaches your ears with a delay. Something similar happens to light as it leaves Earth: its oscillations become less frequent, the beam seems to 'tire.' This is gravitational redshift — a phenomenon Einstein spoke of a century ago.

A photographer tries to capture light interference, but has only one lens and the ability to change exposure time. A similar challenge faces cosmologists studying quantum imprints in the cosmic microwave background — the afterglow of the Big Bang.

In the basements of four European institutes stood seven devices: each can be compared to a tuning fork of unimaginable precision. But instead of sound, they emit 'ticks' made of light: oscillations of a laser locked to a specific frequency by atoms of strontium, ytterbium, or mercury.

A clock isn't just hands. Every atomic nucleus is a natural metronome: it oscillates with nearly perfect regularity. If you lift one such metronome a couple of meters, its rhythm speeds up ever so slightly — that's time warping, as predicted by Einstein.

A beam of light travels through the void, and its own 'ticking'—an invisible clock embedded in the very fabric of spacetime. Physicists have taken this beam and allowed it to measure itself: the gravitational field on the beam is quantized, and the role of the clock is played by the so-called dressing time — the time that the field 'puts on' like a coat.

The compass needle trembles when a truck rumbles by. Similarly, in a tiny gyroscope, a device that senses rotation, any noise drowns out the useful signal. For a long time, engineers couldn't shrink a gyroscope down to fingernail size without sacrificing accuracy. The solution came in a two-loop architecture.

Atomic clocks are pendulums that tick at the frequency of light. Inside a trap, an aluminum ion hangs, and a laser probes it like a tuning fork. If the laser's frequency matches the atom's natural frequency, the atom responds. The clock's rate is checked against this response.

The universe wasn't born alone — alongside it came a mirror twin where everything is reversed: left and right are swapped and time flows from future to past. After the Big Bang, the world, according to Alan Guth's idea of superfast inflation, should have contained equal amounts of matter and antimatter.

An atomic gravimeter works like a very precise scale for gravity: it tosses up a cloud of atoms and watches how fast they fall. But these instruments have a hidden flaw — the shape of the signal they produce is slightly skewed. This skew, like a ruler with a chipped edge, subtly shifts every measurement.

When an aircraft is on final approach or a submarine is navigating blind, the navigator needs an instrument that never loses track of motion. Conventional gyroscopes based on atomic waves are extremely precise, but they have a quirk: the signal repeats like a wallpaper pattern, and after half a step, the pattern starts to look the same. The instrument loses its sense of position.

A tiny particle floats in a laser beam like a speck of dust in a flashlight's glow. Water molecules constantly jostle it — randomly, from all sides. Usually these bumps are a nuisance: the particle twitches, and precise measurements turn into a lottery. But physicists have figured out how to turn the interference into an advantage.

At the center of our Galaxy lurks the supermassive black hole Sagittarius A* — a heavyweight that holds stars in its grip. Astronomers dream of finding pulsars near it: these cosmic lighthouses emit radio pulses with the precision of atomic clocks.

Two clocks in different rooms need to tick in unison. Usually, this is done by sending bright pulses or pairs of photons that arrive one right after the other. But the signal can be so weak that photons barely make it through. In this work, clocks were synchronized with light a billion times dimmer than usual.

Ultra-precise scales that catch the slightest changes in gravity work like a keen ear: they hear even the whisper of distant underground voids or the tremors of the Earth's crust. But such instruments have a hidden enemy: their own internal clocks. Time inside the sensor also jitters, and this jitter smears the picture.

Eclipses in binary stars are like clockwork: a white dwarf (the dense remnant of a star) regularly passes in front of its companion, and the light briefly dims. But for most such pairs, the clock runs fast or slow by seconds over years. Astronomers measured these glitches in 43 systems and found a pattern.

In August 2017, gravitational wave detectors caught a signal from the merger of two neutron stars — the ultra-dense remnants of exploded stars. At that very moment, life on Earth was business as usual: planes were landing, cars were driving, and navigators were plotting routes.

Ordinary atomic clocks tick when an electron jumps between energy levels. But external fields and jolts can disrupt this rhythm. Highly charged ions, atoms stripped of many electrons, barely notice such disturbances: their electrons sit deep and tight. That makes them ultra-stable 'pendulums.' The Cf17+ ion is a curium atom that has lost 17 electrons.