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katydee comments on Help: Is there a quick and dirty way to explain quantum immortality? - Less Wrong Discussion
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The (very brief) explanation I normally use:
So, according to quantum physics, any time something can happen more than one way, it actually happens all those ways, but in different universes. So let's say you flip a coin. In one universe, it comes up heads, but in another it comes up tails. However, it's not really that simple, because there are all kinds of crazy things that can happen but are really unlikely. So it's more like, in almost half the universes, the coin lands heads, and in the other almost half of the universes, the coin lands tails, but in a tiny fraction of the universes the coin actually lands on its side. Anyway, quantum immortality holds that, since you can't perceive universes in which your perceptive abilities no longer exist, you'll always end up in a universe where you're still alive. So imagine you push a button that has a 50% chance of killing you. From an observer's perspective, you will die 50% of the time. However, from your perspective, you'll only perceive those universes in which you don't die, so you'll never actually die, no matter how many times you push the button. This applies to everything, since there's never a true 100% chance of death, no matter how crazy the situation.
This is indeed the standard thought, but be careful not to make a mountain out of a molehill.
"You'll never actually die" only holds if you define "you" to be "you who doesn't die." If you include "you who does die" as also you, then you do die! In short, both the mind-blowing and confusifying powers of quantum immortality are related to the fact that it is a tautology in disguise. It's a question of definitions and not a property of the universe because the universe doesn't really care whether you're alive or dead; dead bodies make great quantum observers, they interact with their environment just fine. But we choose to divide up the possible universes into "us alive" and "us dead," and then only look at what happens in one of those groups, making the answer predetermined by our fairly arbitrary choice of which universes to look at.
The standard response to that fact is "yes, but we choose to care about the universes where we live for a very special reason: you can't think about yourself being dead when you're dead. Cogito ergo sum."
And then I would say: "This is true, but just be careful. Though by definition we won't perceive ourselves as dead, that doesn't mean we can't die, unless we redefine 'we.' Pronouns beyond here become confusing; for a clearer view try to think of things following fuzzy worldlines in spacetime, with events being points in spacetime."
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To make quantum immortality not confusing is to make the mountain back into a molehill, relatively speaking at least. "The big complicated space that we exist in can be thought of like parallel universes, and in some of those parallel universes you'll live forever." Cool!
Much like David Allen, I'm not sure this is truly the case. The path of the coin is completely determined, if you knew all starting conditions perfectly you could predict how it would end up with complete accuracy. And the universe knows all starting conditions perfectly.
Quantum fluctuations would tend to cancel each other out (much like all the air molecules in a room are never only in one corner, leaving the rest of the room in a vacuum, even though that's not strictly impossible), and thus never exert any significant pressure on the coin in any direction, and have no net impact on it's path. I would think that the coin would land on one particular side in nearly 100% of universes, with maybe the tiniest fraction containing a truly extraordinary confluence of random fluctuations all in the same direction that made it land on the other side.
I'm almost sure that it's not actually the case. I only use the example of a coin flipping because most people consider that random and it's easier than having to explain Schrodinger's Cat.
Isn't that an argument against quantum immortality though? As the event that kills you in any given universe is not going to be a random quantum event, but a hard-to-affect deterministic event that kills you in (nearly?) 100% of universes.
It shouldn't matter, since even the most infinitesimal chances are guaranteed to come up somewhere.
I think it is important to clarify the meaning of "chance", as you refer to it.
If I say that the behavior of a flipped coin is almost certainly deterministic, the remaining uncertainty is not part of the system, it is caused by my inability to predict the outcome. This is not the kind of "chance" that you are referring to.
The type of "chance" related to quantum immortality is the probability attached to non-zero quantum wave-function amplitudes.
It is not enough for there to be a conceptual "chance" that quantum wave-functions could influence the outcome of a coin toss. There must be actual reachable sequences of quantum state sets, all with non-zero wave-function amplitudes, that result in alternate outcomes.
It may also not be enough to utilize a hypothetical model of the quantum wave-functions. It may be possible that real low probability wave-functions don't result in universe splits. For example, those world-lines might merge with higher probability world lines, or there might be resolution limits set by the holographic universe, or by quantum foam noise.
With these restriction and granting (just for this argument) that the MWI is the right way to think about the universe, I'll agree with your statment:
I understand this, but thanks for the clarification regardless.
That's pretty much the explanation I gave her. I think the weakness in it is that if you're already confused about MWI it sounds almost like you teleport between universes, rather than there being other universes where someone you identify as yourself never stopped existing in the first place. At least, I think that's one of the parts she wasn't getting.
FWIW, I understand the above explanation, and also that one does not teleport, but that a single instance of, say, me, will avoid all opportunities to die. I can't speak for your friend, but the part that remains unclear to me is, since I am still only consciously aware of the one instance of me, and statistically it's very unlikely to be the immortal one, why I should care that some other one is. :P
How much background do you have in the relevant nerdy stuff though? This is someone who can basically be described as the polar opposite of a nerd. She'd never heard of the idea of multiple timelines/realities except really vaguely from mainstream pop culture, has no idea about quantum or any other types of physics, and afaik has never played a computer game in her life. I would also bet that she's never watched any hard sci fi.
Basically I am curious as to whether it's a problem of inferential distance or whether I just didn't explain it clearly enough :p
As for why you should care, depends on how you view continuity of identity.
More than your friend, but maybe not a lot more. I'm one of the least nerdy nerds I know, and probably the least educated if you normalize age. Computer games yes, hard sci fi rarely, quantum physics pretty much none.
True. I have trouble imagining anything being "me" which does/did/will not share the same stream of consciousness which is doing the imagining.
So do you view the 'you' that wakes up in the morning as the same as the 'you' that went to bed?
DSimon woke up this morning. From that point on, there have been a lot of diverging universes, one of which involved DSimon going to work, one of which involved DSimon getting hit by a car, and one of which involved DSimon founding a new town called "Shinypants" where the president lands Air Force One the very same day and makes him emperor of the world on a whim. There's also many many other divergent DSimons, but let's not worry about them.
So we'll call those derivatives DSimonWork, DSimonDead, and DSimonEmperor. All three of these DSimons feel that they are the same as the DSimon that woke up this morning. However, they do not feel that they are the same as each other.
So actually "the same as" isn't quite the right concept here, because then I would be saying that A = B and B = C but A != C. "Identify with" might be better. The main point though is that this feeling of identification proceeds backwards up the divergence tree, but not sideways across it.
What DSimon said.
erratio:
[Edit: Oops, I misread the above question as asking how much background you have to have to be able to understand this stuff. I certainly don't have much background in physics myself. What I wrote below is true regardless, though, so I'm leaving the comment as it is.]
You have to have a deep mathematical grasp of quantum theory, both formal and intuitive, before you can even start to understand what the controversial issues such as MWI really are about, let alone to form any reasonably grounded opinions about them. Otherwise, you can only fool yourself that you understand anything about these topics, and any beliefs you form about them can be based only on faith in authority or arbitrary whim, not sound reasoning.
The same holds for most other topics in physics. In some areas for science, non-technical pop-science explanations can lead to correct understanding, but in physics, they are worse than useless. (The reason why physicists often display a positive view of pop-science physics books even when they don't stand to profit from their sales is that they raise their status.)
I think that's much too egalitarian. As best as I can tell people vary tremendously in their ability and inclination to grasp certain topics with minimal preparation. What you say is true of the majority but not of everyone. The value to the student of popular accounts also varies tremendously. What you say seems to be true if you pair the worst students with the worst books.
Hm.. this is the second time today that I posted a comment based on a misreading (see the correction above). Makes me wonder about the quality of the work I did during the rest of the day.
Regarding the pop-science approach to physics, I read a lot of such books as a teenager. Since then, I've studied several areas of physics with real mathematical rigor, some in university courses and others just out of curiosity, and in retrospect I must say that the popular books had failed to give me any accurate understanding whatsoever. All I got was either confusion or a somewhat coherent but in fact completely misleading picture of the real thing.
The only cases where physical theories can be explained with some accuracy in layman non-mathematical terms are those that deal with people's everyday experiences, and don't involve any phenomena that are outside of that. When it comes to relativity, quantum theory, cosmology, let alone more advanced and esoteric areas of modern physics, to me it seems impossible to convey any accurate understanding without the mathematics. I have yet to see a pop-science book that would successfully do it, no matter how smart the reader. (I liked this recent LW comment made by a physics about this topic.)
If you subscribe to the view that an illusion of understanding is worse than nothing at all (as I do), it's really hard to find anything positive to say about pop-science physics books. Yet scientists will rarely criticize them, sometimes because they profit by writing them (or hope to do so one day), but more often because they raise the status of physicists and scientists in general in the eyes of the general public. Ultimately, these works allow readers to signal their smarts and sophistication, famous physicists to make lots of money, and other physicists to enjoy the high status brought this way to their profession, but certainly nobody gains any (scientific) knowledge in the process.
A couple of my own experiences. As a kid I devoured Lewis Carroll Epstein's Thinking Physics, which so easy and fun and filled with pictures it is like a comic book, and I devoured it like one (though I always - always - answered each puzzle before seeing his explanation). Talk about popularizations, this was a frickin comic book. I think it took a week of afternoons to get through. In retrospect I don't think it misled me. On the contrary, I think I learned a lot from it. My major as an undergraduate was mathematics but I took two semesters of physics along with the regular physics majors in my freshman year, classical mechanics and electromagnetism, so I think I am in a position to judge that book in light of a "real" education.
Second experience. I never took a course in special or general relativity per se, but I touched on both in a graduate course on differential geometry (I think it was). In light of my graduate level understanding of the mathematics of spacetime, I would not say that I had been misled by popularizations of it (though I could not at this point name you which exact popularizations of it I read - Epstein wrote one but I never read it). It has been many years but what I learned in that graduate course regarding special relativity was a certain metric, a simple, trivial metric, called I think minkowski metric (it's been a while). That metric made the geometric aspects of special relativity utterly trivial, all the familiar amazing geometric predictions of SR regarding lengths and timing just fell right out of it, everything, the flashlight on the train, everything. It was a course on geometry so it didn't integrate the minkowski geometry with the laws of mechanics, force, acceleration, etc. But my point is this: it did not reveal the popularizations that I had read as a teenager to be a load of bull. On the contrary, it confirmed and completed them.
To compare, I have read some of the "best" popularizations of string theory. I do not feel very enlightened by them. There is no illusion of understanding because there is no belief that I understand. In contrast, I find Feynman's QED (the thin book, not the theory) enlightening - as far as it goes. The book has modest goals. It does not pretend to arm you with the ability to carry out real-world predictions. So I find popular books to vary tremendously in their ability to convey knowledge.
Before you get to exotic* applications of quantum mechanics, you should cover the basics. Quantum mechanics forces a view of personal identity that treats the many words equally. This is true even if you endorse collapse, as long as people can occasionally be put in superposition.
In a classical world with randomness, you could imagine that there are lots of parallel copies of people, each with a serial number ("indexical uncertainty") who march in parallel until random events occur whose outcome is determined by the serial number. In such a setting, you could imagine that the invisible serial number is a core part of the person's consciousness and there is no continuity of identity between them. But QM says that quantum sources of randomness are not like that. It is not meaningful to talk of which parallel person gets which result.
* a euphemism for "wrong."