In one of my recent posts, about the Quantum to Cosmos meeting, I was enthusing about the beautiful physics of cold atoms. While writing, I was reminded of a wonderful Java applet about this topic, which I learned about in a colloquium that I saw Carl Weiman give a few years ago, before he won the Nobel Prize (but when people were sure he was going to get it).
The particular applet I was thinking of was one on evaporative cooling, which is a method used to obtain an extremely cold collection of atoms. The way this works is simple in principle, but difficult in practice. First, atoms are contained (or trapped) by a magnetic field in the shape of a bowl, (see screenshot below).
As time passes, occasionally some atoms will borrow enough energy from the group that they are able to escape the trap, taking all that energy with them. Next, after doing this for a while, one gradually reduces the strength of the magnetic field and waits again. The trick is to let the very few atoms carrying just the very highest energies escape at each step. If one repeats, being careful to reduce the magnetic field slowly enough, one can end up with a rather large collection of extremely cold atoms in this way.
The applet is only part of an extensive educational site called Physics 2000 from the University of Colorado, Boulder, physics department. Physics 2000 contains a number of different parts, but the piece that is most directly related to the physics I was discussing in my post can be found in The Atomic Lab, which has excellent pedagogical discussions of Interference Experiments and of Bose-Einstein Condensation.
The discussions are at a level that can be enjoyed by professional physicists (I enjoyed them and learned things from them) but also should be fun and informative for interested non-physicists, including kids. They are arranged as question and answer sessions between two people. Here’s the bit that comes just before one plays with the laser cooling applet, describing the apparatus necessary a couple of steps before evaporative cooling comes in
[Professor] Shining light on your hand makes it get hotter because the light is absorbed and turns into heat. The trick to making atoms colder is to make the light bounce off of them. In fact, it bounces off with more energy than when it hits the atoms.
[Student] That sounds like quite a trick.
[Professor] It is. It took physicists quite a while to figure out how to do it. (Click here to find out more about laser cooling and the winner of the 1997 Nobel Prize for Physics.) You start with the idea that laser light comes in a stream of photons. These photons are very light, so to speak. Compared to an atom, they are like ping-pong balls compared to a bowling ball. But in just the same way you can push a bowling ball around if you shoot a big enough stream of ping-pong balls at it, you can push atoms around by bouncing laser light off them. Try to adjust the laser power and laser position to slow down the atoms.
The applet being talked about here looks like this
I hope you have as much fun as I did playing with the applets.
Incidentally, a measure of Weiman’s commitment to science education is that he is moving to spend most of his time at the University of British Columbia working on just that – quite remarkable for any physicist, never mind a Nobel laureate.