Forget nine lives; if one interpretation of quantum mechanics is right, the cat might have an infinite number of them.
You've never seen a half-dead, half-living cat. You might even want to say, "well clearly quantum mechanics doesn't work because I've never seen a zombie cat, so let's throw out the whole thing." But the problem is that quantum mechanics makes the best predictions of any physical theory of the universe we've ever had (like, literally ever). Somehow, we're going to have to explain why quantum mechanics says there should be zombie cats, when no one has ever seen one. And that's what a guy called Niels Bohr wanted to figure out. He came up with the first real attempt at explaining the zombie cat problem. There must be something special about us, that forces the cat to choose its state (either dead or alive) when we look at it, the cat must be forced to collapse out of its simultaneous alive/dead state and becomes either alive or dead (not both)."Although this does explain why we never see zombie cats, many people today see it as an unnecessary bit of magic to introduce into our laws of nature. Bohr's explanation had some holes in it that no one could quite plug:
(1) What makes humans so special that they can force quantum system (like our electron/detector/gun/cat system) to collapse into a single, well-defined state (like "dead" or "alive")?
(2) Does the cat have the power to collapse the state of the electron/detector/gun combination? Would a monkey?
(3) How about the gun or the detector? Why can't they collapse the state of the electron?
Some people started throwing around some pretty New Agey terms in response to (1), It’s human consciousness that collapses the zombie cat into its alive or dead state! "Leaving aside the fact that it's totally unclear what anyone might mean by "consciousness" or "observer", we still have problems (2) and (3) to deal with. So how do we get mind magic out of the equation? One Word - MULTIVERSE.
We're going to take an electron a tiny particle that can spin either clockwise or counter clockwise and connect it to a gun that's pointing at your head. If the electron is spinning clockwise, a signal will be sent to the gun, and the gun will go off and kill you. If the electron spins counter clockwise, the gun won't go off, and you'll live. If the electron had been spinning counter clockwise, the situation would have been a bit simpler. It turns out that tiny particles like electrons have a weird superpower: they can actually spin both clockwise and counter clockwise at the same time. The best way to picture this is to imagine drawing an analogy with colours: if clockwise is white, and counter clockwise is black, then electrons can be grey. Now let's look at our suicide experiment again, and see what happens if we start our electron off spinning in two directions at once: What happens next? According to quantum mechanics, it turns out that the gun will get "split" into two versions of itself by our electron. One version of it will get the signal from the clockwise spinning electron and go off, and the other won't. The bullet is now flying at top speed (and at the same time, not flying at all), heading straight at you. So, what will happen to you? Just like the gun, the laws of quantum mechanics say you'll be split in two. One version of you gets killed, the other doesn't.
What we've effectively got here are two parallel universes: one in which the electron was spinning clockwise, the gun went off and you were killed, and another in which the spin was counter clockwise, and you survived. This whole setup an electron spinning in two directions at the same time, connected to a gun which is aimed right at you is known as a "quantum suicide" experiment. In one case, you'd feel perfectly fine, having, quite literally, dodged a bullet. And in the other? Well, you can't possibly experience what it's like to be the "dead you" in the quantum suicide experiment, since the dead you has no experience of the world at all. There's no mind to inhabit, since it’s been blown to bits and no consciousness to experience. In fact, the only universe you'll ever be around to witness is the one in which you survive the experiment. So the outcome you're guaranteed to perceive when you run the quantum suicide experiment is the one where you survive. Now imagine taking a new electron, and trying this experiment again. According to quantum immortality, you should also experience the survival outcome in this second round of the experiment. Your conscious awareness would flow only to the living copies, and we could survive any number of potentially hazardous events related to quantum transitions. Everett reportedly believed in this kind of "quantum immortality:' Fourteen years after his death in 1982, his daughter Liz took her own life, explaining in her suicide note that in some branch of the universe, she hoped to reunite with her father.
In fact, if you repeat this experiment 100 times, you should experience the survival outcome each time. You're destined to experience walking away unscathed against all odds (the odds in this case being 1 in 2¹⁰⁰, or about one in 1300000000000000000000000000000). A cool consequence of this is that the quantum suicide experiment could actually allow you to prove to yourself that parallel universes exist. Here's how that would work:
(1)-If there actually exists only one universe, you should expect to die with 50% probability after each experimental run. Repeat the quantum suicide experiment a dozen times, and you're virtually guaranteed to die.
(2)-But if there actually do exist universes with survival outcomes for every round of the experiment, the door for quantum immortality is left open.
So surviving all the way through 100 or more rounds of the quantum suicide experiment would essentially prove to you that parallel universes are real. Ok, so you just got excited about quantum immortality and secured the research grant you needed to build your quantum suicide gadget. You hop inside, and run 100 rounds of the experiment, and come out ecstatic: you've proved that parallel universes exist, and now it's time to tell the whole world and pick up your Nobel Prize! Not so fast. To other people, who weren't in the quantum suicide device with you, you'll certainly seem incredibly lucky.
But no matter how many times you survive the experiment, that's all you'll ever seem to them, because if you try to repeat it another hundred times, you're almost guaranteed to die, from their point of view. To everyone who isn't you, the "dead you" outcomes will certainly seem every bit as real as the "living you" outcome. So although you'll perceive that you survived every round of the experiment, everyone else will see you die in the vast majority of universes. But what about the observers in the universe where you survive all the way through the 100 rounds of the experiment? Would they believe you? Unfortunately not. Raising one eyebrow, they'd turn to you with scepticism and say, "I could believe you're really, really lucky. But if you're actually immortal, you'll be able to repeat that experiment another 100 times:' And if you did, the vast majority of their future experiences would involve staring smugly at your limp corpse, only wishing you were alive so they could say "I told you so" (physicists are assholes). So even if quantum immortality is a thing, you'd only ever be able to prove it to yourself. To everyone else, you'll either seem like one lucky bastard, or a very dead idiot. Some people take quantum immortality even further: every heart attack, cancer, bullet wound, etc, that might spell the end of you is theoretically the product of a huge number of cellular and ultimately, molecular and even subatomic events.
An electron, nudged to the left or to the right, might cause a cancer gene that would otherwise have killed you to stay dormant, for example. And because molecular and subatomic events are governed by the laws of quantum mechanics, those events will also lead to splitting universes in just the same way as our "grey" electron did, leaving the door open to the quantum immortality effect. So hard-core fans of quantum immortality say that we're all destined to experience eventually becoming the oldest person on earth outliving our friends and family, and eventually even the human species and planet Earth as a whole. What's so hard to believe about parallel universes and quantum immortality, you ask? One problem is that the case for quantum immortality depends entirely on the instantaneous death of the experimenter. Think of the case where the gun goes off: for several hundred milliseconds after the gun is fired, you're still alive and fully conscious. So there actually is something to experience in that timeline even if it's just a few fleeting moments of awareness. That's enough to challenge the notion that that timeline is a complete nothing burger of subjective experience. As a result, it's argued, your subjective experience can absolutely fall into the "dead you" timeline, and once it has, there's no escaping your ultimate fate. But the quantum immortality diehards shot back: "Fine, we just have to design an experiment that kills you instantly. No more Mr. Nice Guy: the next time we run this thought experiment, it'll be with a laser beam and not a gun.”
Some say it's also unclear whether the state of "being vaporised" is a valid state of human existence, too. You could certainly argue that there will be versions of you that will "experience" nothingness, whatever that feels like. So it's pretty unclear what kinds of guarantees can be made about your subjective experience of the quantum immortality experiment, at all. The jury's still out on the case for quantum immortality. And the only way to know if it's true is to build a contraption that's currently beyond the reach of modern technology (and which may even be impossible), and to put yourself directly in its line of fire. Suppose you do somehow manage to achieve quantum immortality with your conscious existence following each auspicious branch. You would eventually outlive all your friends and family members because in your web of branches you would eventually encounter copies of them that didn't survive. Quantum immortality would be lonely indeed!
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