The Restless universe | |||||

Introduction to The restless Universe 1 The lawful Universe 4 The intangible Universe An introuduction to The uncertain Universe 1/2 An introuduction to The uncertain Universe 2/2 5.1 Quantum mechanics and chance 1/3 5.1 Quantum mechanics and chance 2/3
5.2 Quantum fields and unification 1/3 5.2 Quantum fields and unification 2/3 5.2 Quantum fields and unification 3/3 -------------------- Other titles in the Physical World series | 5 The uncertain Universe5.1 Quantum mechanics and chance Part 1 of 3 | Part 2 | Part 3 For a printable version of 'The uncertain Universe' click here
The last of these points represents a substantial shift from classical determinism. In classical mechanics the past uniquely determines the present and hence the future. In quantum mechanics this is not so. Even the most complete possible knowledge of the past would only permit the calculation of the probability of future events. Some, perhaps a little naively, saw in this a scientific basis for free will: there was an element of freedom, or at least of chance, in the Universe. The Copenhagen interpretation calls simple realism into question. If the most that you can say about a position measurement you are about to perform is that various values may be obtained, with various probabilities, then it may well mean that the object has no position until it is measured. Note that this is quite different from saying that the object has a position which you don't happen to know - it is as if the object had not made up its mind where to appear until the position measurement has been made. Clearly if you say that the object has An alternative stance is to assume that there is a real world out there, but to admit that it cannot be adequately described in terms of classical concepts such as position or velocity. This is plausible. We have no right to expect microscopic physics to be just a scaled-down version of everyday experience. Given that quantum mechanics deals with a microscopic world well beyond the immediate reach of our senses and intuitions, perhaps the most surprising thing is that we can make predictions at all. From this perspective, the price that must be paid for the mismatch between our classical concepts and the quantum world is astonishingly small, and is reflected mainly in the appearance of probabilities. In philosophical terms, the concept of a real world can be preserved by admitting that certain aspects of it are inaccessible to us, clumsy giants that we are. But in practical, or scientific, terms this makes no difference. It is hard to see how we could ever develop an understanding that was not based on classical concepts, so probabilities seem destined to remain intrinsic and unavoidable, offering the only gateway through which we can glimpse the microscopic world.
| Relevant Links A note on powers of ten and significant figures Suggestions for further reading | |||

S207 The Physical World | |||||