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TO HONOUR the physicist Enrico Fermi鈥攆ather of the sustained nuclear
chain reaction鈥攖he US government has unwittingly repeated one of his rare
mistakes thousands of times and distributed it across the nation.

The United States Postal Service issued a stamp to commemorate Fermi on 29
September, the centenary of his birth. The stamp shows a photo of Fermi, who
died in 1954, standing in front of a blackboard on which he has fluffed the
equation for the 鈥渇ine-structure constant鈥�, a number that determines the
strength of electric and magnetic forces.

Denoted &agr;, the fine-structure constant combines the charge of the electron,
the speed of light and a quantity known as Planck鈥檚 constant. In the picture,
Fermi has accidentally switched the places of the electron charge and Planck鈥檚
constant.

It鈥檚 hard to understand how Fermi could have botched such a fundamental
equation, says Greg Huber of the University of Massachusetts in Boston. 鈥淚t
would be equivalent to Einstein writing c = mE², instead of
E = mc²,鈥� he says. 鈥淚t鈥檚 that crazy a mistake.鈥�

The Postal Service obscured the mistake by cropping part of the equation out
of the image after Albert Wattenberg of the University of Illinois in
Urbana-Champaign, who had worked with Fermi, alerted it to the error. 鈥淲hy have
him standing there next to something incorrect?鈥� says Wattenberg. But the gaffe
appears clearly in the original photograph and in another picture snapped at the
same time, says Huber.

Fermi rarely made such slips, adds Jerome Friedman, a Nobel laureate from the
Massachusetts Institute of Technology, who was one of Fermi鈥檚 students at the
University of Chicago. He thinks it鈥檚 fitting that the stamp portrays Fermi with
chalk in hand: 鈥淔ermi was a wonderful teacher, and that鈥檚 what the stamp
肠辞尘尘别尘辞谤补迟别蝉.鈥�

The photograph on the stamp appears to have been staged, according to James
Cronin of the University of Chicago, also a Nobel laureate and a former student
of Fermi. Cronin thinks Fermi may have goofed as he hastily scribbled the
equation for the sake of the photographer. And were he alive today, Fermi would
react to his own mistake with characteristic good humour, Cronin believes. 鈥淗e
would probably laugh, and he might even say, 鈥楾hat鈥檚 not the fine-structure
constant, that鈥檚 just some number.'鈥�