What can we measure with a good clock?

By Dr Erik Streed

A clock can tell you what time it is, but in the right hands a good clock can do so much more. During the age of sail Captain Cook used a seaworthy mechanical clock to determine his longitude by measuring whennoonoccurred locally compared to that in London.

Our modern satellite navigation systems rely on atomic clocks that use the quantum mechanical properties of a small cloud of atoms to keep time to within a few billionths of a second. Light travels 30 cm in a billionth of a second. Measuring the delay time between signals broadcast from different satellites lets your phone calculate where you are.

Recently researchers at the US National Institute of Standards and Technology in Boulder, Colorado constructed two of the most accurate clocks ever built. Instead of using a cloud of atoms, each clock used a single aluminium atom to measure time.

The aluminium atoms were cooled to near absolute zero temperature to minimise the atom’s motion, which would have slowed down the time they were measuring, dueto Einstein’s Special Theory of relativity.These clocks are so incredibly accurate that it would take nearly a billion years for them to become out of synch by a second.

Einstein’s theory of General Relativity predicted that clocks run slower in areas of higher gravity. The space near a black hole has very high gravity but it would take a lot of time and money to run an experiment there.

Building more accurate clocks means we can measure the subtle effectsofgravity on time here on earth. If we move farther away from the centre of the earth, the effect of gravity is less. The NIST aluminium clocks are so accurate that raising one above the other by just 33 cm results in a measurable difference in time. The higher clock is running faster because it is experiencing less gravity, just like Einstein predicted.

DrErikStreedis an ARC Future Fellow and a Senior Lecturer in the School of Natural Sciences on the Gold Coast campus. He is most noted for capturing the first image of an atom’s shadow.Erikis a graduate of Caltech (BSc) and MIT (PhD).