double slit experiment from QUANTUM by jim al-khalili

Since i can only use adjectives such as weird strange and mysterious just so many times, i will waste no further time with this fanfare and get on with it. what i will describe is a real experiment. first, a beam of light is shone on a screen with two narrow slits in it that allow some of the 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. where the two wave crests meet (or troughs) meet, they combine to form a higher crest or lower trough that corresponds to a more intense light and hence a bright band on the screen. but where a crest of one wave corresponds with the trough of the other, they cancel each other out resulting in a dark patch. it is therefore only because light behaves as a wave washing through both slits simultaneously that the interference pattern appears.
a special atomic gun fires a beam of atoms at a screen with two narrow slits. on the other side, the second screen is treated with a coating that shows up a tiny bright spot whenever a single atom hits it.
of course, atoms should behave in a manner similar to sand, as opposed to spread-out waves, capable of overlapping both slits at once.
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.
nest we open the second slit and wait for the spots to appear on the screen. you would expect a cluster of spots to build up behind each slit like two piles of sand. well, surprise, surprise, atoms dont behave like this. instead we see an interference pattern of light and dark fringes just as we did with light. the brightest part of the screen is in the centre where we would not expect many atoms to be able to reach.
we could have a stab at what is going on. despite the atom being a localized particle (after all, each atom hits the screen at a single point), it seems that the stream of atoms have somehow conspired to behave in a way similar to a wave. they wash up against the first screen, and those that manage to get through the slits interfere with each other's paths. maybe the atoms bump into each other in a way so as to guide each other onto the screen. atoms (we would reason) are not like spread-out waves (such as light and sound).
so here is where this comforting prop is knocked away. to begin with, we see that the pattern of fringes on the back screen is connected somehow to the way the two waves interfere. just as with normal waves, its details depend on the width of the slits, the distance between them and how far away the back screen is.
this in itself is not proof that the atoms are behaving in a wavelike fashion. however, not only has the double-slit experiment been performed with atoms, but it has also been done by firing individual atoms one at a time! there is only ever one atom traveling through the apparatus at one time. each atom that manages to get through the slits leaves a tiny localized spot of light somewhere on the screen.
what we see is quite incredible. the spots gradually build up on the screen and light bands of an interference pattern slowly emerge where there is a high density of spots. in between these bands are dark regions where no or very few atoms land.
the interference pattern cannot be the result of collective behaviour. so what is going on? what makes this result particularly spectacular is that there are paces on the back screen where atoms were arriving when only one of the slits was opened. by opening the second slit we are providing another route for the atoms to go through, so you would expect to increase the chances of atoms reaching these places. instead, with both slits open no atoms arrive at all. somehow, if the atom really does go through only one slit then it must already know whether or not the other one is open and act accordingly!
how does a tiny atom, which leaves the gun as a particle and hits the screen at a definite point go through two slits at once!
With a detector in place we can lie in wait behind one of the slits and see what the atoms actually do. With this detector, the interference pattern disappears. it is as if the atoms do not wish to be caught in the act of going both ways at once, and only travel through one slit or the other. two bands form on the screen adjacent to the slits as a result of particle like behaviour, similar to what happens with sand. with the detector turned off, we have no knowledge of the route taken buy each atom. they revert to their mysterious wave-like behaviour and the interference pattern comes back. here, clearly the act of observing the atom is crucial.

Two roads diverged in a yellow wood,
And sorry i could not travel both
And be one traveller, long i stood
and looked down one as long as i could
to where it bent in the undergrowth;
(robert frost)

a single atom can travel down both road in frost's yellow wood. they can sample all possible experiences simultaneously. this is known as superposition.