Quantum weirdness

We often hear about the weird and wonderful world of quantum mechanics but the nuts and bolts of it are not easy to grasp. This post will attempt to present the fundamental mystery at the heart of quantum physics in a digestible way, based on Richard Feynman’s book “Six Easy Pieces”.

Imagine an electron source (ES) and electron detector (ED) separated by a solid screen (S) with two narrow slits cut in it. The electrons emitted by the source reach the solid screen and the ones which get through the slits carry on to hit the detector on the other side. This setup is referred to in the scientific literature as a “double-slit experiment” – nice and easy.


To kick things off let us imagine that one of the slits is blocked. The electrons which have got through the open slit will hit the detector screen and over time build a pattern similar to the sketch below. The curve drawn against the detector screen indicates how many electrons hit the screen at a particular location. The crest of the curve will align with the straight line projected from the electron source through the centre of the open slit.


So far so good but let us now uncover the blocked slit. The pattern of electrons reaching the detector screen changes to something like the sketch below. Instead of two hills aligned with the slit centres we have a wavy graph with a number of crests and troughs. The location where most electrons are likely to hit the detector screen is half way between the slits.


Surprised? You should be because this is a typical wave interference pattern. It indicates that electrons propagate through space like a wave, squeezing through both slits similar to waves on the surface of water. The high points on the detection curve correspond to locations where crests from both interfering waves meet and amplification occurs. The points where few electrons hit are where the waves cancel each other out.


Now the bolter: if we turned down the electron source so that there was only one electron in transit at any moment the detection pattern, built over time, would remain the same! Many people think that this is what the fuss is all about – individual electrons travel through space like waves but hit the screen as particles – but they are wrong. We have not even started with the truly weird stuff. Let us now imagine that we have added a spot detector (SD) at the slits. Any electron passing through the slits will be detected (“observed”) and then travel on to the detector screen on the other side. After a while we will notice with amazement that the detection curve has changed to the twin-hill shape indicating that we are dealing with particles, not waves.


Yes, the mere fact that we observed electrons at the slits made them behave differently. Instead of traveling as a wave they now appear to move through space in straight lines like particles. This is wacky but let us try to make sense of it. Ok, so the electrons got detected at the slits and from that point on “became” particles which do not interfere with themselves but rather scoot through space like bullets. To expose their trickery we will now move the spot detector from the slits to a point located behind the solid screen and a distance away from it. Now the crucial detail – the spot detector is turned on by a fast switch when the electron has already passed the slits. This is the “delayed choice” version of the double-slit experiment. We would expect that since at the instant of detection the electrons are already past the slits the wave interference will appear. But this is not so – the twin-hill pattern of “particle” detection remains unchanged.


I will reiterate here what this means. The electrons have passed through the slits undetected and, for all we know, were at that moment travelling through space as a wave which should manifests an interference pattern on the detection screen. When we turned on the spot detector the electrons were forced to become particles. But the twin-hill pattern on the detection screen means that the electrons changed the way they had passed through the slits by erasing the interference pattern. It means that our decision to turn on the spot detector changed the events which had already occurred. In other words the information about the detection traveled back in time to alter the past.

Having read the above a few times you will find it impossible to make sense of. Well, you are in good company – it appears that no one at present understands the inner workings of the double-slit affair. The experimental physicists only report on what happens in the labs and theoretical physicists come up with various ideas, which are often even more whacky than the lab results. While we now have more theories than before, no real progress has been made in understanding the underlying issue for the last 80 or so years. We can measure, analyse and correlate the results of the experiments but have no clue why this is the way matter behaves. Weird.


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