(Aside: note to self: tags are really useful for finding old stuff; start using them again.)
But anyway, I'm now revisitng the idea -- mainly because there's one specific thing I want to write about right now -- though I think that anything I do write will be much less structured and more random than what I had been thinking about last time. Of course, since I don't think I ever mentioned just what I was thinking of writing last time, that doesn't mean much to you.
What I want to write about today is Schrödinger's cat. This is one of those bits of physics that nearly everyone seems to know about in broad outline, but far too many have a tendency to get the details horribly wrong. Here's how it goes: you put a cat inside a box with a radioactive nucleus of a known half-life, then have a Geiger counter hooked up to detect the potential nuclear decay, and if this exists, to smash a vial releasing some poison gas to kill the cat. You then wait for the half life to pass, which would classically give you an equal probability of finding the cat alive or dead. But according to quantum mechanics, the nucleus can exist in a supposition of states where it is simultaneously both decayed and not decayed. Therefore, by extention, the cat is simultaneously both alive and dead (and bloody furious) and ho ho ho, aren't those quantum physicists a whacky bunch?
Only it's not that simple. Unsurprisingly. See, quantum mechanics is, at heart, a mathematical beast. The maths behind it all is very beautiful, but almost completely divorced from anything that we would think of as reality. The only thing is, though, that it works. It makes predictions totally out of line with classical physics, and when they get tested experimentally, lo and behold, the quantum ones turn out to be right.
So we go back in time, to the early decades of the twentieth century. The idea of quantisation had recently been introduced to help explain some previously problematic areas (black body radiation and the photoelectric effect, for instance), and a bunch of really smart people (Bohr, Heisenber, Schrödinger, Dirac, et al) took this idea and started running with it. They took the idea of quantisation as a basic tenet and tried to figure out what consequences that must have. And in doing so they created a whole new branch of physics, and discovered a vast rannge of new and interesting things. Which was neat..
The problem was that they went off and created a whole bunch of really nice maths, and they understood the maths and they knew that it worked, but they didn't really know what it all meant. Given how seriously weird some of it was, this is hardly surprising. In fact, to this day nobody really and truly knows with certainty what it all actually means in real terms. See for instance wikipedia's page on interpretations of quantum mechanics which demonstrates the lack of consensus (my own personal viewpoint, for what it's worth, is mainly in line with the Copenhagen interpretation, seasoned with a heavy dose of holism and just a pinch of solipsism at which point it veers off sharply into the realms of philosophy).
So we have these nice mathematical models which we know work, and we have a bunch of people trying to explain them in a way that meshes them with reality. And these interpretation always have flaws and ommisions and are generally less than perfect. And here is where we get back to Schrödinger's poor, abused little kitty. The thought experiment is intended not to say that the cat really would be both alive and dead simultaneously, but to point out one of the holes in the Copenhagen interpretation, concerning the boundary between classical macroscopic systems, and quantum sub-atomic ones.
Edit: And having posted this, I've realised that I'm not remotely happy with it, and that I probably should have written it at a time when I wasn't horrendously tired. But now it's here, it may as well live.