Alien Seas and the Nature of Science

December 22, 2011

Went to John Lewis yesterday. A subterranean memory emerged. When I was nineteen I used to get a strange pleasure wandering through the John Lewis Department Store. It was an eery world, detached from my own. I would get a buzz drifting past thousands upon thousands of things I had no desire  for whatsoever. Vast acres of the uninteresting. It was like sailing on an alien sea, gazing upon lifeforms and liquid shapes I would never understand.

Well, thats the smugness of youth I guess. Now of course I wander into John Lewis and within four yards I am picking something up thinking “mmm, that would be useful”. How things have changed. Over the same thirty eight year period, I have noticed that my waistline has slowly but systematically increased. This correlation must mean something. It clearly proves that the desire for household items has a calorific effect.

Spotting correlations is what science is all about ! Did you know that in the twentieth century there was a clear correlation between the average size of feet in China, and the price of fish in Billingsgate market ? There is as yet no good theory for that one. On the other hand, I had a friend who trained spiders to obey his voice commands. Then he would pull their legs off, and found that they no longer obeyed the commands. As he explained to me, this proves that spiders have their ears in their legs.

In a similar vein, a paper about high redshift galaxies I read the other day showed very clearly that … oh, hang on, is that the time ? Must dash. I am sure you can complete the example and several more of your own.

Cosmology/Glasgow exam technique

April 15, 2010

Along with a large fraction of the UK astronomical community, I am at the National Astronomy Meeting (NAM2010) in Glasgow. Just imagine a well placed bomb. Could be like the Manchester United Plane Crash all over again.

Two fabulous plenary talks this morning from George Esthathiou and Rosie Wyse. Plack has completed its first survey, but George is not allowed to show us the CMB maps yet. We have to wait until October 2012. (Southwood was in the audience, so he had to behave…) But he did give us a taster – Galactic cold dust map, made by combining Planck and IRAS.  Rosie Wyse showed us all sorts of fascinating stellar population data reflecting the merger history of the Milky Way and other nearby galaxies, as expected in the standard CDM cosmology. But she also showed two worrying things. The thick disc is thought to be made by mergers stirring up an older thin disc. But the stars in the thick disc are really old, suggesting the thick disc of the Milky Way has not been affected by mergers since about redshift two. The second niggle was that the distribution of abundances is consistent with the stellar initial mass function being constant through cosmic history – whereas the galaxy formation simulators desperately want early star formation to be high-mass biased. Hmmm.

Brain spinning, I wandered across the corridor to the Hunterian Museum. This has one of my favourite pieces of Glasgow History – the Blackstone Chair. From mediaeval times until the 19th century, this is how Glasgow students were examined. When you were ready, you sat on a special chair surrounded by Profs. They fired questions at you for twenty minutes, with the sand running out just behind your head. Then some flunky stamped a pole on the ground and you were done.

We gotta bring this back.

People, galaxies, and complexity

January 19, 2009

Which is more complex, a person or a galaxy ? I have been to several seminars recently about various aspects of simulating galaxy formation and structure, on a variety of scales from large scale clustering to nuclear gas flow. Work of this kind is getting gradually more impressive, partly because better physics is going in, partly because algorithms are improving, and partly of course because folks are using bigger computers and gobbling up more CPU hours, thus achieving simulations with more resolution, more particles, or more timesteps. Obviously this must lead to more realism… but you get this strange queasy Borgesian feeling … if we simulate in enough detail to get it right, the galaxy in a box becomes indistinguishable from the real thing. What then have we learned ? Surely understanding involves some kind of encapsulation, some boiling down to a deep but simple statement ? Some kind of emergent law ?

At lunch after one such seminar, I was trying to provoke some of the locals with sceptical thoughts of this kind. Tom Abel had a good answer : the aim of massive simulations should not be to make it look right, but to see patterns and consistencies emerge. The simulations are not explanations, but experiments. Nonetheless Tom and his co-workers make very convincing looking – and stunningly beautiful – pictures. Although we love telling each other how little we know, in fact you could make a good case for the idea that we understand galaxies better than we understand people.

In popular talks I often go beyond this, claiming that people are quantifiably more complex than galaxies. (This is all about flattering the punters of course.) Now, you could go about defining  complexity in a number of ways. Its not about specifying the phase-space position of every molecule, otherwise a bucket of water is more complex than an iPod, and that ain’t right. One useful method is to ask how many instructions it takes to make something. To make a person, you take the information in the DNA molecule. The human genome has about three billion base pairs, and each of those needs 2 bits to specify one of four bases. So in total the information needed is about 750MB, or about one CD. Its instructive to compare that to printed text. A character from the alphabet needs 5 bits, but lets add other stuff and round up to one byte per character The book in front of me has 72 characters per line and 46 lines per page. So DNA is equivalent to 5.2 million lines, about 113 thousand pages, or maybe a couple of hundred moderately fat textbooks. To specify a person you don’t need an entire library, but you do need several shelves of books. By contrast, you can do a pretty good job simulating a galaxy with a few pages of C code. I believe that even the goriest codes are around the 50,000 lines mark. So there you go. A person is a thousand times more complex than a galaxy.

Well, I am sure many of you will be squirming and spotting all the cracks in that argument. It does remind me of the old story about Fred Hoyle, who claimed in a lecture that a star was a “pretty simple thing”. From the back of the room, someone called “You’d look pretty simple from ten parsecs, Fred.” (Special no-prize for anyone who can shed light on this urban myth.)

However, its pretty hard to come up with something objective. As I said, we don’t want to get into the list-every-molecule game. Perhaps this says something important about information. I can never resist the feeling that whenever somebody asks “how much information is there in X ?” the answer is “what do you want to know ?”. For this reason, I suspect that the infamous question of whether a black hole destroys information is an incoherent one. But I had better stop there or I will get stomped on by people cleverer than me. So. There you go. Galaxies simpler than people. From our point of view.


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