How atomic clocks have been used to measure the universe's fundamental constants

Why atoms are used to keep track of time and define the meaning of a second

Small variations in the production of human-made devices, such as mechanical watches and pendulums, make them unreliable for the ultraprecise timekeeping required by modern technology, including financial transaction timestamping and GPS. Because atoms obey fundamental laws of nature and never wear out, their light emissions when energized provide a consistent timekeeping signal.

One model of the universe expresses all constants using only time

About seven base units, such as meters for length, have historically been used to describe everything in our physical reality. However, researchers have shown that, through relativistic space-time, clocks can serve as replacements for rulers, scales, and other instruments, allowing everything to be measured in terms of time.

The extreme precision of atomic clocks highlights gravitational time dilation

Scientists at the National Institute of Standards and Technology have constructed clocks made out of stacked strontium atoms, which show differences in the passage of time across just a millimeter of height consistent with predictions from general relativity, where atoms closer to Earth's center tick off time more slowly.

Explore the fundamental constants of the metric system

In 2019, the International System of Units was revised, such that all seven fundamental units are defined in terms of combinations of seven fundamental constants of the universe. These include the speed of light in a vacuum, Planck's constant, Avogadro's number, and the elementary charge of protons and electrons.

Optical clocks track time by measuring electron transitions involving visible light

The frequency of visible light is significantly greater than that of microwaves used in cesium atomic clocks. This makes optical clocks about 100 times more accurate, losing at most one second across the entire age of the universe.

Astronomers use a set of scaffolding methods to measure distances across the universe

Different methods are used depending on how far away an object is—from radar for nearby planets, to parallax for stars, to redshift and supernovae for distant galaxies. These tools allow scientists to chart the universe across unimaginable scales using astronomical units (AU), light-years, and parsecs.

An explanation of isotopes, which can help scientists trace food, water, and farming choices

Atoms of the same element have the same number of protons, but experiments showed that atoms of the same element can move through electromagnetic fields as if they had different masses. These differences in mass come from variations in the number of neutrons, which contribute to the stability of atoms by providing a buffer to the repulsion between protons.

Calibrate to the U.S. government's official clock, synchronized to the atomic standard

The site displays the current time across all American time zones—corrected for network delay—and shows whether your device's clock has drifted. Computer clocks rely on quartz crystals that drift slightly with temperature and age, requiring alignment with atomic clocks (which are so stable that seconds are defined by them).

Delays in the decay of unstable particles provide evidence for time dilation

Physicists used the muon half-life measured in labs to predict how many should reach detectors on Earth's surface after their production in collisions between atmospheric molecules and cosmic rays. Higher detection rates were consistent with a longer half-life due to slower movement through time.

Lasers, semiconductors, medical imaging, and atomic clocks rely on quantum mechanics

Semiconductors, crucial in electronics, operate based on quantum superposition. Lasers use stimulated emission, a quantum process, and MRI machines leverage the quantum spin of hydrogen atoms for imaging. Atomic clocks measure time with unprecedented precision using quantum transitions.

The simple grandeur of atomic theory

Is the universe made up of a continuous substance, or a finite amount of tiny, indivisible atoms? The atomic hypothesis, largely vindicated in the 20th century, was forgotten for nearly 2,000 years after first being formulated by the Greek philosopher Democritus. Learn more about the theory and its implications for science and human life with this reflective essay.

How scientists determine elemental composition based on the light emitted by cooling atoms

When electrons within an atom drop to a lower energy level closer to the nucleus, they lose energy and produce light with energy equal to the difference between the two energy levels. Because these levels and the corresponding emitted light are unique to each element, the light serves as a unique signature that can be compared to known lab samples for identification.

Explaining the unified atomic mass units, which quantifies the mass of atoms and molecules

Until 1961, the atomic mass unit was defined based on oxygen-16, but it was later replaced by the unified atomic mass unit, defined as 1/12th the mass of a carbon-12 atom. Today, the term "atomic mass unit" often refers to the "unified atomic mass unit," denoted by the symbol u.

Type Ia supernovae are 'standard candles' used to accurately measure cosmic distances

These explosions are caused by white dwarfs—remnants of low-mass stars—that reignite and explode, with every explosion brightening and dimming in the same way. By comparing them, astronomers have mapped the locations of galaxies alongside velocity data to determine the universe's expansion rate across time and the existence of dark energy.

Early timekeeping tools included sundials, water clocks, and astrolabes

Astrolabes also provided a person's latitude based on the height of the stars in the sky, thus aiding in navigation. Incense and candle clocks relied on the constant burning rate of fire to track the passage of time based on premarked locations with equal-interval spacing.

Particle accelerators detect subatomic particles using millions of sensors

Calorimeters measure the energy of particles. Tracking devices can measure momentum and charge from the trajectory of particles, which is determined by recording tiny electrical signals. Time-of-flight detectors provide particle velocities directly and particle mass indirectly, giving physicists a suite of properties to compare with those of known matter.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second and are about a billion times as rare.

LIGO can detect changes in spacetime one ten-thousandth the width of a proton

Gravity is the weakest of the fundamental forces in nature, meaning the length of objects only changes by minuscule amounts as gravitational waves ripple through space from distant, albeit massive, cosmic events. LIGO's sensitivity can measure an object shifting its position by one seven-hundred-trillionth of the width of a human hair.

A scale of the universe, from atoms to cosmos

Can you really appreciate just how small protons are, or how massive the universe is? To attempt to grasp it, watch this short video which zooms out from a young woman in California to the very limits of the cosmos using established scales. Then, dive deep into the atoms of her white blood cells for a fully-rounded effect.

Quantum field theory, which has made predictions accurate to 12 decimal places, explained

Instead of being made of indivisible particles, all visible matter in the universe is built from underlying quantum fields—smooth, invisible entities that behave like waves and extend throughout space and time. The theory suggests these fields are fundamental, and particles are disturbances that move through them.

Ernest Rutherford's gold foil and nitrogen experiments revealed nuclear structure

Observing high-angle deflections when alpha particles were fired at gold foil led to an atomic model with a dense, positively charged nucleus. Firing those particles at nitrogen resulted in the emission of hydrogen nuclei—protons—leading to the discovery of the subatomic particle.

The cosmic dark ages: before stars existed

The theories behind the Big Bang suggest the process of the universe beginning to the creation of stable, neutral atoms took just half a million years. But the transition from such atoms to star formation took surprisingly much longer, upwards of 100 million years. Why? This deep dive explores the science behind the unique physical composition of the so-called cosmic dark ages.

Measurements of an electron’s magnetic moment help test the Standard Model

A particle’s magnetic moment quantifies how it responds to the presence of an external magnetic field. An electron’s value can be measured down to 14 decimal places, with certainties in the first 12 allowing it to be used to assess the accuracy of predictive models.

Biological aging tests assess the breakdown of physiological systems

Known as aging clocks, these tests compare various metrics, like DNA methylation patterns and protein profiles, to healthy baselines. However, the non-uniformity of these baselines across individuals limits testing accuracy and prevents their clinical use.

Astatine-211 is a radioisotope that can be used for therapeutic cancer treatment

With less than 30 grams in the Earth's crust at any given time, astatine is the rarest naturally occurring element. Its decay produces alpha particles that can damage diseased cells with the same impact as 10,000 electrons.

Single-cell organisms have circadian clocks that direct daily biochemical routines

Cyanobacteria, a type of light-sensitive bacteria, use this clock to signal when to alternate between making organic compounds through photosynthesis and when to convert atmospheric nitrogen into usable ammonia. Upregulating photosynthetic machinery before dawn helps ensure efficient photosynthesis.

Stars appear to twinkle because turbulence in the motion of molecules in Earth's atmosphere causes light to bend and wiggle rapidly on its way to the surface—a phenomenon called scintillation.

Neutron stars, the remains of some massive stars after they explode in a supernova, are the densest directly observable objects in the universe.

Explore chaos in this simulation of two double pendulums, where small differences in the release angles produce wildly different outcomes.

Radioactive elements can introduce software errors because the ionizing radiation they produce can detach electrons from atoms, and computer code relies on the distribution of electrons.

Scientists can determine Earth's past climate conditions by analyzing air trapped in microscopic bubbles within ice cores—columns of compressed snow layers that build up over hundreds of thousands of years.

Most of the observable universe's volume consists of voids and supervoids—enormous regions of space with little to no galaxies, stars, or light.

Studies suggest when you sleep is as important as how much you get, and syncing with your circadian rhythms may lead to less fatigue and improved long-term health outcomes

The first image of a black hole was captured by creating an Earth-sized virtual observatory from a network of eight radio telescopes.

While the sun's gravity keeps the planets in orbit, their gravity also pulls on the sun, causing it and everything else in the solar system to orbit around a mutual balance point called the barycenter—the true center of the solar system.

Track the location of the Voyager probes, currently between 13 and 15 billion miles away from Earth.

Astronauts have repaired genetic damage in space using CRISPR—a molecular tool that can make precise changes to DNA and is used in single-celled organisms to attack invasive viruses.

Clever Hans was a 1900s horse that was believed to be able to solve math equations, tell time, and read and comprehend German, until a psychology student discovered how he got his answers.

As of 2025, the most cited papers in science include a review of graphene, methods to improve image recognition, streamlining electron density calculations, and optimizing protein studies.

Learn more about the history of fireworks, which dates back to an accidental invention during an alchemist's quest for information about immortality.

English mathematician Ada Lovelace wrote the first computer algorithm a century before computers existed and hypothesized that computing devices would one day be able to compose music—foreseeing the generative AI systems of today.

The US Constitution is widely regarded as the longest-standing written national constitution still in force—see how it has influenced rights worldwide.

See how each of the more than two million known species relates to each other—and to common ancestors—in this zoomable tree of life.

A small portion of The Winstons' 'Amen, Brother,' an obscure '60s gospel record, is the basis for thousands of hip-hop tracks, making it one of the most sampled records ever that's informally known as 'the Amen break.'

The Mona Lisa's celebrity status and repeated restorations have obscured what made the painting so revolutionary for its time

Stanford built a Google Maps for the Roman Empire, revealing how much it cost to travel in the ancient world

Sony set the standard for cylinder-shaped batteries when it repurposed manufacturing equipment used to make rolls of magnetic recording tape.

Budweiser stores yeast derived from the original 1800s strain in a secret bunker, which only the Head of Yeast Management can access—not even the CEO

In recent years a child anxiety expert has created and tested a novel phobia and anxiety treatment approach for children, focused on changing how parents respond to the fears.

The most expensive ad of all time was for Chanel No. 5 perfume and cost roughly $33M to produce in 2004—about $57M in today's dollars.