The Double Slit Experiment

I simply want this to be expounded once in this blog.

I've alluded to the double slit experiment in some of my recent remarks on quantum theory and its philosophical overlay, and I even pasted an image of one hypothetical result of the experiment onto my most recent such discussion. But for this once I'll be explicit.

Imagine that electrons, or photons, are little tiny particles, as Democritus might have imagined. Imagine shooting them (in Chicago-movie machine-gun fashion) at a wall that had two parallel slits in it, and that there is a screen behind the wall, such that each particle that gets through the slit leaves a mark on the screen.

What would you expect to see on the other side of that wall? You'd likely expect to see marks directly behind each slit, corresponding top the shape and size of the split. Some portion of the screen that lay between the two slits would presumably be left unmarked.

That's not what happens. Consider light first. Thomas Young did this version of the experiment back in 1801, and he quite reasonably believed that he had proven that light is a wave, not a stream of particles. Because what shows up on the screen behind this slit wall is an "interference pattern,": that is, a pattern such as above indicating that two distinct waves passed through the wall, one originating (as far as the space behind the wall is concern) from each slit, and these two waves then each had an effect on the space between the slits, interfering with one another.

In the century or so between Young's work and the revolution in physics early in the 20th century, the general consensus was that matter and energy were very different sorts of stuff. One way of stating the difference is/was that energy consists of waves, matter consists of particles. But in 1927, with that revolution well underway, more than 20 years after Albert Einstein had given a formal mathematical equation for mass/energy equivalence, Davisson and Germer did the experiment with beams of electrons. Guess what ... they got an interference pattern.

So matter and energy aren't as different as all that, and electrons can at least in some contexts act like waves.

But things get stranger. When scientists included detectors at each of the slits, in order to determine whether a particular electron passed trough slit A or slit B, they noticed something very odd. The interference pattern disappeared. The screen showed instead mere pile-ups of markings behind each slit. In other words, the electrons, when observed in this manner, act like particles.

Now, imagine an electron that has just been released by the "gun" and has not yet reached the wall. Considering it as a particle, it is in the condition of Schrodinger's famous cat before the box is opened. It is both headed for the slit (it will get through) or headed for the wall (it will "die"). Until some macro-atomic entity -- a human, a computer, a detecting device, gets involved there is no good reason to say it is one or the other. The difference between the experiment run with and without detectors is one of WHEN macro-atomic entities get involved. If they don't get involved until a pattern appears on the screen: interference pattern. If they get involved earlier, when electrons are passing through the slits: two distinct pile-ups.

On the many-worlds interpretation of quantum physics, it is as soon as the macro-atomic object does get involved that the worlds split.  Other interpretations of the physics at stake have other ways of dealing with these strange experimental results.