Skip to main content

Two weird quantum concepts

Quantum physics is famous for its strangeness. As the great Richard Feynman once said about the part of quantum theory that deals with the interactions of light and matter particles, quantum electrodynamics:
I’m going to describe to you how Nature is – and if you don’t like it, that’s going to get in the way of your understanding it… The theory of quantum electrodynamics describes Nature as absurd from the point of view of common sense. And it agrees fully with experiment. So I hope you can accept Nature as she is – absurd.
It's interesting to compare two of the strangest concepts to be associated with quantum physics - Dirac's negative energy sea and the 'many worlds' interpretation. Each strains our acceptance, but both have had their ardent supporters.

Dirac's 'sea' emerges from his equation which describes the behaviour of the electron as a quantum particle that is subject to relativistic effects. The English physicist Paul Dirac discovered that his equation, which fits experimental observation beautifully, could not hold without one really weird implication. We are used to electrons occupying different quantised energy levels. This is bread and butter quantum theory. But all those levels are positive. Dirac's equation required there also to be a matching set of negative energy levels.

This caused confusion, doubt and in some cases rage. Such levels had never been observed. And if they were there, you would expect electrons to plunge down into them, emitting radiation as they went. Nothing would be stable. As a mind-boggling patch, Dirac suggested that while these levels existed, they were already full of electrons. So every electron we observe would be supported by an infinite tower of electrons, all combining to fill space with his 'Dirac sea'.

As you might expect, a good number of physicists were not impressed by this concept. But Dirac stuck with it and examined the implications. Sometimes you would expect that an electron in the sea would absorb energy and jump to a higher, positive level - leaving behind a hole in the negative energy sea. Dirac reasoned that such an absence of a negatively charged, negative energy electron would be the same as the presence of a positively charged, positive energy anti-electron. If his sea existed, there should be some anti-electrons out there, which would be able to combine with a conventional electron - as the electron filled the hole - giving off a zap of energy as photons.

It took quite a while, but in the early cloud chambers that were used to study cosmic rays it was discovered that a particle sometimes formed that seemed identical to an electron, except for having a positive charge - the positron, or anti-electron.

Weird though it was, Dirac's concept was able to predict a detectable outcome and moved forward our understanding of physics. As it happens, with time it proved possible to formulate quantum field theory in such a way that the positron was a true particle and the need for the sea was removed, although it remains as an alternative way of thinking about electrons that has proved useful in solid state electronics.

The 'many worlds' hypothesis originated in the late 1950s from the American physicist Hugh Everett. Its aim is to avoid the difficulty we have of the difference between the probabilistic quantum world and the 'real' things we see around us, which seem not to have the same flighty behaviour. Everett didn't like the then dominant 'Copenhagen interpretation' (variants of which are still relatively common) which said that a quantum particle would cease behaving in a weird quantum fashion and 'collapse' to having a particular value when it was 'observed'. This concept gave a lot of physicists problems, especially when it was assumed that this 'observation' had to be by a conscious being, rather than simply an interaction with other particles.

Like the Dirac sea, 'many worlds' patches up a problem with a drastic-sounding solution. In 'many worlds', the system being observed and the observer are considered as a whole. After an event that the Copenhagen interpretation would regard as a collapse, 'many worlds' effectively has a universe that combines both possible states, each with its own version of the observer. So, in effect, the process means that the universe doubles in complexity each time such a quantum event occurs, becoming a massively complex tree of possibilities.

Some physicists like the lack of a need for anything like the odd 'collapse' and the distinction between  small scale and large - others find the whole thing baroque in its complexity. What would help is if 'many worlds' could come up with its equivalent of antimatter - a prediction of something that emerges from it but not from other interpretations that can be measured and detected. As yet this is to happen. Whether or not you accept 'many worlds', it is certainly a remarkable example of the kind of thinking needed to get your head around quantum physics.

Find out more about Dirac, his equation and the quantum sea in The Strangest Man by Graham Farmelo.

Find out more about the latest thinking on 'many worlds' in Our Mathematical Universe by Max Tegmark.

Comments

Popular posts from this blog

The Art of Logic - Eugenia Cheng ***

This is an important book, though I'm not sure Eugenia Cheng would agree with my logic in saying so. 

Going on the marketing, what we have here is a counter to fake news and dodgy argumentation in the form of mathematical logic. The back cover tells us 'Newspaper headlines and social media use emotions to warp the facts. Politicians and companies master rhetoric to mislead us. What one book could help us make sense of it all?' Admittedly they don't answer their rhetorical question, but I assume the answer is intended to be The Art of Logic. (Did the company behind this book realise it was using rhetoric, though presumably not to mislead us?) 

What we actually have is a combination of a lucid and interesting explanation of the basics of logic with the mathematical equivalent of those books such as Algorithms to Live By that were so popular a couple of years ago. They used the logic of algorithms (differently worded, and, to me, easier to understand), the heart of computer…

Quantum Economics - David Orrell ****

David Orrell's earlier title Economyths is one of my favourite popular science books of all time. Or, perhaps, I should say popular non-science, as Orrell shows just how devastatingly traditional economics uses the tools of science without having a scientific basis. I was, therefore, really looking forward to reading Orrell's new book - until I saw the title. As anyone involved with physics can tell you, there's nothing more irritating than the business of sticking the word 'quantum' onto something to give a pseudo-scientific boost to waffle and woo. Was Orrell doing the same thing? Thankfully, his introduction put my fears aside.

Orrell, a mathematician with a physics background quickly makes it clear that the way he is using quantum theory is not just employing magic words, but involves making use of strong parallels between the nature of quantum objects and concepts like money (more on money in a moment). Yes, this is to some extent a metaphorical use of quantum …

The Ashtray - Errol Morris *****

Wow. When someone suggested I read a book called The Ashtray, written by a documentary film-maker, it didn't strike me that it would be a book that gave deep insights into the history and philosophy of science - while also being a remarkable reading experience. In fact, I almost didn't bother with it, but I'm glad that I did.

The titular ashtray was thrown at the author when he was a grad student - thrown by one of the two best known names in the philosophy of science, Thomas Kuhn, he of The Structure of Scientific Revolutions and the concept of paradigm shifts. Kuhn didn't like the young Errol Morris daring to challenge his ideas and reacted with what some would regard as a less than philosophical reply by hurling a heavy glass ashtray at him.

Part of the reason that reading The Ashtray is a remarkable experience is because it's a book that feels in some ways like watching a documentary. I have to confess I've never seen any of Morris's work, but he uses vis…