Fun With Bose-Einstein Applets

By Mark Trodden | May 31, 2006 9:15 pm

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.

  • adam

    That sort of stuff was being used towards the end of my time as a physics teacher (although not applets at that time). I think that the IOP’s ‘Advancing Physics’ A Level support material has a bunch of animations of physical processes.

    Certainly much better than my diagrams.

  • Mark

    Sounds cool Adam. There was really nothing comparable when I did my A Levels (before the web was used in schools), and it is clear to me that I would have learned a lot from this kind of thing.

  • Mark

    P.S. Adam, that was a seriously fast comment on my post!

  • adam

    I am quick like a startled squirrel.

    Soon to be snoring like an elephant with stuffed sinuses (to add to the animal comparisons).

    When I was teaching at one school (we used the old Nuffield ‘A’ Level course), we used to use programming to explain S.H.M and exponential decay; we’d get the kids to design BBC Basic code that simulated them stepwise (because the course didn’t require calculus so they didn’t get to solve the differential equations but this way they could still understand the important issue of how the shapes of the graphs arose) and then, as it became clear that students were generally getting worse at programming, we got them to do it with Excel. It was always a bit of a headache, though. Purpose-written demos are better (our HoD had a nice one demonstrating heat transfer in terms of microscopic configurations).

    In my days as an A Level student, however, like yours, we got a simple Casio calculator and a bag of cold gravel and counted ourlselves lucky.

  • Jeff Nuttall

    Reminds me of a project I wanted to do, making a site of Flash games that would teach concepts of basic physics–acceleration, momentum, etc.–in an entertaining way. I even thought of a way to make (what I think would be) a fun game about vector addition and subtraction and cross products. (Haven’t figured out a good way to work dot products into it, but what the hey.) I haven’t made any progress on this project beyond the planning stages, though, and I’m not positive I ever will–I have a lot more planned projects I want to pursue than I’m likely ever to have time for. Anyway, though, like I said, I was planning to make games based on basic physics concepts, not the more advanced stuff the Physics 2000 site covers, so it’s not the same thing.

    This Physics 2000 site, though, could come in handy–I’m teaching a third-semester physics class at a community college in the fall, and there’s likely to be some stuff here I can use for the class, directing students to this page and letting them know what applets apply to the concepts we’re currently discussing. I’ll have to take a closer look at the site and correlate the applets there with my syllabus…

  • B

    Hi Mark,

    I remember I heard a similar talk by Weiman shortly after he got the Nobelprize. He showed several of the applets and I ended up spending quite some working time playing around with them :-)

    I also recall he said that the ‘average brain’ is only able to process 7 new information items per hour. I find myself frequently thinking that seminar speakers used up my seven items already with the introduction.

    (I am just sitting in a talk by Antoniadis at the Planck 2006 conference. He already used up all my items for the whole day.)



  • Sean

    I managed to get 27 atoms to Bose-Einstein condense. Woot!

    Also, do you know why thermodynamics works? Because being Maxwell’s Demon is really hard:

  • J

    I’ve got 50 atoms in a BEC before. Top that!

    (Hint there is a way to cheat and remove energy from the system without removing atoms.)

  • twaters

    Badass! During Leno I got a BEC at 34 and a temp diff of 1.525 w/density 1:2 on Maxwell’s demon. More of those!

  • Plato

    Greg Egan’s animation gallery and I am sure others could link many more as well?

    Actually this is what makes it easier for us older folk to understand what the common language may be saying mathematically.

    As I mention also, Thomas Banchoff is readying the mind for “fifth dimensional views?” I mean you know that computer screen analogy and the “two dimensional view” becoming something much more complex?


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About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.


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