I think you are reading too much into my comment. It totally wasn’t about what humanity should collectively give up on, or even what anybody should. And I agree that philosophy is effectively defined as a collection of problems which are not yet understood enough to be even investigated by standard scientific methods.
I was only pointing out (perhaps not much clearly, but I hadn’t time for a lengthier comment) that the core of physics is formalisms and modelling and predictions (and perhaps engineering issues since experimental apparatuses today are often more complex than the phenomena they are used to observe). That is, almost all knowledge needed to be a physicist is the ordinary “non-philosophical” knowledge that everybody agrees upon, and almost all talks at physics conferences are about formalism and observations, while the questions you label “foundational” are given relatively small amount of attention. It may seem that asking “what is the true nature of electron” is a question of physics, since it is about electrons, but actually most physicists would find the question uninteresting and/or confused while the question might sound truly interesting to a philosopher. (And it isn’t due to lack of agreement on the correct answer, but more likely because physicists like more specific / less vague questions as compared to philosophers).
One can get false impression about that since the most famous physicists tend to talk significantly more about philosophical questions than the average, but if Feynman speaks about interpretation of quantum mechanics, it’s not a proof that interpretation of quantum mechanics is extremely important question of physics (because else a Nobel laureate wouldn’t talk about it), it’s rather proof that Feynman has really high status and he can get away with giving a talk on a less-than-usually rigorous topic (and it is much easier to make an interesting lecture from philosophical stuff than from more technical stuff).
Of course, my point is partly about definitions—not so much the definition of philosophy but rather the definition of physics—but once we are comparing two disciplines having common definitions of those disciplines is unavoidable.
I don’t think we disagree all that much; and I meant ‘you’ to be a hypothetical interlocuter, not prase. All I want to reiterate is that the line between physics and philosophy-of-physics can be quite fuzzy. The ‘measurement problem’ is perhaps the pre-eminent problem in ‘philosophy of physics,’ but it’s not some neoscholastic mumbo-jumbo of the form “what is the true nature of electron?”. Rather, it’s a straightforward physics problem that happens to have turned out to be especially intractable. Specifically, it is the problem that these three propositions form an inconsistent triad given our Born-probabilistic observations:
(1) Wave-function descriptions specify all the properties of physical systems.
(2) The wave function evolves solely in accord with the Schrödinger equation.
(3) Measurements have definite outcomes.
De-Broglie-style interpretations (‘hidden variables’) reject (1), von-Neumann-style interpretations (‘objective collapse’) reject (2), and Everett-style interpretations (‘many worlds’) reject (3). So far. there doesn’t seem to be anything ‘unphysical’ or ‘unphysicsy’ about any of these views. What’s made them ‘philosophical’ is simply that the problem is especially difficult, and the prospects for solving it to everyone’s satisfaction, by ordinary physicsy methods, seem especially dim. So, if that makes it philosophy, OK. But problems of this sort divide philosophers because they’re hard, not because philosophers ‘trust their own rationality’ more than physicists do.
I find it a bit tricky to formulate problems in propositions like yours (1) - (3) and insist that at least one must be rejected because of mutual inconsistency. The problem is that the meaning of the propositions is not precise. What exactly does “all properties of physical systems” denote? Is it “maximum information about the system that can be obtained in principle” (subproblem: what does “in principle” mean), or is it “information sufficient to predict all events in which the system is involved, if there is no uncertainty external to the system involved”, or is it something else?
We know that the conditions under which we prepare the system can be summarised in a wave function and we know how to calculate the frequencies of measurement outcomes, given a specific wave function. We know that the knowledge of wave function doesn’t let us predict the measurements with certainty. We even know, due to Bell’s inequalities and the experimental results, that if there is some unknown property of the system which determines the measurement outcome prior to actual measurement, then this property must be non-local. We know that the evolution of systems under observation isn’t described by Schrödinger equation only. All this is pretty uncontroversial.
Now the interpretations tend to use different words to describe the same amount of knowledge. Instead of saying that we can get unpredictably different outcomes from a measurement on a system with some given wave function, one may say that the outcome is always the same but our consciousness splits and each part is aligned only with a portion of the outcome, or one may say that the outcome is not “definite” (whatever it means). This verbal play is the unphysicsy thing with the given propositions.
What exactly does “all properties of physical systems” denote? Is it “maximum information about the system that can be obtained in principle”
You seem to be trying to explain something rather clear with something less clear. The sentence in question is simply affirming that the wave function captures everything that is true of the system; thus (if you accept this view) there are no hidden variables determining the seemingly probabilistic outcomes of trying to measure non-observables. There’s nothing mysterious about asserting that there’s a hidden cause in this case, any more than science in general is going Mystical when it hypothesizes unobserved causes for patterns in our data.
To say that the outcome is not “definite” is to say that it is false that a particular measurement outcome (like ‘spin up’), and not an alternative outcome (like ‘spin down’), obtains. “Definite” sounds vague here because the very idea of “many worlds” is extremely vague and hard to pin down. One way to think of it is that the statistical properties of quantum mechanics are an epiphenomenon of a vastly larger, unobserved reality (the wave function itself) that continues merrily on its way after the observation.
Say there are no hidden variables and the evolution is probabilistic. Does then the wave function capture everything that is true of the system? It seems to me that it doesn’t: it is true that the system will be measured spin up in the next measurement, but the wave function is as well compatible with spin down. But you seem to assert that if I don’t believe in hidden variables then the wave function does capture everything that is true of the system. Thus I don’t find it rather clear. Neither does “epiphenomenon of a vastly larger reality” seem clarifying to me even a little bit.
Say there are no hidden variables and the evolution is probabilistic. Does then the wave function capture everything that is true of the system?
At a given time, yes. But over time, the way a wave function changes may (a) be determined entirely by the Schrödinger equation, or (b) be determined by a mixture of the Schrödinger equation and intermittent ‘collapses.’ Given (a), the apparently probabilistic distribution of observations is somehow mistaken, and we get a many-worlds-type interpretation. Given (b), the probabilities are preserved but the universe suddenly operates by two completely different causal orders, and we get an ‘objective collapse’ interpretation. These are the two options if the wave function captures all the variables.
I am now interested in clarification of “everything that is true of the system”. I have an electron whose spin I am going to measure five minutes from now. Does the proposition “the spin will be measured up” belong to “everything that is true about the electron”? Presume that the spin will indeed be measured up (or I will perceive the world in which it was up or whatever formulation will suit you the best). To me it appears as a true proposition, but there may be philosophical arguments to the contrary (problem of future contingents comes to mind).
Physics-inclined people tend to be 4-dimensionalists, so I don’t think they’ll object to describing wave functions in terms that account for them at all times. Even indeterminists (i.e., collapse theorists) can accept that we can talk about what will be true of electrons in the future, though we can’t even in principle know some of those facts in advance.
Does the proposition “the spin will be measured up” belong to “everything that is true about the electron”?
de Broglie sez: “Yes, that belongs to everything that is true (about the electron’s wave function). But at least one truth about the electron (its position at any given time) is not accounted for in the wave function. (This explains why the Schrödinger equation, although a complete description of how wave functions change, is not a complete description of how physical systems change.)”
von Neumann sez: “Yes. And the wave function encompasses all these truths. But there is no linear dynamical equation relating all the time-slices of the wave function. There are more free-floating brute facts within wave functions than we might have expected.”
Everett sez: “Yes… well, sort of. The formalism for ‘the spin will be measured up’ is a component of a truth. But it would be more accurate and objective to say something like ‘the spin will be measured up and down’ (assuming it was in a prior superposition). Thus the wave function encompasses all the truths, and evolves linearly over time in accord with the Schrödinger equation. Win-win!”
I think you are reading too much into my comment. It totally wasn’t about what humanity should collectively give up on, or even what anybody should. And I agree that philosophy is effectively defined as a collection of problems which are not yet understood enough to be even investigated by standard scientific methods.
I was only pointing out (perhaps not much clearly, but I hadn’t time for a lengthier comment) that the core of physics is formalisms and modelling and predictions (and perhaps engineering issues since experimental apparatuses today are often more complex than the phenomena they are used to observe). That is, almost all knowledge needed to be a physicist is the ordinary “non-philosophical” knowledge that everybody agrees upon, and almost all talks at physics conferences are about formalism and observations, while the questions you label “foundational” are given relatively small amount of attention. It may seem that asking “what is the true nature of electron” is a question of physics, since it is about electrons, but actually most physicists would find the question uninteresting and/or confused while the question might sound truly interesting to a philosopher. (And it isn’t due to lack of agreement on the correct answer, but more likely because physicists like more specific / less vague questions as compared to philosophers).
One can get false impression about that since the most famous physicists tend to talk significantly more about philosophical questions than the average, but if Feynman speaks about interpretation of quantum mechanics, it’s not a proof that interpretation of quantum mechanics is extremely important question of physics (because else a Nobel laureate wouldn’t talk about it), it’s rather proof that Feynman has really high status and he can get away with giving a talk on a less-than-usually rigorous topic (and it is much easier to make an interesting lecture from philosophical stuff than from more technical stuff).
Of course, my point is partly about definitions—not so much the definition of philosophy but rather the definition of physics—but once we are comparing two disciplines having common definitions of those disciplines is unavoidable.
I don’t think we disagree all that much; and I meant ‘you’ to be a hypothetical interlocuter, not prase. All I want to reiterate is that the line between physics and philosophy-of-physics can be quite fuzzy. The ‘measurement problem’ is perhaps the pre-eminent problem in ‘philosophy of physics,’ but it’s not some neoscholastic mumbo-jumbo of the form “what is the true nature of electron?”. Rather, it’s a straightforward physics problem that happens to have turned out to be especially intractable. Specifically, it is the problem that these three propositions form an inconsistent triad given our Born-probabilistic observations:
(1) Wave-function descriptions specify all the properties of physical systems.
(2) The wave function evolves solely in accord with the Schrödinger equation.
(3) Measurements have definite outcomes.
De-Broglie-style interpretations (‘hidden variables’) reject (1), von-Neumann-style interpretations (‘objective collapse’) reject (2), and Everett-style interpretations (‘many worlds’) reject (3). So far. there doesn’t seem to be anything ‘unphysical’ or ‘unphysicsy’ about any of these views. What’s made them ‘philosophical’ is simply that the problem is especially difficult, and the prospects for solving it to everyone’s satisfaction, by ordinary physicsy methods, seem especially dim. So, if that makes it philosophy, OK. But problems of this sort divide philosophers because they’re hard, not because philosophers ‘trust their own rationality’ more than physicists do.
I find it a bit tricky to formulate problems in propositions like yours (1) - (3) and insist that at least one must be rejected because of mutual inconsistency. The problem is that the meaning of the propositions is not precise. What exactly does “all properties of physical systems” denote? Is it “maximum information about the system that can be obtained in principle” (subproblem: what does “in principle” mean), or is it “information sufficient to predict all events in which the system is involved, if there is no uncertainty external to the system involved”, or is it something else?
We know that the conditions under which we prepare the system can be summarised in a wave function and we know how to calculate the frequencies of measurement outcomes, given a specific wave function. We know that the knowledge of wave function doesn’t let us predict the measurements with certainty. We even know, due to Bell’s inequalities and the experimental results, that if there is some unknown property of the system which determines the measurement outcome prior to actual measurement, then this property must be non-local. We know that the evolution of systems under observation isn’t described by Schrödinger equation only. All this is pretty uncontroversial.
Now the interpretations tend to use different words to describe the same amount of knowledge. Instead of saying that we can get unpredictably different outcomes from a measurement on a system with some given wave function, one may say that the outcome is always the same but our consciousness splits and each part is aligned only with a portion of the outcome, or one may say that the outcome is not “definite” (whatever it means). This verbal play is the unphysicsy thing with the given propositions.
You seem to be trying to explain something rather clear with something less clear. The sentence in question is simply affirming that the wave function captures everything that is true of the system; thus (if you accept this view) there are no hidden variables determining the seemingly probabilistic outcomes of trying to measure non-observables. There’s nothing mysterious about asserting that there’s a hidden cause in this case, any more than science in general is going Mystical when it hypothesizes unobserved causes for patterns in our data.
To say that the outcome is not “definite” is to say that it is false that a particular measurement outcome (like ‘spin up’), and not an alternative outcome (like ‘spin down’), obtains. “Definite” sounds vague here because the very idea of “many worlds” is extremely vague and hard to pin down. One way to think of it is that the statistical properties of quantum mechanics are an epiphenomenon of a vastly larger, unobserved reality (the wave function itself) that continues merrily on its way after the observation.
Where’s the ‘verbal play’?
Say there are no hidden variables and the evolution is probabilistic. Does then the wave function capture everything that is true of the system? It seems to me that it doesn’t: it is true that the system will be measured spin up in the next measurement, but the wave function is as well compatible with spin down. But you seem to assert that if I don’t believe in hidden variables then the wave function does capture everything that is true of the system. Thus I don’t find it rather clear. Neither does “epiphenomenon of a vastly larger reality” seem clarifying to me even a little bit.
At a given time, yes. But over time, the way a wave function changes may (a) be determined entirely by the Schrödinger equation, or (b) be determined by a mixture of the Schrödinger equation and intermittent ‘collapses.’ Given (a), the apparently probabilistic distribution of observations is somehow mistaken, and we get a many-worlds-type interpretation. Given (b), the probabilities are preserved but the universe suddenly operates by two completely different causal orders, and we get an ‘objective collapse’ interpretation. These are the two options if the wave function captures all the variables.
I am now interested in clarification of “everything that is true of the system”. I have an electron whose spin I am going to measure five minutes from now. Does the proposition “the spin will be measured up” belong to “everything that is true about the electron”? Presume that the spin will indeed be measured up (or I will perceive the world in which it was up or whatever formulation will suit you the best). To me it appears as a true proposition, but there may be philosophical arguments to the contrary (problem of future contingents comes to mind).
Physics-inclined people tend to be 4-dimensionalists, so I don’t think they’ll object to describing wave functions in terms that account for them at all times. Even indeterminists (i.e., collapse theorists) can accept that we can talk about what will be true of electrons in the future, though we can’t even in principle know some of those facts in advance.
de Broglie sez: “Yes, that belongs to everything that is true (about the electron’s wave function). But at least one truth about the electron (its position at any given time) is not accounted for in the wave function. (This explains why the Schrödinger equation, although a complete description of how wave functions change, is not a complete description of how physical systems change.)”
von Neumann sez: “Yes. And the wave function encompasses all these truths. But there is no linear dynamical equation relating all the time-slices of the wave function. There are more free-floating brute facts within wave functions than we might have expected.”
Everett sez: “Yes… well, sort of. The formalism for ‘the spin will be measured up’ is a component of a truth. But it would be more accurate and objective to say something like ‘the spin will be measured up and down’ (assuming it was in a prior superposition). Thus the wave function encompasses all the truths, and evolves linearly over time in accord with the Schrödinger equation. Win-win!”