Image: George Musser ATOMS of helium (gold) and krypton (red) clump when the temperature is low....

Grant that the universe is filled with atomic-size billiard balls. Then, with a few insightful definitions and some mathematical gymnastics, physicists can provide you with a near-perfect explanation of our everyday world. The theory is called statistical mechanics. Many people know that it limits the amount of work a machine can deliver. But it actually goes much further. Statistical mechanics describes the engines that drive the earth's weather. It governs the temperatures and pressures inside stars and constrains the evolution of the cosmos. It even sheds light on the arrow of time--why we remember the past and not the future. Indeed, Albert Einstein and Richard Feynman saw the theory as the highest achievement of classical physics.

Sadly, many amateurs have avoided this important subject because, in this case, the highest plateau is also the hardest to reach. One cubic centimeter of air at atmospheric pressure contains more than 10 billion billion atoms of various sizes, all smashing into one another at different speeds. No computer can project the exact trajectories of all these particles, and even if one could, no human mind could make sense of it. Therefore, physicists have devised clever but devilishly difficult mathematical methods to extract comprehension from the chaos.

Daniels & Daniels; Source: Molecular Modeling but as the gas heats up, the lighter heliums are torn asunder....

Daniels & Daniels; Source: Molecular Modeling ...and at still higher temperatures, the heavier kryptons fly apart, too.

But the subject is not as abstruse as it seems. The trick is to find models that let you visualize how these random collisions average out to yield the familiar properties of matter, such as temperature, pressure and entropy. The right mental pictures can elucidate the behavior of materials and in turn can help advance amateur projects involving chemistry, sound, heat transfer, crystals and vacuum techniques. That's why I'm pleased to let you know about Molecular Dynamics, an innovative piece of educational software. It doesn't cover every topic within classical statistical mechanics, and it ignores quantum-mechanical effects completely. But it is still the most accessible modeling software I've seen. What is more, the authors of the program at Stark Design in Morristown, N.J., have made it available to Scientific American readers for free until October 2000.

This kind of simulation is nothing new. Many amateur scientists fondly remember writing such programs back in the days of hobbyist computing [see Computer Recreations, by A. K. Dewdney; Scientific American, March 1988], and several limited versions are available on the World Wide Web (such as a Maxwell's demon game).

But Molecular Dynamics takes this all to a new level. It allows you to conduct an impressive array of virtual experiments to see how different atoms interact under all kinds of conditions. The program consists of numerous modules that demonstrate diffusion, osmotic pressure, the relation between temperature and pressure, the distribution of molecular speeds in a gas and many other topics. And you can use the software to discover things that even the most mathematically gifted physicist would be hard-pressed to wrestle from the basic the

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