A Crookes Radiometer is a scientific device (often looking a bit like a lightbulb) with a number of mobile vanes, colored black on one side, and reflective on the other. Inside the glass is a partial vacuum, that enables the effect where light appears to 'push' the vanes around. Interestingly for the Victorians, the vanes move in the opposite direction from the way you'd expect if light had mass.

During my PhD I became interested in Crookes Radiometers, specifically I wondered if you could build something that did the same thing with Neutrons - effectively creating a potential route to micro or nano neutron flux powered systems.

The story is quite fascinating. It turns out that the Victorians were really interested in these devices (now mainly known as "that thing that hangs in your Granny's window"). With giant names in science like Maxwell and Reynolds slugging it out over the pages of academic journals - both had theories that lined up with parts of the observations, but neither quite had the whole picture. As is often the way with the big ego's of science, it got pretty personal! Maxwell wrote a fairly rude review of Reynolds work and in the end, the final repost by Reynolds was never published out of respect for Maxwell after his passing!

Then came Einstein, and in his typical way he simply solved the whole concept in one fell swoop! His paper is a tour-de-force, it can be understood by someone with A-level(high-school) maths (math) and yet explains a previously unknown phenomena (which would later be known as the Knudsen pump) Everyone breathed a sigh of relief and science moved on. The process is not covered by the Navier-Stokes equations and it has tended to be ignored in subsequent thermodynamics literature which seems to like the pretty pictures generated by Navier-Stokes! Leaving these little devices as nothing more than a curio.

As a good PhD student (or one who'd drunk too much coffee) I tried to always trace my sources. Einstein's original paper was published in 1924 in "Zeitshrift für Physik BDXXVII", its easy enough to find papers that reference it. It turned out to be alot harder to actually find the paper! Einstein's estate were notoriously careful to protect it, and so I eventually found a paper copy in Trinity college library! I was pretty excited and made a copy of the original German [zip]!



In order to understand the work, I made a translation of the Einstein paper into English - As far as I can tell, this paper was never translated before! This was in 2019, but on asking at my university library I was advised that the translations were considered derivative works and I couldn't publish my translation without the permission of his estate... until 2026 (70 years after the death of the great fellow) when his words would be in the public domain. So here we are, I am in the clear!

I am not a great physicist, but I do enjoy Einstein, his explanation of Special Relativity is so simple, yet so far reaching. General Relativity is also somewhat accessible (although I haven't read the hard maths yet!). This paper is also a great example of science in action - in it basic tools, nothing more difficult than trigonometry and vectors are used to explain a previously unexplained phenomena. It is truly an honour to read this paper and an inspiration that he can reach forward in time over a hundred years.

There you have it, a previously unpublished Einstein translation ... in my next post, I will use the principle of the Crookes Radiometer to generate a novel method of harvesting neutron energy - watch this space! :D