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Byron Jennings | TRIUMF | Canada

View Blog | Read Bio

Nobody understands quantum mechanics? Nonsense!

Despite the old canard about nobody understanding quantum mechanics, physicists do understand it.  With all of the interpretations ever conceived for quantum mechanics[1], 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.

  1. 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.
  2. Quantum mechanics eschews realism[2]. 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.
  3. 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[3].

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[4]. 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).

To receive a notice of future posts follow me on Twitter: @musquod.

[1] See my February 2014 post “Reality and the Interpretations of Quantum Mechanics.”

[2] Realism as defined in the paper by Einstein, Podolsky and Rosen, Physical Review 47 (10): 777–780 (1935).

[3] Or dice.

  • http://www.brianclegg.net Brian Clegg

    I think you rather misunderstand the ‘nobody understands quantum mechanics’ concept. It’s certainly one you argue with at your peril. After all when Richard Feynman says ‘‘[Y]ou think I’m going to explain it to you so you can understand it? No, you’re not going to be able to understand it. Why, then, am I going to bother you with all this? Why are you going to sit here all this time, when you won’t be able to understand what I am going to say? It is my task to persuade you not to turn away because you don’t understand it. You see, my physics students don’t understand it either. This is because I don’t understand it. Nobody does.” it’s a brave person who argues.

    But the point is that what Feynman meant is that no one understand *why* quantum particles behave the way they do, etc. Not ‘no one understands how they behave’, or ‘no one understands the interpretations.’ It’s quite a different concept. And while you’re welcome to say Feynman was wrong, I’m not inclined to.

  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    Quantum mechanics fails when it assumes vacuum achiral isotropy toward fermionic matter (quarks). Parity violations, symmetry breakings, chiral anomalies, Chern-Simons repair of Einstein-Hilbert action in quantum gravitation reveal vacuum trace chirality toward hadrons. QM has a defective postulate toward mass, hence Milgrom acceleration not dark matter and SUSY being fairy dust.

    Look. The smallest pairs of shoes differentially embedding in trace chiral vacuum are the unit cells of two periodic crystals in paired enantiomorphic space groups. 20 grams each of right-handed versus left-handed quartz compare 6.68×10^22 pairs of opposite shoes. A geometric Eötvös experiment decides it within 90 days. Theory is fundamentally flawed when it requires unending corrections to its predictions “to make it conform to one’s ideas of how the universe should behave.” Proton decay, neutrino see-saw mechanism, dark matter,

    Number 43, Winter 2014
    Page 8 – “anti-de Sitter space-time is unstable”

  • http://tsm2.blogspot.com wolfgang

    >> Quantum mechanics is probabilistic, not determinist.
    Yes, but the Schroedinger evolution is fully deterministic (if you prefer the heisenberg picture, its unitary evolution is fully deterministic too).
    The quest to reconcile the Born probabilities with this deterministic evolution is exactly the problem many people have with qm.

  • Kevin Sinclair


    You say QM is local but how do you explain that the double slit experiment is understood? And entanglement?


  • http://www.vitha.nl Peter Schuttevaar

    Dear Byron,

    I like this article very much for it’s clarity and its conciseness. Nonetheless, I would like to point out to you that the main argument of the article can also be turned against QM.

    The main argument seems to be that such hypothesis like the one of Omphalos create theoretical constructs that conform to observations, but are really designed to match preconceived ideas.

    But to be honest, QM also constructs a lot of theory that match preconceived ideas, namely a series of abstract mathematical notions based on a branch of mathematics called set theory. These notions are regarded to have elegance and beauty. In any case, a special quality is given to them, such that they are considered to be in a preferred category, compared to other theoretical, more physical notions.

    So QM is based on the preconceived notion that nature behaves according to some mathematical constructs and is thus just another Omphalos hypothesis.

    Peter Schuttevaar

  • David

    “But much about the microscopic world is not obviously determinist, the weather in Vancouver for example”

    I’m not sure this is a good analogy. Systems that are non-deterministic because the observer doesn’t have as much knowledge as they *could* have are different from systems where the observer *cannot* have complete information…

  • http://backreaction.blogspot.se/ Bee

    I tend to agree with you though it depends on what people mean with ‘understanding’. I think the math underlying quantum mechanics (and qft) is presently very well understood. There are some things in quantum information that for all I know are not very well understood. Eg, what is a maximally entangled state for n>2 particles. How to define the free energy of a quantum system. And so on. So it’s not like there are no research questions left. But you are right, I think most people, when they say cryptically ‘we don’t understand quantum mechanics’ mean either it’s not realist or not deterministic and they don’t like that because it’s incompatible with their every-day experience.

  • Zeynep

    I think you might have a typo—you possibly meant “macroscopic,” not “microscopic,” when talking about weather in Vancouver.

    Nonetheless, good essay. Thank you.

  • Tim Maudlin

    This post repeats many misconceptions. Einstein;s objection to quantum theory was never based on its indeterminism, but rather on the non-locality of the standard (Copenhagen) interpretation, as was demonstrated in the EPR argument. In fact, no one working in the foundations of quantum theory is motivated by a desire for determinism per se. Rather, one wants a physical theory in which all interactions (including those that happen to be denominated “measurements”) can be analyzed by the same physical theory, whether that theory is deterministic or stochastic. There is nothing contradictory about a “realistic indeterministic” theory, no matter how “realistic” is to be understood, so the notion that 2) is a corollary of 1) is certainly incorrect.

    If you think you understand quantum theory, then you should be able to answer these questions:

    1) Is the quantum state assigned to a system complete, that is, are two systems assigned the same quantum state identical in all physical respects? (Point 1 suggests you believe this. Bohm’s theory, for example denies it. This was the main question in the EPR paper.)

    2) What is the dynamics of the quantum state? The usual dynamical equations (Schrödinger’s equation or the Dirac equation, for example) are both deterministic and linear. Do you think the quantum state ever evolves in accord with these equations? Does it always evolve with these equations (i.e. does the quantum state ever “collapse”)? If it does collapse, how and under what conditions? (If you plan to use the word “measurement” here, please be prepared to give a precise physical account of what sorts of interactions constitute measurement.)

    3) Bell proved (note: this is a proof, a mathematical proof) that no local theory can reproduce the prediction of quantum theory for certain experiments carried out at space-like separation. Given this proof, point 3 is provably incorrect, at least for the sense of “local” that Bell (and Einstein) had in mind. Why are you ignoring what they meant?

    It is, in addition, rather odd that you characterize a claim of Feynman as an “old canard”. Among those objecting to the Copenhagen interpretation, and insisting on foundational problems were Einstein, Schrodinger, Dirac, Gell-Mann and Weinberg. One might conclude that many prominent physicists share this assessment.

  • Gilbert Fernandes

    I never had any problem with the quantum theory. I guess that people that want answers to everything might be troubled with it (and go back to the idea of hidden variables). I don’t need a theory to explain everything. Perhaps a new one will do, or the theory will be improved generations after we die.

    If this is how the world does work, that’s it. You might not like it, but it’s how it works and the scientific truth that matters. This is what Feynmann said.

    I feel quite confortable with the Quantum theory. Everything a little strange about it makes it interesting. We should stop telling people that no one understands it or that it is complex.

  • http://www.abicana.com Knut Holt

    But that a system is evolving probabilistically does not mean that the system does not have a specific state at any point in time, only that the state at a time A does not determine extactly the state at a later time B, and therefore you cannot know the state at that later time exactly either before you observe it.