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

[LURCH]

Oh, very nice, Count. You really are evil, aren't you?!

 

[reilly]

Jive

The fact that 1. nobody has tried to answer my questions, or 2. that nobody has suggested that my questions are dilatory helps to convince me even more that MWI is jive.

I know that my questions are not dilatory, as I've discussed them with quite a few physicists, some of whom think I'm out of my mind due to my disdain for MWI, and some who agree with me.

I'm convinced that MWI is simply a romantic attempt to get around the use of probability by ascribing some type of reality to each possible event in a probability system -- this could have been as easily done several hundred years ago as now. If that was the case , the idea lay dormant for several centuries, as it still should in my opinion. Where's Occam?


I'll stay more open minded if somebody could answer my questions, or indicate that they are nonsense.
Regards,
Reilly Atkinson

I'll ask three questions:

1. How many universes are there -- what kind of Cantorian infinity are we talking?

2. If there is no splitting, what conservation law attends to the constancy of the number of universes?

3. Can anyone explain, without contradictions, Deutsch's rather odd notion of shadow photons?

Regards,
Reilly Atkinson

 

[Count Iblis]

The fact that 1. nobody has tried to answer
...

I'll stay more open minded if somebody could answer my questions, or indicate that they are nonsense.
Regards,
Reilly Atkinson

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?

 

[JesseM]

I'll ask three questions:

1. How many universes are there -- what kind of Cantorian infinity are we talking?

2. If there is no splitting, what conservation law attends to the constancy of the number of universes?

3. Can anyone explain, without contradictions, Deutsch's rather odd notion of shadow photons?

Regards,
Reilly Atkinson

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:

Q11 How many worlds are there?

The thermodynamic Planck-Boltzmann relationship, S = k*log(W), counts the branches of the wavefunction at each splitting, at the lowest, maximally refined level of Gell-Mann's many-histories tree. (See "What is many-histories?") The bottom or maximally divided level consists of microstates which can be counted by the formula W = exp (S/k), where S = entropy, k = Boltzmann's constant (approx 10^-23 Joules/Kelvin) and W = number of worlds or macrostates. The number of coarser grained worlds is lower, but still increasing with entropy by the same ratio, i.e. the number of worlds a single world splits into at the site of an irreversible event, entropy dS, is exp(dS/k). Because k is very small a great many worlds split off at each macroscopic event.

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.

 

[xantox]

Where's Occam?

Occam is in the fact that MWI is just that "every closed system evolves according to the Schrödinger equation".

Worlds are defined by decoherence, this was studied by Gell-Mann, Hartle and others.

 

[Hans de Vries]

I'll stay more open minded if somebody could answer my questions, or indicate that they are nonsense.
Regards,
Reilly Atkinson

Discussions like these tend to depress me especially if they start
involving solipsism, That makes me run away as fast as I can
These discussions are more appropriate in the philosophy section.
How can one claim (referring to Deutsch), that, at the same time,

A) Different worlds do not interfere.
B) Each path in a path integral belongs to a different world.

The latter means that they interfere maximally and that the single world we
live in is just undetectable noise in an interference pattern.

The total number of worlds which should exist at the same time to support
the ideas floating around during these discussions must dwarf the number
of elementary particles in the universe. Nobody bothers?
What leap of faith is needed to believe in solipsism, especially if the solipsist
never manages to prove his/her claim by performing any miracles, "acts of god"
or other such things, but instead has to argue and discuss endlessly and
mostly fruitlessly with the products of his own imagination (the other people)
to try to convince them that they just own their entire existence to his or
her consciousness? Must be frustrating...

And then, when the solipsist finally manages to convince a product of his/her
imagination about solipsism, then it is only to convince it that it itself is the
sole source of the universe, and, the poor solipsist, well, he/she just becomes
degraded to a product of imagination...

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.Regards, Hans

 

[JesseM]

How can one claim (referring to Deutsch), that, at the same time,

A) Different worlds do not interfere.

When does Deutsch say this? http://findarticles.com/p/articles/mi_m1511/is_n10_v16/ai_17449599/pg_2 suggests otherwise:

An extremely large number--perhaps an infinite number--of these parallel worlds exist, says Deutsch. They reveal their existence whenever a particle has the opportunity to follow more than one path--which, of course, is essentially always the case. When a photon, for example, goes through one of the two slits, something exceedingly strange happens. In our universe, Deutsch says, the photon goes through one slit. But in some other universe, it goes through the other. These alternate realities thereafter continue to exist, with identical pasts but different futures. ("There is another universe that is as real as ours," Deutsch said during an interview, "in which I fail to get through to you today, and we'll only be talking tomorrow.") The interference pattern we see in the two-slit experiment, says Deutsch, arises because the two universes interact.

Such interference between worlds, he points out, is detectable only under very carefully controlled conditions. The interference is a rare example of two universes that have briefly diverged--in this case when the photons went through different slits--and then merged into one reality, leaving only the interference pattern as evidence of their once independent status. Normally, outside the bounds of an experiment specifically designed to create interference, particles that split off into separate realities will go off and collide with various other particles, and these with still others, in an unending, diverging cascade. The chance that any two of these many different worlds, each marching to the beat of its own capricious quantum drummer, will subsequently evolve along identical paths is vanishingly small. So while many David Deutsches occupy these assorted worlds, they will never meet. Only the evanescent warp and woof of quantum mechanical interactions stitch these universes together.

The total number of worlds which should exist at the same time to support
the ideas floating around during these discussions must dwarf the number
of elementary particles in the universe. Nobody bothers?

This seems more like an "argument from incredulity" than an actual scientific or philosophical argument.

 

[Hans de Vries]

When does Deutsch say this? http://findarticles.com/p/articles/mi_m1511/is_n10_v16/ai_17449599/pg_2 suggests otherwise:

I'm aware what Deutsch says. I've seen his video's and studied his
experimental setups, which can be explained entirely from classical
optics without any need for MWI claims at all.

He claims the interference is extremely small so that entirely different
worlds can coexist without notably disturbing each other. That is,
you don't run into a car passing in another world while swimming in
the pool.

These ideas have little or nothing to do with what the path integral
formalism says and they defy quantum mechanics and most of its
applications.

There's a huge industry using molecular modeling algorithms based on
the notion that the wave function is a distributed charge/spin density
and that it is the whole wavefunction which contributes to the electric
and spin /angular momentum magnetic fields which determine the
properties of molecules and solid state materials.

In David Deutsch's picture the atom is a core with a classical particle
rotating around it, taking one path in one world and a different path
in another world. This is not correct.

This seems more like an "argument from incredulity" than an actual scientific or philosophical argument.

With a single human body containing ~10^30 elementary particles
with all of them splitting up into endless numbers of paths at the
femtometer/attosecond scale, the number of different worlds add
up very fast, for a single human, let alone for an entire world.Regards, Hans

 

[Count Iblis]

Hmmm, I thought that the wavefunction of the entire muliverse is supposed to be in an eigenstate of the Hamiltonian, so we can put H|psi> = 0, so it doesn't evolve in time, it doesn't split, it is static.

 

[JesseM]

I'm aware what Deutsch says. I've seen his video's and studied his
experimental setups, which can be explained entirely from classical
optics without any need for MWI claims at all.

Plenty of quantum phenomena can't be explained with classical optics, like entanglement or quantum computing.

He claims the interference is extremely small so that entirely different
worlds can coexist without notably disturbing each other. That is,
you don't run into a car passing in another world while swimming in
the pool.

These ideas have little or nothing to do with what the path integral
formalism says and they defy quantum mechanics and most of it's
applications.

Are you familiar with the phenomenon of decoherence in ordinary QM? If you have a small subsystem A interacting thermally with another system B, the interactions can make it so that the reduced state of the subsystem A becomes arbitrarily close to a "mixed state" in which there is virtually no interference between the different elements of the superposition for A (see my post #15 on this thread for more). As I understand it, this is basically how the many-worlds interpretation explains why macroscopically different "worlds" don't noticeably interfere with one another.

In David Deutsch's picture the atom is a core with a classical particle
rotating around it, taking one path in one world and a different path
in another world. This is not correct.

Why "classical particle"? The paths in the path-integral picture don't behave like the paths of classical particles. Presumably you could in principle make correct predictions about the probability an electron will be detected at different positions around the nucleus by summing all possible paths, I think Deutsch is just adding the idea that each of these paths is a real electron, a philosophical gloss which shouldn't change the physical analysis.

With a single human body containing ~10^30 elementary particles
which each splitting up into endless numbers of paths at the femtometer,
attosecond scale, the number of different worlds add up very fast,
for a single human, let alone for an entire world.

Yes, I get that. But again, why is this anything more than an argument from incredulity, i.e. "I find it incredible there could be so many versions of me"? If the universe is spatially infinite there would also be an infinite number of slightly different versions of you at sufficiently great spatial distances, is this a good scientific or philosophical argument for believing space must be finite?

 

[xantox]

In David Deutsch's picture the atom is a core with a classical particle rotating around it, taking one path in one world and a different path in another world. This is not correct.

Attributing this view to Deutsch seem incorrect too. Do you have a quote where he did actually state this?

With a single human body containing ~10^30 elementary particles with all of them splitting up into endless numbers of paths at the femtometer/attosecond scale, the number of different worlds add up very fast, for a single human, let alone for an entire world.

Since in MWI worlds are just an emergent feature of the wavefunction, arguing on the basis of their number does not seem quite significant.

 

[Hans de Vries]

Plenty of quantum phenomena can't be explained with classical optics, like entanglement or quantum computing.

The quantum computing videos from David Deutsch I've seen could
be explained by classical optics. You might see people agreeing here
including those who consider MWI to have attractive sides (vanesch).

Are you familiar with the phenomenon of decoherence in ordinary QM? If you have a small subsystem A interacting thermally with another system B, the interactions can make it so that the reduced state of the subsystem A becomes arbitrarily close to a "mixed state" in which there is virtually no interference between the different elements of the superposition for A (see my post #15 on this thread for more). As I understand it, this is basically how the many-worlds interpretation explains why macroscopically different "worlds" don't noticeably interfere with one another.

It's hard to see how there can't be interference, non interference occurs
only in orthogonal states, 90 degrees for bosons and 180 degrees for fermions.
This means there are at most two different states per particle which do
not interfere.

For the rest, everything is based on interference. From propagator theory:
A wavefunction moves in a certain direction because all other directions
are interfered out destructively, that is, there's no motion without interference.

Why "classical particle"? The paths in the path-integral picture don't behave like the paths of classical particles. Presumably you could in principle make correct predictions about the probability an electron will be detected at different positions around the nucleus by summing all possible paths, I think Deutsch is just adding the idea that each of these paths is a real electron, a philosophical gloss which shouldn't change the physical analysis.

The propagators of massive particles reflect continuously (the interacting
left and right chiral components), but this is not the picture Deutsch gives
with "In our world the particle goes through one split and in another world
it goes through another split" This is a classical picture.

Yes, I get that. But again, why is this anything more than an argument from incredulity, i.e. "I find it incredible there could be so many versions of me"? If the universe is spatially infinite there would also be an infinite number of slightly different versions of you at sufficiently great spatial distances, is this a good scientific or philosophical argument for believing space must be finite?

What bothers me is that the different worlds do not interact as predicted
by quantum mechanics, but rather they all exist mostly independently without
disturbing each other. Especially since older ideas which claimed that particles
can only interfere with them self and not with other particles of the same kind
have been proven wrong by experiment.

Regards, Hans

 

[JesseM]

The quantum computing videos from David Deutsch I've seen could
be explained by classical optics. You might see people agreeing here
including those who consider MWI to have attractive sides (vanesch).

Do you remember a post by vanesch where he discusses this? In any case, surely you're not arguing that all quantum phenomena can be explained by classical optics (violations of the Bell inequality obviously can't, for example), so if you agree the quantum formalism is needed for certain situations, then whatever your interpretation of the quantum formalism, wouldn't you apply the same interpretation to any situation which physicists analyze using QM, like the double-slit experiment?

It's hard to see how there can't be interference

From what I remember, the interference terms in the "reduced state" for a certain subsystem (which are apparently the off-diagonal terms in the density matrix) never actually disappear completely, but they do decay exponentially. For example, this paper says:

The quantum decoherence process is elegantly expressed in the framework of the reduced density matrix of the quantum register. When no coupling to the environment is present, the reduced density matrix simply follows a Heisenberg-type evolution. As soon as the coupling to the environment is introduced, the off-diagonal terms of the reduced density matrix of the register decay with respect to time. This is often referred to as phase damping. In the simplest case of a single two level system connected to an environment, the off-diagonal elements of the reduced density matrix decay exponentially in time as ~e^−q(t) , where t is the time and the function q(t) depends on the strength of the coupling to the environment.

I have never studied decoherence formally so I don't claim to understand why this is true or even precisely what it means, I'd suggest you might at least want to do some of your own reading on the subject instead of dismissing it based on my layman's summary, as far as I know decoherence is a widely-accepted consequence of applying the rules of QM to the problem of a quantum system which is in thermal interaction with a larger environment.

The propagators of massive particles reflect continuously (the interacting
left and right chiral components), but this is not the picture Deutsch gives
with "In our world the particle goes through one split and in another world
it goes through another split" This is a classical picture.

I don't quite understand your objection here, are you just objecting that he makes it sound like there are only two paths involved? If so he'd probably say he was simplifying for a general audience, in fact you have to integrate over an infinite number of distinct paths through each slit.

What bothers me is that the different worlds do not interact as predicted
by quantum mechanics, but rather they all exist mostly independently without
disturbing each other.

Again, I think you really need to delve into the theory of decoherence to understand why many-worlds advocates say the different worlds interact only weakly (they don't say that they aren't interacting at all, I've seen a quote by Deutsch where he points out that the interference terms never disappear completely even with decoherence).

 

[xantox]

What bothers me is that the different worlds do not interact as predicted by quantum mechanics, but rather they all exist mostly independently without disturbing each other.

The classical picture is an approximation, Deutsch considers worlds unsharp and affecting each other. To discuss his views, it is perhaps more appropriate to refer to his actual papers, eg. "The Structure of the Multiverse", arxiv:quant-ph/0104033

"if reality – which in this context is called the multiverse – is indeed literally quantum-mechanical, then it must have a great deal more structure than merely a collection of entities each resembling the universe of classical physics.[...] "

"Since a generic quantum computational network does not perform anything like a classical computation on a substantial proportion of its qubits for many computational steps, it may seem that when we extend the above conclusions to the multiverse at large, we should expect parallelism (ensemble-like systems) to be confined to spatially and temporally small, scattered pockets. The reason why these systems in fact extend over the whole of spacetime with the exception of some small regions (such as the interiors of atoms and quantum computers), and why they approximately obey classical laws of physics, is studied in the theory of decoherence (see Zurek 1981, Hartle 1991)."

"For present purposes, note only that although most of the descriptors of physical systems throughout spacetime do not obey anything like classical physics, the ones that do, form a system that, to a good approximation, is not only causally autonomous but can store information for extended periods and carry it over great distances. It is therefore that system which is most easily accessible to our senses – indeed, it includes all the information processing performed by our sense organs and brains. It has the approximate structure of a classical ensemble comprising ‘the universe’ that we subjectively perceive and participate in, and other ‘parallel’ universes."

 

[Hans de Vries]

The quantum computing videos from David Deutsch I've seen could
be explained by classical optics. You might see people agreeing here
including those who consider MWI to have attractive sides (vanesch).

Do you remember a post by vanesch where he discusses this?

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


Regards, Hans

 

[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?