Charming Interview: Grand Old Men of c.20th Physics

[exchemist]

I found this 1982 interview between Friedrich Hund (of Hund's Rules) and Paul Dirac (of the Dirac Equation, bra-ket notation etc) on YouTube. At the time Dirac was 80 - he died 2 years later - and Hund, who looks in better shape, was 86 - he lived to 101.

This may not mean a lot to many readers, but to those of us that learned physics or chemistry, these are two well-known figures. Especially Dirac, who got a Nobel Prize for incorporating special relativity into QM, thereby accounting for spin of the particles like the electron, and for predicting antiparticles like the positron. You can occasionally hear Dirac's Bristolian accent coming through slightly. Like most mathematicians I have known, he chooses his words with care and speaks with great clarity. (So does Hund, to a large extent - but then he has the advantage of being German and so has spent a lifetime working out what he is going to say before he opens his mouth, since the verb comes at the end.)

Dirac was Lucasian Professor of Mathematics at Cambridge, a chair founded by Charles II and occupied by Isaac Newton and Stephen Hawking, among others. There was a joke at Cambridge that there was a unit called the dirac, which meant speaking at a rate of one word per hour.

It's interesting that Dirac was still working, apparently being interested in the idea that the relative strengths of gravitation and the other interactions might not be constant. He evidently held a low opinion of the "renormalisation" necessary in QFT. People seem to have got over this subsequently. (Don't ask me about that. It's probably one for @Polymath257 or possibly @ratiocinator.)

Anyway, a little physics curiosity.

 

[exchemist]

I've found another video, this time a long one (over an hour) of Dirac giving a lecture in Rome, in 1975, on his work that led to his Nobel Prize.

Although long I found this really interesting, as being a chemist I never learnt the details of how Dirac succeeded in incorporating special relativity into QM and why it was that this led naturally to accounting for spin and the expectation that there should be antiparticles. So here is my synopsis.

He explains all this in most interesting way. Evidently he approached the topic as mathematician, dissatisfied by the way that Schrödinger and Heinsenberg's formalisms did not treat treat space and time the same way, as required by relativity. It was this lack of symmetry that disturbed him. There was also the problem of the ugly square root term expressing energy in the relativistic relation E = √(m²c⁴+p²c²) which made it very hard to manipulate. So he set about "playing" as he puts it, with recasting the equations in a form that might lead to symmetrical treatment. In the end he managed, by dint of using matrices to express the variables. But this then raised the question of what the quantities in the rows and columns might represent, physically. He realised that part of the answer could be the spin property of the electron, which was known but hitherto had been assumed to be something separate from the QM treatment. So, without setting out to do so, he had found that spin is actually intrinsic to the relativity-compatible formulation of QM he had derived!

The other mystery quantities seemed to imply -ve energies, which he could not make sense of at first. But then he considered the idea, from chemistry, of the "hole" in the shell of electrons which accounts for the reactivity of the halogens. The hole can be thought of as a +vecharge embedded among the -ve electrons, reducing their number by one. So he conceived the idea of the vacuum being not emptybut full of electrons with -ve energies which we can never see, and that by pulling one out to a +ve energy, where we can see it, we would leave behind a "hole" which would behave like an electron but with +ve charge.

However nobody in that era dreamt of proposing new particles. But he was in contact with some experimenters who had seen tracks in cloud chambers during radioactive decay, which looked like electrons but bent the wrong way in a magnetic field. So then he had the courage to say he thought these could be +ve charged versions of the electron, as predicted by his equations - the positron.

What was interesting to me was his entirely mathematical, rather than physical, approach. He made a mathematical structure that did what he felt satisfied him - and then starting wondering what the new quantities in his equations could possibly mean!

At the end of the video, he has a crack at the problem of infinities in "normalisation" of QFT. This seems to have been something he felt very strongly showed something is wrong with QFT, - just as, half a centruy before, he had had the conviction something was wrong with Schrödinger and Heisenberg's formulations of QM.