Confundo! Two slits and a…thing

For the duration of this article, I would request you to take a metaphorical spade, dig a six feet deep hole in the far recesses of your logical brain, and bury your common sense there. Because if we are to dive into the complexities of this particular theory, logic is the last thing of any help. The theory I am talking about is of course, quantum theory: the most confusing, most wibbly-wobbly, and therefore, in my opinion, the most astounding theory to grace the history of physics.

It all started with an experiment with light. You take a light beam, pass it through two slits and voila! Instead of the pattern your logical brain would suggest (fig i), you actually get something of the form of fig (ii).

Let there be light!
Let there be light!

Well, that’s because light behaves as a wave! You say. It’s a standard interference pattern! That’s all high school physics, I know. The experiment displayed the wave nature of light. But there was also a lot of evidence supporting that light is made up of particles called photons. (Links below if you are curious.) The question now was if light is made of photons and photons are indeed particles, if you send single photons through the slits, would you get the same result? The logical brain again says no. If you throw balls through two holes in a wall you would just get two piles of balls not a pattern. To which quantum theory just gives a smug smile. Ah logic. Ridiculously boring.

And then a team of scientists finally performed the experiment with photons, in Paris in the 1980s. Any guesses on the result? Ten points to Ravenclaw if you guessed that the pattern observed after sending millions of photons one by one through the slits was the same as the pattern in fig (ii). Now sit back and take a moment to comprehend these results. You send a photon through the silts. It hits a point on the screen or photographic plate or whatever leaving a mark. The second photon does the same but arrives at a different location. Then the third, then the fourth and so on until millions of photons later the pattern begins to emerge. It’s as if the photon has gone through both the slits at once, interfered with itself and arrived at a point. But even if the photon somehow performs this feat, how does it know exactly where to place itself? Why don’t all the photons follow the same trajectory and arrive at the same spot? And so the problem was laid out and physicists all over lost their nights’ sleep to come to a solution. And they did come to a conclusion which was somewhere along the lines of this:

'I can tell you what will happen, but I can't tell you why'
‘I can tell you what will happen, but I can’t tell you why’

Of course there were some people who just said maybe it’s a weird property of light. Maybe that is all there is to it. And quantum theory gave a condescending smile again. Because soon the experiment was being repeated with other particles, electrons and then finally atoms and the results were the same every single time. After the predictability of Newton’s laws, the uncertainty that quantum theory brought to the table changed forever the way we look at physics.

Until next time.

Links:

Photoelectric effect; Compton effect;

Most of the inspiration for this article comes from “Schrodinger’s kittens” by John Gribbin. Even if you don’t read the entire thing, I recommend reading the prologue.

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