God's Atom

Doug Natelson is thinking about fortuitous physics, inspired by some solid state examples:

Every now and then you stumble across a piece of physics, some detail about how the universe works, that is extremely lucky in some sense. For example, it's very convenient that Si is a great semiconductor, and at the same time SiO2 is an incredibly good insulator - in terms of the electric field that it can sustain before breakdown, SiO2 is about as good as it gets.

From my own field of physics, I would suggest the rubidium atom. Rubidium isn't a substance you run across every day, and that's a Good Thing-- like all the alkali metals, it reacts explosively when it comes in contact with water. In spite of that, it's become the workhorse atom of modern atomic physics, thanks to a large collection of fortuitous properties.

For one thing, rubidium happens to have a very convenient transition between its ground state and first excited state. Transitions between these two states are driven by light with a wavelength of 780 nm, which turns out to be a very convenient spot for diode lasers. That means that it's relatively cheap to do laser cooling and trapping experiments in rubidium.

Rubidium also has remarkably nice collisional properties. The interactions between atoms are characterized by a quantity called the "scattering length," and for rubidium, the scattering length is positive, and moderately large. This means that it's possible to form large Bose-Einstein Condensates of rubidium, and indeed, the first atomic vapor BEC was made in rubidium in 1995. If that scattering length were negative, as it is in lithium, it would be impossible to make a large BEC with normal measures.

The scattering length also happens to be more or less the same for two different states of rubidium atoms, allowing all sorts of interesting experiments involving two-species condensates. A wealth of interesting physics has been uncovered by looking at the behavior of mixed samples of the different states.

Another collisional property of interest is known as a "Feshbach resonance," which is important because it allows you to tune the scattering length by varying the magnetic field. If you put rubidium atoms in a small-ish uniform field (around 155 gauss), you can change the magnitude, and even the sign of the scattering length. This means you can form a large condensate, and then investigate what happens when you change the interactions between the atoms, allowing the study of all sorts of interesting physics.

Finally, on a more practical side, the collisional properties of rubidium are such that interactions between atoms do not change the frequency of the "clock transition" analogous to the hyperfine transition in cesium that is the basis for our definition of the second. This has led some people to argue that atomic clocks based on rubidium would be superior to clocks based on cesium. (The development of frequency-comb technology probably renders this moot, but there's still some interest in rubidium clock studies.)

All of this stuff put together has led some people to refer to rubidium as "God's atom." (I think this originates with Eric Cornell, but I'm not 100% sure...) Whether it's a sign of divine providence or not, the properties of rubidium are certainly an example of fortuitous physics.