A while back I mentioned that quantum mechanics is beset by questions of interpretation: the mathematical relations in it correspond well to experiments. Better than any other area of physics, in fact, but that doesn't mean we know what they're telling us. They seem to suggest that systems are in multiple states ("superposition") until a mysterious event ("measurement") forces them to choose just one. As Wikipedia can attest, there's no shortage of suggestions for what's really going on there. The so-called Copenhagen interpretation has historically had the most weight, so much so that its philosophical precepts are often seamlessly integrated into the introductory QM curriculum to the extent that undergrads don't even know their "objective" physics is coming with a helping of philosophy (hint, kids: it always does). The squares of coefficients in expansions of a wave function (the piece of math that's taken to be a complete description of a physical system) are to be taken as the probability of the system collapsing into a given possible state during measurement. Thus the notion of a collapse is built right into that interpretation.
The interpretation I want to focus on here is called the many worlds interpretation (MWI). This one rejects the notion of a collapse from many states to one. In the MWI, there's nothing magical about measuring something, no (meta)physical difference between that state before and after. Instead of things being in a superposition of every possible state and then dropping out of it, states always remain in superpositions. Sometimes this is stated as the universe "splitting" whenever a measurement occurs but this is sloppy language that implies the existence of multiple universes. The split is merely the smooth evolution of wave functions simultaneously down different streams of possibility. MWI only involves one universe but there are many stories playing out concurrently (many superposed worlds) in that universe. Schrödinger's cat is dead and alive, both before and after we peek at it. In peeking, our state (I see it alive vs. I see it dead) gets mixed up with the state of the cat and we ourselves are in a superposition. We exist in many different worlds but since these worlds or branches of our existence can't interact we're only aware of one of the possibilities.
Now that we've got that background down, we're ready for the good stuff. There's a thought experiment, perhaps most closely associated with physicist Max Tegmark, called quantum suicide. It posits that if the MWI is correct suicide is pretty much impossible. Tegmark envisions a (pretty sick) experimental apparatus in which a machine gun is aimed at an experimenter and hooked up to a tool that measures the spin of a certain particle along a certain axis when the gun's trigger is pulled. If the spin of the particle is found to be up, the gun fires a single bullet. If the spin is down, nothing happens. The experimenter lives in one possible outcome, he dies in the other. So what happens when we try it out? Tegmark argues that we have a superposition of the two possibilities: 1) spin-up, experimenter dead and 2) spin-down, experimenter alive. However, since presumably conscious experience ceases in the first possibility, one's conscious experience can't travel down that road. That doesn't mean that this possibility doesn't occur (both possibilities play out), it means that you can't experience it. This is intimately related to that continuity of consciousness notion I've mentioned before.
In practice, this would imply that you--the experimenter--can pull the trigger as many times as you want, the gun will never seem to fire and kill you. Your lack of dying will get exceedingly improbable after a while, effectively proving the MWI to be the correct interpretation of quantum mechanics. However, someone watching you do the experiment can see you die (their conscious experience is free to flow down that branch). So while you can't experience a world in which you're dead, others can. Which intuitively makes sense. I should note that not everyone agrees with Tegmark's conclusions but let's ignore those buzzkills for a bit.
For a moment, let's accept the (admittedly absurd) conceit that the new Large Hadron Collider in France could destroy the world. What might we expect to see if MWI is correct? I assume this would be a bit like a massive quantum suicide experiment--or rather, more like a worldwide cult suicide attempt. If operating the LHC inevitably leads to the destruction of the planet and the death of everyone on earth, none of our conscious experiences could ever travel down a branch of existence in which the machine works. In other words, the only observation we could ever have is the machine breaking. Every single time we try to use it. Is this what we've observed every time we try to use it?
Well, it's hard to be sure because the first time they tried taking it out for a real spin last year they broke it.
On 19 September, just nine days after protons were circulated in both directions of the €3bn LHC, an electrical connection between a dipole magnet (one of 1232 that bend the protons around the ring) and a neighbouring quadrupole magnet (one of 392 that focus the proton beam) failed during circuit tests in the last of the LHC’s eight sectors. At the time, a current of 8.7 kA was being pushed through superconducting cables the width of a stick of chewing gum to generate the enormous magnetic fields required to bend protons at high energies.
But they're going to start it up again next month. So we'll see. If it breaks again and again maybe we'll have some slim evidence that 1) the LHC is destroying us all in another branch of reality and 2) MWI is correct, in the same way that an endless series of clicks from Tegmark's quantum gun would indicate that MWI is correct and he can never lose a game of quantum Russian roulette. Or it could mean that the LHC is a multi-billion dollar piece of shit.
Of course, most of this is utter nonsense. But everyone's allowed to be a crackpot once in a while.
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