Clocks play an incredibly important role in our lives. This post is about a recent development, based on nuclear physics, which promises the most accurate clock ever [1].
A Short History of Clocks
To measure time, we need an event that repeats. Thus the earliest clocks were made of pendulums, hour glasses, etc. The relatively recent advent of electronics brought clocks made of quartz crystals and transistors. These timepieces each present some problems. For one, their accuracy is limited: even a good quartz clock, for example, goes off time by about half a second every day. Also, each one, due to manufacturing differences, is unique: no two quartz crystals lose exactly the same amount of time. This poses problems for synchronization.
Clocks go Atomic
Interestingly, Nature gives us clocks that are identical copies of each other: atoms. A Cesium atom from China is indistinguishable from a Cesium atom from India, or from anywhere else in the universe for that matter. How can an atom be used as a clock? Roughly speaking, by driving one of its electrons periodically between two well-defined energy levels. This repeating action, which is carried out by using a laser which puts out photons with the same energy as the difference between the atomic energy levels, gives us a clock.
Using these ideas, 'atomic clocks' have made fantastic progress; Cesium clocks lose about a a second in 1.4 million years; the best atomic clocks lose one second in 30 billion years (more than twice the age of the universe)! One might ask at this point if this isn't overkill now - why even bother building better clocks?
The answer is that a precise measurement of time can be translated into a measurement of other quantities. For example, a good clock can be used to measure distances accurately: this is why atomic clocks form the basis of the GPS: the more accurate the clock, the better you can localize your car in the parking lot.
Super-accurate atomic clocks are also used to look for very slow changes, such as predicted by some scientists, in the fundamental constants in the universe. (These look like constants right now, but may in fact be changing slowly with time - but this is yet to be confirmed). So we do need better clocks.
Problems with Atomic clocks
A major limitation of atomic clocks is that the atomic energy levels are very sensitive to noise, and background electric and magnetic fields, which are usually present in the apparatus. So the atoms have to be levitated in space (using electromagnetic fields which we need to know well), away from as many things as possible so the 'clock levels' are not affected. This makes most atomic clocks quite bulky - though some chip-scale designs have emerged lately.
Why would you need a chip-scale clock, when GPS gives us access to the time? There might be situations where GPS is not available (down in an oil well, or in outer space, for example).
Clocks go Nuclear
Interestingly, the problems faced by clocks based on atomic electrons can be tackled by clocks based on atomic nuclei. Because atomic nuclei (protons and neutrons) are (five orders of magnitude) smaller than electron orbits, they are relatively more insensitive to background electromagnetic fields and chemical environments. They are therefore expected to be highly accurate, sensitive and portable.
The problem is that the nuclear energy levels are usually too energetic to be driven by the lasers we presently have (they typically need gamma rays - so that's a motivation for you to build a gamma ray laser, in case you are itching to make one).
However, there is a freak pair of energy levels in the Thorium 229 nucleus, which were being hunted for for more than 25 years, and have recently finally been identified using a laser at 148nm [2]. This is the first time an atomic nucleus has been excited using a laser! Among other things (see below) this is expected to lead to a clock which loses 1 second in 300 billion years - so you don't have to worry about being late for work anymore.
Bonus
i) The energy levels in Thorium could also be used to make a 'nuclear laser'. Don't ask me what that is.
ii) The nuclear clock is sensitive to dark matter. Since no one has ever detected dark matter, this new clock is an exciting development in the context of ongoing dark matter searches.
[1] A Brief History of Timekeeping by C. Orczel. (I have not read it; I will probably review it when I do).
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