Astrophysics: What Hit Siberia?

Whatever it was that rocked Central Siberia back in 1908, echoes of the explosion still reverberate in scientific argument. Farmers 40 miles away from the center of the blast were knocked down by the pressure wave and burned by the flash; trees 30 miles from the center were blown over. But no one is yet sure what actually happened.

Writing in Nature, Physicist Clyde Cowan of Catholic University of America, along with Geophysicist Chandra Atluri and Nobel Prizewinning Chemist Willard Libby of U.C.L.A., offer the most ingenious theory so far. After disposing of previous guesses (If it was a meteor, where is the crater? If it was a comet, why was it not seen approaching?), Libby & Co. suggest that what caused the big bang may well have been a hunk of antimatter that must have wandered into the solar system from some distant galaxy.

Antimatter, which has thus far been created on earth only as infinitesimal particles in giant synchrotrons, reacts violently when it comes into contact with true matter. One product of the Siberian reaction would have been a vast number of free neutrons, many of which would have joined with nitrogen atoms, turning them into radioactive carbon 14. Calculations showed that the explosion would have increased the carbon-14 content of the earth's atmosphere by about 7%. That heightened radioactivity could be expected to show up in vegetable matter a short time later.

Dr. Libby and his colleagues peeled the annual rings of wood from the trunk of an Arizona Douglas fir. The rings formed in 1909, one year after the explosion, showed a small but unmistakable excess of radioactivity. This indicates, say the authors guardedly, that about one-seventh of the energy in the Siberian explosion came from antimatter.