This week, UCLA scientists showed that unspooling scotch tape can emit X-rays. Most puzzling to the scientists was that the X-rays are emitted in discrete quanta. Why, they ask?

At least superficially, the phenomenon seems reminiscent of lightning bolts, another emission of electromagnetic spectrum radiation that happens in bursts. If the accumulation of charge was enough for zillions of electrons to travel the length of the lightning bolt, and thus the potential difference was clearly larger than the threshold for the electrons to travel the length of the lightning bolt even before the last charge difference accumulated, why did the lightning bolt not start earlier (and consist of one less electron)? The answer must be that the potential difference threshold to start a flow of electrons must be higher than the threshold to continue one. An atomic-scale equivalent of the fact that dynamic friction coefficients are lower than static friction coefficients. Why?

In order to start a lightning bolt, an electron needs to travel from source to sink; the probability amplitude for such an event is rather small, given the few paths that add up to that long and unusual movement. But to *continue* a lightning bolt, each electron needs only move to the spot previously occupied by the adjacent one. Which would itself make it more likely for the next electron to move over, and so on, so these are not uncorrelated events. Which makes the probability of a zillion electrons each moving a bit immensely higher than the probability of one electron making the whole trip on its own.

It seems the same explanation ought to apply to the Scotch-tape induced X-rays.

Alex Backer, 10/28/2008