Rosio Pavoris

I said I was going to do this, so I am~

The double-slit experiment is a classic experiment, first done in 1801 by the English Thomas Young, who was looking to solve the issue of whether light is particles or waves once and for all. Originally it was done with light, but it can be done with electrons or protons or neutrons or whatever as well.
First, let’s look at our set-up. It’s a box.

If we’re dealing with light, it’s a simple box out of which no light can escape. If we’re dealing with electrons, it’s a conductive plate over which the electrons can travel.
The important part is that O is the source of light or electricity or whatever we’re dealing with, and S1 and S2 are slits or openings in a central wall through which lights or electrons can travel. The back (green, here) is a detector type plate, which registers when light or electrons hit it.

Let’s think about what behavior we should expect, here.
If we’re dealing with particles, we can expect that O would fire a particle in a random direction, and if it happens to be aimed at a slit, it will perhaps bounce off the slit and hit the detector plate behind it. There will always be just one impact on the detector plate at a time.

You can see the area we’d expect the most particles to land in that picture there.
If we count the number of particles that hit a certain position and plot it on a graph, we get something similar to the one on the right. P₁ would be the particles that went through S1, and P₂ would be the ones that went through S2.
Of course, we can’t really know, when we look at a place where an electron hit, which slit it went through, so the actual graph will be the sum of those graphs, with two obvious peaks.
(I stole the graph picture from a scan of a Physics textbook someone put online, since I don’t really have the software to plot this sort of thing. This explains why it’s shoddy-looking.)

Now, what if it’s a wave? Well, it’d look a bit more complicated.

As you can see, a new wave would be started at each slit simultaneously, and this leads to a complicated-looking interference pattern, with light hitting the detector plate continuously.
The result looks something like the graph on the right. P₁₂ is the sum of two wave patterns. In some places the waves cancel each other out, in others they amplify each other, as you’ve no doubt seen in highschool Physics or Mathematics.
It’s a beautiful interference pattern.

It’s quite meaningless to talk about which slit every thing that was detected came through, since we aren’t dealing with discrete chunks.

Now, if we actually perform the double-slit experiment, the pattern we get on the detector looks something like this:

Obviously a wave interference pattern. Problem solved, then, right? Light is a wave!
Well, no, not quite.

See, we know light exists as particles, since we know about photons through things like the photoelectric effect. And electrons make the same pattern, and we know those are particles too. So why does it do this?

Another experiment we could try is to do this with electrons, but this time, we’re going to keep track of which hole each electron went through. We could do this by, for example, shining a really bright light in the first chamber, and looking at the slits from the other side. If the light in one slit dims for a bit, we know an electron has passed through, since it blocked out the light when it passed.

Someone did an experiment similar to this, and found something very interesting: when we watch the electrons like this, they generate a pattern like in graph 1.
If we keep track of which slit the electrons pass through, they behave like particles. If we don’t, they behave like waves.

And this isn’t just true when we use this particular set-up, in case you’re concerned that since light is so unusual itself, it may have done something weird to the electrons. If we observe the path of the electron in any way, it stops behaving like a wave, and acts like a normal particle.

So why does it do this?

I’m not actually sure.
The Copenhagen interpretation (remember Heisenberg and Bohr?) posits the existence of probability waves, essentially meaning that until the position of a particle is determined, it essentially exists in all places at once, just with a higher probability in some areas.
So unless someone actually determines which slit the particle goes through, it essentially goes through both slits at once, which creates this wave-like interference pattern.

It’s all very interesting, but unfortunately it also means Physics is incredibly counter-intuitive and hard to approach without explicitly using mathematics, which makes the whole field a bit inaccessible to the common man.