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## The 10 best physicists – no. 10 – Paul Dirac

At number 10 in “The Guardian’s” 10 best physicists is English theoretical physicist Paul Dirac.

## Dirac’s brief biography

Dirac was born in Bristol in the south-west of England in 1902. He died in 1984. He was brought up in Bristol. His father was Swiss-French, his mother was English. He did his undergraduate degree at Bristol University studying engineering. However, he was unable to find work as an engineer, and so instead undertook a second degree, this time in mathematics, at the same institute. He then went to Cambridge to do his PhD, working on General Relativity and Quantum Mechanics, under the supervision of Ralph Fowler. The title of his PhD thesis was simply “Quantum Mechanics”.

The front cover of Paul Dirac’s PhD Thesis, submitted in 1927 to Saint John’s College, Cambridge.

## Dirac’s main achievements

Dirac’s place in this top 10 list is due to two main things, his prediction of the existence of antimatter, and for the equation which describes the motion of a fundamental particle such as an electron when it is travelling near the speed of light. Both of these will be described in more detail in future blogs. Dirac won the Nobel prize for Physics in 1933, he shared it with Erwin Shrödinger “for the discovery of new productive forms of atomic theory”.

### Antimatter

The theoretical prediction for which Dirac is most famous to people outside of physics is his idea of antimatter, which of course has become a firm favourite of science fiction. His basic idea was that every fundamental particle has an anti-particle. So, for example, an electron has an anti-particle which would have the same mass and the opposite electric charge. We call this anti-electron a positron. A proton would have an anti-proton and so on. Anti-matter was predicted by Dirac in 1928 and was experimentally verified in 1932 with the discovery of the positron.

### The Dirac equation

Dirac is most famous amongst physicists for what is now known as “Dirac’s equation”. This is an equation which describes the relativistic behaviour of an electron, and therefore unified quantum mechanics with special relativity. Relativistic means travelling near the speed of light.

The terms in this equation need a little explaining. Rather than explaining them in this blog, I will do so in a series of future blogs, as I will need to give some background. Not only do I need to explain the terms in this equation, but this equation cannot be understood in isolation, one has to also understand Schrödinger’s equation.

For example, the term $\psi(x,t)$ is the so-called “wave-function” of the particle, and $\nabla^{2}$ is the so-called Laplacian. $i \text{ is the imaginary number, that is } \sqrt{-1}$. Now you see why I need to give some background!!

You can read more about Paul Dirac and the other physicists in this “10 best” list in our book 10 Physicists Who Transformed Our Understanding of the Universe. Click here for more details and to read some reviews.

Ten Physicists Who Transformed Our Understanding of Reality is available now. Follow this link to order

### 23 Responses

1. on 19/07/2013 at 05:56 | Reply Charlie T

Why so low!

• Where would you put him?

2. on 19/07/2013 at 08:07 | Reply briancleggauthor

I’d probably put him a little higher. He also showed that matrix mechanics and quantum mechanics were equivalent, effectively unifying the quantum physics of the day. Just one small point on the bio – he did his engineering degree at Bristol, not Cambridge (see, for instance, http://www.nobelprize.org/nobel_prizes/physics/laureates/1933/dirac-bio.html) He was also very likely to have been on the autistic spectrum with many tales of his impressive lack of interpersonal skills. I particularly like the story of his introduction to Richard Feynman where after one of his hallmark uncomfortably long pauses, Dirac is said to have remarked: ‘I have an equation. Do you have one too?’

• Certainly his theoretical contributions were immense. Unifying wave mechanics and matrix mechanics was a phenomenal achievement. He is often said to be England’s greatest theoretical physicist.

I was discussing this list with my son (who has just finished his AS physics course). He hadn’t heard of Paul Dirac, and I suspect most non-physics graduates haven’t.

We then had a discussion which went along the lines of my saying that I thought most people would be able to agree on the “top 4” in the list, but the remaining 6 of the 10 could come from several dozen names.

I will check my facts again over Dirac’s undergraduate studies. I thought I had read he did Engineering at Cambridge then Maths at Bristol when he wasn’t able to find a job as an engineer. But I could well be wrong, I’m wrong about most things.

• There is an allusion to his lack of personal skills in the caption to the image. It quotes Einstein as saying about him “This balancing of the dizzying path between genius and madness is awful”.

I found a link to a Daily Mail article (it was traumatic to follow a link to the Daily Mail but when needs must) about his lack of social skills, and how he was humanised by his wife whom he didn’t marry until he was in his 40s. As you say, he was happy to allow pauses in conversation to stretch into minutes, many minutes.

• You are right about his undergraduate degrees, the two are from Bristol. I’ve edited the blog accordingly. Hang head in shame……

• I remember in a 2nd year tutorial our tutor trying to explain to us the mathematical connection between Schrödinger’s wave mechanics and Heisenber’s matrix mechanics. I didn’t understand it during the hour tutorial, and I never went back to try and understand it as we didn’t need to know the connection. We just needed to know how to solve problems using either approach. I’m tempted to go back and try and understand it now, it’ll be a fun challenge.

3. Reblogged this on readearthanduniverse and commented:
Rankings

4. “He also showed that matrix mechanics and quantum mechanics were equivalent, effectively unifying the quantum physics of the day.”

I’m pretty sure that the equivalence between matrix mechanics and wave mechanics (perhaps that is what you mean) was demonstrated by Schrödinger (who, of course, created wave mechanics).

• No it wasn’t. Dirac showed the equivalence.

• Are you sure? From Wikipedia (not the best source, admittedly): “he formulated the wave equation (stationary and time-dependent Schrödinger equation) and revealed the identity of his development of the formalism and matrix mechanics” and also “A third paper in May showed the equivalence of his approach to that of Heisenberg”. The German version is a bit more detailed 🙂 and provides references: “In dieser berühmten Artikelserie (Annalen der Physik Bd. 79, S. 361, 489, 734, und Bd. 81, S. 109, 1926) bewies er auch gleich die Äquivalenz seiner Formulierung mit der Matrizenmechanik von Heisenberg und Born.”

I’m pretty sure that Schrödinger was the first to do this (Dirac might have done it as well, of course). I’ll have to look into my copy of Moore’s biography of Schrödinger.

• on 22/07/2013 at 07:28 briancleggauthor

I think in a sense they are both right. As far as I can see Schrödinger showed they were equivalent but it was Dirac created the unified equations.

• Certainly in the tutorial that I didn’t follow it was Dirac’s approach that our tutor tried (in vain) to explain to us. As I say, it was too much to understand in an hour’s 2nd year tutorial, and as we didn’t need to know the underlying connection I never went back to it.

• What are the dates on the two pieces of work? Schrödinger’s and Dirac’s?

• on 22/07/2013 at 11:16 briancleggauthor

I think Schrödinger’s paper was 1926 and Dirac’s (according to this: http://www.lucasianchair.org/20/dirac.html) was 1927. So S was first but D actually pulled it all together.

5. Dirac probably had the Asperger syndrome. Experts differ as to whether this should be classified as a type of autism (which includes a wide range of syndromes, with no known cause for any of them). He famously used to introduce his wife as “Wigner’s sister”. Well, she was, but still.

• Yes, I think that is pretty much agreed upon. And physics / maths departments tend to have a higher proportion of people with Asperger syndrome than most fields.

• on 22/07/2013 at 06:53 | Reply briancleggauthor

That’s why I said ‘on the autistic spectrum’ rather than was autistic. Given Simon Baron Cohen classes Aspergers as such I don’t think it’s controversial to do so.

• Go to a theoretical physics conference and you’ll see several people on the autism spectrum, probably with asperger’s. Of course we now think Newton was on the autism spectrum too. He was certainly very odd.

• on 22/07/2013 at 07:04 briancleggauthor

Asperger himself said: ‘autistic individuals, as long as they are intellectually intact, can almost always achieve professional success, usually in highly specialised academic professions, with a preference for abstract content. We found a large number of people whose mathematical ability determines their professions [were on the spectrum]…’

• I don’t think it’s controversial, but the practice is not uniform. It seems to be moving from Asperger syndrome as a separate entity to classifying it as part of the autistic spectrum. Given that there is some overlap with other forms of autism, that the spectrum already includes quite a few different types, and that no-one knows the cause of any of them, this seems to make sense. (There is probably a genetic element; it is a bit more common that a child has autism if a sibling does, at least for some forms (I have 2 children with autism). However, it has not been linked to a specific genetic defect or whatever (and hence there is no DNA test for it) while this has happened for several other genetic diseases.)

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