Why I don’t find quantum contextuality all that bizarre

A lot of people think contextuality is one of the oddest (if not the oddest) things about quantum mechanics.  It’s certainly a little odd, but here’s why I don’t think it’s actually as odd as everyone thinks it is.  In a nutshell, contextuality is the idea that the outcomes of quantum measurements can depend on the context in which the measurement is made.  As an analogy, it might be a bit like saying that the temperature outside my house depends on how I look at the thermometer.

There are two reasons that I don’t find this all that strange.  First, we know that at the quantum level measurement is a potentially disruptive act, i.e. it’s very difficult to measure a system without disturbing it since we are working with such small systems.  I don’t find that odd.  In fact, to me, it makes intuitive sense.  The second reason I don’t think it’s odd is because it seems to me that, in a way, all physics is contextual.  One could interpret contextuality as being related to reference frames.  For example, even in classical physics one can get inconsistent results if one switches to a non-inertial reference frame.  The difference in the quantum situation is that the measurement disturbs the system (which I’ve already said doesn’t seem all that weird to me).  If you disturb a system in a different way, the result of your measurement will naturally be different.  I see no reason why that’s all that strange.

What I do find strange about quantum physics are entangled states and photons (photons are really strange if you stop to think about them).


2 Responses to “Why I don’t find quantum contextuality all that bizarre”

  1. You are in good company. John Bell did not find contextuality all that strange, which is why he emphasised nonlocality instead.

    However, I think your dismissal is a little too quick. For one thing, I don’t think disturbance is relevant because contextuality is about counterfactuals, i.e. different possibilities for what we might choose to find out about the system as it is now, rather than about the different states the system would end up in if we did actually try to find those things out.

    More broadly, I think the version of contextuality promoted by Rob Spekkens is one of the key features of quantum theory. The catchphrase for what it says is that the mapping from epistemic to operational is many to one, i.e. for any ontic model you can make a change to the way you do an experiment that does not alter the statistics of the experiment, but must necessarily alter our knowledge about what is going on in reality. This is strange because it violates the principle of the identity of indiscernibles. To put it another way, if some change that we make necessarily changes what we think is going on in reality, then you would expect to be able to detect that change by some experiment. Contextuality thus sets up another tension in quantum theory akin to the one we get from Bell’s theorem. If reality really is nonlocal then it has to be rather delicately fine-tuned in order to ensure that those nonlocal influences cannot be used for signalling. Similarly, if something changes our knowledge about reality then reality must be fine-tuned in order to avoid our change in knowledge causing a change in the predictions we can make.

    • quantummoxie Says:

      Actually, I think disturbance in measurement is intimately related to counterfactuals. Consider this: since the act of measurement disturbs a system and there is some element of randomness in the result of that measurement (i.e. we can choose the basis, but not necessarily the result of any specific measurement), it is completely natural to debate counterfactuals in this context (no pun intended). In other words, in my view, counterfactuals are non-definite precisely because of measurement disturbance.

      I’ll have to ponder Rob’s version of contextuality before I can say for certain what I think of it.

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