Is wave function a real physical thing?

[Ken G]

Some work has been done in this thread attempting to supply a scientific meaning to "physically real." That is of course essential if we wish to give a scientific answer to "is the wave function physically real." But I think the actual problem here is that there is only one scientific meaning to "physically real" that holds up to the process of science itself, and that is essentially the meaning "whatever we say is physically real." In other words, "reality" is our word, and we are the ones who say what it shall mean to us, it is not a word picked out of some pre-existing language that we only dimly understand because that is just not what language is.

As such, it is simply the wrong way to ask the question to frame it is as "which of the devices and concepts invoked by science are the ones that are actually physically real, and which ones are not", the question must be posed as "what whall we mean by physically real such that the devices and concepts we wish to manipulate may be regarded as having that property." In short, the question is not if the quantum state is physically real, it is, given the observed attributes that quantum states have (ability to be nonlocally entangled, inability to provide simultaneous answers to all hypothetical questions that could be put to the state) do we want the quantum state to be physically real, or don't we? You must answer that question first, or you are spinning your wheels. Because the simple truth is, some do want that state to be physically real, because they want to be able to talk about it that way, and others do not, because they want to be able to talk about physically real things as having properties that quantum states do not.

Framed like this, we put the question in its proper context: it is not a physics question, and it is not even a philosophy question-- it is a sociology question. We don't get to decide what a quantum state is, that is constrained by how it is found to behave, but we do get to decide what we want "physically real" to mean, in our modern scientific society. And that meaning is not handed to us in advance because nothing is handed to us in advance in science, we have to explore every nook and cranny ourselves, and it is not common sense because science is constantly challenging the idea that common sense is likely to be correct. We have to be scientists first, and then we have to ask what is physically real, and when we do that, the answer always comes out something like "what we mean by physically real is nothing but our current understanding of the situation, whatever that is." So I would say that the quantum state is indeed physically real, but not because it has attributes that conform to what is physically real-- instead, what is physically real must conform to the attributes of the quantum state, because why would we want our most fundamental understanding of nature to be not what we mean by what is physically real?

 

[vanhees71]

I strongly disagree. Real is what can be objectively observed and quantitatively measured. It's not a social construct or something like this. In history of science there have been independent dicoveries of the same thing (e.g., renormalization of QED has been discovered independently by Tomonaga, Feynman, and Schwinger). The results of meausrements are independent of who performs them and independent from religious, political, or philosophical believes. Also the observable consequences of quantum theory are independent of the interpretation the one or the other physicist follows. In some sense, this huge thread and all our discussions are pretty unscientific but rather philosophical or (at best) metaphysical!

 

[atyy]

I'm not a solipsist. Of course, nature exists well without us. How do you come to the conclusion that the validity of quantum mechanics implies that nature doesn't exist without us humans?

The validity of quantum mechanics does not imply that nature does not exist without us. However, I think your proposal that nature is irreducibly stochastic causes problems for solving the measurement problem.

If there is an observer-independent reality that is describable by physical laws, then there should be hidden variables in order to solve the measurement problem. Such a solution would probably use classical probability (Kolmogorov's axioms), which is always compatible with determinism. This is why I agree with the idea that reality always seems to have something to do with determinism. So if one rejects any possibility of determinism, and insists on irreducible stochasticity, it seems that one is rejecting solutions to the measurement problem, and therefore rejecting that there is a law-like observer-independent reality.

Or can there be solutions to the measurement problem which reject classical probability (Kolmogorov's axioms)?

 

[bohm2]

Some work has been done in this thread attempting to supply a scientific meaning to "physically real." That is of course essential if we wish to give a scientific answer to "is the wave function physically real." But I think the actual problem here is that there is only one scientific meaning to "physically real" that holds up to the process of science itself, and that is essentially the meaning "whatever we say is physically real." In other words, "reality" is our word, and we are the ones who say what it shall mean to us, it is not a word picked out of some pre-existing language that we only dimly understand because that is just not what language is.

Do you believe that some clear definitions of "realism" is even required to draw EPR or Bell's conclusions? As an aside, Gisin recently tried to give some definition of realism:

A theory is realistic if and only if, according to the mathematical structure of this theory, the collection of all physical quantities written in the system unambiguously determines the probabilities of all possible measurement outcomes.

A possible definition of a Realistic Physics Theory
http://arxiv.org/abs/1401.0419

Given this definition both classical mechanics and QM are "realistic". That is the problem with defining realism. It can mean different things to different people.

 

[Ken G]

Do you believe that some clear definitions of "realism" is even required to draw EPR or Bell's conclusions?

If you are saying that "realism" is not the crucial issue in EPR/Bell, I agree-- the crucial issue is what the experiments are doing that are not what we might have expected. EPR and Bell are just means to help us understand the significance of the observational data, as must always be true in science. Philosophical implications are also important, because we want to gain a sense that there is some lesson here, and we need a philosophical framework to decide what that lesson is. But philosophical frameworks are erected by us, not by nature. So I think we must distinguish the EPR/Bell conclusions as what nature is telling us, from how we choose to frame the significance of the data. It is the latter that depends on our goals, whereas the former is much less subjective.

Given this definition both classical mechanics and QM are "realistic". That is the problem with defining realism. It can mean different things to different people.

Yes, we should not regard the meaning of "realism" as handed to us, it is very much a project of our own to decide what we wish that word to mean. Gisin seems to be taking the scientific approach that I also am advocating-- let's choose a meaning that fits our scientific understanding, rather than reverse engineer our understanding to fit our preconceptions. Of course, scientific understanding is a moving target, so if we adopt this approach, it means that what we regard as "real" will also be a moving target. This elicits discomfort from many, but scientists shouldn't recoil from that, discomfort is like a signpost to where we need to go rather than what we need to avoid.

 

[microsansfil]

Hi Ken G,

Some work has been done in this thread attempting to supply a scientific meaning to "physically real." That is of course essential if we wish to give a scientific answer to "is the wave function physically real."
...

You give an interresting epismetic analysis of the use of the sentence "physically real", that coming out of the metaphysical assertions based on a-priori of scientific realism. “This is how humans are: We question all our beliefs, except for the ones that we really believe in, and those we never think to question.”

however, what is the usefulness for science to seek to build a response to this metaphysics question : "is the wave function physically real." ?

Patrick

 

[vanhees71]

The validity of quantum mechanics does not imply that nature does not exist without us. However, I think your proposal that nature is irreducibly stochastic causes problems for solving the measurement problem.

If there is an observer-independent reality that is describable by physical laws, then there should be hidden variables in order to solve the measurement problem. Such a solution would probably use classical probability (Kolmogorov's axioms), which is always compatible with determinism. This is why I agree with the idea that reality always seems to have something to do with determinism. So if one rejects any possibility of determinism, and insists on irreducible stochasticity, it seems that one is rejecting solutions to the measurement problem, and therefore rejecting that there is a law-like observer-independent reality.

Or can there be solutions to the measurement problem which reject classical probability (Kolmogorov's axioms)?

Again, I don't understand your conclusions. The probabilities of quantum theory obey Kolmogorov's axioms. Why shouldn't they? Then, if Nature is intrinsically stochastic why do you need hidden variables to make our description realistic? If it's stochastic then some or even any observables can be indetermined. Only the probability for a measurement outcome then is "real". So what's then "the measurement problem"?

I don't say that there aren't hidden variables or that there isn't a non-local deterministic (and causal!) theory. I only say that so far we haven't found it, and so far I even don't see any necessity to look for one, since quantum theory works so far very well. Perhaps some motivation might be to find a consistent quantum theory of gravitation?

 

[atyy]

Again, I don't understand your conclusions. The probabilities of quantum theory obey Kolmogorov's axioms. Why shouldn't they? Then, if Nature is intrinsically stochastic why do you need hidden variables to make our description realistic? If it's stochastic then some or even any observables can be indetermined. Only the probability for a measurement outcome then is "real". So what's then "the measurement problem"?

The measurement problem is that within quantum mechanics, if there is no observer, there are no measurement outcomes, and thus no observer-independent reality.

I don't say that there aren't hidden variables or that there isn't a non-local deterministic (and causal!) theory. I only say that so far we haven't found it, and so far I even don't see any necessity to look for one, since quantum theory works so far very well. Perhaps some motivation might be to find a consistent quantum theory of gravitation?

But if you don't reject hidden variables or the possibility of a non-local deterministic theory, how can you come to the conclusion that nature is intrinsically stochastic?

 

[stevendaryl]

In quantum theory the objective entities are the states, and they evolve even deterministic over time. Why is the many-particle wave function of non-relativistic physics not realistic, only because it's a function on configuration space? It tells you the position probality density for $N$ particles and thus is a function of the $3N$ position coordinates (and $N$ spin components if you have particles with spin). For a classical $N$-body system you need $6N$ phase-space coordinates (e.g., position and canonical momentum coordinates). Is this then also not realistic (despite the fact that it's classical and thus only an approximation, but within classical physics, I'd consider this as a realistic description)?

Well, classically, a phase-space description is equivalent to particle density and current density as a function of 3-space.

 

[Ken G]

however, what is the usefulness for science to seek to build a response to this metaphysics question : "is the wave function physically real." ?

One can certainly go the road that science is separate from metaphysics, and just avoid a question like that on the basis that it doesn't help you solve an equation or predict an experiment. That is often called the "shut up and calculate" approach to science. Yet, I must say that I've never actually met someone who only does that, even among those who claim to be of that philosophical bent. It's just not what draws us to science-- we wish to understand, we wish to learn the lessons. The learning process often feels like a sense of discomfort and confusion, followed by a sense of a light bulb coming on, a veil being lifted. That is a description of a metaphysical experience, you just can't get a "Eureka" moment by shutting up and calculating. But this presents a problem-- is not that "Eureka" moment something personal, so how can that mesh with the objectivity of science? This is a fundamental conundrum that is at the heart of questions like "is the wave function real", I don't know how to make that conundrum go away. In fact, I don't think we should make that conundrum go away, we are supposed to wrestle with it-- what do we want these words to mean, such that we get a sense of understanding and learning the lessons of science, more so than just being able to predict the outcomes? What are the lessons of the quantum state that are telling us something different from what we thought was true about reality? That's the question of importance, reality is something we are supposed to figure out, not already know.

 

[Dale]

Closed for moderation

EDIT: we will leave this closed. Everyone has had a chance to say their piece.