From Bern to Solvay

I spent last week at the International Space Science Institute (ISSI), Bern, Switzerland. I was there with a team of scientists aiming to understand better the role of energetic ions in solar flares.

Our team went really well, excellent discussions, progress on our key questions, but by Friday afternoon everybody else had left. I worked for a little bit on the team website before skiving off to become a tourist for a few hours. (Complete aside: let me mention how surprised I was by the etymology of skive. As one of those few thousand words we all had in common in school, and a very important one, I always assumed it was a Scots word; not according to the Oxford English Dictionary).

Albert Einstein lived in Bern for a few years at the very start of the 20th century, famously working as a patent clerk at the same time as puzzling over questions like, "how would the world look to somebody travelling with a beam of light?" A scientific paper he wrote in 1905 earned him the Nobel Prize in Physics and it probably isn't even the most important paper he published that year. The time he spent in Bern represents an extremely important episode in the history of science and it's no surprise that the apartment he lived in is now a tiny museum, the Einsteinhaus.

On Friday afternoon I visited the Einsteinhaus. There are nice displays on his family life, his career, his fame. Among them my eye was caught by a mounted print of the photo at left, showing attendees at the 1927 Solvay conference. It's a very famous photo, showing many of the biggest names in 20th century physics all together at the same meeting. Like most physicists I've seen it often, in various places, and noted for example CTR Wilson, our great Scottish genius of experimental physics, second from right at the front.

In the Einsteinhaus I took some minutes to look again at this picture. It's amusing to see the faces that go with those famous names. Who looks confident, bold, who shy and diffident? At the far left-hand side I was slightly surprised to spot two gentlemen almost side by side whose names are often met together in the textbooks, Messrs Peter Debye and Irving Langmuir. Langmuir is right at the left end of the front row, with a rather natty cane. The moustachioed Debye is peering over Langmuir's right shoulder.

In very hot gases, like those met in the outer layers of the Sun, collisions between atoms are sufficiently violent that they completely remove electrons; ionise the atoms. The constituents of the gas are electrically charged. We call the gas a plasma - a term first used by Langmuir, apparently. Plasma behaves in all sorts of new and interesting ways compared to the ordinary, neutral gas we're surrounded by here at sea level on Earth.

The Debye length is the greatest distance over which an electric field can persist in a plasma - because its constituents are electrically charged, if we try to impose an electric field the charges will just move to short it out. Langmuir waves are something like sound waves but with electric fields playing a central role. Their natural frequency, the plasma frequency, is one of the basic numbers that describe a plasma. The Debye length is another. In the first few pages of any book on plasma physics we meet the names of Langmuir and Debye so it tickled me to find the two of them side by side in the 1927 Solvay Conference picture.

Both were Nobel Prize winners, for Chemistry rather than Physics. Langmuir later became rather notorious for attempts at rain-making. The great German scientist Arnold Sommerfeld was once asked what was his greatest discovery. "Debye," he replied.