One of the most remarkable features of the world we live in is that we can make measurements that are consistent across space and time. By "consistent across space" I mean that you and I can look at the outcome of a measurement and agree on what that outcome was. By "consistent across time" I mean that you can make a measurement of a system at one time and then make the same measurement of that system at some later time and the results will agree.
... um, no. We can't reliably do either of those things. You need to add some qualifiers.
The rest, I didn't understand. That's to be expected, though.
Well, I didn't say we could do it reliably. :-) But we can do it. You can look at something and say, "It's green" and I can look at the same thing and agree, "Yes, it is green." And then we can look at the same thing a minute later and say, "It's still green." The remarkable fact is not that we can do this 100% of the time, but that we can do it at all.
Thanks, this helped me fill in some gaps. In Ron Garret's piece that you linked above, a comment has a link to a very nice article by Aharonov et al titled Can a Future Choice Affect a Past Measurement's Outcome?. (Hint: yes.)
Just FYI, I am Ron Garret. Also just FYI, the Aharonov study does not show that future choices can affect a past measurement's outcome. If this were possible, you could use it to send yourself information about the future of (say) the stock market and become the richest person on earth.
I don't understand Aharonov's experiment enough to say what it does or doesn't show. But your argument surely does not disprove his claim, since he is talking about particular circumstances, not making a general claim that there is some method that will tell you general truths about the future such as what the stock market is going to do. In fact, he does not appear to be saying that you can send yourself information at all, in a form which will be intelligible to you before the future events.
So I read the paper, and it is kind of a cool experiment, but it does not show that "future choices can affect a past measurement's outcome." Explaining why would require a separate article (maybe time to re-open main!) But the TL;DR version is this: if you want to argue that A affects B then you have to show a causal relationship that runs from A to B. If you can do that, then you can always come up with some encoding that will allow you to transmit information from A to B. That's what "causal relationship" means. But that is (unsurprisingly) not what Aharonov et al. have done. They have merely shown correlations between A and B, and then argue on purely intuitive grounds that there must have been some causal relationship between A and B because "Bell's theorem forbids spin values to exist prior to the choice of the orientation measured." While this is true, it's misleading because it implies that spin values do exist after a strong measurement. But that is not true. There is no fundamental difference between a strong and a weak measurement. There is a smooth continuum between weak and strong measurements, and at no point during the transition from weak to strong does the spin value begin to "actually exist" (a.k.a. wavefunction collapse).
That's what "causal relationship" means.
I disagree. Following Pearl, I define "A causes B" to mean something like: (DO:A) raises the probability of B.
Bob's choice in the evening to make strong measurements along the beta-axis, raises the probability of Alice's noon measurements along the beta-axis measurements having been the ones that showed the best correlation. It doesn't raise the probability of any individual measurement being up or down, but that's OK. Even on a many worlds interpretation, where perhaps every digital up/down pattern happens at some "world" and the overall multi-world distribution is invariant, "probability" refers to what happens in our "world", so again that's OK.
Correlation can only be observed after the fact, in the evening, not at noon. So isn't this just a case of Bob affecting Bob+Alice's immediate future, where they go over the results? Why do I say Bob's choice affected Alice's results? Because correlation is a two-way street, and in this case there isn't much traffic in the forward direction. Alice's measurements only weakly affect Bob's results.
(DO:A) raises the probability of B.
Yes, but there's still some terminological sleight-of-hand going on here. It is only fair to say that a future A affected a past B if P(B) is well defined without reference to A. In this case it's not. Because B is defined in terms of correlations between measurements made at T1 (noon) and measurements made at T2 (evening) then B cannot be said to have actually happened until T2.
correlation is a two-way street
No, it's an n-squared-minus-one-way street. It appears to be a two-way street in one (very common) special case (two macroscopic systems mutually entangled with each other), but weak measurements are interesting precisely because they do not conform to the conditions of that special case. When you go beyond the conditions of the common special case you can't keep using the rhetoric and intuitions that apply only to the special case and hope to come up with the right answer.
if P(B) is well defined without reference to A
You're right. Good point.
it's an n-squared-minus-one-way street
Don't you mean n-factorial? Anyway, ... hmm, I need to think about this more.
Don't you mean n-factorial?
Yeah, probably. It's actually probably N!-1 because you have to trace over one degree of freedom to obtain a classical universe. But the details don't really matter. What matters is that it's >>N.
I disagree: if you interpret EPR experiments as wavefunction collapse rather than many worlds, then you can conclude that either one measurement affects the other, or both affect each other. But you cannot come up with any encoding that will allow you to transmit information.
Yes, of course that's true. But collapse is only an approximation to the truth. It is a very good approximation in many common cases. But the Aharonov experiment is interesting precisely because it is a case where collapse is no longer a good approximation to the truth, and so of course if you view it through the lens of collapse things are going to look weird. To see why collapse is not always a good approximation to the truth, see the references in the OP.
To be honest, I never saw how it would be "self evident" that not only there is some "objective reality" out there but that we also have an accurate representation of it. How would we know our representation is accurate? We don't have access to an objective observer, we don't even have access to a non-human observer. Immanuel Kant said, back in the 18th century, that in truth, the "laws of nature" are "laws of human perception [of nature]".
I recently wondered if the idea that our perception of reality is an accurate representation of the underlying objective reality was actually a commonly held idea around here. In Eliezer's short story "Three Worlds Collide", he has the characters say as much, because they assume that the periodic table and mathematics would be the same for every other species. But there is no reason to assume that is true.
I like your article, but have some naive questions: Where starts the original research part? That is how differs your article from existing interpretations of QM?
Will your interpretation of QM present any new testable predictions? Will it allow any new useful things?
What is the nature of observer in it?
None of this is original research on my part. My only contribution is pedagogical. QIT doesn't make any predictions that QM doesn't make because it's an interpretation, just another way of looking at the math. But the reason it's a better way of looking at the math is that it solves the measurement problem. It explains measurement in terms of entanglement. It reduces two mysteries to one. IMHO that's progress.
How far does it go in solving the measurement problem? Can you derive the Born rule? Can you settle the single/many world dichotomy?
Can you derive the Born rule?
Yes.
Can you settle the single/many world dichotomy?
That depends on what you mean by "settle". The only thing that you can definitively say is that the transition between the quantum and the classical is gradual, not abrupt. Because of this, any statement about a classical world is necessarily an approximation of some sort, and all approximations break down if you lean on them in the right way. Copenhagen breaks down most easily because it only applies under some very particular circumstances. Those circumstances happen to be very common, which is why Copenhagen is not completely useless, but nowadays it is common to do experiments under which the Copenhagen approximation conditions do not apply. Multiple-worlds is mathematically tenable, but it has some very serious problems as an explanatory theory and it makes predictions that even its adherents seem unwilling to accept.
Personally, I find the rhetoric of QIT/relational-QM/Ithaca to be far less taxing on my intuition than multiple-worlds. These interpretations acknowledge that classical reality is a slice of the wave function, that there are many different ways to slice up the wave function to obtain a classical reality, and therefore there are many potential classical realities. But there is one classical reality that is privileged to me because it happens to be the one that I'm living in, which is to say, it's the reality that is mutually entangled (and therefore classically correlated) with the massively-mutually-entangled system that is me. In an absolute sense I am no more or less real than all the other potential mes that you get by slicing up the wave function in different ways, but I don't care about that except in the abstract. Day-to-day, what matters to me -- this me, the one that is writing these words -- is what is correlated with (this) me.
The cool thing about this is that if you are reading these words -- the ones written by this me -- then you are entangled with me and therefore classically correlated with me and therefore we are both emerging from the same slice of the wave function, and so the exact same argument applies to you: both of us can proceed on the assumption that our classical reality is the One True Classical Reality even though we can both understand in the abstract that this isn't really true, and that by doing the right kinds of quantum experiments we can actually demonstrate to ourselves that it isn't really true. For me personally, that makes QIT the best approximation to use because it's the one that applies in the greatest variety of circumstances and has the fewest conceptual problems. But it's ultimately a matter of personal preference.
Multiple-worlds is mathematically tenable, but it has some very serious problems as an explanatory theory and it makes predictions that even its adherents seem unwilling to accept.
Doesn't the QIT you describe make the exact same predictions, also the Russian roulette you mentioned?
But there is one classical reality that is privileged to me because it happens to be the one that I'm living in, which is to say, it's the reality that is mutually entangled (and therefore classically correlated) with the massively-mutually-entangled system that is me. In an absolute sense I am no more or less real than all the other potential mes that you get by slicing up the wave function in different ways, but I don't care about that except in the abstract.
But there's no single privileged future you, right?
Doesn't the QIT you describe make the exact same predictions, also the Russian roulette you mentioned?
Nope.
But there's no single privileged future you, right?
There is no single privileged future me now, but when my future becomes my present there will be. (Also, see note below.)
You can actually do this experiment: listen to a geiger counter, or tune an old-school TV to an inactive channel and watch the snow on the screen. The math says that during this process there are an inconceivably vast number of you's being split off every time the geiger counter clicks (or fails to click) or every time you perceive a light or dark pixel on the screen. But you will only ever experience being one of those you's. Yes, all those other you's do exist, but the you that you perceive yourself to be can never interact with any of them, so they may as well not exist for the one you that you perceive yourself to be. And so the one you that you perceive yourself to be may as well live your life as if all those other you's didn't exist even though they really do.
(NOTE: there is really no such thing as "now", and you don't even have to go quantum to see that. Simultaneity gets tossed out the window with special relativity. There is my "now" and there is your "now" and they will not, in general, be the same.)
Hi lisper,
I found your paper easy to follow and maybe insightful (I'll have to read it more carefully the second time) but like qmotus, I don't understand your reasoning in this thread. I'm assuming MWI is just an interpretation of unitary QM, so makes all the same mathematical predictions as other non-collapse theories. And the roulette story is just one way of looking at it, from the perspective of what I consider my (classical) self and what I call the future.
Since you are not claiming that QIT makes different mathematical predictions than MWI, how can you claim they make different predictions at all?
QIT and MWI don't make any different predictions that are testable in a single classical universe (obviously, because QIT and MWI are just different interpretation of QM, so they both make the same predictions for all observables, namely, the predictions made by QM).
QIT and MWI are simply differences in perspective -- the God's eye view (MWI) versus the mortal's-eye-view (QIT). Neither view is "correct", but since I (the thing engaged in this conversation) am a mortal, I choose the mortal's-eye-view as more relevant for day-to-day decision making. But as I keep saying, it's ultimately a matter of personal preference.
The problem with quantum roulette is that it takes a prediction made from a God's-eye-view and tries to apply it in a mortal's-eye-view context. Yes, God will be able to see that there is a you that survived the process and went on to live the life of Riley. But whether or not you will be able to see that is a very open question. (God will also be able to see a lot of branches of the multiverse containing your friends and loved ones mourning your untimely death.)
Note that playing quantum roulette successfully depends crucially on the speed with which you can kill yourself. Trying to play by slitting your wrists, for example, doesn't work because once you see that your wrists are slit you can't roll that back. So the success of the enterprise depends entirely on killing yourself fast enough that you don't become aware of your imminent and (in the relevant branches of the multiverse) unavoidable death. How fast is fast enough? Well, that is (literally!) the sixty-four-million-dollar question. Unless you have an answer that you are very confident is the correct one, it seems to me like an imprudent risk to take.
(This comment is a reply to another branch of this discussion as well.)
Yes, God will be able to see that there is a you that survived the process and went on to live the life of Riley. But whether or not you will be able to see that is a very open question
I disagree. To keep things simple, let's suppose that the bullet, if it hits, really will kill the participant with practically 100% certainty and will do so practically immediately (I'll come to this a bit later). In that case the only outcome the participant can expect to experience, and that they will experience with certainty, is that the gun didn't fire. This is exactly what happens if you take the mortal's-eye-view; God, as you mentioned, will notice that elsewhere in the multiverse, the participant did get hit. Now, whether the participant cares about their loved ones or the copies that die in the attempt is a matter of preferences, but if we're simply talking about which outcome to experience, this is how it goes, I think.
Note that playing quantum roulette successfully depends crucially on the speed with which you can kill yourself. Trying to play by slitting your wrists, for example, doesn't work because once you see that your wrists are slit you can't roll that back. So the success of the enterprise depends entirely on killing yourself fast enough that you don't become aware of your imminent and (in the relevant branches of the multiverse) unavoidable death. How fast is fast enough? Well, that is (literally!) the sixty-four-million-dollar question. Unless you have an answer that you are very confident is the correct one, it seems to me like an imprudent risk to take.
With this I agree, which is why I think the quantum Russian roulette or quantum suicide scenarios are mostly interesting as a thought experiment, as they're intended to be. But there are practical situations that are somewhat analogous: think, for example, about a terminally ill patient who faces an almost certain death within several days. Should they expect to survive or continue to experience things, and if so, in what way? My understanding is that according to quantum mechanics, there are all kinds of weird scenarios with non-zero probability that make "survival" possible, such as simply surviving one more day indefinitely despite all odds, being miraculously cured, or maybe being resurrected by a hyper-advanced future civilization in a simulation. Note that, in principle, this probably applies to any possible life-and-death situation.
I used the word "experience" a number of times there, which brings me to a point you made in another comment:
Notice (!) that when you start to talk about "noticing" things you are tacitly bringing consciousness into the discussion, which is a whole 'nuther can o' philosophical worms.
I don't think this can of worms is that bad. We have a pretty good grasp of what it means to be conscious, even if we can't define it exactly; and also we're (at least I am) pretty confident that it's a purely physical phenomenon with nothing supernatural and thus subject to the laws of QM. I think that's enough. Where it does get a bit problematic is when we're talking about scenarios like the one with the terminally ill patient; presumably there's also a possibility that the patient's consciousness degrades until it no longer makes sense to call them conscious, since there's probably no clear line anywhere separating conscious and non-conscious in this way. (This might also imply that if we should expect to die, we should expect to do so by very slow decay, like patients with Alzheimer's, which doesn't sound too good to me.)
I agree with most of what you say. Consciousness is not supernatural. But it is still problematic because:
the only outcome the participant can expect to experience, and that they will experience with certainty
"Only outcome you can experience" is not quite the same thing as "Will experience with certainty." Let's go back to the case where you survive in both branches. The outcome you do experience is the only outcome that you can experience. The trick is that this is really two statements disguised as one. After the event there are two you's, you1 and you2. The outcome that you1 do experience is the only outcome you1 can experience, and the outcome you2 do experience is the only outcome you2 can experience. This remains true (I believe) even if one of those experiences is the null experience of having your consciousness enter the cosmic void.
Reasonable people could disagree, I suppose. We can never know what the null experience "feels like" because by definition it doesn't feel like anything. Personally, I find even the possibility that this argument could be correct to be sufficient reason for me to avoid playing quantum roulette. But everyone needs to choose their own risk posture.
It was two statements: "only outcome the participant can expect to experience" because I think that no, it is not possible to experience a null experience; and "will experience with certainty" because I believe quantum mechanics, when interpreted literally, means that the experience will exist.
As I said, I don't find the Russian roulette a particularly interesting scenario in reality, nor something that I would like to try myself; it's because I think this applies to other life-and-death situations as well that I think the basic question is important.
In that case the only outcome the participant can expect to experience, and that they will experience with certainty, is that the gun didn't fire
Yes, that's the point. Every future version of you will of course call themselves "you".
Note that playing quantum roulette successfully depends crucially on the speed with which you can kill yourself. With this I agree, which is why I think the quantum Russian roulette or quantum suicide scenarios are mostly interesting as a thought experiment, as they're intended to be.
Although I don't want to advocate performing the roulette experiment, I do disagree. If it's a quantum certainty that all future branches of you die off, perhaps due to a conservation law, then only those versions of you which didn't go down that branch will be conscious.
Even if it isn't certain, because it seems like we are more likely to experience the branches that match our classical explanations in the following scenario after a few minutes I would expect to be version 3. Version 1 is of course impossible and only with a very short-sighted definition of self do I need to consider version 2.
Yeah. I meant that I don't find the roulette scenario very relevant since I believe that we're much more likely to experience some other scenario where this property of the quantum multiverse becomes relevant, like the terminal illness one I described. Most of us won't play the roulette.
Anyways, there's a flaw in lisper's original argument: death is not unavoidable even after wrists have been slit.
Oh, come on. Surely you do not dispute that there are ways of dying that are both unavoidable and non-instantaneous. What difference does it make what the details are?
If I decide to open my wrists, there are many ways that I can still keep going: I may simply faint and wake up in a hospital, the paramedics having arrived just in time despite all odds; quantum fluctuations may spawn a hitherto unkown angelic being who heals me; or a highly advanced future civilization may decide to run an afterlife simulation for 21st century earthlings that I end up in. As far as I know, these are all scenarios with a non-zero probability according to quantum mechanics and that this is in principle generalizable to any other life-and-death situation, although I have to admit that my understanding of QM is somewhat fuzzy. Feel free to correct me.
quantum fluctuations may spawn a hitherto unkown angelic being who heals me
Quantum fluctuations may also spawn the ghost of Karl Popper who will wag his finger at you and remind you that unfalsifiable statements aren't terribly useful.
Heh, I would definitely like to see that.
That said, I do believe that what I said is true if we assume that quantum mechanics is a complete theory, and pretty much all evidence so far points towards it. It's a fairly common idea among physicists nowadays, actually, that not every single prediction needs to be falsifiable. David Deutsch has also mentioned that most fiction or something arbitrarily close to it is probably real in some part of the quantum multiverse.
Once you start to invoke "hitherto unknown angelic beings" and give up on falsifiablity you are basically in a religious dispute and I don't see much advantages to this new religion over the existing traditional ones.
The point was to illustrate that there can be ways to survive a seemingly inevitably fatal situation that are extremely unlikely but still have a non-zero probability of occurring and that, therefore, will happen in some Everett branches (assuming MWI is true). Being rescued by an angel is probably one of the least likely ways for somebody to survive after slicing their wrists, so I would bet on simply waking up in a hospital instead.
I don't think claims like that need to be empirically falsified. Quantum mechanics is falsifiable, and so far it's withstood every test. I suppose you could try to prove that survival probability in some case or in some way is zero by math alone, but I don't think that's true.
to illustrate that there can be ways to survive
Well, from my point of view an unfalsifiable illustration doesn't really illustrate anything. "There could be a god and she could save me" is a fully generic answer to absolutely anything.
You can just ignore the angelic being thing if it bothers you too much. Even so, I'd argue that at least in almost every slit-wrists scenario, there is a non-zero probability of being rescued by modern medicine. But do not that I'm not saying that the angelic being will in fact appear somewhere! That one would follow from quantum mechanics being a complete theory and MWI (or QIT) being a correct interpretation, both of which are surely debatable (and even then it would only happen in a very small minority of all worlds).
I wonder where you would draw the line with falsifiability though. For example, according to quantum mechanics there is a non-zero probability (and this one I'm quite certain about) that when you perform a double-slit experiment, all the photons will hit the detector in just the right way to give results that agree with the world being classical. Is this claim falsifiable? I guess not, but it's still true.
Even so, I'd argue that at least in almost every slit-wrists scenario, there is a non-zero probability of being rescued by modern medicine.
So, replace slit wrists with standing in the center of Hiroshima on August 6, 1945 around 8:15 looking up at the sky.
I wonder where you would draw the line with falsifiability though.
In the usual way: is it possible to observe an empirical result which will either prove of disprove the claim in question?
In your example it is possible to observe the classical result from the double-slit experiment, so I don't know why you think it's not falsifiable.
So, replace slit wrists with standing in the center of Hiroshima on August 6, 1945 around 8:15 looking up at the sky.
All right, that's more difficult. So are you sure that there's no way, miraculous or non-miraculous, to keep existing in that situation?
In your example it is possible to observe the classical result from the double-slit experiment, so I don't know why you think it's not falsifiable.
You would have to run the experiment very many times to see the classical result even once. In practice it's not possible to test it. And what if my claim is not true - how would you show that?
Your default assumption regarding the classical result in a double-slit experiment seems to be that it is actually allowed by the formalism of quantum mechanics. So why does your default assumption seems to be that the formalism of quantum mechanics says that the angel thing is impossible?
So are you sure that there's no way, miraculous or non-miraculous, to keep existing in that situation?
Divine intervention can solve ALL problems.
So why does your default assumption seems to be that the formalism of quantum mechanics says that the angel thing is impossible?
That is not my assumption and QM says nothing about angelic beings. My problem is that deux ex machina, in the form of an angel or not, is the answer to absolutely everything and so is useless.
As to why the "angel thing" is unlikely, let me introduce you to Bertrand Russel. You see, he had a teapot...
That is not my assumption and QM says nothing about angelic beings. My problem is that deux ex machina, in the form of an angel or not, is the answer to absolutely everything and so is useless.
Maybe you misunderstand my point. I'm not proposing the angel thing as an answer to anything. I'd rather say that I'm trying to present a problem, actually.
Unfortunately, I don't understand the math of QM enough to make a convincing argument here. But basically my logic is this: the probability that instead of an angel, a disembodied brain will materialize in front of me is apparently non-zero according to our current knowledge, since the Boltzmann brain paradox is something that is taken seriously by physicists. I don't see any fundamental difference between this and a blonde guy dressed in white with wings (perhaps made of cardboard) who's very skilled at treating slit wrists materializing instead, so I assume that also has a small but non-zero probability. (Also, the probability that I would survive a nuclear explosion by tunneling to a safe distance is non-zero, I believe, by the same logic that the probability of me tunneling to the room next door is non-zero, which I've also heard physicists say.)
Now, the possible existence of Boltzmann brains is actually generally considered a problem, but scientists who work on that problem, as far as I know, usually don't deny that our current understanding of physics says that Boltzmann brains are a real thing; instead, they think that it either shows a flaw in the theories that predict them, or that our understanding of those theories is incomplete in some way (Sean Carroll has written about this). I don't know, but I suppose it's also possible that the world simply really is a weird place.
basically my logic is this: the probability that instead of an angel, a disembodied brain will materialize in front of me is apparently non-zero according to our current knowledge
You are basically arguing that "the probability of X is non-zero for all X". That is true (or unfalsifiable) in the same sense that solipsism is true (or unfalsifiable). No one can prove to you that it ain't so, but it's an entirely uninteresting and useless position to hold.
It's not non-zero for all X. If I claim that "according to QM, there's a non-zero probability two entangled particles have the same spin", one could just produce the math necessary to show that this claim is false. Likewise, if my claim about QM saying that the probability of an angel appearing is non-zero, I'm sure it's possible to do that (it might be difficult, though). The same with those Boltzmann brains, but physicists seem to think that their probability really is non-zero. Solipsism, on the other hand, is completely immune to such attacks.
The important thing here is really whether what I'm claiming follows from the math of quantum mechanics or not. Physicists don't try to falsify every single prediction that quantum mechanics makes, because they already think there's enough evidence (well, most do) that the theory is correct. There obviously is no such evidence for solipsism.
The probability of our current understanding of QM to be false is non-zero, too.
Things that QM forbids might be allowed by the next theory that follows it (what does Newtonian physics say about quantum tunneling? Absolutely impossible.)
You are granting QM the status of absolute, final truth and I see no reason for that. There is non-zero probability that it is mistaken :-P
I tried to be careful about this. In an earlier post, I said: "But do not that I'm not saying that the angelic being will in fact appear somewhere! That one would follow from quantum mechanics being a complete theory and MWI (or QIT) being a correct interpretation, both of which are surely debatable (and even then it would only happen in a very small minority of all worlds)."
That said, I think it's quite a radical position to assign much probability to the possibility that QM is wrong. So far it's consistent with all the evidence that we have, and there's no evidence to support any of the competing theories or modifications, like objective-collapse theories. Because of this, a hypothetical improved theory might also contain all the weird stuff that QM does, plus some more.
On the other hand, I sometimes wonder if the people working on these theories would be more inclined to question them if they thought more about all the absurd implications they potentially have. (They might not: I recall reading a statement somewhere by Steven Weinberg who thought that eternal inflation or some other multiverse-predicting theory is a miserable theory, but the best that there is and possibly correct.)
I think it's quite a radical position to assign much probability to the possibility that QM is wrong.
I don't think so, with the slight change from "QM is wrong" to "QM is limited in its applicability, just like Newtonian physics".
But in any case, in this thread you operate in a black-and-white world of "zero probability" and "non-zero probability". I am pointing out that the set of zero-probability events is empty.
I don't think so, with the slight change from "QM is wrong" to "QM is limited in its applicability, just like Newtonian physics".
Fair enough. I suppose one could say that the problems with combining QM and general relativity suggest that QM needs to be modified, but so far we don't have experimental evidence for anything but pure QM, I believe.
But in any case, in this thread you operate in a black-and-white world of "zero probability" and "non-zero probability". I am pointing out that the set of zero-probability events is empty.
Well, our discussion spun off from a comment where lisper claimed that surely there are situations where death is inevitable. I countered by saying that as far as I know, there's always some way to survive that has a non-zero probability according to quantum mechanics, so that's where the emphasis on zero vs non-zero probability originally came. Note that the view advocated in the original post views the formalism of quantum mechanics as an accurate description at all scales and also says that all events with a non-zero probability will happen (but that the ones we end up not observing are not something we should care about).
I think we agree but I was trying to make a bigger point than your reply captures. I doubt that you will even experience the terminal illness assuming there are many more possible futures where you stay healthy and anti-aging science advances than ones where you are miraculously saved at the last minute, by aliens or luck.
That makes the roulette scenario relevant to our experience. Because if you have the conviction to pull the trigger if something doesn't go your way you have the three options I laid out. So most likely you don't even have to try - assuming you are sure you will.
Well, I'd rather say that I will perceive being every one of them; it's just that no future me will perceive being more than one of the future mes. The terminology gets quite confusing here, but I think the Quantum Russian Roulette you mentioned (and quantum suicide and immortality, by extension, for example) is one situation where this aspect of quantum theory becomes somewhat apparent, which is why I think it would be interesting if you elaborated a bit more on how you think the predictions QTI and MWI make differ from each other.
I will perceive being every one of them
It depends on what you mean by "I". This is the crux of the matter. MWI takes a God's-eye perspective and looks at the whole wave function. On that view, there are many you's (i.e. many slices of the wave function that contain macroscopic systems of mutually entangled particles that perceive themselves to be you).
QIT takes the perspective of the-you-that-you-currently-perceive-yourself-to-be. You will only ever perceive one of that kind of you.
For the purposes of making decisions it makes more sense to take the latter perspective because it's the-latter-kind-of-you that is making the decisions and has to live with the consequences.
I would say that a major difference between MWI and various collapse interpretations is that there are situations where according to collapse interpretations there most likely will be no future you; but according to MWI there surely will, although their amplitude is low (the aforementioned Russian roulette is one such situation, for instance). I find it somewhat difficult to think about those from the perspective you advocate.
according to MWI there surely will
No. Not "will". IS. If you're going to take the God's eye view then you have to let go of your intuitions about time along with your intuitions about classical reality. The wave function is a static four-dimensional thing. Time emerges from the wave function in exactly the same way that classical reality does. You have to be careful not to apply terminology from the mortal's-eye-view to the God's-eye-view. That's how you get yourself into trouble.
UPDATE: Here is a popular article about how time emerges from entanglement.
I think what I said applies when you take a first-person point of view. If you're a participant in a quantum suicide experiment, then if you expect a collapse interpretation to be an accurate description of reality, you should expect to eventually be hit by a bullet and die. But both MWI and QIT predict that you will continuously notice that the gun doesn't fire. The difference is not in the point of view taken, it's in the fact that the parts of the wavefunction that contain a (from first-person eye-view) future version of the participant actually are there.
But both MWI and QIT predict that you will continuously notice that the gun doesn't fire.
No, that's not quite true. QIT predicts that if you notice anything then you will notice that the gun didn't fire. But QIT does not guarantee that you will notice anything. You could just die.
Notice (!) that when you start to talk about "noticing" things you are tacitly bringing consciousness into the discussion, which is a whole 'nuther can o' philosophical worms.
See also my response to akvadrako.
Only commenting on one point here.
The argument that something is "logically incoherent" has been used to justify many a false conclusion about the observed universe, don't do that.
Your other argument against time travel is better, but not airtight: it violates not the conservation of energy but the dominant energy condition in general relativity, Basically, for something to disappear, all of its mass has to vanish somewhere and no faster than with the speed of light. So maybe you get turned into a neutrino stream and or something. A better reason for why time travel (but not timeline forking) is incompatible with General Relativity is the uniqueness of the metric. But this is becoming a discussion of a real science, not philosophy.
don't do that
If you were to ban every mode of argument that has ever been used to justify a false conclusion then it would be impossible to argue for anything.
this is becoming a discussion of a real science
Heaven forfend! ;-)
I don't buy your first argument against time-travel. Even under the model of the universe as a static mathematical object connected by wave-function consistency constraints, there is still a consistent interpretation of the intuitive notion of "time travel":
The "passage" of time is the continuous measurement of the environment by a subsystem (which incidentally believes itself to be an 'observer') and the resulting entanglement with farther away parts of the system as "time goes on" (i.e. further towards positive time). Then time-travel is a measurement of a "past" state or described differently (but the same thing) an entanglement between a subsystem (the location in the past the traveler visited) and its surroundings, which does not respect the common constraint that entanglement propagates at speed of light (because the traveler came from some future location (and its past light-cone) which is -- "surprisingly" -- entangled with the past). While violating common understanding of space-time, it is not logically impossible in this understanding of the universe.
This time-travel allows interaction with the past (which are not different from observations anyway).
Do I overlook something here?
No, what you say is correct, but you don't even need to bring entanglement into it at all: moving faster than light is the same thing as moving into the past (in some reference frame). This is why information can't propagate faster than light.
The kind of time travel that I'm talking about here is not merely sending information into the past but sending yourself into the past, that is, sending your body into the past. But that's not possible because your body is on the most fundamental level made of entanglements, and entanglements define the arrow of time.
I was asked to write up a pithy summary of the upshot of this paper. This is the best I could manage.
One of the most remarkable features of the world we live in is that we can make measurements that are consistent across space and time. By "consistent across space" I mean that you and I can look at the outcome of a measurement and agree on what that outcome was. By "consistent across time" I mean that you can make a measurement of a system at one time and then make the same measurement of that system at some later time and the results will agree.
It is tempting to think that the reason we can do these things is that there exists an objective reality that is "actually out there" in some metaphysical sense, and that our measurements are faithful reflections of that objective reality. This hypothesis works well (indeed, seems self-evidently true!) until we get to very small systems, where it seems to break down. We can still make measurements that are consistent across space and time, but as soon as we stop making measurements, then things start to behave very differently than they did before. The classical example of this is the two-slit experiment: whenever we look at a particle we only ever find it in one particular place. When we look continuously, we see the particle trace out an unambiguous and continuous trajectory. But when we don't look, the particle behaves as if it is in more than one place at once, a behavior that manifests itself as interference.
The problem of how to reconcile the seemingly incompatible behavior of physical systems depending on whether or not they are under observation has come to be called the measurement problem. The most common explanation of the measurement problem is the Copenhagen interpretation of quantum mechanics which postulates that the act of measurement changes a system via a process called wave function collapse. In the contemporary popular press you will often read about wave function collapse in conjunction with the phenomenon of quantum entanglement, which is usually referred to as "spooky action at a distance", a phrase coined by Einstein, and intended to be pejorative. For example, here's the headline and first sentence of the above piece:
This sort of language is endemic in the popular press as well as many physics textbooks, but it is demonstrably wrong. The truth is that measurement and entanglement are actually the same physical phenomenon. What we call "measurement" is really just entanglement on a large scale. If you want to see the demonstration of the truth of this statement, read the paper or watch the video or read the original paper on which my paper and video are based. Or go back and read about Von Neumann measurements or quantum decoherence or Everett's relative state theory (often mis-labeled "many-worlds") or relational quantum mechanics or the Ithaca interpretation of quantum mechanics, all of which turn out to be saying exactly the same thing.
Which is: the reason that measurements are consistent across space and time is not because these measurements are a faithful reflection of an underlying objective reality. The reason that measurements are consistent across space and time is because this is what quantum mechanics predicts when you consider only parts of the wave function and ignore other parts.
Specifically, it is possible to write down a mathematical description of a particle and two observers as a quantum mechanical system. If you ignore the particle (this is a formal mathematical operation called a partial trace of an operator matrix ) what you are left with is a description of the observers. And if you then apply information theoretical operations to that, what pops out is that the two observers are in classically correlated states. The exact same thing happens for observations made of the same particle at two different times.
The upshot is that nothing special happens during a measurement. Measurements are not instantaneous (though they are very fast ) and they are in principle reversible, though not in practice.
The final consequence of this, the one that grates most heavily on the intuition, is that your existence as a classical entity is an illusion. Because measurements are not a faithful reflection of an underlying objective reality, your own self-perception (which is a kind of measurement) is not a faithful reflection of an underlying objective reality either. You are not, in point of metaphysical fact, made of atoms. Atoms are a very (very!) good approximation to the truth, but they are not the truth. At the deepest level, you are a slice of the quantum wave function that behaves, to a very high degree of approximation, as if it were a classical system but is not in fact a classical system. You are in a very real sense living in the Matrix, except that the Matrix you are living in is running on a quantum computer, and so you -- the very close approximation to a classical entity that is reading these words -- can never "escape" the way Neo did.
As a corollary to this, time travel is impossible, because in point of metaphysical fact there is no time. Your perception of time is caused by the accumulation of entanglements in your slice of the wave function, resulting in the creation of information that you (and the rest of your classically-correlated slice of the wave function) "remember". It is those memories that define the past, you could even say create the past. Going "back to the past" is not merely impossible it is logically incoherent, no different from trying to construct a four-sided triangle. (And if you don't buy that argument, here's a more prosaic one: having a physical entity suddenly vanish from one time and reappear at a different time would violate conservation of energy.)