Everyone needs to know the time. Ever since the 17th century Dutch inventor Christiaan Huygens made the first pendulum clock, people have found good reasons to measure time more accurately.
Finding the right time is important in many ways, from running a railroad to trading milliseconds on the stock exchange. For most of us, our clocks are now checking against a signal from atomic clocks such as those on board the GPS (Global Positioning System) satellites.
But a recent study From two teams of scientists in Boulder, Colorado, these signals could be made much more accurate by paving the way for us to more accurately define the second. Atomic clocks could actually become so accurate that we could begin to measure previously imperceptible gravitational waves.
Brief contemporary history
Modern clocks still use Huygens’ basic idea of an oscillator with resonance – like a pendulum of fixed length that always moves back and forth at the same frequency, or a bell that rings with a specific tone. This idea was greatly improved in the 18th century by John Harrison, who realized that smaller, higher frequency oscillators had more stable and cleaner resonances, which made clocks more reliable.
Nowadays most everyday clocks use a small piece of quartz crystal in the form of a miniature tuning fork with a very high frequency and stability. Not much has changed in this watch design over the past hundred years, although we’ve done better to make it cheaper and more reproducible.
The massive difference these days is the way we review – or “discipline” – quartz watches. Until 1955, you had to keep correcting your clock by comparing it to a very regular astronomical phenomenon like the sun or the moons of Jupiter. Now we are disciplining clocks against natural vibrations within atoms.
The atomic clock was first built by Louis Essen. It was used to redefine the second in 1967, a definition that has remained the same since then.
It works by counting the flip frequency of a quantum property called spin in the electrons in cesium atoms. This natural atomic resonance is so sharp that you can tell if your quartz crystal clock signal is deviating in frequency by less than a part in 10¹⁵, that’s a millionth of a billionth. One second is officially defined as 9,192,631,770 cesium electron spin flips.
The fact that we can produce such precisely disciplined oscillators makes frequency and time the most accurately measured of all physical quantities. We send signals from atomic clocks around the world and into space via GPS. Anyone with a GPS receiver in their mobile phone has access to an amazingly accurate timing device.
If you can accurately measure time and frequency, then there are plenty of other things you can also measure accurately. For example, by measuring the spin-flip frequency of certain atoms and molecules, you can determine the strength of the magnetic field they are experiencing. So if you can accurately determine the frequency, you have also accurately determined the field strength. The smallest possible Magnetic field sensors work like that.
But can we make better clocks with which we can measure frequency or time even more precisely? The answer could still be exactly what John Harrison found that the frequency should be higher.
The cesium spin-flip resonance has a frequency equivalent to microwaves, but some atoms have nice sharp resonances to optical light, the frequency of which is a million times higher. Optical atomic clocks have shown extremely stable comparisons, at least when a pair of them are only a few meters apart.
Scientists are considering whether the international definition of the second could be redefined to make it more precise. To do this, the various optical clocks that we would use to keep time accurate must be trusted to read the same time even when they are in different laboratories Thousands of miles apart. So far, such remote tests have been carried out not much better than for microwave clocks.
Now use a new method to link the clocks with ultra-fast lasersResearchers have shown that different types of optical atomic clocks can be placed a few kilometers apart and still match within 1 part of 10¹⁸. This is just as good as with previous measurements Pairs of identical clocks a few hundred meters apart, but about a hundred times more precise than before with different clocks or long distances.
The authors of the new study compared several clocks based on different types of atoms – in their case ytterbium, aluminum, and strontium. The strontium clock was at the University of Colorado and the other two were at the U.S. National Institute of Standards and Technology.
Hanacek / NIST, Author provided
The study connected the clocks with a laser beam through the air for 1.5 km from building to building, and that connection proved to be as good as an optical fiber under the street, despite the turbulence in the air.
But why do we need such precise clocks? Although the atoms in the clock are supposed to be exactly the same wherever the clock sits and whoever is looking at it, tiny useful differences can arise when the time measurements are so accurate.
According to Einstein’s general theory of relativity, gravity is distorts space-timeand we can measure this distortion. Optical clocks have already been used to detect the difference in the earth’s gravitational field through move only an inch in height.
With more accurate clocks, you could perhaps feel the creeping tension in the earth’s crust and Predict volcanic eruptions. Gravitational waves generated from distant Black Hole Mergers have been seen – perhaps we can now see much weaker waves from less catastrophic events with a pair of Satellites with optical clocks.