Quantum Immortality Is Useless

And now for something completely different.

Quantum immortality is a controversial speculation supported by a handful of quantum physicists and abused by many science fiction authors. In essence, it states that the many-worlds interpretation of quantum mechanics implies that conscious beings are immortal. In this post I’ll attempt to show that quantum immortality is completely useless, even if it really works as described.

You Are The Cat

The idea of quantum immortality was derived from the quantum suicide thought experiment, which is a close cousin of the famous “Schroedinger’s undead cat” experiment. Instead of a cat in a box + poison triggered by radioactive decay, the quantum suicide thought experiment involves a mad scientist and a lethal weapon (which I will refer to by the unscientific name “gun” from now on). This gun has been cleverly modified so that when one pulls the trigger, it will measure the spin direction of a passing photon and fire only if the direction is, say, “up”. This amounts to a 50% chance of the gun actually firing.

So the mad scientist points the gun at his head and pulls the trigger. Depending on the spin direction of a random photon, either the gun fires and kills the scientist, or it just goes “click” and leaves him unharmed and free to go on doing what he must because he can.

According to the Everett many-worlds interpretation, each run of this experiment will cause the Universe to branch into two alternate world-lines – one where the scientist lives, and one where he dies. However, the scientist will only experience the world where he survives, because he obviously won’t have any conscious experiences in the worlds where he’s dead. Even if the experiment is repeated an arbitrary number of times, the scientist will only experience the world(s) where he survives.

From the point of view of the surviving copy (or copies), he’ll be immortal.

That’s the essence of the quantum immortality argument. You can’t experience the world-lines where you die, so you can expect to experience only the one where you survive – indefinitely.

Who Wants To Live Forever?

As a matter of fact, I do. Unfortunately, quantum immortality is unlikely to help me achieve eternal life. In fact, the concept is pretty much useless, except as a quaint “what if?” brain-tickler. Lets look at that quantum suicide thought experiment again :

Everett branches

Gun stock photo credit: Fastfood @ sxc.hu

If the many-worlds interpretation is correct, there will indeed always be at least one world-branch that avoids the fatal outcome, one copy of you that survives. Even if you get thrown into an active volcano, there will be an Everett branch where Harry Potter swoops in at the last moment and carries you to safety on his magical flying broomstick (depending on your cultural preferences, this may or may not be worse than being burnt alive by magma). The proponents of quantum immortality will point at that lucky branch and clap their hands giddily.

But – and here’s the kicker – you don’t get to choose in which Everett branch you end up. Yes, there may well be a world-line or two where you live forever, but what are the chances that you – this particular version of you, reading this blog post – are in that world-line? It would be extremely generous to say they’re rather slim.

As for the “you can only experience the worlds where you survive” thing, it’s just plain fallacious. It’s like a sales-person asking you “Will you be paying by check, cash, or credit card?” when you haven’t even decided if you actually want to buy their product. The question intentionally leaves out a valid option – not buying anything – and tries to make you pick one of the options most beneficial to the sales-person.

Similarly, the idea that you can only experience the Everett branches where you are alive incorrectly implies that being dead is somehow not a valid option. Unfortunately, it is. Magic aside, there is no overriding reason why a conscious creature couldn’t just… die, and thus stop being conscious (though that might change sometime this century).

Even if Everett was right and there are numerous alternate worlds, and in some of them a version of you is subjectively quantum-immortal, that doesn’t change your chances of survival one bit.

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30 Responses to “Quantum Immortality Is Useless”

  1. Thomas Riker says:

    From a quantum immortality standpoint, whenever you attempt to use a destructive teleportation machine, perhaps in an attempt to escape from the surface of a desolate planet; when the machine is engaged, you will only experience an Everett Branch where the machine somehow fails to dematerialize you. You might be horribly maimed, but the attempt will fail. An outside observer will probably observe you vanish from the surface of the planet and materialize successfully in the shiny space ship, which is why they will go ahead and leave and not bother looking for you. Better hope you packed some food, because it will be many years before they come back. In that time, the copy of you on the space ship will have screwed things up with your girlfriend. Yes, life sucks and then you can’t die.

  2. Hugh Everett says:

    That depends on probabilities. You’ll experience the teleportation branch if the probability of it succeeding is high. You would only experience the failure to transport branch if A and B terminated on re-materialisation, or somewhere in transmission.

    Actually you’d experience all branches, but each branch in which you survived would have a likelihood of being experienced equal to the probability of that branch occuring.

  3. Hugh Everett says:

    If B and C terminated, I mean.

  4. Sam_Spade says:

    But the probability of each branch occuring is 100%. Flip a coin – one version of you sees heads, one sees tails. Keep flipping, and the number of versions keeps multiplying. Each version invents this idea of “probabiltiy” to explain why his own history happens to be H-T-H-H-T-H, or whatever.

    The real core of the weirdness — at time t+1 there are two replicas of you, each with an equally valid claim to being the real you. From an objective perspective, there’s no paradox — two people claim to be the same person. But if you’re talking about many-worlds, there is no objective perspective. And subjectively, it makes no sense at all. The best we can do is invent this idea of probability : 50% chance I’ll end up at location B, 50% at location C. It IS the best we can come up with, but it’s also wrong. The right answer is 100% B/100% C.

    Arguments against quantum immortality rely on the flawed 50/50 idea. “I toss the coin, and if I end up as the version of me that dies, that’s my bad luck.” Trouble is, there’s no coin.

  5. John Laurie says:

    The way I see it, it all comes down to first person experience.

    I have a first person perspective of being ‘me’. So any other versions of me will always have that ongoing continuing first person experience I am having now.

    By default, ‘I’ can only have the first person experience of comtinuing to live and anyone having that first person experience of being me is ‘me’.

    While I will see others die, I am only experiencing this in the third person.

  6. Davide Pintus says:

    No offense, but I’m fairly sure you missed the point.
    The point is not that there is a 50% chance to survive, but that you are only conscious of universes where you are still alive

  7. kurt says:

    The main point is that, if there any subset of possible futures in which you are alive, your consciousness will live on, and that each of those will be the subjective self. It will be a continuous existence in which everyone around you dies, but you somehow manage to carry-on.

    It would be similar to living a simulation in which you are the central star and you never die. Even if, among the multitude of possible futures for a person right now, there is only one in which that person survives over the next minute (gets nuked at home), there will be continuity from the current moment to that single future, and the person alive at the single future walking around the wreckage will remember precisely one history. The mapping is injective from the present to the future univeres. Several futures occur for each present. One of these futures is bound to feature you alive. If all possible futures from this instant feature you dead, then you’re dead. But given the virtually infinite number of possible futures, the chances are at least one features you alive.

    That’s quantum immortality as I understand it.

  8. Richard says:

    I think people get confused by this when you regard yourself as some thread of consciousness living on, rather than some future state of yourself looking back. There is no thread connecting you to the past. There is only right now and your memories.

    Given that your physical state can be duplicated at any point, in any simulation, or in the multiverse or an infinite universe or similar universe created after this one dies or in the past. All you need is a bit of common sense here and answer the following question:

    How many universes are there? a) 0 b) 1 c) all of them, every one. Every possible mathematical state that can be described.

    Well we can all agree on at least one, and I am of the opinion that everything existing is a much simpler model than one thing existing.

    Regarding the teleportation machine:

    a) You would look back on the memories of yourself successfully teleporting. That would be your experience.

    b) Its impossible to experience being dead.

    c) Can you experience being horribily maimed? Undoubtedly so – presuming your faculties are intact, but I suggest it doesnt seem very survivable for long. I suspect you would look back on that as a dream or a simulation.

    When I finish building my doomsday machine we will all be able to put this to the test.

  9. Donathan says:

    I know this post is incredibly old at this point, but it really does seem like you’ve misunderstood what Quantum Immortality is.

    Just because you observe someone else dying, that doesn’t disprove the theory. Of course people die, and everyone who supports this theory acknowledges that. The theory hinges on the idea that whatever worldline you are currently in is the one in which you aren’t dead yet, and it will always be like that because you can’t observe the worldlines in which you have died.

    Flip a coin – heads or tails. Heads you live, Tails you die instantly. Every time you flip the coin, your universe is split into two separate timelines for each outcome. While both of these timelines do indeed exist, and there is a timeline that goes on in which you are dead, you are unable to observe the outcome of your own death so your consciousness lands in the timeline in which you live. Though it is a 50% chance of landing on either, in this thought experiment at least, you will continue to land on heads for as long as you flip the coin even if it is 100 times in a row. This is because you can’t perceive being dead and therefore you will only exist in the worldline in which you survive.

  10. Talon says:

    You are very confused. Let me attempt to clarify.

    First, understand that existence is information. You are equivalent to the current state of the universe (which includes its understanding of the past). Note that the term “universe” here refers to your experiences (i.e., solipsism), as everything you know about the universe (including, say, any laws of physics) are derived from them. The process of going forward in time is the process of accumulating information. This is called conditioning (as in conditional probabilities).

    Second, understand that you are an equivalence class (a bunch of things grouped together). Everything in the future that is “you” is something that, when it looks back at its past, will see your current state. In other words, “you” is everything that believes it is you. A future you is a state of the universe that is in agreement with past you, that contains the information from your past state.

    Now, let’s answer some questions.

    “what are the chances that you – this particular version of you, reading this blog post – are in that world-line?”

    100%. By defnition, that “world-line” is an extension of your current existence. All of them are “you.” That’s why they’re called future versions of you!

    Now, you’re probably disagreeing and saying, “I don’t experience all these outcomes; I only experience one world-line!” Well, every future version of you has just one world-line, its unique past (state). When you’re asking, “Which one will I be?” you’re thinking of questions like, “Tomorrow, will I have won the lottery?” Here’s the right question:

    “What is the probability that a future version of me will look back at its past and see this event?”

    This is what it means to be YOUR world-line. Remember, you are everything you know, which is your state, which contains your past, which is the one, unique world-line you feel you are experiencing. So, in order to even ask the question, this world-line needs to be defined, which means the event has to be in your past, which means you have to be experiencing after it. This is fundamentally incompatible with “death” (meaning a permanent end to your experiences).

    And now you see that, in order to even have a world-line to talk about, you have to be experiencing. That is what quantum immortality means.

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