Sunday, November 12, 2006

The last shall be first

Some are born great, some achieve greatness, and some have greatness thrust upon 'em," says Shakespeare's Malvolio, reading from Maria's letter.

One might apply these words to the elements of which the world is made.

Oxygen is born great. Forged in the hot interior of stars, oxygen has from the first moment of its birth a promiscuous tendency to combine with other atoms. This appetite for union springs from its hunger for electrons, which it will happily share with other elements in molecular marriage.

For example, oxygen will combine with two hydrogen atoms to form water, that gushy substance so essential to life. Water has been detected in meteorites, suggesting that wetness might be deliciously primeval.

Oxygen also throws in its lot with silicon and aluminum to form the rocks that make up the Earth's crust. It constitutes 35 percent of our entire planet by weight, more than any other element. When gravity was pulling the Earth together out of the dust and gas of the presolar nebular, it was oxygen that did much of the "heavy lifting," dragging its companion elements into planetary sphericity.

Carbon achieved greatness. Of the materials of the Earth's surface it amounts to a mere drop in the bucket, a few hundredths of a percent. Who would have predicted 4 billion years ago that carbon would be the basis for life and intelligence, and ultimately the transformation of the planet?

Unlike oxygen, shy carbon is reticent in making alliances, contentedly inclined to mind the advice of Shakespeare's Polonius: "Neither a borrower or a lender [of electrons] be." It usually resides in exclusive affiliation with other carbon atoms, in unions as hard as diamond or as soft as graphite. But when it does throw in its lot with other elements, the marriages are likely to be interesting.

The chemistry of no other element is as subtle and various as the alliances of carbon. Entire courses at universities -- organic chemistry -- consider nothing else. At the heart of every organic molecule is a carbon backbone, a chain or ring of carbon atoms that represents that element's narcissistic affinity for its own kind.

Organic molecules tend to have low melting and boiling points, and therefore usually exist as liquids or gases at the temperatures of the Earth's surface. Sometime more than 3 billion years ago, compounds of carbon, stewing in the soft envelope of the early Earth, found the means to self-catalyse their production -- and life made its appearance on the planet.

All terrestrial life is carbon based, and life trumps the inorganic. Reluctant carbon was the tortoise to oxygen's hare.

Silicon is carbon's heftier cousin, residing just below it in the periodic table and sharing carbon's chemical affinities. One might think that it would also share carbon's subtle and prolific chemistry; indeed, some folks have speculated that other planets might harbor silicon-based life. But carbon's lighter weight makes its compounds more reactive at the temperatures of the Earth's surface. Silicon is heavier, and its compounds achieve the same degree of reactivity only at hot subterranean depths. At Earth's surface silicon compounds are as inert as sand.

Certainly, there's plenty of silicon about, vastly more silicon than carbon. About a quarter of the Earth's surface rocks are silicon by weight. Sand is mostly silicon dioxide, and what could be cheaper and more plentiful. But for all of its promise, and all of its plenty, silicon never achieved the greatness of carbon.

Until it had greatness thrust upon it.

Silicon has electrical properties that make it a wonderful substrate for computer circuits. Its ability to conduct electricity can easily be modified by adding impurities to pure silicon crystals. Silicon oxide, which forms on the surface of silicon when exposed to oxygen at high temperature -- a kind of silicon "rust" -- is an excellent insulator. All of which makes it relatively easy to fabricate electrical circuits in silicon on a microscopic scale.

Carbon, in the form of intelligent life, extracted pure silicon from the rocks and helped it achieve its potential as -- the silicon chip.

Of course, life has lifted other elements from obscurity. What would the history of civilization be without artifacts of copper (bronze) and iron, for example? What of silver and gold? But none of these elements has the potential of silicon. A microcomputer chip the size of a thumbnail is more complex than, say, the Eiffel Tower or the Hoover Dam. The day may not be far off when silicon-based thinking machines surpass the intelligence of our own carbon-based brains.

And because silicon is less reactive than carbon, computer chips have a kind of stability and -- dare I say it? -- immortality that our carbon-based brains lack. A few hundred years down the line, when computers rule the roost, stolid silicon will have trumped supple carbon, by playing a waiting game and following Polonius's other bit of advice: "To thy own self be true."

Further Reading

A delightful exploration of the elements is P. W. Atkins' The Periodic Kingdom: A Journey Into the Land of the Chemical Elements.

And of course Primo Levi's The Periodic Table is a must read, a classic of scientific humanism.

Discuss this essay and more over on the Science Musings Blog.