These Atomic Clocks Wouldn’t Lose A Second In 13.8 Billion Years

The most precise clocks ever built are now testing Einstein, hunting dark matter, and reshaping how we define time itself.

What if a clock began tracking time at the very beginning of the universe? This clock would have to be so precise that it never gained or lost even a single second by today. That’s 13.8 billion years of near-perfect timekeeping.

Timekeeping sounds like that sounds more like a comic book than actual science, but scientists now report the construction of clocks capable of reaching roughly this level of precision. These clocks are currently being used to hunt for dark matter, test Einstein’s theories, and measure Earth’s gravity.

Amazingly, these optical atomic clocks are now so sensitive that moving one just a centimeter higher or lower (about the thickness of your pinkie finger) changes how fast it ticks. Researchers can measure that difference. A review published in the journal Optica lays out how these ultra-precise timepieces work and why they’re about to change the very definition of time itself.

The basic idea is simple. Atoms make better clocks than anything humans can build mechanically. Every atom of the same type is identical, governed by unchanging laws of physics. When electrons jump between energy levels inside an atom, they absorb or emit light at frequencies that never vary. These frequencies act as perfect pendulums for keeping time.

Traditional atomic clocks use cesium atoms and tick at microwave frequencies around 9 billion times per second. Optical atomic clocks use atoms like strontium, ytterbium, and aluminum. They tick at frequencies in the visible light range, hundreds of thousands of times higher. That’s like upgrading from a ruler marked in inches to one marked in thousandths of an inch.

How Optical Atomic Clocks Work

Building clocks this accurate requires extreme measures. Scientists trap individual atoms using electromagnetic fields and cool them with lasers to within a millionth of a degree above absolute zero. At these frigid temperatures, the atoms barely move.

Two main designs have emerged as winners. Single-ion clocks trap one atom at a time in what’s essentially an electromagnetic cage. One top-performing aluminum-ion clock recently hit an uncertainty of about five and a half parts in a billion billion. Optical lattice clocks trap thousands of atoms at once in a grid made of laser light. One leading strontium lattice clock reached about 8 parts in a billion billion.

Both need ultra-stable lasers that stay locked on a single frequency. Scientists achieve this by bouncing laser light between mirrors in carefully designed cavities, some cooled to near absolute zero. Another key technology is the optical frequency comb, which won its inventors the 2005 Nobel Prize. These devices convert the insanely high optical frequencies into radio frequencies that normal electronics can measure.

All of this begs the question: what can you do with a clock this good?

Hunting For Dark Matter

One of the most notable applications is searching for dark matter. This mysterious stuff makes up about 85% of the universe’s matter but doesn’t interact with light in normal ways. That’s why we can’t see it.

Some theories suggest dark matter consists of ultralight particles that create fields permeating all of space. These fields might cause tiny wobbles in the fundamental constants of physics. Things like the fine-structure constant, which governs how atoms behave.

Here’s where optical clocks come in. Different atoms depend on these constants in different ways. So by comparing multiple optical clocks based on different atoms, researchers can search for these wobbles. If dark matter passes through Earth, it might show up as a brief glitch in how the clocks compare to each other.

Clock networks spanning continents have looked for these signals. They haven’t found dark matter yet, but they’ve ruled out some theories and set the tightest limits yet on others.

The clocks are also testing whether fundamental constants actually stay constant (as their name suggests), or whether they might change over time or with position in space. So far, if they’re changing, it’s too subtle for today’s best clocks to catch. But the search continues.

Even Einstein’s general relativity is getting stress-tested. One experiment at Tokyo Skytree compared two strontium clocks separated vertically by just a few hundred meters. It measured the gravitational effect on time more precisely than earlier satellite experiments, despite using a height difference thousands of times smaller.

Why Scientists Want To Redefine The Second

Since 1967, the official definition of the second has been based on cesium atoms. One second equals 9,192,631,770 cycles of microwave radiation from a specific transition in cesium-133.

Optical clocks now beat the best cesium clocks by about 100 times. So the international metrology community is working toward redefining the second using optical clocks instead. The target date is sometime in the 2030s.

The tricky part isn’t the technology. It’s getting the world to agree on a single definition. Many different optical clock systems now perform at similar levels. Which one should define the second?

The simplest option: pick one optical transition and fix its frequency, just like we do now with cesium. That transition becomes the new gold standard.

However, with so many good options (strontium, ytterbium, aluminum, and others), getting countries to agree on just one is tough. Some researchers have proposed an alternative. Define the second using a combination of several different optical transitions. This would use the strengths of multiple systems but would also be unprecedented for the International System of Units.

Either way, this will be the first time we’ve changed the definition of the second in over 60 years.

Measuring Gravity With Time

Einstein predicted that clocks run at different speeds depending on their position in a gravitational field. The deeper you are in a gravity well, the slower time passes.

On Earth’s surface, for every meter of height difference, clocks differ in rate by about one part in 10 quadrillion. Optical clocks are now good enough to measure this over height differences of just centimeters. In laboratory settings at JILA and NIST, researchers have even detected the gravitational effect on time across height differences at the millimeter scale, about the width of a grain of sand.

This opens up a new way to measure Earth’s shape and gravity field. Traditional surveying uses instruments called gravimeters to measure gravitational acceleration, then does math to figure out gravitational potential. Optical clocks measure potential directly. Put two clocks at different heights, and the difference in how fast they tick tells you the potential difference.

This technique, called chronometric leveling, could solve a longstanding problem. Different countries use different sea level references to define their national height systems. Some disagree by several decimeters even within the same continent. A system based on gravitational potential measured by clocks would eliminate these inconsistencies.

Researchers have already tested the concept. In one experiment, they used a portable strontium clock to measure the gravitational potential difference between two locations in Italy about 90 kilometers apart and 1,000 meters different in elevation.

With better portable clocks, chronometric leveling could create detailed gravity maps of mountainous regions. Some researchers think future clocks might even detect time-varying changes in Earth’s gravity caused by phenomena like tides in the solid Earth itself.

Portable Atomic Clocks: Taking Precision Time Outside The Lab

The catch with all these amazing clocks is that they’re huge. The record-breakers occupy entire rooms, with racks full of lasers, electronics, and control systems. They need vibration isolation, temperature control, and teams of PhD physicists to keep them running.

Several groups have shrunk optical clocks down to the size of a van. These portable versions trade a bit of performance for mobility, but they’re still better than the best cesium clocks. One portable clock traveled from Japan to the UK and Germany for international comparisons. Others have been used for those gravity-measuring experiments in Italy.

For applications needing even smaller systems, researchers are building compact clocks based on simpler technology. These use warm vapor instead of laser-cooled atoms. They’re not as precise as the big lab systems, but they’re good enough for many applications and can fit in a backpack.

One recent demonstration put a portable optical clock on a ship for several weeks. Despite the rocking, vibration, and salty air, it kept working. Another demonstration flew on a suborbital rocket, a test run for future space missions.

Why bother making them portable? GPS outages are one reason. Modern infrastructure depends dangerously on satellite signals that are weak and easy to jam or disrupt. Portable atomic clocks can keep accurate time when GPS fails. They also work in places GPS can’t reach: underwater, underground, inside buildings.

As the technology matures and gets cheaper, portable optical clocks might become common. The laser and photonics industry is already developing compact optical frequency combs and miniature reference cavities. What once filled a room might eventually fit in a briefcase.

The precision of optical atomic clocks has improved by more than 100 times every decade since the 1990s. They’ve gone from laboratory curiosities to tools that can probe fundamental physics, map Earth’s gravity, and will soon redefine the second itself. And they’re still getting better.

For most people, a clock that wouldn’t lose a second over the age of the universe sounds like overkill. But that extreme precision is exactly what’s needed to answer some of science’s biggest questions. Like whether dark matter is real, whether the constants of nature actually stay constant, and whether Einstein got everything right about gravity.

Turns out the best way to explore the universe isn’t always to look out into space. Sometimes you just need to measure time really, really well.

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