A video going around TikTok has people confused about quantum immortality, and believing that it might mean that they will live forever.
"Quantum immortality suggests that nobody ever actually dies; that consciousness never experiences death," host of the Your One Black Friend podcast Joli Moli said on TikTok. "Instead, whenever you die in one universe your consciousness just gets transferred to a parallel universe where you survive."
In a fun thought much more linked to sci-fi than the singular sci, @Joli.Artist went on to speculate that people's consciousness could transfer from worlds where there was an apocalyptic event that wiped out all life, to one where that didn't take place. Here, Joli says, the only clues would be slight differences between the worlds and "new Mandela effects".
"You're going to find yourself on Reddit talking about 'since when did Pizza Hutt have two "T"s'," she said, adding that in this new reality, they would argue it had always had two of them.
She goes on to say that apocalypses happen all the time, such as the dinosaurs being wiped out 65 million years ago, citing as evidence for the multiverse theory that such a destructive event hasn't happened in the interim. You just keep going (getting older and older, surviving apocalypse after apocalypse?) until, presumably, only you remain.
While it's a nice (and then horrifying, if you really think it through) thought that you will never die, but simply switch to a universe where you haven't died, is there anything to it? Should you throw away your gym equipment and get into knife juggling without first googling how to do that?
Well, the idea of quantum immortality has been knocking around for a while, and is seen variously as untestable nonsense to an interesting thought experiment. As with all stories like this, it's easiest to go back to Schröedinger's cat.
According to the Copenhagen Interpretation of quantum mechanics, particles can exist in a "superposition" of several states at the same time. When the particle is observed or measured, however, the system collapses and the particle is found in just one of the possible states.
In 1933, Schröedinger proposed a thought experiment which he believed showed the absurdity of this, by scaling it up to the size of a cat. In the hypothetical experiment, a cat is placed in a box with radioactive material that has a 50/50 chance of decaying over an hour and being detected by a geiger counter which would smash a vial of poison if radioactive material was present, killing the cat dead.
The problem, as Schröedinger saw it, was that for an hour in that box – until the result is measured by opening up the box and having a glance at the annoyed and/or dead animal – the cat is both alive and dead at the same time.
One resolution to this – known as the "many-worlds interpretation" – gets around this problem by saying that both outcomes take place. In this hypothesis, all possible outcomes of quantum measurements take place across many, probably infinite, universes.
The theory, of course, is criticized as unfalsifiable – the universes would not interact with each other, making it impossible to test – and occasionally "one of the most implausible and unrealistic ideas in the history of science".
There is one way that has been proposed as a test, which is misinterpreted by some to mean that immortality is real, known as the "quantum suicide experiment".
In the quantum suicide experiment, the role of Schröedinger's cat is played by a human experimenter. There are various versions of it, with the pop-culture favorite being the "quantum gun".
In the "quantum gun" experiment, a gun is made so that it fires a bullet depending on two possible spins of a quantum particle. If the particle is measured to be in one position the gun will fire, if it is in the other there will just be a click.
The experimenter would then fire the quantum gun while pointing at a sandbag, hearing a mix of bangs and clicks. However, if the gun is then pointed at the experimenter by an assistant – and it can kill the experimenter in such a way that they die before they learn the outcome of the measurement – something else would happen, according to the many-worlds interpretation (MWI).
"Since there is exactly one observer having perceptions both before and after the trigger event," a paper on the topic reads, "and since it occurred too fast to notice, the MWI prediction is that [...] will hear 'click' with 100% certainty."
The observer at the barrel end of the experiment would only be able to observe clicks. They would hear click after click, perhaps moving away from the barrel to have it fire once more at the sandbags before moving their head in front of the barrel again and hearing more clicks.
This isn't to say that the experimenter survives from the perspective of everyone else. Each time the system collapses, the shot fires and doesn't fire, according to the MWI. So in most of these worlds, the experiment ends with a dead boss on the floor of the lab, while from the perspective of one version of the experimenter all the gun did was click. Unfortunately, this would only be useful information in that world, while the others would merely have seen a scientist die in a stupidly dangerous experiment.
The experiment setup, like the infamous cat experiment, was meant to scale up quantum outcomes to larger sizes. While some – including the origin of the MWI, Hugh Everett, who believed it up until his death – believe that it guarantees immortality, this is a huge leap from the effects of quantum mechanics that we don't understand yet.
For instance, it's not clear why – except in very specific experimental circumstances – your consciousness would transfer between worlds, just because you are dead in this one. While it may give comfort to know that there is another you out there who didn't die, what reason is there to believe that you only exist in the one where you are alive, rather than being another corpse to be cleared up in the universe where you didn't make it. As for the suggestion that we would know that we were in a different universe due to Mandela Effects, we are truly well into the area of science fiction and wild speculation.
In short: Quantum immortality? Don't stake your life on it.