![]() When the microwave source is tuned to the right frequency, oscillations of the caesium atoms can be measured by a magnetic field or laser. The development of early caesium clocks was made possible by the invention of coherent microwave sources called masers (the predecessors of lasers) in the 1950s. ![]() Caesium Clocks and the Advent of International TimeĪlthough the universe provides ready-made atoms with their own characteristic frequencies, building an atomic clock requires a way to produce electromagnetic radiation at a single frequency, or ‘coherent radiation.’ The most commonly used standard is the oscillation of a soft, metallic element called caesium (or cesium) as it is exposed to microwaves – the same kind of radiation we use to heat our food. As with much of the quantum world, this should seem strange – imagine if you could only hear a handful of acoustic frequencies – could only hear changing sounds in “steps”, not in a smooth range! However, the fact that atoms only respond to discrete frequencies of light allows a single frequency to be selected and used as a reference for keeping time. A fundamental principle of quantum mechanics dictates that atoms can only absorb and emit light at a discrete set of frequencies. In contrast with conventional clocks, atomic clocks tune their ticks to the oscillations of atoms as they absorb particular frequencies of light. In a grandfather clock, the frequency refers to the number of times per second a pendulum swings back and forth as it mechanically drives the clock’s hands for a modern quartz watch, the frequency of interest is the number of electrically driven vibrations per second in a crystal, a signal that is converted into electronic pulses that are displayed digitally. Frequency expresses how often a periodic signal repeats itself. ![]() While atomic clocks are technologically more complicated than the average timepiece, their operating principle is more or less the same – time is kept by precisely measuring the frequency of a signal. However, advances in physics and engineering over the past fifty years have decreased that uncertainty with the development of the gold standard of timekeeping: atomic clocks. It’s ten seconds to midnight on New Year’s Eve, but by whose watch? Like all standards, measurements of time are arbitrary, and only as good as the precision of each ‘tick.’ As no clock is perfect, each will eventually speed up or slow down, making that stroke of midnight a bit fuzzy. ![]()
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