QM says no observer, no existence

[gptejms]

Pick your flavor !

Let * stand for my consciousness, and @ stand for yours.
At a certain point, we have observed each other and we're in the following state:

|mybody*> |yourbody@> (a |1> |U>+ b|2>|V>)

I took an entangled system to give the issue more spin (!).

Suppose I will observe the 1/2 system, and you will observe the U/V system.

After I interact with the 1/2 system, this gives:

|yourbody@> (a |mybody1>|1>|U> + b|mybody2>|2>|V>)

but now the body to which my consciousness is associated, due to the interaction of the measurement, doesn't appear in a product state anymore, so my consciousness has to be assigned to one of both terms (the law of consciousness, if you want to).

|yourbody@> (a |mybody1*>|1>|U> + b|mybody2>|2>|V>)

This means that my consciousness observed the result "1" (and this happened with a probability |a|^2), and it is associated with a body state of my body that has "1" in its physical brain as a souvenir of the measurement.

Now, you will interact with the U/V system:
a |mybody1*>|1>|yourbodyU>|U> + b |mybody2>|2>|yourbodyV>|V>

Again, your conscious body state doesn't appear in a product, which is forbidden. Your consciousness will be assigned one of your bodystates at random, say V, to make it spicy:

a|mybody1*>|1>|yourbodyU>|U> + b |mybody2>|2>|yourbodyV@>|V>

This happens to your consciousness with probability |b|^2

So both of us consciousnesses, in our own little subjective experiences, are convinced that there was some real observation (I think it was 1 and U, you consciously think it was 2 and V), and everything I can try to find out about it, by asking YOUR BODY, will give me the impression that I'm right ; to you, exactly the same thing will happen. But we each now live in our separate terms (branches - worlds).

Patrick.

So the consciousness has not split---yours is one branch(1 and U) and mine is in the other branch(2 and V).Or is it in both?After all the fellow that tells you that he observes U(the other me) is a meaningful live fellow!The you in my branch is also alive for me to tell me that 2 was observed.Don't you agree that this all very weird and can not be taken seriously.
You didn't answer the probability part.

 

[vanesch]

So the consciousness has not split---yours is one branch(1 and U) and mine is in the other branch(2 and V).Or is it in both?

As I said, pick your flavor. Of course it cannot be in both: we don't observe consciously to be in a superposition (that's the whole riddle to be solved!). Now, you can say that when a conscious body gets entangled with something else (by interaction), the actual consciousness will be assigned to ONE of the term in the superposition, according to the Born rule (take that as a postulate, it answers your probability question). You are now free to assign a NEW consciousness to the other states or not. These are different flavors of the interpretation. It doesn't really matter, because you can never know if a living being has a consciousness or not. If you want it to have one, just assign a "new" consciousness to the other branches then (which, itself, will not know it is a "new" one because the bodystate it is associated with will remember everything from the "old" body, except of course a different measurement result).
[/quote]

After all the fellow that tells you that he observes U(the other me) is a meaningful live fellow!The you in my branch is also alive for me to tell me that 2 was observed.Don't you agree that this all very weird and can not be taken seriously.
You didn't answer the probability part.

I agree that this is very weird, however I don't see why it cannot be taken seriously. (who said again: "we all agree that your theory is crazy. We are discussing if it is crazy enough!") After all, if quantum theory is correct (and there is a unitary evolution - a hamiltonian) for every interaction in the world, there's almost no way to avoid this. There is no logical error in this, and it explains all our conscious subjective experiences.
I did answer the probability part (and that's where my view is NOT the one of the average MWI supporter): it is a postulate: "when the body associated with a consciousness, gets entangled with something else, the consciousness will now be associated with ONE of the body states in the entanglement, with a probability that is given by the Born rule". Take it as the MWI version of the projection postulate.
EDIT: the hard-core MWI fan will hope that we do not have to *postulate* this but that it comes out "naturally" of the formalism ; I think that this cannot be done, and that's where I'm a heretic wrt MWI.

You might add: "and a new consciousness will be created and associated with each of the viable bodystates in the other terms", at your will.
You might also consider that there is only one consciousness, namely yours (mine!), and that all other bodies are just not associated with a consciousness. There's no way to find out as is philosophically well known. Probably the version with the "new" consciousnesses is the most "symmetrical", but it is just a matter of taste.

cheers,
Patrick.

 

[nameta9]

"Irreducible complexity". Maybe the problem is in the concept that observer plus system being observed cannot really be further decomposed without eliminating everything. An odd theory put forward to justify "creationism", but in this case may very well apply. The system as a whole is "irreducible", so you have irreducible complexity where you can't take any part away without modifying the system into something else.

 

[vanesch]

"Irreducible complexity". Maybe the problem is in the concept that observer plus system being observed cannot really be further decomposed without eliminating everything.

This is not just an "odd theory", it is fundamental in quantum theory (and well known). The hilbert state space of a composed system is the kronecker product of the individual hilbert state spaces: H = H_A (x) H_B.
Now, in order to be able to talk about the state of each system individually (A or B), the global state must be a product state of a state in H_A and a state in H_B. However, it is well known that the very large majority of states in H CANNOT BE WRITTEN IN SUCH A PRODUCT FORM (in that case, one says that systems A and B are entangled). In such a case, there's no way of assigning an individual state to system A and system B. There are *statistical* descriptions (using reduced density matrices) that can limit one's attention to A or to B, but they are not correct when both systems are considered (they do not correctly give the correlations between measurements).

cheers,
Patrick.

 

[nameta9]

This also confirms that the "marxist-sociological" theories that science is a social "construction" really applies. If you were the only observer in the universe, then the laws of physics would change (and probably wouldn't even exist) as it is all dependent on your subjective view and experience. The other "observers" confirm science, but all civilization as a whole could be looked upon as only one observer, hence the laws of physics don't exist. And "irreducible complexity" may even apply to all objects; hence even an atom if taken away an electron becomes something else (anyways it remains an odd theory).

Did the creationist oddballs know that stuff of hilbert ?

 

[vanesch]

This also confirms that the "marxist-sociological" theories that science is a social "construction" really applies. If you were the only observer in the universe, then the laws of physics would change (and probably wouldn't even exist) as it is all dependent on your subjective view and experience.

I don't understand this. Why would the laws of physics change or not even exist if I were the only observer ? What stops objective laws of physics from determining my subjective experiences ?

Did the creationist oddballs know that stuff of hilbert ?

As it isn't written in the Bible, I think they don't

 

[nameta9]

Vague concept not worded right. Imagine really if you were the only person in the entire universe. So you have books, instruments and laws of physics. But you completely depend on your "subjective-psychological" state. I think that under these conditions after some time, you would start losing all references as only "other" people / communications somehow provide a reference. I think a single pesron observer would end up "changing" the laws of physics because he wouldn't be able to reason straight for very long.

 

[vanesch]

I think a single pesron observer would end up "changing" the laws of physics because he wouldn't be able to reason straight for very long.

Now let us assume for a moment that there are other bodies, which act as persons act, but which are not conscious. Only, the behaviour of these bodies is exactly as if they were conscious, and you ARE indeed the only person in the world with a consciousness. What does that change from the actual situation you think you are in ?

 

[Canute]

Quite. Every theory must be consistent with solipsism being true, since if it is not then proving that theory true will involve proving solipsism false, and this cannot be done. This renders any theory that assumes solipsism to be false unprovable in principle. This seems a regularly overlooked issue in physics. For example, even if materialism is true we can never know this, since its truth cannot be known by experience, and to know it to be true by reason would involve falsifying solipsism. Btw, it can be argued that solipsism is not falsifiable because it's not exactly false, even if it's not exactly true either. Many people hold this view.

Earlier (Vanesch) you wrote - "Of course it cannot be in both: we don't observe consciously to be in a superposition (that's the whole riddle to be solved!)." I wonder though. Is this not just an assumption? It's my suspicion that we can observe ourselves to be in a superposed state, and that this is the answer to many riddles.

 

[vanesch]

It's my suspicion that we can observe ourselves to be in a superposed state, and that this is the answer to many riddles.

Well, maybe exceptionally (which would then account for a lot of "supernatural" phenomena :-).
But I fail to see how we observe ourselves to be in superposed states in the "standard" situation: it would mean that we observe outcomes of experiments in superposition: the answer on the voltmeter was observed to be 2, 5, 18 and 21 Volt at the same time. Sorry, but this happens to me only when I'm seriously drunk (ah, that's maybe the key: ethanol (and other substances) momentarily suspend the Born rule :-)

 

[nameta9]

Now let us assume for a moment that there are other bodies, which act as persons act

But that is exactly the point. As long as there are other people/communications or similar (even if fake) we retain some reference. Imagine waking up and no one is in the world anymore. You walk in a town and then everything is void of people. Now this situation (twilight zone?) would completely modify your "subjective" view of all. How long would you last ? what meaning would the "laws of physics" have in this context ? How long could you reason properly ? Now an entire civilization can be viewed as this lone "observer". It really seems to confirm the "marxist-sociological" idea that "science is a social construction".

 

[Canute]

Well, maybe exceptionally (which would then account for a lot of "supernatural" phenomena :-).
But I fail to see how we observe ourselves to be in superposed states in the "standard" situation:

In the standard situation maybe not. Maybe not even after a few beers. But do the descriptions of conscious states below not have something suggestive of superposition about them? After all, to suggest that the universe is a quantum fluctuation in a superposed state, or something like that, seems pretty uncontentious these days. When people say that reality is 'nondual' they mean something very like that it exists as a superposition of complementary appearances or aspects. Thus Jung's "no qualities, because it has all qualities."

"Nothing is the same as fullness. In the endless state fullness is the same as emptiness. The Nothing is both empty and full. One may just as well state some other thing about the Nothing, namely that it is white or that it is black or that it exists or that it exists not. That which is endless and eternal has no qualities, because it has all qualities." (Carl Jung, VII Sermones ad Moruos)

"When we encounter the Void, we feel that it is primordial emptiness of cosmic proportions and relevance. We become pure consciousness aware of this absolute nothingness; however, at the same time, we have a strange paradoxical sense of its essential fullness. This cosmic vacuum is also a plenum, since nothing seems to be missing in it. While it does not contain in a concrete manifest form, it seems to comprise all of existence in a potential form. In this paradoxical way, we can transcend the usual dichotomy between emptiness and form, or existence and non-existence. However, the possibility of such a resolution cannot be adequately conveyed in words; it has to be experienced to be understood." (Stanislav Grof, The Cosmic Game
State University of New York)

 

[vanesch]

Now this situation (twilight zone?) would completely modify your "subjective" view of all. How long would you last ? what meaning would the "laws of physics" have in this context ? How long could you reason properly ?

I don't know. You claim that an individual without contact with its peers necessarily goes bezerk. I'm not even sure of that, but let us for the sake of argument say that it is. The probable origin of this is that we are, as individuals, a social species and that somehow psychologically we do not function well if we are alone. So be it. But clearly solitary beings have no problem with that, such as leopards, who meet their "peers" only once a few years, for a copulation or so, and then go on on their own again.

Now an entire civilization can be viewed as this lone "observer". It really seems to confirm the "marxist-sociological" idea that "science is a social construction".

What goes wrong with this reasoning is that you transpose the (assumed) property of individual humans, namely that they cannot reason properly if on their own, to an entirely different animal, namely a civilisation (of which you still have to demonstrate that it even *has* a subjective experience!). There's no reason to think so, because our entire civilisation's subjective experience (if it exists!) will be "used" to live in solitude. It never met another one !

But there's something far more basic that goes wrong with this "science is a social construction". Boiling water burns your finger, whether you're alone or whether you're in a group. Rocks fall down. This is not part of a "social construction". Calculational procedures give certain results (which you can cross check by different methods yourself), and experimental procedures give also certain results, which you can also crosscheck yourself. I don't see how the agreement or disagreement between both is socially decided on matters where science is clear. I'm not talking about front level research. I'm talking about all the well-established stuff.

cheers,
Patrick.

 

[nameta9]

There is something deeply "arbitrary" in science. Science depends on an "observer", but in our case this observer is a human mind/body. Now if you modify this observer maybe by changing its neural circuits or the way it processes sense information or adding other sense organs you end up having a completely different observation of the universe. Now this observer+universe is a completely different system, hence science is arbitrary in that it depends on the structure of the observer. In many posts in these forums and in www.scienceforums.net I have often mentioned the fact that we may be able to modify our own neural circuits or the way our mind organizes the information or adding new sense organs since what we really have in the end is just electrical signals going into an arbitrarily evolved/designed brain. We could add trillions of new sense organs by just modifying the neural organization of our minds hence we would have an infinite number of different universes. We would use our present science to kick start the process.

 

[vanesch]

There is something deeply "arbitrary" in science. Science depends on an "observer", but in our case this observer is a human mind/body.

What I think you mean is that PERCEPTION is deeply arbitrary. But science is more than just perception: it is the construction of a mathematical representation of a hypothetical ontology that is a 1-1 mapping to its perception. Now, of course because this perception is arbitrary (up to a point), this representation can change. But I'm somehow convinced that the underlying mathematical object that is represented (in different ways) is unique - and IS in fact, the objective, ontological world.

Let me take a childish illustration of what I'm trying to say. Imagine that space is given by 3-dim Euclidean space, and that there are different creatures, with different eyes, which "see" the world as rather hyperbolical, or elliptical or rasterized or whatever. They will come up with different theories of their vision (if we limit their perception to their optical vision) with different laws according to what is "distance" and so on, but if they are analysed deep enough, one will finally notice that all those complicated objects are just inequivalent representations of 3-dim Euclidean space.

THIS underlying structure is objective, and all our possible perceptions (no matter how crazily we are wired up) are limited to representations of said underlying structure.

Now if you modify this observer maybe by changing its neural circuits or the way it processes sense information or adding other sense organs you end up having a completely different observation of the universe. Now this observer+universe is a completely different system, hence science is arbitrary in that it depends on the structure of the observer.

I just tried to argue that although "superficially" the world looks totally different, the underlying object should be the same, and with enough thought, observation and analysis, one could hope that different observer/universe systems lead to finally the same deeper understanding: these are the laws of nature, and it is the most fundamental working hypothesis of physics that these laws exist (that this mathematical object exists).

We could add trillions of new sense organs by just modifying the neural organization of our minds hence we would have an infinite number of different universes. We would use our present science to kick start the process.

yes, but they would all be limited to being representations of the mathematical object that is at the heart of the workings of the universe (this mathematical object are the "ultimate laws of nature")
The very fact that we would "use our present science to kick start the process" is the very antithesis of the statement that "science is but a social process" !

 

[Canute]

Perhaps the question is not: Is there or is there not an underlying reality? This is an all or nothing choice equivalent to the unresolved background dependent/independent paradox, which may be an undecidable metaphysical question for all we yet know. Perhaps the question could be : If one takes away all observer-actualised differences between universes, or if one take away all observers, one by one, then when at last the last one is taken away what is left?

What is really real and so would remain even if unobserved or unconceptualised? Would quantum events ever happen? Would matter still exist? Would space and time still exist? Would 'Being' still exist? How could we ever know?

 

[vanesch]

What is really real and so would remain even if unobserved or unconceptualised? Would quantum events ever happen? Would matter still exist? Would space and time still exist? Would 'Being' still exist? How could we ever know?

If nothing is there to know, the last question doesn't make sense of course. But I'd say "yes" to all the others, realising that this is an unfalsifiable working hypothesis.

I think the simplest indication that at least this is a sensible working hypothesis is: the world looks as if it had a history. By that, I mean: with all we know, from different disciplines, it really does make sense to say that the earth, for instance, is something like 5 billion years old. Too many things fit together with this picture for it just to be decided by "social convenience". You can of course not exclude the big conspiracy, that the Earth is in fact 7 minutes old, but that everything was put together TO MAKE YOU BELIEVE that you are already there for several years, and that dinosaur bones were put in the ground etc... but that is a much less fruitful hypothesis.
Things just fit together when we say that the Earth was there even before there were humans, or dinosaurs, or trilobites. And know what ? It seems to have followed about the same laws of nature than those that we socially convened upon. We cannot go back to know for sure, of course, but this hypothesis fits within what we observe as "remnants of the past", which is the best we can do.
Now from an MWI quantum view, does that mean that "the wavefunction got observed before we were here" ? You don't need that. The universe can be objectively in a quantum state where there are terms where the Earth didn't even devellop and others where the Earth did devellop, but life never formed, and others where the dinosaurs didn't get exterminated. We simply don't happen to observe that branch (and to a certain point, we couldn't, because our bodies aren't present in those branches). This view implements automatically the antropological principle in some way.

 

[nameta9]

yes, but they would all be limited to being representations of the mathematical object that is at the heart of the workings of the universe (this mathematical object are the "ultimate laws of nature")
The very fact that we would "use our present science to kick start the process" is the very antithesis of the statement that "science is but a social process" !

True, but we are always assuming that our mental-sensory informational organization is the fundamentally true and basic one. We would modify another mind's neural circuits using our science but we would really not know how the other mind views and sees the universe unless we see "through" that new mind. And it may be that the new mind reveals completely different fundamental relationships, or even logical processes or emotions/sentiments etc. This new mind is at an equal level with ours. We are just as fundamental as any of the trillions of other possible minds. So the new mind can look at ours and say look at how unfundamental the human mind is "and they thought they knew the laws of physics". Of course there is an initial master-slave configuration in that we modify the other mind, but through the other mind we may appear just as arbitrary. It may be that there are no mathematical invariants in the end or that mathematics may just be one of many ways to organize information.

I wrote some other similar threads:


http://www.scienceforums.net/forums/showthread.php?t=6944

https://www.physicsforums.com/showthread.php?t=71298

 

[oldtobor]

What an interesting relationship with the whole concept of intelligent design and evolution. So then, what if another mind who has a more fundamental or knowledgeable view of the universe designed our mind and we are the "slave" of the master-slave configuration ? Also evolution itself somehow creates more complex organisms from simpler ones, the "hierarchy" is actually inverted from almost nothing creating a human mind. So we don't need a "master" with a better mind to "design" another as evolution shows that nothing at all evolved (designed?) us not a superior being as god.

It is like saying an inferior being can design a superior being, a modified mind can know more than its designer just as a blind process like evolution created superior minds after a long chain of events starting from carbon atoms. Simple carbon atoms in a sense "designed" by themselves and the process of evolution a mind. An inferior thing designed a superior one that knows "more" and "deeper" than the originator.

 

[vanesch]

And it may be that the new mind reveals completely different fundamental relationships, or even logical processes or emotions/sentiments etc. This new mind is at an equal level with ours. We are just as fundamental as any of the trillions of other possible minds.

Well, before playing Dr. Frankenstein yourself, you could already look at what variants of our minds "observe". Go to your nearest closed psychiatric hospital, to study "other minds with other perceptions of the world" I don't know if they produce a lot of sophisticated science on which they have social consensus (of the style "I'm Napoleon" - "No, *I* am Napoleon" ;-)

 

[nameta9]

This may end up being very important in the future when we start to modify our neural circuits. I was even thinking if we had 2 minds A and B where A sees the world as the human and B sees the world as modified mind. So A will always interpret all that B sees within its reference system and B will interpret things according to its. But a system composed of A + B could see things a bit like each but it would already be a completely different system. It is like if you take a human mind and you change its fundamental beliefs ( or even memories) like "matter is made up of atoms". It would be a completely different view of the world, hence a different system; A+B is neither really A or B.

 

[vanesch]

It is like if you take a human mind and you change its fundamental beliefs ( or even memories) like "matter is made up of atoms". It would be a completely different view of the world, hence a different system; A+B is neither really A or B.

Could it be that your B is a complete nutcracker ?
There are probably zillions of ways to make CRAZY minds. But do you have to consider a madman's propositions as just an alternate form of science ?

 

[Canute]

Vanesch - You wrote "In MWI quantum view, does that mean that "the wavefunction got observed before we were here" ? You don't need that. The universe can be objectively in a quantum state where there are terms where the Earth didn't even devellop and others where the Earth did devellop, but life never formed, and others where the dinosaurs didn't get exterminated. We simply don't happen to observe that branch... "

Doesn't this imply that the branch we are in had to be observed to be actualised. If it existed in the past then this was because it was observed, if it was not observed in the past then it 'was' in a superposed state and it is our current observations that create our evolutionary past.

This seems relevant here, regarding the . Speaking of the consistent histories approach to quantum cosmology Lee Smolin writes:

"While the ‘classical’ world we observe, in which particles have definite positions, may be one of the consistent worlds described by a solution to the theory, Dowker and Kent’s results showed that there had to be an infinite number of other worlds too. Moreover, there were an infinite number of consistent worlds that have been classical up to this point but will not be anything like our world in five minutes’ time. Even more disturbing, there were worlds that were classical now that were arbitrarily mixed up superpositions of classical at any point in the past. Dowker concluded that, if the consistent-histories interpretation is correct, we have no right to deduce from the existence of fossils now that dinosaurs roamed the planet a hundred million years ago."

Lee Smolin
Three Roads to Quantum Gravity (44)

On the importance of social organisation to the systemisation of our knowledge, or on science as social process, Tobias Dantzig, in his book "Number -The Langauge of Science" (worth a read on some of these issues IMO - it was a favourite of Einstein) writes:

"Of the absolute and immutable world which exists outside our consciousness we know only through theological speculations: accepting it or rejecting it are alike futile to a natural philosophy. But just as futile and sterile is the acceptance of the crude reality of our senses as the arch-reality, the only reality. It is convenient for a systematic exposition, to be sure, to regard the newly born child, or the primitive man, or the animal, as the embodiment of such an arch-reality. We can go still further and imagine, as did Helmholtz, Mach and Poincare, an intelligent being who has been deprived of all but one of his senses, say sight, and speculate on the type of universe that such a being would construct. Such speculations are tremendously fascinating in that they allow free rein to our power of resolving our sensations into their constituents, and then regarding the concept as a synthesis of these arch-sensations. But to accept such a synthesis as reality, as the reality, has, to my way of thinking, one fatal defect: it postulates the existence of an individual intellect; whereas the very process of coordinating these sensations involves thought, which is impossible without the vehicle language, which in turn implies an organized exchange of impressions, which in turn presupposes a collective existence for human beings, some form of social organisation."

Tobias Dantzig
Number - The Langauge of Science

 

[Canute]

Vanesch - you wrote - "In MWI quantum view, does that mean that "the wavefunction got observed before we were here" ? You don't need that. The universe can be objectively in a quantum state where there are terms where the Earth didn't even devellop and others where the Earth did devellop, but life never formed, and others where the dinosaurs didn't get exterminated. We simply don't happen to observe that branch... "

Doesn't this imply that the branch we are in had to be observed to be actualised? If it existed in the past then this was because it was observed, if it was not observed in the past then it 'was' in a superposed state and it is our current observations that create our evolutionary past.

Speaking of the consistent histories approach to quantum cosmology Lee Smolin writes:

"While the ‘classical’ world we observe, in which particles have definite positions, may be one of the consistent worlds described by a solution to the theory, Dowker and Kent’s results showed that there had to be an infinite number of other worlds too. Moreover, there were an infinite number of consistent worlds that have been classical up to this point but will not be anything like our world in five minutes’ time. Even more disturbing, there were worlds that were classical now that were arbitrarily mixed up superpositions of classical at any point in the past. Dowker concluded that, if the consistent-histories interpretation is correct, we have no right to deduce from the existence of fossils now that dinosaurs roamed the planet a hundred million years ago."

Lee Smolin
Three Roads to Quantum Gravity (44)

On the importance of social organisation to the systemisation of our knowledge, or on science as social process, Tobias Dantzig, in his book "Number -The Langauge of Science" (worth a read on some of these issues IMO - it was a favourite of Einstein) writes:

"Of the absolute and immutable world which exists outside our consciousness we know only through theological speculations: accepting it or rejecting it are alike futile to a natural philosophy. But just as futile and sterile is the acceptance of the crude reality of our senses as the arch-reality, the only reality. It is convenient for a systematic exposition, to be sure, to regard the newly born child, or the primitive man, or the animal, as the embodiment of such an arch-reality. We can go still further and imagine, as did Helmholtz, Mach and Poincare, an intelligent being who has been deprived of all but one of his senses, say sight, and speculate on the type of universe that such a being would construct. Such speculations are tremendously fascinating in that they allow free rein to our power of resolving our sensations into their constituents, and then regarding the concept as a synthesis of these arch-sensations. But to accept such a synthesis as reality, as the reality, has, to my way of thinking, one fatal defect: it postulates the existence of an individual intellect; whereas the very process of coordinating these sensations involves thought, which is impossible without the vehicle language, which in turn implies an organized exchange of impressions, which in turn presupposes a collective existence for human beings, some form of social organisation."

Tobias Dantzig
Number - The Langauge of Science

 

[nameta9]

no observer, no existence, problem has no solution. The observer is a billion neurons with lungs, oxygen, mental states, 10 years higher education etc. The observer is very complex. There is nothing elementary in "elementary particle physics". Each "human mind" is irreducibly different to each other in the sense that a physicist would say that the electrons are the fundamental particles, a psychologist would say mind comes first and mental states are more fundamental, a marxist would say money, power, social relations is really behind all. In reality each position is irreducible, they can be mixed but then you have a different system. Our limits are in our irreducible minds, paradigms. We can't really know how another mind feels sees without being the other mind completely, but then we would no longer be ourselves hence no way to know. If you mix the minds it is neither yours or the others.

 

[vanesch]

Doesn't this imply that the branch we are in had to be observed to be actualised? If it existed in the past then this was because it was observed, if it was not observed in the past then it 'was' in a superposed state and it is our current observations that create our evolutionary past.

Yes, almost. And all those other branches still exist too in this view, and our branch "existed" amongst others, so we didn't "create" it. We *picked* it. It is as if you step on a train, and you say that you "created" the train because if you would have picked another one, you wouldn't see this one.

Speaking of the consistent histories approach to quantum cosmology Lee Smolin writes:

"While the ‘classical’ world we observe, in which particles have definite positions, may be one of the consistent worlds described by a solution to the theory, Dowker and Kent’s results showed that there had to be an infinite number of other worlds too. Moreover, there were an infinite number of consistent worlds that have been classical up to this point but will not be anything like our world in five minutes’ time. Even more disturbing, there were worlds that were classical now that were arbitrarily mixed up superpositions of classical at any point in the past. Dowker concluded that, if the consistent-histories interpretation is correct, we have no right to deduce from the existence of fossils now that dinosaurs roamed the planet a hundred million years ago."

Lee Smolin
Three Roads to Quantum Gravity (44)

Who am I to critique Lee Smolin of course But I think he misses the point that the branch with the dinosaurs was there already (amongst many other ones). So we DO have the right to conclude that dinosaurs roamed the world, IN THE BRANCH WE PICKED NOW.

cheers,
patrick.

 

[Canute]

Are you saying that we create the history of this branch in retrospect, as Dowker suggests may be the case, or, rather, that consciousness was present at all times in the history of this branch, doing the picking?

Would you agree that saying that all possible branches existed in the past is the same as saying that all branches (with a finite probability of existing) were at that time superposed?

 

[vanesch]

Are you saying that we create the history of this branch in retrospect, as Dowker suggests may be the case, or, rather, that consciousness was present at all times in the history of this branch, doing the picking?

No, I'm saying that the branch (= term in the superposition) was present all the time, and that your consciousness picks out that term NOW.

Would you agree that saying that all possible branches existed in the past is the same as saying that all branches (with a finite probability of existing) were at that time superposed?

Yes, and they still are in superposition.

What I grossly mean is this:

t1 = 5 billion years ago
|psi_universe(t1)> = |earth formation> + |Bearth formation> + |binary star formation>

(I consider 3 "branches": namely one where the Earth formed, one where another planet (much bigger) formed, and one where there were no planets, but instead of the sun, a binary star system formed)

t2 = 150 million years ago:

|psi_universe(t2)> = (|dinosaurs> +|giant trilobites>+|nolife>) + (|instable solar system> + |wonky life>) + |binary star>

The parentheses indicate possible evolutions from the previous branches.

t3 = 65 million years ago:

|psi_universe(t3)> = ({|dinosaurs dead> + |dinosaurs still around>} +|giant trilobites>+|nolife>) + (|instable solar system> + |wonky life>) + |binary star>

etc...

t4 = 1945

|psi_universe(t4)> = ({|dinosaurs dead, hitler wins WWII> + |dinausaurs dead, hilter looses WWII> + |dinosaurs still around>} +|giant trilobites>+|nolife>) + (|instable solar system> + |wonky life>) + |binary star>


t5 = today

|psi_universe(t5)> = ({|dinosaurs dead, hitler wins WWII> + |dinausaurs dead, hilter looses WWII, you look at physics forum> + |dinosaurs still around>} +|giant trilobites>+|nolife>) + (|instable solar system> + |wonky life>) + |binary star>

You happen to be in the highlighted branch. That's all.

cheers,
Patrick.

 

[Canute]

I'm still not sure what you consider to be the role of consciousness in all this. Do you mean that this branch is "highlighted" by consciousness, that consciousness picks from amongst the possibilities and actualises one of them? Or do you mean that all the possible branches or universes exist forever in superposition, as a cosmic wave-function, all of them as real as this one, and that we just happen to find ourselves here in this one? I suppose the question is: Would this branch exist if consciousness did not exist?

 

[vanesch]

all of them as real as this one, and that we just happen to find ourselves here in this one?

That's the essence of the MWI view indeed. I don't "believe" in it, I just find it the interpretation that comes closest to the ACTUAL quantum formalism. All the rest supposes some extra physics (except the also acceptable, purely epistemological view that all we do is scratch symbols on a sheet of paper, crank out numbers and observe that they correspond to readings on measurement apparatus, without giving a meaning or reality to any of this - but I personally don't like to think of things that way because it is a big "inspiration killer" !). I'm HOPING for some extra physics, but as long as we don't have such extra physics, I stick to what the actual formalism tells us ; I don't see how to do anything else, honestly.

cheers,
Patrick.

 

[vanesch]

Would this branch exist if consciousness did not exist?

I'd say: why not ?

 

[Tournesol]

That's the essence of the MWI view indeed. I don't "believe" in it, I just find it the interpretation that comes closest to the ACTUAL quantum formalism.

The actual formalism requires use of Born rule, and that is quite
tricky for MWI poponents to justify.

 

[Canute]

I'd say: why not ?

Hmm. I'd say: why? And also how? I though MWI depended on observers having a role, but maybe I'm wrong on that one.

 

[vanesch]

I though MWI depended on observers having a role, but maybe I'm wrong on that one.

MWI is of course encompassing a lot of flavors, but I think that they all agree on these points (I'm a bit a heretic amongst MWI, you'll see):

1) the state of the system (universe...) is given by the wavefunction, and it evolves according to the unitary Schroedinger equation, period (no collapse).

2) Observers manage to observe only ONE term in the wavefunction

All the different flavors try to extablish 2) FROM WITHIN the unitary formalism ; I'm convinced that this is doomed (that's my MWI-heresy) and that you have to postulate it externally.

But MWI is "less" observer-dependent than observer-collapse theories, because MWI postulates that the state of the universe follows its deterministic evolution equation, independent of what might be observed or not ; collapse models, where the observer actively collapses the wavefunction, have of course a specific dynamical role for the observer, which happens to modify the wavefunction of the entire universe.

Now, true MWI-ers hope that "observer" will be some kind of emergent phenomenon within the unitary QM evolution, but, as I said, I think this is impossible.

cheers,
Patrick.

 

[vanesch]

The actual formalism requires use of Born rule, and that is quite
tricky for MWI poponents to justify.

This is correct, and that's where I think that MWI goes wrong, in fact. I think the Born rule is needed as an extra postulate (and that all "derivations" of the born rule sneaked it in through the back door).
But that doesn't do away with the possibility (taken on by MWI proponents) that the wavefunction evolves unitarily, period. (no ontological projection postulate).

If you then let your consciousness "associate" with a term in the wavefunction, according to the Born rule, by postulate, you solve what MWI-ers do not succeed in deriving.

So the Born rule becomes the "evolution of (non-physical) consciousness", while the Schroedinger equation is the "evolution of the physical wavefunction".

It solves at least the issue of why certain interactions collapse the wavefunction, when you call them observation, while those same interactions evolve unitarily if you don't call them "observation".

 

[Canute]

Are you suggesting that the "collapse" of the wave function is not "out there" but rather in consciousness, and that "out there" there is no collapse?

 

[vanesch]

Are you suggesting that the "collapse" of the wave function is not "out there" but rather in consciousness, and that "out there" there is no collapse?

In a way, yes. It appears to us that there is "collapse" because we are now only associated to ONE term in the wavefunction, and as thus, can only couple significantly to this single term. The other terms, except for very very peculiar dynamics, will remain "orthogonal" for ever and we'll never observe their effect anymore. We can then, from our point of view, just take out that term alone and pretend that it is the entire wavefunction: the wavefunction, from our point of view, has "collapsed" into this single term.
Epistemologically of course, this IS a collapse. But if you insist on an ontology (such as I try to do), I'd say that there is a difference between: "the state doesn't collapse, but to us, it appears to do so" (which means that the observer, in the end, does NOT do something to the wavefunction), and "the observer collapsed the wavefunction (of the universe)" - I feel too modest to claim so :-)

cheers,
Patrick.

 

[El Hombre Invisible]

In a way, yes. It appears to us that there is "collapse" because we are now only associated to ONE term in the wavefunction, and as thus, can only couple significantly to this single term. The other terms, except for very very peculiar dynamics, will remain "orthogonal" for ever and we'll never observe their effect anymore. We can then, from our point of view, just take out that term alone and pretend that it is the entire wavefunction: the wavefunction, from our point of view, has "collapsed" into this single term.
Epistemologically of course, this IS a collapse. But if you insist on an ontology (such as I try to do), I'd say that there is a difference between: "the state doesn't collapse, but to us, it appears to do so" (which means that the observer, in the end, does NOT do something to the wavefunction), and "the observer collapsed the wavefunction (of the universe)" - I feel too modest to claim so :-)

cheers,
Patrick.

When you say the wave function continues uncollapsed 'out there', and collapse is purely a property of observation, do you mean that the wave function is completely unaltered by that observation?

I mean, if you looked in the box to see that Shrodinger's cat was dead, then came back an hour later and looked again... could it be alive?

 

[vanesch]

When you say the wave function continues uncollapsed 'out there', and collapse is purely a property of observation, do you mean that the wave function is completely unaltered by that observation?

No, it is not completely unaltered of course, because measurement is always associated with an interaction (of the device with the system). So you will introduce some unitary evolution associated with that interaction ; it is even exactly that interaction which will give rise to the "split" of the terms (and the obligation of choosing again "one term" to live in, according to the Born rule). "Ideal" measurements (pre-measurement interactions a la von Neumann) however, limit the interaction to entangling the state of the measurement system with the different eigenstates of the system under study, without altering first the state of the system under study.

If the system under study is in the state: a |1> + b|2> and you measure the 1/2 property (|1> and |2> are the eigenstates of the measurement operator), then you get, in an ideal pre-measurement interaction:
|m0> (a|1> + b|2>) evolves into a|m1>|1> + b|m2>|2>

A "dirty" measurement could first evolve |1> and |2> into different things, such as |A> and |B> and we'd have:
|m0> (a|1> + b|2>) evolves into a|m1> |A> + b|m2> |B>

This gives you the same measurement results, but the state after the "dirty" measurement is not the projection anymore of the state onto the "measurement eigenvector".

I mean, if you looked in the box to see that Shrodinger's cat was dead, then came back an hour later and looked again... could it be alive?

No, because the second time you look, you *remember* the result of the first measurement, so your brain is still entangled with the "dead cat" term, and you now re-measure that result. The state of your brain remembering "dead cat yesterday" is a complicated quantum state, which is essentially orthogonal to the state where you remember "live cat yesterday", and its evolution from yesterday to today will keep that essentially orthogonal under MOST possible time evolutions. This orthogonality suppresses completely the possible term in which you "remember dead cat yesterday" AND "live cat today". It is this EFFECTIVE orthogonality which makes that you cannot "switch branches" and that once in a branch (a term of the wave function), you're stuck with it for the rest of your days - so that you can just as well only work with that single term (projection).

But *in principle* one could evolve your brain states and all that goes with it (what you wrote down on paper, etc...) "dead cat yesterday" and "live cat yesterday" into two NON-orthogonal states (by un-doing the measurement interaction), and, indeed, "ressurect" the cat. However, in order to do so, you'd have to FORGET that you saw the cat dead yesterday, so you wouldn't feel any contradiction in seeing the cat live today. Such an evolution is far far far beyond our technical means, but it is, in principle, possible and that is what MWI distinguishes, as a physical theory, from any "collapse" theory.
The microscopic version, where we DO have the technology, is called "delayed quantum erasure". We don't do it with brains, but simply with photons.

cheers,
Patrick.

 

[El Hombre Invisible]

No, it is not completely unaltered of course, because measurement is always associated with an interaction (of the device with the system). So you will introduce some unitary evolution associated with that interaction ; it is even exactly that interaction which will give rise to the "split" of the terms (and the obligation of choosing again "one term" to live in, according to the Born rule).

So when that wave function (one in which two states are possible) is altered after measurement, is it altered such that the measured state is the only possible state, or is it just more probable than before? I assume the latter, since the former would constitute a collapsed wave function, no?

No, because the second time you look, you *remember* the result of the first measurement, so your brain is still entangled with the "dead cat" term, and you now re-measure that result. The state of your brain remembering "dead cat yesterday" is a complicated quantum state, which is essentially orthogonal to the state where you remember "live cat yesterday", and its evolution from yesterday to today will keep that essentially orthogonal under MOST possible time evolutions. This orthogonality suppresses completely the possible term in which you "remember dead cat yesterday" AND "live cat today". It is this EFFECTIVE orthogonality which makes that you cannot "switch branches" and that once in a branch (a term of the wave function), you're stuck with it for the rest of your days - so that you can just as well only work with that single term (projection).

Well, what if the second measurement is made by someone else? If it is the 'dead cat yesterday' state that ensures the state evolves into 'dead cat today' when I look again, then if I don't look again today but someone else does, they don't have that brain state of the cat being dead yesterday to stop them remembering the cat being alive today (assuming I didn't tell them).

Another few questions, related to both of the last points. If the wave function can be altered by detection, can it not be altered by non-conscious detection? Say, the photon detector in the cat experiment?

Also, do not the indiviual components of the system have their own wave functions, such as the photon detector in the cat experiment? For instance, if the photon detector can record if it detects a photon, then up until that lone photon is actual emitted towards it, the detector must surely be in a state of 'no detection' - because so far there is nothing to be detected. In which case, at what point can the wave function collapse into one of several states? Once the photon is emitted? Once it reaches the mirror? Once it reaches the detector?

Sorry if I don't really get it. I'll be getting more into this anyway soon, but it'll be good to have some kind of grounding. And to talk about it on this forum in the meantime of course.

 

[vanesch]

So when that wave function (one in which two states are possible) is altered after measurement, is it altered such that the measured state is the only possible state, or is it just more probable than before? I assume the latter, since the former would constitute a collapsed wave function, no?

Eh, no. The measurement device is now just coupled with each of the possible outcomes, and in each term, a different outcome is present. As, in the end, your body will also couple to these situations, your body will experience all possible outcomes, but you will only be aware of ONE of these bodystates.

Well, what if the second measurement is made by someone else? If it is the 'dead cat yesterday' state that ensures the state evolves into 'dead cat today' when I look again, then if I don't look again today but someone else does, they don't have that brain state of the cat being dead yesterday to stop them remembering the cat being alive today (assuming I didn't tell them).

This is entirely correct. So the other person looking will get entangled, and one of his new states will see the cat live (and my alter ego with my other bodystate remembering that the cat was live - only, that's not the bodystate *I* am aware of), and the other state will see the cat dead, and thus end up entangled also with the bodystate of my body which *I* am aware of, and which remembers the cat to be dead.
Have a look at my journal (to the left of this post), where you will see a few formal examples of this.

Another few questions, related to both of the last points. If the wave function can be altered by detection, can it not be altered by non-conscious detection? Say, the photon detector in the cat experiment?

Of course. So one state of the photon detector will "see the photon" and the other state of the photon will "not see the photon". The photon detector just happens to get entangled and gets into a superposition of 2 states, just as any other quantum object.

Also, do not the indiviual components of the system have their own wave functions, such as the photon detector in the cat experiment? For instance, if the photon detector can record if it detects a photon, then up until that lone photon is actual emitted towards it, the detector must surely be in a state of 'no detection' - because so far there is nothing to be detected. In which case, at what point can the wave function collapse into one of several states? Once the photon is emitted? Once it reaches the mirror? Once it reaches the detector?

You got it: it never collapses :-)

Sorry if I don't really get it. I'll be getting more into this anyway soon, but it'll be good to have some kind of grounding. And to talk about it on this forum in the meantime of course.

The viewpoint is that *everything* is a quantum object, has a wavefunction associated to it, and can as thus appear in a superposition (and get entangled with other stuff): photons, detectors, computers, displays, eyes, brains, people, friends.
These wavefunctions evolve according to quantum prescription: a unitary evolution given by the hamiltonian of self and interacting energy terms.
It is just a weirdness of life that we do not seem to experience this consciously: so our consciousness seems only to associate with ONE term in the entire superposition. Which term ? Assigned randomly. With what probability ? Born's rule.
This is the essence of the MWI views ; but most MWI proponents even try to do away with the last postulate, and try to *derive* it from unitary QM - something I'm pretty convinced is doomed to fail.
If you think I'm ripe for the asylum, there are many serious people thinking about these things, and I'd even say more: there are many people accepting this *without realising it*.
All string theorists do so, for instance, because they still accept unitary QM, and they try to explain the quantum states of a BLACK HOLE. Now, if you consider objects the size, mass and life time of a black hole to be quantum objects, surely human bodies are tiny microscopic things compared to that, no ? So all people doing quantum gravity cannot but accept MWI at least up to the level far beyond the scale of human beings, which means that the superposition principle must apply strictly to human bodies.

 

[Canute]

This is very interesting. It seems to be a complete reversal of the usual way of looking at the observer/object relationship, interchanging their traditional roles, as I understood them. It almost seems to work, but I still have some problems with it.

If all possible future states of the universe are superposed and are not 'collapsed' by observers, then the question arises of why conscious observers end up in the particular universes they do. This permanently uncollapsed and superimposed multiverse would be like a vast possibility space of events, in which all events that don't break the rules, whatever they are, happen. Anything not disallowed would be compulsory.

I'm ok with that picture of the multiverse, more or less. But how are observers related to their observed universes? In other words, how is it determined which universe an observer will end up in? Before we look at the hypothetical cat it exists not just in a superposition of alive and dead. It exists is all possible states, sitting, standing, facing left or right, licking it left paw or its right, behaving like this or like that and so on. Why should we observe just one of these states and thus end up in a universe with this state of the cat rather than that state?

Is it that some states are more probable than others? In this case wouldn't all observers converge on the most probable universe, or, equivalently, wouldn't universes (collections of cats and other things) converge (collapse) on their most probable state?

This relates to El Hombre's point about how an observation is made. If we cannot observe the state of the cat until after we have branched into a universe in which the cat is in one particular state rather than a superposition of states, then what caused that branching? The branching would have to occur before the observation, and it would not be caused by the observation. Why would the observer branch off in this particular direction instead of one of the many others?

I think what I'm saying is that there seems to be a problem of causality in this MWI. But I'm still trying to get my head around it.

 

[vanesch]

If all possible future states of the universe are superposed and are not 'collapsed' by observers, then the question arises of why conscious observers end up in the particular universes they do.

Well, that's where "orthodox" MWI and I differ: orthodox MWI "hopes to show that", while I think that you cannot do so and that you simply have to accept that as a basic postulate, in the same way as that you have to accept that the physics evolves according to a hamiltonian. So the point is: just take this as a fundamental postulate on how "consciousnesses" behave.
It is the fundamental "one term out of all" problem of quantum theory.
Of course, a real collapse theory does NOT have that problem, but they have others, the most pressing that they violate causality and/or special relativity (an example of such a collapse theory is Bohmian mechanics).

This permanently uncollapsed and superimposed multiverse would be like a vast possibility space of events, in which all events that don't break the rules, whatever they are, happen. Anything not disallowed would be compulsory.

Yes, that's the view, indeed. And it opens possibilities for a real application of the anthropic principle: we can only be in branches where intelligent life develloped, for instance. So calculating how "improbable" it was that life spontaneously develloped on Earth is meaningless in this view, just as it is meaningless to calculate what the chances are that you were born in the recent past (less than say, 100 years ago).

I'm ok with that picture of the multiverse, more or less. But how are observers related to their observed universes? In other words, how is it determined which universe an observer will end up in? Before we look at the hypothetical cat it exists not just in a superposition of alive and dead. It exists is all possible states, sitting, standing, facing left or right, licking it left paw or its right, behaving like this or like that and so on. Why should we observe just one of these states and thus end up in a universe with this state of the cat rather than that state?

Just take it as a postulate. "the law of consciousness" :-)

Is it that some states are more probable than others? In this case wouldn't all observers converge on the most probable universe, or, equivalently, wouldn't universes (collections of cats and other things) converge (collapse) on their most probable state?

There is not a "most probable" universe that is observed; otherwise you would not find the Born rule, but you'd find deterministically EACH TIME the same outcome, namely the one that the highest amplitude. But in fact your argument was the one DeWitt used more or less as one of the first attempts to explain the appearance of the Born probabilities in MWI: he found that all terms in the wave function in which observers do NOT, on average, observe Born probabilities, have a Hilbert norm tending to 0 in the limit of t-> infinity.
So all "branches" in which observers would systematically find deviations from the Born rule will vanish "in the norm" at t-> infinity, meaning that those branches that will get there are OK, with an observer in them agreeing on the Born rule.
The problem with that is 2-fold (but it is a nice try :-): first, what about an observer right now ? What does he care about t-> infinity ? And 2) the Hilbert norm is the Born probability, so this is a circular argument: branches in which observers observe highly unlikely Born probabilities will, according to Born probability, have a vanishingly small probability. Yeah.
Also this argument doesn't explain why we only have to observe one term in the first place.
There have been other arguments made to derive the Born rule from the unitary structure of QM, but they all contain extra hypotheses (which, to me, means that these extra hypotheses are simply the logical equivalent of postulating the Born rule - so let us postulate it right away !)

This relates to El Hombre's point about how an observation is made. If we cannot observe the state of the cat until after we have branched into a universe in which the cat is in one particular state rather than a superposition of states, then what caused that branching?

The *interaction* of your body with whatever information carrier (= entangled state) about the cat. There is a physical interaction you have when you make an observation: you *look* at something (light interacts with your retina etc...). Now, IF the thing you're interacting with 'carries information about the cat state" then it means it is entangled with it. And if your body now interacts with this carrier, it means your body gets entangled with it. Apparently, our consciousness doesn't support "being entangled" and has now to make a choice between the different terms of the entanglement, according to the Born rule.

The branching would have to occur before the observation, and it would not be caused by the observation. Why would the observer branch off in this particular direction instead of one of the many others?

Why do you say that the branching has to occur before observation ??

It is exactly DURING the observation that the branching occurs, no ?

 

[El Hombre Invisible]

I don't think you're ripe for the asylum, Patrick, but I do detect some dangerous-sounding assumptions in this view. Personally, I find the idea of a postulate that observation of a single state is simply how consciousness behaves something of a leap of faith, not because the reasoning is in any way faulty - I know more about it from this one thread than I knew about it before, so I bow down to you - but that it makes special claims about consciousness when a) we know too little about it - even how it arose is still contentious theory; and b) this 'specialness' (to coin a word) of consciousness is a little too close to a religious view of self. To make such assumptions is perhaps jumping the gun a little?

In explaining how two objective observers would agree on the same measurement, you omitted to explain how their states would become entangled. How each separately become entangled to the system under observation I can follow, but not how they themselves concur. I'm picking up, though, on some idea of a kind of 'universal' state - that the observation by one person creates a branch of this wave function such that everyone in this branch measures the same state. If I am right, I would rather come to understand MWI as a logical consequence of the evolution of such states, than assume MWI is true and therefore such paradoxes are resolved. On discussion of the cat experiment, a lecturer advised me last week that part of the problem is applying the wave function view of a single particle to a complex body (in this case a cat) comprises many, many more orders of magnitudes of particles. I'm inclined to believe him, and applying it to the universe as a whole is taking it so much further.

Your smiley suggests you knew I didn't mean the 'out there' wave function but the measured term of it. What I was getting at was: can a single term be thought to be 'observed' by a non-conscious apparatus, such as photon detector, in the same way one is observed by the experimenter? Because if not then, again, this makes special claims for consciousness that I'm not comfortable with (my problem, but surely an understandable one).

Your answer to my question about how measurement alters a wave function is perhaps an answer to a different question, presumably because the question I asked makes no sense. \o/ I was referring to the wave function that gives the probability of the photon being in the detector or not in the detector alone. Is this solitary wave function altered by observation of the cat, or no, or does that make no sense? Again, apologies for my slight grasp of all things Schrodinger. I've only got about as far as distances of electrons from nuclei. )o: I don't even know what the wave function of a photon being in a photon detector would look like because I presume there's no term for Epot for a photon. But I still relate wave functions to 'probability a particle is at x,y,z,t'. My bad, I'm sure.

 

[Canute]

Why do you say that the branching has to occur before observation ?? It is exactly DURING the observation that the branching occurs, no ?

If it takes a finite time for information to travel from the cat to my consciousness then I cannot be conscious of the cat being in some state or other until a finite time after the cat has adopted that state. This seem to mean that the universes in which the cat is dead and the universes in which it is alive must branch before I can consciously observe the cat in either state. If so then what determines which branch I will end up in? Could it be my expectation of which state I will observe? Or are advanced and retarded waves relavant here, allowing the observation and the branching to be effectively simultaneous but apparently sequential?

 

[vanesch]

If it takes a finite time for information to travel from the cat to my consciousness then I cannot be conscious of the cat being in some state or other until a finite time after the cat has adopted that state. This seem to mean that the universes in which the cat is dead and the universes in which it is alive must branch before I can consciously observe the cat in either state.

"universes branching" is a fancy word for single terms becoming a sum in the wavefunction.

So let us look at this (I'm pushing some OTHER problems under the carpet here, related to what's called the "preferred basis problem").

Begin state: time t0. The state is a product state:
|radioactive atom>|bottle with poison>|live cat>|ignorant me>

Time evolution: time t1.

the radioactive atom's state has now evolved into two states: |radioactive atom> and |decayed atom+photon>. This is due to the unitary evolution within the "atom" hilbert space ; it doesn't affect, for the moment, anything else. The systems remain hence, still in a product. As long as *I* am in a product state with the rest, "my branch didn't split".

(a | radioactive atom> + b |decayed atom+photon>) |bottle with poison>|live cat>|ignorant me>

Time evolution: time t2: we'll spare the details, but the photon triggered a detector that activated a hammer and shattered the bottle. THIS is of course now an interaction between the bottle and the atom (through a lot of intermediary steps which I left out here). The interaction causes an entanglement between the bottle state and the atom state.


(a | radioactive atom>|bottle with poison> + b |decayed atom+photon>|broken bottle with poison>) |live cat>|ignorant me>

Time evolution time t3: the poison is inhaled by the poor cat. This clearly is an interaction of the cat with the poison:

(a | radioactive atom>|bottle with poison>|live cat> + b |decayed atom+photon>|broken bottle with poison>|dead cat>) |ignorant me>

This is an interesting moment: the cat got into its "Schroedinger state". Note that the information didn't get to me yet, so I'm still ignorant about it, and hence the universe didn't "branch" from my point of view. But taking that the cat is an observing being, from the cat's point of view, the universe just branched, because it's not in a product state anymore. It was the interaction (cat-poison) which did the entanglement and hence the branching. Let us say that this is an extreme case of observation... from the cat's viewpoint.

Time t4: I look at the cat:
Now, I look at the cat. This is an interaction: light scattering off the cat gets into my eyes, which trigger nervecells etc...

a | radioactive atom>|bottle with poison>|live cat>|I_see_cat_alive> + b |decayed atom+photon>|broken bottle with poison>|dead cat>|I_see_cat_dead>

Note that NOW, I'm not in a product state anymore with the rest of the world: so I have to choose now in what branch to live. This happens according to the Born rule: the part of me that sees the "cat live" has a Hilbert norm |a|^2, while the part of me that sees the cat dead, has Hilbert norm |b|^2.
I am assigned one of the branches with these respective probabilities, so I will consciously experience now "I_see_cat_alive" with probability |a|^2, and "I_see_cat_dead" with probability |b|^2, but only ONE will be consciously experienced.
So for all that matters to me now, I can just as well claim that the state is now (if I saw the cat live):
| radioactive atom>|bottle with poison>|live cat>|I_see_cat_alive>

(this is the effective projection postulate)

but the real state of the universe is still:
a | radioactive atom>|bottle with poison>|live cat>|I_see_cat_alive> + b |decayed atom+photon>|broken bottle with poison>|dead cat>|mybody_sees_cat_dead>

where I changed the denomination of the last term, because the "I" experience was now in the other term.
You can, if you want, assign a NEW consciousness to the other state of mybody. But *I* do not experience this.

If so then what determines which branch I will end up in? Could it be my expectation of which state I will observe? Or are advanced and retarded waves relavant here, allowing the observation and the branching to be effectively simultaneous but apparently sequential?

There is no "objective" branching of the world, only "relative to an observer". So when *I* get involved, the world branches relative to me. When the cat gets involved, it branches relative to the cat, but maybe not to me.

 

[vanesch]

Personally, I find the idea of a postulate that observation of a single state is simply how consciousness behaves something of a leap of faith, not because the reasoning is in any way faulty - I know more about it from this one thread than I knew about it before, so I bow down to you - but that it makes special claims about consciousness when a) we know too little about it - even how it arose is still contentious theory; and b) this 'specialness' (to coin a word) of consciousness is a little too close to a religious view of self. To make such assumptions is perhaps jumping the gun a little?

Let us say that exploring all the miriads of mystical possibilities of MWI/consciousness related issues, makes Harry Potter live a dull trivial life (hey, I should write novels about it, and become rich - quantum mechanics course included ). You could for instance insist on the difference between mortal consciousnesses which follow Born's rule, and divine consciousnesses which don't, and hence experience the entire multiverse , or even wilder stuff. Never thought that physics got so fun.

But you could indeed also be scared that you completely loose all of scientific rigor when you slide that slippery slope (I guess that's your mindset). Let me repeat that I think that the MWI+consciousness view follows, as far as I can see (which is maybe not far enough), without any alternative from:

1) an assumption of ontology of the wavefunction
2) the assumption that the superposition principle is universally valid (quantum mechanics rules "all the way up") and dynamics is strictly unitary.
3) the assumption that the dynamics of the universe is local and causal
4) the fact that we do not observe the world in a superposition
5) the assumption that we cannot instantaneously change the state of the universe

1) means that you think that the wavefunction correctly describes the world "out there" - that QM is a correct theory and that there IS a world out there.
2) means essentially that you cannot avoid human bodies being described by quantum wave functions, and them getting into superposed states. In fact, all of quantum theory develloped so far make this assumption (especially string theory)
3) this assumption eliminates non-local hidden variable theories, such as Bohm's theory, which have a natural explanation for "projection" but with the price to pay that they are non-local and (via SR) hence non-causal.
4) ok, I guess we can agree upon that fact.
5) this means that my mind is not somehow powerful enough to change the state of the universe, including the Andromeda galaxy. So I cannot ontologically "collapse the wavefunction".

I think that if you ask many physicists about each assumption individually, they all agree with it (except maybe with 1). But when presented with the unavoidable conclusion then, which is this MWI stuff, they often refuse it !

My hope is in fact that 2) and 3) are subtly interwoven and not individually true, and that gravity might do something there, inducing a genuine collapse, with subtle violations of SR, but in a controlled way. I only have to work it out yet in detail


cheers,
Patrick

PS: before tackling your other points, have a look at the cat stuff I posted before.

 

[selfAdjoint]

Yeah #1 is the key.

1) means that you think that the wavefunction correctly describes the world "out there" - that QM is a correct theory and that there IS a world out there.

Many of us don't see any sufficient reason to believe it. Oh, I believe in "the world out there"; I just don't believe QM is a complete and correct theory of it. Or if it is, the question is beyond conceivable experimental tests.

 

[vanesch]

Many of us don't see any sufficient reason to believe it. Oh, I believe in "the world out there"; I just don't believe QM is a complete and correct theory of it. Or if it is, the question is beyond conceivable experimental tests.

Ok, but that's a strange viewpoint too. The best theory we have doesn't possesses anything that describes the world "out there", even though it "is" out there. So this is the QM interpretation: "QM is nonsense, but it works in the lab ; and we don't know anything else"
This never happened before. Each time people thought that ("it's nonsense, but it works") we got stuck with it: the existence of atoms, the existence of EM fields, spacetime, ... the wave function ?
Of course, there has to be a first time for everything, but what is the reason to take up the point of view that "strangely enough, QM works in the lab, but it doesn't contain anything which descibes the world" ? Does it lead to contradictions or inconsistencies ? I don't think so.
Why take on that view then ? Because if you assume that it is somehow right enough to contain something which describes the world, you arrive at weird conclusions ? I don't think it is justified, when trying to make sense of a theory (when trying to make sense of actual, standard, quantum theory) to start from the viewpoint that it is wrong, just because the conclusion you arrive at is so weird. After all, _we don't know anything else for the moment_, so it is just wishful thinking that whatever will replace QM will REMOVE the weirdness. There's nothing in sight that can rise hope it will do so.
So, or we're stuck in a strange situation, which is: there's a world out there, but we haven't gotten a clue of what it is like. The only thing we have, are a set of calculational rules which fit perfectly with whatever we measure, and which claims that it contains a mathematical object that is the state of the world, but this is in fact not the case because if it were, we'd have very weird conclusions indeed about this world.
What if we transposed this reasoning to another domain ? Say, biological evolution. So, we have a theory, called evolution, which explains nicely many aspects of the biological world, and the bones we find in the ground. It even claims that human beings are descendents of apes which is a weird conclusion indeed. So although this theory explains nicely different aspects of the biological world and many lab experiments, we take it that a future theory will remove this weirdness of humans being descendents of apes while keeping all the nice results...
(ok, this last paragraph was a piece of rethoric :-)

 

[selfAdjoint]

This never happened before. Each time people thought that ("it's nonsense, but it works") we got stuck with it: the existence of atoms, the existence of EM fields, spacetime, ...

...the luminiferous ether, phlogiston, crystalline spheres,...