Problems with Many Worlds Interpretation

[kith]

I don't know Bohmian mechanics well enough to comment about that, but each point in the phase space of a classical particle theory specifies the position and momentum of each particle.

How do you deduce how much particles you have and how do you know they act in 3D space given only a state in abstract phase space? You have to add the knowledge that the corresponding experiment is done by an observer in 3 dimensions. So if we can't deduce this from a classical state, is it really reasonable to demand that we can deduce it from the state vector/ray in the MWI?

This is only about the factorization question and not about the emergence of a preferred basis once we know how to factorize our Hilbert space, about the ermergence of probabilities or about the emergence of single outcomes.

 

[Darwin123]

Hello,

While the majority of physicists embrace the Many Worlds interpretation of quantum decoherence, I am holding out hope for the Copenhagen interpretation or better yet, a undiscovered interpretation.

I am a retired physicist. I haven't met any physicist who promotes that Many Worlds interpretation of quantum decoherence. That is to say, in my universe there are very few physicists that endorse the Many World's interpretation of Quantum mechanics.

Most physics students in my universe learn the Copenhagen interpretation of quantum mechanics. I have met many other physicists in my universe that endorse the Copenhagen interpretation of quantum mechanics. My opinion is that there are better interpretations of quantum mechanics. I am really interested in Quantum Decoherence explanations of the so called wave collapse. For a while, I was really interested in the Stochastic Electrodynamics explanations of quantum mechanics. Recent experiments have shown that stochastic electrodynamics may not be a valid theory. However, I keep hoping.

Most of your objections to the multiworld interpretation are really objections to quantum mechanics. In the multiworld interpretation, for instance, there is a universe somewhere where someone very much like you has already turned into a sun. You are not that universe or you wouldn't be reading this. However, in the Copenhagen view there was always a very small probability that you would turn into a sun. You are lucky it didn't happen, or you couldn't read this.

The turning into a sun scenario is a type of quantum tunneling. You are suggesting that the atoms of your body have a finite probability of tunneling from a low density quantum state (you right now) into a high density quantum state (a sun). This is a possibility no matter what interpretation you pick. The probability is astronomically small, but one can estimate it with quantum mechanics. I hope that I didn't frighten you!

Quantum tunneling has been proven in many, many experiments on a small scale. How one interprets it is epistomology. However, the phenomenon has been observed. Caclulating the odds that an object like you will turn into a sun within the next 5 minutes involves extrapolating from a small scale to a large scale. Maybe extrapolating it doesn't work. However, the interpretation doesn't change the theory.

The main objection to Multiworld Interpretation isn't on your list. It isn't even wave interference, per se. It is the uniqueness of the basis. The set of possible universes seems to be set by the experimental apparatus in one universe. This is more mathematics than physics.

The Fourier decomposition isn't unique. The basis functions of the Fourier transform are not unique. Yet, the basis functions define the set of universes. This is a logical paradox, not just something that appears to be improbable. So very few physicists subscribe to the Multiworld's approximation.

I conjecture there is a way around this objection. In any case, I have lost interest in the Multiple World's interpretation of quantum mechanics because it is not mathematically consistent. There still seems to be a type of "multiworld theory" in general relativity and cosmology. I have a mild interest in this because there are hypothetical experiments that can be done, and may someday be done. However, this subset of general relativity theory is just an analogue of the multiworlds theory of quantum mechanics.

I am really interested in what made you think that multiworld interpretation was a commonly accepted theory by physicists.

There are a lot of physicist want to bees. Depak Chopra is not a physicist. Timothy Leary was not a physicist. Carlos Castenada was not a physicist. Gene Roddenbury was not a physicist.

 

[Hurkyl]

Dynamics is nothing but a unitary transformation from one point in the Hilbert space to another. As long as all points in the Hilbert space look the same (which is one of central claims in the paper), such dynamics does not bring anything interesting.

But we can distinguish between symmetries that preserve the laws of physics and those that do not. This is the most important idea behind a symmetry based analysis of physical theories.

 

[bohm2]

In case others haven't come across the papers, note that Schwindt does mention and references 2 authors (M. Dugic and J. Jeknic-Dugic) that made the same argument. On of the papers by these 2 authors was posted previously in this thread. I believe that Ilja Schmelzer also makes a similar argument in some of his papers.

 

[Demystifier]

Ok, but why isn't David Wallace's FAPP and arguments from functionalism sufficient?

I don't see how functionalism can resolve the basis problem.

 

[Demystifier]

Yes, I get this. I also get that he tries to prove that we can't get our 3D world experience out of it. What I don't get is why this should be something inherent only to the MWI. As Schwindt himself mentions in section 6.3, how do we get our 3D world experience from the point in 3n-dimensional configuration space of Bohmian mechanics? And I'd like to add: how do we get our 3D world experience from the point in 6n-dimensional phase space of classical mechanics?

Please forgive my sloppiness regarding some technical details, I appreciate your corrections. ;-)

Yes, that's an important question to answer.

Note that he discusses Bohmian mechanics (BM) in TWO contexts. First at page 18 where he explains why BM does not have the basis problem. Second at page 21 where he suggests that it might. So which argument is correct? Both!

Namely, the second argument is a CONDITIONAL argument, valid only if one "takes mathematical universe serious" (see the end of page 20). Indeed, the title of section starting at page 20 is "Is the world a mathematical structure?". He correctly points out that if it is, then Bohmian mechanics also has a problem. But so has classical mechanics in configuration or phase space. Yet this is not really a problem for Bohmians, or for classical physicists, because they typically don't think that "the world is a mathematical structure". Such thinking is more typical for many-world people.

 

[Demystifier]

But why do you need to separate the worlds?

To explain the illusion of wave-function collapse, which, indeed, is the main motivation for studying many worlds in the first place.

 

[Demystifier]

But we can distinguish between symmetries that preserve the laws of physics and those that do not. This is the most important idea behind a symmetry based analysis of physical theories.

He discusses that issue as well. What matters in MWI is not the Hamiltonian (with its symmetries), but the wave function (as a function of time). If you know the wave function, you don't need the Hamiltonian. A wave function is a particular solution, and in general it does not have any symmetries which the Hamiltonian does.

 

[Dmitry67]

BTW, what is the current position of BM with the issue about the cosmology. AFAIK, there is a hidden rest frame in BM, right? But in expanding Universe no objects 'at rest' in some frame can have timelike worldlines globally. For example, let's say that here, on Earth, the hidden rest frame is the same as 'rest to CMB'. So we have a 'hidden' center of the universe in BM :) But outside of our Hubble space the 'NOW' in the hidden rest frame, normally spacelike, becomes timelike.

 

[Demystifier]

In case others haven't come across the papers, note that Schwindt does mention and references 2 authors (M. Dugic and J. Jeknic-Dugic) that made the same argument. On of the papers by these 2 authors was posted previously in this thread. I believe that Ilja Schmelzer also makes a similar argument in some of his papers.

As I said, his (Schwindt's) argument is not new, but in my opinion, nobody before presented this argument in such a clear form.

In particular, his analogies (with Minkowski space in strange coordinates, as well as with classical phase space) are brilliant. Also, his terminology (nirvana and samara basis) is fun.

 

[Dmitry67]

To explain the illusion of wave-function collapse, which, indeed, is the main motivation for studying many worlds in the first place.

Demystifier, could you explain it in more details?

Decoherence shows that taking some basis and somehow isolating somehow 2 systems (observer and the cat), we can explain what an observer perceive.

I am putting a stress on the words some/somehow, because in fact, these parameters are free, decoherence doesn't put any constraint to limit your choice (expect the observer must have high number degrees of freedom).

WHY do you need any constrains on these parameters?

 

[Demystifier]

BTW, what is the current position of BM with the issue about the cosmology. AFAIK, there is a hidden rest frame in BM, right? But in expanding Universe no objects 'at rest' in some frame can have timelike worldlines globally. For example, let's say that here, on Earth, the hidden rest frame is the same as 'rest to CMB'. So we have a 'hidden' center of the universe in BM :) But outside of our Hubble space the 'NOW' in the hidden rest frame, normally spacelike, becomes timelike.

You misunderstood something about general relativity. In the fame in which CMB is homogeneous and isotropic, there is no center of the Universe, there is no horizon, and the notion of "hubble space" does not make sense.

 

[Demystifier]

Demystifier, could you explain it in more details?

Decoherence shows that taking some basis and somehow isolating somehow 2 systems (observer and the cat), we can explain what an observer perceive.

I am putting a stress on the words some/somehow, because in fact, these parameters are free, decoherence doesn't put any constraint to limit your choice (expect the observer must have high number degrees of freedom).

WHY do you need any constrains on these parameters?

The point is that you have to do it somehow. (Unlike you, I emphasize the word "have"). On the other hand, MWI in its minimal form (i.e., without the additional structure) tells you that you shouldn't, because otherwise you ruin the mathematical structure of MWI. That is the problem.

 

[Dmitry67]

You misunderstood something about general relativity. In the fame in which CMB is homogeneous and isotropic, there is no center of the Universe, there is no horizon, and the notion of "hubble space" does not make sense.

Of course I understand it - you had probably misunderstood my post.
But I've heard that BM has a special rest frame, is it true?

 

[Dmitry67]

The point is that you have to do it somehow. (Unlike you, I emphasize the word "have"). On the other hand, MWI in its minimal form (i.e., without the additional structure) tells you that you shouldn't, because otherwise you ruin the mathematical structure of MWI. That is the problem.

No, it doesn't say that you shouldn't.
It says that you can do it any way you want
Feel the difference.

 

[Demystifier]

But I've heard that BM has a special rest frame, is it true?

Some variants do, some variants don't.

 

[Dmitry67]

Some variants do, some variants don't.

Hm...
Then how many flavors of BM exist,
and are all of them compatible?
Of course, I am interested only in relativistic BM

 

[Demystifier]

No, it doesn't say that you shouldn't.
It says that you can do it any way you want
Feel the difference.

Yes, Schwindt discusses that variant of MWI too. In second paragraph of Sec. 5 he says:
"The Many World Interpretation is therefore rather a No World Interpretation (accord-
ing to the simple factorization), or a Many Many Worlds Interpretation (because each of
the arbitrary more complicated factorizations tells a different story about Many Worlds
[7])."

But the many-many world interpretation is certainly not how most MWI experts see MWI, because they typically believe that decoherence fixes the basis. What Schwindt shows is that it doesn't.

 

[Demystifier]

Hm...
Then how many flavors of BM exist,
and are all of them compatible?
Of course, I am interested only in relativistic BM

There are a few of them. They are not intrinsically equivalent, but they all reproduce standard measurable predictions of relativistic quantum theory.

 

[Dmitry67]

Exactly! But one story is special for you, because of your consciousness.
So we have a constructive disagreement - our axioms are different, I assume a special role of consciousness aka 'qualia'.

 

[Demystifier]

Exactly! But one story is special for you, because of your consciousness.
So we have a constructive disagreement - our axioms are different, I assume a special role of consciousness aka 'qualia'.

I don't have a problem with your axioms, indeed they seem viable to me. My point is that your additional consciousness axiom makes the total axiomatic system more complicated than typical MWI-believers would like. So you are a not-typical MWI-believer, which I think is good.

 

[Dmitry67]

Thank you. Minor question about BM: say, there are 2 observers in 2 different inertial frames. Do they agree on the 'trajectories' of BM 'particles'? (Of course, they can't observe these trajectories because they are hidden, but observers can calculate them).

Probably they would agree in BM flavor with hidden special rest frame, but what's about flavor without special hidden rest frame?

 

[Ilja]

Of course I understand it - you had probably misunderstood my post.
But I've heard that BM has a special rest frame, is it true?

Yes it has. Some people try to get rid of it, but I don't think it is a good idea (sorry, Demystifier).

But to have a preferred frame in cosmology does not mean that there has to be a hidden center of the universe. It is simply a preferred foliation in the GR sense, not an inertial frame in the SR sense.

Trajectories in different frames are trajectories with completely different initial values and there is no reason at all to suspect that they have something in common. But observers will not think that their own rest frame, based on the quite arbitrary Einstein synchronization, has some relation with the hidden preferred rest frame. So I think they will agree that the CMB frame is the closest candidate for the hidden preferred frame and use it, independend of their own speed, for computations.

 

[Demystifier]

Thank you. Minor question about BM: say, there are 2 observers in 2 different inertial frames. Do they agree on the 'trajectories' of BM 'particles'? (Of course, they can't observe these trajectories because they are hidden, but observers can calculate them).

Probably they would agree in BM flavor with hidden special rest frame, but what's about flavor without special hidden rest frame?

The two observers agree on the trajectories, both with and without special hidden rest frame.

 

[Dmitry67]

Ilja, thank you for understanding correctly and answering my question about BM vs cosmology.

Trajectories in different frames are trajectories with completely different initial values and there is no reason at all to suspect that they have something in common.

But if different observers don't agree on the trajectories, can you call BM particles 'real'? BM insists that in some sense, 'particles' are what makes otherwise empty Universe wavefunction 'real' in some branches?

Related question, if trajectories are different, what guarantees that different observers agree on the outcomes of the experiments? If for Bob cat is dead, what guarantees that for Alice flying at 0.99c, particles also fall into the 'dead cat' branch, making it real?

UPD: answers from Ilja and Demystifier are different, I'll wait for the clarification

 

[Hurkyl]

He discusses that issue as well. What matters in MWI is not the Hamiltonian (with its symmetries), but the wave function (as a function of time). If you know the wave function, you don't need the Hamiltonian. A wave function is a particular solution, and in general it does not have any symmetries which the Hamiltonian does.

A wave function doesn't have the symmetries of Hilbert space either.

The objection is to the claim that "all wave functions look the same". This can only make sense if you forget the laws of physics. But the 'brilliant analogies' you lauded earlier demonstrate quite clearly that if you forget about the laws of physics, you can't discern what information is meaningful.


But it strikes me that there's another way to respond that makes the emptiness of this line of reasoning more obvious. Yes, a point in state space is not enough to describe a physical system. You need something more. That 'something more' is the laws of physics.

Nobody worth their salt should have ever claimed that MWI considers a state vector to be a complete description of the universe. Instead, the description is the triple

(state vector, Hamiltonian, Schrödinger equation)​

(or some equivalent formulation).

The last component is nearly always left implicit, and the middle one usually so. But if you're going to start talking about symmetry groups and consider two descriptions in the same orbit as equivalent systems, it is a terrible mistake to forget about them.

 

[Demystifier]

The statement by Ilja quoted by Dmitry makes no sense to me. So I'll also wait for Ilja's clarification.

 

[Demystifier]

But it strikes me that there's another way to respond that makes the emptiness of this line of reasoning more obvious. Yes, a point in state space is not enough to describe a physical system. You need something more. That 'something more' is the laws of physics.

Yes, but the claim is that a TRAJECTORY in phase space should be enough. Namely, if you know the trajectory, the laws of physics seem superfluous.

 

[Demystifier]

Dmitry67, today appeared a new paper that might interest you:
"The role of the observer in the Everett interpretation"
http://arxiv.org/abs/1211.0196

Also, I have a question for you. If, except the wave function, one also needs consciousness, then does this consciousness needs a preferred (Lorentz) frame? Of course, it is difficult to answer this question without an explicit mathematical model of consciousness (which is the reason why I prefer Bohmian trajectories over consciousness as an additional structure needed in MWI), but perhaps the question still makes sense to you.

 

[Hurkyl]

Yes, but the claim is that a TRAJECTORY in phase space should be enough. Namely, if you know the trajectory, the laws of physics seem superfluous.

Anyone who makes that claim without implicitly assuming the laws of physics are fixed (or believes the structure of the state space includes the laws of physics) is either being very silly, or hasn't fully thought things through yet.

I'm sure there are people who would make the claim without this assumption, or haven't yet realized its importance. These people are wrong.

 

[Dmitry67]

Thanks, I'll take a look.

Regarding your question, I suppose consciousness resides in some Lorentz frame - the frame where consciousness holder (brain) is located. Don't ask me what if brain is made of relativistic components - I don't know :)

This answer is on the surface, deeper analysis requires an understanding what consciousness is. For example, I can't answer the following question:

In infinite Universe, there are infinitely many physical copies of myself in identical states, even very very far from each other. Are these exact copies share the same consciousness or not? I don't know how '=' is defined for consciousness.

 

[Demystifier]

Dmitry, thanks for the honest answer!

 

[Ilja]

But if different observers don't agree on the trajectories, can you call BM particles 'real'? BM insists that in some sense, 'particles' are what makes otherwise empty Universe wavefunction 'real' in some branches?

I do not say that different observers don't agree.

If different observers agree about the hypothesis that the CMBR frame is the preferred one, then they will compute the same trajectories. The hypothesis about the CMBR frame as the preferred one is a reasonable one, the hypothesis that the own frame obtained by Einstein synchronization is a preferred one seems, instread, quite stupid.

Related question, if trajectories are different, what guarantees that different observers agree on the outcomes of the experiments? If for Bob cat is dead, what guarantees that for Alice flying at 0.99c, particles also fall into the 'dead cat' branch, making it real?

This is a completely different question. That different observers agree on the outcomes is caused in BM by the simple fact that the trajectory of the configuration is the same. It has to be computed in the true prefferred frame.

If you fail to make the correct hypothesis about which frame is the preferred one, you compute some stupid meaningless trajectories which have nothing in common with reality. But, because the probabilities in quantum equilibrium will not depend on this error, it usually does not matter.

 

[Ilja]

The two observers agree on the trajectories, both with and without special hidden rest frame.

And this does not make sense to me.

Somewhere Valentini has made the point that outside the quantum equilibrium you have no problem of sending FTL messages in BM. And these messages of course define a preferred frame. (At least if you want to preserve a world with causality but without causal loops.)

 

[Demystifier]

Somewhere Valentini has made the point that outside the quantum equilibrium you have no problem of sending FTL messages in BM. And these messages of course define a preferred frame. (At least if you don't want to preserve a world with causality but without causal loops.)

I guess you wanted to say "... if you want to preserve a world with causality but without causal loops."

 

[Ilja]

I guess you wanted to say "... if you want to preserve a world with causality but without causal loops."

Indeed. Corrected.

 

[Demystifier]

Indeed. Corrected.

OK. Then I would like to add that my covariant BM does NOT preserve causality. Or more precisely, it does not preserve the usual relativistic causality, but replaces it with a more general notion of causality.

 

[Ilja]

OK. Then I would like to add that my covariant BM does NOT preserve causality. Or more precisely, it does not preserve the usual relativistic causality, but replaces it with a more general notion of causality.

Of course, relativistic causality cannot be preserved, but classical causality of an absolute time (preferred frame) without causal loops can be preserved, and this is the usual version of causality of dBB theory, non-relativistic or relativistic.

Relativistic causality in the weak sense (only about information transfer, not about reality) one obtains in quantum equilibrium.

 

[kith]

Indeed, the title of section starting at page 20 is "Is the world a mathematical structure?". He correctly points out that if it is, then Bohmian mechanics also has a problem. But so has classical mechanics in configuration or phase space. Yet this is not really a problem for Bohmians, or for classical physicists, because they typically don't think that "the world is a mathematical structure". Such thinking is more typical for many-world people.

So why do (some?) many-worlds people think that the world is a mathematical structure if this isn't even a meaningful concept in classical mechanics?

 

[martinbn]

Later in the paper he explains that only the wave function is what matters, in the sense that if wave function at all times is given, then the Hamiltonian is irrelevant. The Hamiltonian only serves to determine wave function at all times for the case when it is not already known.

Ok, but I still don't understand. Given the wave function for all times i.e. given a "curve" in the Hilbert space is not the same as just the Hilbert space. Vectors do not all look the same, some are on the curve and some are not, those on the curve are ordered and so on. The curve gives the Hamiltonian and its eignvectors are different in a way than the others and so on. Also my question why the factorization was not answered. At one point he says "let's ignor the why question and do the factorization anyway".

 

[mfb]

So why do (some?) many-worlds people think that the world is a mathematical structure if this isn't even a meaningful concept in classical mechanics?

You can fully define a classical system by the Hamiltonian and a vector (given for a specific time) in phase space. I think this is very similar to a mathematical structure. The existence of the system (if it corresponds to the universe) could be similar to the "existence" of the vector (1,5,2) in R³.

 

[kith]

You can fully define a classical system by the Hamiltonian and a vector (given for a specific time) in phase space.

You can't tell wether you have 3 particles moving in n dimensions or n particles moving in 3 dimensions from this. So you have to assume the division into subsystems.

 

[Dmitry67]

So why do (some?) many-worlds people think that the world is a mathematical structure if this isn't even a meaningful concept in classical mechanics?

http://arxiv.org/abs/0704.0646

 

[mfb]

You can't tell wether you have 3 particles moving in n dimensions or n particles moving in 3 dimensions from this.

So what? I do not care ;).

The Hamiltonian will be easier to write if you group those degrees of freedom in groups of 6.

 

[kith]

http://arxiv.org/abs/0704.0646

Thanks, I will read this when I have the time

So what? I do not care ;)

That's certainly a valid point of view, if you are willing to accept that the time evolution in our world of experience is only due to the choice of coordinates.

 

[Ilja]

So why do (some?) many-worlds people think that the world is a mathematical structure if this isn't even a meaningful concept in classical mechanics?

I think the very idea to interpret the world as a mathematical structure is an implicit consequence of the positivistic rejection of interpretations, metaphysics, and philosophy.

The point is that there is a natural and justified wish to consider interpretation, metaphysics and philosophy. So those who officially, ideologically accept this rejection, in fact want to consider them. So what to do them? They try the most innocent (from the positivistic point of view) metaphysics. Which metaphysics is the most innocent one, from point of view of those who reject metaphysics? It is the metaphysics which does not add any new methaphysical structure, which, does not openly argue for additional structure except the one which is given by the mathematics of the theory.

So, one takes the mathematics of the given theory, does not accept anything in contradiction with these mathematics. And tries to interpret the mathematics metaphysically.

That means, one rejects preferred frames (addtional structure), ether interpretations of fields (additional structure), Bohmian trajectories (addtional structure) but accepts spacetime interpretations (four-dimensional mathematics instead of 3D reality) despite closed causal loops, nontrivial topologies and singularities (they are holy as part of mathematics) frequency interpretation (mathematics of probability theory as part of nature) instead of Bayesian interpretation (where the same mathematics is only used to describe incomplete human information) and wave function realism (also no new mathematics).

So the mystification of the mathematics of the actual physical theories seems to me a quite natural consequence of the antimetaphysical prejudices of the last century, which have not played an important role in classical physics.

 

[Dmitry67]

dBB is in fact incompatible with MUH, but not because of the preferred frame. Incompatibility with MUH is not about having additional structures, no matter how ugly they are, it is about having something that can't be defined as formula in principle.

 

[Ilja]

dBB is in fact incompatible with MUH, but not because of the preferred frame. Incompatibility with MUH is not about having additional structures, no matter how ugly they are, it is about having something that can't be defined as formula in principle.

I would not care about such a compatibility. But there is nothing in dBB which cannot be expressed in formulas. It is the interpretation where all the mysticism of the other interpretations is replaced by clear and simple precise mathematics. The guiding equation, the equation which defines the evolution of a subsystem in terms of the larger system and the configuration of the environment, which replaces all the mystery about collapse - simple mathematics.

 

[Fredrik]

Max Tegmark made a poll on preferred interpretation of QM during a conference Fundamental Problems in Quantum Theory, 1997, and got:

Copenhagen 13
Many Worlds 8
Bohm 4
Consistent Histories 4
Modified dynamics (GRW/DRM) 1
None of the above/undecided 18

http://arxiv.org/abs/quant-ph/9709032v1

I think polls like this are kind of pointless, mainly for these two reasons:

1. There are no standard definitions of these interpretations. In fact, it's hard to find two people who mean the same thing by "the CI" or "the MWI". There isn't even a standard definition of the term "interpretation of QM", so different people have different ideas about what sort of things an interpretation should say.

2. A lot of physicists haven't thought enough about interpretations to have an informed opinion. So anyone's guess is as good as theirs.

Edit: I didn't even realize that this post was on the first page. I thought I was I was replying to a comment made recently.

 

[Dmitry67]

I would not care about such a compatibility. But there is nothing in dBB which cannot be expressed in formulas. It is the interpretation where all the mysticism of the other interpretations is replaced by clear and simple precise mathematics. The guiding equation, the equation which defines the evolution of a subsystem in terms of the larger system and the configuration of the environment, which replaces all the mystery about collapse - simple mathematics.

As you remember, Universe wavefunction in dBB is exactly the same as in MWI (this fact is well known as "dBB is MWI in chronic denial"). As unavoidable consequence of MUH is that empty dBB branches are real.

The word "real" is confusing, but it is fair to say that in dBB some branches are empty, while one is "tagged" with "particles" (dBB people tend to say that "it is real, while empty branches are not" - but you can't say it under MUH - they are just different, but both "real")

Evolution of the wavefunction branch with a dead cat, or better, poor experimenter slowly dying from poison, is controlled by the same laws as the 'tagged' one - dBB particles don't affect the wavefunction. In empty branch, neurons in experimenter's brain are sending panic signals... Vision becomes blurry, pain grows stronger and...

Now comes the pure magic. dBB says: don't worry about him. Yes, it looks almost exactly as 'real', but it is empty. He does not feel any pain. That experimenter is not conscious at all! But why if wavefunction of his brain is the same? Because there are no particles inside! But these particles don't affect the wavefunction! Still, they are the undetectable but they are the most important part of the consciousness!

So here is a non-physical axiom of dBB (with the assumption of MUH): while all branches are real, non-tagged branches are somehow... ehhh... existence-challenged :) Earth in empty branch is populated with perfect examples of http://en.wikipedia.org/wiki/Philosophical_zombie