Friday, September 12, 2014

Are parallel universes real?

Since I gave my Google Tech Talk on quantum mechanics nearly four years ago I get a steady stream of email from people asking questions about it.  There's one question I keep getting over and over which I thought I'd just try to answer once and for all so I could point people to this answer instead of improvising a new one from scratch every time.

Note that this blog post assumes that you have either watched the video or read the associated paper.  If you haven't, what follows will probably not make a lot of sense.

The question I keep getting is some variation on the following theme: What is the relation of the QIT/zero-worlds interpretation of QM to interpretation X, where X is usually many-worlds, but is sometimes relative state.  Riffing off this I'll get questions about the implications of QIT for time-travel, the relationship of QM to consciousness, and whether or not we might be able to influence the results of quantum measurements with our minds.

The short version of the answer is: QIT/zero-worlds is nothing more than a different way of looking at the math than what is usually presented in the popular press.  It is a way of looking at the math that makes sense to me (and apparently, based on the feedback I get, makes sense to a lot of other people as well).  But that is all it is.  There is no breakthrough here (except, perhaps, a pedagogical one).  It turns out that all this stuff was actually known as early as the 1930s.  Why Feynman was still saying that no one understood quantum mechanics in the 1960s I do not know.  It is certainly not true today.  But the point is that despite the somewhat sensational rhetoric ("You don't really exist; you are living in a simulation running on a quantum computer") nothing really changes as a result of QIT except your perspective.  You are still every bit as real (or not) as you were before.  Time travel, ESP, and telekinesis are still every bit as impossible as they were before.

The other short version of the answer is that many-worlds/relative-state/whatever are all equally valid ways of looking at QM.  The only one that isn't equally valid is Copenhagen.  To be sure, Copenhagen is a reasonable approximation to the truth for many practical purposes, just as Newtonian mechanics is a reasonable approximation to the truth (which is, to the best of our current knowledge, general relativity) for many practical purposes.  But Copenhagen is conceptually wrong, just as Newtonian mechanics is conceptually wrong.  There is no "force of gravity" and the wave function never collapses.  The challenge is to explain why it appears to do so.  That is what QIT does (IMO).

Let's take a moment to review the problem that QIT (and other interpretations of QM) purport to solve: QM is one of the two most successful scientific theories ever (the other being GR).  No experiment has ever disagreed with a prediction made by QM.  However, the mathematics of QM seem to be fundamentally at odds with the apparent nature of reality.  The Shroedinger equation is continuous, deterministic, and time-reversible.  Moreover, it describes a world where objects can exist in superpositions of states, a phenomenon which can be experimentally demonstrated through interference experiments.  By way of contrast, the world appears to consist of material objects which at all times exist in some particular state and never in a superposition.  Moreover, the process of making a measurement appears to be discontinuous, non-reversible, and also involves some fundamental randomness which is nowhere to be found in the Shroedinger equation.  The apparent contradiction between the theory and the manifest nature of reality has historically been called the "measurement problem."

QIT solves the measurement problem by observing that you can describe measurement as a purely quantum process.  When you do this, the following facts emerge (and this is what the Google tech talk and associated paper are about):

1.  Measurement and entanglement are the same physical phenomenon.  Measurement is nothing more than the mutual entanglement of a large collection of particles (or, to be strictly correct, of systems that manifest themselves as particles under certain circumstances).

2.  Once two particles are entangled, it is not possible to "undo" that entanglement except by bringing the two particles physically together.  If there were any other way to "undo" an entanglement, then it would be possible to transmit information faster than light.

3.  The apparent randomness that results from a quantum measurement is just that: apparent.  In actual fact, the entropy of a system that has undergone a quantum measurement does not change.  The reason that there seems to be randomness is that when you draw a line between the particle being measured and the measurement apparatus, you end up with positive entropy (i.e. randomness) in the measurement apparatus and corresponding negative entropy in the particle being measured (which is possible because the state of the particle is a complex number).

4.  The reason that two measurements made on the same physical quantity produce the same result is not that the measurements are a faithful reflection of some underlying physical (or metaphysical) "element of reality" as Einstein put it.  Instead, if you look at the quantum mechanical description of two separate measurements on the same system what you end up with is a mathematical description that looks exactly the same as two classical systems in classical correlation with each other, but that says nothing about the actual state of the system being measured (except that it is now entangled with the measurement apparatus).

5.  The apparent non-reversibility of a measurement is likewise not fundamental, but merely practical.  Reversing a measurement is possible in principle, but to reverse a measurement, you have to reverse all of the entanglements that produced that measurement to begin with.  Reversing even a single entanglement is extremely difficult.  Reversing a macroscopic number of them (and you really do have to get them all, every single last one), while possible in principle, is not possible in practice.

In other words, there is no measurement problem.  All of the apparent contradictions between the mathematics of QM (continuous, deterministic, time-reversible) and measurement (discontinuous, random, non-reversible) can be understood purely in terms of quantum mechanics itself.  Furthermore, all of this (except possibly the bit about negative entropies) was known in the 1930s.  So why has QM been considered so intractably mysterious for so long?  Indeed, why is QM *still* considered by many to be intractably mysterious?

I don't really know, but I suspect it's because people don't want to accept what the math is telling them.  The math says, essentially, that you don't really exist (or, if you prefer, your existence is not unique -- it turns out these are two equivalent ways of saying the same somewhat ineffable thing).  This is not the first time this has happened.  The exact same kind of conceptual stumbling block delayed the discovery of relativity for decades.  The fact that Maxwell's equations predicted the existence of electromagnetic waves moving at a fixed velocity c was known in the mid-1800s.  But no one took this seriously until 1905, because it was just obvious that time and space are absolute and so there just had to be some fixed medium through which electromagnetic waves propagated and relative to which the predicted speed c was to be measured.

The similarly obvious (but nonetheless false) assumption that everyone gets hung up on today is that the universe is, in point of metaphysical fact, what it appears to be: the whole of creation, populated by material objects that exist in particular places at particular times.  The answer to the puzzle: how can such a universe arise from quantum mechanics is, quite simply: it doesn't.  It appears to, but this is an illusion.  To be sure, the illusion is quite compelling, but it is false.  It is every bit as false as the illusion that space and time are two distinct things (which can also, it should be reiterated, be quite compelling).

It is worth pointing out that the fact that the underlying truth is very different from what we naively perceive it to be is evident long before you get to quantum mechanics.  You think that the chair you are sitting on is a solid object, but in fact it is mostly (>>99%) empty space.  The reason is appears to be solid is that the electrons in the outer shells of the atoms that make up the chair repel the electrons in the outer shells of the atoms that make up your body (or your pants).  So even in a pre-Shroedinger world, things are very different than they appear.

OK, so atoms aren't solid, but they are still (in a post-Rutherford but pre-Shroedinger world) classical.  They exist at definite places at definite times.  It makes sense to distinguish this particular hydrogen atom that is part of a water molecule in your little finger from that hydrogen atom which is undergoing nuclear fusion in the core of the sun.  It is obvious that atoms are classical material objects.  We can even take pictures of them and move them around nowadays.  The evidence that atoms are classical is overwhelming.  How could it not be true?

Well, it's not true.  Not only is it commonplace nowadays to take pictures of atoms and move them around, it is also commonplace to do interference experiments with them.  And not just atoms, but enormous molecules have been observed to interfere.  And yet, it is obvious (and at this point that phrase should be ringing alarm bells in your head) that somewhere between a buckeyball and you there must be a line where the world really does become classical because it is obvious that you are classical.

Sorry to be the one to break this to you, but you're not.  The evidence that you are classical is indeed overwhelming, just as the evidence that space and time are absolute is overwhelming.  But in fact neither is true.  The reason you can take a picture of an atom is not that the atom is really there, but because in the process of taking the picture your camera becomes entangled with the atom.  Then, when you look at the picture, you become entangled with the camera.  The reason you think that there's an atom there is because you are a large system of mutually entangled particles, hence quantum mechanics predicts that any particular part of you will behave as if it were a classical system in classical correlation with every other part of you.  The net result is a system where every piece of it agrees that there is (or is not) an atom there.  And asking your fellow humans to corroborate your intuitions doesn't help, because they too are large systems of mutually entangled particles, and as soon as they look at the same picture you have looked at, they too become entangled with it and with you and with the original atom, and so every part of that system (you plus your collaborators) will agree that there was an atom there (or not).

So is the atom "really" there?

The problem with this question is that it seems like the answer should be either "yes" or "no", but this too is false.  The nature of this question is more like this one:

Was Darth Vader (or, if you prefer, Anakin Skywalker) "really" Luke's father?

One the one hand, it seems that the answer should be "yes" because, in the Star Wars universe, Anakin/Vader was Luke's father.  But, of course, the Star Wars universe is fictional, so what does it mean for a fictional character to "really" have any particular attribute?

The answer, IMO, is to simply observe that fictional characters like Luke Skywalker and Harry Potter are in a different "ontological category" from (classically) real things like George Lucas or J.K. Rowling.  Well, the quantum wave function is also in a different ontological category than classical reality.  Fiction "emerges" from (classical) reality in much the same way that classical reality "emerges" from the wave function.  (The reason I hedge with "much the same way" is that there is one important difference: fiction and classical reality can both be described as classical computational processes, i.e. the math involves only real numbers, whereas the quantum wave function can only be described with complex numbers.  So the process by which classical reality emerges from the wave function is mathematically different (but conceptually similar) from the process by which fiction emerges from classical reality.)

So is the atom "really" there?  Well, to you it is.  It is every bit as real as you yourself are, and for the exact same reason: because the atom is part of the system of mutually entangled particles of which you are a part.  (This is sometimes called the "relative state" interpretation of QM.)

But let's take a different example.  Instead of asking whether the atom is really "there" let us ask instead if one of its electrons is "really" spin-up or spin-down (or, equivalently, if some photon it emits is "really" polarized vertically or horizontally).  You measure it, and the result is spin-up.  Your friend measures the same electron and agrees, yep, it's spin-up.  So you and your friend have become mutually entangled with this electron and hence are behaving just like a pair of classically correlated classical systems, just as QM predicts.

But, while QM predicts that you will be classically correlated, it does NOT (and cannot) predict what the outcome of your measurements will actually be.  That can only be done probabilistically, which seems at odds with QM (which is, if you will recall, purely deterministic).  To understand this we have to dig a little deeper into the math.  I've hinted at this before when I said that in order to extract a description of the classical world from the wave function you have to "trace over certain degrees of freedom".  That is just a fancy way of saying, "discard some of the information about the system."  Consider the full QM description of a particle that has been measured.  Part of that description is the state of the particle, and the other part is the description of the measurement apparatus.  To extract the state of the measurement apparatus you "trace over" (i.e. discard) the parts of the description that describe the state of the particle being measured.  What you are left with is not one classical world, but two: one in which the measurement apparatus says spin-up, the other in which it says spin-down.  But (and this is the crucial point) in neither of these descriptions is the spin of the particle actually spin-up or spin-down.  It can't be.  There is no description of the state of the particle being measured, because we had to throw it out in order to extract a description of (something that looks like) a classical universe, and that actually turns out to be a description of two classical universes.  That is where the "multiple worlds" interpretation comes from.

So do these universes "really exist"?  Again, in my opinion that's like asking whether Darth Vader is "really" Luke's father.  Classical universes are what you get when you take the quantum wave function and throw out parts of it.  That is the mathematical fact.  You can interpret this mathematical fact however you choose, with one exception: you cannot reasonably conclude that the classical universe that you live in is "all there is" because a complete description of the (classical) state of the universe is only, and can only ever be, a partial description of the underlying quantum state.

So what about all those other universes?  Are they real?  Well, from the perspective of the quantum wave function, yes, they are.  A classical universe is just a "slice" of the wave function (i.e. the whole wave function with parts of it discarded) and the wave function doesn't care which way you slice.  It's rather like if someone wrote an alternate Star Wars universe where Darth Vader was not Luke's father.  The existence of such an alternate Star Wars universe would have no bearing on whether Darth Vader was Luke's father in the original Star Wars universe (the answer there would remain "yes") nor would it have any bearing on whether Darth Vader was Luke's father in the "real" universe in which both Star Wars universes were embedded (as fiction): the answer there would remain that the question is meaningless because mixing ontological categories makes no sense.

David Deutsch, for whom I have the utmost respect (I think he's actually one of the best popularizers of science ever) is a fierce proponent of the proposition that all classical universes are equally real.  I respectfully disagree with him.  It is true that they are all equally real from the perspective of the wave function.  But I don't have the perspective of the wave function, and neither do you.  You and I live in this universe, and so to us, this universe is more real than any of the other myriad universes that emerge from the wave function.  There may be a transporter in the Star Trek universe, but that doesn't help Luke Skywalker escape from Emperor Palpatine because Luke can only take advantage of (and hence only cares about) what exists in his universe.

What about the possibility of communicating between universes?  Wouldn't that be cool?  If those universes are "as real as we are", shouldn't that be possible?  Well, unfortunately, no, it's not.  The way in which classical universes emerge from the wave function makes communication between them impossible.  You can prove this mathematically, just as you can prove that quantum entanglement can't be used to send information faster than light.  This is another reason I believe that parallel universes can safely be regarded as less real than our own universe, at least by us.  But reasonable people can (and do) disagree.

There's a lot more to say about this topic, but this post has already become longer than I intended it to be.  I'll write more if there's interest, but I want to leave you with a parting thought (well, more of an exercise actually): remember that I said that measurements were in principle reversible.  Imagine that we could actually carry out this program of undoing the myriad entanglements that constitute your making a particular observation.  What would be the subjective sensation, i.e. what would it "feel like" if this were done to you?

89 comments:

Don Geddis said...

I'm with Deutsch and the multiple-world interpretation, but most of your lengthy post doesn't actually distinguish between QIT and MWI. You spend the first section on quantum vs. classical, and then some on how Copenhagen (the most popular!) doesn't actually work.

But for people who are past all that, and just looking to choose between QIT and MWI, is it merely a matter of intuitive comfort? If I'm comfortable with MWI, would there be any additional reason for me to consider switching my conceptual framework to QIT instead?

Ron said...

> But for people who are past all that, and just looking to choose between QIT and MWI, is it merely a matter of intuitive comfort?

Well, it is a matter of intuitive comfort since the difference between QIT and MWI is purely rhetorical, but I'd take issue with your use of the word "merely". The question of whether or not I actually exist, or whether or not an infinite number of copies of me actually exist (whatever that could possibly mean) matters a lot to me.

To me the biggest conceptual hurdle of MWI is not the existence of parallel universes, but the fact that the supposed "shadow particles" that actually affect our universe are precisely those that propagate according to the macroscopic configuration of our universe. That rule strikes me as arbitrary and without any conceptual foundation. QIT has no such problems.

BTW, even Deutsch concedes that multiple worlds is only an approximation to the truth in his latest book:

"Universes, histories, particles and their instances are not referred to by quantum theory at all – any more than are planets, and human beings and their lives and loves. Those are all approximate, emergent phenomena in the Multiverse." [Deutsch, David (2011-03-31). The Beginning of Infinity: Explanations that Transform The World (Penguin Press Science) (p. 292). Penguin Books Ltd. Kindle Edition. p291]

Don Geddis said...

Thanks for attempting to answer my question. I may not be following your words. What does "shadow particles that actually affect our universe are precisely those that propagate according to the macroscopic configuration of our universe" refer to?

I thought MWI was basically just: the wavefunction is "real", it just evolves deterministically, everything is always in superposition, "particles" are distributed throughout space, and (like you say above), the classical world illusion comes because you, the observer, get entangled by the observation. Which necessarily implies that you the observer also remain in superposition, necessarily having seen both outcomes of the experiment. Which is often informally described as "multiple worlds", but as you say from Deutsch, it's really just a single multiverse, and it's just a convenient labelling to call them "multiple worlds".

But that's what I thought MWI was. Your quote from Deutsch doesn't seem in conflict with it, to me. It seems to describe the actual original MWI interpretation.

Ron said...

> What does "shadow particles that actually affect our universe are precisely those that propagate according to the macroscopic configuration of our universe" refer to?

See: http://blog.rongarret.info/2009/04/on-shadow-photons-and-real-unicorns.html

(I know you read that post because you left a comment on it :-)

> that's what I thought MWI was.

This is part of the problem. Everyone agrees on the math. The trouble is translating the math into a natural-language story, so the words matter. The problem with *your* formulation of MWI is:

"Which necessarily implies that you the observer also remain in superposition"

The math clearly says this. So why don't I feel like I'm in a superposition? Why does it feel to me as if the results of quantum experiments are definite but random results?

Deutsch's formulation is different from yours, and his formulation answers that question. His formulation doesn't refer to superposition. According to Deutsch, the multiverse *consists of* an infinite number of classical universes, of which you inhabit one, and copies of you inhabit the rest. The reason the outcomes of quantum measurements appear definite is that they *are* definite in each universe. The reason they appear random is that the outcomes are *different* in each universe, so the outcome you see depends on the universe you are in.

The problem with *that* formulation is that it has a devil of a time explaining interference. (Go pick up a copy of "The Beginning of Infinity" and read chapter 11 to see what I mean.)

BTW, "The Beginning of Infinity" is a terrific read. Highly recommended.

Don Geddis said...

Very helpful, thanks. I went back and re-read that earlier post of yours. As I essentially implied with my comment on that post, I'm not sure that Deutsch's "shadow photons" are the same as Everett's Multiple Worlds. That's why it confused me when you said "the biggest conceptual hurdle of MWI is ... shadow particles".

On the other hand, I accept your followup question: if observers remain in superposition, why does the world feel classical? And why does QM appear random? I agree that those are the primary challenges for an intuitive QM interpretation.

Ron said...

> As I essentially implied with my comment on that post, I'm not sure that Deutsch's "shadow photons" are the same as Everett's Multiple Worlds.

Are we reading the same comment? Because I don't get that at all from what you wrote. (And why don't you think that they're the same?)

> it confused me when you said "the biggest conceptual hurdle of MWI is ... shadow particles".

Well, you edited out some pretty crucial words there :-)

Let me try to make it clearer: the biggest conceptual hurdle in the many-worlds interpretation is describing the rules under which one universe interacts with another. The reason this is challenging is that there is a tacit implication that every individual universe in the multiverse is classical, and that's wrong. Classicality is an approximation, and there's no way around that. At macroscopic scale and 300K it's a damned good approximation, but it's an approximation nonetheless. The instant you insist that particles are real, you lose. (IMO of course. Deutsch would vehemently disagree.)

Don Geddis said...

I haven't (yet) read the Deutsch stuff that you're referring to, so I'm just going off your own description here. But wow, that is not how I would have described MWI to someone else. I would never mention "shadow photons", I completely agree that the classical world is only an approximation, I don't think that particles are local (I assume that's what you mean by "particles are real"). The rules by which one universe interacts with another, are nothing more (or less) than the standard QM math for superposition. Because "world" in "multiple worlds" is just an approximation, perhaps somewhat similar to the way that "species" describes useful categories of life forms (but isn't precise when you get to the edge cases).

But that doesn't sound at all like what you are describing as Deutsch's view, and/or your description of MWI. (And are those two things the same? Do you think Deutsch is describing "standard" MWI?) On the other hand, I'm not quite sure whether the framework I already understood, is the same as what you are calling QIT (given that I don't really understand the details of QIT).

I guess I need to read and study Deutsch's new book, and your QM paper, to see how it relates to what I thought I already understood (which, until now, I believed was "standard MWI").

VMC said...

Just a quick note of thanks, Ron, both for your YouTube video on QIT and your linked paper. I will not pretend to grasp the math at this time, no background in it. But I appreciate your efforts to clarify the issues surrounding QM's application to "reality."

Best wishes from SoCal.

Scott

Ron said...

My pleasure, VMC, and thanks for the kind words. Just out of curiosity, what part of the math do you not think you understand? If you know basic high school algebra and understand what a complex number is then you should be able to understand the basics of QIT.

VMC said...

LOL! Well, let's start with this:
(ΨU + ΨL)/√2

I read the above as "Strange Maserati Emblem" unintelligible PLUS "Repeat Strange Maserati Emblem" lost-me divided by the square root of 2.

I understand that complex numbers allow for exponential expressions to yield negative numbers and that they are useful for describing curves that go into the below-zero realm on the number line.

I do understand multi-dimensionality through working with arrays in programming, but all-in-all, I am an English major who could never have done any math at all had it not been for computers and spreadsheets. In any case, I am studying your paper and will try to get better in the math realm. My hope is to have as clear a view of the implications of QM on the "real" as you seem to have, Ron. In any case, that's about where I am at with it.

Ron said...

Don't let the Greek letters freak you out. The Maserati emblem is just a Greek letter called Psi, but that doesn't matter. You could just as easily call it X. What matters is:

1. It's a function (generally of space and time, but that doesn't really matter. It's a solution to the Shroedinger equation too, but that doesn't really matter either.)

2. Its value is a complex number

3. Its significance is that its value represents the amplitude of finding a particle at a particular place and time, and the amplitude is the "square root" of the probability. You square the magnitude of the amplitude to get the probability.

That's really all there is to it. The square root of 2 is there to make the total probability when you add two wave functions come out to be 1. If you were adding three wave functions there would be a square root of three there instead.

Publius said...

Triple Existential Crisis?

>The math says, essentially, that you don't really exist (or, if you prefer, your existence is not unique . . ..
>.. . . how can such a universe arise from quantum mechanics is, quite simply: it doesn't. It appears to, but this is an illusion. To be sure, the illusion is quite compelling, but it is false

It appears we have a triple existential crisis possibility here
1) one's life is meaningless [the original]
2) one doesn't really exist (or is not unique) [new]
3) one's life is an illusion [new]

Ron, have you suffered any day-to-day angst upon realizing the above? Often, the more knowledge, the more grief.

> In actual fact, the entropy of a system that has undergone a quantum measurement does not change.

Any commentary you can share here regarding a linkage to information entropy? Say in the context of Maxwell's Demon? [see especially reference 7 on the demon page]
Perhaps the measurement apparatus creates a bit (and positive entropy kT ln 2) and the particle creates a negative bit (and negative entropy -kT ln 2)?

P.S. pilot waves...

Ron said...

> Ron, have you suffered any day-to-day angst upon realizing the above?

No, absolutely not. The exact opposite, in fact. For starters, I don't believe #1, and never have. As for 2 and 3, they don't really change anything. Just because I happen to know that I'm made of bits instead of things doesn't make me any less real with respect to the things I care about. If anything, coming to understand QM has brought a rather profound (if subtle) sort of inner peace because now I feel like I know the Mind of God (speaking metaphorically of course).

> Perhaps the measurement apparatus creates a bit (and positive entropy kT ln 2) and the particle creates a negative bit (and negative entropy -kT ln 2)?

Yes, that is in fact exactly what happens. Did you not read the post? It's point #3:

"3. The apparent randomness that results from a quantum measurement is just that: apparent. In actual fact, the entropy of a system that has undergone a quantum measurement does not change. The reason that there seems to be randomness is that when you draw a line between the particle being measured and the measurement apparatus, you end up with positive entropy (i.e. randomness) in the measurement apparatus and corresponding negative entropy in the particle being measured (which is possible because the state of the particle is a complex number)." [Emphasis added]

Publius said...

>> 1) one's life is meaningless [the original]

> I don't believe #1, and never have.

You've been holding back on us. Forget QIT. Answer this question:

What is the Meaning of Life?

Ron said...

> You've been holding back on us.

No, just because I believe life has meaning doesn't mean that I know what that meaning is. (Look up "non-constructive proof.") And even if I knew the meaning of life, it does not follow that I can articulate it in a way that someone else could understand. It's challenging enough explaining quantum mechanics. That's a cakewalk compared to the meaning of life.

Nonetheless, I'll take a quick whack at it here. For a much better answer read David Deutsch's book, "The Beginning of Infinity".

First, the word "meaning" has two meanings (see?) and I presume you mean meaning as in "purpose" rather than meaning as in "definition". So to avoid ambiguity, I'm going to use the word "purpose" rather than "meaning" (though it turns out that the purpose of life is found in its definition -- see below).

Second, the question itself is ambiguous. It can mean "What is the purpose of life in general?" Or it can mean, "What is the purpose of human life?" Or it can mean, "What is the purpose of *my* life (or *your* life)?" I think the last question is the one most people really care about, but that, of course, is the hardest one to answer. Personally, I believe the answer to the last question can be found in the answer to the first, and that the purpose of life is to be found in its definition: Life is (by definition) the reproduction of information, and IMO that is also its purpose. What makes this answer more profound than it appears to be at first glance is that it turns out that not all information is created equal. Some information is "better" than other information because it actually corresponds to the way the world actually works, and so it can be used to manipulate one's environment in the service of goals, and so it can be used to promote the reproduction of information, i.e. life.

A more poetic way of putting it: life is a process, and its purpose is simply to participate in it. But to fully participate in it you have to *understand* it, and that makes it profound and interesting and meaningful.

You asked.

stechert said...

Shadow particles: qm :: epicycles: Copernicus. By the way, I think it's time to kill the Buddha.

VMC said...

Life has one intrinsic meaning: love. No other thing makes life worth the time other than love. So that is the best purpose or meaning to life I can dowse out of it.

Love. Figure shit out the best you can. Have fun doing it. That's it. If you need more meaning than that, make some good shit up and build a temple in Salt Lake.

Publius said...
This comment has been removed by the author.
Publius said...

The Meaning Of Life

>A more poetic way of putting it: life is a process, and its purpose is simply to participate in it. But to fully participate in it you have to *understand* it, and that makes it profound and interesting and meaningful.

That is very rational and well-reasoned.

You put yourself in good company. It reminds me of the message of Ecclesiates, traditionally credited to Solomon (traditionally, the wisest man who ever lived). I would summarize the message of Ecclesiates as "enjoy life and obey God's commandments." Ref. Eccl 9:9-7, 12:13

Of course, the real meaning of life is~`!.&
LOST CONNECTION

Ron said...

> That is very rational and well-reasoned.

Why, thank you! :-)

Don Geddis said...

Ron: Finally had a chance to read "Beginning of Infinity" over the holidays. Good stuff, although most of it (by now) wasn't especially new to me.

I encountered two big surprises that I didn't expect. Near the end, he's very critical of Jared Diamond's "Guns, Germs, and Steel" (and the subsequent "Collapse"), which is a book that I was very impressed by. The criticisms I had heard before were about Diamond unscientifically dismissing possible group/racial IQ differences (because the subject is painful and politically incorrect, not necessarily because it isn't a piece of the puzzle). But that's not what Deutsch complains about. I don't think Deutsch quite convinced me, but he has left me a good deal more uncertain about Diamond's thesis.

The second, was his chapter on the benefits of plurality voting ("winner take all"), over what seems intuitively to be a more "fair" proportional voting system with a parliament. And the benefits of a "two-party system", vs. again the intuitively appealing benefits of a strong multi-party culture.

I had never heard a perspective defending two-party plurality, and criticizing multi-party proportional parliaments. Very enlightening.

(Of course, the main topic of the book is science and explanations and memes, but that's old ground for us here.)

Natasha Dunlop said...

A bit of a tangent, but connected, could i ask if you guys might give us some comments on a face book conversation?


Natasha Dunlop’s idea for how the universe came into being;

Closed Door Universe Idea

Once observed the multiverse collapses and resolves to a single event pathway leading to the observation.

For anyone who wants to read more;
see next comment

Natasha Dunlop said...

....Continued for anyone who wants to read more;
I first posted it on Online Philosophy Club in September 2013.

17 Sept 2013

This is a response to the question of how the universe emerged within the extremely narrow parameters which enable it to support life.
This stab at an answer suggests that the only outcome of a quantum multiverse can be life, and that no other outcome is possible.
We look back through time and space attempting to see the origins of our universe and ultimately our own existence. We observe a clear linear path, a single universe flowering from a surprisingly fortuitous set of universal constants, and unfolding step by step with each node and path leading onwards towards greater complexity and life.
Imagine that the clear path that we look back upon, is only one of many, of a multitude of paths covering every option conceivable, every possible fluctuation, but that now, crucially to this idea, every one of these paths has been closed and is invisible to us, except for one.
In this idea I envisage the universe developing as a quantum multiverse, in which all available possibilities are explored and taken simultaneously, and multiple universes exist alongside one another. This multiverse state exists only initially, I envisage it as being unstable. The event which causes it to crystallize, to resolve, is the act of observation / recording.
Once observed the multiverse collapses and resolves to a single event pathway leading to the observation.
To the observer no other pathway existed, and the single observable pathway appears to have been driven by chance and probability.
What this infant hypothesis does not yet attempt to state is the exact nature of the observation. Is the simple perception of an early creature's environment enough to resolve the quantum universe, or does it need to be a recording of the universal constants, in which case the development of highly intelligent life and complex culture is required?
Is the resolving event a single event resulting in a single universe, or is it resolved multiple times by different creatures / cultures / individuals?
Is it possible that further universal quantum doors close within the time frame of the life of an individual, and if so is that individual's perception and memory of their past altered in some way?
This is a very early idea, wide open with further questions. I welcome your thoughts.

Natasha Dunlop said...

The comments in the face book conversation;

.... Richard Woods commented;

That sounds like a reasonable hypothesis, there's certainly a lot of evidence that quantum effects are materialised at the macro scale. That the cosmic scale would also be subject to quantum effects seems to follow. This reminded me of a very good Google Tech Talk about quantum mechanics which starts by describing the well known double slit experiment and goes on to explaining quantum entanglement and quantum erasers. The conclusion is what your hypothesis reminded me of. https://www.youtube.com/watch?v=dEaecUuEqfc


.... I commented;

Thanks for reading and responding to my idea Rich. Great link. I see that Ron Garrett has a blog - blog.rongarret.info - and I might see if he might comment on our thoughts. Seeing link my guess is that he will disagree with the idea, seeing it based on Copenhagen in terms of the element of collapse / crystallisation. He certainly sees one classical universe as untenable doesn't he, in favour of multiple or zero, whether he would see final outcome of hypothesis as a single classical, or as a quantum? Anyway I'll try and ask.

Another quote from the link that links to idea and measurement;
Maths tell us that "Measurement is a continuum. It's not a dichotomy".
Guess this feeds into the who what can be the observer, and does it make the idea of an observation leading to crystallization of a single universe less plausible?

Ron said...

> could i ask if you guys might give us some comments on a face book conversation?

Not quite sure what sort of comment you're looking for here. But you might find this interesting:

http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html

Don Geddis said...

@Natasha Dunlop: I suspect you guessed correctly, that your theory is built on the Copenhagen interpretation, which attempts to make consciousness and/or conscious observation "special". But that's probably the worst and least plausible of all QM interpretations. (As you yourself say: "...does not yet attempt to state ... the exact nature of the observation." That's an alarming weakness, for something that is so central to your theory.)

Nonetheless, the intuitions behind your theory remind me a lot of the Anthropic principle. You may find it interesting that it's a feasible candidate for explaining some apparently arbitrary parts of this universe ... but it doesn't require quantum mechanics at all!

Natasha Dunlop said...

Many thanks Ron and Don for your comments and for your suggested links and further reading.

Peter Donis said...

I'm coming to this thread very late, via a link from a discussion on Hacker News, to post a few comments/questions on aspects of physics discussed in this article.

The only one that isn't equally valid is Copenhagen.

I don't think this is correct. The Copenhagen interpretation uses the same underlying mathematical machinery and makes the same predictions for all experimental results as all of the other interpretations of QM. So it's just as valid given our current state of knowledge.

What's different about Copenhagen is that it leads to different expectations about what a future theory that has current QM as an appropriate approximation will look like: Copenhagen leads you to expect that such a theory will have an explicit collapse (i.e., non-unitary process) somewhere in it (as in, for example, the GRW stochastic collapse model), whereas interpretations like the MWI lead you to expect that no such thing will ever be part of a more complete theory. But we don't currently have any way of testing these possibilities by experiment.

To extract the state of the measurement apparatus you "trace over" (i.e. discard) the parts of the description that describe the state of the particle being measured.

I don't think this is correct. You trace over degrees of freedom that are not involved in the experiment (for example, degrees of freedom describing the environment--this is done in analyses of decoherence, for example), but that's not the case for the degrees of freedom that describe the particle being measured.

If you are measuring one of a pair of entangled particles, then you would trace over the degrees of freedom that describe the other particle, the one you are not measuring; but you wouldn't trace over the degrees of freedom that describe the particle you are measuring.

That is where the "multiple worlds" interpretation comes from.

The MWI does not require you to trace over the particle being measured. The two classical worlds emerge from decoherence of the two branches of the superposition after the measurement interaction entangles the particle and the measuring apparatus, and the apparatus then becomes entangled with its environment. And the reason you can "undo" measurements in quantum eraser experiments is precisely that you don't allow decoherence to occur--you have to keep the measured system and the measuring apparatus isolated from everything else.

I'll leave further comments in your follow-up article as well.

Ron said...

> The Copenhagen interpretation uses the same underlying mathematical machinery

No, it doesn't. Copenhagen holds that wavefunction collapse is an actual physical process, which is non-unitary, and hence takes the Born rule as an *axiom*.

> and makes the same predictions for all experimental results

That's not quite true. It's is true that Copenhagen makes the same predictions for all systems where the end result is a measurement on a single particle. But it fails for measurements on entangled particles, not mathematically, but *rhetorically*. Copenhagen barely even acknowledges the *existence* of entangled particles, treating them as an intellectual curiosity rather than as the central physical phenomenon that they actually are. If you don't believe me, look at the third volume of the Feynman lectures. He doesn't mention entanglement *at all*. (This is ironic considering that Feynman went on to invent quantum computing, though that happened 20 years later.)

> > To extract the state of the measurement apparatus you "trace over" (i.e. discard) the parts of the description that describe the state of the particle being measured.

> I don't think this is correct.

Of course it is correct. If you have a description D of a system S that consists of two parts, P and A, and you want to know what is the state of A then you have to discard the part of D that describes P, which leaves you with the description of A. How could it possibly be otherwise?

> The MWI does not require you to trace over the particle being measured. The two classical worlds emerge from decoherence

Semantics. Tracing is a mathematical operation, decoherence is a physical one. "Decoherence" is just the name given to the process of creating a large network of mutual entanglements. Tracing is the math you do in order to describe the state of a part of such a system.

Peter Donis said...

Copenhagen holds that wavefunction collapse is an actual physical process, which is non-unitary, and hence takes the Born rule as an *axiom*.

So do all of the other interpretations. I know there have been attempts to derive the Born rule in the context of, for example, the MWI, but I'm not aware of any that have been successful.

it fails for measurements on entangled particles, not mathematically, but *rhetorically*.

There is no such thing as a "rhetorical" prediction. The mathematical prediction is the prediction. The "rhetorical" part is just a story in ordinary language that some people choose to tell about what's going on. It's not part of the theory and it's not part of the prediction.

If you have a description D of a system S that consists of two parts, P and A, and you want to know what is the state of A

This is not what's going on in a measurement. You want a description of both P and A, because that's how you know that the measuring device correctly reflects the state of the measured system.

If, as I said, you are measuring one particle of an entangled pair, then you have a description of a system S that consists of three parts, P1, P2, and A. You trace over P2 (the particle not being measured) in order to make predictions about the outcome state of the subsystem P1 x A.

Don Geddis said...

Peter: "The mathematical prediction is the prediction. The "rhetorical" part is just a story in ordinary language that some people choose to tell about what's going on. It's not part of the theory"

I'm not sure that I understand what you're trying to say here. Don't all the competing "interpretations" of quantum mechanics, share the same underlying mathematics? They all agree on the QM equations, they all agree on the "mathematical predictions". That isn't what this discussion is about.

The only thing they disagree about, is the "rhetorical ... story in ordinary language". Isn't that what we're discussing here? How to intuitively think about the mathematics?

If you want to reduce the discussion to mere mathematics, then why are you even talking about "Copenhagen"? Copenhagen adds no new mathematics to QM theory, so if you only want to discuss mathematics, you can't even have a conversation about Copenhagen.

Don Geddis said...

Also, Peter: "Copenhagen [is] just as valid given our current state of knowledge."

I don't think that's true either. You seem to be relying on the theory being "consistent" with currently known evidence. The requirement of consistency is certainly necessary for a plausible theory ... but it's hardly sufficient. (There are obviously an infinite number of theories consistent with any possible data set, but they aren't all equally plausible.)

If all you require is the lack of falsification, then you're vulnerable to claims like Russell's teapot.

Or, to give a closer analogy, consider the theory that all photons emitted by our sun, in a direction opposite to the earth, simply vanish from the universe and violate conservation of energy. We have no way of observing any of these photons, so we can't get any concrete evidence to distinguish this silly theory from our current theories of physics. And one can imagine some future luck, perhaps, where maybe there is a mirror that happens to be placed 1000 light years away, which might reflect some of those photons back to earth. So you might posit that someday, in the future, we actually will observe evidence about whether those sun-emitted photons away from the earth just vanish from the universe, or else act like every other photon we've observed.

But it is not the case that, until we observe such a luckily-placed mirror, we must necessarily conclude that the "photons vanish" theory of physics is just as plausible as the standard one, merely because of consistency with current evidence. That is far too weak a standard.

The Copenhagen interpretation is in a similar situation. Yes, it matches the QM equations for those situations we can currently test. And then it also posits a ridiculously complex and implausible mysterious new physics that is untestable, on the basis of exactly zero evidence.

There are many, many reasons to reject Copenhagen ... and being "consistent" is hardly a strong enough endorsement to overcome the drawbacks.

Ron said...

@Peter:

> This is not what's going on in a measurement. You want a description of both P and A, because that's how you know that the measuring device correctly reflects the state of the measured system.

Ah. There's yer problem right there. :-)

You say you want a description of both P and A. And you can have that, but it will be a quantum description, i.e. it will be Schroedinger's cat.

What you really want (or at least what most people really want) is a *classical* description of both P and A, i.e. a description that simultaneously says "A in in the indicating-spin-up state" and "P is in the spin-up state". But you can't have that. The only way you can get a classical description out of a quantum one is to ignore (i.e. trace over) part of the system. If your system is fully decomposable into A and P and you want a classical description of the state of A, then you have no choice but to ignore the state of P because that's all there is.

> There is no such thing as a "rhetorical" prediction.

Of course there is. Here is an example, quoting from Feynman:

"If an experiment is performed which is capable of determining whether one or another alternative is actually taken, the probability of the event is the sum of the probabilities for each alternative."

This is a rhetorical prediction because it does not define what is meant by "an experiment is performed." It is precisely this vaguery that leads to all the hand-wringing over Schroedinger's cat, Wigner's friend, delayed-choice experiments, and quantum erasers. At the extreme, if you take a common but informal definition of "performing an experiment" then you end up with the EPRG paradox.

Feynman's quote is one of the most prominent manifestations of the Copnehagen interpretation, and it is wrong. It is a reasonable approximation to the truth in a narrow set of circumstances (single unentagled particles and an overly restrictive notion of what an "experiment" is), but conceptually it is 100% wrong.

Peter Donis said...

Don't all the competing "interpretations" of quantum mechanics, share the same underlying mathematics? They all agree on the QM equations, they all agree on the "mathematical predictions".

Yes, that's my point.

That isn't what this discussion is about.

I think it is. The title of this post was "Are parallel universes real?" You can't say something is real unless the mathematical predictions (provided they agree with experiments) are only compatible with it being real. But if there are multiple interpretations of QM that all agree on the mathematical predictions, and some say parallel universes are real while others don't, then you can't say they're real. You can only say your preferred interpretation says they are.

If you want to reduce the discussion to mere mathematics, then why are you even talking about "Copenhagen"?

Because Ron did in the article; he said it wasn't a valid interpretation. I'm saying it is. Ron thinks it isn't because it "fails rhetorically"; but my point is that there is no such thing as "fails rhetorically". Copenhagen, as you agree, uses the same math and makes the same predictions as all other interpretations. That means it's as valid as any other interpretation. If you don't happen to like the "rhetoric" that Copenhagen uses, that's a matter of personal preference, not physics.

Yes, it matches the QM equations for those situations we can currently test. And then it also posits a ridiculously complex and implausible mysterious new physics that is untestable, on the basis of exactly zero evidence.

No, it doesn't. The Copenhagen interpretation is just an interpretation of QM. It doesn't make any predictions that QM doesn't make.

If there are people who are using something they call "Copenhagen", but claiming it predicts new physics beyond what the QM math predicts, then their "Copenhagen" is not an interpretation of QM; it's a different theory (because it makes different predictions). In principle we could distinguish the two theories (standard QM and whatever "Copenhagen" such people are using) by doing an experiment for which the two make different predictions. But if such a "Copenhagen" exists, it's not the one we're talking about in this discussion, since we have all agreed (I think) that the "Copenhagen" we are talking about is an interpretation of QM and makes the same predictions that QM makes (and no others).

Peter Donis said...

You say you want a description of both P and A. And you can have that, but it will be a quantum description, i.e. it will be Schroedinger's cat.

Until it's measured, yes. After measurement, no. After measurement, I can make predictions about all future measurements that I can possibly make by using the state of the system P x A that describes the result I observed. That's what the math of QM says to do.

What you really want (or at least what most people really want) is a *classical* description of both P and A, i.e. a description that simultaneously says "A in in the indicating-spin-up state" and "P is in the spin-up state". But you can't have that.

Sure, I can. I just described how.

What you would like to say here is that the state of the "full system" has multiple terms, each of which describes a P eigenstate and the corresponding "indicator" state of A; and that the state I described above is not the "right" state to use, because it only includes one term of that "full state". However, you have no experimental evidence for this; you only say it must be that way because you believe that unitary evolution is never violated. But our actual evidence doesn't say unitary evolution is never violated; it only says unitary evolution isn't violated in reversible processes. We do not have any evidence that irreversible measurements are also unitary.

This is a rhetorical prediction because it does not define what is meant by "an experiment is performed."

Not in the quote you gave; but the standard definition in QM is that an "experiment is performed" when an irreversible interaction takes place.

I realize that many people working in quantum information theory don't talk this way, but the key processes studied by quantum information theory that give rise to all the talk about things like "parallel universes" are reversible; that's the whole point. If you can "quantum erase" an interaction, then the interaction is reversible. But such an interaction, from the standpoint of "collapse" interpretations like Copenhagen, doesn't even count as a "measurement" (or as an "experiment being performed", in Feynman's language). Yes, that means that "measurement" and "experiment" are very bad terms to use in this connection, since the reversible interactions that happen inside quantum information labs certainly meet the ordinary person's definition of "measurements" or "experiments". But that's an issue of language and terminology, not physics.

You might also be tempted to say that, from the standpoint of an interpretation like the MWI, all interactions are in principle reversible. I agree that, from the standpoint of the MWI, that's true; but we certainly do not have experimental evidence that all interactions are reversible. We only have experimental evidence that particular tightly controlled interactions in systems that are carefully isolated from their environments are reversible.

Peter Donis said...

If all you require is the lack of falsification, then you're vulnerable to claims like Russell's teapot.

Rather than try to respond to this here, I'll just point at an article I wrote on Physics Forums:

https://www.physicsforums.com/insights/fundamental-difference-interpretations-quantum-mechanics/

In short: the reason people differ so strongly about QM interpretations is that there is no intepretation of QM that doesn't require you to give up some strongly held intuition. Different people choose different ones to give up.

Don Geddis said...

@Peter: "You can't say something is real unless the mathematical predictions are only compatible with it being real."

I disagree completely. Math is never "only compatible" with anything being "real". I think you're confusing arguments at different levels. Math is a model, and the model makes predictions, and you compare the model's predictions to observations of reality. But whether the model's entities are "really real" is not something that you can ever find in the math itself.

"You can only say your preferred interpretation says they are."

This is always true for every scientific theory. There are always an infinite number of theories compatible with past evidence, which always make different predictions about the future. You can't say that the earth is billions of years old, when Last Thursdayism is equally as compatible with all available evidence. It is only "your preferred interpretation" which says that the earth is more than a week old.

Ah, but what you seem to be missing, is that there yet remain ways to discuss a plausibility preference among theories, even when all the theories remain compatible with all available evidence. That isn't the limit of our ability to compare theories.

"Copenhagen ... makes the same predictions as all other interpretations. That means it's as valid as any other interpretation."

No, it doesn't. Because there are other arguments to prefer one theory over another, besides mere consistency. (E.g., you never bothered to address my "photons headed away from earth just disappear" theory.)

"If you don't happen to like the "rhetoric" that Copenhagen uses, that's a matter of personal preference, not physics."

No, it's far more than "personal preference", sorry. Perhaps whether it is "physics" or not is a matter of social convention and labeling. But it's at least "metaphysics". If you don't want interpretations to be part of physics, that's fine. But that doesn't license you to perform bad metaphysics.

"It doesn't make any predictions that QM doesn't make."

Sure it does. It predicts a "collapse", as a new actual event that happens in the real world.

Don Geddis said...

@Peter: "we certainly do not have experimental evidence that all interactions are reversible."

You're putting the burden on the wrong side. Every experiment we've been able to run, has shown reversible interactions. Copenhagen posits that there must be some interactions that are theoretically (not just practically) irreversible. There has never been any experimental evidence for even a single example of an interaction which is provably irreversible.

You're doing a kind of God of the gaps bad argument: if an interaction is complex enough that we are currently unable to attempt to reverse it, then you want to claim that it is surely there where this magic new irreversibility takes place. Alas, every attempt to demonstrate any actual irreversibility has failed, and the size and complexity of particle configurations that instead demonstrate reversibility, continues to grow as our technology grows.

"I'll just point at an article I wrote on Physics Forums"

I appreciate your attempt, but I don't like your perspective at all. This isn't just a personal preference thing. You need to look at all the other scientific theories of the world, and what it means to claim that a mathematical model is "physically real". Are atoms "real"? Are electrons "real"? Is the moon "real"? Is gravity "real"?

Meanwhile, in your alternative proposal, we do already have a scientific theory which is about one's state of knowledge: Bayesian probability theory. Changes in probability calculations are explicitly about changes in your internal state of knowledge, not about changes in the "real world". But QM theory doesn't work like that. It's a plausible hypothesis to explore, but attempting to interpret QM as mere statements about our internal knowledge (like probability theory) quickly fail. There is very clearly more going on out in the real world, then merely our current local knowledge of it.

Peter Donis said...

whether the model's entities are "really real" is not something that you can ever find in the math itself.

I didn't say it was. I said it was a matter of interpretation: in order to say that the model's entities are "really real", there must be no valid interpretations of the math in which they aren't.

There are always an infinite number of theories compatible with past evidence, which always make different predictions about the future.

That's irrelevant to what we're talking about here. We're all using the same theory and the same predictions: standard QM. We're talking about different possible interpretations of that same theory and those same predictions. The fact that there might be an infinite number of other theories that yield different predictions but are compatible with the same evidence is a different discussion (and I have no more interest in it than you appear to have).

there are other arguments to prefer one theory over another

You keep on missing my point: we are not talking about different theories! We are talking about different interpretations of the same theory.

It predicts a "collapse", as a new actual event that happens in the real world.

No, it doesn't That's a different theory, not an interpretation of standard QM. "Copenhagen" as an interpretation of standard QM does not say collapse is an actual event. There are theories that do, but they're not standard QM (an example would be the GRW model that adds "stochastic collapse" to standard QM, and is a different theory from standard QM).

Once again, you keep on mixing up "different theories" with "different interpretations of the same theory". We're only talking about the latter.

Every experiment we've been able to run, has shown reversible interactions.

Really? So when you cook an egg, that's a reversible interaction? Somebody has reversed it?

The correct statement is that every experiment involving a quantum system with a small number of degrees of freedom, carefully isolated from its environment so it doesn't decohere that we've been able to run, has shown reversible interactions. But that is very, very far from being "every experiment we've been able to run", period.

Peter Donis said...

You need to look at all the other scientific theories of the world, and what it means to claim that a mathematical model is "physically real".

I have. And I don't see any other scientific theory of the world that has the same issue QM has: that there is no interpretation in which things work the way our normal everyday intuitions say they should work. Relativity is the other theory people often say things like that about, but relativity is a classical theory; you still have a mathematical model that has both of the aspects I gave as desirable in the article: the states in the model directly correspond to the states of the systems being modeled, and they are complete and describe a world that looks like our ordinary world. The only "weirdness" that you have to accept is that some of the relationships between what different observers in relative motion observe are not quite what you expected (time dilation, length contraction, relativity of simultaneity). But the 4-dimensional spacetime world model shows exactly how all this fits together.

we do already have a scientific theory which is about one's state of knowledge: Bayesian probability theory.

Yes, and one way of looking at QM is that it is an extension of probability theory that allows negative probabilities (by using complex numbers as amplitudes, which are "square roots" of probabilities).

attempting to interpret QM as mere statements about our internal knowledge (like probability theory) quickly fail.

I don't know where you're getting this from. Interpretations of QM that treat the quantum state as describing our knowledge of the system are perfectly valid interpretations; they use the same math and make the same predictions as other interpretations.

Peter Donis said...

Perhaps whether it is "physics" or not is a matter of social convention and labeling. But it's at least "metaphysics". If you don't want interpretations to be part of physics, that's fine. But that doesn't license you to perform bad metaphysics.

I'm not sure how you tell what is "bad metaphysics" unless it leads you to a theory of physics that makes wrong predictions. And, as I've said several times now, that's not what we're talking about here. If all we're talking about is different interpretations of the same theory and the same predictions, then the only meaning I can give to "bad metaphysics" is that you don't like the interpretation. Which, as I said, is a matter of personal preference.

Just to be clear: I'm not saying people shouldn't explore different interpretations or talk about them. Of course they should. But unless and until somebody figures out how to go from "different interpretation" to "different theory"--until somebody figures out how to extend standard QM in a way that will make different predictions for different interpretations (or what used to be different interpretations before the extension), then there is no way to say that any interpretation is "right" or "wrong". The value of having different interpretations is that they might suggest different possible extensions of standard QM, i.e., different research directions. But believing that one particular research direction is the one that's going to succeed is not the same as it having already succeeded. When physicists talk like the MWI or "parallel universes" are established fact, they are mistaking the former for the latter.

Ron said...

@Peter:

> "really real"

I think you need to go back and re-read the OP more carefully. Pay particular attention to the fifth paragraph. "Really real" is a red herring.

Copenhagen is deficient in the same way that Ptolemaic epicycle theory is deficient. Note that I deliberately chose the word "deficient" rather than "wrong" or "false". This is not about truth, this is about *quality*, if not scientific quality, then semantic and pedagogic quality. Epicycle theory is consistent with all the observed data. But it is deficient because it fails to explain *why* there are epicycles.

Copenhagen fails in the exact same way because it hinges on this concept of "wavefunction collapse" which is (supposedly) a physical phenomenon that happens when an "experiment" is performed, but it doesn't tell you what an experiment is. It turns out that you *can* actually specify what an experiment is (it has to do with macroscopic entanglement) but Copenhagen actively rejects this advance because it leads to some logical consequences that are highly unintuitive i.e. our classical universe is not all there is. But space and time being interchangeable is pretty frickin' weird too when you first encounter is and yet somehow people manage to wrap their brains around that.

Ron said...

@Peter:

> an article I wrote on Physics Forums

"The state describes our knowledge or potential knowledge"

See, that's exactly the problem. Which is it? It it a description of our knowledge, or is it a description of our potential knowledge? Because these are not at all the same. And if it's potential knowledge, then potential with respect to what counterfactuals?

Either way there is no escape from the EPRG paradox. I don't think that you have taken this fully on board.

Peter Donis said...

Epicycle theory is consistent with all the observed data.

No, it isn't. It was when Copernicus first proposed his alternative model, but by the time Kepler was analyzing Tycho Brahe's data, epicycle model predictions had fallen outside the error bars in the data.

it is deficient because it fails to explain *why* there are epicycles.

Just like general relativity fails to explain *why* spacetime curvature is produced by stress-energy. It just says it is.

Or just like QM fails to explain *why* there are probability amplitudes. It just says that there are.

I'm unimpressed by arguments of this sort, because, in the absence of some way to experimentally test the alternatives, they boil down to personal preference. If you honestly don't see that, then we'll just have to agree to disagree.

It it a description of our knowledge, or is it a description of our potential knowledge?

It depends on the specific version of category #1 of interpretations. I wasn't trying to pin that down in the article. I personally favor "knowledge" over "potential knowledge" because of the issue of dealing with counterfactuals, which you mention. But that's just my personal preference.

Either way there is no escape from the EPRG paradox.

Sure there is. If the quantum state only represents our knowledge about the system, then a pair of EPR measurements just means our knowledge about the system has been updated with a particular set of correlations. This interpretation is completely agnostic about what is going on "behind the scenes" to produce those correlations. So there is no paradox.

I understand that you don't like this interpretation because, as I said in the article, it says that the description of the system in terms of a quantum state is incomplete. Basically, you find it less unpalatable to believe in many worlds than to believe that QM is an incomplete theory. My preference is the other way around. But we have no way of resolving this disagreement by experiment, which means it can't be resolved at this point.

Peter Donis said...

Copenhagen fails in the exact same way because it hinges on this concept of "wavefunction collapse" which is (supposedly) a physical phenomenon that happens when an "experiment" is performed

As I've already pointed out several times in response to Don Geddis, if there is such a version of "Copenhagen", it appears to be making a physical prediction that standard QM does not make, which means it's not an interpretation of QM, it's a different theory. I'm not talking about that version of "Copenhagen". I'm only talking about the version where "collapse" is a statement about how our knowledge of the system changes when we do an experiment. If you don't think that counts as "Copenhagen", then I'll drop the label, not the interpretation.

It turns out that you *can* actually specify what an experiment is (it has to do with macroscopic entanglement)

Which just pushes the vagueness into the word "macroscopic". Practically speaking, it ends up meaning "irreversible", which is the definition I gave before. But nobody has a rigorous definition of what counts as "macroscopic", any more than anyone has a rigorous definition of what counts as an "experiment".

Ron said...

> I'm only talking about the version where "collapse" is a statement about how our knowledge of the system changes when we do an experiment.

Yes. That version predicts that the EPRG experiment should produce faster-than-light communications. Which it doesn't. So it's wrong.

> Which just pushes the vagueness into the word "macroscopic".

No. It fundamentally changes the description of the phenomenon from an incorrect one (collapse is a dichotomy -- it either happens or it doesn't) to a correct one: the phenomenon called "collapse" is one extreme of a continuum. At the other extreme is a single entangled pair. In the middle lies a multitude of interesting phenomena, arising from mutually entangled sets of N particles for 3<=N<=10^23.

Don Geddis said...

@Peter: "in order to say that the model's entities are "really real", there must be no valid interpretations of the math in which they aren't"

You seem to have suggested a rather silly requirement. I doubt you can offer me any math from any field of science, in which there are "no valid interpretations" of the math where the described entities are "not real". You're just not exercising your imagination sufficiently. Every mathematical theory, can be "interpreted" as describing non-real entities. You think this somehow specially singles out quantum mechanics, but it doesn't.

""Copenhagen" as an interpretation of standard QM does not say collapse is an actual event."

I don't think I believe you. Perhaps you can offer some references to your claim here. Or even a coherent description of what you think the Copenhagen interpretation actually is.

"So when you cook an egg, that's a reversible interaction?"

We have reversed specific (small scale) interactions. And we have other interactions that are not practical to reverse (such as an egg). What we don't have -- zero evidence -- is for any interaction that is theoretically impossible to reverse. There is zero evidence that it is impossible in theory to uncook an egg. (There is only evidence that it is very difficult to do in practice.)

"I don't see any other scientific theory of the world [...] there is no interpretation in which things work the way our normal everyday intuitions say they should work"

This is just laughably false. As Ron says -- and even you realize -- claiming that time is relative matches the normal everyday intuitions of exactly nobody. Requiring the universe to act according to human-scale everyday intuitions is not a reasonable criteria for good science.

"I'm not sure how you tell what is "bad metaphysics" unless it leads you to a theory of physics that makes wrong predictions."

And you've been given a half-dozen concrete examples, that you've carefully avoided reacting to: half of the sun's emitted photons disappearing from the universe; Last Thursdayism; Ptolemaic epicycles; grue and bleen. It is trivial to come up with stupid theories that are (merely) consistent with past observation. (And even ones that agree on future predictions.)

I've said it many times already: there is far, far more to good metaphysics than mere consistency. You can start with, for example, Occam's razor or Kolmogorov complexity or overfitting a curve.

"then the only meaning I can give to "bad metaphysics" is that you don't like the interpretation. Which, as I said, is a matter of personal preference."

Nope. There is much more to metaphysics than mere consistency. You being unaware of other criteria, doesn't mean they don't exist. And it also doesn't mean that people are free to just choose whatever theory they "prefer".

Peter Donis said...

That version predicts that the EPRG experiment should produce faster-than-light communications.

No, it doesn't. It's an interpretation of QM; it uses the same math and makes the same predictions as any other interpretation of QM. The math of QM says that FTL communication is impossible in an EPR experiment. So all interpretations of QM also say that.

At this point I don't think further discussion will be useful. Even though I have repeatedly said (and Don explicitly agreed) that all interpretations of QM use the same math and make the same predictions, you both persist in talking about "interpretations" making different predictions. I can't usefully respond if we can't agree on what we are talking about.

Peter Donis said...

What we don't have -- zero evidence -- is for any interaction that is theoretically impossible to reverse.

That depends on what you think is "theoretically impossible". Which depends on what theory you are using and what you think its domain of validity is.

Requiring the universe to act according to human-scale everyday intuitions is not a reasonable criteria for good science.

I agree. I just think that on the counterintuitiveness scale, QM scores a lot higher than relativity. But that's just my personal opinion, since "counterintuitiveness" is not a quantifiable property. (Also, I admit my opinion is skewed compared to that of the average person, since I have spent many years studying both theories so my intuitions have been significantly recalibrated.)

Ron said...

> No, it doesn't. It's an interpretation of QM; it uses the same math and makes the same predictions as any other interpretation of QM.

Did you actually read the description of the EPRG experiment?

Don Geddis said...

Peter, please stop avoiding the critical question. Tell me, specifically, why you don't take "Last Thursdayism" as seriously as the standard model of the origins of the solar system and earth. I'm assuming that you don't, and you also likely don't think it is merely a matter of "personal preference".

I challenge you to offer a principled reason for rejecting Last Thursdayism, that isn't also relevant to the plausibility of the Copenhagen interpretation, in a way different than mere consistency with observation.

I think you're asserting claims that you don't actually believe. You've said that QM interpretations are a mere matter of preference. And you've justified Copenhagen by nothing more than it being consistent with QM equations. And you've acted as though that is enough. But I'm sure that, in comparing theories in other domains, you're concerned about a whole lot more than mere consistency.

So tell me why. Why do you reject Last Thursdayism?

(I'm also waiting for a link to a description of your concept of Copenhagen, where collapse is not a physical event.)

Peter Donis said...

Did you actually read the description of the EPRG experiment?

Do you mean your description in your article? If it is claiming that some interpretations of QM make different predictions than others, then it's incorrect.

The descriptions of EPR-type experiments that I have read in the actual QM literature are interpretation agnostic; they describe the preparation procedure, the measuring apparatus, the math of standard QM and what it predicts for the results, and the actual results compared to the predictions.

Peter Donis said...

@Ron, are there other posts from me still in moderation? I only see one from yesterday, but I thought I had posted several. It's possible that blogger's comment page tricked me and didn't post them at all, if so I'll just repost.

Ron said...

@Peter:

> > Did you actually read the description of the EPRG experiment?

> Do you mean your description in your article? If it is claiming that some interpretations of QM make different predictions than others, then it's incorrect.

So that would be a "no" then. If you refuse to even look at the argument then I don't see any point in engaging you further on this.

> are there other posts from me still in moderation?

No.

Peter Donis said...

> are there other posts from me still in moderation?

No.


Ok, I must have fat fingered something when I tried to post. Will repost as needed.

Peter Donis said...

@Ron:
So that would be a "no" then. If you refuse to even look at the argument then I don't see any point in engaging you further on this.

So that would be a "yes" in response to my question, did you mean the description in your article?

If so, I am unable to figure out what part of the article you think is relevant to what I've been saying. Most of what you say there seems to me to be interpretation agnostic. At the end of section 3 you say that there is a contradiction in a "commonly told story" about QM, but I don't see how that "commonly told story" relates to anything I've said.

I admit, though, that I am not a "naive subject" for your article to begin with, since my mind is already contaminated, so to speak, with a lot of knowledge of QM, acquired over many years, so it might just be that the way I've developed of understanding it doesn't match up well with the way you are describing in the article. In which case discussion is probably not going to be fruitful. But see some further comments below in response to Don.

@Don:
I'm also waiting for a link to a description of your concept of Copenhagen, where collapse is not a physical event.

For a textbook that, IMO, does a good job of presenting QM interpretations (though that's by no means all it does), you could try Ballentine:

https://www.amazon.com/Quantum-Mechanics-Modern-Development-2nd/dp/9814578584

The second edition (which is what I've linked to) has been updated to discuss fairly recent experiments, including quantum information experiments.

As far as "Copenhagen" is concerned, if the interpretation (or type of interpretation, more precisely) that I describe as option #1 in my Physics Forums article doesn't pattern match with your definition of "Copenhagen", then I'll drop the term, not the interpretation. I'm not interested in defending the term "Copenhagen", or every claim that has been made in the literature under that umbrella. If my use of that term has caused confusion, I apologize; terminology is often a source of confusion in this area. The type of interpretation I described as option #1 in my article is the type I have been referring to.

Peter Donis said...

@Don:
please stop avoiding the critical question

I disagree that the question you're asking is "the critical question". I think "Last Thursdayism" is irrelevant to this discussion, for reasons I've already explained, but I'll try once more.

"Last Thursdayism", as compared to our standard model of the solar system (that it is 4.6 billion years old), is a different theory; it uses different laws (actually, it's not clear what "laws" it uses, but they certainly aren't the standard ones used in astronomy and cosmology) and presents a different theoretical model, which just happens to make the same predictions as our standard one. But the model itself is easily distinguishable; if any "Last Thursdayists" (or young Earth creationists, if we want to take a close real-world example) were able to actually build a mathematical representation of their model, distinguishing it from our standard model would be even easier.

In other words, "Last Thursdayism" isn't a different "interpretation" of standard solar system physics; it's a different physical theory.

However, what we're discussing here is different interpretations of the same physical theory--standard quantum mechanics. It's not just that all of the interpretations make the same predictions; it's that all of the interpretations use the same underlying theoretical model. The only difference between the interpretations is what story they tell in ordinary language about how the model produces the predictions, or what the "underlying reality" is beneath the predictions. But none of that is part of the actual theoretical model.

Or, to put it another way, people who take their preferred interpretation of QM to be making additional claims about what is "really there" are often not actually describing an interpretation of QM; they're describing a different theory from standard QM, one which takes standard QM and adds more stuff to its theoretical model--stuff which can be used to make additional predictions, over and above those that QM makes.

The model you have been using the term "Copenhagen" to describe is such a model: it takes standard QM and adds to its model the claim that "collapse" is a real physical event. That drives additional predictions: for example, if nothing else is done to the model, it predicts, as you and Ron have said, that FTL communication should be possible. (I say "if nothing else is done" because there are attempts in the literature to construct models in which collapse is a physical event, but which have other stuff also added to the model to remove the prediction of possible FTL communication. AFAIK such models have not made much headway; I just use them to illustrate the possibilities in the abstract "model space" in question.)

To be clear, I'm not saying that this latter type of effort--adding stuff to the theoretical model of standard QM, driven by the hints provided by some particular intepretation, in order to generate additional predictions--is not worthwhile. It is. In fact, it's exactly what theorists should be doing, in order to try to resolve the QM interpretation debate in the only way that any such debate can be finally resolved: by figuring out how to test it by experiment, and then running the experiment to see which way Nature votes. I am only saying that this has not happened yet, and unless and until it does, talking of any particular interpretation of QM as though it were established fact is, as Sherlock Holmes would say, theorizing in advance of your data.

Ron said...

@Peter:

> I don't see how that "commonly told story" relates to anything I've said.

Then I don't see how anything you've said relates -- or could possibly relate -- to this blog post.

You do understand that there is a thing called the "measurement problem"? You may or may not consider the measurement problem to be a problem, but you do understand that some noteworthy people do consider it a problem? That indeed some people today consider it an unsolved problem? And that there is a thing commonly referred to as the Copenhagen Interpretation which holds itself out as a solution to the measurement problem? (Well, it's not really a solution, more of a dogma that the measurement problem cannot be solved and we just have to make our peace with that, but whatever.)

Peter Donis said...

You do understand that there is a thing called the "measurement problem"?

Yes. I'm disputing the claim you appear to be making, that this problem is solved. What I think you should be saying is that, if your preferred interpretation of QM turns out to be correct, then the measurement problem will be solved. But the only way to show that it's correct is to figure out how to distinguish between it and other interpretations by experiment, which hasn't been done yet.

Don Geddis said...

@Peter: Well, thanks for providing a kind of link, I guess ... but if your only reference is that I need to spend $50-$100 to buy a textbook in order to understand some internet argument ... well, perhaps it won't surprise you that I'm not optimistic enough about the likely success, to make it worth the effort of trying that hard.

Your "option #1" in your paper, seems to imply that there is some "interpretation" of quantum mechanics where the wave function is somehow only about our knowledge of the particles, and any update of the wave function is really only about an update in our internal knowledge, and none of it has anything to do with what is "really" out there in the universe. You appear to want to make some analogy with probability (only using complex numbers, allowing "negative" probability, whatever that might mean).

This thing you are describing certainly doesn't seem to be the standard Copenhagen interpretation. Nor have I heard of it before. (And I'm not yet convinced that you're offering a valid QM interpretation; but since a >$50 textbook seems to be the only access to your idea, perhaps we'll have to let that go.)

As Ron says, it isn't clear that you have addressed the QM measurement problem. Moreover, I can't match your description of your preferred interpretation ("only complex probabilities; only about your internal knowledge; no new predictions") with any of the standard interpretations of quantum mechanics. It isn't clear to me that any of the couple dozen major proposed QM interpretations listed there, matches what you have been describing as your preferred interpretation.

Have you made up something new on your own? Have you misinterpreted what everybody else has been saying all along? Or is there yet another option? Is yours one of the minority interpretations of quantum mechanics?

Don Geddis said...

@Peter: ""Last Thursdayism" ... is a different theory; it uses different laws ... they certainly aren't the standard ones used in astronomy and cosmology ... But the model itself is easily distinguishable; if any "Last Thursdayists" ... were able to actually build a mathematical representation of their model, distinguishing it from our standard model would be even easier."

I don't think you get it. Last Thursdayism uses exactly the same physical laws as the standard theory. That's exactly why all the predictions going forward are identical. The only difference is the starting point: the standard theory says that the origin of the universe was billions of years ago in a single point as a big bang. Last Thursdayism says that the origin of the universe was a week ago, where all matter and energy began at that point in the distribution and configuration that they happened to have, one week ago.

Neither theory tells you what happened "before" the universe began; neither theory tells you why the universe began, in that particular state. They only tell you the starting state, and then how the physics evolves after that. They happen to begin with different starting states, but all of the laws of physics are completely identical between the two.

So, try again: why do you not consider Last Thursdayism a mere matter of "personal preference"?

Don Geddis said...

@Peter: Ah, I was able to search a bit from your hinted link to Ballentine. That seems to suggest that your preferred interpretation has nothing to do with Copenhagen, but is instead what is apparently known as the ensemble interpretation of QM.

Ron said...

@Peter:

> I'm disputing the claim you appear to be making, that this problem is solved.

Well, your dispute has zero credibility with me since by your own admission you have not even looked at the proposed solution.

Why are you even here? Do you really have nothing better to do than to argue against a position that you haven't even bothered to learn what it is you're arguing against?

Peter Donis said...

That seems to suggest that your preferred interpretation has nothing to do with Copenhagen, but is instead what is apparently known as the ensemble interpretation of QM.

As I mentioned in my article, that is one of the interpretations that falls into my category #1, yes.

Peter Donis said...

by your own admission you have not even looked at the proposed solution.

I don't have to read your article to know what the interpretations you are talking about say. I've already mentioned that I've been studying QM for many years.

Why are you even here?

At this point I've said all that I came to say, so I'll bow out.

Peter Donis said...

@Don:
Last Thursdayism uses exactly the same physical laws as the standard theory. That's exactly why all the predictions going forward are identical. The only difference is the starting point

I do have one quick comment on this: in General Relativity (which is the physical laws I am referring to, since that's the basis for our best current models in cosmology), you can't arbitrarily choose the starting point like this. Spacetime can't just "stop" at last Thursday; that would violate conservation laws. You have to keep on analytically extending the model until it "naturally" tells you it stops. That was the basis for my statement that "last Thursdayism" can't be using the same physical laws as our standard model.

Peter Donis said...

You appear to want to make some analogy with probability

I should clarify this as well: it's just a reference to the idea that quantum mechanics is a generalization of ordinary probability theory in which probabilities can be negative. A good quick exposition by Scott Aaronson is here:

https://www.scottaaronson.com/democritus/lec9.html

A more comprehensive treatment in an actual paper is here:

https://arxiv.org/pdf/1402.6562.pdf

Don Geddis said...

@Peter: "I don't have to read your article to know what the interpretations you are talking about say. I've already mentioned that I've been studying QM for many years."

I don't think that's accurate. You began commenting in this thread by making some incorrect claims about the Copenhagen interpretation. When called on it, you retreated by saying that you didn't want to get hung up on terminology, but were actually only defending something that now (perhaps?) seems to be the Ensemble interpretation. Your record on accurately describing alternative QM interpretations is not impressive.

"you can't arbitrarily choose the starting point like this"

I don't see why not. GR tells you, once you have a given configuration of matter and energy, how it evolves forward in time. It really has nothing to say about the initial state of the universe, at the beginning. Even in standard cosmology, there is nothing in the theory that explains why the universe began in the exact state it did at the big bang, in precisely that size, with precisely that amount of energy. It's just taken as a given: that was the (arbitrary) starting state, and then the universe evolved according to our known physical laws after that.

I'm just giving you a different starting state. There are no "conservation laws" that apply prior to the start of the universe: neither in Last Thursdayism, nor in the standard cosmology. It isn't even meaningful to talk about the situation "before" the start of the universe, as time itself is inside the universe. So conservation laws aren't meaningful across the beginning of the universe either.

Peter Donis said...

You began commenting in this thread by making some incorrect claims about the Copenhagen interpretation.

No, we were using the term "Copenhagen interpretation" to refer to different things. Since there is no universally accepted definition of the term "Copenhagen interpretation" (as the Wikipedia page on that interpretation, which is linked to in the page on QM interpretations that you linked to, points out), that's always a potential issue.

GR tells you, once you have a given configuration of matter and energy, how it evolves forward in time.

No, GR tells you a four-dimensional spacetime. If you pick a particular spacelike hypersurface in that spacetime, and consider the data on that surface as "initial data", you can evolve it forward to determine the 4-geometry to the future of that hypersurface. But you can also evolve it backward to determine the 4-geometry to the past of that hypersurface. And since GR includes conservation laws that basically say you can't create or destroy spacetime curvature or stress-energy in any infinitesimal volume of spacetime, "chopping off" the 4-geometry to the past of your chosen initial hypersurface and only considering that surface and its future is not a valid solution.

Peter Donis said...

Even in standard cosmology, there is nothing in the theory that explains why the universe began in the exact state it did at the big bang, in precisely that size, with precisely that amount of energy. It's just taken as a given: that was the (arbitrary) starting state, and then the universe evolved according to our known physical laws after that.

No, that's not what standard cosmology does. Standard cosmology says that the "Big Bang" state is the earliest state of which we have reasonably reliable knowledge. It does not say that was the first state of the universe and there were no states before it. (Yes, many pop science books and articles on cosmology say that, but they are wrong. This question comes up often on Physics Forums, which is why I have the response down pat by now.) The whole point of models like inflation is to say what came before the state we call the "Big Bang"; and one of the main reasons why "eternal inflation" models are proposed is that in them, the universe (or more precisely the eternally inflating "multiverse" that spawned our universe) has no "initial state": it existed infinitely into the past.

Peter Donis said...

This thing you are describing certainly doesn't seem to be the standard Copenhagen interpretation.

Since you gave Wikipedia as a source, here is an excerpt from the Wikipedia page on the Copenhagen interpretation:

A wave function Ψ represents the state of the system. It encapsulates everything that can be known about that system before an observation; there are no additional "hidden parameters".

If you click on the "state" link in the above, it says:

"A quantum state provides a probability distribution for the value of each observable, i.e. for the outcome of each possible measurement on the system. Knowledge of the quantum state together with the rules[clarification needed] for the system's evolution in time exhausts all that can be predicted about the system's behavior."

And a little bit later on (in the fifth principle):

The description given by the wave function is probabilistic.

In other words, the wave function describes what can be known about the system. There is no claim that it is the physically real state of the system (if it were, it wouldn't be "probabilistic"). So this version of "Copenhagen", at least, falls into category #1 in my Physics Forums article.

https://en.wikipedia.org/wiki/Copenhagen_interpretation#Principles

Don Geddis said...

"there is no universally accepted definition of the term "Copenhagen interpretation""

There may be some minor variations of interpretation that all get labeled "Copenhagen". That does not mean that any arbitrary variation is acceptable. All correct "Copenhagen" interpretations of QM posit that "wavefunction collapse" is a physical event, precipitated by a "measurement". You denied this well-known feature. You were using the label to refer to something that it does not, in fact, refer to.


"quantum mechanics is a generalization of ordinary probability theory in which probabilities can be negative"

Oh, I know all about how the math of QM, can be seen as a generalization of the math of probability theory -- by way of adding negative (and complex) probabilities. But you went much, much farther than that. There is a Bayesian interpretation of the math of probability theory, that says that the math describes the evolution of your state of internal knowledge, not any feature of external reality.

You made a claim that the math of QM can similarly be seen, by analogy, as not referring to external reality, but only referring to internal knowledge.

I can't make any sense of that claim. It's easy to see how the bare math is a generalization of probability math. It's not at all obvious what it could possibly mean that negative or complex "probabilities" describe something about the evolution of my internal state of knowledge.

I don't (yet) believe that the analogy successfully carries across for the interpretations.

Don Geddis said...

"the wave function describes what can be known about the system. There is no claim that it is the physically real state of the system"

I disagree with your epistemology here. "What can be known" is a statement about objective reality, independent of any particular observer. It is very, very different, than a Bayesian probability interpretation describing the state of knowledge of some specific observer.

(I don't agree that your "category #1", as described in your paper, actually applies to any QM interpretation. Although you kind of weasel out of it by using the phrase "or potential knowledge" in the first sentence, the rest of your definition just talks about changes in the state of "our knowledge".)

Ron said...

@Peter:

> I don't have to read your article to know what the interpretations you are talking about say.

But you do have to read my article to know what is under discussion because, as you yourself just pointed out:

> there is no universally accepted definition of the term "Copenhagen interpretation"

Also:

> The Copenhagen interpretation is just an interpretation of QM. It doesn't make any predictions that QM doesn't make.

And yet:

> What's different about Copenhagen is that it leads to different expectations about what a future theory that has current QM as an appropriate approximation will look like: Copenhagen leads you to expect that such a theory will have an explicit collapse (i.e., non-unitary process) somewhere in it

I don't want to quibble over whether there is a meaningful distinction between an "expectation" and a "prediction". We agree that this is the distinguishing feature of Copenhagen. And it is demonstrably false.

Peter Donis said...

There is a Bayesian interpretation of the math of probability theory, that says that the math describes the evolution of your state of internal knowledge, not any feature of external reality.

I'm not sure I would describe the Bayesian interpretation that way. For example, say I have a Bayesian model of a coin. My prior is that the coin is fair, so I assign equal probability mass to heads and tails. But, if the coin keeps coming up heads flip after flip, at some point that data will overcome my prior and I will conclude that it's a two-headed coin (strictly speaking, I will assign a very high posterior probability to heads).

Now, one could argue that the prior was a statement purely about my internal knowledge; but as soon as actual data starts accumulating, my updated posterior probabilities aren't just about my knowledge any more. They take into account actual data that is objective--as many people as desired can observe the coin flips and agree on their results. And after a large enough number of flips that were all heads, even someone with a very different prior from mine--for example, someone who thought it was extremely likely that the coin had two sides both tails, so they assigned almost all the probability mass to tails and almost none to heads, would eventually be forced to conclude that the coin was actually two-headed (though it would take them more flips than it would take me).

So while I think it can be said that Bayesians interpret probabilities as being about knowledge, I don't think it can be said that that knowledge is "purely internal" to one person. It can be widely shared knowledge that is based on objective data, and so reflects features of external reality, not just the internals of a person's mind.

You made a claim that the math of QM can similarly be seen, by analogy, as not referring to external reality, but only referring to internal knowledge.

Similar remarks to the above would apply here.

Peter Donis said...

But you do have to read my article to know what is under discussion

I had to read this article, yes. It already said more than enough for me to know what kind of interpretation you were discussing in the paper. You say at the start of the article that if one has not either watched your video or read your paper, the rest of the article probably won't make a lot of sense; but it made sense to me just fine.

I don't want to quibble over whether there is a meaningful distinction between an "expectation" and a "prediction".

It's not a quibble; it's a crucial point. Here are my words that you quoted: "different expectations about what a future theory that has current QM as an appropriate approximation will look like". Can you honestly not see the difference between that and "predictions that current QM makes"?

Don Geddis said...

"as actual data starts accumulating, my updated posterior probabilities aren't just about my knowledge any more"

But of course they are. The entire evolution of the probability mass that you are describing, is an evolution of your internal model. Nothing in the external objective reality has changed. The coin was always two-headed. You just didn't know it, and so you are coming to learn this fact. But this is a change inside of you, not a change in external reality.

"I don't think it can be said that that knowledge is "purely internal" to one person."

Again, of course it is. Your "correct" Bayesian calculation, is based on your individual priors, and your specific, local observations. It has nothing (necessarily) to do with any (correct!) calculation that any other person might make, as they will (most likely) have both different priors and also different sets of observations. Each calculation is local to some specific observer, and refers to the evolution of knowledge with that observer.

"It can be widely shared knowledge that is based on objective data, and so reflects features of external reality"

Well it's certainly the case that people can adopt procedures to more closely align their internal models, with external reality. And in so far as external reality is consistent, we should of course expect that multiple observers engaging in such procedures should likely converge on similar internal models, since that's kind of the whole point of such alignment.

But none of that distracts from the basic fact that Bayesian probability calculations are about observer-specific evolution of internal knowledge.

"Similar remarks to the above would apply here."

You keep making this analogy as though it is straightforward and obvious. I'm telling you that I'm unable to follow this connection even a single step. I don't understand what negative or complex probabilities refer to, if applied to my internal reasoning and knowledge. And I don't understand how a wavefunction evolution in a distant experiment that I don't observe, is somehow supposed to be describing changes to my internal models.

As far as I can tell, this analogy (beyond the basic math extension) falls apart completely. I can't see any way that QM describes some internal modeling process, in the way that probability does.

Peter Donis said...

it is demonstrably false.

It is if you assume that the quantum state is physically real, i.e., if you assume that an interpretation in my category #2 is correct.

I don't think it is if an interpretation in my category #1 turns out to be correct. Or, to put it another way, it might turn out that whatever more fundamental theory we end up discovering, that has current QM as an appropriate approximation, does not describe reality using quantum states, but using something different. If that happens, explicit collapse might not turn out to be demonstrably false.

Peter Donis said...

The coin was always two-headed. You just didn't know it, and so you are coming to learn this fact. But this is a change inside of you, not a change in external reality.

it's certainly the case that people can adopt procedures to more closely align their internal models, with external reality. And in so far as external reality is consistent, we should of course expect that multiple observers engaging in such procedures should likely converge on similar internal models, since that's kind of the whole point of such alignment.

I would agree with these two statements, put together. Yes, everyone's individual model is inside their head; but the models can be updated based on data to more closely align with external reality.

I don't understand what negative or complex probabilities refer to, if applied to my internal reasoning and knowledge.

They mean that you are allowing for different alternatives to interfere--i.e., two possible ways something could happen could reduce the total probability instead of increase it. We already know this is possible, so why can't your internal reasoning processes allow for it?

Ron said...

@Peter:

> > it is demonstrably false.

> It is if you assume that the quantum state is physically real

No, it is not necessary to assume that. Non-unitary collapse is enough to produce FTL communications. It doesn't matter *how* the collapse happens, it is sufficient *that* it happens.

Don Geddis said...

"two possible ways something could happen could reduce the total probability instead of increase it. We already know this is possible, so why can't your internal reasoning processes allow for it?"

Because that's not how internal reasoning works. My internal knowledge update process follows ordinary probability (with some errors, of course), not quantum pseudo-probability. The QM math of complex amplitudes does not match the evolution of my internal knowledge.

For that matter, I don't even know how to meaningfully understand a claimed statement like "I am 0.2+0.1i confident that the sun will rise tomorrow." I don't know what claim that is intending to make about my reasoning.

Peter Donis said...

My internal knowledge update process follows ordinary probability

Why does it have to? Your internal reasoning process can use any math you like. There's no reason why you have to restrict it to "ordinary probability".

I don't even know how to meaningfully understand a claimed statement like "I am 0.2+0.1i confident that the sun will rise tomorrow."

Complex numbers are probability amplitudes, not probabilities. You would take the squared modulus of the complex number to obtain the probability. So if your amplitude for the sun rising tomorrow is 0.2 + 0.1i, then you are (0.2)^2 + (0.1)^2 = 0.05, or 5 percent confident that the sun will rise tomorrow.

Peter Donis said...

you are (0.2)^2 + (0.1)^2 = 0.05, or 5 percent confident that the sun will rise tomorrow.

Oops, forgot to take the square root at the end; that should be sqrt(0.05), or about 22% confident that the sun will rise tomorrow.

Peter Donis said...

I don't understand what negative or complex probabilities refer to, if applied to my internal reasoning and knowledge.

Looking at the Scott Aaronson lecture I linked to earlier, I see that it's a bit unclear on this as well. He says " what would it mean to have "probability theory" with negative numbers?", but just a little later, when talking about a vector of probabilities of N different outcomes, he says "the probabilities had better be nonnegative". So he (and I) were using the word "probability" to refer to two different things, which I admit is confusing.

It might be better to describe the QM extension of probability theory as having "probabilities", which are still the way they are in ordinary probability theory (real numbers in the range 0 to 1), and "amplitudes", which are complex numbers whose norms (when the numbers are appropriately normalized) give probabilities.

Don Geddis said...

"Why does it have to? Your internal reasoning process can use any math you like."

It doesn't "have" to. You can make a machine that does any computation you wish. Machines don't have to be "rational", and neither does your reasoning process.

The important point is that complex amplitudes do not, in fact, describe my internal reasoning process. So a QM interpretation claim that "the evolution of this waveform models the observer's internal reasoning process" is demonstrably false. The waveform evolution does not, in fact, model the reasoning process of humans.

Peter Donis said...

Oops, forgot to take the square root at the end

Oops again, I was right the first time. The square root applies to the vector of amplitudes for all of the possibilities; in this case, it would mean that an amplitude of 0.2 + 0.1i for the sun rising, with a squared modulus of 0.05 (or 5 percent probability for the sun rising) requires that the amplitude for the sun not rising must have a squared modulus of 0.95, so that the two squared moduli sum to 1.

Peter Donis said...

complex amplitudes do not, in fact, describe my internal reasoning process

Hm. So if you were watching qubit measurements instead of coin flips, and the experimenter was applying various quantum logic gates to produce interference effects, you would refuse to update your internal model to make correct predictions because your internal reasoning process is only allowed to use ordinary probabilities?

Don Geddis said...

"you would refuse to update your internal model to make correct predictions because your internal reasoning process is only allowed to use ordinary probabilities?"

No, of course not. You're greatly confusing things at very different levels.

I can have internal models of all sorts of different things out in the world. I could have a model of a small boy named David, who (let's say) sincerely believes in magic. I can pour liquid from a tall thin glass, into a short fat glass, and David will believe some of the liquid has vanished. And I can reason, knowing that David has not yet reached the childhood developmental milestone to acquire the knowledge of conservation of liquid volume, that if I pour the liquid back into the tall thin glass, David will once again think there is more volume of liquid.

But the fact that I can reason about another entity, doesn't mean that "failure to conserve liquid volume" describes my reasoning.

Similarly, I can easily set up an internal model to calculate and predict the strange quantum logic of qubits. That does not mean that my internal reasoning process itself is using quantum logic. Internally, it is still using probabilities. It is only modeling and describing something externally, out there in objective reality.

You have once again (accidentally) provided yet more evidence that the quantum wavefunction describes physical reality. Me being able to have an internal model of that external reality, does not in the slightest mean that quantum logic is in any way a description of my own internal reasoning.

Peter Donis said...

No, of course not.

Then it seems that I'm as unable to understand your position as you are to understand mine. So at this point I really am going to bow out.