Understanding MWI: A Newbie's Guide to Quantum Physics and the Multiverse

[JesseM]

https://www.physicsforums.com/showthread.php?p=1070970#post1070970

OK, I thought you were saying that some significant aspects of quantum computation itself could be explained through classical optics, this is just a discussion of a "curious feature about a beam splitter". Anyway, see my comment above--if one agrees with Deutsch that at least some quantum phenomena, like the fast factorization of large numbers using algorithm[/url], would be most naturally understood in terms of the many-worlds interpretation (Deutsch sometimes talks about quantum computers achieving their rapid speeds by running huge numbers of computations in parallel, in different 'worlds'), then it would be strange not to extend this to all phenomena that physicists analyze using QM, even if some of these phenomena can also be analyzed using classical optics. As an analogy, if you believe that spacetime curves in the neighborhood of a black hole, you wouldn't say that other phenomena involving gravitation don't involve curved spacetime just because some of them can also be analyzed perfectly well using Newtonian gravity.

 

[Hans de Vries]

OK, I thought you were saying that some significant aspects of quantum computation itself could be explained through classical optics,

No, I wouldn't say so, although Shor's algorithm has also been
implemented with http://www.sciencenews.org/articles/20010519/fob4.asp". What we are
discussing here is MWI and the claim of other universes "hiding
on hyperplanes via some yet to discover theory of quantum
gravity" according to Deutsch.

this is just a discussion of a "curious feature about a beam splitter".

Well... Its actually what Deutsch calls quantum computing in his online
course, although it is best understood using classical optics. Nevertheless
he makes statements like "The computing is done in another universe"
Subsequently you get people claiming this as "The prove of the MWI"

http://www.quiprocone.org/Protected/Lecture_2.htmRegards, Hans

 

[JesseM]

No, I wouldn't say so, although Shor's algorithm has also been
implemented with http://www.sciencenews.org/articles/20010519/fob4.asp".

The article seems to be talking about algorithm[/url] rather than Shor's algorithm, but that's a minor quibble obviously, it's certainly interesting that any of the sort of speedups associated with quantum computers might be achievable using classical optics. But the article doesn't go so far as to say all quantum computations could be achieved with classical optics, instead it says "some other theorists had previously argued that a computer using classical physics can perform as well as any quantum computer in some calculations that involve only interference."

What we are
discussing here is MWI and the claim of other universes "hiding
on hyperplanes via some yet to discover theory of quantum
gravity" according to Deutsch.

Where did he make this claim? It sounds like he is speculating about future theories here rather than discussing the issue of interpreting our existing theory of QM, which is all that the MWI purports to do. It is of course possible that QM will turn out to be just a sort of approximation to some ultimate theory of quantum gravity or "theory of everything", and that untestable elements of existing interpretations (like the multiple 'worlds' of the MWI, or the FTL pilot wave of Bohmian mechanics, or the backwards causality of the transactional interpretation) will correspond to actual testable elements of the new theory.

Nevertheless
he makes statements like "The computing is done in another universe"
Subsequently you get people claiming this as "The prove of the MWI"

Well, anyone who thinks that any experiment can "prove" an interpretation is obviously confused or at least speaking sloppily--the most you can really argue is that certain physical results are more elegantly explained using one interpretation over another.

 

[Fra]

Why doesn't an MWI approach go back to the origins of probability theory, particularly conditional probability -- as in chains of events--, circa the 17th century? (I'll bet it actually does, but was jettisoned, so to speak, for whatever reasons, one of which I would guess was cumbersomeness. )

Regards,
Reilly Atkinson

Going back to the origin and axioms of probability theory is the way to go IMO. Glad to hear more people share this view. This is also where most of my serious philosophical objections are rooted (the effiency of applicability of the axioms of probability to reality).

The concept of objective probabilities makes no sense except as special cases. Also the concept of unitarity is highly suspect as it is relevant only the in concept of closed systems, but the whole point is that how do we deduce that we have a closed system? There is bound to be an uncertainty, and the limiting case where this uncertainty is insignificant certainly limits the domain of applicability? I personally think to solve these things, a reconstruction of the formalism should be made starting from the probability concepts and first principles.

/Fredrik

 

[Fra]

Forget this "splitting", "number of universes" etc. You just have the postulates of QM without wavefunction collapse. How can an observer collapse the state of the entire universe by just observing?

IMO. The only thing that collapses is the projection that lives inside the observer, which I see no more weird than similar to a bayesian update.

Why my view of the world change when I receive more information about it, is quite obvious. Whatever the world REALLY is, still has to be projected onto my perspective.

I think these words of Niels Bohr's still stands:

"It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature...".

/Fredrik

 

[Hans de Vries]

Where did he make this claim? It sounds like he is speculating about future theories here rather than discussing the issue of interpreting our existing theory of QM, which is all that the MWI purports to do. It is of course possible that QM will turn out to be just a sort of approximation to some ultimate theory of quantum gravity or "theory of everything", and that untestable elements of existing interpretations (like the multiple 'worlds' of the MWI, or the FTL pilot wave of Bohmian mechanics, or the backwards causality of the transactional interpretation) will correspond to actual testable elements of the new theory.

Here:

http://uk.arxiv.org/ftp/quant-ph/papers/0104/0104033.pdf

This is reminiscent of the infinity of ways in which one can slice (‘foliate’) a spacetime into spacelike hypersurfaces in the general theory of relativity. Given such a foliation, the theory partitions physical quantities into those ‘within’ each of the hypersurfaces and those that relate hypersurfaces to each other.
...
...
Hence the theory presented here and the classical theory of foliation must in reality be two limiting cases of a single, yet-to-be-discovered theory – the theory of the structure of the multiverse under quantum gravity.

Regards, Hans

 

[Fra]

I didn't follow this thread so forgive me for making fragmentary comments. (I do not adhere to MWI or any other particular camp)

Amusing when two solipsist aid each other in a discussion. At some point in time one would expect the two to get bitterly fighting about who is the real "source of the universe" and who is a product of imagination.

I think the conclusion they should arrive at is that there exists different views. I see no logical conflict in this. And those views that are favoured by the environment is those that will persist?

Who is real and who is a projection is mutual. I think of the identity of the observer as beeing the projection of the environment. Due to encoding constraints, all information about the environment can not possibly be projected onto a small observer, only a limited projection is sustained constituting the observer. But I guess that is also the key to explain the non-trivial dynamics we witness.

The solipsist may finally reach the agreement, that they consistently disagree about certain things. But generally two solipsists in the same environment will generally agree on the major part making up classical reality, mediated by the environment.

While the the objectivists will keep hunting their own tail failing to see that it is impossible to find an objective view that projects perfectly identically on two different observers

I don't see solipsism in physics having anything to do with fantasies or imagination in the negative sense. I merely see it as the state of the observer encodes the projection of the environment. Like Zurek said "What the observer knows is inseparable from what the observer is".

Meaning that "what views are valid" in the solipsism view? really means what observers will be seleceted/favoured in this environment? While it's easy to imagine that ANY observer has the chance, there is certainly going to be a selection that favours particular observers/particles/structures. And thus these "solipsist" views will come to dominate the environment, and thus giving appearance of agreement and objectivity. An unfit view or particle, will quickly destabilise in this environment so these contradictory views will not be a problem since they will be unlikely/rare.

/Fredrik

 

[xantox]

Well... Its actually what Deutsch calls quantum computing in his onlinecourse, although it is best understood using classical optics.

How one-photon realizations could be best understood using classical optics, and how classical optics could help explaining aspects of a quantum theory?

 

[vanesch]

I think you guys are forgetting where MWI comes from: it doesn't come from "observation" or anything. It is just a straightforward application of the axioms of quantum theory to the physical system that is "the observer", assuming that this observer is just as well part of "the physics" as anything else. So it is the application of quantum theory to "big systems". Whether that is allowed or not is left in the middle. It might be that quantum theory doesn't apply, the way we know it, to these big systems. A good reason might be that gravity plays a role in these bigger systems. But given that we don't have anything else yet that replaces quantum theory, we apply it, knowing very well that we apply quantum theory outside of its "proved scope of applicability".

So what are these famous axioms ? It is 1) the principle of superposition, which says that if |A> is a state that the system can be in, and |B> is a state that the system can be in, then any linear combination a |A> + b |B> is also a state that the system can be in. And it is 2) the fact that the time evolution of the system is given by a unitary operator U(t,t').

Well, if you apply that bluntly to "Joe saw the red light go on" and "Joe saw the green light go on", and you realize that "the red light go on" was: the particle hit detector 1, and "the green light go on" was "the particle hit detector 2", then it is obvious that you arrive very quickly at situations where Joe's situation is described as:
a |Joe saw the red light go on> + b |Joe saw the green light go on>

And it is difficult to interpret this. We know that it will end up in one way or another that Joe has |a|^2 chance to see a red light, and |b|^2 chance to see a green light. We know that Joe won't see both.

This is what *elementary quantum theory tells us*. And MWI stops there, while other interpretations go on fiddling, because they don't like what they see.

MWI says that the ACTUAL QUANTUM STATE of Joe is now the above superposition, but that "a Joe conscience" will only be aware of one of the Joe states, which means that there are now "two Joe's" around, and if you happen to be a Joe, you have |a|^2 chance to be the first one, and |b|^2 chance to be the second one.

Projection interpretations say that "upon observation" we have to re-interpret the superposition a |Joe saw the red light go on> + b |Joe saw the green light go on>
as just a statistical mixture of possibilities, of which only one "really happened" of course.
However, this last statement has a difficulty: why should the "Joe" superposition be interpreted as a statistical mixture, while (|s> + |p>) of an electron, not ?
Because we know, in quantum theory, that there is an observable difference between a statistical mixture of 50% s and 50% p, and the superposition |s> + |p>.
So SOME superpositions are "genuine" superpositions, and others are "just statistical mixtures", but quantum theory doesn't tell us which one is which ? It depends on the declaration of some system to be an "observer" or not ? Is an electron an observer ?

Still other interpretations change entirely the formalism of quantum mechanics (such as Bohmian mechanics), and give then a more "Newtonian" mechanical interpretation to the added parts to the formalism. The difficulty here is that these extra formal elements have been introduced just for the sake of giving a mechanistic interpretation without having any dynamical significance, and moreover destroying certain symmetries in the laws of nature.

This is why, personally, I prefer MWI *as an interpretation of quantum theory*, in that it tries to follow as faithfully as possible the fundamental axioms of quantum theory. But I realize that this means that we apply those axioms at scales far beyond its proven domain of applicability! Only, we don't have anything else. And moreover, out of this view doesn't come anything that is in blunt contradiction with observation. So is MWI "true" ? No idea ! I don't know if quantum theory applies to human bodies for instance. MWI is probably the view on QM which is most consistent with its formalism and at least, it doesn't lead to any contradiction. But it is of course far from "proven" - in fact, there's no way to prove it, beyond proving quantum theory correct on "larger and larger scales".

That's why I find the claims by Deutsch a bit disturbing: for the moment we haven't gotten any proof that quantum theory is applicable at the human scale or beyond. We even have a serious difficulty: gravity. So we haven't established the applicability of quantum theory at a scale which is assumed in MWI.

I would even say: imagine that we find that QM is limited in scope, and that we have to replace it with something else on a larger scale. That won't mean that we will not be studying QM anymore as an effective theory, just as we still use Newtonian mechanics. I would say that even then, MWI would be a good "view" on that approximate QM. We would treat it with a smile probably, because we might, for instance, know that due to lack of unitarity on a larger scale, the worlds "collapse" or whatever. But I think it would still be the best view on linear quantum theory. Simply because it sticks to its postulates all the way. So maybe MWI is only "valid" for a few milliseconds or whatever in this new theory.

 

[xantox]

vanesch, I completely agree with your post. But I didn't notice that Deutsch claimed to have proven MWI or anything like that, he seem to me just strongly supporting, the same view you just presented, that MWI is logically more consistent if we want to accept quantum mechanics literally (and we don't have much alternatives right now).

 

[Hans de Vries]

How one-photon realizations could be best understood using classical optics, and how classical optics could help explaining aspects of a quantum theory?

This is not the point. I'm simply objecting to the lingo he uses, like:
"The outcome of this experiment depends on events in another universe" ...
while he describes a simple optical interference experiment.

One can have an interpretation hypothesis but don't preach it as being
an absolute truth. In my opinion this is a lack of respect towards those
students who can't yet distinguish between the scientifically proven
facts here and his personal hypothesis / pet theory.


Regards, Hans

 

[reilly]

Yes, I know a small of QM.

I like to think that I know a bit about QM, both the physics and the math. Enough to land me a job teaching QM -- I even managed to get my PhD with a dissertation involving QED, and learned my QM at Harvard and Stanford. Why, I've even lectured at Harvard, and the Fermi Lab -- when it was Argonne National Lab -- on relativistic QM and radiative corrections. I feel I'm safe in saying I understand things like superposition, and spin, and how to calculate cross sections, why I even understand both Fermi-Dirac and Bose-Einstein statistics, and complex angular momentum... So,

Frankly, I fail to see how the "axioms -- or whatever you want to call them --" imply the MWI approach, as some seem to imply.

Worse yet, I believe in wave-function collapse; it occurs in people's brains as we gain knowledge of which alternative actually happens. There is absolutely no doubt that such a mental collapse occurs; we've all experienced such a collapse or change in mental state many times. You are stuck for a moment seeing someone you might have known once. Then "Aha, yes that's Ed from my previous job", That is, we get a change in mental state as our knowledge changes. And, people in the neurosciences are understand more and more how this collapse" occurs.

This knowledge-based approach was championed by the Nobelist Sir Rudolph Peierls. It ties into what I like to call the Practical Copenhagen Interpretation -- PCI. That is, use the Schrodinger Eq, or appropriate variations thereof to compute wave functions; and use Born's idea that the absolute square of the wave function is a probability density. Use standard probability theory to continue; leave the collapse to the neuroscientists. That is, QM uses standard probability theory -- what else could it be?

After having written about this in many threads, I'm delighted to find an ally in Fra.

I assume, in a rejoinder, that you can compute 9-j symbols and fractional parentage coefficients, compute, say, a cross section for double pion photoproduction from a hadron, or get the exact solutions to the two-level atom interacting with the quantized E&M radiation field.

Shadow photons? Your explanation appears to be rather disjoint from Deutsch's discussion in, as some denote it, FAR. That his discussion is poetic is open to some doubt.

Regards,
Reilly Atkinson

(Sorry about all that name-dropping. Sometimes it just happens)

How familiar are you with the mathematical structure of conventional (non-MWI) QM? Do you understand the idea that a quantum system is assigned a quantum state which evolves over time according to the Schroedinger equation, and that each quantum state involves a "superposition" of different possible eigenstates which correspond to particular measurement outcomes, with each measurement "collapsing" the system's state onto one of the eigenstates with a probability of collapsing into any eigenstate proportional to the square of its amplitude in the superposition before the measurement? If you are, then as I understand it the MWI twist on this is that there is no "collapse" on measurement, that the universe is assigned a single state which remains in a massive superposition, and that each macroscopically-distinct element of the superposition will appear as a distinct "world" to its inhabitants. So the question of the number would be somewhat subjective, depending on how coarse-grained a measure of "macroscopically-distinct" you use...the Everett FAQ says in question #11:

The FAQ also says in questions 6, 7 and 19 that worlds do "split" in the sense of their being multiple macroscopically-distinct later states for a single earlier state, so your question 2 wouldn't really apply. As for your own question 3, are you familiar with the Feynman path integral or sum-over-paths formalism in conventional QM, where the probability of measuring a particular outcome is calculated by doing a certain type of sum of all possible pathways leading up to that outcome, and allowing the different pathways to interfere with one another? I think Deutsch's talk about "shadow photons" is just a poetic way of discussing this, but with Deutsch believing that each path is actually taken by an alternate version of the photon.

 

[Count Iblis]

Well, if you apply that bluntly to "Joe saw the red light go on" and "Joe saw the green light go on", and you realize that "the red light go on" was: the particle hit detector 1, and "the green light go on" was "the particle hit detector 2", then it is obvious that you arrive very quickly at situations where Joe's situation is described as:
a |Joe saw the red light go on> + b |Joe saw the green light go on>

And it is difficult to interpret this. We know that it will end up in one way or another that Joe has |a|^2 chance to see a red light, and |b|^2 chance to see a green light. We know that Joe won't see both.

Why not interpret the entangled superposition as the situation Joe was in before he saw the light as that state is related to the suprposition under a unitary transformation? If you want to do a measurement on Joe before he saw the light, you can still do that measurement after he interacted with the light as long as he is in that superposition.

If L is the operator that measures what light Joe is seeing with eigenvalues 0 (no light), 1 (red light), and 2 (green light), then the operator ULU^(-1) with U the time evolution operator applied to the superposition shoud yield 0, so this suggests that the (entangled) superposition is just the old Joe (plus environment) who hasn't seen the light yet.

 

[JesseM]

I assume, in a rejoinder, that you can compute 9-j symbols and fractional parentage coefficients, compute, say, a cross section for double pion photoproduction from a hadron, or get the exact solutions to the two-level atom interacting with the quantized E&M radiation field.

No, I have only an undergraduate education so far. Are these things relevant to the topics under discussion now? Look, I didn't ask you your background because I wanted to start a physics pissing contest, sorry if you were offended but I just asked because of course I have no idea what a given username on this forum might know (unless I happen to remember from previous interactions with them), and my answers to your questions did depend on certain background knowledge.

Shadow photons? Your explanation appears to be rather disjoint from Deutsch's discussion in, as some denote it, FAR. That his discussion is poetic is open to some doubt.

Can you explain what specifically in Deutsch's explanation doesn't fit with the idea that he is granting equal reality to all the paths in the path integral?

 

[reilly]

Apologies to JesseM

Yes, I did not need to wave my credentials, and apologize for so doing. When I was in graduate school and then when I was teaching, physics was a contact sport -- more than once I got hammered when giving a seminar, and more than once did the hammer thing myself. I still, after more than 40 years, have a Pavlovian response when there's even a suspicion that my credentials or ideas are being challenged. Much to my chagrin, I do not always keep my cool under such circumstances.

I know there are many who agree with me when I say that you never really understand QM until you have taught it, which means first a dissertation or long paper based on QM. In other words, you have to do it. Book learnin' is not enough. Got to deal with hbars, and 2pis, and signs, and tons of algebra with reality checks.

Your questions are perfectly reasonable.

Forget about shadow photons unless you want to get hooked into a long chain of contradictions.

Regards,
Reilly Atkinson

No, I have only an undergraduate education so far. Are these things relevant to the topics under discussion now? Look, I didn't ask you your background because I wanted to start a physics pissing contest, sorry if you were offended but I just asked because of course I have no idea what a given username on this forum might know (unless I happen to remember from previous interactions with them), and my answers to your questions did depend on certain background knowledge.

Can you explain what specifically in Deutsch's explanation doesn't fit with the idea that he is granting equal reality to all the paths in the path integral?

 

[reilly]

Fra -- Right on

Great to have an ally. The Bohr quote is great -- thanks.
Regards,
Reilly

IMO. The only thing that collapses is the projection that lives inside the observer, which I see no more weird than similar to a bayesian update.

Why my view of the world change when I receive more information about it, is quite obvious. Whatever the world REALLY is, still has to be projected onto my perspective.

I think these words of Niels Bohr's still stands:

"It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature...".

/Fredrik

 

[JesseM]

Yes, I did not need to wave my credentials, and apologize for so doing. When I was in graduate school and then when I was teaching, physics was a contact sport -- more than once I got hammered when giving a seminar, and more than once did the hammer thing myself. I still, after more than 40 years, have a Pavlovian response when there's even a suspicion that my credentials or ideas are being challenged. Much to my chagrin, I do not always keep my cool under such circumstances.

Thanks for that, and no offense taken, I know from personal experience that it's especially easy on the internet to jump to conclusions about someone's tone or about what they're implying.

I know there are many who agree with me when I say that you never really understand QM until you have taught it, which means first a dissertation or long paper based on QM. In other words, you have to do it. Book learnin' is not enough. Got to deal with hbars, and 2pis, and signs, and tons of algebra with reality checks.

Your questions are perfectly reasonable.

Forget about shadow photons unless you want to get hooked into a long chain of contradictions.

Well, I wouldn't disagree with the idea that the best way to learn physics is by doing. But interpretations like the MWI or Bohmian mechanics involve a fair amount of mathematical elaboration too, I think it'd be a mistake to dismiss some element of an interpretation as self-contradictory just based on what one of the interpretation's advocates says in a nontechnical discussion aimed at a broad audience (of course you might also argue that thinking about 'interpretations' is pointless if they make no new predictions, even if the interpretation doesn't involve any internal contradictions).

 

[vanesch]

Worse yet, I believe in wave-function collapse; it occurs in people's brains as we gain knowledge of which alternative actually happens. There is absolutely no doubt that such a mental collapse occurs; we've all experienced such a collapse or change in mental state many times. You are stuck for a moment seeing someone you might have known once. Then "Aha, yes that's Ed from my previous job", That is, we get a change in mental state as our knowledge changes. And, people in the neurosciences are understand more and more how this collapse" occurs.

This knowledge-based approach was championed by the Nobelist Sir Rudolph Peierls. It ties into what I like to call the Practical Copenhagen Interpretation -- PCI. That is, use the Schrodinger Eq, or appropriate variations thereof to compute wave functions; and use Born's idea that the absolute square of the wave function is a probability density.

But the point is that the absolute square of the wave function isn't ALWAYS a probability density! So when does it *become* a probability density ? This is the main issue which leads to considerations of an MWI approach.
See, in the double slit experiment, when the wave function is a superposition of "is in left slit" and "is in right slit", it is NOT a probability density. For if it were, we wouldn't have any interference, as we would simply have that the final probability to have a hit in position X would be P(X | A) P(A) + P(X|B) P(B) where A stands for slit 1 and B stands for slit 2.

So along the way, the wavefunction evolves from something that clearly is NOT a probability density (when it is at the two slits) to something that IS a probability density (when it "hits the screen"). It is this undocumented "change in nature" of the wavefunction (from "a physical entity which is not a probability" to "a probability") which stands in the way of a pure "bayesian collapse" view on the measurement problem.

You can get around this in several ways. You can insist on the bayesian nature all the way, but then, in order to get out interference, you have to introduce extra elements. That's what the Bohmians do. The particle DID go through just slit 1 or just slit 2, and it was a complicated dynamics with a *physical* wavefunction which NEVER becomes a probability distribution which guides it to the interference image.
Or you can insist on the non-bayesian nature of the wavefunction all the way, accepting it as a purely physical state, and that's what MWI does for instance.
You can also alter the dynamics of the wavefunction, and give it a physical status all the way, at which point the "collapse" is just part of the (non-linear) dynamics.

But you cannot "sneak in a change in PoV" which is implicitly done when the wavefunction goes from (certainly not a probability function) on microscopic level to (a probability function) at the Heisenberg cut, without at least some explanation where this change came about, because the question that comes up then is:
why can't I consider, from the start, an electron orbital as just a probability density to find the electron ? And you know very well that if you do so, that you do not get out the same results than when you keep the electron wavefunction as a wavefunction and not a statistical mixture of positions.

 

[Fra]

I agree with part of what Reily says but also with part of what Vanesch says. I don't know if my view is the same as Reilly's all the way but this is what I personally think:

A plain bayesian view _alone_ is insufficient (agreeing with Vanesh), but it's IMO most probably part of progress, but not end of story! That's why I think we need to go back to probability theory and think again, we need more...

IMO, the clearly identifiable problem is that the probability space is not clearly coupled to the current information and history like I think it should, this is why the arbitrary decomposition between coherent or non-coherent mixtures occur. I don't see this as a pure QM problem, it goes deeper and may relax unitarity because the probability space itself is uncertain and dynamical. I think this falls in the category that Vanesch calls the non-linear approach.

This is what I personally see as the route forward in the spirit of "minimum speculation".

But the point is that the absolute square of the wave function isn't ALWAYS a probability density! So when does it *become* a probability density ?

I'd choose to say it becomes a probability density when more information about the "microstructure" of our system is collected, or when the probability space has stabilised (more or less the same thing). But of course this is a non-unitary process. The evolution isn't effectively unitary until the probability space has stabilised.

This is reasonably similar to some decoherence based ideas but one still need to quantify in terms of knowledge how certain we are that we have a closed system? usually we don't know and I consider that a fact, it just _seems to be closed_, and I suggest we quantify this. Also considering the system + observer, doesn't solve the point, it just moves the point.

I imagine the "probability space" as something that is emergent as the observer evolves in the environment, once the observer is equilibrated in it's environment I think the residual uncertainty will most probably be the ordinary unitary QM. But that's clearly a special case.

I can't wrap my head around unitary evolution in the general case. The unitary pictures needs to be selected.

/Fredrik

 

[confusedashell]

I no longer understand half of this thread:P but anyway thanks a lot guys.
I've decided not to put too much faith into MWI, not only because it's bizarre, but I was handed some evidence and tests done against it which convince me it's bull****.
I think MWI is built pure on wishful thinking of some and convictionby others.
Since there's NO proof or anything indicating MWI to be true, I think I'll go on living in reality without "reconsidering" how it works:PP

Thanks a lot

 

[Fra]

Confused, I'd suggest you try to spend some time thinking on your own. There is usually no replacement for thinking on your own and coming up with our own conclusions. Learning from others is great but some things you just have to decide for yourself. If you feel you can not make such a decision I suggest you read up on the foundations and the philosophical issues with measurements and go through the pain :) Chances are you haven't appreciated the question to start with, when faced with the possible answer to these questions when then of course make no sense because it's difficult to relate to.

/Fredrik

 

[xantox]

This is not the point. I'm simply objecting to the lingo he uses, like: "The outcome of this experiment depends on events in another universe" ...
while he describes a simple optical interference experiment. One can have an interpretation hypothesis but don't preach it as being an absolute truth.

OK, I agree that he should use some precautions in introducing the interpretation. However, concerning the experiment, in lecture 2 he's describing a single-photon setup, so as it can be correctly viewed as a quantum computation without classical analog.

 

[confusedashell]

Confused, I'd suggest you try to spend some time thinking on your own. There is usually no replacement for thinking on your own and coming up with our own conclusions. Learning from others is great but some things you just have to decide for yourself. If you feel you can not make such a decision I suggest you read up on the foundations and the philosophical issues with measurements and go through the pain :) Chances are you haven't appreciated the question to start with, when faced with the possible answer to these questions when then of course make no sense because it's difficult to relate to.

/Fredrik

Good advice.
it's just, MWI, seems retarded to me, from common sense and what i know of science and quantum physics someone on here provided me links to proof against MWI so.
Dno why it's even wasted more money on researching, guess its like... faith lol:P

I maen in one universe, you'd take lastic surgery to look like ur mom after rapin and killin her, just because it's physically possible. I maen seriously how the **** can anyone believe in **** like this

 

[Demystifier]

Vanesch, have you been thinking about writing a long pedagogic paper on MWI where all FAQs would be clearly and honestly answered? I strongly believe that you could do that very well, perhaps better than any already existing paper on that subject. Next time when somebody asks a question, you just tell him - go and read Section XX in my paper on arXiv. If you do that paper well (as I am sure you could) you could even publish this paper in a journal like Foundations of Physics.

 

[xantox]

Good advice.
it's just, MWI, seems retarded to me, from common sense and what i know of science and quantum physics someone on here provided me links to proof against MWI so.

There are no "proofs" against MWI around. Also, common sense is useless, since many scientific theories like relativity, also go against common sense and they are nevertheless correct. There are two kind of people who could be interested in MWI. The first, is scientists and philosophers of science, and they of course need to deal with MWI seriously, and it is not a waste of time at all. The second, is everyone of us in their daily life, and in this case you don't need anything like an interpretation of quantum theory as a foundation for your actions and moral life like you're trying to. Better go around with your girlfriend!

I maen in one universe, you'd take lastic surgery to look like ur mom after rapin and killin her, just because it's physically possible.

No, you would not do that even in MWI.

 

[confusedashell]

if u don't do every possible action then MWI is ultimately wrong. so mwi is ultimately wrong. Common sense:) 2+2 still gon be 4 even if some scifi professor makes up some untestable claims.
Untesteable = non science in my eyes. Philosohpy at best in MY eyes.

I declare MWI dead and won't waste more time on it, but please use this thread to share ur ideas for and against it:)

http://www.boingboing.net/2004/04/26/many-worlds-theory-i.html << against MWI

 

[Fra]

If MWI is the wrong answer, to what question is it the wrong answer? :) All I know is that it does not seem like the answer to any questions of mine. But then I need to be humble enough to confess I can not and should not speak for others.

Perhaps the trick is to pose the right question, and then MWI may seem more reasonable. My heavy impression from talking to people on here as well as reading the posts is that ultimately we disagree on the formulating of the core questions and what is the eye of the problems that determines the direction of our efforts. This is reflected in the questions we ask.

Like has been pointed out many times, choosing the questions and finding the answers are closely related.

/Fredrik

 

[xantox]

if u don't do every possible action then MWI is ultimately wrong. so mwi is ultimately wrong. Common sense:)

Wrong :-). Look, when you decide to do something, you're using your rational faculties, isn't it? You have some reason to do it, it's not like you do it at random. Just like a computer will say that 1+1=2 in almost all universes, and not any result at random. So even in MWI, in almost all other universes, you will use your reason in exactly the same way, so as you will not do nasty things if you are not a nasty person to start with.

PS/ That boingboing article is relating known factoids which were proven wrong. Afshar experiment didn't invalidate anything, since standard quantum theory predicts exactly the same results. Since MWI has the same predictions as standard quantum theory, it can't be invalidated by such experiment.

 

[vanesch]

if u don't do every possible action then MWI is ultimately wrong. so mwi is ultimately wrong. Common sense:) 2+2 still gon be 4 even if some scifi professor makes up some untestable claims.
Untesteable = non science in my eyes. Philosohpy at best in MY eyes.

I declare MWI dead and won't waste more time on it, but please use this thread to share ur ideas for and against it:)

http://www.boingboing.net/2004/04/26/many-worlds-theory-i.html << against MWI

This famous "Afshar experiment" and its erroneous analysis was what Hans de Vries was referring to earlier, and reduces to a classical optics experiment, misunderstood.

The so-called transactional interpretation has 2 problems: first of all, things need to come "from the future", and second, I've never seen it being expanded beyond the single-particle situation. Now, maybe I'm wrong on the second point.

In any case, there cannot be an invalidation of MWI without also an invalidation of quantum theory on a certain level. It is not impossible, and it would surely be interesting news, but as long as quantum theory is assumed correct, there's no way to invalidate MWI. Copenhagen is even worse, because Copenhagen can handle it both ways (it has the extra freedom of choosing where to put the Heisenberg cut).

So you will NEVER read that MWI has been falsified. You might read one day that *quantum theory* has been falsified. MWI falls then with it. Copenhagen can still survive.

People claiming that MWI has been proven, or that MWI has been falsified, independent of quantum theory itself, are obviously not knowing what they are talking about: it is an *interpretation* of the formalism. As such, it makes exactly the same predictions as the formalism. Only, MWI needs the validity of the quantum formalism for macroscopic systems, which might not be applicable. If it is not applicable, then we've found a LIMIT to the applicability of the *quantum formalism*. That's much bigger news than an interpretation being correct or not!

 

[reilly]

vanesch -- I disagree; I have no idea why the absolute square of a wave function can be anything else but a probability density, given Born's ideas.

Your probability density argument disagrees with both QM and classical electrodynamics; with any system subject to a linear wave equation. Huygens' Principle guarantees that the absolute square of the wave function, or intensity , contains interference terms. Why? Always, according to Huygens, a solution to a wave equation can be expressed as a sum, so the intensity contains interference terms -- if the sum has two or more terms.

Another way to look at it is to go from a scattering problem -> Fraunhofer Diffraction.

Forgetting that initial conditions for a diffraction problem are tricky, one could state that the initial value of the wave function is 0, except for two disjoint finite intervals with values, w1 and w2. Use a Feynman propagator, the Lippman-Schwinger or resolvant approaches, and effectively what you see is sort of equivalent to two beams, one from each slit, and these beams interact and thus scatter.

Re Bayesian collapse: we talk about the probabiity of an event -- or more than one. All QM normally does is to predict an event, and nothing after the event. There's no real problem of Bayesian collapse or inference. Suppose you are in traffic and figure the odds of making the next light, are, say 2-1. Once you get to the light, you know, so the probability estimate is irrelevant -- unless you are considering a series of lights, or ..

And the QM dynamics guarantee that the norm of the solution is invariant under time translations , except for time dependent Hamiltonians,..., so, it seems to me, that

1. QM gives a perfectly reasonable probability system -- with wave functions or more generally, density matrices. The absolute square of any solution of a linear wave equation provides a valid probability measure.

2. If, as we do in probability, talk about events, much of the mystery of measurement goes away.Prob. theory assumes only that we can recognize events, and distinguish between them. Einstein talked about events and their relationships, but didn't say much about the details of reading a clock. It seems to me that if we keep things simple we can do much of what we want to do -- simple being the use of probability theory's and Einstein's events, and assuming that we can make the appropriate measurements.

3. A theory of measurements is, in my view, way beyond us. But that's not a problem as we do pretty well without one- true classically and "quantumly". Would it be better to have such a theory? Yes, of course.

Regards,
Reilly

But the point is that the absolute square of the wave function isn't ALWAYS a probability density! So when does it *become* a probability density ? This is the main issue which leads to considerations of an MWI approach.
See, in the double slit experiment, when the wave function is a superposition of "is in left slit" and "is in right slit", it is NOT a probability density. For if it were, we wouldn't have any interference, as we would simply have that the final probability to have a hit in position X would be P(X | A) P(A) + P(X|B) P(B) where A stands for slit 1 and B stands for slit 2.

So along the way, the wavefunction evolves from something that clearly is NOT a probability density (when it is at the two slits) to something that IS a probability density (when it "hits the screen"). It is this undocumented "change in nature" of the wavefunction (from "a physical entity which is not a probability" to "a probability") which stands in the way of a pure "bayesian collapse" view on the measurement problem.

 

[Hans de Vries]

OK, I agree that he should use some precautions in introducing the interpretation. However, concerning the experiment, in lecture 2 he's describing a single-photon setup, so as it can be correctly viewed as a quantum computation without classical analog.

Despite the single photon setup, nothing in the outcome of the experiment
depends on the quantum behavior of photons. That is, an experiment with
bursts of sound waves or water waves would lead to the same interference
result.

An experiment which does show single photon quantum behavior for instance
uses two detectors at both outputs of a beamsplitter (typically a Wollaston
prism) and demonstrate that only one of the two detectors goes of after
a single photon went through the beamsplitter.


Regards, Hans

 

[xantox]

Despite the single photon setup, nothing in the outcome of the experiment depends on the quantum behavior of photons. That is, an experiment with bursts of sound waves or water waves would lead to the same interference result.

Yes, but since in this setting there is no water, I think we have to explain what happens by using single photons, and here the classical explanation fails. It is not about understanding interference, since it's a course in quantum computation. If we send each photon through a beam splitter, followed by a mirror, followed by a beam splitter, and try to interpret this by using classical bits, we would have a coin flip followed by a NOT operation followed by a coin flip, which doesn't yeld the identity operation performed by the quantum system.

 

[vanesch]

vanesch -- I disagree; I have no idea why the absolute square of a wave function can be anything else but a probability density, given Born's ideas.

Your probability density argument disagrees with both QM and classical electrodynamics; with any system subject to a linear wave equation. Huygens' Principle guarantees that the absolute square of the wave function, or intensity , contains interference terms. Why? Always, according to Huygens, a solution to a wave equation can be expressed as a sum, so the intensity contains interference terms -- if the sum has two or more terms.

The difference between a classical wave superposition and a quantum interference is the following: in the classical superposition of a wave from slit 1 and a wave from slit 2, we had not a PROBABILITY 50% - 50% that the "wave" went through slit 1 or slit 2, it went physically through BOTH. In classical physics, intensity has nothing to do with probability, but just the amount of energy that is in a particular slot. HALF of the energy went through slit 1 and HALF of the energy went through slit 2 in the classical wave experiment.

But we cannot say that "half of the particle" went through slit 1 and "half of the particle" went through slit 2, and then equate this with the *probability that the entire particle* went through slit 1 is 50% and the probability that it went through slit 2 is also 50%, because if that were true, we wouldn't have a LUMPED impact on the screen, but rather 20% of a particle here, 10% of a particle there, etc...

Now, it is so that the application of the Born rule to a quantum optics problem gives you in many cases just the classical intensity as a probability, which instores somehow the confusion between "classical power density" and "quantum probability" but these are entirely different concepts. The classical power density has nothing of a probability density in a classical setting, and a probability has nothing of a power density in a quantum setting.

The classical superposition of waves has as much to do with probabilities as the superposition of, say, forces in Newtonian mechanics. Consider the forces on an apple lying on the table: there's the force of gravity, downward, and there's the force of reaction of the table, upward. Now, does that mean that we have 50% chance for the apple to undergo a downward fall, and 50% chance for it to be lifted upward ? This is exactly the same kind of reasoning that is applied when saying that two classical fields interfere is equivalent to two quantum states interfering. The classical fields were BOTH physically there (just as the two forces are). Their superposition gives rise to a certain overall field (just as there is a resultant force 0 on the apple). At no point, a probability is invoked.

But in the quantum setting, the wavefunction is made up of two parts. If this "made up of two parts" is interpreted as a probability at a certain point, you'd be able to track the system on the condition that case 1 was true, and to track the system on the condition that case 2 was true. But that's not what is the result when you compute the interference of the two parts. So when the wavefunction was made up of 2 parts, you CANNOT consider it as a probability distribution. But when the interference pattern is there, suddenly it DID become a probability distribution. THAT'S where a sleight of hand took place.

Forgetting that initial conditions for a diffraction problem are tricky, one could state that the initial value of the wave function is 0, except for two disjoint finite intervals with values, w1 and w2. Use a Feynman propagator, the Lippman-Schwinger or resolvant approaches, and effectively what you see is sort of equivalent to two beams, one from each slit, and these beams interact and thus scatter.

Absolutely. But classically, that would mean that BOTH beams are physically present, and NOT that they represent a 50/50 probability!

Re Bayesian collapse: we talk about the probabiity of an event -- or more than one. All QM normally does is to predict an event, and nothing after the event. There's no real problem of Bayesian collapse or inference. Suppose you are in traffic and figure the odds of making the next light, are, say 2-1. Once you get to the light, you know, so the probability estimate is irrelevant -- unless you are considering a series of lights, or ..

And the QM dynamics guarantee that the norm of the solution is invariant under time translations , except for time dependent Hamiltonians,..., so, it seems to me, that

1. QM gives a perfectly reasonable probability system -- with wave functions or more generally, density matrices. The absolute square of any solution of a linear wave equation provides a valid probability measure.

This is ONLY true when ACTUAL measurements took place! You cannot (such as at the slits in the screen) "pretend to have done a measurement" and then sum over all cases, which you can normally do with a genuine evolving probability distribution.
If the wavefunction decomposes, at a time t1, into disjoint states A, B and C, then, if it were possible to interpret the wavefunction as a probability density ALL THE TIME, it would mean that we can do:

P(X) = P(X|A) P(A) + P(X|B) P(B) + P(X|C) P(C)

and that *doesn't work* in quantum theory if we don't keep any information (by entanglement for instance) about A, B or C. So when we have the wavefunction expanded over A, B and C, we CANNOT see it as giving rise to a probability distribution.

Now, of course, for an actual setup, quantum theory generates a consistent set of probabilities for observation in the given setup. But the way the wavefunction behaves *in between* is NOT interpretable as an evolving probability distribution.
So the wavefunction is sometimes giving rise to a probability distribution (namely at the point of "measurements"), but sometimes not.

Now, you can take the attitude that the wavefunction is just "a trick relating preparations and observations". Fine. Or you can try to give a "physical picture" of the wavefunction machinery. Then you're looking into interpretations.

 

[Fra]

What I personally meant with "going back to the origin and axioms of probability theory" for the resolution, and that I thought Reilly also referred to is something very basic, but that I think is causing a lot of problems.

It's the idea that while we cannot with arbitrary precision determine the future, we CAN with arbitrary precision determine the probability for any possibility. This simple statement occurring for example in the first chapter of Diracs "The Principles of Quantum Mechanics" is IMO where we lay ground for the future problems.

Then there are attempts to deal with this problem with density matrixes and non-coherent mixtures and decoherence. But so far I've personally found that insufficient.

This I see touching the philosophy of probability theory and information theory, and is thus not QM-specific as such.

The real problem I see is that the usual argument is the frequentist idea that we can find the probability by carrying out an infinite number of experiments. Why this is highly unsatisfactory should be clear if you consider processing power and information capacity. We can not make imaginary use of information the belongs to the future.

My personal questions starts here. How can we improve the foundation?

I am trying to find a possible revision of the probability formalism which introduces a fuzzier(non-unitary) formalism, where the probabilities as we know them today will be more appropriately seen as "subjective estimated" probabilities, induced from *incomplete information* and thus the probability space itself is not yet known.

The idea I favour is that these estimated probabilities are encoded in the microstructure of the observer, and that the probabilities simply correspond to uncertainty in the observers microstructure and state, which in turn is a reduced projection of the environment.

This will I hope yield standard probability as emergent. And the idea is to assign also a physical basis for the formalism itself, and the probability space. Usually this is carelessly done and one imagines infinite amounts of data and infinite experiments to justify the probability, but rarely does one consider in what physical structures this information is encoded? Some decoherence ideas considers the environment as an information sink, which records everything in correlactions, but that is also a problem because the observer only sees a fraction of the environment. So there is still something missing.

Reilly is this anything like you had in mind as well or what did you mean with "going back to probability theory"

/Fredrik

Edit: Another issue is that I don't think it's in general valid to consider the environment as an infinite information sink. One can imagine a case where the observers informationcapacity is comparable to the environment, then the sink idealisation fails. And I guess the key is that an observer does not have an a priori knowledge of the size of the environment where it's immersed. The only way to find out is to interact and try to learn something.

 

[Fra]

Edit: Another issue is that I don't think it's in general valid to consider the environment as an infinite information sink.

This is probably a very good approximation when it regards human particle physics experiments, because then in effect we control and monitor the effective environment of the localised experiment and we could probably quite well in principle inform us about correlations in the environment with the system.

But this idealisation doesn't seem near as appealing if one considers cosmology or gravity interactions, or cases where the observers complexity is far less than the complexity of the system under study.

/Fredrik