In discussions about the Everett interpretation of quantum mechanics, one of the concerns I often see expressed is for the perverse low probability outcomes that would exist in the quantum multiverse. For example, if every quantum outcome is reality, then in some branches of the wave function, entropy has never increased. In some branches, quantum computing doesn’t work because every attempt at it has produced the wrong result and people have concluded it doesn’t work. In other branches, you as a macroscopic object might quantum tunnel through a wall.
Of course, for enthusiasts, this comes with a hopeful aspect. Because in some branches, you would go on living indefinitely, no matter how improbable it might be. Hugh Everett himself was reportedly a believer in quantum immortality and so had little concern about the unhealthy lifestyle that led to his early demise in this branch. The idea is that if every outcome happens, then there are versions of you reading this that will live until the heat death of the universe.
This is vividly illustrated in the infamous quantum suicide thought experiment. One version described by Max Tegmark involves rigging up a gun to fire if a certain quantum event happens. Say the quantum event has a 50% chance of happening in any one second. You then put your head in front of the gun and begin the experiment. In half of all worlds where you begin the experiment, you die in the first second, but you go on living in the other half. In half of that remaining half you die in the next second, but go on living in the other half.
For you as the experimenter this goes on indefinitely with increasingly improbable outcomes leading to your survival. Of course, in virtually all worlds you leave behind grieving friends and family who are less convinced. But for you subjectively, if many-worlds is reality, you continue living until the experiment ends.
(Before getting too comforted by the possibility of quantum immortality, it’s important to remember that this is more of a side-life than an afterlife. Most of the versions of you will still experience an approaching death. It’s also worth noting that a you a million years from now would likely have evolved into something utterly strangle and unrecognizable to the you of today. And there’s no guarantee this ongoing existence would be pleasant. Indeed, under many-worlds, some would inevitably be hellish.)
One question that often comes up in discussions about this is whether reality allows for these infinitesimally low probability outcomes, or whether there is some inherent minimal discreteness at the base of reality that prevents it. There’s nothing in the math to indicate it, but of course the math, at least the math we have today, is a description of reality that is likely only an approximation.
However in a recent interview with Curt Jaimungal, David Wallace, a proponent of the many-worlds interpretation, may have provided another reason to doubt these outcomes: quantum interference. (Note: if the embed doesn’t work right, the relevant remarks are at around the 1:21 mark. Also you don’t have to watch the interview to understand this post, but it is an interesting discussion.)
To understand Wallace’s point, it helps to realize some important points about how quantum decoherence works. Decoherence is the process of the quantum particle losing its wave like nature and becoming more particle like. This happens because as it interacts with the environment, the phase relations which keep the wave coherent become disrupted. The wave becomes fragmented. We call the fragments “particles”. This leads to the famous (infamous?) quantum interference effects disappearing. (As shown by the double slit experiment.)
But the word “disappearing” here in reference to the interference effects should be understood to mean “become undetectable”, not that they cease to exist entirely. Under decoherence the interference never goes away entirely. Like the wave overall, it becomes fragmented, and settles into an underlying “noise”. (Note: this is actually a difference in predictions between collapse and non-collapse interpretations that should, in principle, be testable. Of course, figuring out a way to do the test is another matter.)
Wallace’s point is that infinitesimally low probability outcomes should be swamped out by this remnant interference from higher probability outcomes, meaning that they should be prevented from existing. If so the branches where entropy never increased, where quantum computing never works, or to use his example, where he as a macroscopic object quantum tunnels through a wall, shouldn’t exist.
What does this mean for quantum immortality? I don’t know that it wipes it out entirely. Many of the initial survival scenarios may be very low probability, but not profoundly low ones, and so may not be swamped by interference from the other branches. But it does seem like it shortens the duration and overall makes it less certain, even once someone accepts the existence of the other worlds. So there may be versions of you reading this that live for decades or centuries beyond the normal lifespan, maybe even millenia, but probably not until the end of the universe.
Still, the implications are interesting and fun to speculate about. If there is a version of me alive in the far future, I wonder if he (it?) will remember these speculations.
What do you think of Wallace’s point? If we assume many-worlds is reality, does the idea of quantum immortality seem plausible? Or are there other reasons to doubt it?
SelfAwarePatterns 
I would strongly advise you not to try this yourself! Personally I cling to wavefunction collapse as a real physical process and am warming to Penrose’s idea of gravitationally-induced collapse. Since Penrose makes actual predictions of the collapse time, it should be testable.
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Definitely do not try at home!
Philip Ball had an article in Quanta a few years ago about empirical results closing in on objective collapse models. Last I heard physicists had reached the point of achieving superposition effects in tiny macroscopic objects. But haven’t kept up, so not sure where things stand today. I do prefer physical collapse to anti-real stances.
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“you continue living until the experiment ends”
Doesn’t that depend a lot upon the meaning of “you?”
In a way, it could mean there is no “you.”
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Only, I think, if we stick to “you” as a fundamental concept. But just based on biology and neuroscience, “you” seems like an emergent thing. From that perch, I’m not sure if many-worlds does that much violence to it. It also helps if we discuss the worlds the way Deutsch does, as all the worlds already existing, so there’s no splitting of the self, just various versions of the self diverging from each other.
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What is amazing about genetics is how few generations it takes for “you” to have near zero amount of the unique DNA of an ancestor. It waters down quickly.
I think much the same would be the case. Probably within a few seconds the difference between “you” and the “you’s” would be significant. I mean some of those you’s could even be dead.
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Certainly. Some of them would have died in the womb, others early in childhood. Some might have been conceived one second earlier with different sperm, and so be a completely different person. Others might have grown up in a very different environment where we’d struggle to see ourselves in them. But others would have typed everything I have right up until this last word. There would be no sharp line, no clear boundary, between us and the other, just a scale of sameness vs difference.
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“No real you” would be my take on it. Much like Buddhist Anattā.
https://en.wikipedia.org/wiki/Anatt%C4%81
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It seems to me that the existence of many worlds is a belief. Many (scientists and otherwise) do not believe in the many worlds interpretation. Deutsch (and presumably others ) have argued its existence from the double slit experiment but I don’t suppose this counts as “evidence”.
In some ways it seems to me that “many worlds” may be about as likely as Russell’s Flying Teapot. Or creationism. Both to many worlds and deities I would take the stand of Thomas and ask to touch the cloth.
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Consider Russell’s teapot compared to a lost space probe, also in the same solar orbit. We have no more evidence for one than the other. But I think we’d agree that the probability of these two are very different. For the second case, we have many probes that have been launched over the decades, and not a few where communication has either been lost, or the probe finished its mission and we lost contact. So in the second case, the history of space exploration makes it far less unlikely.
To me, that’s the difference between deities and a quantum multiverse. We have a physical theory which has been tested to several decimal places. If we take the raw theory by itself, we get the other worlds. We normally add extra assumptions to make the other worlds go away, but those additional assumptions themselves seem to be in the same category as the assumptions about deities, assumptions we make for a metaphysics we’d prefer.
That doesn’t mean the other worlds exist, but I take their existence to be far less unlikely than Quetzalcōātl‘s.
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I don’t think the physical theory is actual evidence of the existence of a multiverse?
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The multiverse concept raises significant philosophical questions about testability and the nature of science. If we cannot observe other universes, can the multiverse hypothesis truly be considered scientific? TBH I’m not at all sure it is any more or less likely to exist than a deity – particularly if you include in your definition of deity a computer scientist in some other universe who has created us in virtual reality!
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For decades general relativity predicted the existence of black holes, but none had yet been observed, so they remained only a theoretical possibility. Einstein in particular thought they were just perverse solutions to his equations. During those decades, would we have been right to assume black holes existed? No. But would we have been right to dismiss them as fanciful speculation? With hindsight it’s easy to say no.
A physical theory makes predictions. Typically some of those predictions can be tested while other can’t. The ones that can be tested validate the structure of the theory. It doesn’t rule out that there may be more structure there that we’re not aware of yet, but it does give us confidence in the structure we do know about. If that structure then makes predictions we can’t test yet, it probably isn’t right to just accept them as reality. But is it right to dismiss them as mere speculation?
That’s where we appear to be with bare quantum theory and the quantum multiverse.
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So I listened to David Wallace, and it makes sense to me. But then, I give Wallace’s view more credit on the basis of “David Wallace said that very low probability branches wash out,” than on the basis of tentatively understanding why that seems to follow from what I already knew.
However, he also said that it would be different if you have a series of moderate-probability events such that a certain combination of results is highly unlikely. Such as doing a million spin detection experiments and getting spin up in all cases. The speed of decoherence gives each result enough stability so that the next can proceed. Tegmark’s quantum suicide gun would seem to be of this type.
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Thanks! That’s a good point I’d overlooked.
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We have a “physical theory” which sounds about as likely to be true as the existence of a deity. The theory may have been tested to several decimal places, but am I not right in thinking that there is no evidence whatsoever of a multiverse? In which case does the testing of a theory get us much further?
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Note: Just repeating here what I said above.
For decades general relativity predicted the existence of black holes, but none had yet been observed, so they remained only a theoretical possibility. Einstein in particular thought they were just perverse solutions to his equations. During those decades, would we have been right to assume black holes existed? No. But would we have been right to dismiss them as fanciful speculation? With hindsight it’s easy to say no.
A physical theory makes predictions. Typically some of those predictions can be tested while other can’t. The ones that can be tested validate the structure of the theory. It doesn’t rule out that there may be more structure there that we’re not aware of yet, but it does give us confidence in the structure we do know about. If that structure then makes predictions we can’t test yet, it probably isn’t right to just accept them as reality. But is it right to dismiss them as mere speculation?
That’s where we appear to be with bare quantum theory and the quantum multiverse.
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If we “assume many-worlds is reality” (I don’t), then quantum immortality seems plausible, but then we need a different understanding of “you”. “You” should be “you at time t with your singular past and all of your futures”, which means taking an action now which would be detrimental to the vast majority of your futures in hopes that one tiny minority of your futures would benefit would be stupid. And the fact that you find yourself having lived a charmed life, taking extreme risks and winning every time, would not change the fact if you try to continue the pattern you are responsible for a vast amount of suffering in future “you”s.
*
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I agree taking a lot of risks isn’t intelligent. But then I would say that regardless if we’re talking about one world or an infinity of them. Living recklessly even in a single world doesn’t seem rational.
But in a many-worlds scenario, we’d also have to accept that we have no control over what our other selves do in other branches. This branch has a fairly prudent Mike in it, but there would likely be other Mikes who died as a kid or young adult for taking stupid(er) risks. Or ones who are now facing much more serious health issues due to having started smoking or drinking heavily in early life.
All of which is to say, I don’t think we should worry overmuch about the other worlds in our day to day life decisions, even assuming they’re true. We should focus on what we can control and experience.
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But what does this mean for the quantum reality where we all have hot dogs for fingers? And if there’s a version of me who lives forever, is there also a version of hot-dog-fingers me who also lives forever?
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Someone’s been watching Marvel’s multiverse stuff?
It’s worth noting that it would have to be coherent and physically possible to exist in the quantum multiverse. That allows for a lot, but not sure it enable cartoon universes, Alligator Loki, or hot-dog fingers. But maybe I’m just being too closed minded…
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It was Everything, Everywhere, All at Once. But anyway, one of the things that bugs me about the many worlds interpretation are all the potentially absurd universes. I’m also thinking of Rick and Morty here. Basically, I want a many worlds interpretation that safely excludes a Rick and Morty style multiverse, and your post and what you’re saying now kind of help get me there.
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I really need make an effort to watch Rick and Morty at some point. Everything, Everywhere, All at Once was a pretty cool movie. But the multiverse in most fiction is silliness. It’s kind of like the Ant Man Quantumania movie, which took the word “quantum” as an excuse to just go for maximum psychedelic weirdness. It’s an entertaining fantasy, but definitely fantasy.
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Eh… not sure I’d recommend Rick and Morty that highly. It’s a weird mix of high concept Sci-Fi and low brow humor, with the scale tipped more toward the low brow humor.
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I can see that. I did try an episode or two years ago and didn’t feel the pull. But it gets acclaim and I’m always wonder if I’m missing something.
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Well, you know I can’t take this stuff seriously. “The idea is that if every outcome happens, then there are versions of you reading this that will live until the heat death of the universe.” Versions of me that I can never know about? That’s nothing I can bring myself to care about, and I’m not sure it makes sense to identify myself as something I can never know about. That seems to violate the very meaning of self.
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Assuming they exist, I think there are things we do know about them. Some will be just like you, others slightly different in a spectrum of similarities to differences. Consider replicant Roy Batty’s existential monologue at the end of Blade Runner (emphasis added):
If quantum immortality is true, Batty could consider the moments to not be lost in time, just lost in this branch of the wave function. He could envision other versions of those moments would continue to exist.
I don’t want to oversell this. There could be new data at any time that rules out the other worlds. (Alternatively a way could be found to communicate with them.) My personal philosophy skews more Epicurean, but it is an interesting possibility.
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“Assuming they exist, I think there are things we do know about them.”
That says it all right there. 😀
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Many-worlds theory strikes me as a collapse theory that does not want to commit to any particular collapse,, and so embraces all possible collapses, which in my view is a bad way to avoid the problem of collapse. But I’m curious whether all the worlds are underpinned by a common physics, so that, for example, entropy would be a part of every one of them. This would attenuate any expectations of immortality across the board, since nothing could last forever.
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The definition of the collapse is all the outcomes but the one observed disappearing. We could characterize many-worlds as many relative collapses. (In fact, Everett’s original paper was called the “Relative State’ Formulation of Quantum Mechanics.”) The benefit is we’re now talking about something in the mathematics, rather than just an ad hoc bolt on, which is the classic approach.
There are theories that some of the laws of physics weren’t set in the early universe. That might have an effect with the bubble universes of eternal inflation (if eternal inflation is a thing). The question would be when quantum physics begin. If it was before the final laws are set, then that would imply branches with different laws (albeit very distant from us in Hilbert space). I think Sean Carroll recently admitted it as a possibility.
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You’ve probably already seen this video, but just in case you haven’t…it’s not about quantum immortality, per se, but relevant to this discussion.
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Thanks. I hadn’t seen that one before, at least not that I remembered. As usual I’m pretty much in agreement with Carroll. Which does mean accepting that “you” is a more complex concept than we’ve been able to get away with until now.
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