Saturday, January 8, 2011

Schrődinger’s Cat

 Taking a break from reading philosophy over my summer holiday, I thought, as you do, that a little quantum physics might be relaxing. Have you heard of Schrődinger’s Cat? This is the name of both a book by John Gribbin, written in the early 1980s, and a thought experiment proposed by Erwin Schrődinger, an Austrian scientist, in the 1930s. This was a time when things were hotting up in the world of quantum physics, and the thought experiment was meant by Schrődinger to show the absurdity of what all the current mathematics and thinking were showing: but did it?

In the thought experiment there is a closed box which contains a live cat and phial of poison. Now, the mathematics showed that a radio-active atom might decay or it might not; there is a precise fifty-fifty chance that one of the atoms in a lump of radioactive material will decay in a certain time. In the cat’s box, there is a trigger which will break the phial of poison and kill the cat if and when the radioactive decay occurs. In the everyday world we would be content to say that the cat is either dead or it is alive. But in the theoretical world of quantum physics, the radioactive decay does not either occur or not occur unless it is observed. Thus, the cat is neither dead nor alive until we open the box to check. Before that, it is in a kind of limbo, an indeterminate state.

'Pandora's cat' - alive last time I checked
You can see that this was – and still is – a rather shocking conclusion. Einstein baulked at it, and spent much of his later career trying (unsuccessfully) to prove the science wrong. We non-scientists might find it either absurd, or exciting in a science-fiction kind of way. Let’s think about this: nothing is real unless it is observed (you might suspect that we are straying into philosophical realms again, and you might be right...).  In the world of the atom, how do scientists study what goes on? Everything is ridiculously small - Gribbin gives this example:  imagine the head of a pin in the centre of St Paul’s Cathedral. Floating up in the extremities of the roof are some dust motes. The pin head is like the nucleus of an atom, and the dust motes are its electrons. That’s how much empty space is in the atoms which make up everything in the world. To observe the behaviour of electrons, we need to shine light on them, which means bouncing photons of light off them. A photon won’t much disturb a big thing like a house, but it (or electromagnetic energy with a short wave length) will have a huge effect on a small thing like an electron. So the very act of observing the electron or atom will knock it about and change its nature.

The quantum physicists developed a conclusion they called ‘the uncertainly principle’, which says that there is no such thing as an electron which possesses both a precise momentum and a precise position. We can measure one or the other but not both. We cannot know the present in all its details (said Heisenberg).  As Gribbin says:

To Newton, it would be possible to predict the entire course of the future if we knew the position and momentum of every particle in the universe; to the modern physicist, the idea of such perfect prediction is meaningless because we cannot know the position and momentum of even one particle precisely. (p. 157)
In quantum physics the observer interacts with the system [being observed] to such an extent that the system cannot be thought of as having independent existence...we, the observers, are in a very real sense part of the experiment – there is no clockwork that ticks away regardless of whether we look at it or not...all we know about are the results of experiments...It is interesting that there are limits to our knowledge of what an electron is doing when we are looking at it, but it is absolutely mind-blowing to discover that we have no idea at all what it is doing when we are not looking at it. (p. 160-161)

And this is before I even get to Einstein’s Time and the news that since a photon travels at the speed of light, for a photon time has no meaning. To the photon the Big Bang and now are the same; everything in the universe is connected to everything else by a web of electromagnetic radiation...and, by the way, it is possible to imagine experiments where particles move backwards and forwards in time. Then there are all the ‘virtual’ particles that come into existence from nothing and go out of existence, and appear to ‘know’ that we are watching them.

So if you’re following this so far (and haven’t dismissed it all as a crazy thought experiment that is merely a game), consider that “by observing the photons of the cosmic background radiation which is the echo of the Big Bang, we may be creating the Big Bang and the universe”.  (p.212)

And then the intriguing possibility – considered by respectable physicists, remember, not (or not only) sci-fi fiction writers – that the theories suggest that there are many worlds existing parallel to our everyday world, possibly an infinite number of them; “existing in some way sideways across time from our reality, parallel to our own universe but forever cut off from it.” (p.235) – Gribbin calls this the ‘Many Worlds’ theory, a term not uncommon in philosophy and one long pondered (starting with Plato) in various forms, well before the quantum physicists started to find physical experiments which suggest it. Just by the way.

In the 'Many Worlds' theory (also called 'Everett's Interpretation') when the box is opened, the observer and the already-split cat split into an observer looking at a box with a dead cat, and an observer looking at a box with a live cat. But since the dead and alive states are decoherent, there is no effective communication or interaction between them.

Please appreciate that I estimate that I understand perhaps half of Gribbin’s excellently-written book, and that quantum physics can’t be explained in a short review, least of all by me. But the tantalizing ideas are absorbing. Gribbin’s book finishes with the state of play in 1983:

The success of the Aspect team’s experiments...has eliminated all but two of the possible interpretations of quantum mechanics ever put forward. Either we have to accept the ‘Copenhagen interpretation’, with its ghost realities and half-dead cats, or we have to accept the ‘Everett interpretation’ with its many worlds. It is, of course, conceivable that neither of these two ‘best buys’ in the science supermarket is correct, and that both these alternatives are wrong. ..but if you think easy route out of the dilemma, remember that any such ‘new’ interpretation must explain everything that we have learned since Planck’s great leap in the dark, and that it must explain everything as well, or better than, the two current explanations. That is a very tall order indeed...In the absence of a better answer, we have to face up to the implications of the best answer we’ve got. (p.253)

I can’t wait to read Gribbin’s sequel, written about 15 years later: ‘Schrődinger’s Kittens.’

If you want more information, you could start with Wikipedia, and this BBC page gives a brief overview. If you're really impressed with the whole question, you might like to buy this T-shirt.

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