Rainer Plaga: On your quantum immortality: you're right about traffic accidents and the like. But I would guess that the fact that nobody ever gets older than, say, 300 years is a deterministic law of nature, or in other words that there is no single branch in which a 300 year old James exists. Isn't that plausible?

James Higgo: The fact that nobody over 300 exists in this universe is unsurprising. But this is one of quite a few universes. And perhaps they're hiding. Will you concede there is a one in a thousand billion chance that you will live over 300 years?

Rainer Plaga: I don't know what the chance for a branch with a 300 year old James is. But how do you know it is not equal 0?

James Higgo: Anything that's possible exists, according to David Deutsch. Sorry to appeal to his authority.

Rainer Plaga: Yeah, anything *that's possible* but I guess nobody knows whether a 300 year old James *is possible* according to the laws of nature.

James Higgo: But it's possible that all the molecules in my body just rearranged themselves by chance into the form of a teapot, so it's certainly possible that you don't die.

Rainer Plaga Hello James,

I see what you are saying now. Probably you are right. Unfortunately even when we're 300 years old we will have a hard time to convince our fellow ``branchians'' that the reason for our extreme longevity is MWI! (Analogous to Max's experiment). Could there be something extremely improbable that led to the survival of the human race? (Your idea's analogy to my A-bomb experiment) An example would be: if we could prove that it is extremely improbable that a time span of 70 million years goes by without a nearby supernova that would wipe out the human race. This would be an argument in favour of MWI. This was only an example, one can show that it is quite probable that no nearby SN happens within 70 million years.

But perhaps we can find an example that works. Actually: this would also explain the absence of extraterrestrial intelligence (Fermi's paradox).

All the best Rainer

James Higgo: Some fine ideas. So I take it you're a convert, O Immortal one!

Rainer Plaga: I believe

Actually there is a corollary: It is impossible to commit suicide. Each time you try you will find that some stupid accident or strange improbable force kept you from doing it.

Rainer

James Higgo: Absolutely. And you will miraculously survive terminal cancer.

Max Tegmark Thanks for the [QTI] article. Quite interesting. In the motorcycle case, you still get a few seconds between the quantum decision and your loss of consciousness, so the surviving version of you would have to live with the distressing knowledge that the other version actually had time to become aware of its imminent death and that 3 seconds of original life actually got killed. I made the loss of consciousness virtually immediate in the gun case to avoid such unpleasantness. But you could of course easily cook up an accident scenario reducing the time delay. The ideal accident should in my opinion have * no lead time * certainty of death (you don't want to wake up as a cripple).

Max Tegmark This immortality issue has bothered me too, and a number of other people also brought it up after this article came out. I agree with you that if the argument were flawless, I should expect to be the oldest guy on the planet (at least), severely discrediting the Everett hypothesis. However, I think I've found a flaw. After all, dying isn't a binary thing where you're either dead or alive - rather, there's a whole continuum of states of progressively decreasing self-awareness. What makes the quantum suicide work is that you force an abrupt transition. I suspect that when I get old, my brain cells will gradually give out so that I keep feeling self-aware, but less and less so, the final "death" being quite anti-climactic, sort of like when an amoeba croaks. Do you buy this?

James Higgo Max, I agree this is a flaw - did your note cross with my mail asking about Alzheimer's? But I think there muse be a solution if only we understood the concept of consciousness.

James Higgo Max Tegmark, in a recent e-mail, pointed out that progressive loss of consciousness was a problem for the quantum theory of immortality. I just realised, while reading Huw Price's book, that this is an arrow-of-time issue. If you can degenerate into an amoeba, the amoeba can regenerate into you, and there will always be 'subsequent' universes in which you have recovered from whatever damage could be inflicted, to regain your consciousness. I need to read Dennett on this, but consciousness seems to be just a collection of memories, and this can always be preserved and reproduced. Any comment on these thoughts, Max?

Max Tegmark That's an interesting point. I agree that you'll make a spectacular recovery from your brain-damaged amoeba-like state in a number of branches, but just like a broken egg only comes together again in a negligible fraction of all branches, the same will apply here. So a TYPICAL expectation for my subjective future is probably that I will gradually fade away without ever making much of a comeback. Cheers, Max ;-)

James Higgo But Max, this is a very important point. If you *know* that no matter how far you degenerate there is *always* a subsequent universe in which you are 'you' again, then the immortality problem is still there. 'Typically' we disappear in a puff of smoke. So what? Weak anthropic principle - we always end up in a universe in which we exist (which normally means, one with a history which fits the laws of physics).

Gilles Henri Maybe the question of identity is not so trivial. Who is "you"? In the "everything" hypothesis (that I share with Deutsch if I understand correctly James' previous mail) only some states are connected together from "past" to "future" following a classical "consistent history" (following Griffith), defining a possible world story. The notion of "you" can be followed by temporal continuity. But it is not so obvious if you consider very rare, quantum events that branch you on completely different worlds. What allows to say "you are 'you'" during such process? You are you because of your memory of your past, which implies an arrow of time, which implies classically allowed evolution... By the way I defended on another forum (initiated by Jacques) the idea that in the "everything" hypothesis time is really not existing, since the total wavefunction of the Universe is most probably stationary. Each "instant" we observe is only a classical subcomponent of this wavefunction, apparently connected to "past" and "future" states (much like Deutsch again if I understood it correctly). So the question of one's "future" is really a question of how many "classical" states can exist and which of them do contain a "you-like" (to be defined..) with a certain age?

James Higgo Your world-view is very similar to mine. We only see an arrow of time as we are 'creatures in time'. Time is not an objective feature of the block universe. If you adopt Huw Price's Archimedean standpoint in 'nowhen' and look through Deutsch's 'multiverse' at a certain angle, you might happen (by a one in a ten to the billion chance) to string certain 'snapshots' together so you saw a 'world' evolving in time. The difference is that I am initerested in the practical question: whatever consciousness is (I'm with Dennett), can I expect to die or should I expect to be conscious for ever? I have, as yet, heard of no reason why not.

Jacques Mallah Max, you may be just one step away from seeing the error of your ways. You seem to realize that measure is the issue. Why can't you realize that even if the amoeba went one step further and disappeared completely, or if you were to consider the amoeba to be some individual other than yourself, it would not help you in the slightest to be effectively immortal? Most of your measure is in your pre-suicide perceptions.

Hal Finney It seems that there are two ways to increase the measure of copies and near-copies of yourself which have favorable experiences. One is to try to make good decisions. Some people say that in a many-worlds model decisions are irrelevant, since everything will happen. However this ignores the fact that different branches have different measures. Even though we know that all things will happen, by making good decisions we increase the fraction which have good outcomes, thereby increasing the measure of those instances of ourselves which are having favorable experiences. The other method is by killing yourself when things go wrong. This approach is more controversial, but many people seem to have an instinctive understanding of its value. People often do kill themselves when things get sufficiently bad. By doing so they are increasing the fraction of their near-copies which have good experiences. They are not thinking in those terms, but that is the effect of their actions.

Bruno Marchal I quite agree with that. But it is necessary to be cautious. Suppose that my goal is to prove Goldbach conjecture (or any unproved big mathematical conjecture). I am not very gifted in mathematics, so I decide to proceed in the following way. I use a big array of quantum particles, let us say 2^32 particles. Each one is prepared in a superposition like 1/sqr(2)(O + 1). Then I read (measure) each particle following the order given by the array, and I decode the result in the computer-keyboard-base. In case I understand what I read as a proof of Goldbach conjecture : I am done. If not, then I kill myself with a gun (let us say). Let us suppose now that there is no such proof. With MWI, I will survive ANYWAY. For example I will survive because in some "worlds" the bullet will go through by brain without affecting it thanks to the tunnel effect. Unfortunately, in the majority of the world where I survive by tunnel effect, my brain will be damaged because the probability of a clean tunnel effect is little compare to a less clean tunnel effect. In case there is a proof of G. conjecture less than 2^32 bits, then I can expect to survive in good shape in a world with the proof ONLY IF the probability to survive (even in bad shape) by tunnel effect is much less than 1/2^32. To sum up : to use succesfully the quantum suicide, you need to compare carefully the probability of your gain with the probability to survive annihilation. (and of course you must realize that according to your friends your quantum suicide is just suicide, and there are infinitely other ethical problem).

(Note : More on this in my 1998 PhD Thesis, or in my 1988 or 1991 paper, which I intend to put on a WEB page as soon as possible. In my thesis I show that the MWI is directly deductible from the thesis "I am a machine", so that empirical quantum mechanics is a confirmation of mechanism. I show that most of the qualitative aspect of quantum mechanics (indeterminism, mon-locality, etc.) are derivable fromarithmetic + mechanism. Consciousness including the appearance of matter emerge from arithmetical truth. This give a kind of "many (computational) histories, no universes" interpretation of number theory.

James Higgo Fantastic stuff! Someone who has 100% the same understanding of the situation as I do! But surely, then, Bruno, you also wonder about the issue of whether we will ever experience death. I reiterate what I discussed with Max above: no matter how 'dirty the tunnel', there will always be subsequent branches in which your brain regenerates fully. Can you shed any light on this issue? The point you make about your relatives grieving in most universes is accurate and this is the only reason I can think of (bar my genetic programming, and my genes can take a jump) for not indulging in quantum suicide.

Bruno Marchal Well ... I do think that nobody experience (absolute, first person) death. Because either we survive clinical (3d person) death, in which case we do not experience death, or we don't survive clinical death, in which case we do not experience death because to make an experience you must survive it ! (Disappointing answer I guess, it doesn't depend on MWI or Mechanism). Topologically, time-life is open, we cannot experience the border of life.

JH> I reiterate what I discussed with Max above: no matter how 'dirty the tunnel', there will always be subsequent branches in which your brain regenerates fully. Can you shed any light on this issue?

OK James, I can try. As Hal Finney put it, it is a question of (conditional) measure. Suppose you jump out of the window which is just 5 meter high. In that case, the probability you will survive is rather high. The probability you will be wounded will be rather high too. Nevertheless, the probability you will recover is not very high. I mean it is just a classical probability. The problem with the "dirty tunnel" is the fact that you survive in bad shape. And once you have survived, probabilities are classical. And, although you are right in saying that there will be subsequent branching in which you will recover fully, you cannot take that into account because your expectation for that personnal recovering is very low. ... Unless you reiterate the quantum suicide. But here, the problem is that you are taking the risk to find yourself in a so bad shape that you will not be able to reiterate the suicide. In that case, you will be stuck (for example paralysed) in a universe, perhaps for a very long time. For the long run you are surely (with Mechanism or with MWI) making a point.

JH>Another point - committing suicide is always going to be against your genetic program as your genes act as if they wanted their structure to appear in as many universes as possible. Form their point of view it is better to have you arbitrarily unhappy but existing than to have you decompose.

I think it is plausible that in most future the genes will lose the battle against other form of memes. We can hope that earth will at least be used as a kind of carbon-life and genes museum. But most practical computationalist will explore the multiverse by exchanging their expensive carbon based body for radio-waves and silicon hardware. From OUR point of view it is better to be happy without genes than unhappy with genes ...

>JH …mechanism is one valid way of looking at things but not the only…

I am indeed convinced that mechanism is not the only way to look at things. The beauty of mechanism is that with mechanism, it is even necessarily so. I mean mechanism entails the consistency of non-mechanism. A little like the fact that consistency of arithmetic entails the consistency of the non consistency of arithmetic ( a form of second Godel's theorem).

I have put the thesis on my web page. You can load it at http://iridia.ulb.ac.be/marchal

James Higgo Many thanks for your detailed reply. Of course you are right that it is trivial to say that we will not experience death. But I think we need to be clear that classical probability is merely the way we perceive the relationship between universes in MWI. I am grasping at an idea that the very definition of consciousness is such that to be conscious now means that your consciousness is unbounded in time... but it will take more time for my consciousness to formulate that idea coherently.

I like your idea about genes losing to memes. It will be one hell of a battle. Once you are downloaded onto a more robust platform - presumably the mechanism by which consciousness is immortal - you will not be striving to replicate yourself. However the remaining gene machines will be doing so with tooth and claw.

The thesis is impressive and the images came out perfectly. My French is lousy but is C.2.3 not Max's experiment? Did you think of it first, simultaneously, or later? I look forward, anxiously, to the English version.

Gilles Henri I propose a new game based on the same idea as quantum suicide, but much more awarding. It can make you for sure the richest guy in the world. The only condition is to find enough players who are convinced by MWI. Each one gives say 10000 euros (let's be modern). In fact he can give his whole fortune. The total amount of money is gathered in one single pot. Each player is assigned a different binary number, and the game can start. Each bit is tested at turn by measuring the spin of an unpolarized electron on a z-axis. The +1/2 value corresponds to 1, the -1/2 to 0. All players whose bit does not correspond to the measured value are killed. Of course following the MWI, each player ends up in the world where his own number has been drawn, and he (or she) takes the jackpot for himself. It is perfectly fair since all the other players keep on living in another world. If you can convince maybe 1 million persons (?), this makes a lot of money...

Some drawbacks: 1)You are also sure that the (N-1) other players will have been killed in your world. So it's better not to play with friends. 2)You must accept the idea that all your relatives and friends will loose you in all the worlds where you lost the game (although of course they continue to live with you in your world). This could be a good reason not to play by compassion with the other worlds... 3)But it is not worse that the simple suicide! So the game could be played with all guys ready to commit suicide (with the hope that in the world where there are winning, they are happy enough to keep on living ! Although as we say in French "l'argent ne fait pas le bonheur..."). A (less moral ? but much more effective ) way to win is to organize the game without taking part in it, but sharing the money with the winner. You may have to convince a tribunal that the (N-1) killed persons are living and happy in some world..Ready to play?

James Higgo Gilles, did I not recommend a Tontine to you the other day? You have just described it, a form of pension arrangement available since the 1700s in Italy and still available in France today but illegal in the UK for obvious reasons. I actually intend to buy one when I'm next in France - putting my money where my mouth is.

Gilles Henri Yes, but my game is much more efficient: it makes you win billions of euros (or any currency ) in a few minutes! Of course you may have trouble with the law, even in France or in Italy. By the way if you really come in France I would be happy to meet you there!

James Higgo As we live forever there's no hurry. Efficiency is an invalid concept when you have an infinite amount of time! I shall be in Paris again May, but I shall be in StMoritz on 29th Jan and Val d'Isere at the end of March, and that's closer to Grenoble. If you're near any of the above or feel like a visit it would be very fine to meet you.

Jaques Mallah Would you have us believe that if only I could hook up a device to my head, that could measure my neurones to see if they are giving out (which is of course a quantum process), and instantly kill me if they are, then since only the few copies of me with healthy brains will exist, that I would be immortal? Ridiculous.

James Higgo You propose a neat solution. Where can I buy this device?

Wei Dai It depends on whether you want to preserve the illusion of forward-flowing consciousness. If you don't, then you should expect to (I) experience exactly what you are experiencing right now. If you do, then you have to figure out how this flow actually works. There are two possibilities: either (IIa) some (most) threads terminate or converge into null experiences (corresponding to unconsciousness), or (IIb) that doesn't happen (maybe the threads merge somehow). In the first case you should expect to experience nothing. In the second case you should expect to experience immortality. (IIb) is probably most appealing to most people, but I prefer (on an intellectual level) (I) because it is simplest.

James Higgo (IIb) is not what I think. I think both (I) and (IIb). There is no 'natural flow of time', so a forward-flowing consciousness is doubly meaningless. In my view, the overwhelming majority of threads end in 'null experiences' but from any point you can possibly find yourself at on any thread there will always be an 'escape thread' along which your consciousness 'survives'. In a multiverse where everything possible exists, this absolutely MUST be true. The only question in my mind is whether I will experience a smooth flow of consciousness, always along those 'escape threads'. Nothing I believe contradicts my experience or, to the best of my knowledge, any generally accepted physical principle, once you accept MWI.

Victor Stenger I think the problem is that you are treating macroscopic, many body systems as if they are pure quantum states. This is often done in discussions of Schroedinger's cat, but was not meant to be taken that literally by Schroedinger. The quantum entanglement problem is still there, and Schroedinger was right in pointing it out, but it applies to coherent quantum states rather than most macroscopic systems which are incoherent mixtures. This is the problem with all the quantum consciousness business too, since the brain is a very hot, incoherent, "macroscopic" system even at the level of neurons and synapses. (See my discussion in The Unconscious Quantum). So, maybe you can make one of your atoms immortal, but not likely the whole assemblage.

Rainer Plaga Vic's criticism seems justified in the sense that you didn't prove immortality, only extreme longevity seems very plausible. On the other hand I do not understand his point about mixtures. There are no ``true mixtures'' in the MWI, only one pure state of which we perceive a very small part. So he seems to be outside strict MWI with his argument.

James Higgo Agreed. Also, Vic seems to imply that there are two types of system - quantum and non-quantum. A 'quantum effect' can make a difference to a hot, wet classical system.

Gilles Henri Jacques, I basically agree with what you say, excepted for the last sentence. I doubt that the measure of a conscious thought is a simple concept. Consciousness is *not* a physical parameter like others. It is *not* measurable by an external apparatus. It is a subjective feeling that we can only experience for ourselves, not for other human people. We just infer logically that the other people are conscious because of the similarity of their behavior with ours.

Russell Standish > > In Tegmark's experiment you would know if you had a quantum machine > > gun firing at you - because you would be damaged by it, and would feel > > the bullets, although not killed by it. The would also be other human > > beings looking on who would know that there was a machine gun, and > > congratulate you on your miraculous ability to survive. > > > > In the case of vacuum decays, surely these would have some kind of > > distribution of energies, and some of the lower valued decays (say of > > the order of the KT event, or even more wimpish at about the level of > > a Nuclear Amageddon) would tend to be noticed by the surviving > > concious observers. > >> > Unless the distribution is vanishingly small at these lower energies, > > observational experience would rule out events of this nature.

James Higgo Restate the Tegmark experiment so that instead of a machine gun there is something that instantly and certainly obliterates you. In any other situation, you will always have a branch leading away in which you are alive, right up until the instant of death.

Russell Standish Yes, but I don't know of any process that can kill a person with absolute certainty. If you know of one, then you have a counter example to the Quantum Theory of Immortality. I'm sure there would be a few people in this discussion group who would be interested.

James Higgo OK - you are in Universe 1. That universe is connected to Universes 1.1, 1.2, 1.3.... 1.infinity by a relationship which we conventionally call "one planck-time". Let us say that in some of those universes (1.a) you are alive, and in the others, (1.b), you are dead. The mechanism doesn't really matter. Having a faulty machine gun or a death wish on a motorcycle, or a Tegmark experiment or whatever, there will always be some where you live, and therefore are conscious of, and some where you die, and therefore they are immaterial to you.

Russell Standish It seems to me that the Tegmark suicide experiment is approached rather idealistically. It is quite possible to survive a gunshot wound to the head. The most likely outcome the researcher performing the Tegmark suicide experiment would experience is a few clicks, followed by a bang, then waking up in a hospital with half her brains blown out! I certainly take the proposition that one can never experience one's own death seriously. My question is whether there are any dead-end paths in the multiverse - i.e. is it possible for one to experience a situation where it is 100% certain that you will die. I haven't been able to think of one - even if you are on the guillotine with the blade dropping - it is still possible for the blade to jam just millimetres from your neck!

James Higgo ... or millimetres through your neck. Yes, this is a problem. We could spend eternity paralysed and pretty miserable. In fact, we do in some universes. But in quite a few - those in which technology continues to develop, i.e. no WWIII or supernova - we could download ourselves onto a more robust platform. I agree that there is no reason why there would be a branch in which you are certain to die; it is more likely that in all branches you are certain not to.

Russell Standish Actually, another thought I had on the Quantum Theory of Immortality, was to look at the actual distribution of human lifetimes. There is a theory about distributions of improbably events, and a write up of it appeared in one of New Scientist's Inside Science supplements maybe a year or two ago. The point was that if there was a definite time span for human life (in a statistical sense), then the distribution of ages when people die should drop off at least as fast as an exponential. In reality, the distribution drops off a lot slower - i.e. people never seem to die of "old-age" they always die of something - disease, misadventure, whatever. This certainly seemed to point to the possibility of human lives on average being extended considerably more than the current upper bound (about 120 years or so, if you discount the accounts in the bible). Actually, none of this is really necessary from the QTI perspective - an exponential distribution still has finite probability of generating any length of lifetime - only one that goes to zero after finite time would rule out QTI - and as has been argued this case seems rather implausible.

Gilles Henri A potential caveat (which would make this perspective less attractive than what could be expected) is that we are made of a finite number of atoms. So most probably the number of different mind states we are able to experiment is also finite; so even if the theoretical lifetime is infinite, we won't be able to remember our life after some time, and we will only be able to experiment a finite interval of ages. A possibility is that you could evolve biologically to more and more complex systems, which could record more and more data. I think we are lacking here a real theory of consciousness that could tell us which possibilities are compatible with physical laws, and what is the maximal complexity (if any?) you can reach . Of course MWI isn't saying that everything imaginable is realised in some world, only what is possible physically (to be defined!) is realised.

James Higgo Both points are valid, and I think we all agree that neither of them provides any theoretical reason why we should not expect that an arbitrarily large number of arbitrarily old Gilleses, Russells and Jameses are out there. The really interesting issue from my point of view is whether we can be sure that we will 'end up' as one of them, and if so, how our consciousness evolves from the point where 10^500 universes have us alive and riding our motorcycle to the point just afterwards where we are dead in 0.9*10^500 universes, and alive (but possible battered) in 0.1*10^500 universes. Although we can see this in the Tegmark equation, this is pretty hard to conceptualise and any new ideas would be welcome.

Rainer Plaga I think Henri's point is correct. When we say ``immortality'' this is probably not age=infinity but age=large number. However this is very speculative you come into questions like the heat death of the universe etc. For me these questions are not so pressing, as the pressing question: Is MWI true? We have to find tests! Somebody who has a proof that tests are impossible should publish it. (If the proof is convincing MWI will become ``many words'' for me). A last point: You only focus on the positive. Tomorrow I'll go on a plane. If MWI is true some fraction of me will go through the agony of a drawn out plane crash.

James Higgo Yup. You will crash. Sorry. If you're coming to London give me a call. On Henri's point, we have finite number of atoms, but I am not certain that there is a finite number of configurations for those atoms in relation to their environment. You must be a positivist if lack of proof will cause you to discount MWI. As Goedel showed, many true hypotheses are unprovable.

Rainer Plaga >On Henri's point, we have finite number of atoms, but I am not certain that

>there is a finite number of configurations for those atoms.

Yes, that's a very difficult question. There is an article by F.Dyson on RevMod Phys ca. 1979 whether life ever has to stop in an expanding universe and he reaches the conclusion that it does not have to.

>As Goedel showed, many true hypotheses are unprovable.

OK, true in principle. I have only a very modest question: Can one get experimental evidence for the existence of other branches? (In last consequence this means communication). This is most important question in physics, I feel. There are two answers possible in principle:

a. this is impossible in principle because...

There is no paper which evens aims to give such a proof. Max thinks this is true but without giving a demonstration.

b. the simplest device possible for this feat is...

David Deutsch gives a very sci-fi device which is certainly non-realisable in the immediate future.

I can't understand why people who favour MWI do not agree on the importance of this question. Even I my proposal is wrong, and a correct proposal would be much more difficult to realise: the exp. proof of parallel branches is clearly more important than the exp. detection of a Higgs boson, so about 5 billion $ would not be too high a price to realise such a proposal.

The fraction of myself that will die tomorrow says goodbye, it was nice to have made your email acquaintance.

Rainer

James Higgo Perhaps we believe MWI so strongly that we're not so determined to prove it. Eventually everyone will accept it anyway as it is the most elegant, simplest and most consistent (with my world-view) interpretation. Enjoy the trip.

Gilles Henri On infinite mind states: In my opinion, brain states are highly degenerated in the sense that typical mind states are probably linked to macroscopic neural states, corresponding to many microscopic configurations. It is further probable that many neural states can correspond to the same apparent mind state, since we are continuously (in most worlds!) loosing a lot of neurones without suffering dramatical consequences. As we have around 10^10 neurones, I would guess that the total number of mind states would be lower (and probably much lower) than 2^(10^10), many of them not corresponding to sensible thoughts. This would let some room for the memory of a very long life, but not for an infinite one...

On proving MWI: In my opinion again, the difficulty of "proving" MWI is that genuine quantum effects are only visible in interference terms, as the decoherence theory tells us. That's why Tegmark's suicide experiment, although correct in the MWI, can not be considered as a proof of it, even in the worlds where the experimentalist survives. As already mentioned, it could be merely considered as the realisation of a very unlikely probability, which is also compatible with the wave function collapse interpretation. The only proof of MWI would be by a statistically significant deviation from the classical evolution after a measurement, due to interference with the "ghost" universes which are supposed to have disappeared in the WFCI. It should be reproducible in any world (some kind of macroscopic Aspect's experiment). Unfortunately, the decoherence theory tells us that these deviations are VERY VERY small, so I cannot figure out how to make them significant... The best argument for MWI is still that of logical self-consistency. Well, we are quite confident that gravitational waves do exist, although nobody has been able to detect them up to now!

James Higgo Jaques, what you say is fine for 1000 Bobs and Toms but not for an infinite number. The probability goes from 'negligible' to 100%. The real question is whether it is possible to be one of the dead Bobs, or whether yet you will always find that you are and ever will be a surviving Bob.

Jacques Mallah Why should it make any difference whether there are 1000 Bobs or infinitely many? If there is one Bob every mile on an infinite road, and every other Bob commits suicide today, there are only half as many Bobs left, even though there is still an infinite number. The set of Bobs would lose half its measure, and if you are a typical Bob, you are 50% likely to die. You do seem to at least realise that quantum suicide may not work the way Tegmark says it should. That's a good sign. The real question is not whether it is possible to 'be dead'. Of course, death implies a lack of being. Rather, the point is that the total measure of Bobs would decrease, and that a typical (i.e. randomly chosen) Bob conscious observation would be twice as likely to be before the suicide than after.

Gilles Henri Jacques, your sentence " if you are a typical Bob, you are 50% likely to die" has not the same meaning as in common life. As all worlds where you don't exist have no signification for you, you can say by anthropic principle that you (i.e. at least one of your possible future) are sure to continue to live in a world where you survived suicide, unless the quantum probability of this event is strictly zero. You are not sensitive to the fact that you exist only in a very small fraction of the possible Universes. Gilles

James Higgo Jaques, Gilles said exactly what I would have said. Has anyone given Alzheimer's much thought? The concept of a progressive loss of consciousness is a troubling one for the QTI.

Jaques Mallah There certainly seems to be a lot of confusion about the anthropic principle, for some reason. What the anthropic principle is, in the formulation that makes sense, is a better version of the Copernican principle. It states that we should assume our conscious observations to be typical; effectively, drawn at random from the set of all such conscious observations that exist. Obviously, in regions of space or time in which there are no conscious observations, we will not find ourselves there. Perhaps your confusion is as follows: how should we count multiple instances in which the same observation occurs, as one observation or as many, for the purpose of finding the probabilities? But the answer is clear: as many. It makes no difference whether observations are the same or not. Indeed, if the answer were 'one', then the effective probabilities for each observation in quantum mechanics would be equal, instead of depending on the wavefunction. So when some Bobs die, the total measure decreases, and the fractional measure of Bobs decreases. There is really no difference between the MWI and non-MWI in this respect. If your life is so painful that you'd rather not exist than live, commit suicide; otherwise, don't. You q-suiciders seem to have some weird notions of identity. You seem to think that there is a well defined 'you' and that for some odd reason, the measure of that thing's thoughts is conserved in time."

James Higgo Being sympathetic to Buddhist philosophy, I deny absolutely that there *is* such a thing as a well defined 'me'. All I am interested in is the 'flow' of consciousness. I see no reason why this should ever be 'interrupted'.

Jacques Mallah To those who still believe in quantum suicide, the most dangerous crackpot idea I've seen in physics: how can you really understand nothing about the simple concept of the measure of a conscious thought? Even if suicide reduces the measure of the set of universes in which I exist, why is that bad? Is it detectable by me? Do I notice if I kill myself off in some universes? Can I "feel" when my measure changes? I don't see how. Think about the example with the spacially infinite universe. That gives the same measure distribution, but may be easier to understand for some people. When some thoughts have more measure than others, is that noticeable? Only in an effectively statistical sense. It is just like when you measure the spin of an electron. You either get spin up or spin down; that does not tell you the effective probabilities. But by no means must the effective probabilities be equal! The measure distribution is determined by the wavefunction in QM. In the example with the infinite universe, it simply means that more 'copies' of you might see spin up than spin down. Similarly, if more 'copies' see age 30 than age 300, the thoughts associated with age 30 have more measure. A typical thought associated with you-like beings is more likely to be drawn from the set of age 30 thoughts. This is all very easy to understand from the point of view of the infinite universe: the fact that other you-like beings exist out there has no bearing on your own limited experiences. You are not immortal, and the set of you-like beings has a limited expectation value of age I am emphasizing here that there is a set of 'you-like beings' rather than calling them all 'you' because you q-suiciders seem to have some weird notions of identity. You seem to think that there is a well defined 'you' and that for some odd reason, the measure of that thing's thoughts is conserved in time. There is no reason to even think that personal identity is such a well defined notion; I am different now than I was yesterday, and tomorrow there is a small component of the wavefunction in which the atoms that now compose me have assumed the configuration of those that now currently compose you.

Gilles Henri Jacques, I basically agree with what you say, excepted for the last sentence. I doubt that the measure of a conscious thought is a simple concept. [...] Yes, consciousness is not a simple idea, but: measure is. It's like saying that the mass of a dog is a simple concept. You don't need to know much about dogs, or how they differ from wolves, to understand what I mean when I say that Fluffy has half the mass of Rover.

Gilles Henri I think all this discussion shows the contradictions implied by the idea that "you" are entirely equivalent to a computation that can be duplicated. Nobody would say that two identical billiard balls are "the same" object, because there is always some level at which they are distinguishable. That is the same for human beings : even if you could create a perfect copy of your quantum state (what I suspect is impossible because the wavefunction itself is not measurable for a single system), this quantum state would immediately evolve differently because of its different environment. It will just be a "delayed twin". The identity is not only a computational property, but a physical one. Moreover the properties of consciousness (in the human sense) make it essential to oneself to represent one's own existence-this is the main criterion to distinguish it from other ways of handling information. If you are conscious of yourself, you are also conscious of your position in the Universe and you cannot confuse yourself with another one in the same macroscopic Universe. So any reasoning implying a "duplication" of your identity is wrong from the very beginning, or more exactly has no sense. It has only sense in MWI, where by definition all copies of yourselves can't interact.

James Higgo My real question is: will our consciousness always 'flow' into those branches of the multiverse in which we still exist, i.e. will we experience a continuous flow of consciousness ad infinitum. Assuming MWI, I can't see why not; and this is a good argument against MWI, say some, or a very interesting result, say others.

John Mazetier An odd and interesting question. My first hurdle is in contemplating the utter Vastness of the multiverse, if it is indeed so. Every nexus of quantum probability a shearpoint to more than one universe? This is hard enough to imagine, let alone use as a conduit for even *one* multi-branching mind!

James Higgo I'm convinced that its enormity is the reason so few have taken the idea seriously. Remember Copernicus?

John Mazetier My reaction is just that, a reaction. I take it as nothing more than a highly questionable guide, since so much about the universe is counter intuitive. However, picture the gallant Sir James riding his steed across the heath, having a fine day, and ***Shearpoint*** here are the gallant Sir James on their trusty...well, one is really an old mare, barely able to trot, and alas James 2 has become a petunia plant, confusing the hell out of his horse... What is wrong with this picture? To put a finer point on my unease about the MWI thesis, from what perspective do we determine that a set of events is an actual history in MWI? If the only possible connection between universes is an individual shearpoint, and there can be no further "leakage" between universes, what is the meaning of "universe"? Definitionally, we cannot have a universe of shearpoints. So, it would have to consist exclusively of non-quantum probabalistic events, and how many of those can exist in a single universe? Perhaps the answer is hinted at in the quotation below:

'We consider Everett's work to be useful and important, but we believe that there is much more to be done. In some cases too, his choice of vocabulary and that of subsequent commentators on his work have created confusion. For example, his interpretation is often described in terms of "many worlds," whereas we believe that "many alternate histories of the universe" is what is really meant. Furthermore, the many worlds are described as being "all equally real," whereas we believe it is less confusing to speak of "many histories, all treated alike by the theory except for their different probabilities." To use the language we recommend is to address the familiar notion that a given system can have different possible histories, each with its own probability; it is not necessary to become queasy trying to conceive of many "parallel universes," all equally real. (One distinguished physicist, well versed in quantum mechanics, inferred from certain commentaries on Everett's interpretation that anyone who accepts it should want to play Russian roulette for high stakes, because in some of the "equally real" worlds the player would survive and be rich.)' - Murray Gell-Mann, 1994

Within any given macro object lie the potential for billions of alternate histories. How much can the macro-histories affect the macro realm? Do we picture a writhing sea of micro-universes coming and going, whose net effect is the larger history we see? Does not this last surmise look suspiciously like AM? I donno. I'm pretty new at this. Maybe it does not matter. In an alternate universe, I've got the all the answers!

James Higgo Of course, there is a >0 chance I will become a petunia. If we believe MWI, that means that in some universes I do in fact turn into a petunia, accompanied perhaps by wailing and gnashing of teeth. But I think our difficulty with these ideas stems form the fact that we still thing that there is an objective reality. Well, there is, but as it is *everything* there is no point describing it unless you take a subjective viewpoint. If you get the 'angle' right, you can glance through the block universe and see anything you like. On a more day-to-day level, we experience these 'shearpoints', quite literally, all the time. One moment my toast has not popped up, the next it has. Indeed, it could be the case under any QM interpretation that the toast may become a small bunch of roses. Of course, I'd be surprised as the probability (in MWI say the measure) of toast-> rose in my subjective time is so small.

Ed Weinmann The most accurate way of describing immortality under MWI would be to say, that in all those worlds in which one continues to exist, one's consciousness (assuming one is not somehow rendered unconscious, in that particular world) will also continue to exist. No "flowing" from one world to another is necessary, or possible. You would retain all the memories (again, barring some events that would wipe that memory, in that particular world) that constitute your personality, that occurred in the world-branch leading out to the most recent split. Now, the big question: will there be at least one world in which you survive forever? Answer: not known. This is the substance of "things hoped for", rather than of things known.

James Higgo That 'big question' is answered as everything possible exists in the MWI. So a universe with a billion-year-old Ed exists. And no matter where you are in the MWI, by an MWI version of cosmological principle, you will always have such a universe 'subsequent' to you. The big question is whether you can be sure that you will seamlessly 'end up' in that universe, as universes in which you do not exist - e.g. one in which there is a vacuum collapse - cease to become relevant to you.

Ed Weinmann It seems to me that whether a billion-year old Ed is *possible*, then, would be the last area, of anything resembling religious faith, that would be open to me. If it is possible, then I would *have* to end up a billion years old, in that particular world. There would be no need to worry about "seams", since the branching occurs automatically... if it is possible that there be a billion-year old Ed, then there will have to be such an Ed. Counterfactuality, whether produced by blind chance or a divine providence, is gone. I am reminded of a line from one of the Greek tragedies... maybe someone can help me out with the author, which I *think* is Sophocles, and the work, which I can't remember at all: "Oh Zeus, whether you were compulsion of nature, or intelligence of mankind To you, I prayed."

James Higgo Precisely. As there is everything in MWI, there is no room for God.