Wheeler's delayed choice doesn't change the past

[stevendaryl]

Well Copenhagen states the state is subjective, the ensemble states its a conceptual very large number of possible outcomes (actually infinite - but I don't know about you but I can't imagine that) associated with each observable.

But it seems to me that there is a "preferred basis" problem with such an ensemble view. In a classical use of ensembles, you have some undetermined variable, such as the precise location of the system in phase space, and the ensemble consists of many similar systems in which this variable has a definite value (but different in different elements of the ensemble). So you could try to do something similar with quantum uncertainty: You have an uncertain variable, such as the spin of a particular particle along the z-axis. And then you have an ensemble of similar systems that differ in the value of that variable.

But here's where the basis problem comes in, it seems to me. Why in the ensemble do the individual systems have definite values for spin in the z-direction, as opposed to, say, spin in the y-direction? You could answer: Because spin in the z-direction is what we're measuring---there would be a different ensemble for each kind of measurement you might want to perform.

Okay, that's fine for some purposes, but presumably the measurement device and the observer doing the measurement are themselves quantum systems, although very complex ones. Presumably what it is that you're trying to measure is a fact about the configuration of atoms making up the measurement devices. And the fact that the outcome was "spin up in the z-direction" rather than "spin up in the x-direction" is again a fact about the configuration after the measurement. So it seems to me that in principle, it should not be necessary to specify what is being measured, because that is itself a property of the state.

The other view is every theory, every single one has things that are simply accepted as true - that improper mixed states become proper ones is simply one that's accepted as true

Well, I'm fine with it just being an additional assumption of the theory. However, there is no way for that transition to take place in a basis-independent manner. The usual assumption, in agreement with our intuitions and observations, is that the transition happens so that after the transition to proper mixed states, macroscopic variables--whether cats are alive or dead, whether you are in London or Beijing--have definite values. That seems to me to be a breaking of the basis-independence of the quantum formalism.

I know that there is a reason for "preferring" a position basis for macroscopic objects, because only in the position basis are interactions local. That's a clue, but it isn't an answer. The question is whether adding a transition from improper to proper mixed states involves some violation of quantum mechanics, or is it derivable from quantum mechanics, or is it merely consistent (thought not implied by) quantum mechanics? I don't think that just assuming that it happens actually answers the questions.

 

[RUTA]

Is this still science? Is this something beyond standard QT or just another mathematical technique to evaluate path integrals? I'm a bit sceptical ;-)). Last but not least the preprint (on a theoretical subject) is written in Word...

RBW specifically and adynamical explanation in the block universe in general focus on empirical science rather than metaphysics. We want to move physics beyond theories like string theory and inflation where you have to abandon empirical verification, as we argue at length in chapter 1. But, in the context of this thread, just read https://www.physicsforums.com/insig...elayed-choice-no-counterfactual-definiteness/ to see how adynamical explanation in the block universe resolves the mystery of delayed choice that results from dynamical explanation in the mechanical universe. Yes, RBW has implications for quantum gravity (theory beyond QT as we detail in chapter 6), but there is no mention of RBW in the Insight — you don’t need RBW to appreciate the explanatory power of the “God’s-eye view” over the “ant’s-eye view” (Wilczek, F. : Physics in 100 Years. Physics Today 69(4), 32-39 (2016)). You can read Parts 3 and 4 on GR as well to appreciate that. As for the last sentence, I fail to see the relevance. Five respected foundationalists (to include Rovelli) gave the thumbs up on the book or Oxford UP would never have given us a contract. Several others since have sent us their input and/or given us blurbs (you can read those on Amazon). Does it matter what word processor we used to write it?

 

[DrChinese]

... Acausal behavior would be the end of science as we know it!

I'm sure there were lots of people who thought the same when the probabilities of QM appeared on the scene. And yet, here we are, nearly 100 years later and the science just keeps getting better and better.

 

[RUTA]

Acausal behavior would be the end of science as we know it!

Moving “Beyond the Dynamical Universe” is a paradigm change, yes. I’ve been doing foundations of physics for 23 years and it’s my impression that most in the community believe experiments like delayed choice will lead to a new scientific worldview. Here is a nice quote from Richard DeWitt that sums up the atmosphere in foundations of physics (Worldviews: An Introduction to the History and Philosophy of Science, DeWitt, R., 2004, p. 304, Blackwell Publishing):

In the past, fundamental new discoveries have led to changes -- including theoretical, technological, and conceptual changes -- that could not even be imagined when the discoveries were first made. The discovery that we live in a universe that, deep down, allows for Bell-like influences strikes me as just such a fundamental, important new discovery. ... If I am right about this, then we are living in a period that is in many ways like that of the early 1600’s. At that time, new discoveries, such as those involving Galileo and the telescope, eventually led to an entirely new way of thinking about the sort of universe we live in. Today, at the very least, the discovery of Bell-like influences forces us to give up the Newtonian view that the universe is entirely a mechanistic universe. And I suspect this is only the tip of the iceberg, and that this discovery, like those in the 1600’s, will lead to a quite different view of the sort of universe in which we live.

 

[Alfredo Tifi]

I just cannot parse this statement at all...:

"Obviously, deductions or interpretations would be more mingled with the theory on the bench than it occurred [in] the past."

did you leave out some words, or punctuation, or is it just me ? .

I missed [in] before "the past". But I see it is not enough to make sense of this thought of mine. I'll try to express it better.
First, usual theories address what is worth observing under circumstances that are partially generated by the experimenter's will and strategy, in the form of an experimental setting. The "quasi-deterministic" or "classic" character of the experimental apparatus and constraints can be obtained only operating in the macroscopic domain. Let's think to a particle accelerator. About this part of the experimental circumstances and constraints which is under our control, we suppose that are independent of the theory we want to probe. There is another part of the experimental circumstances that are "decided" and acted by and under the control of the "nature of things". The separateness of the two parts in determining the experimental circumstances warranted the realism of science. We were sure that what we were "observing" was not an artefact of our apparatus or preconception, but rather an expression of a sort of "freedom" of nature whether to correspond, or not, to our theory-based guesses.
If we go to devise experiments at the nanoscale or quantum scale, we must renounce at determinism, to the control of setting, of constraints, to the deterministic tuning of experimental variables. We are not anymore observing how our commanded actors influence and interact with common people on the stage. We would be engaged in simply collecting signals from the microscopic world: a certain particle decays, gamma rays are emitted at certain frequencies etc. We don't care about initial conditions because these are always statistically defined at most. An experiment wouldn't be anymore a "natural outcome" as determined by an artificial input entered into the black box. We will just observe outputs coming from the black box and try to make sense of them. If a classical experiment is the study of "if I do something then something happens", in an experiment at the quantum scale we haven't anymore the control of the "if I" part. The "if" part and the "then" part are both theory dependent footprints, to be interpreted within the theory before we can check the correlations as matching/not matching the theory itself.
We don't need to renounce at realism and believe that the way we rationally choose to interpret the signals and footprints could ever affect what the hell occurs in the black box. We don't need to renounce at studying phenomena. But we must adopt new concepts which are farther from our point of view and closer to the "nature of things" in studying phenomena. Sum over histories is possibly one of these.

 

[zonde]

Moving “Beyond the Dynamical Universe” is a paradigm change, yes. I’ve been doing foundations of physics for 23 years and it’s my impression that most in the community believe experiments like delayed choice will lead to a new scientific worldview. Here is a nice quote from Richard DeWitt that sums up the atmosphere in foundations of physics (Worldviews: An Introduction to the History and Philosophy of Science, DeWitt, R., 2004, p. 304, Blackwell Publishing):

In the past, fundamental new discoveries have led to changes -- including theoretical, technological, and conceptual changes -- that could not even be imagined when the discoveries were first made. The discovery that we live in a universe that, deep down, allows for Bell-like influences strikes me as just such a fundamental, important new discovery. ... If I am right about this, then we are living in a period that is in many ways like that of the early 1600’s. At that time, new discoveries, such as those involving Galileo and the telescope, eventually led to an entirely new way of thinking about the sort of universe we live in. Today, at the very least, the discovery of Bell-like influences forces us to give up the Newtonian view that the universe is entirely a mechanistic universe. And I suspect this is only the tip of the iceberg, and that this discovery, like those in the 1600’s, will lead to a quite different view of the sort of universe in which we live.

I don't see that the quote you gave suggests that we should move away from scientific approach and adopt some acausal philosophical view of reality.

 

[RUTA]

I don't see that the quote you gave suggests that we should move away from scientific approach and adopt some acausal philosophical view of reality.

Who said anything about moving away from scientific approaches? Acausal explanation is still scientific.

 

[OCR]

I'll try to express it better.

Success... and, thank you.

 

[DrChinese]

I don't see that the quote you gave suggests that we should move away from scientific approach and adopt some acausal philosophical view of reality.

If it works, it works. There's no sense in calling an approach that is aesthetically unpleasing to you "unscientific". I will point out that all interpretations posit probabilities - akin to throwing dice - as the BEST that can be offered as an explanation for quantum outcomes. That includes Bohmian Theories.

So I don't see what the objection is to an acausal theory if it provides explanatory power. If I observe the spin of a particle to be up, and can't explain that outcome deterministically, how is that NOT acausal anyway?

 

[bhobba]

it should not be necessary to specify what is being measured, because that is itself a property of the state.

Ahhh. But that's not what the Born rule says is it - so 'seems to me' is obviously wrong. Intuition and QM are sometimes not good friends.

You keep mentioning basis independence - that in fact is the key assumption of Gleason and the root of that very Born Rule.

THanks
Bill

 

[zonde]

Who said anything about moving away from scientific approaches? Acausal explanation is still scientific.

All our reasoning about reality is built on causal relationships. Say we believe that our records at present tell us something about experiments and their results in the past. And actually any information about the past is result of causal process. So acausal relationship is such that does not leave any trace of it in the future. So we can't gather knowledge about such relationship.

 

[vanhees71]

Moving “Beyond the Dynamical Universe” is a paradigm change, yes. I’ve been doing foundations of physics for 23 years and it’s my impression that most in the community believe experiments like delayed choice will lead to a new scientific worldview. Here is a nice quote from Richard DeWitt that sums up the atmosphere in foundations of physics (Worldviews: An Introduction to the History and Philosophy of Science, DeWitt, R., 2004, p. 304, Blackwell Publishing):

In the past, fundamental new discoveries have led to changes -- including theoretical, technological, and conceptual changes -- that could not even be imagined when the discoveries were first made. The discovery that we live in a universe that, deep down, allows for Bell-like influences strikes me as just such a fundamental, important new discovery. ... If I am right about this, then we are living in a period that is in many ways like that of the early 1600’s. At that time, new discoveries, such as those involving Galileo and the telescope, eventually led to an entirely new way of thinking about the sort of universe we live in. Today, at the very least, the discovery of Bell-like influences forces us to give up the Newtonian view that the universe is entirely a mechanistic universe. And I suspect this is only the tip of the iceberg, and that this discovery, like those in the 1600’s, will lead to a quite different view of the sort of universe in which we live.

Well, I don't believe much in "philosophy of science". If there is a paradigm change this must come from reproducible objective observations not from scholastic thinking about "angles on the head of a pin".

 

[vanhees71]

So I don't see what the objection is to an acausal theory if it provides explanatory power. If I observe the spin of a particle to be up, and can't explain that outcome deterministically, how is that NOT acausal anyway?

As often, this confusion comes from not clearly distinguishing between "deterministic" and "causal"

Deterministic means that all observables have precise values at any time, no matter whether we have prepared the observed system to have these values or not. That's the worldview of classical physics (both in point-particle mechanics and classical field theory).

Causal means that if the state of a system is known over the entire history of the past, the state is determined at any later time. On a fundamental level, even a much stronger assumption holds in all physics (including QT): If the state of the system is known at time $t_0$ than it's also known at any later time $t>t_0$.

The difference between classical and quantum physics is that the preparation in a pure state, i.e., the complete preparation such that the values of a complete set of compatible observables are determined, in the quantum case does not imply that all observables have determined values. In QT, indeed it's impossible to prepare a system in a state, where this is the case, and all the complete state preparation implies are the corresponding probabilities for the outcome of measurements of any observable of the system.

In short: All of physics is causal. If there is no causality, physics (and all natural sciences) as we know it, is simply gone since the scientific method hinges on the assumption that there are causal natural laws and the objective observations are reproducible, i.e., when preparing a system in any pure states leads to the very same outcome of measurements of all observables. If this was not the case, particularly QT wouldn't make any sense as a physical theory since you can test the probabilistic (and only probabilistic!) predictions it makes about the outcome of measurement only if these probabilities are fixed. If the causality assumption is invalid, this wouldn't be the case but everything would be indetermined completely, i.e., we'd not be able to reproduce any previous empirical finding in the world.

 

[bhobba]

Acausal behavior would be the end of science as we know it!

What's your view of the Wheeler-Feynman absorber theory?

I am not a fan of that or the transactional interpretation, but that scientifically means diddly squat. As usual I agree with you but I don't think the situation is quite as clear cut as you said - or maybe its is - but you can best explain what you mean because I think its a bit context dependent ie we can't send information backward in time - that would be exactly as you say.

Thanks
Bill

 

[microsansfil]

Hi all

To my mind, Copenhagen says nothing but: Microscopic entities are not „real“ in the sense that we cannot speak about them directly; we cannot describe these entities at all, we can only represent them by means of a quantum mechanical formalism; as distinguished from macroscopic entities which allow at least an approximate classical description.

indeed, a "microsystem" is not perceptible, by us humain being, as a pebble can be. To study "microstates" that are not perceptible to us, we need recording devices that are capable of capturing brands (for example buble chamber), following interactions with "microstates", that are perceptible to us. We have to abandon our naïve realism assumptions, which is the idea that the senses provide us with direct awareness of objects as they really are (wave, corpuscle, trajectory, ..). In all intellectually honesty, we should not forget the mediation of our brains when building our scientific knowledge.

Best regards

 

[RUTA]

Well, I don't believe much in "philosophy of science". If there is a paradigm change this must come from reproducible objective observations not from scholastic thinking about "angles on the head of a pin".

For someone who claims not to believe in philosophy of science, you certainly practice it liberally in this thread! I started teaching physics as an undergraduate physics major in 1981. I continued as a grad student then as an assistant prof, associate prof and finally a full prof. I’ve pretty much taught every subject in physics over those 36 years, publishing and refereeing journal articles, presenting at conferences, and recently writing a book representing 23 years of research. That research was in foundations of physics which includes publications in physics journals fitting dark energy and dark matter data, and refereeing papers written by notables such as Aharonov, Smolin, Price, and Elitzur, for example. I was fortunate to learn early in my career that, like Max Planck said, “There is no physics without some metaphysics,” and metaphysics is philosophy. The understanding that any practicing physicist is necessarily making philosophical assumptions has helped me greatly in crafting a worldview based on physics, because I was able to confront and challenge my philosophical assumptions in my practice of physics. I was also lucky to have a colleague in philosophy of physics whose PhD thesis was on the Bell inequality. Without him challenging my philosophical assumptions in physics for the past 23 years, we would never have put together the approach to quantum gravity outlined in “Beyond the Dynamical Universe.” Take it from this old timer, there is much about physics to learn if you’re willing to engage in philosophy of physics :-)

 

[bhobba]

Take it from this old timer, there is much about physics to learn if you’re willing to engage in philosophy of physics :-)

Eloquently expressed and I will not argue with it.

But surely you have noticed there are many physicists, and those like me interested in physics, that find philosophy leaves them cold. Of course it doesn't make it right - or wrong either - its just the way some people are.

What I have noticed on this forum there are those who like Bohr and more currently Wallace who has a PhD in both, are very imbued with philosophy and those like Dirac, Feynman and Weinberg that are not. I am in the second camp, but that does not make me correct - its just the way me and similar people are. You and others are different - and it's just the way they are.

I don't know what view will eventually make progress - as Feynman said in the past sometimes a philosophical view like logical positivism made progress as done by Einstein and Heisenberg, and other times just fooling around with equations like Dirac did made progress. When progress isn't being made its because all the 'tricks' that worked in the past now don't work - they would have been tried and because progress has paused obviously are now not working.

I have zero idea what will work to make progress in the future - the type of things you suggest, or what Dirac did - who knows. All I think is necessary is to understand we all are different and understand those differences.

And yes many have wryly noted that those anti philosophy like Feynman and Weinberg have in fact adopted a philosophical position. So I think the question is not so much whether you practice philosophy or not - but what one you have chosen.

Thanks
Bill

 

[stevendaryl]

Ahhh. But that's not what the Born rule says is it - so 'seems to me' is obviously wrong. Intuition and QM are sometimes not good friends.

Yes, I know that. The Born rule assumes a split between the observer and the thing being observed. But surely observers are quantum systems, as well.

The derivation of Gleason's theorem is the same way. It's not really assuming basis-independence. Given a split between observer and thing being observed, you can equally well assume any correlation between the states of the observed and the states of the observer. But the split itself is not basis-independent.

 

[DrChinese]

Eloquently expressed and I will not argue with it.

But surely you have noticed there are many physicists, and those like me interested in physics, that find philosophy leaves them cold. ...

I'd go further, and say your statement applies to the majority of physicists. And in another recent thread, the OP asks "What is the value of "delayed choice" experiments?" It wouldn't make sense if all physicists believed the exact same things, and similarly thought that all future research should focus on the same things. They'd all be going down the same road!

I remember reading articles written prior to 1965 arguing that the universe was "steady state" or "continuous creation"... essentially on aesthetic grounds of one type or another. I don't think the question of whether those - or the newfangled "big bang" theory - kept most physicists up at night back then. And certainly not any more than the philosophy of QM interpretations keep most physicists up at night today. But there have always been those whose curiosity DOES keep them up at night. And some of those have gone on to answer some of those deep questions. Again: I don't think it is reasonable to predict where the next big discovery is coming from... or who is going to make it. Or whether it will follow "causal" ideas or not.

Honestly, I scoff at the idea that we live in a causal universe in the first place. Other than the "deterministic" movement of bodies due to general relativity, a reasonable person can question the role of causality versus chance in most of science:

1. Evolution.
2. Human behavior.
3. Quantum mechanics.
4. The initial conditions of our universe.
5. The weather and forecasts for our planets future.

So I'm not sure where anyone gets the idea that the above subjects cannot be studied unless we first assume causality. All we need is to be able to see patterns and pattern exceptions. That would be enough to make a scientific advance.

 

[stevendaryl]

Yes, I know that. The Born rule assumes a split between the observer and the thing being observed. But surely observers are quantum systems, as well.

The derivation of Gleason's theorem is the same way. It's not really assuming basis-independence. Given a split between observer and thing being observed, you can equally well assume any correlation between the states of the observed and the states of the observer. But the split itself is not basis-independent.

A little elaboration on that: In an observation or measurement, you have two systems, the observer system (measuring device, or experimenter, or whatever) and the observed system, (an electron, say). To say that the set up is measuring property $A$ of the observed system is to say that there is an interaction between the two systems such that different values of $A$ in the observed system leads to macroscopically distinguishable states of the observer system (typically, in an irreversible way). The quantum formalism is independent of the basis for the observed system, but not for the composite system.

 

[bhobba]

Yes, I know that. The Born rule assumes a split between the observer and the thing being observed. But surely observers are quantum systems, as well.

Yes - that is the well known issue talked often about here with MW:
https://arxiv.org/abs/1210.8447

I think it's wrong because structure emerges naturally eg gravitational fields cause clumps of matter to form.

IMHO there is nothing to gain rehashing it again - its been done to death.

The derivation of Gleason's theorem is the same way. It's not really assuming basis-independence. Given a split between observer and thing being observed, you can equally well assume any correlation between the states of the observed and the states of the observer. But the split itself is not basis-independent.

I think you can dig up a proof and see the assumptions made. It's usually called non-contextuality - but is really saying the probabilities do not depend on what basis a vector may be part of eg basis independence.

Thanks
Bill

 

[stevendaryl]

I think you can dig up a proof and see the assumptions made.

Yes, and from the very beginning, it is assuming a split between observer and observed. He poses the problem as how to derive expectation values from states.

 

[bhobba]

Yes, and from the very beginning, it is assuming a split between observer and observed. He poses the problem as how to derive expectation values from states.

To cut this short - the issue is this. Yes you do have a cut between what is observed and what does the observing. The decoherence paradigm depends on it. The issue though is this - what if the cut is placed differently. General system cuts don't really make a difference in physics - you choose the simplest one. We do not have theorems if this breaks down in QM.

Thanks
Bill

 

[bhobba]

Honestly, I scoff at the idea that we live in a causal universe in the first place.

I think its very definition dependent personally - such things always worry me.

Thanks
Bill

 

[RUTA]

But surely you have noticed there are many physicists, and those like me interested in physics, that find philosophy leaves them cold. Of course it doesn't make it right - or wrong either - its just the way some people are.

All physicists make philosophical assumptions, whether they choose to acknowledge them or not. Most physicists can't be bothered to explore their philosophical assumptions and they don't need to be, they're still able to do their physics. Philosophy doesn't just leave me cold, it irritates the hell out of me! I want to do physics, not argue endlessly about my philosophical assumptions! I have told my philosophy colleagues on many occasions that I've explored my assumptions, made my choices and now I'm doing physics accordingly. They, on the other hand, don't ever stop such arguing b/c that's what it means to do philosophy. That's what they enjoy. A physicist needs to know where to draw the line in dealing with philosophers, so they can get on with doing physics. But, in PF threads about the ontology underlying delayed choice, it is precisely those philosophical assumptions that must be articulated. So, I provide my philosophical assumptions when responding to such questions on PF. I believe those assumptions are interesting, or I wouldn't bother sharing them, but I spend most of my time doing physics in accord with those assumptions (e.g., fitting the SCP Union2.1 SN data in accord with modified Regge calculus or fitting THINGS galactic rotation data, ROSAT/ASCA data on galactic cluster mass profiles and Planck CMB anisotropy data using GR contextuality, or fitting nested interferometer data on photon detections based on modified lattice gauge theory), rather than debating them. When someone starts a PF thread on dark energy or dark matter, I articulate their assumptions then show how different assumptions lead to different physics.

I suspect we agree that philosophy for the sake of philosophy, i.e., without the concordant physics, is of little or no interest to us :-)

 

[stevendaryl]

To cut this short - the issue is this. Yes you do have a cut between what is observed and what does the observing. The decoherence paradigm depends on it. The issue though is this - what if the cut is placed differently. General system cuts don't really make a difference in physics - you choose the simplest one. We do not have theorems if this breaks down in QM.

The cut is a red herring, in my opinion. The empirical predictions of QM can (in principle, at least) be obtained without a cut along the following lines:

1. You figure out the Hilbert space $\mathcal{H}$ and Hamiltonian $H$ for the whole shebang: observers, measuring devices, environment, observed system, everything.
2. You pick a coarse-graining for the macroscopic states. This can be done lots of different ways, but you can summarize it by coming up with a set of Hilbert space projection operators $\Pi_j$ corresponding to each macroscopic state, $j$
3. You pick an initial state, $|\psi\rangle$
4. Then the probability of being in macroscopic state $j$ a time $t$ later would be $\langle \psi| e^{iHt} \Pi_j e^{-iHt} |\psi\rangle$

This way of looking at it, QM is a stochastic theory for the evolution of the macroscopic state. So the business of state preparation, observables, etc., would be seen as a shortcut, or rule of thumb.

 

[Lord Jestocost]

Honestly, I scoff at the idea that we live in a causal universe in the first place.

I think its very definition dependent personally - such things always worry me.

Thanks
Bill

Nothing to worry!

„The position is that the laws governing the microscopic elements of the physical World - individual atoms, electrons, quanta - do not make definite predictions as to what the individual will do next. I am here speaking of the laws that have been actually discovered and formulated on the old quantum theory and the new. These laws indicate several possibilities in the future and state the odds on each. In general the odds are moderately balanced and are not tempting to an aspiring prophet. But short odds on the behaviour of individuals combine into very long odds on suitably selected statistics of a number of individuals; and the wary prophet can find predictions of this kind on which to stake his credit - without serious risk. All the successful predictions hitherto attributed to causality are traceable to this. It is quite true that the quantum laws for individuals are not incompatible with causality; they merely ignore it. But if we take advantage of this indifference to reintroduce determinism at the basis of world structure it is because our philosophy predisposes us that way, not because we know of any experimental evidence in its favor.“

Sir A. S. Eddington in „THE NATURE OF THE PHYSICAL WORLD“, Cambridge, 1948

 

[Stephen Tashi]

I believe Hawking is exaggerating in telling us (page 83):
«...the Universe doesn't have just a single history, but every possible history, each with its own probability; and our observations of the current state affect its past and determine the different histories of the Universe, just as the observations of the particles in the double slit experiment affect the particles' past.»
Do you agree?

From the viewpoint of logic and pure mathematics, that's true by definition!

If we take the (metaphysically) controversial viewpoint that correct physics is organized as mathematics, then what "exists" in mathematical problems is certain given information and deductions made from that information. So if there are phenomena that (in the sense of common language) "existed" in the past but are not present or deducible from the current "given" information they don't exist in the sense of being possible subject matter in a mathematical problem. If, from the current given information, we can infer only certain probabilities for a past event, then what exists in the mathematical problem is those probabilities.

(e.g. If we are given only that Bob has 3 apples and Alice has twice as many apples as Bob then how many eggs Bob ate for breakfast does not exist as part of the given information.)

From a classical point of view, one might seek to refute this proof-by-definition experimentally by secretly recording some event and then arguing the contradiction that some other physicist would later treat the event as non-existent. However, the total information for that experiment includes the fact that the event was recorded by us.

From a classical point of view, one could argue from (empirical) induction that observed trees that fall make a sound and have other effects - therefore unobserved trees that fall make sounds and have similar effects even if these effects are unknown in our present state of knowledge. That gives the philosophical satisfaction of being able to say that the past includes "definite but unknown" events. From the mathematical point of view, that affects the existence of given information insofar as axioms based on such an induction add to what can be deduced.

I'll distinguish 3 definitions for the "existence" of the past - or anything else
1) The common language meaning - whatever that is!
2) Existence in the sense of specific given or deducible information in a mathematical problem
3) Existence in the mathematical sense of "given but unknown" (e.g. There exists an x such that...) This would include contexts where a mathematical theory has axioms about unobserved events having definite outcomes etc.

In the sense of 2), the quotation above says that information about the current state of the universe does not allow us to deduce a unique past history, so that unique history does not exist (as given information) in any correct physical theory. Whether the author of that quotation had this interpretation in mind, I don't know.

In the sense of 3) the quotation would assert that a mathematical theory of physics that assumes the past has a definite but unknown state is self contradictory or, at best, no better than a theory that omits such an assumption.

 

[bhobba]

I suspect we agree that philosophy for the sake of philosophy, i.e., without the concordant physics, is of little or no interest to us :-)

Yes.

My issue with philosophy isn't that I think its useless (I do - but that is just an opinion and I acknowledge as always opinions are like bums - everyone has one - but it doesn't make it right) it's I don't often actually even understand it. To me it's not discussing angels dancing on a pin - its discussing what I find often not even understandable. Take complementary - I can't even make sense of it. And that's something pretty basic. We had a philosopher come here discussing his view of MW - to me it was gibberish, and other science advisers thought the same. This makes it really really hard IMHO.

Check this out:
https://www.scienceandnonduality.com/wp-content/uploads/2014/09/einstein_tagore.pdf

When I read it I thought - is this guy for real? Why did Einstein even waste his time. To counter this down the ANU I did a course in philosophy as a postgraduate student for a year at their center for continuing studies. Practically it was good because it let me have official access to their library - but nobody really worried anyway - they more or less let anyone go in and read. I mentioned a lot of things I thought very profound like Noether's Theorem - but was consigned to the merely material. The maddening thing was my teacher, Petra was her name, had a very keen mind. So they are actually quite intelligent - but on a different wavelength.

I have come to the conclusion its simply, to paraphrase another saying, Physicists are from Mars, Philosophers from Venus.

Thanks
Bill

 

[bhobba]

This way of looking at it, QM is a stochastic theory for the evolution of the macroscopic state. So the business of state preparation, observables, etc., would be seen as a shortcut, or rule of thumb.

Docoherent histories maybe?

It was what Feynman thought in the end.

Thanks
Bill

 

[Lord Jestocost]

Check this out:
https://www.scienceandnonduality.com/wp-content/uploads/2014/09/einstein_tagore.pdf

When I read it I thought - is this guy for real? Why did Einstein even waste his time.

With all due respect, it seems that you have no idea at all who Rabindranath Tagore was. Your remark is misplaced.

 

[bhobba]

If we take the (metaphysically) controversial viewpoint that correct physics is organized as mathematics,

I don't even know what that means. Do you mean the laws are written in the language of math? If so I agree. If not can you be more specific? Then the rest you wrote likely will be clearer.

Thanks
Bill

 

[bhobba]

With all due respect, it seems that you have no idea at all who Rabindranath Tagore was. Your remark is misplaced.

Could be since I understood nothing of what he said. Of course I looked up who he was and what he did which was of great importance in India and probably elsewhere - but that doesn't change I couldn't understand it at all. Again I emphasize that means nothing - quite possibly just my own shortcomings. I did however understand what Einstein said - do you think Einstein understood him?

Thanks
Bill

 

[Stephen Tashi]

Do you mean the laws are written in the language of math?

Yes, and further (from that viewpoint) what "exists" in a physical situation is exactly what exists in the correct mathematical formulation of that situation. So if information about something doesn't exist in the mathematical formulation (e.g. which slit the particle went through) then this something does not exist physically. (I'm not saying this is my personal viewpoint, but I think it is a defensible viewpoint.)

 

[bhobba]

Yes, and further (from that viewpoint) what "exists" in a physical situation is exactly what exists in the correct mathematical formulation of that situation. So if information about something doesn't exist in the mathematical formulation (e.g. which slit the particle went through) then this something does not exist physically. (I'm not saying this is my personal viewpoint, but I think it is a defensible viewpoint.)

I liked it because I now understand what you said - or think I do anyway. But I hold a different view.

My view is the connection between applied mathematics and what its applied to is hard to pin down. That's the understanding part of a theory - it takes a while to build up. Now let's stay on topic with this thread. In the delayed choice experiment on the surface it looks like something really weird is going on - but when analysed carefully that isn't he case:
http://quantum.phys.cmu.edu/CQT/chaps/cqt20.pdf

The purpose of the math is to keep our thinking clear and on the straight and narrow. It turns out our laws, for reasons I do not know, are written in the language of math. Its very strange that God is a mathematician - but he seems to be. Its so strange I believed at one time in Penrose's view - but changed my mind when I realized what a strange view I was led to - but it is just so seductive.

Thanks
Bill