## Who needled Buffon?

I already mentioned Buffon’s needle a long time ago, with a picture of the page of my grandfather’s own copy of the multi-volume Histoire naturelle. Recently, I had a closer look at what Buffon actually wrote, and realized that, far from dropping unit-lengths needles to compute π, he was suggesting dropping well-calibrated needles to compute 1/2, or more precisely to create a fair game between two players, one of which bets on whether a needle dropped by the other will cross the boundaries of the strips of wood on the floor?

In fact, since he uses the “value” π=22/7 just a few pages before,

one can see that computing π must have been very far from his mind. The question thus arises: who did first interpret Buffon’s question as one that may lead to an estimate for π?

Equally recently, I noticed the following variant of the problem, which may already be known: suppose a player is allowed to throw a dart randomly anywhere on the complex upper half-plane (this may seem too “infinite”, but the disc model of the hyperbolic plane will work just as well if you want to throw darts to a circular target…), but that the dart is instantaneously moved, as soon as it touches the target, to the corresponding point of the fundamental domain of $SL_2(\mathbf{Z})$

(in principle, this is an operation that can be performed very efficiently by some variant of the euclidean algorithm, though in practice there are numerical stability problems for points very close to the boundary of the fundamental domain). Now the bet with the other player is whether the point in the fundamental domain will have imaginary part larger than some fixed a>0.

As Buffon, we ask: which value of a will ensure that this is a fair game? If we assume that the point in the fundamental domain is uniformly distributed for its probability Poincaré measure, which is $3\pi^{-1} y^{-2}dxdy$, it is easy to see that the answer is $a=6/\pi$, so we have some kind of hyperbolic Buffon needle. To make this into a “computing π procedure”, one may for instance “throw darts” successively at the points

$x_j=\frac{j+i}{N}$

on the segment of horocycle at height $1/N$, for some large $N\geq 1$, and look at the corresponding points $y_j$ in the fundamental domain. Then it is is a well-known result (which follows here most easily from non-trivial bounds on Hecke eigenvalues of Maass cusp forms) that as $N$ grows, the points $y_j$ become equidistributed in the fundamental domain for the Poincaré measure, and hence that the proportion among them which have imaginary part larger than $a$ converges to $1/a$. Hence if we look at the median of this sequence of imaginary parts, we will find that it converges to $6/\pi$ as $N$ tends to infinity…

## Neils Bohr

What does it say about the state of science publishing when the paperback edition of a popular science book, appearing two years after the hardback edition with glowing blurbs, published by Oxford University Press, written by an actual physicist who is also an OBE, etc, etc, speaks of “Neils Bohr” at least three times in 70 pages?

## Journals

Two cents on the current journal/Elsevier controversy: this recent article in the ETH online magazine indicates that commercial publishers are suing ETH for providing a scanning service, where researchers in Switzerland (members of one of the libraries belonging to the Nebis consortium) are able to ask that the ETH library scan and send them by email any article available in the library (sometimes for a fee; this service is highly convenient to access articles not available online because the stacks of the main library at ETH are not accessible to its users.)

Note that Springer and Elsevier are both explicitly mentioned as two of the plaintiffs in that case.

## With many thanks to Dickinson State College

I said in the last post that I didn’t know anything about Arthur Schuster before reading his quote on hearing the shape of a bell. Actually that was not quite true: he is mentioned three times in the book of Max Jammer on the history of Quantum Mechanics, and one reference leads to the source of the quote. This was a report for the 1882 meeting of the British Association for the Advancement of Science, held in Southampton in August 1882. The full report of the meeting can be read online, and Schuster’s paper starts on page 120, the baffled skillful mathematician appearing at the end of this first page (I’ve also prepared a PDF of these two pages).

Now I have to thank whoever decided to withdraw Max Jammer’s book from the library of Dickinson State University (née Dickinson State College), which is where the copy I recently got from BetterWorldBooks came from…

The holder for the library slip

is still in the book, and it was apparently only borrowed twice, once in 1978 and once in 1982 (or maybe 1992).