Sir

The 10 September Daedalus column was, as usual, fascinating and charming ( Nature 395, 120; 1998 ). But it raises the possibility of a grave academic injustice dating from nearly 20 years ago.

At the time, I was a sophomore in a prestigious university in the American Midwest (we'll leave out the institution's name, but its initials are University of Chicago). I recall with chagrin breaking up on the rocks of an introductory quantum mechanics examination (I had an A grade going into the final) in which one of the questions was an explanation of the Crookes radiometer, as dealt with by Daedalus.

Never mind what I answered; you're not going to get me to admit to that. But the correct answer, I was told by a tired-looking professor, was that photons hitting the black side of the vane were experiencing an inelastic collision, while those hitting the white side were colliding elastically. The inelastic collision imparted a momentum, P (derived from the photon), onto the vane, but in order to conserve momentum the elastic one had to impart a 2P momentum. Hence, motion in the direction of the black side of the vanes, QED.

I existed at the time in a perpetual state of indignation, which was not helped by the intrusion of classical mechanics into my world of hamiltonian operators and eigenfunctions (any misuse of terms, by the way, I blame on my teachers).

Now, however, Daedalus provides a mechanism of action for the Crookes radiometer that deviates from the given wisdom of that fateful day, nigh on two decades ago. Have I been wronged? Should I bring this case before the university's ethics committee? I am, of course, willing to dispense with any considerations of academic collegiality or respect for a talented researcher/teacher. I'm a journalist now, I need neither.

I await a reply with high expectations of twisting that A out of said professor. Better late than never.