Somehow I don't find that very reassuring :-P

Explanation on its way.

OK, in order to understand what Quantum Suicide (QS) refers to, you need a little background. I'll be as brief as possible.

A state in quantum mechanics is described by the Wave Function. The Wave Function is merely a mathematical construct that contains all the known information about the state (of whatever you're looking at). But, the Wave Function doesn't directly tell you about what you will *observe* that state doing. The Wave Function gives you a *probability*. Nothing more. So, in our world, looking at a ball flying through the air does nothing to affect where it is. But, in QM, observing a state, in a way determines where the object is. The Wave Form is said to "collapse" because, once you observe it, it is no longer a probability.

Now, this notion of collapse causes all kinds of problems. Where was the device before you observed it? What happens after you observe it? Does it uncollapse? How can (while conserving Energy/Momentum) a particle *instantly* collapse into a certain location?

This notion of collapse is called the Copenhagen Interpretation. But, there are other interpretations. The next-most common is the Many-Worlds Interpretation. This interp. basically says, "Wait, Collapsing Wave Functions makes no sense. Instead, every possibility must be true, forming a new universe."

So, QS is a thought-experiment that derived from this MW Interp.

Now to answer your question. And, I apologize in advance if you found all of the above unnecessary.

So, in the QS, a gun is fired based off of some Quantum Event. There are two outcomes, "fire" and "hold," creating two new universes. In one, you're dead. In the other, you're alive. Obviously, you're only aware of the universe in which you're alive. QS goes on to say that you can generalize any event into one which will kill you and one which will not. This is how Quantum Immortality is arrived at.

IMHO, QS=>QI is bogus for a number of reasons. The most practical is: why are there no immortals in "our" universe? We're stuck in the only one where *no-one* has even made it past 200? This is because there are some logical/mathematical inconsistencies with trying to break every quantum event into a binary killing process.

tl;dr Copenhagen and Many-Worlds are both reasonable interpretations and while Quantum Suicide is interesting, I don't think you'll find many professionals who actually think it leads to immortality.

Oh, wow. That was much longer than I intended. Let me know if I even answered your question : )

@abge - Have to say I think he's correct.

The way I see it, QS implies that there does exists some universe were you never die. It is merely the case that, to the best of my knowledge, in this one I know other people have died and thus conclude I will one day die.
I have no evidence for this, and it is quite possible I will be immortal.

@figle

If you could break down every single wavefunction into the eigenvectors a|Die>+b|Kill>, then I agree. In some universe you'd live forever. But, this is a *huge* if. There is no indication, as far as I know, that you could reasonable break events down in this matter.

Perhaps I should have explained it this way from the start, but I didn't want to get too technical.

In order for there to be QI, you'd have to show that every single wavefunction is a superposition of |Psi> = d|Die>+l|Live>. Even if this were true (which I doubt it is for various reasons), you'd need to show that lim t->infin. l=/=0. Again, no reason to suppose this is true.

So, no, I do not think Quantum Mechanics, in any interpretation, allows for QI.

wait, i know a little bit of probability as applied in physics.

So we know all the atoms/molecules of gas in a room could concievably all move into the ocrner of the room, however the probability of this happening is so low that you'd have to observe them for longer than the Age of the Universe (AotU) - and i don't mean several days longer, i mean several orders of magnitude longer, millions or perhaps even billions of AotU...

Now the two binary outcomes may not be equally likely, as a wave function collapse describes the probability, it does not mean a 50/50 outcome for every observation. it could be a gun with a 90/10 chance of killing you...

As people get older it becomes increasingly likely that SOMETHING will kill them. (entrophy leads to their body breaking down, and the repair mechanisms failing, though this is not neccesary, it is a consequence of our haphazard evolution)

So it is argueably possible that we are living in a Universe where no-one has survived to older than 200 BECAUSE it is more likely in any universe that people will die before 200 - infact it is soo much more likely that we don't expect to see it happen ever, just like the gas particles gathering in one corner of the room.

I don't see the idea of QS or QI as wrong, just unimportant and fairly uninteresting.

that is to say, assuming i can write any wavefunction as "|Psi> = d|Die>+l|Live>" that doesn't really have any major impact on what we observe in reality - thus it is not interesting.

@Ora - Much better way of putting it.
My point was not that these are impossible, but that they have such crazily minute probablities that you discount them.

@orathaic

I'm glad you chimed in. I agree with everything you said, but I'm not sure you actually answered the question.

First off, there are two things being discussed here. QS and QI. I agree that QS is certainly possible in the MWI. But, I very strongly disagree that QS=>QI.

Also, I will admit that the 200 year old example doesn't make much sense, but in my defense, I wrote that in three in the morning.

As to your second post: " that doesn't really have any major impact on what we observe in reality - thus it is not interesting."

Whether or not it's interesting is completely irrelevant. The question specifically asked about QS and this is what QS says happens.

So, did this answer your question, baum?

I think so. Your problem with the extrapolations leading to QI is that the extrapolations are based on the assumption that everything, no matter how improbable, is possible. The QI proposition suggests that in every wave function, some possibility of "not die" is always included.
However, you say no, the possibility "not die" is not always included.

Adjusting for the nuances and really minor details laity like me would not get, is that about right? In layman's terms?

"However, you say no, the possibility "not die" is not always included."

Yes. Furthermore, I may go so far as to say that the possibility of "not die" doesn't even make sense, because you can't easily transition from an isolated system (such as the QI thought experiment) into the real world.

Now, I could be completely wrong, but this is my best understanding : )

Amen to abgemacht's last comment. Just because the math allows for something in theory does not mean that reality directly follows suit.

Simple example: Consider a square, with area s^2. All you know about it is its area. This could be one of two squares, each equally mathematically feasible. One has side length s, one has side length -s. While imaginary math is useful in certain contexts and does provide meaningful information about our universe, you won't be bumping into any squares with negative side lengths in your next home improvement project.

That's why I take assertions about multiple universes with a grain of salt. Just because the math allows for them does not mean that we should assert their actual existence other than as a mathematical construct.

Interesting comparison. I was thinking of it more like Zeno's paradox of the hare and the tortoise.
Just because each decision it is possible for you to still be alive, the rate at which these decisions occurs isn't mentioned, and thus the rate may tend to an infinite value.

Ugh. I came up with the idea of quantum suicide on my own a few years back, and was so excited for myself. I found out a couple weeks later some stupid physicist came up with it about a decade before. TheGhostmaker coined the word preadime to address my frustrations; preadime being a verb that describes something that was stolen from someone before they even had it. Damn physicist preadimed my idea.

I considered the possibility of strapping explosives to my head (the most thorough way I could think of to reliably obliterate my consciousness) and measuring the spin of an electron at the press of a button. If Many Worlds interpretation is real, then I could give very convincing evidence by standing in front of a live audience and consistently measuring an up spin in at least one universe, while unfortunately exploding my head in the vast majority of universes.

I seriously hope that this is not possible, because it would suggest that everyone's stream of consciousness is likely doomed to a life of serious maiming, as one would appear to survive all events that have even the slightest chance of preserving one's consciousness. Of course, this theory makes serious physical and philosophical assumptions.

Luckily for you, as long as you keep measuring an electron spin in the same direction, the waveform doesn't uncollapse, so if you make it through the first round, you're pretty safe : )

But, more to the point, you could give yourself very convincing evidence in ONE of the universes. In the other, you'd just make a big mess. That's really the problem with multi-worlds. If you can't interact with them, do they really solve any of the problems associated with copenhagen interpretation?

@Chris Surely pregiarism (as in plagiarism)?
@point - Christopher Nolan made a very interesting point on that one (and yes I know where he made it too but won't reveal that due to the side effects it would lead to) where he talks about the fear such an event would bring.
Could you ever do such a thing, not knowing if your consciousness would be the one that lived or died?

If quantum suicide were reality, then blowing your head off guarantees your current consciousness would be the one to survive as well as the one to die. It would split into two new conciousnesses with the complete memory of the previous one. One of them would potentially suffer for as long as it took the body to die, and the other would go on going "was the detonator a dud or did I just prove quantum suicide?"

What Draug said is true. If there are Many Worlds, then our consciousnesses are constantly splitting without us knowing it. QS would be no different.

Now there's a concept. Conciousness travels down the path of least resitance in a multiverse. every decision we make splinters off other universes is the idea, but out conciousness that we perceive is the one that has the longest potential (i.e. infinity) as that is the path of least resitance. Hmmm...

Can I just say that it makes me smile every time I see my name as the subject of a thread at the top of the forum.

I don't think my understanding of the many-worlds theory is the conciousness 'splits' off down any path, of one become two.

It is that many worlds exist, perhaps infinitely many, and that before a 'wave function collapses' two particular universes are indistinguishable after the collapse they give different results.

as to "Whether or not it's interesting is completely irrelevant. The question specifically asked about QS and this is what QS says happens."

Ok, so yes, i suppose i did miss the point of the question. Does this imply that any observer will live forever? I give it that it is required in this many-world theory but vanishingly small, and irrelevant to the point of not being observable.

In this i DO ascert that it is possible to find some probable state ( of a wave function) which corresponds to a surviving being.

Now do i know how to find this correspondance? No.

However, I stand by my position that QI is not interesting. Even with my assumption that it is correct, if there are infinitely many Universes and EVER possible state exists, then the question merely become 'Is it possible that Observer A is still alive in one?' And the answer is neccesarily yes, it is guarenteed that Observer A is still alive in infinitely many Universes, if that is possible at all, however improbable.

Yeah, that's why I tried to devise a situation where one's consciousness could be reliably obliterated in a very short span of time, such that my stream of consciousness would could only take me down the universes in which I did not detonate the explosives. You could make multiple explosive belts and show they weren't duds by running the experiment afterward while not around your head to see them explode.

abgemacht, are there no testable (in one universe) differences in predictions between Many Worlds and the Copenhagen interpretations?

No, not that I'm aware of. That's why they are "interpretations."

Or I should ask, would convincingly demonstrating the the truth of Many Worlds in a minority of universes not advance scientific knowledge and physical application in said universes?

Of course it would, but in the sense that knowing anything advances science. It would not, for instance, have a foreseeable affect on device design, which is what I do.

@orathaic

"It is that many worlds exist, perhaps infinitely many, and that before a 'wave function collapses' two particular universes are indistinguishable after the collapse they give different results."

Yes, but you are (probably) in both of those Universes that give different results, no?

"However, I stand by my position that QI is not interesting"

I agree, in the sense that QI wouldn't help us actually do anything. But, like Quantum Entanglement, it is fun to talk about.

" it is guarenteed that Observer A is still alive in infinitely many Universes, if that is possible at all, however improbable."

Yes, but after a certain amount of time, I don't think you'd find the observer in any of them.

I actually take that back. It turns out that QE is very useful. Just in ways I don't understand at all.

"Yes, but you are (probably) in both of those Universes that give different results, no?" - yes, and probably observing both, but in some you may be dead...

but unlike quantum entanglement - which people have found useful in designing a system which can detect whether it has been intercepted and read, hence making an unbreakable encryption technique - QI is most likely useless.

"Yes, but after a certain amount of time, I don't think you'd find the observer in any of them. "

I don't think you're getting the concept of infinite universes....

I mean it has been shown on a small enough scale that a system can dynamically decrease it's own local entropy. Now that was one copy of a system which in the 100,000 other copes increased their entropy, but if it is possible it will happen given enough Universes... so I don't know if immortality is possible, but IF it is then an Immortal me exists in one Universe.

An immortal and perfectly healthy individual exists in one of them. No scars and no injuries ever. So an "unbreakable" you exists in one of them. In short, we are all Superman in one universe. Of course, if your a man, you have boobs in at least one just as every woman has a penis in at least one.

and nothing tastes like chicken...

"It would not, for instance, have a foreseeable affect on device design, which is what I do. "

That is more what I was getting at. I like that you very diplomatically inserted the word "foreseeable" in there.

"I don't think you're getting the concept of infinite universes...."

Does Many Worlds necessarily require an infinite number of universes or merely an *extremely* large finite number? I think it's also important to consider the continuity of consciousness of an observer. The matter that could be defined as an observer would be limited by its light cone on a physical level, and the continuity of its stream of consciousness on a psychophilosophical level. Are you envisioning quantum tunnelling on such a scale that an observer has a nonzero chance of enduring a supernova and then surviving in a vacuum for eternity? Could the biological processes that underlie consciousness be sustained indefinitely purely due to quantum phenomena? I can grant that some type of stasis might be possible, but that could hardly be called a conscious observation of quantum immortality.

I think the whole point of the concept of quantum immortality isn't that it is possible that somewhere an immortal you exists, but more that you will *inevitably* be immortal (or at least live for an unreasonably long time) from your own point of view due to Many Worlds and the continuity of your stream of consciousness.

"surviving in a vacuum" should be "surviving in space"

It's not about survivng the supernova but whether or not the supernova actually happened. Maybe a Deus Ex Machina occurs and the sun about to blow suddenly settles down because something unforeseeable occurs to stop the countdown to destruction.

@Everyone

Let's back up for a second and make sure we agree on the basics. Specifically, what MWI says.

To the best of my understanding, MWI says that every time a quantum state is measured, the current universe splits. These new universes are identical except for the result of that one measurement. For instance, if I were to measure an electron's spin right now, in one universe it would be Up and in another it would be Down. These new universes are then free to split off the next time a quantum state is measured.

Do we all agree that's basically correct?

@Chris

If what I said above is true, then under the MWI there would have to be an infinite number of universes. Consider measure the position of an electron. Position has a continuous eigenspectrum, so there are an infinite number of places it can be (within a certain area.) Not only is it infinite, but it is uncountably infinite. So, just based off of one event, you now have an uncountably infinite number of universes.

But again, please make sure my understanding of MWI agrees with yours (especially orathaic) because I could be terribly wrong.

abgemacht, you just blew my mind, is it actually well known that position is completely continuous? I mean, theoretically, a line segment can hold an infinite number of points, and that a particle's wave function can be described by a continuous probabilistic position. However, is it not plausible that at some extremely small scale the spatial dimensions are not continuous, but comprised of discrete positions? I've only an amateur interest in modern physics, so I hope I'm making sense.

@Chris

I see exactly what you're saying. It seems very weird (and quite frankly, unfair) that some things are fundamentally discrete and others are continuous.

Yes, Position is completely continuous. Electron Spin, for instance has an eigenspectrum of two discrete eigenvectors (up and down). Position has an infinite number of eigenvectors within some range. While we can say that an electron has some probability of being Up, we can only say it has some probability of being located within some *range* of values.

I believe that Position is always continuous. Some things like Energy, are continuous when you are in a vacuum and become discrete when restrict a particle's movement. Say, by trapping it in a potential well. This is why electrons have orbitals around an atom. The Potential of the nucleus restricts e movement and you are left with discrete "allowed" energy levels.

Em, yes, Abgemacht what you said seems perfectly right, and i'm happy to agree with you in the definition of MWI.

also thanks for the uncountable infinite example, I coudln't think of one and it is reasonable. (the idea that a bound state wave function - for an electron - has finite energy solutions, and thus it's position is known to some degree as it is bound to an atom, as compare to a free electron which has continious solutions as it can occupy any point in space)

However based on the concept of the plank lenght Chris is right to suggest the position MAY be discrete. I can't actually tell you that i understand the maths, nor how quantum mechanics works it space and time is quantised (nor in fact how gravity works, but no-one can tell you that so i'm in good company there...)

So in short, i've got very little to say other than I don't know.

I am supposing a continous Universe spectrum which is infinite in it's potential for distuinguishability. So if all the numbers between 0 and 1 (an uncountably infinite number of numbers...) represent all the Universe in Many-Worlds, then as the first quantum state is measured 0 and 1 become distinct, and then we'll probably name the next disctint universe 0.5... and there is infinite potential distinction.

Therefore everything which can possibly happen does... i think.

PS we can't measure the plank lenght, but for some reason theoretically think it's the shortest meaningful lenght. Thus mathimatically proven to discretize space, IF the physics behind that math is correct for this Universe...

But, definition has a very solid mathematical definition. If position was was actually discrete with very small step sizes, then it wouldn't be continuous, it would be discrete.

Now, it may turn out that we're wrong. But at the moment, we aren't saying position is continuous as an approx. for it actually being discrete. The current knowledge on the subject is that position is truly discrete.

Also, I'm not sure what you mean by Planck length. As far as I know, there is no physical significance to the PL. It's just a number.

Also, if position is bothering you, try to remember that an electron isn't a particle that moves from one place to another. It is a travelling wave that, when measured, collapses to a certain point in space.

" but for some reason theoretically think it's the shortest meaningful lenght."

If this is true, then I'd have to agree with you. But, I have to admit, I'm not too familiar with the concept.

"In general relativity, spacetime is assumed to be smooth and continuous—and not just in the mathematical sense. In the theory of quantum mechanics, there is an inherent discreteness present in physics. In attempting to reconcile these two theories, it is sometimes postulated that spacetime should be quantized at the very smallest scales. Current theory is focused on the nature of spacetime at the Planck scale. Causal sets, loop quantum gravity, string theory, and black hole thermodynamics all predict a quantized spacetime with agreement on the order of magnitude."

http://en.wikipedia.org/wiki/Spacetime#Quantized_spacetime

The physical significance of Planck's lengths, and indeed Planck's time is unknown, but I think I've heard it been said that these are the smallest measurable lengths of space and time, so speaking of smaller units is meaningless, as all measurable events would happen at multiples of Planck's length and Planck's time.

Sorry to bring this back from the dead, but I didn't want to leave this unanswered.

I had a chat with one of my professors (who's research is in Quantum Information Theory). Basically, he said that QM falls apart in the Plank's length regime. Or, to quote him, "The walls on the house don't change color; the whole house falls down."

Not only that, but there really isn't a coherent theory that replaced QM at that level. Some combination of String/Relativity that I know nothing about. But, it clearly isn't complete yet.

So, position is continuous up to the Planck's Length. After that, there doesn't seem to be agreement on what happens. So, I guess you could say that position is continuous until the point that "position" no longer has any meaning.