Wednesday, July 12, 2006

About Time

Scientific American is a treasure. I was browsing some old issues at the library today, and came across a special issue (September, 2002), on Time (as in space-time, not the magazine). Lots of good stuff, including Paul Davies and Antonio Damasio.

What I did read was "A hole at the heart of Physics" by George Musser.The article is behind the pay-wall, but google has a cache here. Its on how Philosophers may be able to help Physicists decipher Time.

There is some stuff about General Covariance, which is supposedly fundamental to Relativity, but which I could make neither head nor tail of, and so obviously cannot be important.

However, the bit about the Arrow of Time was quite nice. I have come across this stuff before, and Wikipedia explains it here.The basic question is why cups can break, and glass shatter, but broken cups never re-assemble themselves. This is a mystery because the basic laws of Physics seem time-invariant. They work whether time moves forward or backward (we could run a movie of two ball-bearings colliding backwards or forwards, and would not know which is which). One explanation is that this is a result of the Second Law of Thermodynamics, and this where the article points out something that I should have noticed long ago- its so simple.

The basic idea is that there are more ways for a system to be disordered than to be ordered. If the system is fairly ordered now, it will probably be more disordered a moment from now. This reasoning, however, is symmetric in time. The system was probably more disordered a moment ago, too. As Boltzmann recognized, the only way to ensure that entropy will increase into the future is if it starts off with a low value in the past. Thus, the second law is not so much a fundamental truth as historical happenstance, perhaps related to events early in the big bang. (Italics added)

Other theories for the arrow of time are similarly incomplete. Philosopher Huw Price of the University of Sydney argues that almost every attempt to explain time asymmetry suffers from circular reasoning, such as some hidden presumption of time asymmetry.

Well, I thought that was pretty neat.

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