Thursday, May 3, 2007

Sonoluminescence and Other Creatures

As I've mentioned a few times in class, physics is an open science. The number of unsolved problems in physics are quite stupendous, and some of them seem to be questions that we really ought to know about.

One really fine example of how limited our knowledge is in physics is the fact that we are currently unable to provide a complete understanding of how fluids flow. Physicists are unable to supply a law that governs the behaviour of turbulence (which we sometimes encounter during airplane trips). The extremely famous physicist, Werner Heisenberg, who pioneered quantum physics and gave us the infamous Heisenberg Uncertainty Principle once quipped, "when I meet God, I am going to ask him two questions: Why relativity? and why turbulence? I really believe he will have an answer for the first."

Many of the open questions in physics are rather esoteric, but some of them can be rather spectacular. You can check out a list of unsolved problems in physics at Wikipedia, and you can scroll down to click a few of the links at the bottom which give more insight into the nature of these problems.

I'd just like to point out something weird and intriguing in that whole list: sonoluminescence. Sonoluminescence arises when a liquid is excited by ultrasound. Bubbles are formed within the liquid in the presence of the ultrasound (remember that sound waves are pressure waves? They create differences in pressure in the medium, which in this case is the liquid. The rarefactions are areas of low pressure, and in special cases a bubble can form at a rarefaction, which will pop when it becomes a compression), and when they burst, somehow or another they release light.

This process is really very strange if you think about it: why would a collapsing bubble give off light? It becomes even weirder when some scientists actually believed that the temperatures within the bubble actually reached as high as one megakelvin, or 1,000,000 K.

Sonoluminescence is still an unsolved problem today, but you can check out some of the mechanisms that have been proposed by physicists in the Wikipedia entry. In the mean time, don't forget that although what you are studying seems to be written in stone, physics as a science is rather lost at the moment. Everything seems to make sense at some level, but each individual piece is difficult to string together to give a definite picture of the universe, which is more or less the ultimate goal of physics. I hope you can click some of the links available in this post here. Most of the open questions are explained in nice and easy terms.

Happy reading!

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