Measuring how Earth's gravity bends time

You'd be forgiven for thinking that Professor Jun Ye is from a science-fiction story: he and his team in Colerado are measuring the warping of the space-time continuum itself...

Einstein's theories of relativity claim that space and time are deeply intermeshed, and that things that affect one, affect the other. For example, heavy objects such as the Earth bend spacetime to form the gravitational field that keeps us all comfortably on the ground. And bizarre as this sounds, in fact, General Relativity also predicts that gravitational fields bend time too, and slow it down.

This isn't just a theory either: experiments with satellites and even aeroplanes have found that, as you go upwards - and gravity decreases in strength slightly - clocks tick faster compared to when they're on the ground.

But, to measure this, we need very precise time-measuring devices: atomic clocks. Unlike traditional clocks, which use pendulums and springs, or digital clocks that use the vibrations of a piece of quartz, atomic clocks use the energy levels of atoms to slice up time. In a similar idea to a pendulum, electrons around these atoms click between two energy states. By fine tuning a laser, scientists can probe the natural "swing time" between these two states. Since that swing time is constant: dictated by the rules of quantum mechanics, this lets us measure the passage of time.

This is what Prof Jun Ye and his team at the University of Colorado are doing. But instead of building one atomic clock, they are making many in the same machine. Their device comprises layers of strontium atoms, like a stack of pancakes, and interrogates each layer individually.  Because the layers are arranged vertically, those closer to the Earth should tick slower than those higher up.  And indeed, that is what the scientists see.

The really astonishing fact is how precisely the team are able to measure this difference. The layers of atoms are less than 1mm apart, meaning that we can measure the effect of gravity on the dilation of time at microscopic spatial resolution. To give an idea of how precise this is, the bottom of the stack loses 0.0000000000000000001 seconds for every second the top clock clicks.  If this experiment were to have been started at the Big Bang, it would have lost less than a tenth of a second by now.

Measuring gravity on such a fine scale opens many doors. Ye is hopeful that this technology could be used to detect magma flows and predict volcanoes, help us navigate to Mars, or even probe the mystery of dark matter.

But perhaps more exciting is that with a push to build clocks which are a hundred times more accurate than this (which Ye thinks might be possible in the next decade), we may be able to measure the interaction of gravity and quantum mechanics. At this scale, atoms start to behave more like waves but currently we don't have a good understanding of what happens when part of this wave is in a stronger gravitational field than another. Prof Ye hopes that we will start to be able to probe the intersection of gravity and quantum mechanics through measuring time.

For all the potential applications, it is still an impressive engineering feat. Technologies from disparate parts of science have consolidated in this laboritory to measure how Earth distorts time itself on the smallest of scales. Where it goes from here, only time will tell...