Meaning of "realism" in quantum mechanics

Question

When physicists doing work in quantum measurement, decoherence, Bells' inequalities etc. use the term "realism" what exactly do they mean?

I'm looking for answers targeted towards, say, someone whose had formal education in physics, including multiple years of QM, but is not well versed in the literature in this sub-domain.

I see popular accounts of some results in this field describing the conclusions of experiments as, to paraphrase, "disproving realism" and I'm suspicious that physicists are using this term in a localized manner that does not carry with it all of the implications that the term "realism" has in ordinary English.

Examples

The wikipedia page on the Legget inequalities, provides a good example -- "They are fulfilled by a large class of physical theories based on particular non-local and realistic assumptions". As I understand it the Legget results are a statement of physics, and thus, if this use of the term "realistic" is appropriate, it must unpack into something specific within the context of quantum mechanics.

A phys.org article on an EPR type of experiment uses the term "realism" alot, and only defines it as "the view that reality exists with definite properties even when not being observed", which I don't find helpful since it doesn't specify which classes of properties it is referring to.

Answer 1

Unfortunately, exactly how you define realism sometimes depends on whether or not you are trying to disprove it.

For instance you might label a theory as having realism if measurements passively reveal a preexisting property. And there is a lot to unpack there. First we will review some quantum mechanics.

Basically if you measured the $\hat z$ component of a spin 1/2 system twice in a row then the second time you will get the same result as you got the first time.

The actual state after the first measurement is one that must give that result for that kind of measurement (the $\hat z$ component of that spin 1/2 system). You don't have to debate realism for this, it is just what Quantum Mechanics predicts. The state sometimes perfectly and reliably predicts a particular measurement outcome.

So lets now discuss another example. First you measure the $\hat z$ component of a spin 1/2 system and then instead of doing another $\hat z$ measurement you measure the $\hat x$ component of the same spin 1/2 system. Now you get a result of $\pm \hbar/2$ but now you can bring realism in since multiple results are predicted.

One approach is common if you want to disprove realism. You say that realism means the system that just underwent a measurement of the $\hat z$ component of a spin 1/2 system actually has

1. a spin of $+\hbar/2$ for a $\hat x$ component of the spin, or
2. that it has a spin of $-\hbar/2$ for a $\hat x$ component of the spin.

And furthermore, that a $\hat x$ measurement, if done now, would reveal that property. You need the last part about revealing, since you can't just throw the word actual around without some experimental consequences or else it is meaningless.

And now you can argue that realism makes predictions. And they are predictions that disagree with Quantum Mechanics. But that's exactly how you define realism if your goal is to disagree with Quantum Mechanics.

If you don't want to disagree with Quantum Mechanics, you can still have realism. You just have to say that measurements change the state of the system rather than passively revealing something.

For instance in Bohmian Mechanics they can be realists about position, and then they say that spin measurement outcomes are determined solely by the spin state of the system, the type and calibration of device used, and the position.

So someone using Bohmian Mechanics could say they have realism because they were a realist about enough things to totally determine the results (states and position), but they didn't try to be a realist about other things (like components of spin) besides the things that were enough to determine all the results.

And no one one should try. Because the results you get for different measurements (e.g. two $\hat z$ and an $\hat x$) can depend on the order you do them ($\hat z,$ $\hat z,$ $\hat x$ always have the two $\hat z$ agree with each other, and $\hat z,$ $\hat x,$ $\hat z$ can have the two $\hat z$ disagree with each other). So clearly what we call a measurement is an interaction that changes the state and not a passively revealing of knowledge. It can change a state from an eigenstate of $\hat\sigma_z$ into an eigenstate of $\hat\sigma_x.$ You can not expect noncommuting operators to passively reveal preexisting eigenvalues, that would not make sense for the noncommon eigenvectors.

It's not so different than the colloquial idea that things appear a certain way becasue they already were a particular way and that the correspondence is pretty tight.

In Quantum Mechanics when you have multiple results possible for one state, it's hard to have a tight correspondence. If you add something in addition to a state to make a tight correspondence you can get something just like realism ... if you want. But you can't have more than is needed to determine the results because then you go beyond a tight correspondence to an inconsistent theory.

So Bohmian Mechanics as an example has to stop with states and position and doesn't have spin measurements passively reveal preexisting components of spin. It just has a state and a position.

Answer 2

I know this is an old post, and maybe the following content did not exist back then, but I found it much clearer than the answers provided above, so for purposes of helping others like me, without further ado, Wikipedia:

Realism in the sense used in physics[6] is the idea that nature exists independently of man's mind: that even if the result of a possible measurement does not exist before the act of measuring it, that does not mean it is a creation of the mind of the observer (contrary to the "consciousness causes collapse" theory in quantum mechanics).

A mind-independent property does not have to be a value of a physical variable, such as position or momentum. A property can be potential (i.e. can be a capacity): in the way that a glass object has the potential (or capacity) to break, if subjected to a particular force, but otherwise will not actually break.

Even though the result of striking a glass object with a hammer does not exist before the act of striking it, that does not mean the broken glass is a creation of the observer. A particle accelerator is a sophisticated type of hammer, and the target particles are liable to end up as a heap of broken shards.

Such a response, i.e. breaking, is a conditional response: a response to a particular application of force. Applied to quantum systems, Schrödinger recognised that they too have a conditional response: a tendency to respond (i.e. a specific probability of responding) to a particular measuring force with a particular value.[7] In a sense, they are pre-programmed with a particular outcome.

Such an outcome would be realistic in a metaphysical sense, without being realistic in the physicist's sense of local realism (which requires that a single value be produced with certainty).

A related concept is "counterfactual definiteness", the idea that it is possible to meaningfully describe as definite the result of a measurement which, in fact, has not been performed (i.e. the ability to assume the existence of objects, and assign values to their properties, even when they have not been measured).

from here

BTW, reading about the most famous difference in opinion between Plato and Aristotle might provide additional understanding.

Answer 3

There are two viewpoints used amongst physicists in order to do quantum mechanics. The first one is the operational viewpoint. In this viewpoint, what exactly matters is not the underlying nature of quantum mechanics, but the stress is there on calculating expectation values etc. Whereas realists tend to believe that there is something deeper going on, and that there is a second layer to quantum mechanics underlying it. In a way, the operationalists oppose realism, as they don't believe in any underlying theory/the underlying theory doesn't have deeper consequences. That might be the reason for anti-reallism that you are mentioning.