Wednesday, August 18, 2010

Arrow of time, thermodynamics, and another transit to Yokohama

The second law of thermodynamics is the only physical law that is time-irreversible. All other laws, such as Newton's equation of motion or the Maxwell equations of electromagnetism, are insensitive to the direction of time (for example, the Earth is revolving around the Sun counterclockwise, and if you reverse the time, it would revolve clockwise, and this situation is perfectly fine). The second law of thermodynamics tells us that entropy (of a close system) can only increase with time, if you let the system alone. It's like your room getting messier and messier until you make your mind to clean it up. Why only the second law of thermodynamics has this arrow of time is still an unresolved problem in physics. Ludwig Boltzmann, the founder of statistical mechanics, once provided a microscopic justification, which was found to be rather incomplete, and he allegedly committed suicide because of this. Boltzmann's explanation is, however, what is commonly seen in most of textbooks on thermodynamics. Here I translate it to a more familiar example using a deck of cards. Suppose you start with a deck of 52 playing cards nicely ordered from the ace of spades to the king of clubs. You then shuffle it 100 times so that the cards are pretty much randomly ordered. Now you keep shuffling it and see if any order pops up. You may occasionally get lucky to see some partially ordered sequence (e.g., all aces in one pace), but your cards would stay looking randomly ordered for most of times, because there are so many combinations of card order that look random. How many different combinations of card order do we have? It's 52x51x50x….x1 = 52! ~ 8x10^67. If you're not familiar with the scientific notation, it's about 80000000000000000000000000000000000000000000000000000000000000000000 (67 zeros after 8) [try WolframAlpha for the exact result], and most of these combinations are pretty much randomly ordered. The initial order you started with is just one out of this huge number, so the probability of coming back to the initial order (the ace of spades to the king of clubs) by randomly shuffling is extremely small. It's not zero, but really, really small. Therefore, things are usually got more disordered with time (i.e., entropy increases), because a more random state is more likely to happen. But the probability of getting back to the initial state is not exactly zero, so some rigorous people are not satisfied with this sort of explanation based on thermodynamic improbability, and the debate over the origin of the arrow of time continues…

Now, why am I talking about this? Isn't this blog supposed to be about the Shatsky Rise cruise? OK, the reason is that we had another medical emergency, and we're currently heading to Yokohama again (this time, fortunately, no death is involved). Having two medical diversions in one cruise is pretty unusual. Will Sager, who has much longer sea-going experiences than I, told me that he has been on ~40 cruises over 33 years and had never had bad luck to this degree before. This cruise is only my 10th cruise, and this is actually my very first cruise as a chief scientist, and look what an experience I'm having! But if you think about the above thermodynamical improbability, the likelihood of having two medical diversions in one cruise may not be so small… Or I should think the other way around... Having similarly bad luck in future would be even less likely, so my future cruises may go entirely trouble-free!? Maybe it's time to start writing another sea-going proposal.

NSF has been very sympathetic to our situation, and because no further extension is possible this year (we need to get back to Honolulu by September 14th to give OBS to another cruise), they agreed to have another Shatsky Rise cruise sometime during late 2011 to early 2012 to finish any unaccomplished portion of our planned seismic survey. The science party is all grateful for this thoughtful decision.

1 comment:

  1. I wonder whether a large 'confiscated drug boat' could be acquired permanently for your department's research needs! Or have I been watching too many American movies? ;-)

    Anyway, thanks for sharing your science and your travails! It even got me thinking