Quantum Immortatlity
Everything is possible, the quiet trick at its centre.
When I first learned about the multiverse, something in the floor gave way.
The argument was simple, and that was the seductive part. If the universe really runs on probability, and if every branch that probability allows is somewhere realised, then anything with a non-zero chance of happening happens to someone, somewhere. A coin lands heads in one world and tails in another. Both are real. Nobody collapses into the loser.
Follow that one step further and it gets personal. A bullet fired at your head is a physical system like any other. There is a fantastically small but non-zero probability that its particles tunnel clean through yours and you walk away. Vanishingly unlikely, but not zero. And if it is not zero, then in some branch of the wavefunction, it happens. You survive.
Now run it again. And again. In every branch where the bullet does its job, there is no “you” left to notice. The only branches in which anyone is around to have an experience are the ones in which you lived. So from the inside, from the only vantage point you will ever occupy, you never die. The probability of you witnessing your own death is zero. Not low. Zero.
That is quantum immortality. I took it seriously for a while. Then I stopped, and the stopping turned out to be more interesting than the believing.
Where the idea comes from
The branching picture is Hugh Everett’s. In 1957, as a graduate student at Princeton, he proposed reading quantum mechanics with the collapse taken out.¹ In the standard story, a measurement forces a particle to choose: the cat is alive or dead, the spin is up or down. Everett’s move was to say the choosing never happens. The wavefunction just keeps evolving, and every outcome is realised, each in its own branch. You see one result only because there is now a version of you in each branch, and each version sees just the one it is in.
Everett himself, by several accounts, believed this meant he would never subjectively die.² He never published the thought. The job of writing it down fell to others. Euan Squires put a version in print in 1986, Hans Moravec described a “doomsday device” along the same lines soon after, and Max Tegmark gave it the clean formulation that everyone now argues about.³
The quantum gun
Tegmark’s apparatus is worth describing because it strips the idea to its bones. Picture a gun wired to a quantum measurement. Each time the trigger is pulled, the device measures the spin of a particle. Spin down, it fires. Spin up, it clicks. A clean fifty-fifty, and quantum rather than mechanical, so each pull puts the shooter into a superposition of dead and not-dead.⁴
To anyone watching, this is grim and brief. Click, click, bang. The experimenter is gone within a few pulls. But Tegmark’s point is about the experimenter’s own stream of experience. In half the branches at each pull, that stream ends. In the other half it continues. And a stream of experience cannot register its own ending. So the experimenter, following their own perspective forward, hears click after click after click. The gun, from the only seat they will ever sit in, never goes off.
Moravec had already noticed the loneliest implication. Everyone you love dies on schedule, from your point of view, because you are not riding their branches. You alone keep waking up. You become the one immortal at a funeral that never ends.
I remember finding this vertiginous. It has the shape of a proof. It uses real physics. And it seems to make death, the one fixed point, optional.
Where the spell breaks
Here is the move that took me longest to see, and it is the one that matters.
Not all branches are equal. This is not a footnote to the many-worlds picture, it is the centre of it. The branches carry weights, given by the squared amplitude of the wavefunction, and those weights are what the Born rule turns into probabilities. David Wallace, who has done more than anyone to make Everett’s picture rigorous, spends much of The Emergent Multiverse on exactly this: a rational agent living in a branching world is compelled, on pain of incoherence, to set their expectations according to branch weight.⁵
That changes everything about the gun. Yes, there is a branch where you survive a hundred pulls. Its weight is one in 2¹⁰⁰, a number so small it has no business in any decision you would actually make. Quantum immortality only sounds like immortality because it quietly stops counting. It says: ignore every branch where you died, then notice you are alive in what remains. But a rational person does not get to discard the overwhelming bulk of the probability and then act surprised at what is left. By any measure you would use for any other choice in your life, the surviving branch is not where you are. It is a rounding error you have promoted to a destiny.
So the honest reading is almost the reverse of the one that seduced me. You are not guaranteed to live. You are guaranteed that some unimaginably thin sliver of the wavefunction contains a survivor, while essentially all of the weight sits with the branches where the gun did exactly what guns do.
The state you wake up in
Suppose you grant the whole thing anyway. Suppose you insist on following the survivor. Look at what surviving involves.
A bullet has many more ways to leave you alive and ruined than alive and fine. The surviving branches are dominated by the maimed: brain-damaged, bleeding, conscious in ways you would not wish on anyone. Iterate the experiment and you do not drift towards eternal youth. You drift towards the narrowest, most broken corner of the possible, because that is where the survivors increasingly are. The fantasy sold you endless life. The mechanism, taken at its word, delivers something closer to endless injury.
Tegmark saw this too, and made a sharper point about ordinary death. The thought experiment works only because the gun is binary and instant: alive one moment, gone the next, with a clean branch point in between. Real dying is not like that. It is gradual, a slope of fading awareness rather than a switch.⁶ There is no crisp quantum coin flip between a living you and a dead one when the cause is age, or illness, or a body slowly giving out. So even on its own terms, the argument does not reach the conclusion people want from it. It does not make you immortal. At best it describes a strange edge case involving an idealised gun that does not exist.
Which multiverse, anyway
There is a deeper problem under all of this. Everett’s branching is one way to read quantum mechanics. It is not the only one, and it is not settled fact.
The mathematics that everyone agrees on is silent about what is really happening underneath. Copenhagen keeps the collapse. Bohmian mechanics adds definite particle positions guided by a pilot wave, with no branching needed. Objective-collapse models like GRW make the wavefunction physically collapse on its own. Relational and QBist readings make the whole question of a single objective state more subtle still. These are live disagreements among serious people, and quantum immortality is a conclusion you can only reach by first committing to one contested interpretation and then making a logical slip inside it.
It is worth saying, too, that the “infinite multiverse” of late-night conversation is a blend of several different ideas. The branches of Everett are not the bubble universes of cosmic inflation, which are not the same as the notion that every mathematical structure is its own world. The feeling that “everything is possible” borrows a little from each and checks none of them. Quantum immortality rides specifically on the first, and the first is exactly the one where branch weight quietly sinks the argument.
Back here
What is left, once the door closes again?
This branch. The one you are reading in. From where you stand, its weight is close to one, and the version of you sitting here is not a rounding error to itself. That is the part the fantasy gets backwards. Quantum immortality feels like an escape from a single finite life into infinite ones. What it does instead, followed through to the end, is hand you back the one life you have, and quietly remove the consolation that some other you carries on regardless.
I think that is why the idea is more comforting than true. It lets you treat this life as a draft, one timeline among countless others, none of them quite load-bearing. The maths it leans on says the opposite. The weight is here. The stakes are here. There is no thicker branch of you somewhere else making up for the thin one.
I am glad I walked through that door when I was young, and gladder I walked back out. The multiverse, if it is real, is not a safety net. It never promised you would land. The most it ever said was that somewhere, with vanishing weight, a version of you might. Everywhere that counts, the only honest plan is to live as though this is the world that happened. Because, by every measure you will ever use, it is.
Notes
Hugh Everett III, “’Relative State’ Formulation of Quantum Mechanics,” Reviews of Modern Physics 29 (1957), 454–462.
Peter Byrne, The Many Worlds of Hugh Everett III (Oxford University Press, 2010). Everett is reported to have privately held a version of the immortality view without ever publishing it.
Euan Squires, The Mystery of the Quantum World (1986); Hans Moravec, Mind Children (1988), the “doomsday device” scenario; Max Tegmark’s independent formulation followed in the late 1990s.
Max Tegmark, Our Mathematical Universe (2014), where he sets out the conditions a genuine quantum-suicide setup would have to meet: a truly quantum trigger, death faster than awareness of the outcome, and near-certain lethality if it fires.
David Wallace, The Emergent Multiverse: Quantum Theory according to the Everett Interpretation (Oxford University Press, 2012), including the treatment of quantum immortality and the Deutsch–Wallace decision-theoretic derivation of the Born rule.
Tegmark’s reply to the “subjective immortality from ordinary death” objection: dying is normally gradual, a continuum of decreasing consciousness rather than a single binary event, so the clean branch point the argument needs is absent.