Who is Schrodinger's cat? Arguably the world's most famous purely hypothetical feline. Never lived, but some say he's both dead and alive. At the same time. Ask your nearest physicist.
Erwin Schrodinger was a Nobel winning German physicist who died in 1961. The cat was part of a thought experiment he devised to explain one of the fundamental ideas of modern physics: Heisenberg's Uncertainty Principle.
Shorn of jargon, the Uncertainty Principle says something very simple: the act of measuring something changes the result of that measurement. Heisenberg showed that simultaneously determining both the position of an electron and the speed at which it is moving is impossible. If you can measure its speed accurately, that measurement will itself make its location wildly uncertain. And vice versa.
Put another way, measurement decides the state of the electron.
This is not such an esoteric idea. Examples abound, and not just among electrons. Imagine an anthropologist visiting a remote tribal village to study its inhabitants. His very presence disturbs the villagers, who will behave differently with this stranger in their midst. So by simply observing, the anthropologist affects what he wants to observe; and thus can never hope to get a true picture of life there.
This is all very well with tiny particles nobody can see anyway, and maybe also with distant tribals. But what about everyday objects around us? What about, say, cats?
Well, that very question occured to Schrodinger. His famous thought experiment goes something like this. Let's say we have a sealed box with a cat in it. Also in the box is a device that can randomly emit marbles. In the course of a minute, the chances are exactly 50-50 that it emits one. If it does, the marble breaks a vial and releases a poisonous gas into the box. Kitty is instantly asphyxiated. Otherwise, nothing happens.
We put the box somewhere far away, where we have no way to tell what's going on inside it. Suppose we turn on the device for exactly one minute. Question: what happens to the cat?
It must seem like a trivial question. The answer is that we don't know. We cannot predict whether a marble was actually emitted. So we don't know if the cat is alive or dead.
But if we walk up to the box and open it to hear -- let's hope -- the loud miaow of a very puzzled cat, only then do we actually know that it has survived its uncertain ordeal.
Before then, the best we can say about the cat is the non-sequitur that it is either alive or dead. But that's not really such a non-sequitur. It is entirely consistent with the laws of physics to think of the cat, before we open its box, as being both alive and dead, with a probability of 50 per cent for each state. Here's the point of the experiment: our act of opening the box and observing the cat -- taking a measurement, in other words -- is what puts the cat definitely into one of those states.
So what's the point, you want to know. What's so earth-shaking about this cat shut in a box?
There are many points, actually: the effect of measurement, the idea of uncertainty, the fact of indeterminacy (of that, perhaps another time). But probably the deepest and yet simplest point is this interesting view of the world: reality takes shape only when, precisely when, we sense it. Until then, it's uncertain. That's the Principle.
The anthropologist gets a picture of tribal behaviour only when he actually observes them, even if that changes the way they behave. We really know the fate of that poor cat only when we open Schrodinger's box.
All of us have wondered on these lines. Is my image in a mirror really there if I cannot see the mirror -- if I've turned my back to it, for example? Does a tree falling in a forest make a sound, if nobody is there to hear it?
Is there reality without observation, existence without consciousness?
Schrodinger's cat showed that the laws of physics might answer that last question with "no". That may be too extreme a view for most people's tastes, people who believe reality surrounds them without needing to be looked at. Then again, Schrodinger's cat wasn't real himself.