## Monday, February 04, 2013 ... /////

### Hendrik Antoon Lorentz: an anniversary

Hendrik Antoon Lorentz died 85 years ago, on February 4th, 1928. He was born in Summer 1853, almost 160 years ago, to a wealthy nurseryman who re-married when Hendrik's mother died. Educated as a protestant, he was a natural freethinker and his career was fast as he was already the boss of physics in Leiden at age of 24 (Johan van der Waals was offered the job but picked Amsterdam).

He has tried to make contributions to hydrodynamics, victims of the war (these efforts led to no genuinely helpful outcomes), and scientific bureaucracy but we of course remember him primarily because of his improvements of the electromagnetic theory, his help to the eradication of the luminiferous aether by showing how to derive Maxwell's equations in materials from the vacuum ones, the Lorentz force acting on a charged particle, and early formulae for the Lorentz contraction and Lorentz transformations that would become essential in Einstein's special theory of relativity a few years later.

He was also one of a few enthusiastic supporters of Einstein when he was looking for the general theory of relativity.

During my high school years, I was somewhat obsessed with Einstein's memoirs that I found in the bookshelves – probably a book that used to belong to my grandfather. Among other things, I learned what Einstein would say about Planck, Lorentz, Poincaré, and other top physicists of his era (and the previous eras).

Lorentz (maybe even more so than Poincaré) was very important in creating the "intellectual atmosphere" that made it easier for Einstein to discover special relativity. He still couldn't find relativity himself – although Lorentz was clearly one of the first men after Einstein who understood it.

According to Einstein, the key insight due to Lorentz that directed Einstein's thinking was the realization that there is fundamentally just 1 electric field strength vector $\vec E$ and one magnetic field strength vector $\vec B$ at each point – and all the other vectors are "derived". Yes, Lorentz was the guy who showed that the extra vectors $\vec D,\vec H$ in various materials result from effects such as polarization etc.

I believe Lorentz was the first guy who wrote the vacuum Maxwell's equation (using just $\vec E,\vec B$) with the bonus claim that they're valid everywhere – even in other materials. These equations are so simple and crisp that they really make it hard to imagine that the electromagnetic phenomena result from some messy phenomena in a complicated environment, the luminiferous aether, that was believed to exist by many physicists (including giants – and, well, including Lorentz himself) in the 19th century.

But the simplicity of the vacuum Maxwell's equations was an irresistible playground for Einstein who would decide to settle the question what happens with all these fields from the viewpoint of other reference frames.

Lorentz would also deduce that the Lorentz force$\vec F = q(\vec E+\vec v\times \vec B)$ is acting upon every charged particle.

Unlike Einstein, Lorentz did follow experiments such as the Michelson-Morley experiment and he tried to construct explanations why the aether wind wasn't observed. (The only reason to think that Einstein might have known about the experiment at all is that he referred to a paper by Lorentz that had previously discussed the experiments.) He got very close to special relativity when he decided that objects should be squeezed by his factor$L = L_0 \cdot \sqrt{1-\frac{v^2}{c^2}}.$ Aside from the Lorentz contraction, the other Lorentz's claims about what happens to times and distances at speed $v$ were wrong, however. On the other hand, Lorentz had also found the Lorentz transformations as a "change of spacetime variables" that left Maxwell's equations intact. At that time, the teaching of group theory had to be really bad – symmetries weren't a recognized ruler of the laws of physics – because Lorentz couldn't realize that these (or any) coordinate redefinitions formed a group. That's why he was also unable to make the great insight – Einstein's later insight – that these coordinate redefinitions are physically equivalent to the change of the inertial frame (and they should supersede the Galilean transformation).

All these insights seem so obvious today that it's hard to imagine how someone could have ever failed to make the "final steps" that were needed to find special relativity. But it's only easy to say "it's trivial" with the hindsight. I am pretty sure that the future generations will also say that we will have been (is that a legitimate tense?) very stupid because all of us are unable to understand certain things – unfortunately, I can't tell you what these things are.

In 1910, Lorentz reorganized his life, dropped all the bureaucratic university jobs, and became a curator in a science museum. He would follow physics without any change, however, and his Monday morning lectures were legendary. His daughter Geertruida de Haas-Lorentz became a physicist herself. She would do some work on electrons in Brownian motion - the "first woman in electrical noise theory".

#### snail feedback (7) :

H A Lorentz (dutch) is mistakenly credited for some findings which were actually due to the (danish) physicist L V Lorentz a few decades earlier, see http://arxiv.org/abs/hep-ph/0012061 for details.

But no doubt that H A Lorentz was a great physicist

Right except that the Dane's surname is spelled Lorenz. ;-)

http://motls.blogspot.cz/2006/02/physics-as-danish-enterprise.html?m=1

I corrected the spelling about 1 second after posting - but maybe Lorentz transformations have large delays on the internet ;-)

future generations will also say that we "were" very stupid because . . .