-----Original Message----- From: math-fun-bounces+andy.latto=pobox.com@mailman.xmission.com [mailto:math-fun-bounces+andy.latto=pobox.com@mailman.xmission.com]On Behalf Of Henry Baker Sent: Sunday, July 16, 2006 9:23 PM To: David Wilson Cc: math-fun Subject: Re: [math-fun] Physics question

There seems to be another analog to this system: semiconductors.

The electrons act a little like a gas & bounce around, while the places they leave are called "holes". However, they aren't identical: electrons have higher mobility than holes, which is why NPN transistors are faster than PNP transistors.

When I was very young, one of my favorite books was _Many_Moons_, by James Thurber. One of the things the Royal Astronomer (or possibly the Royal Mathematician) brags to the king that he has done is calculating the Speed of Darkness. This was just meant to be silly, but I was intrigued by it. If a room is brightly lit by a lightbulb, and you turn the bulb off, it gets dark, but not instantaneously. It will be some amount of time before the far corner of the room is dark, so there is a meaningful speed of darkness. At the time, I was proud of myself for figuring out that this speed would be exactly the speed of light. While that's true if you're only considering direct illumination, if you take into account the fact that some of the reason that the far corner is light is not through direct illumination, but via reflections off of the walls, the matter becomes more complicated. Andy Latto andy.latto@pobox.com