I'm not a fan of Many-Worlds, but this article seems to overlook the fact that the many worlds get "thinner" as they continue to split off. The total weight of all worlds sums to 1, and that continues to be the case across time.
warty dog gave one possible answer--there might be a very large (but finite) multiplicity of worlds, and the count of worlds in each state is decreasing.
I don't believe that, though. Amplitudes yield probabilities, but the underlying interferences are at odds with the notion of "parallel universes": the whole reason to postulate "many worlds" within a quantum-theoretic context is because they interact!
Here's what I know. Quantum mechanics is a perfectly sensible and intuitive mathematical formalism, but the metaphysical underpinnings (whatever they turn out to be) are profoundly alien. So much so, in fact, that I have no inclination at all to lean on my naive intuitions about how the universe is "supposed to work". You tell me I am wrong because all of these worlds are equally real. I see no reason to grant you that supposition, nor do I require a precise or rigorous notion of what "equally real" means to express that skepticism. Whatever it means, having phantom worlds of lesser ontological weight would not be especially weird on the scale of quantum mechanics. I'm also not persuaded that your games with cardinalities gives a better notion of size than the games you reject playing with measures.
Fundamentally, you are making an appeal to intuitions that are not as strong as the conclusions you draw from them. Incidentally, your reductio ad absurdum on the self-indexing assumption persuaded me (against my prior intuitions) that it can't be correct, or at least it requires serious revision when dealing with infinite sets.
I prefer to think of the worlds as splitting than as coming into existence. They were all there before, just most of them were identical to each other. It’s not that one electron splits into two electrons as when it encounters two possible paths it could take, it’s that a wave follows both those paths, but both parts of the wave existed before they split. You could image an infinite (or very large number of) identical electrons moving together and splitting into new paths, and the “thickness” of the worlds being just how many electrons follow each path. The discreetness of the picture isn’t quite right but I think it’s pretty close and gives better intuitions than thinking of new copies of the world being created.
Lower measure states contribute less to a superposition (like a ripple having little effect on a mighty wave) , and are less likely to be observed. Both are factors only if the "worlds" are still coherent. What measure means under decoherence is another matter. What it means ethically..low measurements of womanly still real to their occupants.
Imagine multiple balls with numbers written on them. for every ball with number 1 there is 2 balls with number 2, 4 balls with number 3, 8 balls with number 4 and so on. The last group of balls has number N on it, and there are more of them than all the other balls combined.
Suppose that an equiprobably random ball from all these balls was picked and given to you. Naturally you would expect that then number on it would be around N.
But suppose that the ball was not picked equiprobably, that a more nuanced procedure was used: a weighted sample, where every ball with number i+1 is twice less likely to be picked than ball with number i. Now you are indifferent between all the numbers on the ball that you've got.
According MWI, we are dealing with the latter case.
This is needed to make MW work as an empirical theory, one that can predict observations. But it's not clear how it applies to consciousness, ethics , or anthropics. Should we discount the ethical weight of low measure worlds? Do their inhabitants not feel real..or are they relative zombies? And how does low measure reduce your Anthropic probability of finding yourself in a world?
“ Should we discount the ethical weight of low measure worlds? ”
I would think that when we get to the point where we can take moral actions with different consequences in different worlds, we can worry about it then. As it stands there’s no reason to think we will ever have that level of control considering we have no way to “split” macro objects.
“ Do their inhabitants not feel real..or are they relative zombies?”
They would feel and be real, since the laws of physics in all worlds are the same. At least, that would be a physicalist interpretation.
"They would feel and be real, since the laws of physics in all worlds are the same."
Everything would be physically the same except the measure...which is physical. We don't know what consciousness supervenes on, so why shouldn't it supervenes on measure?
The probability distribution has to add up to one, so if "our" world increases or decreases in probability, that affects all the others. That does not quite add up to voluntary control over their probabilities, because MWI is deterministic. However, most people think you should consider your actions carefully even if they are deterministically inevitable. MWI kind of makes things worse, because you can't prevent possible outcomes ,only lower their probability.
I agree this is the only hope MWI has, but I have never known what it is supposed to mean. As someone asked below, are low-amplitude creatures less conscious? If not, then this is no solution at all. If so, then the problems become severe (*we* are of relatively low weight, by this time, after all. Are we relatively unconscious?).
It's true, as you say below, that there could be some weird, undefined sense in which this could be made to make sense. But then, it's hard to evaluate a theory with undefinable concepts necessary for it to work. That doesn't mean it's wrong, but it certainly does mean it's not as knock-down obvious as its advocates pretend (which I gather you agree about, for one reason or another -- maybe not this).
This seems like the correct response. I also know next to nothing about quantum mechanics but it seems like we should expect the probabilities of being in worlds to be distributed "per stirpes" if you will, rather than per capita.
Many people have already brought up the measure / "reality fluid" objection, but there's an interesting moral reason to want something like this to be true as well.
Suppose you have one doggy treat, and want to give it to your dog either now or later.
If you give it now, only one dog will enjoy the treat. If instead you resolve to give it in one second, after the world has split a million times, then one million copies of you will give the treat to the dog, and one million dogs will get to enjoy it.
Therefore, it is morally imperative to give the dog to the treat later. And this applies forever, or at least until there's a ~100% chance the dog will die in the next second (or whatever).
I think if you don't want to be bowled over by crazy moral conundra like this, you need to accept the reality fluid interpretation.
I'm suspicious of trying to figure out what the world is like in non-moral respects based on our moral intuitions. But I just find the reality fluid idea so crazy! How can two different things, both real, have different amounts of relaity fluid? What is reality fluid supposed to be?
You would need need RF, and also the idea that it is morally relevant. But why would it be morally relevant? if something like hedonic utilitarianism ...which you seem to be assuming ..is true, a neat solution is to assume that everything is felt less intensely in low RF/measure worlds, so the thousand dogs have a milli hedon, each. But that's a kind of zombie argument, apart from lying on a continuum.
I think if you combine this approach to MWI with SIA (or SSA for that matter) you end up being inconsistent with the Born probabilities anyways, so this argument already presupposes that physics works differently from how it actually does.
Except we all ARE at the end of the universe. The end of the universe so far. That’s what the present is. 100% of currently conscious people are conscious at the leading edge of these universe divisions.
In his podcast with Alex O'Connor, I understood David Deutsch to be pointing out that the "splitting" that occurs in MWI is *local*, each divergence does not produce an entire extra universe along with it. I won't claim to have understood him but it seems like the structure of the multiverse is very complicated and you'd need a better understanding of its implications to make an argument like this.
This ties into the reality fluidity concepts mentioned in other concepts, where it’s more like there’s a fixed number of identical universes that stop being identical. Outside of that local area where the split happened, they’re still identical in every other way.
For those of us who view the cosmos as one entity, splitting worlds are immaterial. We create reality as we traverse it. The end of a cycle is the beginning of another one. One world or many worlds. It's whatever we choose.
The central premise of MWI is that quantum amplitude is a real physical quantity that measures the degree to which something happens. As time passes quantum amplitude tends to break up into non-interacting blobs, which effectively become independent worlds with their own history and future, but the total amount of amplitude remains the same.
(Actually it's the squared magnitude of quantum amplitude that is conserved and measures happeningness, the amplitude itself is a complex number which is why destructive interference can happen)
I may be misunderstanding the argument, but won't the SIA avoid this? If on theory A 10 people exist at the end of the world and 1 person exists before the end of the world, while on theory B only 1 person exists and is before the end of the world then the SIA gives a 11:1 prior in favor of A. Finding yourself not at the end of the world would then update you 11:1 in favor of theory B, meaning the posterior is the same for both theories. This would presumably extrapolate to whatever number of people many worlds says there are.
I suppose the Boltzmann brains might ruin this, but I'm not sure.
this would ignore causality, the existence of any particular world and also cardinality of the set of existing worlds is conditional on previous worlds, the total probability of all worlds at all times sums to 1 by law of total probability.
to go back to the population lineage example, you are assuming that the likelihood of existing at a specific time only depends on the number of people existing at that time when it also depends on the the number of people in the previous generation, so it is a marginal probability. to get the probability that you correspond to any specific person in the lineage, for the later generations you would have to multiply through the probabilities of the prior generations as well, which becomes increasingly small, in fact, the probability of being in any generation is equal, because the probability of corresponding to any particular person is increasingly small as the number of people in that generation grows larger.
I think there are two objections to this argument:
1) Doesn't it assume SSA? On SIA, you should expect the total number of people to be extremely large, and the MWI follows that. And just as, say, Adam and Eve can't conclude that they'll never have children because it would make their birth rank less likely on SIA, we can't conclude that we won't have a vast number of "quantum children" via world-splitting.
2) Given that there are infinitely many observers involved, we have a measure problem concerning how to weight them. I think that, in order to be consistent with the Born Rule, MWI is just going to say something kind of weird about how to weight the probabilities that doesn't make it more likely for you to come later. Maybe you could argue that the necessity of this weighting is itself an argument against MWI, though just about any weighting scheme has to be weird in some ways.
1. No. I agree on SIA, MWI beats single-universe versions of every other theory. But many worlds does this in a way that makes it very unlikely you'd be so early. So you being very early is highly unlikely on many worlds.
2. Yeah, I think this will just end up super weird. It seems hard to see what the principled motivation is for thinking the probabilities line up with squared amplitude--people just like that because if fits the math.
But doesn't MWI + any multiverse theory beat the multiverse theory alone? Like, if we consider SC to be the the theory that the string theory landscape exists, but each of those universes has objective collapse rather than many worlds, and we let SM be the theory that the string theory landscape exists, but each universe splits in the way described by MWI, then surely SM should come out more likely than SC for the same reason that the MWI beats any single-universe theory.
I think this is a really clever objection to Many Worlds, and I agree with Joseph that the world-thinning response would resolve the problem (though I don’t have enough familiarity with theoretical physics to know whether it’s true).
The first thing to note is that MWI is more than one theory. (In fact, quantum mechanical AI is different to Tegmarks mathematical multiverse, and is several different theories itself).What splittng is...how complete and irrevocable it is ... varies between particular theories. So does the rate of splitting, so does the mechanism of splitting.
The way that you are doing anthropics assumes that full decoherent splitting occurs at every interaction. If that were so there would be no evidence of the various phenomena based on coherent superposition. The doubly maximal concept of many worlds...full decoherence at every elementary interaction (not just observations, which are relatively high level interactions)...has been ruled out.
Scott Aaronson expands on the point:-
"David Deutsch, one of the founders of quantum computing in
the 1980s, certainly thinks that it would. Though to be fair, Deutsch
thinks the impact would “merely” be psychological – since for him,
quantum mechanics has already proved the existence of parallel uni-
verses! Deutsch is fond of asking questions like the following: if Shor’s
algorithm succeeds in factoring a 3000-digit integer, then where was
the number factored? Where did the computational resources needed
to factor the number come from, if not from some sort of “multiverse”
exponentially bigger than the universe we see? To my mind, Deutsch
seems to be tacitly assuming here that factoring is not in BPP – but
no matter; for purposes of argument, we can certainly grant him that
assumption.
It should surprise no one that Deutsch’s views about this are
far from universally accepted. Many who agree about the possibil-
ity of building quantum computers, and the formalism needed to
describe them, nevertheless disagree that the formalism is best inter-
preted in terms of “parallel universes.” To Deutsch, these people are
simply intellectual wusses – like the churchmen who agreed that the
Copernican system was practically useful, so long as one remembers
that obviously the Earth doesn’t really go around the sun.
So, how do the intellectual wusses respond to the charges?
For one thing, they point out that viewing a quantum computer in
terms of “parallel universes” raises serious difficulties of its own.
In particular, there’s what those condemned to worry about such
things call the “preferred basis problem.” The problem is basically
this: how do we define a “split” between one parallel universe and
another? There are infinitely many ways you could imagine slic-
ing up a quantum state, and it’s not clear why one is better than
another!
One can push the argument further. The key thing that quan-
tum computers rely on for speedups – indeed, the thing that makes
quantum mechanics different from classical probability theory in the
first place – is interference between positive and negative amplitudes.
But to whatever extent different “branches” of the multiverse can
usefully interfere for quantum computing, to that extent they don’t
seem like separate branches at all! I mean, the whole point of inter-
ference is to mix branches together so that they lose their individual
identities. If they retain their identities, then for exactly that reason
we don’t see interference.
Of course, a many-worlder could respond that, in order to lose
their separate identities by interfering with each other, the branches
had to be there in the first place! And the argument could go on
(indeed, has gone on) for quite a while.
Rather than take sides in this fraught, fascinating, but perhaps
Note that both arguments can be right, to an extent. Deutsch can be correct that a single universe is unable to account for the power of quantum computation, and Aaronson, can be correct that nothing is implied about a unique decomposition into large scale quasi classical universes as expected by the science fictional version of many worlds.
The point in relation to anthropics is that of only a partial.splitting between components that continue to influence each other occurs, it could well be reasonable to do anthropics in a coarse grained way , over fuzzy blobs of amplitude, leading to far fewer "yous".
I'm not a fan of Many-Worlds, but this article seems to overlook the fact that the many worlds get "thinner" as they continue to split off. The total weight of all worlds sums to 1, and that continues to be the case across time.
What does it mean for a world to be thinner than another if they're equally real? They both exist.
warty dog gave one possible answer--there might be a very large (but finite) multiplicity of worlds, and the count of worlds in each state is decreasing.
I don't believe that, though. Amplitudes yield probabilities, but the underlying interferences are at odds with the notion of "parallel universes": the whole reason to postulate "many worlds" within a quantum-theoretic context is because they interact!
Here's what I know. Quantum mechanics is a perfectly sensible and intuitive mathematical formalism, but the metaphysical underpinnings (whatever they turn out to be) are profoundly alien. So much so, in fact, that I have no inclination at all to lean on my naive intuitions about how the universe is "supposed to work". You tell me I am wrong because all of these worlds are equally real. I see no reason to grant you that supposition, nor do I require a precise or rigorous notion of what "equally real" means to express that skepticism. Whatever it means, having phantom worlds of lesser ontological weight would not be especially weird on the scale of quantum mechanics. I'm also not persuaded that your games with cardinalities gives a better notion of size than the games you reject playing with measures.
Fundamentally, you are making an appeal to intuitions that are not as strong as the conclusions you draw from them. Incidentally, your reductio ad absurdum on the self-indexing assumption persuaded me (against my prior intuitions) that it can't be correct, or at least it requires serious revision when dealing with infinite sets.
https://gwern.net/modus
I prefer to think of the worlds as splitting than as coming into existence. They were all there before, just most of them were identical to each other. It’s not that one electron splits into two electrons as when it encounters two possible paths it could take, it’s that a wave follows both those paths, but both parts of the wave existed before they split. You could image an infinite (or very large number of) identical electrons moving together and splitting into new paths, and the “thickness” of the worlds being just how many electrons follow each path. The discreetness of the picture isn’t quite right but I think it’s pretty close and gives better intuitions than thinking of new copies of the world being created.
Lower measure states contribute less to a superposition (like a ripple having little effect on a mighty wave) , and are less likely to be observed. Both are factors only if the "worlds" are still coherent. What measure means under decoherence is another matter. What it means ethically..low measurements of womanly still real to their occupants.
Some are, alledgedly, less real than others.
Imagine multiple balls with numbers written on them. for every ball with number 1 there is 2 balls with number 2, 4 balls with number 3, 8 balls with number 4 and so on. The last group of balls has number N on it, and there are more of them than all the other balls combined.
Suppose that an equiprobably random ball from all these balls was picked and given to you. Naturally you would expect that then number on it would be around N.
But suppose that the ball was not picked equiprobably, that a more nuanced procedure was used: a weighted sample, where every ball with number i+1 is twice less likely to be picked than ball with number i. Now you are indifferent between all the numbers on the ball that you've got.
According MWI, we are dealing with the latter case.
This is needed to make MW work as an empirical theory, one that can predict observations. But it's not clear how it applies to consciousness, ethics , or anthropics. Should we discount the ethical weight of low measure worlds? Do their inhabitants not feel real..or are they relative zombies? And how does low measure reduce your Anthropic probability of finding yourself in a world?
“ Should we discount the ethical weight of low measure worlds? ”
I would think that when we get to the point where we can take moral actions with different consequences in different worlds, we can worry about it then. As it stands there’s no reason to think we will ever have that level of control considering we have no way to “split” macro objects.
“ Do their inhabitants not feel real..or are they relative zombies?”
They would feel and be real, since the laws of physics in all worlds are the same. At least, that would be a physicalist interpretation.
"They would feel and be real, since the laws of physics in all worlds are the same."
Everything would be physically the same except the measure...which is physical. We don't know what consciousness supervenes on, so why shouldn't it supervenes on measure?
The probability distribution has to add up to one, so if "our" world increases or decreases in probability, that affects all the others. That does not quite add up to voluntary control over their probabilities, because MWI is deterministic. However, most people think you should consider your actions carefully even if they are deterministically inevitable. MWI kind of makes things worse, because you can't prevent possible outcomes ,only lower their probability.
I agree this is the only hope MWI has, but I have never known what it is supposed to mean. As someone asked below, are low-amplitude creatures less conscious? If not, then this is no solution at all. If so, then the problems become severe (*we* are of relatively low weight, by this time, after all. Are we relatively unconscious?).
It's true, as you say below, that there could be some weird, undefined sense in which this could be made to make sense. But then, it's hard to evaluate a theory with undefinable concepts necessary for it to work. That doesn't mean it's wrong, but it certainly does mean it's not as knock-down obvious as its advocates pretend (which I gather you agree about, for one reason or another -- maybe not this).
This seems like the correct response. I also know next to nothing about quantum mechanics but it seems like we should expect the probabilities of being in worlds to be distributed "per stirpes" if you will, rather than per capita.
Many people have already brought up the measure / "reality fluid" objection, but there's an interesting moral reason to want something like this to be true as well.
Suppose you have one doggy treat, and want to give it to your dog either now or later.
If you give it now, only one dog will enjoy the treat. If instead you resolve to give it in one second, after the world has split a million times, then one million copies of you will give the treat to the dog, and one million dogs will get to enjoy it.
Therefore, it is morally imperative to give the dog to the treat later. And this applies forever, or at least until there's a ~100% chance the dog will die in the next second (or whatever).
I think if you don't want to be bowled over by crazy moral conundra like this, you need to accept the reality fluid interpretation.
Sorry for the late reply lol!
I'm suspicious of trying to figure out what the world is like in non-moral respects based on our moral intuitions. But I just find the reality fluid idea so crazy! How can two different things, both real, have different amounts of relaity fluid? What is reality fluid supposed to be?
You would need need RF, and also the idea that it is morally relevant. But why would it be morally relevant? if something like hedonic utilitarianism ...which you seem to be assuming ..is true, a neat solution is to assume that everything is felt less intensely in low RF/measure worlds, so the thousand dogs have a milli hedon, each. But that's a kind of zombie argument, apart from lying on a continuum.
MWI asserts that the sum of measure over branches remain constant (equal to 1) over time. There's no discrete point where timelines "split".
I think if you combine this approach to MWI with SIA (or SSA for that matter) you end up being inconsistent with the Born probabilities anyways, so this argument already presupposes that physics works differently from how it actually does.
A video I found with some more objections to many-worlds:
https://www.youtube.com/watch?v=6Qbyxg95ebw
Except we all ARE at the end of the universe. The end of the universe so far. That’s what the present is. 100% of currently conscious people are conscious at the leading edge of these universe divisions.
In his podcast with Alex O'Connor, I understood David Deutsch to be pointing out that the "splitting" that occurs in MWI is *local*, each divergence does not produce an entire extra universe along with it. I won't claim to have understood him but it seems like the structure of the multiverse is very complicated and you'd need a better understanding of its implications to make an argument like this.
This ties into the reality fluidity concepts mentioned in other concepts, where it’s more like there’s a fixed number of identical universes that stop being identical. Outside of that local area where the split happened, they’re still identical in every other way.
If splitting means coherent super positions, they are not just local, but reversible and basis dependent! Not very world like , in other words.
For those of us who view the cosmos as one entity, splitting worlds are immaterial. We create reality as we traverse it. The end of a cycle is the beginning of another one. One world or many worlds. It's whatever we choose.
This is the Doomsday argument but adapted to many-worlds, right? (And I think Doomsday is a bad argument)
Hello Matthew I wanted to ask a question on why you think the Problem of Evil doesn’t disprove a God ?
The central premise of MWI is that quantum amplitude is a real physical quantity that measures the degree to which something happens. As time passes quantum amplitude tends to break up into non-interacting blobs, which effectively become independent worlds with their own history and future, but the total amount of amplitude remains the same.
(Actually it's the squared magnitude of quantum amplitude that is conserved and measures happeningness, the amplitude itself is a complex number which is why destructive interference can happen)
I may be misunderstanding the argument, but won't the SIA avoid this? If on theory A 10 people exist at the end of the world and 1 person exists before the end of the world, while on theory B only 1 person exists and is before the end of the world then the SIA gives a 11:1 prior in favor of A. Finding yourself not at the end of the world would then update you 11:1 in favor of theory B, meaning the posterior is the same for both theories. This would presumably extrapolate to whatever number of people many worlds says there are.
I suppose the Boltzmann brains might ruin this, but I'm not sure.
this would ignore causality, the existence of any particular world and also cardinality of the set of existing worlds is conditional on previous worlds, the total probability of all worlds at all times sums to 1 by law of total probability.
to go back to the population lineage example, you are assuming that the likelihood of existing at a specific time only depends on the number of people existing at that time when it also depends on the the number of people in the previous generation, so it is a marginal probability. to get the probability that you correspond to any specific person in the lineage, for the later generations you would have to multiply through the probabilities of the prior generations as well, which becomes increasingly small, in fact, the probability of being in any generation is equal, because the probability of corresponding to any particular person is increasingly small as the number of people in that generation grows larger.
I think there are two objections to this argument:
1) Doesn't it assume SSA? On SIA, you should expect the total number of people to be extremely large, and the MWI follows that. And just as, say, Adam and Eve can't conclude that they'll never have children because it would make their birth rank less likely on SIA, we can't conclude that we won't have a vast number of "quantum children" via world-splitting.
2) Given that there are infinitely many observers involved, we have a measure problem concerning how to weight them. I think that, in order to be consistent with the Born Rule, MWI is just going to say something kind of weird about how to weight the probabilities that doesn't make it more likely for you to come later. Maybe you could argue that the necessity of this weighting is itself an argument against MWI, though just about any weighting scheme has to be weird in some ways.
1. No. I agree on SIA, MWI beats single-universe versions of every other theory. But many worlds does this in a way that makes it very unlikely you'd be so early. So you being very early is highly unlikely on many worlds.
2. Yeah, I think this will just end up super weird. It seems hard to see what the principled motivation is for thinking the probabilities line up with squared amplitude--people just like that because if fits the math.
But doesn't MWI + any multiverse theory beat the multiverse theory alone? Like, if we consider SC to be the the theory that the string theory landscape exists, but each of those universes has objective collapse rather than many worlds, and we let SM be the theory that the string theory landscape exists, but each universe splits in the way described by MWI, then surely SM should come out more likely than SC for the same reason that the MWI beats any single-universe theory.
I think this is a really clever objection to Many Worlds, and I agree with Joseph that the world-thinning response would resolve the problem (though I don’t have enough familiarity with theoretical physics to know whether it’s true).
The first thing to note is that MWI is more than one theory. (In fact, quantum mechanical AI is different to Tegmarks mathematical multiverse, and is several different theories itself).What splittng is...how complete and irrevocable it is ... varies between particular theories. So does the rate of splitting, so does the mechanism of splitting.
The way that you are doing anthropics assumes that full decoherent splitting occurs at every interaction. If that were so there would be no evidence of the various phenomena based on coherent superposition. The doubly maximal concept of many worlds...full decoherence at every elementary interaction (not just observations, which are relatively high level interactions)...has been ruled out.
Scott Aaronson expands on the point:-
"David Deutsch, one of the founders of quantum computing in
the 1980s, certainly thinks that it would. Though to be fair, Deutsch
thinks the impact would “merely” be psychological – since for him,
quantum mechanics has already proved the existence of parallel uni-
verses! Deutsch is fond of asking questions like the following: if Shor’s
algorithm succeeds in factoring a 3000-digit integer, then where was
the number factored? Where did the computational resources needed
to factor the number come from, if not from some sort of “multiverse”
exponentially bigger than the universe we see? To my mind, Deutsch
seems to be tacitly assuming here that factoring is not in BPP – but
no matter; for purposes of argument, we can certainly grant him that
assumption.
It should surprise no one that Deutsch’s views about this are
far from universally accepted. Many who agree about the possibil-
ity of building quantum computers, and the formalism needed to
describe them, nevertheless disagree that the formalism is best inter-
preted in terms of “parallel universes.” To Deutsch, these people are
simply intellectual wusses – like the churchmen who agreed that the
Copernican system was practically useful, so long as one remembers
that obviously the Earth doesn’t really go around the sun.
So, how do the intellectual wusses respond to the charges?
For one thing, they point out that viewing a quantum computer in
terms of “parallel universes” raises serious difficulties of its own.
In particular, there’s what those condemned to worry about such
things call the “preferred basis problem.” The problem is basically
this: how do we define a “split” between one parallel universe and
another? There are infinitely many ways you could imagine slic-
ing up a quantum state, and it’s not clear why one is better than
another!
One can push the argument further. The key thing that quan-
tum computers rely on for speedups – indeed, the thing that makes
quantum mechanics different from classical probability theory in the
first place – is interference between positive and negative amplitudes.
But to whatever extent different “branches” of the multiverse can
usefully interfere for quantum computing, to that extent they don’t
seem like separate branches at all! I mean, the whole point of inter-
ference is to mix branches together so that they lose their individual
identities. If they retain their identities, then for exactly that reason
we don’t see interference.
Of course, a many-worlder could respond that, in order to lose
their separate identities by interfering with each other, the branches
had to be there in the first place! And the argument could go on
(indeed, has gone on) for quite a while.
Rather than take sides in this fraught, fascinating, but perhaps
ultimately meaningless debate..."..Scott Aaronson , QCSD, p148
Note that both arguments can be right, to an extent. Deutsch can be correct that a single universe is unable to account for the power of quantum computation, and Aaronson, can be correct that nothing is implied about a unique decomposition into large scale quasi classical universes as expected by the science fictional version of many worlds.
The point in relation to anthropics is that of only a partial.splitting between components that continue to influence each other occurs, it could well be reasonable to do anthropics in a coarse grained way , over fuzzy blobs of amplitude, leading to far fewer "yous".