The Grand Final Interpretation of quantum mechanics

 The 2007 Rugby World Cup was won by South Africa. Who won in 2003? England. What about in 2005? There was no tournament in that year so nobody won. It is a stupid question. 

And yet this is very like a question in fundamental physics that has bothered people for over 80 years. One school sticks to the line that it is a stupid question, and there is nothing more to say. The other school finds this deeply unsatisfying, and asks: “But if there had been a tournament in 2005, who would have won it?”

The first school says: “We can make a good guess at the different teams’ chances, but we can never say for certain who would have won a tournament that did not happen.” This does not pacify the second school, which says (more or less): “That is not good enough. Your World Cup reality, with definite results only when tournaments are held, cannot be the whole story. There must be an underlying reality, with clear results for the times when tournaments were not held; it is just that we do not know what they are.” 

Before descending into quantum physics, there is a crucial idea that was not available to the quantum pioneers in the 1920s, which could possibly have prevented the question from arising in the first place. It is simply that information comes in discrete chunks – there is no such thing as truly continuous information. In the last decade or so most of us have become accustomed to the idea that documents, music, and photographs, as well as things we think of as “data”, fill up a number of the available bits in our computer memories, digital cameras and music players. Nobody would now expect a camera to have unlimited picture resolution, not because current technology is not good enough, but because a picture with unlimited resolution would need an infinite number of bits, and this will never be possible, whatever the technology. In the same way, to extrapolate slightly from the World Cup, not only must there be a finite number of sports tournaments and winners, but when we ask any question about anything, the answer can only be a finite number of bits of information.  

The problem in physics has arisen when describing things like the location of a particle. All our instincts tell us that a moving particle has a definite location at every possible instant. In practice, of course, we can only find out about its location a finite number of times, with the equivalent of video footage. Normally we can mentally fill in the gaps between the video frames with an arbitrary number of intermediate frames and there is no problem. It is like filling in the gaps in a slow-motion television replay of a rugby match.

However, in some elementary physics experiments, like sending particles through slits, there is no way to fill in the gaps that makes sense. We have to say that, between frames, a particle must have been in two (or more) places at once. It is like watching a special effect in a movie achieved by digital trickery, except that we are in full control of the camera.

With hindsight, the explanation is not that the physical world must be weirder than the world of our experience; it is that our instincts about particles always having a definite location are wrong. We feel we ought to be able to fill in the gaps – to make the information continuous – when we now know that information can only take the form of discrete chunks.  

What happens in practice is that however ingeniously we set things up to catch weird particle behaviour on camera, we never do – any weirdness always happens between the frames. Indeed, whenever particles are caught on camera, we find that we can predict accurately what we will see. In some circumstances we can only predict the probabilities of various results, but we can then predict the probabilities accurately, and this seems to be as much as it is possible to do. 

Is that good enough? Any physical theory has to be checked by comparing its predictions with information obtained from the “real” world around us. If the job of physics is to describe the world around us, and it can match and predict any information that we can possibly receive from the world, surely it has done its job.

Though it is usually expressed in different ways, historically this view has been a central part of what is known as the Copenhagen Interpretation of quantum mechanics, named after the home town of the institute for theoretical physics founded by the great quantum pioneer Niels Bohr.  

Many distinguished physicists have been unhappy with this – the most famous advocate of Anything-But-Copenhagen interpretations being Albert Einstein. Their line is essentially that there must be more to the “real” world than just the information available about it. If we instead accept that we can never know more about the “real” world than information, indeed that reality is that information, the objection disappears.  

It is unfortunate that the Copenhagen view is usually described as including a central role for “observation” or “measurement”, implying that there are conscious or technical actions that extract information from some continuous underlying sub-reality. Certainly the information needs to be generated in some way, bit by bit. Quite how the information is generated – how future possibilities are turned into results – is another story, but perhaps it is intrinsically no weirder than how England won the World Cup in 2003.

And as we discover who wins the next major sporting event, perhaps we can curb our joy or despair at the final whistle by contemplating the deep link between how sporting results are decided and the mysteries of quantum physics.

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