Friday, 20 February 2015

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.

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