The Quantum Immortality Experiment Nobody Is Allowed to Run

In August 1947, at the Nuremberg trials, American judges wrote the rules for what scientists can and can't do to your body in an experiment. Point five says no experiment should be run if there's reason to believe it will kill the person in it. Except, in the actual wording, perhaps in those experiments where the experimental physicians also serve as subjects. Fifty years later, a physicist described that kind of experiment. It could show whether you can ever experience your own death. But even if someone survived it, nobody else in the room would learn a thing. The physicist is Max Tegmark, and he put it in a paper he posted in 1997, which came out in the journal Fortschritte der Physik the next year. He called it quantum suicide. It's a thought experiment, something to reason through on paper, not something anyone should try. So picture a gun in a physics lab wired to a detector. Every time someone pulls the trigger the detector measures the spin of a single particle which has an even chance of coming up one way or the other one way the gun fires the other way you just hear a click put a sandbag in front of the barrel and you get a random string of bangs and clicks like a coin being flipped over and over nobody argues about that part now in the thought experiment the The experimenter tells her assistant to pull the trigger ten more times, and this time she takes the sandbag's place. If quantum physics works the way it's often taught, where a measurement picks one outcome at random and the other one is gone, she's very likely dead within the first few pulls. Surviving all ten is a one in one thousand and twenty-four shot. But in 1957, a physicist named Hugh Everett proposed that nothing gets picked. Both outcomes happen. And the world, including everyone in the lab, splits into versions that stop affecting each other. That's the many worlds interpretation. And under many worlds, every pull splits the lab. In some worlds, the gun fires. In one world, it clicks and clicks and clicks. And the only version of her still able to hear anything, hears 10 clicks in a row. So why would anyone even describe something this extreme? Well, Tegmark's paper says, the textbook version and many worlds make the same predictions for every experiment except this one. The clean, safe way to tell them apart is out of reach. According to the Stanford Encyclopedia of Philosophy, a decisive experiment would need two versions of something big something you could actually see to interfere with each other and that's an impossible task for today's technology there is a slow legal route in a 2007 piece for the journal Nature Tegmark wrote that many worlds can be falsified by future lab experiments because no matter how large a system physicists probe the theory says they will never see the wave function collapse but every result like that can only say collapse still hasn't shown up it can't hand anyone a final answer so the gun is the one shortcut Tegmark could come up with and there are three reasons nobody runs it the strangest of the three has nothing to do with rules the first reason is the one in the title go back to Nuremberg the doctor's trial put 23 German physicians and administrators in the dock and the verdict laid out 10 points that became known as the Nuremberg Code. The exception for doctors testing on themselves sits right there in point five. It looks like a door left open. A research review on scientists who experiment on themselves by Hanley, Baines and Church calls that clause the only known direct address to self-experimentation in medical ethics. The same review found no law that forces a scientist experimenting on themselves to check with an ethics committee first. And only about a third of the ethics committees that answered had a formal policy on it at all. So is the door actually open? Not really, because the other points are still standing right next to it. Point one, consent. she passes easily. It's her idea. But point six says the risk should never be bigger than the humanitarian importance of the problem the experiment solves. Think about what this one solves. If Tegmark is right, it settles an argument between two readings of the same equations. Readings that agree on every other result in physics. Nobody gets cured and nothing gets safer. Point 7 says researchers have to protect the subject against even remote possibilities of injury, disability, or death. The quantum gun is a machine whose entire job is to make death a possibility on every pull. And point 9 says the person in the experiment has to be free to end it. In this lab, half the time, the gun ends it for her. So an ethics board wouldn't be looking at a brave volunteer. here, it would be looking at a device built around death, answering a question that helps nobody. And that still isn't the part that stops physicists. Suppose someone ignores every rule and does it anyway, what does the rest of the world find out? Tegmark answered that himself in the same paper. His line is that you'd convince yourself many worlds is correct, but you could never convince anyone else. Stand next to the assistant for a second. From where the assistant is standing, every pull is a coin flip. Most of the time the experiment ends early, and it ends the way you'd expect. If it doesn't, if the assistant watches 10 clicks in a row, the assistant has seen something with odds of about 1 in 1024. That's rare. But it's the kind of rare that happens. Line up a couple thousand of these attempts in a couple thousand labs, and a survivor or two is exactly what the textbook version predicts. So the assistant writes down, got lucky. The only person who thinks anything got proven is the one who took the sandbags place. And science runs on other people checking. So say a second lab tries to repeat it. Same problem. The new volunteer either dies, which the textbook version predicted, or survives and convinces only herself. You could run it a hundred times in a hundred countries, and the pile of evidence that anyone else can look at never gets any bigger. And even the survivor's own experience is weaker evidence than it sounds. In that same nature piece, Tegmark describes a gambler watching a card that could fall either way. In either of the two worlds that result, the gambler is unaware of the other outcome and feels as if the card fell randomly. So from the inside, a universe that splits and a streak of plain luck feel exactly the same. That also answers something people ask about this idea all the time. If quantum immortality were real, wouldn't we see people who are 150 or 300 years old walking around? Well, no. From your side of things, everyone else keeps dying at the normal rate, because in the world you're watching, their luck runs out like it always does. The idea only ever makes its promise to the person on the inside. So there's no crowd of impossible old people out there to find. And the other question people ask, whether a strong, dreadful déjà vu could be a moment where you died in some other branch, runs into the same wall. In many worlds, once the branches split and the difference spreads into the world around you, they stop affecting each other in practice. No signal comes back from the branch where the gun fired. So there's nothing in the physics that could carry that feeling across. Which brings us to the third reason, and it's about whether the gun even works the way the idea needs it to. If you're finding this useful, hit subscribe. This channel keeps going after the strangest experiments in physics, space, and the brain, and follows them to where the evidence actually ends. Tegmark put three conditions on the experiment, and the author Colin Bruce quotes them in his 2004 book Schrödinger's Rabbits. first the random choice has to come from a quantum event not an ordinary one so that you really split into a dead version and a living version a random number from an ordinary computer program is really the result of fixed steps so there'd be no split at all just one world and one very bad outcome second it has to kill you or at least knock you unconscious faster than you can become aware of the result and third it has to be virtually certain to kill you not just injure you that That second condition is the sneaky one. If the bullet took even a moment to do its work, there'd be a version of you who hears the bang and knows what just happened. And that version is having an experience in the branch where the gun fired. The whole trick depends on the dying branches having nobody left in them to notice anything. Now look at how people actually die. Hospital ward isn't a clean click or bang. A car crash on a highway isn't certain. Almost no real death is instant, total, and set off by a single particle. So by Tegmark's own conditions, the gun in the lab is nothing like everyday life, and the idea that you personally can never die doesn't follow from the experiment at all. But one philosopher looked at that gap and saw something much worse. On the 27th of June 2001, in Canberra, the philosopher David Lewis gave the Jack Smart lecture about many worlds and Schrödinger's cat. It was published in 2004 in the Australasian Journal of Philosophy. Lewis walked through the logic of surviving in the branches where you don't die. He called the step that makes it work the corrected intensity rule. Here's what that rule does with the gun. After one pull, there are two branches with equal weight, one where she's dead and one where she heard a click. The normal rule says she should expect each about half the time. The corrected rule throws out the branch where she's dead, because she wouldn't experience anything there, and then shares out all the expectation among the branches where she's still around, which leaves only one outcome to expect, the click. And then Lewis wrote a sentence that most versions of this idea leave out. A terrifying corollary has gone unmentioned. If death usually isn't instant, then most of the branches where you survive are ones where you only barely make it. Lewis put it plainly, you should expect to suffer repeated harms. He means losing things one at a time, the people you love, your health, your body and your mind. And he sums it up in one line, eternal life on such terms amounts to a life of eternal torment. So think about what that does to the fantasy. Quantum immortality usually gets sold as living forever. Lewis took the same logic seriously and ended up with a hospital bed you never get to leave, while everyone you know dies on schedule around you. If you've ever survived something you really shouldn't have, a crash, a diagnosis, a close call you still think about, this is the version of the idea that makes that memory feel a lot less comforting. So the ethics say no, and the physics say the gun is nothing like real life. But there's one group you'd expect to say yes, and that's the people who actually believe in many worlds. If anyone should be willing to pull the trigger, it's them. They won't. The Stanford Encyclopedia's entry on the many worlds interpretation was written by Lev Vaidman, who defends the idea. In it, he goes through Quantum Russian Roulette with himself in the chair. In his version, surviving a round comes with a cash prize, which is why the living Lev is rich. And his answer is short. I should not agree to play Quantum Russian Roulette, because the measure of existence of worlds with Lev dead will be much larger than the measure of existence of the worlds with a rich and alive Lev. That phrase, measure of existence, is doing a lot of work, so here's what it means. In many worlds, not every branch counts the same. Each one carries a weight that comes straight out of the quantum math, and Vaidman says an observer should set their expectations for any experiment in proportion to those weights. It's the same number that tells a lab how often a result shows up in every normal measurement. His point is that you don't get to throw the heavy branches away just because you're not conscious in them. After 10 pulls of that gun, 1023 of every 1024 parts of that weight belong to worlds where the experimenter is dead. Her family is in those worlds. Her assistant is in those worlds too. So the people with the best reason to pull the trigger are the ones who believe the theory, and one of its own defenders is telling them to count the worlds where they're gone. And if all of this sounds like overthinking a thought experiment, that's a fair reaction. But a philosopher as careful as Lewis took it on in a major lecture for a reason. You can't disprove it by watching someone else, and the only person who could confirm it can't share what they saw so the experiment nobody is allowed to run turns out to be the one experiment nobody could learn from the rules block it real deaths don't fit it and even a perfect run would leave the answer locked inside one person's head looking exactly like luck to everyone watching it asks whether you can ever experience your own death and the only witness who could answer is the one person who can never report back. Every close call you've already lived through feels the same from the inside under both theories. And no lab and no committee on earth can tell you which one you're living in.