Quantum mechanics

The following text is an introduction to quantum mechanics for the layperson. See Quantum mechanics/Advanced for a more technical exposition.

Quantum mechanics is a physical theory which explains and predicts the behaviour of matter and energy at very small scales - behaviour which is often unusual, and sometimes extremely counter-intuitive, deeply in conflict with the mental models most people have of how the physical world works. It is perhaps the single biggest building block in the revolution in physics in the 1900-1920 period which overthrew classical physics and created the physics of today.

Quantum mechanics, and the understanding of quantum entities (i.e. things which operate under the laws of quantum mechanics) that it provided have also been an utterly indispensible tool in the creation of much of today's modern technology. In particular, the entire semiconductor electronics field is based on quantum mechanical principles - and without semiconductor electronics, the now-ubiquitous miniaturized and cheaply mass-produced electronic devices of today (computers, cell-phones, cameras, etc) would be utterly impossible.

Quantum mechanics is extremely important not only for the technology it has given us, though. What the scientists who uncovered quantum mechanics found was that many of the principles that appear to hold at the large scale at which we experience physical reality are not fundamental i.e. they do not exist as basic attributes of reality. In doing so, they have deeply affected our understanding of the very nature of reality.

For example, the 'rules' that we perceive as governing the behviour of reality often only exist as high-level statistical compilations. To give a simple analogy of this particular aspect, if one only could see the results of flipping a coin if it were done 100 million times, one might gain the (false) impression that any time you flip a coin a given number of times, exactly half the time one will get tails, and half heads. This is of course not true, if the number is small: flip a coin three times, and on average, one quarter of the time you will get the same face showing all three times.

Principal findings and predictions

Among the principle findings and predictions of quantum mechanics are:

• Light, and all electromagnetic radiation, is not emitted in a continuous stream of energy, but in fixed very small units, called quanta - from which the theory derives its name.
  • There is a fixed relationship between the wavelength of a light quantum, and the amount of energy it contains.

• Light seems to be compromised of waves (i.e. travelling perturbances) in the electro-magnetic field, but it also (most paradoxically) also appears to have characteristics of particles (i.e. what is a particle anyway?); it is for this reason that the quanta of electromagnetic waves are also called photons, with the -on ending reserved for particles.
  • Not only do things usually thought of as waves have particle-like aspects, but things usually thought of as particles (e.g. electrons) also have wave-like aspects; this wave-particle duality now seems to be an inherent aspect of all quantum entities.

• The measurement of one characteristic of a quantum entity inherently affects the values of other characteristics of that entity; this is the famous Heisenberg Uncertainty Principle. This is not due to a simple lack of subtlety in the design of experiments, but is a fundamental attribute of all quantum entities; to measure one aspect of one of them necessarily changes the value of others.

• Many processes at the quantum level are only somewhat deterministic; i.e. while their behaviour, when measured in large numbers, follows some law (as in our coin-flipping example), individual events are not predictable. For example, with a large amount of a radioactive element, it is possible to accurately predict how many of those atoms will decay in a given amount of time. It is, however, impossible to predict when any particular atom will decay.
  • In an even more astonishing result, this behaviour was shown in Bell's Theorem to be fundamental; i.e. there cannot be any complex lower-level mechanism, one we simply have not understood yet, which could make such predictions.

• Bell's Theorem, and experiments based on it, have shown that the nature of space is somewhat different than we have understood it to be. Two particles created in a single quantum event appear to share some mysterious instantaneous connection, no matter how far apart they may later travel, so that one 'knows' instantly when some important change happens to the other. A relatively recent discovery, the implications and technological possibilities of this are still being uncovered today.

This list is of course not complete, I just wanted to get what I have so far up so you all can see it. We need to cover e.g. the double-slit stuff, too, although I suppose that's a logical result of the wave/particle duality.