Quantum mechanics

The following text, which is under construction, 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 behavior of matter and energy at very small scales - behavior 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 1900-1925 period that clearly showed the limitations of 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 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. Also lasers and medical diagnostics tools such as MRI (magnetic resonance imaging) would not have existed without the insights provided by quantum mechanics.

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 behavior of reality often only exist as large-number statistics. To give a very simple analogy of this particular aspect, if one only could see the results of flipping a coin a 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 of flippings 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 light, and the amount of energy the light quantum contains.

• Classical physics holds that light is comprised of waves, i.e., travelling perturbances in the electro-magnetic field, but it also (most paradoxically) appears to have characteristics of particles (this raises the question: what is a particle anyway?); it is for this reason that the quanta of electromagnetic waves are also called photons—light particles—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 is seen as an inherent aspect of all quantum entities. As a matter of fact, also macroscopic objects are believed to show this wave-particle duality. A tennis ball served at a hundred and fifty miles per hour has wave character, but its wavelength—which can be easily computed by the laws of quantum physics—is too small to be of significance in real life.

• A particle cannot have a well-defined position and simultaneously a well-defined speed; this follows from the famous Heisenberg Uncertainty Principle. This principle states that certain pairs of physical properties (like position/speed or time/energy) cannot have simultaneously well-defined values. On top of this comes the problem that performing a measurement on a quantum system affects the system; if one measures one aspect of one of the system, it necessarily changes the value of others. The measurement of one characteristic of a quantum entity inherently affects the values of other characteristics of that entity. This is not due to a simple lack of subtlety in the design of experiments, but is a fundamental attribute of all quantum entities.
• Many processes at the quantum level are only seemingly deterministic; i.e. while their behavior, 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 if, and when, any particular atom will decay.
  • In an even more astonishing result, this behavior 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, and causality, is different than we (and Albert Einstein) 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. One particle 'knows' in an instant when some important change happens to the other particle, which may be further away than information traveling with the speed of light can possibly bridge in this instant. A relatively recent discovery, the implications and technological possibilities of this are still being uncovered today.