Tuesday, March 31, 2009

Heading out

Tonight is my last night in Fairbanks. It's been a great six and a half months. I didn't get as much writing done as I originally hoped, but I did get a fair bit, and I did accomplish my health goals, so overall I'm very happy.

On the humble inquiry theme, I'll just put a few thoughts here about a rather interesting problem in thermodynamics that is not widely appreciated. Thermodynamics is generally considered to be a very solid, well-established part of physics, but there is an aspect of it that is extremely problematic: the time-reversibility of physical law means that the second law of thermodynamics, (disorder always increases), therefore predicts that the disorder in the past should also be higher. Yet we know that is not the case. I had realized this long ago after reading about it in the Feynman Lectures on Physics, but it wasn't until I read Robin Hanson's comments in the Overcoming Bias blog that I considered just how "crazy" the standard explanation for this really is.

The "explanation" is, of course, that at some point in the past the disorder was very low. I put "explanation" in quotes, because it really isn't an explanation at all. It's just a statement that disorder was lower in the past without giving any reason for that. With the big bang, it's assumed that the disorder must have been low, but so far no one has come up with a compelling reason why that should be so. The center of an explosion is not normally known for being a nice, orderly place. Where did the order come from?

One suggestion that has been made is discussed by Feynman in his Lectures on Physics: maybe we just happen to live in a more ordered fluctuation in a much larger unordered region. But if that were the case, you would not expect to see the same amount of order in every direction equally, especially in such a large volume as our visible universe. In other words, it's much more likely that you would have a fluctuation that resulted in a single solar system than a galaxy, let alone 80 billion galaxies. The "random fluctuation in a larger volume" argument is not popular because of this.

It's sobering to realize that as we watch the Universe unfold, everything we see is the unwinding of the "spring" that was wound up at some point in the past. To paraphrase Feynman, even a falling drop of water cannot be completely understood until the mystery of the beginnings of the universe are reduced from speculation to scientific understanding. We're still a long way from that.


Update: I recently watched an interesting lecture that Roger Penrose gave at Princeton in 2003 which talks about this issue. He gives an estimate of the volume of the phase space for our observable universe of 1010123. One over that enormous number is an estimate of the likelihood of the special conditions that would have to be present at the Big Bang in order to give rise to a universe with the amount of order we currently observe. That number (10^10^123) is so huge that it overwhelms the anthropic principle: as I mentioned above, you don't need anything near that much order to get observers like us. There must be something else going on, but so far, no one has come up with a good explanation for all that order.


Further update: Sean Carroll gives a fascinating interview on edge.org which covers many of these same issues. It's well worth watching.

No comments:

Post a Comment