Log in

No account? Create an account
delirium happy

Just keep on trying till you run out of cake

Previous Entry Share Next Entry
Schrödinger's cat
delirium happy
A while back, I posted about the possibility of writing about physics type stuff here in my journal. I actually had a bunch of stuff in my head for what I wanted to write, and had awritten somewhere around about 1000 words of the first entry, but then my computer died, making me lose what I'd written, and I lost heart. I also decided that I didn't want to write what I thought I'd wanted to write anyway, since I want to turn all that into a book (or failing that, a website) someday.

(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.

  • 1
I'm probably utterly wrong, but isn't it like the idea that you can keep halving a distance infinitely so that you can say mathematically that you could never actually get from point A to point B...whereas in "reality" of course you would get from point A to point B?

Sort of. Let me think of a closer analogy. OK. So imagine that ou're eating a sandwich. At the point before you start when the sandwich is sitting on a plate, the sandwich is independant and clearly not a a part of you. Whereas after you've eaten it and digested it, and the various molecules from the sandwich have gone all over your body to a whole host of different tissue, then the sandwich has ceased to be a sandwich and has become part of you. At some point in between, there must be a transition point where it goes from being "not you" to being "you", but trying to pin down exactly where that point is would be like grasping at smoke.

In a simmilar vein, a single nucleus is clearly quantum, whereas a cat is clearly not quantum. Somewhere in between there must be a transition point, but the Copenhagen interpretation (at least, the original version; there may have been updates and patches, I'm not sure) didn't have anything to say about where this transition point was.

Yay! I hope you'll do more posts like this.

What do you think of John Gribbin? About 7 or 8 years ago, when I thought I wanted to be a physicist, his books were my favorites. I've never asked someone who actually knows what they're doing whether he's any good, though!

I'm honestly not sure. I know I read some of his stuff around about 7 or 8 years ago as well (In Search of Schrodinger's Cat springs to mind) and I enjoyed them greatly back then, and they probably helped maintain my interest in physics, but I haven't read any of them since. The thing is, any book on quantum mechanics for a lay audience is going to have to rely upon the author's personal interpretation, and as I mentioned, those are never perfect. From what I remember his interpretations were fairly canonical though, which is probably a good thing. I'd have to read some of his books again before I could give a proper answer though.

  • 1