Sunday, November 12, 2006

Drops of uranium you can play with

Below is a very interesting youtube video of a demonstration on the International Space Station. It involves a large sphere of water that is practically free-floating. The experiment shows how liquids actually behave under circumstances where surface tension and the liquid's self-attraction are signficantly stronger than outside influences like gravity. One reason I find this interesting is that this is a rough approximation to how an atomic nucleus behaves.

When physicists try to predict the properties of carbon-12 or other fairly light nuclei they typically account for the forces of all 6 protons and 6 neutrons individually. This is a difficult thing to do computationally as it requires doing math on a matrix with somewhere around 10^12 elements. To use methods like these on uranium-238 would require a matrix with 10^238 elements. This is not possible with modern computers, super or not. So instead they use models which approximate the behavior of the nucleus as a whole, and then worry about quantum mechanical corrections to this approximation. One important (rough) approximation of the nucleus is the liquid drop model. It basically says that the nucleus is governed by 4 effects:
1) Particles are attracted by the surrounding particles.
2) Particles at the surface are not surrounded, so don't have all the attractive energy.
3) Protons repel each other. Neutrons don't
4) An even mix of protons and neutrons is more attractive than a surplus of either one.
If you take a medium sized nucleus with equal numbers of protons and neutrons, then 4) has no effect and 3) has a small effect. What you have left is a drop of liquid very much like the one in that video. The way the sphere deforms and oscillates in 3 dimensions on the video also occurs in actual nuclei. It's also interesting to look closely at the droplet that is ejected from the sphere just after the puff of air. Energy is added and a portion of the sphere elongates until there are two distinct centers of mass, then the liquid connecting them shrinks to nothing and both peices close back into spheres. In short, it's nuclear fission.


Post a Comment

<< Home