Atomic clocks so precise that they can measure gravity

An old version of an atomic clock with ultra-stable ytterbium network at NIST. The Ytterbium atoms are generated in a furnace (large metal cylinder on the left) and sent to a vacuum chamber in the center of the photo to be manipulated and probed by lasers. The laser light is routed to the clock by five fibers (for example, the yellow fiber at the bottom center of the photo). (Credit: Burrus / NIST)

Time, like money, only seems important when time is running out. But for physicists, time is always a big problem. Relativity tells us that the passage of time depends on the circumstances in which you measure it: clocks run faster at the top of the mountains than at ground level, for example, and the faster you go, the slower your clocks will slow down. Time depends on the space.

But now, thanks to technological advances in the field of atomic clocks – the most precise time-measuring devices we have designed – we can reverse the trend and determine physical parameters more precisely by studying the passing of time. Our understanding of space will depend on time.

The new clocks, described today in the newspaper Nature, will also promise what the best atomic clocks usually promise: improve timing, communication and navigation technologies. But in addition to their understanding of the physical space around them, the devices could also help find gravitational waves, test predictions of relativity, and chase dark matter. All this, just from super accurate clocks.

Atomic Clocking In

All this may seem rather complicated (and it's true), so let's start with the basics. As the authors of the document usefully point out, "The evolution of time is followed by counting the oscillations of a reference of frequency, such as the revolutions of the Earth or the oscillations of a pendulum. By referencing atomic transitions, frequency (and hence time) can be measured more accurately than any other physical quantity.

The best-known name in the field of atomic clocks is the National Institute of Standards and Technology (NIST), and that's where the current research comes from. The last clocks are based on 1,000 ytterbium atoms, cooled to a level near absolute zero, trapped in 1-D grids (columns) consisting of laser beams. According to all measurements of atomic clock performance – minimizing errors in atomic frequencies, ensuring tick stability and reproducibility of its measurements – NIST researchers produced clocks of incredible precision . Their error bars are in the order of 10^-18, or a billionth of a billionth.

The shape of things

In fact, these atomic clocks are so precise that they are sensitive to gravitational influences. Normally, the relativistic quirks of gravity that alter the flow of time are too small for us to realize – but not anymore. As the authors have said, "If these clocks were compared on a long base or used for remote comparisons with other clocks in the world, the measurement would be limited by the gravitational knowledge on the surface of Earth." That is, they are so precise that the only thing that could change their ticks and their tocs would be gravity itself. Clocks higher up, farther away from the mass of the Earth, would strike faster than the lower clocks, thanks to relativity.

It is a major agreement for the science of geodesy, which measures the shape and gravitational influence of the Earth. Our current image of the precise surface of the planet depends on satellites and computer modeling, which offer a very good resolution, up to a few centimeters. But these atomic clocks would reduce this resolution to one centimeter. Armed with two of these clocks, researchers could compare the sea level on two different continents, the precise height of a mountain or any other measure depending on the height (and therefore the gravity) that 's going on. they wish to perform.

And since atomic clocks are very sensitive to gravity, they can serve as a type of detector for any related activity. The gravitational waves, when they cross us, and this planet, appear in the readings of these clocks. Extremely subtle experimental tests of Einstein's theories are also possible. And most importantly, technology could help detect minute amounts of dark matter, that invisible material that only interacts with gravity and makes up most of the matter in the universe.

To be clear, these are just possibilities. The authors have just built these atomic clocks and show their accuracy. But, now that they know that technology works as well, a new future for physics discoveries may well come to us. It's time.