Thursday, April 05, 2007

Right-left -- Part 2

Why did life adopt one form of handedness rather than the other? Scientists have been pondering the question for decades, without success.

One theory suggests that the handedness of organic molecules had its origin in radiation from a supernova that bathed the Earth as life had its beginning. All supernova radiation is polarized; that is, the light is emitted in a corkscrew fashion, either to the right or left. Research has shown that polarized radiation can cause chemical reactions to favor one chiral form of molecule rather than its mirror-image twin.

If this is true, then the handedness of life had its origin in an exploding star.

Other scientists have proposed the weak nuclear force as the source of nature's handedness. This is the force that governs the radioactive decay of a neutron in an atomic nucleus into a proton and an electron. It is the only fundamental force of nature (gravity, electromagnetism, and the strong nuclear force are the others) with a built-in twist; the electron always emerges with a left-handed spin.

Experiments confirm that left-spinning electrons can weakly bias chemical reactions toward one chiral form or another. If this is the source of the handedness of biomolecules, then the twist of our visceral organs may be connected to an asymmetry at the heart of matter.

But the handedness of life may have a much simpler source. Stirring of chemical solutions can cause reactions to go one way rather than another. Perhaps life got started in a "warm little pond" that was stirred by wind or currents. Maybe in some complicated way the twists of life are related to the spin of the planet itself.

Whatever the explanation turns out to be, the lesson of chirality is clear: We are all part of a piece. Conch shells, honeysuckle vines, the human heart, exploding stars, whirling planets, decaying neutrons: no part of this twisted, beautiful world can be abstracted from any other.