Measurement: A Very Short Introduction by David J. Hand

Author:David J. Hand
Language: eng
Format: epub
ISBN: 9780191085024
Publisher: OUP Oxford
Published: 2016-08-05T04:00:00+00:00

Quantum measurement

This chapter would be incomplete without a mention of the role of measurement in quantum theory. This theory describes the behaviour of the universe at very small scales. These are scales completely beyond our everyday experience, so it should perhaps not be surprising that behaviour at these scales often runs counter to our intuitions. After all, if the air has the consistency of treacle to a small flying insect, how much more strange should we expect things to seem at the scale of subatomic particles?

Quantum mechanics tells us that certain pairs of properties cannot simultaneously be measured with arbitrary accuracy. This is not because the measurement process necessarily disturbs the object being measured (even though this is also true—at a minimum, to measure something we must bounce a photon off it, and this will have an effect). Rather it is due to the fundamental nature of the universe. An example is given by the position and momentum of a small particle. We can measure the position as accurately as we like, but the relationship between the two means that the accuracy is gained in terms of a cost of inaccuracy in knowing the momentum. And vice-versa. Furthermore, it turns out that it is not meaningful to speak of the true value of an attribute prior to measuring it. Rather, the very action of measurement ‘collapses’ the attribute to a particular value. This is clearly very different from classical physics, where the properties of objects have values, simply waiting to be measured. While the world of quantum mechanics is a truly strange one from the perspective of classical physics, countless experiments, and indeed machines built using the theory, show that it is essentially correct. It may be strange, but when tested in experiments its predictions are right.

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