# Atomic clocks measuring time in millimetres

Constantly running late? There’s now a legitimate reason to blame it on your clock…
21 February 2022

## CLOCK

CS Lewis once said “The future is something which everyone reaches at the rate of sixty minutes an hour.” But is that always true? While it certainly seems that time runs slowly whenever I’m in a boring meeting, physics tells us that in fact, the passage of time is not as constant as we once thought. Professor Jun Ye and his team at the University of Colorado are building incredibly precise atomic clocks to try to measure these distortions in time, as Robert Spencer found out...

Robert - In the 2014 movie Interstellar, the protagonists visit a planet deep in the gravitational pull of a black hole. Despite spending only a few hours on the surface, when they return to their colleague in a spacecraft, they find 2 decades have gone by.

Jun - People find that hard to believe; That time is all relative. There's no absolute time.

Robert - It's called time dilation and it all depends on where you're standing. The key is gravity.

Jun - Space and time are interconnected. When we get close to a massive body, both space and time will be curved.

Robert - That massive body could be a planet or a star. Curved space causes objects to fall towards the body, like a ball falling to the ground or a spacecraft into a black hole. The curvature of time is more mind boggling.

Jun - As you approach a massive body, the time that you measure will slow down.

Robert - It's all the results of Einstein's theory of general relativity. But it's not just a bunch of equations on a Blackboard or a plot point for sci-fi.

Jun - This was a theory, however it's being tested over time and is found to be consistent with experimental findings.

Robert - To test this theory, all you need is to take a clock to a place with weaker gravity and one to a place with stronger gravity and compare how fast they tick. For example, objects in orbit will clock up a different amount of time to us on the surface of the earth.

Jun - And the atomic clocks on board of satellites need to take into account of this time dilation effect.

Robert - In fact, if they didn't GPS would stop working in a matter of hours. But we don't need to go to a black hole or even into space at all to measure this effect. If I put a clock upstairs in my living room and one downstairs in my bedroom, there's a difference.

Jun - A clock downstairs will tick slower than a clock upstairs because downstairs is closer to the center off the earth.

Robert - And that's why I was late to work this morning. My alarm was delayed due to relativistic effects.

Jun - That's not necessarily true.

Robert - No, perhaps not. The effect would be so tiny that we wouldn't be able to detect it. Or could we? Jun Yee has been building the world's most precise clocks and he's gotten pretty good at it. Unlike clocks we may think of, which use the ticking of a pendulum to measure time, Ye's clocks use the ticking of electrons around atoms.

Jun - The atom we use is called a strontium atom.

Robert - They use lasers, shooting, very precise photons at these atoms to interrogate them.

Jun - We can use the frequency of the photon as a handle to tell the time.

Robert - And with high accuracy in measuring time, comes the ability to measure the slightest of changes in gravity. In fact, rather than needing the tens of hundreds of miles to satellites or the few yards to upstairs, Ye's team can measure the difference in gravity between two clocks separated by...

Jun - Just a mere millimeter.

Robert - Yes, that's right. 1 millimeter. The difference in the speed of time here is close to nothing. Each second, the lower clock loses.

Jun - 0.0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 seconds.

Robert - That's 19 zeros.

Jun - This is incredibly precise.

Robert - But Jun Ye and his team are not satisfied with simply measuring gravity and time to this level of accuracy. By measuring the curving of space itself, he hopes to build tools with very real applications.

Jun - You can turn that into geological survey tools to sense the change in Earth.

Robert - By measuring how the mass moving underground distorts time itself. He hopes to be able to predict volcanic eruptions or measure glacier's melting. Or perhaps we could measure changes in gravity not caused by normal mass.

Jun - We may be able to shed light on the mysterious matter called dark matter. It's in our universe, but has eluded our detection.

Robert - Perhaps the most incredible is that when you start measuring gravity on such small scales, you start to probe the relationship between general relativity and quantum mechanics.

Jun - Quantum mechanics describes the microscopic part of the world. How atoms, photons & electrons evolve. General relativity is typically associated with the macroscopic view of the world.

Robert - Getting these two theories to play along together has left physicists scratching their heads for decades. Now measuring the intersection of these models seems within reach.

Jun - It will be fantastic if we can start to connect the very microscopic world of quantum mechanics with the very macroscopic world of general relativity.

Robert - The gravity of this discovery cannot be understated. It may bend our understanding of atoms, space, and time itself. Or perhaps it'll just confirm what we thought to higher and higher precision, or something in between. It's all relative.