It all began with the discovery, at the turn of the 20th century, of wave-particle duality. The discovery that light, which had classically been thought of as a wave, could be described as a particle, rapidly dubbed the photon, was exciting enough, but perhaps even more exciting was the prospect of describing particles as waves. Experiments were quickly done to demonstrate that particles exhibited wave-like behaviour, such as diffraction and interference, and in no time at all Schrödinger had developed his wave equation, which could be used to derive the mathematical wave function that would describe the behaviour of any particle. The exact significance of this wave function was established in the famous Solvay Conference of 1927 – that the wave function would allow one to determine the probability of finding the particle in a particular position, or having a particular quantity, or being in a particular quantum state. This fundamentally probabilistic interpretation thoroughly put out Einstein, who muttered that “God does not play dice”, but it was nonetheless broadly accepted, and had some rather curious consequences, none more so than the measurement postulate.
The measurement postulate goes thusly. If you have a particle that can be in a number of different states, it’s impossible for you to predict what state that particle will be in at a particular moment in time. All you can do is calculate the probability of finding it in a given state when you make a measurement to determine its state. Consequently, before you make that measurement, you can describe the particle as being in all possible states; this is fine mathematically, because waves can be superimposed upon each other to form what is called a wave packet. Then, when you make your measurement and you observe the particle being in a particular state, all the states that the particle isn’t in cancel out and the wave packet collapses into the single state that you’ve observed. This is a rather elegant bit of maths, but it does rather fly in the face of reason, and this is where Schrödinger’s Cat comes in.
So, let’s say you have a cat, said Schrödinger, and you place this cat in a lead casket. Also inside this casket you place a canister of cyanide which is connected to an apparatus containing a single, unstable particle, set up in such a way that when the particle decays the cyanide is released, poisoning the cat. As such, the cat’s life is directly connected to a spontaneous quantum event. You shut the casket. Now, the particle can be in two possible states: a decayed state and an undecayed state, and you have no way of knowing which. In keeping with the measurement postulate, you can describe the particle as being in both states, but in this case the welfare of the cat is connected to those states. If the particle is undecayed, the cat is alive; if the particle is decayed, the cat is dead. So, if you describe the particle as being in both decayed and undecayed states, then you’re also describing the cat as being in both dead and alive states. Of course, you’ll never see the cat in both states – any time you open the casket the wave packet collapses and you’ll see that the cat is either alive or dead, and indeed, you can calculate the probability of finding the cat in either state using the particle’s wave function. You can only describe the cat as being both dead and alive so long as you don’t look at it.
But this is absurd, I here you cry. Surely the cat is only ever either alive or dead, and all you have to do to find out which is look inside the casket. Of course, you're quite right. The purpose of Schrödinger’s thought experiment was not to assert that the cat is both dead and alive at the same time, but to demonstrate the absurdity of taking quantum mechanics at face value. But this has rather interesting implications for the nature of science in general.
Quantum mechanics clearly isn’t ‘true’ in the traditional sense of the word, since the way it operates doesn’t actually make sense. But it nonetheless works incredibly well when it comes to the business of describing the nature and behaviour of things that are very small. It seems to me that the further and deeper we delve into physics, the less interested our physics becomes in any kind of truth. Long gone are the days when we could say, with any degree of certainty, what is; now we can only talk about what works. And as our science slips deeper and deeper into an epistemological apathy, who’s to say what’s true at all, anymore, or whether it even matters? So with science, as with life. Shouldn’t we forget about the absolute assertions of truth and chase, instead, those butterfly dreams of what works? Let us embrace uncertainty, then – truths are too evanescent, anyway.
The illustration was drawn by the illustrious Kim Lee Dang, alias Nobo13, (2013)