How Are Atomic Clocks So Accurate at Keeping Time?
An atomic clock is a device that measures time using the microwave signal emitted by atoms – or the electrons in atoms – when they change energy levels.
Did you know atomic clocks only have an error of 1 second in a span of about one-hundred million years? This makes them one of the most accurate devices in human history, at least when it comes to keeping time.
What Do Atomic Clocks Measure?
Atomic clocks measure the actual length of a second, which is the base unit we use to calculate time.
According to the International System of Units (SI), the time unit "second" is calculated by the time an atom in a defined state - comprised of the element caesium-133 - takes to oscillate 9,192,631,770 times.
We can also refer to the true definition, which is much more complex.
“The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.”
Please note, this only applies to a cesium atom currently at rest at 0 Kelvin.
Why Do We Even Have Atomic Clocks?
An estimated 400 clocks worldwide contribute to International Atomic Time (TAI), a time standard we use to maintain Coordinated Universal Time (UTC). And since UTC is the standard we use to keep all the clocks in the world synced, atomic clocks are pretty important.
Precise measurements – like those from atomic clocks – also help operate satellite navigation systems like GPS, Galileo, and GLONASS.
How Do Atomic Clocks Work?
You know what a pendulum in a grandfather clock is, right? It is a harmonic oscillator that uses gravity to swing back and forth, and this motion helps the clock keep time.
Well, the natural way that atoms oscillate works similar to a pendulum in an atomic clock.
Yet, atomic clocks are more precise because natural atomic oscillations happen at a higher frequency and as a result they are more stable.
This is why atomic clocks are one of the most accurate devices we have to tell time. But what you may not know is while they are identical in principle and function - like traditional clocks - there are many types.
We’re going to explain the most common.
Atomic Clock Principles
The first thing to understand is atoms can be in different energy states, called hyperfine levels or hyperfine structure in scientific terms. In cesium, there are a pair of energy states, so to make it easier, let’s refer to these as state A and state B, respectively.
First, atoms are heated and bundled into a beam. A magnetic field removes atoms in state B so only state A atoms remain. This is a crucial step.
Next, the state A atoms pass into something called a resonator. Inside they are pelted with microwave radiation, which causes some of those atoms to change state, becoming state B.
The atoms are passed behind the resonator, and another magnetic field extracts atoms that remained in state A.
Next, a detection system collects and counts the atoms left in state B.
This is where things get complicated.
The microwave radiation’s frequency will determine the total percentage of atoms that change their state after passing through the resonator. If the radiation frequency is closer to that of the atom’s oscillation then more will change state.
This is exactly how time is determined using an atomic clock. The clock is tuned – or its microwave frequency – to match the oscillation of the atoms and then the outcome is measured. After 9,192,631,770 oscillations have occurred, it can be counted as a whole second.
This is Why Atomic Clocks are so Accurate
By measuring the oscillation of atoms, atomic clocks remain precise, but they’re not perfect. They experience an error of 1 second every one-hundred million years or so.
Today, the NIST-F1 atomic clock in Colorado is considered to be one of the most precise clocks in the world.
It is called a cesium fountain clock. This means the lasers – or beams – bundle the atoms into a bustling cloud, cool them off, and toss them around. What this does is slow the atoms movement so that they can be measured easier and much more precisely.
Are There More Accurate Timekeeping Devices?
Scientists are actually working on a type of clock that is more accurate than the atomic clocks we use today. It is called an optical atomic clock and it measures atomic oscillations using light from the visible spectrum. Obviously, this is where it derives its name.
This is possible because the resonance frequency of light waves is fifty-thousand times greater than the frequency of microwave radiation. Scientists hope to use this to measure oscillation, and it will be much more precise.
An optical clock is believed to have an error of 1 second in about fifteen billion years. You can imagine how valuable such a clock would be.
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