A light source illuminates two narrow adjacent slits in an otherwise impenetrable barrier, the image of the light that passes through the two slits is detected on a screen. Due to the wave-like nature of light both slits act as light sources which leads to two interfering waves behind. The result is a very characteristic interference pattern on the screen: white bands for constructive interference and black gaps where the waves cancel each other out (see this simulation). We are all familiar with that kind of interference from sound waves coming from two stereo speakers – the volume of the sound depends on where we are relative to the speakers, it is loudest in the middle.
If we were to carry out the same experiment with electrons that are usually treated as particles (like tiny little marbles), we would expect only 2 white bands on the screen as the particles go through either the left or the right slit and then hit the screen accordingly. However, we end up of with exactly the same interference pattern as we saw with light waves.
You might say “that's clear, the electrons bounce off each other behind the barrier thus creating some kind of interference pattern on the screen". But the interference pattern prevails even if we fire the electrons at the barrier one at a time, so there is no way they could bounce off each other. Obviously individual objects like electrons have nothing to interfere with, so how does the interference pattern come into existence?
Each electron is interfering with itself, one after another, producing an interference pattern in time!
Here is the explanation: each electron behaves like it would be going through both slits at the same time, an idea that contradicts our everyday experience of discrete objects. Clearly, the electrons that are usually treated as particles in this case behave like waves (more precisely waves of probability). This is a general phenomenon: subatomic particles sometimes behave like particles and sometimes like waves - this strange characteristics became known as 'particle-wave dualism'.
To understand this we have to apply the concept of a wave-function again, remember from the last post: the natural state aka wave-function of any given system is a super-position of all possible quantum states.
Let's translate that into what we encounter in the given experimental setup: the natural state of an electron passing through the two slits is a super-position of all possibilities: it goes through the right slit, it goes through the left slit, it goes through both slits, it goes through neither of the two slits – all at once.
It is those different possibilities or probability waves that are interfering with each other resulting in the interference pattern on the screen. Obviously the electron is not in a well-defined state while going through the slits, it acts as being a wave of probabilities.
In the last post we saw how an observer would lead to a collapse of the wave-function, so what do you think is the effect of say a detector which we place at one of the two slits? The detector measures what the electron “really does”, whether the electron goes through only one of the slits (particle nature) or through both (wave nature).
Magically, the moment we switch on the detector the interference pattern disappears and two white bands emerge – exactly like you would expect from individual particles passing through one of the two slits. There's no more interference of probability waves, in other words: the wave-function collapsed, the electron behaves as a particle this time.
In this altered experimental setup (which now includes performing a measurement at one of the two slits) the probabilistic wave nature of the electrons is eliminated : only the particle nature remains and individual particles cannot produce interference patterns.
It seems that if the experimentalist choses a setup that has a more deterministic touch (measurement via detector), then the particle nature is found. Without the detector (i.e. the experimentalist does not measure which slit the electron goes through) the probabilistic wave nature is found.
A quantum entity such as an electron has a dual potential nature, but its actual (observed) nature is one or the other depending on the way it is measured. It seems that somehow the electron 'knows' what the experimentalist intends to measure and then behaves accordingly. I wonder how it does that trick ...
Clearly the double-slit experiment indicates that there is a much deeper relationship between the observer and the observed system, at least at the subatomic level, which is an extreme break from the idea of an objective reality.
But who am I telling? Meditators know, or should I say experience this for thousands of years already.
Here is a very nice animation of this wave/particle dualism and the effect of an observer, taken from 'What the bleep do we know', part 2:
Some remark on the side: The fact that sub-atomic 'particles' like electrons, photons or protons behave like probability waves if you don't look is the reason why the sun is able to produce light through nuclear fusion (although it's actually not hot enough to do so) or plants produce oxygen via photo synthesis. Without that seemingly nerdy behaviour of those particle-waves life on earth would not be possible.
The last post in this series describes an extension of that experiment to demonstrate how causality breaks down on a quantum level .... now that is REALLY weird stuff :-)