eli5 what’s the difference between a regular clock and an atomic clock?
In: 26
The official scientific standard for 1 second is the time interval of 9192631770 vibrations of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.
So an atomic clock is that precise versus something mechanical like precision gears and springs.
All clocks need *something* to move or change at a regular, predictable rate. It’s those movements that we count to measure the passage of time. For example, one property of pendulums is that for a given weight and length, no matter how high the pendulum goes it will always swing in the same period. So, you tune the pendulum so that the period is 1 second, meaning it will swing back and forth once per second – and attach that to a [mechanism](https://upload.wikimedia.org/wikipedia/commons/2/29/Anchor_escapement_animation_217x328px.gif) that turns the gears that turn the clock hands.
The problem with these is that you still need a spring to keep tension and turn the gears, and as the spring loses tension and runs out of energy, it changes the rate at which that the mechanism will move, so the time will creep off. And, pendulums and springs change with things like weather and heat, which also makes your time less accurate. Of course, that’s assuming they were built perfectly to begin with!
Digital clocks use a tiny [quartz crystal oscillator](https://upload.wikimedia.org/wikipedia/commons/thumb/9/91/32768_Hz_quartz_crystal_resonator.jpg/220px-32768_Hz_quartz_crystal_resonator.jpg) to track the time. It vibrates just like a tuning fork. The tines (or forks) of the oscillator are carefully crafted to resonate at a particular frequency when electricity is connected through the crystal due to [piezoelectric effects](https://en.wikipedia.org/wiki/Piezoelectricity). That’s its own ELI5 but the important part is electricity makes the the crystal vibrate, and the size and shape are designed to make it vibrate a very specific frequency. That frequency is *32768 times each second*, which is very fast but that doesn’t matter. What matters is that the frequency is known, and a tiny computer chip can keep track of that and count the clock forward on time.
But, like a pendulum or spring the quartz crystal would need to be *perfect* to keep perfect time. Most aren’t. Even very high quality crystals will change slightly with temperature and pressure and whatnot.
An atomic clock uses cesium atoms as the oscillator instead of a quartz crystal. When you beam microwaves at the cesium atom at just the right frequency, the atom will absorb the microwave and transition into a higher energy state. The atom then emits light back out at *exactly* that frequency, or wavelength. Frequency is the definition of changing over time, so…that’s the oscillation they measure. It’s *super* reliable and a fundamental part of physics so it’s very very very very very accurate.
Clocks generally work due to something that *oscillates*. A lot of systems have a natural frequency at which they tend to oscillate, like ringing a bell. The trick is to make them do so exactly the same way over long periods of time.
For example, a grandfather clock has a pendulum that swings back and forth and “wants” to go at a specific rate based on the length of the pendulum. But a lot of factors (friction, temperature, etc.) can make this system not behave perfectly. Pretty darned good, but there’s lots of room for improvement. Mechanical watches use an escapement mechanism instead of a pendulum since it’s rather impractical to put a pendulum in a wristwatch.
In many digital clocks, the oscillation is based upon a thin quartz crystal that expands/contracts a little when exposed to a voltage, and creates a pulse of current when it contracts. If you excite it with a changing voltage close to the frequency it “wants” to naturally oscillate at, it will do so with great accuracy. The combination of the quartz crystal and the circuit it is in creates an oscillating system.
These crystal based electronic clocks are a big improvement over mechanical clocks, but there are still various factors that limit how precise they will be over long periods of time.
An atomic clock takes this idea to another level. Instead of using something mechanical or electronic, atoms that can be in one of two energy states are used. They have a frequency at which such transitions naturally happen when excitation energy is supplied. If you bombard these atoms with microwave radiation that is at a frequency close to their natural oscillation frequency and then tune the microwave frequency to maximize the number of atoms changing state, the microwave frequency you end up with will precisely match the rate at which they naturally “want” to oscillate.
Making a setup like this is not trivial, but mostly it simply won’t work at all unless you get things right. Once it works, it tends to behave quite dependably and provides an extremely precise time reference.
There are a few different ways you can keep time:
Use the rotation of the Earth. This is a sundial or measuring the position of the stars to track the passage of time. Due to natural variations in the Earth’s orbit, this method is not super accurate, and varies widely over the year and across different latitudes.
Similarly, you could use the orbit of the Moon, however that requires the Moon be visible in the sky, and generally is very hard to get precision closer than a day, and, without telescopes, closer than around a week.
Another option is to use the burn rate of a candle, incense, or similar objects. The accuracy of this depends on how perfectly you can make the devices. The more variation you have, the less accuracy you get.
You can measure the rate at which objects are fall through a hole. The two most common methods here are sand hourglasses and water clocks.
The other major version of a gravitation clock is a to use a pendulum, such as in a grandfather’s clock. This works as it turns out that a pendulum swings with a consistent period based on its length, so you can measure the swings to track time. This was later improved by using electromagnet to help keep the pendulum swinging and also to track the swings better.
Much more recently, it was discovered that a quartz crystal will vibrate at a specific frequency when an electric current is applied. Measuring these vibrations allow for fairly accurate timekeeping, so long as you can apply electricity. This method is much better than prior methods as it doesn’t require any large moving parts and isn’t suspectable to most motion. Overall, this is probably the most common time keeping method used today, found in almost all watches, computers, and clocks. Even if other methods could be used, these devices will normally use a quartz oscillator for some internal timekeeping, such as a computer’s clock cycle.
Even more recently, only discovered in 1879 and starting widespread usage in the 1960s, are atomic clocks. This uses caesium-133 which happens to have an atomic transition between energy states at an extremely accurate frequency when exposed to microwave radiation and can be easily measured. A full scale atomic clock measures time to a precision of 9 billionths of a second and an accuracy of about 15 decimal points, many orders of magnitude better than quartz crystal clocks.
From those, there are a few derived methods. That is, these methods only work *because* one of the previous methods was used, typically a quartz crystal or atomic clock:
The electric grid uses AC power that cycles back and forth in flow direction on a regular basis. Most of the world uses either 50hz or 60hz AC. The precision of this depends on how well the electric grid maintains its power load and supply, though in most developed nations, it will be within a few cycles per day on average.
Many major nations broadcast a radio signal that specifies the exact time according to their standard on a regular basis. In the US, this occurs at least every second using the official atomic clock. Any device can listen for this signal to detect the current time, and is how many self-setting clocks work. If you need more precision than the second, you need to use a local version, such as a quartz crystal, and adjust your tracking when you get the signal.
GPS, and similar systems, work by broadcasting the time, based on an atomic clock on the satellite, and a satellite identifier. As such, you can listen for the GPS signal to grab the time to an extremely high precision, but, again, only with intermittent data, around a second. Tracking at higher precision requires a local time keeping that gets adjusted. Due to the prevalence of GPS systems, this has started to heavily take over from the dedicated radio signal for self-setting clocks.
Latest Answers