With two cats at home, it’s always a delight when they compete to rub themselves against my shin. So I sometimes wonder, why would anyone dream up a cat? “DD’s lost it", you’re thinking. But I would submit that the world’s best-known cat, at least among physicists, is an imaginary one.

And get this: she never lived, but people say she’s both dead and alive.

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This feline was the creation of the great German physicist Erwin Schrödinger, who died 50 years ago. Seeking to understand Heisenberg’s Uncertainty Principle, fundamental to modern physics, he thought of a cat. I mean, the last thing I expected to hear about in college was a purely hypothetical cat. Yet, generations of students the world over know this one well.

Stripped of mathematical jargon, the Uncertainty Principle says something simple: The act of measuring something affects that measurement. For example, it is impossible to determine both the location of an electron and the speed at which it moves. If you measure its speed accurately, this process of measurement itself makes its location hard to pin down, and vice versa. The uncertainty in one measurement, Heisenberg tells us, depends on the uncertainty in the other.

Looked at another way, measurement decides the state of the electron.

This is not such a mysterious idea. Imagine an anthropologist visiting a tribal village to study its inhabitants. His very presence will disturb the state of the village: We all behave differently when strangers come visiting. By observing, the anthropologist affects what he wants to observe. He never gets a “true" picture of the village.

Sure, but why is this important? Traditionally, physics nurtured the idea that nature’s laws tell us the past and future. If we can fully describe the state of the universe right now, for example, we can deduce its state at any other time. Heisenberg shattered this romantic notion. Not only is there uncertainty in the properties of things, the act of measuring properties itself increases uncertainty. You cannot determine the state of the universe at a given time; life is not predictable.

Now this is fine with tiny particles such as electrons that nobody can see anyway. What about ordinary objects?

What about, say, cats?

That very question occurred to Schrödinger. His famous thought experiment went something like this. Put Lajwanti, the cat, into a box. Also put in a device that, when turned on, might or might not emit a single electron. That is, over a minute, the chances are exactly 50-50 that it emits an electron. If it does, it also releases a poisonous spray, goodbye Lajwanti. If it doesn’t, she lives to fight another minute.

Seal the box and put it far enough away that you can’t tell what’s going on inside. Turn on the device for exactly one minute. What happens to the cat?

Trivial question, right? The answer: We don’t know. The Uncertainty Principle reminds us that we can’t predict the behaviour of the device: Even if we pinpoint the location of its every electron, we have no idea about their motions, no way to determine their behaviour during that minute, no way to tell if one will be emitted. Thus, we don’t know if Lajwanti is alive or dead.

Until, of course, we walk over to the box and open it to hear—let’s hope— the loud miaow of a bewildered cat. Only then do we actually know that she survived her uncertain ordeal.

With the box sealed, we know only that Lajwanti is either alive or dead. This must seem blindingly mundane. But it is entirely consistent with the laws of physics to think of her, before opening the box, as simultaneously alive and dead. Here’s the crucial idea: the act of opening the box and looking in on Lajwanti—taking a measurement, in other words—is what puts her definitely into one of those two states: alive, we hope.

What’s the point? What’s so profound about a cat shut into a box?

Well, there’s the effect of measurement, the idea of uncertainty, and more. But perhaps the deepest, yet simplest, point is this: Reality takes shape only when we observe it.

We know an electron is emitted only when we detect it. The anthropologist learns something about tribal customs only when he actually observes a tribe, even if that affects their behaviour. We find out poor Lajwanti’s fate only when we open Schrödinger’s box.

Haven’t we all wondered on these lines before? If I turn my back to the mirror, is my image really there? If there’s nobody to hear, does a tree that falls in a forest make a sound?

Is there reality without observation, existence without consciousness?

Schrödinger’s cat shows that the laws of physics might answer those questions with “no". That may be too extreme for people who believe reality surrounds them without needing to be looked at.

Then again, Lajwanti herself isn’t real.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers will explore the joy of mathematics, with occasional forays into other sciences. Comments are welcome at dilip@livemint.com