Strangeness of matter at the level of atoms

Go to interactive animation of  the double slit experiment for electrons
This interactive animation was done using paperjs.  It will only work on browsers supporting HTML5 canvas

The behavior of matter and its movement in the macroscopic world around us, forges in our brains, quite early in our lives, notions of what particles and waves are. Simple observations reinforce the distinction between  an object  and a wave - such as the  flight  of a ball  we throw up into the air or the wavy ripples  in a pond when we drop a pebble. A clear line of distinction emerges in our perception of behavior of  an object in comparison to a wave, one of which is the adding up and cancelling nature of waves ( think sea shore - waves either add up to form bigger waves or cancel each other). An individual object in contrast, doesn't  add and cancel itself like collections of moving water molecules do as in the case of waves.

 This distinction is blurred when we zoom into the microscopic world of atoms. Atoms (and particles that go to make atoms) seem to behave both like particles and waves. A simple experiment that shows this blurring of particle and wave boundary, is a double slit experiment done with electrons. A striking aspect of this experiment is that a single particle (not a collection, just one particle) such as an electron can exhibit wave like properties, that is, it can add and cancel with itself. Are you wondering, "what on earth does that mean?". Well, many of us thought exactly the same way.

The crux of the experiment illustrated below is an electron that is shot out from an electron gun, one at a time and has two potential pathways (two holes in an obstructive plate) before it can get to a screen where its impact is recorded (of course there are many potential pathways such as striking the plate etc., but I am ignoring them - not relevant to this experiment). Each electron is alone in transit from the time it leaves the electron gun to the point it impacts the screen, so there is no scope for interaction with other electrons or any other matter (the experiment is done in vacuum). The impact location of electrons on the screen, after several runs of this experiment, each time with one electron in transit, yields an impact pattern on the screen that is identical to a pattern that would be if two water waves went through the holes and interfered with each other at the point of contact with the screen - a pattern of adding and cancelling of waves. That is, an electron would never impact a location on the screen where two waves cancel each other - it would only impact the screen where two waves add up. This seems to indicate that a lone electron, while it only takes one path physically (easy to prove - described in the animation), seems to "virtually" take all potential paths simultaneously like a wave would, and in those places where the waves would cancel at the point of impact on the screen, an electron would never show up! And even more striking is that, the instant we place a detector to know which path it physically takes, it drops its wave like behavior and leaves an impact pattern on the screen like an object in the macroscopic world would do (say tennis balls in an equivalent experiment with two holes big enough to let them through ). This experiment is not a thought experiment. It was successfully performed in 1989 by Akira Tonomura and his team from the Hitachi Advanced Research Laboratory in Japan. There is a video of this experiment on youtube.

This bizarre behavior of matter at the atomic level has been used to great advantage by many to imbue a mystic quality to nature, but thanks to physicists, we can explain it within the domain of rational reasoning. However, unlike the explanation of most physical phenomena, where we can explain "why something happens a particular way", in this case of the strange behavior of matter that we are about to see, we have no answer, from a physical model point of view, to the question, "why is it that way?" . Instead, we have to rely on mathematics to explain the strange behavior.

In Feynman's own words,  'I am going to tell you what nature behaves like. If you will simply admit that maybe she does behave like this, you will find her a delightful and entrancing thing. Do not keep saying to yourself, if you could possibly avoid it, 'but how can it be like that?' because you will get 'down the drain' into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that.' - from the Character of Physical Law

While the paragraph above hardly sounds like anything that one would expect from one of the greatest physicists of our time, the inability to offer a good model explaining the reason for this strangeness, is offset, to some degree, by a surprisingly unorthodox method Feynman himself devised (almost quirky in a way - twiddling with little arrows that represent the likelihood of an event, and connecting them together), that can explain the strange behavior of matter at the atomic level (this work won him the Nobel Prize for Physics in 1965). Not just the strangeness of matter - strangeness of light too, as we shall see in future. While his original work requires a lot of mathematics to comprehend, he wrote a book QED, for everyone to understand - it is a remarkable book, just remarkable.

With that said, the animation of double slit experiment illustrated in the link below, is based on the simple and clear explanation of the experiment by Jonathan Allday in his book Quarks, Leptons and the Big Bang. He has also written a book recently,  Quantum reality - theory and philosopy  that describes a modern version of the original Young's double slit experiment, done with light (the interference pattern in the original experiment was used to demonstrate the wave nature of light).

Go to interactive animation of  the double slit experiment for electrons
This interactive animation was done using paperjs.  It will only work on browsers supporting HTML5 canvas

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