Despite the old canard about nobody understanding quantum mechanics, physicists do understand it. With all of the interpretations ever conceived for quantum mechanics, this claim may seem a bit of a stretch, but like the proverbial ostrich with its head in the sand, many physicists prefer to claim they do not understand quantum mechanics, rather than just admit that it is what it is and move on.
What is it about quantum mechanics that generates so much controversy and even had Albert Einstein (1879 – 1955) refusing to accept it? There are three points about quantum mechanics that generate controversy. It is probabilistic, eschews realism, and is local. Let us look at these three points in more detail.
- Quantum mechanics is probabilistic, not determinist. Consider a radioactive atom. It is impossible, within the confines of quantum mechanics, to predict when an individual atom will decay. There is no measurement or series of measurements that can be made on a given atom to allow me to predict when it will decay. I can calculate the probability of when it will decay or the time it takes half of a sample to decay but not the exact time a given atom will decay. This lack of ability to predict exact outcomes, but only probabilities, permeates all of quantum mechanics. No possible set of measurements on the initial state of a system allows one to predict precisely the result of all possible experiments on that state.
- Quantum mechanics eschews realism. This is a corollary of the first point. A quantum mechanical system does not have well defined values for properties that have not been directly measured. This has been compared to the moon only existing when someone is looking at it. For deterministic systems one can always safely infer back from a measurement what the system was like before the measurement. Hence if I measure a particle’s position and motion I can infer not only where it will go but where it has come from. The probabilistic nature of quantum mechanics prevents this backward looking inference. If I measure the spin of an atom, there is no certainty that is had only that value before the measurement. It is this aspect of quantum mechanics that most disturbs people, but quantum mechanics is what it is.
- Quantum mechanics is local. To be precise, no action at point A will have an observable effect at point B that is instantaneous, or non-causal. Note the word observable. Locality is often denied in an attempt to circumvent Point 2, but when restricted to what is observable, locality holds. Despite the Pentagon’s best efforts, no messages have been sent using quantum non-locality.
Realism, at least, is a common aspect of the macroscopic world. Even a baby quickly learns that the ball is behind the box even when he cannot see it. But much about the microscopic world is not obviously determinist, the weather in Vancouver for example (it is snowing as I write this). Nevertheless, we cling to determinism and realism like a child to his security blanket. It seems to me that determinism or realism, if they exist, would be at least as hard to understand as their lack. There is no theorem that states the universe should be deterministic and not probabilistic or vice versa. Perhaps god, contrary to Einstein’s assertion, does indeed like a good game of craps.
So quantum mechanics, at least at the surface level, has features many do not like. What has the response been? They have followed the example set by Philip Gosse (1810 – 1888) with the Omphalos hypothesis. Gosse, being a literal Christian, had trouble with the geological evidence that the world was older than 6,000, so he came up with an interpretation of history that the world was created only 6,000 years ago but in such a manner that it appeared much older. This can be called an interpretation of history because it leaves all predictions for observations intact but changes the internal aspects of the model so that they match his preconceived ideas. To some extent, Tycho Brahe (1546 – 1601) used the same technique to keep the earth at the center of the universe. He had the earth fixed and the sun circle the earth and the other planets the sun. With the information available at the time, this was consistent with all observations.
The general technique is to adjust those aspects of the model that are not constrained by observation to make it conform to one’s ideas of how the universe should behave. In quantum mechanics these efforts are called interpretations. Hugh Everett (1930 – 1982) proposed many worlds in an attempt to make quantum mechanics deterministic and realistic. But it was only in the unobservable parts of the interpretation that this was achieved and the results of experiments in this world are still unpredictable. Louis de Broglie (1892 – 1987) and later David Bohm (1917 – 1992) introduced pilot waves in an effort to restore realism and determinism. In doing do they gave up locality. Like Gosse’s work, theirs was nice proof in principle that, with sufficient ingenuity, the universe could be made to conform to almost any preconceived ideas, or at least appear to do so. Reassuring I guess, but like Gosse it was done by introducing non-observable aspects to the model: not just unobserved but in principle unobservable. The observable aspects of the universe, at least as far as quantum mechanics is correct, are as stated in the three points above: probabilistic, nonrealistic and local.
Me, I am not convinced that there is anything to understand about quantum mechanics beyond the rules for its use given in standard quantum mechanics text books. However, interpretations of quantum mechanics might, possibly might, suggest different ways to tackle unsolved problems like quantum gravity and they do give one something to discuss after one has had a few beers (or is that a few too many beers).
 See my February 2014 post “Reality and the Interpretations of Quantum Mechanics.”
 Realism as defined in the paper by Einstein, Podolsky and Rosen, Physical Review 47 (10): 777–780 (1935).
 Or dice.
 See the blog http://www.quantumdiaries.org/2011/07/22/science-and-simplicity/ for more details.