Putting Einstein to the Test

In 1905, a young Albert Einstein published his world-changing special relativity theory. Along with the famous equation E = mc², the theory made a startling statement about time: The faster you move relative to a motionless observer, the slower time passes for you. The phenomenon became known as time dilation.

In 1915, Einstein took things to the next level. According to the audacious new theory of general relativity he published that year, time is also affected by gravity. The more strongly you feel the force of gravity, Einstein argued, the slower time passes for you. This effect became known as gravitational time dilation.

Einstein’s theories revolutionized our understanding of the universe. Special relativity revealed that space and time are united in an all-permeating fabric known as space-time. General relativity uncovered a tight connection between two seemingly unrelated realms: gravity and time. And both made surprising predictions that challenged people’s common intuitions and experiences. For example, if you had a twin who went on a long spaceship ride at close to the speed of light or who spent time near a black hole and then rejoined you on Earth, according to relativity, you would now be older than your own twin!

It wasn’t until the invention of portable atomic clocks in the 1950s that scientists could experimentally test many of Einstein’s predictions about time. This was a big deal, because if the theories’ predictions proved to be even slightly off, these discrepancies could reveal important new information about the universe.

Scientists have since compared the ticking rates of atomic clocks on airplanes, satellites and skyscrapers to ones on the ground. Each test confirmed Einstein’s predictions that moving clocks tick more slowly than stationary ones, and clocks in higher gravity tick more sluggishly than ones in lower gravity. These experiments have upped the bar for anyone looking to overthrow Einstein’s theories and discover new physics.

Though not designed as a test of Einstein, GPS satellites — the ones that control the blue dot on your phone — all carry atomic clocks and are subject to the effects of relativity. These satellites hurtle so fast through space that special relativity indicates they should fall behind earthbound clocks by 7 microseconds (millionths of a second) per day. Meanwhile, the lower gravity in medium Earth orbit should speed them up by 45 microseconds per day compared to clocks on Earth. Combining the two effects, atomic clocks aboard GPS satellites run 38 microseconds per day faster than earthbound clocks.

When military scientists and engineers launched the positioning satellites, they realized they needed to correct for relativity’s effects to get the system to provide accurate time on Earth. Because it has worked for so long, GPS could be seen as the longest-running test we have of Einstein’s theories. 

At a much smaller scale, clock scientists have also observed gravitational time dilation in the lab. In 2010, then-NIST physicist David Wineland and colleagues ran two optical clocks based on aluminum ions at heights that differed by about a foot. The clocks ticked at slightly different rates, due to the slight difference in gravity at the two locations.

In 2024, JILA physicist Jun Ye and colleagues pushed the bounds further by using an optical lattice clock to measure a gravitational effect at the submillimeter scale, about the thickness of a human hair.

Indeed, relativity has become one of the most rigorously tested physical theories of all time. But two planned experiments will soon raise the bar far beyond any tests done to date.

In the first, a team of scientists at NIST and other institutions will install two high-performance atomic clocks on the International Space Station (ISS), which orbits at 370-460 kilometers above Earth. In the lower gravity of space, the clocks will run slightly faster than ones on Earth, as do the clocks on GPS satellites. However, because the ISS clocks will be much more precise than GPS clocks, they will allow for a relativity test 50 times more stringent than any done before.

In the second experiment, which could achieve a similar level of precision, a team of scientists at NIST and the University of Colorado will drive a clock to the top of Mount Blue Sky, a 4,348-meter peak that rises west of Boulder, Colorado. The researchers plan to compare the ticking rate of that clock to a near-identical one in the Boulder lab of NIST physicist Andrew Ludlow, whose research group is leading the effort.

The experiment, planned for later in 2025, won’t be easy. Running even one optical lattice clock is a demanding task that typically requires scientists to be on hand tweaking dials and turning knobs. Doing this in a remote and rugged location rather than the controlled environment of a physics lab will be an additional challenge. And simultaneously operating two clocks that are 56 kilometers apart adds more layers of complexity.

On top of that, there is no direct line of sight from the mountaintop to NIST. Ludlow’s team will need to send timing signals from the lab through an optical fiber to a location outside Boulder that can also intercept laser signals from the mountain-based clock.

Indeed, it will be a complicated experiment that pushes the limits of the possible. But if the researchers pull it off, the combined power of ultra-accurate optical clocks and a nearly three-kilometer height difference will put Einstein to one of his most stringent tests yet — and allow scientists to peer deeper than ever into the realm where gravity meets time.

Next: Mapping the Earth