Really interesting example of quantum mechanics from the book

This is quite a long extract so I'll try to cut it down to the most relevant parts. There are some really useful diagrams that I may take pictures of and blog them because they explain this very well. I cannot upload photos onto this blog via my mobile though, so will have to do it another time.

"First, a beam if light is shone on a screen with two narrow slits in it that allow some light to pass through to a second screen where an interference pattern is seen. This is a sequence of light and dark bands that are due to the way the separate light waves emerging from the two slits spread out, overlap and merge before hitting the back screen."

I remember being shown this experiment in a gcse science lesson.

"Next, a similar experiment is carried out using sand. This time the second screen is placed below the one with the slits and gravity does the work. As the sand falls onto the first screen, separate piles gradually build up on the lower one beneath the two slits. This is not surprising since each grain of sand must pass through one or the other of the two slits; we are not dealing with waxes now and there is no interference. The two piles if sand will be if the same height provided the two slits are of the same size and the sand is poured from a position above their mid-point."

"Now for the interesting part: repeating the trick with atoms. A special apparatus - let us call it an atomic gun for want of a better name - fires a beam if atoms at a screen with two appropriately narrow slits. On the other side, the second screen is treated with coating that shows up a tiny bright spot wherever a single atom hits it. [...] First, we run the experiment with just one slit open. Not surprisingly, we get a spread of light spots on the back screen behind the open slit. [...] Next, we open the second slit and wait for the spots to appear on the screen. If I asked you now to predict the distribution if the bright spots that build up you would naturally guess that it would look like the two piles on sand. [...]

Instead, we see an interference pattern of light and dark fringes just as we did with light. [...]
With a detector in place that records which slit each atom passes through, the interference pattern disappears. It is as though the atoms do not wish to be caught in the act of going both ways at once, and only travel through one slit of the other. Two bands form on the screen adjacent to the slits as a result of particle-like behaviour, similar to what happens with the sand.

With the detector turned off we now have no knowledge of the route taken by each atom. Now that their secret is safe, the atoms revert to their mysterious wave-like behaviour and the interference pattern comes back!"

I find this so fascinating! I dont even know what to say in order to evaluate this extract.

"How can we assess the legitimacy or truth of an account of a phenomenon that we can never, even in principle, check? As soon as we try, we alter the outcome. [...] Physicists have been forces to admit that, in the case of the double slit trick, there is no rational way out. We can explain what we see but not why. However strange you may find the predictions of quantum mechanics, it must be emphasized that it is not the theory - mankind's invention - that is strange, but rather Nature herself that insists on such a strange kind of reality on the microscopic scale".