You’ve probably heard of atomic clocks, if you haven’t, then it’s probably about time you did. Atomic clocks have many uses within scientific studies for example, it’s certain where incredible precision is needed to sink their atom-smashing instruments, but atomic clocks are also used within the world we experience every day. including the Internet, where exact timings are critical however, they are probably most commonly known for their use within the global positioning system or GPS.
For a GPS satellite to work out your precise position the timings of the signals it sends and receives have to be just right. the signals travel at the speed of light, which means an error of even a single microsecond in timing translates to an error of 300 meters on the ground. the timing has to be so well controlled that even tiny effects, because of relativity need to be tracked. if we didn’t know Einstein’s equations GPS wouldn’t work. to keep the precious time, GPS satellites carry atomic clocks the more accurate the clock, the better GPS can calculate your location, but how do they work?.
At the heart of any clock is a frequency reference, an object that oscillates at a constant rate. in olden days a pendulum was the frequency reference on swing of the pendulum equaled 1 second. for pendulums are prone to manufacturing errors or victim to changes in temperature and gravity, so all in all pendulum clocks are not so great for keeping accurate time.
In 1927 the quartz clock was invented, a major leap forward for timekeeping quartz or silicon dioxide(sio2) is a crystal with some special property, when jolting with electricity which course resonate or vibrates at very precise frequencies. many materials can be formed into plates that will resonate, but quartz has an exceptional trick up its sleeve it is also what is known as a piezoelectric material.
When quartz crystal is subject to mechanical stress such as, bending for hitting it produces an electrical charge, so as it resonates it generates pulses that keep time like the swinging of a pendulum. quartz clocks are still used in all sorts of devices today including watches, mobile phones, computers oven timers and even pacemakers. with about 2 billion quartz oscillators sold every year. shaped like a tiny tuning fork the quartz oscillators are standardized and designed to vibrate at 32,768 times per second, but this frequency alters very slightly due to temperature and pressure changes. and so like a pendulum over time they still lose well time.
So in order to make the super accurate atomic clocks used today, a new strategy was needed. physicists knew that atoms oscillated at very high and very specific rate. not only that, but atoms from the same element will oscillate at exactly the same rate. virtually immune to environmental factors and with manufacturing errors being a relevant, atom seemed like the perfect oscillator. in 1967 the standard was decided and a second was officially defined at 9,192,631,770 oscillations of a caesium 133 atom. this number is what’s called its resonant frequency, but bombarding a cesium atom with radio waves tuned to that exact same frequency the season will resonate absorbing some of that energy and changing to a higher energy state. a bit like an opera singer smashing a glass with a high note.
The idea was to use this change in energy state to indicate that the radio wave frequency was exactly right. let me explain they kept the courts from before as it is still a very good oscillator, but instead use its resonance and the electrical pulses it produces to determine the frequency of a radio transmitter. radio wave frequency is measured in Hertz or waves per second like counting the number of electrical pulses in a quartz clock. we can now count the number of radio waves to determine when a second has passed.
So for example if we know that a piece of quartz resonates at 32,768 times per second, then the radio transmitter will transmit a frequency of 32,768 Hertz, when we’ve counted the same number of waves, then we know that a second has passed. keeping this frequency exactly the same though is very difficult, because as we know quartz changes its frequency due to environmental factors and so, in turn this alters the radio wave frequency. this is where the season comes in.
Like I said before when a cesium atom resonate, it absorbs some of the energy from the radio waves and changes to a higher energy state. these higher energy state cesium atoms are detected after they’ve passed through the radio waves. if the radio wave frequency is exact, then the higher energy cesium atoms continue to flow into the detector at their peak rate. if the frequency isn’t quite right, then fewer detections are made, so another jolt of electricity is sent to the course oscillator, altering the frequency so it’s just right to make those cesium atoms start resonating again. just like tuning a radio.
This method of feeding output information back in is called a feedback loop, and basically means the atomic clock is self adjusting. and a fun fact, the National Institute of Standards and Technology or NIST in the US has an atomic clock that is now so precise that if one of these have been running since the Big Bang it will now be less than a second off.