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.

Comments

Popular posts from this blog

Why I hate opera

If I'm honest, the title of this post is an exaggeration to make a point. I don't really hate opera. There are a couple of operas - notably Monteverdi's Incoranazione di Poppea and Purcell's Dido & Aeneas - that I quite like. But what I do find truly sickening is the reverence with which opera is treated, as if it were some particularly great art form. Nowhere was this more obvious than in ITV's recent gut-wrenchingly awful series Pop Star to Opera Star , where the likes of Alan Tichmarsh treated the real opera singers as if they were fragile pieces on Antiques Roadshow, and the music as if it were a gift of the gods. In my opinion - and I know not everyone agrees - opera is: Mediocre music Melodramatic plots Amateurishly hammy acting A forced and unpleasant singing style Ridiculously over-supported by public funds I won't even bother to go into any detail on the plots and the acting - this is just self-evident. But the other aspects need some ex

Is 5x3 the same as 3x5?

The Internet has gone mildly bonkers over a child in America who was marked down in a test because when asked to work out 5x3 by repeated addition he/she used 5+5+5 instead of 3+3+3+3+3. Those who support the teacher say that 5x3 means 'five lots of 3' where the complainants say that 'times' is commutative (reversible) so the distinction is meaningless as 5x3 and 3x5 are indistinguishable. It's certainly true that not all mathematical operations are commutative. I think we are all comfortable that 5-3 is not the same as 3-5.  However. This not true of multiplication (of numbers). And so if there is to be any distinction, it has to be in the use of English to interpret the 'x' sign. Unfortunately, even here there is no logical way of coming up with a definitive answer. I suspect most primary school teachers would expands 'times' as 'lots of' as mentioned above. So we get 5 x 3 as '5 lots of 3'. Unfortunately that only wor

Which idiot came up with percentage-based gradient signs

Rant warning: the contents of this post could sound like something produced by UKIP. I wish to make it clear that I do not in any way support or endorse that political party. In fact it gives me the creeps. Once upon a time, the signs for a steep hill on British roads displayed the gradient in a simple, easy-to-understand form. If the hill went up, say, one yard for every three yards forward it said '1 in 3'. Then some bureaucrat came along and decided that it would be a good idea to state the slope as a percentage. So now the sign for (say) a 1 in 10 slope says 10% (I think). That 'I think' is because the percentage-based slope is so unnatural. There are two ways we conventionally measure slopes. Either on X/Y coordiates (as in 1 in 4) or using degrees - say at a 15° angle. We don't measure them in percentages. It's easy to visualize a 1 in 3 slope, or a 30 degree angle. Much less obvious what a 33.333 recurring percent slope is. And what's a 100% slope