Famous Slit Experiment

I have been thinking about your elderly couple of electrons, entering the lecture theatre via one of two doors, I said to Pliny. And it reminds me of the famous Slit Experiment in Quantum Mechanics.

Ah, said Pliny, I know the experiment you mean. When electrons are fired at a screen through one slit they behave like particles, but when they are fired through two slits they behave like waves.

That is the one, I said. You didn't happen to observe whether the elderly couple came in both doors at once, did you?

No, said Pliny, more's the pity. I wasn't looking at the other door.

What a shame, I said. But of course if you had been you would not have noticed anything untoward. The rest of the famous Slit Experiment involves an observer observing one of the slits. And the amazing thing is that when there is an observer, the particles revert to behaving like particles again.

That is not the amazing thing, said Pliny. The amazing thing is how, when you are not watching, they do not.

True, I said. Did you at any point stop watching?

Almost at once, said Pliny. Because the lecture was so interesting.

What interests me is that if the elderly couple had entered unobserved they could well have entered by both doors, like waves, I said. And then of course they would have been able to occupy multiple seats.

Good heavens, said Pliny. Now that I recall, the lecture theatre did seem to contain an inordinate number of them.