Inside a lab in Boulder, Colo., James Chou turns a crank underneath a table, raising one clock 33 centimeters higher than another, identical clock. As the table moves, the tick rate speeds up by 3.5 trillionths of a second.
The purpose of the aluminum ion clocks at the National Institute of Standards and Technology is not to tell time, but to measure time — with spectacular precision. They are so precise, in fact, that they won’t lose a second for over a billion years, says Chou, a postdoctoral researcher at NIST.
In 2008, Chou, along with researchers Till Rosenband and Dave Wineland, also at NIST, set to work on making the most precise clock in the world. A complex system of lasers, ion traps, and cables, the clock is strung across a table, and looks like nothing you’d hang on your living room wall. Nearby, signs warn visitors of laser danger.
Unlike a grandfather clock, which measures seconds based on the swing of a pendulum, the quantum logic clock uses a laser to count those ticks on a much more precise scale. Chou and his team set the laser to oscillate at a frequency of 1.12×10^15 times per second, a million times faster than it takes the light from this page to reach your eyes. They do this by bouncing it off an aluminum ion, which can only absorb light at that frequency. The oscillation of the laser becomes the “tick” of the clock, like the swing of a pendulum, only billions of times faster.
With this technique, the researchers parse time into much finer units. And by doing so, they can see Einstein’s theories of special and general relativity up close.
Einstein’s theories of special and general relativity both explain why time is not constant. Special relativity tells us that nothing can exceed the speed of light, regardless of your frame of reference, or how fast you are moving. It also tells us that time, unlike light speed, is not absolute. As one object speeds up relative to another, time slows for the moving object. Space and time are not experienced identically by everyone. The differences are miniscule, but they exist.
General relativity dictates that space and time are part of a single, grid-like fabric called spacetime. Objects with a lot of mass, like the earth, bend that fabric like a bowling ball on a trampoline, creating curves in spacetime. The grid stretches, causing time to slow in response to this gravitational pull.
The NIST clocks are a demonstration, and confirmation, of these theories. As the clocks move, their motion and their distance from the center of the earth’s gravity influence how fast they tick. Even though Chou has only moved the clock 33 centimeters further from the center of the earth’s gravity, the ticks slow down by trillionths of a second. Similarly, a small increase in velocity, like when Chou moves the clock back and forth, is enough to cause the tick rate to slow down.
Previously, this could only be seen on much larger scales, like clocks on GPS satellites running faster than clocks on earth. The NIST aluminum ion clock shows that time is moving measurably faster or slower based on even the slightest changes in gravity or velocity.
Driving the team was curiosity, but also a spirit of playfulness. “Ever since Einstein’s theory of relativity, people have looked for deviations in what he predicted,” Wineland says. Watching time speed up or slow down in the lab indicates that physics has relativity figured out.
Sean Carroll, a theoretical physicist at the California Institute of Technology, says that this finding drives home that the laws of physics can apply at any size.
“To me, it means a lot that we can measure the fact that spacetime is curved here in my house,” he says. “This abstract idea from Einstein … it really happens. It’s measureable. It’s always a good thing to get data that tests these ideas.”
The NIST clock has not been put to practical use yet, but scientists hope to one day use it to study geophysics by measuring shifts in the earth’s shape, and also to possibly design a better GPS system.
“Time is so pervasive to all of us,” Rosenband says. “For hundreds of years we have tried to measure time more accurately. In the past that has enabled us to make advances like the GPS system. That’s pretty amazing.”