Most clocks, from wristwatches to the systems that run GPS and the internet, work by tracking regular, repeating motions.
To build a clock, you need something that ticks in a perfectly repeatable way. In a pendulum clock, that tick is the regular swinging of the pendulum: back and forth, back and forth, at nearly the same rate each time.
Our team of physicists studies whether an even better kind of clock could one day be built from the atomic nucleus. Today’s best clocks already use atoms to keep extraordinarily accurate time. But in principle, a clock based on a nucleus – the tiny, dense core at the center of an atom – rather than an atom’s electrons, could keep a steadier rhythm because it would be less sensitive to environmental disturbances such as temperature changes. In our research, published in the journal Nature, we measured and interpreted a unique nuclear property of thorium-229 in a crystal that could help make such nuclear clocks possible.
Ultraprecise clocks are more than scientific curiosities. They play key roles in navigation, communications and international timekeeping. Improvements in timing accuracy can also open doors to new science.
How atomic clocks work
In an atomic clock, researchers shine a laser on a material and carefully tune the light until it triggers a specific atomic response, typically by pushing or exciting an electron from one energy level to another. They can tell this has happened because the atoms absorb the laser light most strongly when its energy is exactly right.
That absorption happens at an exquisitely precise frequency. Frequency is how often something repeats over time. For a pendulum, it is the number of back-and-forth swings each second. For light, it is the number of wave cycles that pass each second. A light wave’s frequency also determines its energy and, in the visible light range, its color.
By detecting when atoms absorb the laser light most strongly, scientists can use the laser as a metronome. Rather than counting swings, these clocks count light waves.
To ensure the tick rate stays constant and the clock remains accurate, scientists closely match the laser’s energy to the energy needed to excite an electron in an atom.
Because the electron excitation energy is set by the laws of physics, atomic clocks based on the same atom tick at the same rate everywhere in the universe – even E.T. would agree with your clock.
Using this energy to calibrate a clock, like atomic clocks do, does not come without consequence, though. If anything changes the energy of the atom, like an unaccounted for magnetic field or the temperature of the room, the clock will tick at a different rate.
Deep inside every atom is something even smaller: the nucleus. Today’s atomic clocks keep time by tracking changes in an atom’s electrons. A nuclear clock, by contrast, would use an excitation in the nucleus itself, which is far more compact.
Because a nucleus is 10,000 times smaller…
