WANTED: SCHRÖDINGER’S CAT. DEAD OR ALIVE. OR BOTH.
SCHRÖDINGER’S CAT DEAD OR ALIVE. OR BOTH.
Erwin Schrödinger evidently didn’t care too much about cats. But who can blame him when his attempt at a high-calibre joke went so long without appreciation. Ross Harper puts an end to the confusion and shows that physicists really need to work on their sense of humour.
KNOWN TO BE IN POSSESSION OF POISONOUS AND RADIOACTIVE SUBSTANCES. APPROACH WITH GREAT CAUTION.
Let’s get a cat. Now let’s stick it in a box. Now let’s throw in some confusion over whether or not the cat inside is living or dead. Congratulations! You’re an expert in quantum physics. Feel smarter? Didn’t think so.
The problem is, this frequently quoted thought experiment by Erwin Schrödinger is rarely fully understood (or at least not fully appreciated). The scenario was in fact designed to highlight a flaw in quantum physics, not as a tool to explain it, or to make things in any way clearer. Time and time again, however, this pesky little hypothetical is rote-learnt and regurgitated as a (perhaps ill-considered) display of knowledge. But no more, I say! This article should hopefully clear up a few things, and arm you with the ability to silence that annoying dinner party know-it-all. So what’s going on? Well, we take a cat and put it in a box – that much is true. But this box also contains a flask of poison. The flask is rigged to a device that will release the poison when one of the atoms in a small amount of radioactive substance (also inside the box) breaks down or ‘decays’. During the experiment, there is a chance that one of the atoms will decay, releasing the poison, and thus killing the cat. There is also the equally probable chance that no atom will decay (leaving our four-legged hero unharmed and free to continue wondering why it’s in a box at all). Upon closing the lid, we have no way of telling whether the cat is alive or dead. Either scenario is equally probable, and until we lift the lid to see, we just don’t know. Now comes the science. A fundamental principle of quantum physics is something called ‘quantum superposition’. This is the idea that, until measured, a system exists in all its theoretically possible states simultaneously. So something that is normally either black or white can instead be considered a kind of grey. In our case, the quantum superposition principle implies that, while in the box, the cat’s state is actually a combination of living and dead. This conclusion is obviously ludicrous. It goes against common sense and conjures up images of deranged cat-zombies clawing their way through cat flaps (when everyone knows cat-zombies don’t have claws). Unfortunately, most people don’t go much further with Schrödinger’s experiment, but walk away with a message that goes something like, “There’s this cat, and it’s kind of alive, but also kind of dead. Who knows? Science is crazy!” This interpretation isn’t exactly stretching the boundaries of knowledge. Clearly in today’s society, the juxtaposition of life and death is a nonsense easily swallowed. But Schrödinger was making an entirely different point altogether. The fact is, quantum mechanical effects are only really relevant on a very small scale – for example, at an atomic level. Electrons (the small particles whizzing around the centre of an atom) can be located in different positions, and have a different probability of being found in particular places at a given time. So taking into account quantum superposition, until it’s observed, we consider a single electron to be
everywhere, rather than in just one specific place. This has an important implication: by measuring the position of an electron we actually force it into that location, and in doing so, change the system. It’s a pretty cool idea. By simply observing the world around us, we are changing the way it acts (cue delusions of grandeur). So let’s get back to our cat-in-a-box. The creature’s life depends on whether or not a radioactive atom decays; the two are inextricably linked. At this small scale, the atom is governed by quantum effects, and can therefore be considered as both decayed and un-decayed
at the same time. But hang on a minute: this also means quantum effects will indirectly govern the cat, right? So, it too is in a state of… living death? Schrödinger is messing with us! He has purposefully paired a quantum system (the radioactive atom) with a more normal system (the cat) to leave us wondering. He raises an important question: when does a quantum system stop existing as a superposition of states and adopt just one? We’re not supposed to accept the conclusion of the experiment: we’re supposed to laugh at how ridiculous it is. Clearly there is still some fine-tuning to be done with quantum theory. As it happens, there are some problems with Schrödinger’s experiment – in particular, the question of what constitutes a measurement. We’re led to believe the only measurement taken is the action of us opening the box and looking inside (at which point, the system would be forced into one particular state). However, isn’t the poison-releasing device a piece of apparatus designed to measure radioactivity? And what about the cat itself? Is there a minimum level of intelligence required from the observer? The term ‘ measurement’ is ambiguous, and as a result, things start to fall apart a little bit. (The debate continues to rage today.) But don’t lose faith in old Erwin. He hasn’t failed us. Oh no. His aim was to spark discussion, and spark discussion he most certainly has. Just don’t ask him to look after your cat while you’re on holiday. So the next time you find yourself in the presence of an insufferable know-itall preaching the gospel of half-alive, half-dead cats, quoting Schrödinger, and explaining how complicated it all is and why you couldn’t possibly understand, just take a moment, furrow your brow and say, “Dude… it’s a joke”. [Disclaimer: No cats were harmed in the writing of this article.]