You Are Smarter than Quantum Physicists

Fairly recently, I started reading up on quantum mechanics (QM) to brush up my understanding of the topic and, quite surprisingly, I’ve found it ripe with analogies to my typical interests: software development. The one that stands out particularly well relates to the very basics of QM and the way they were widely misunderstood for many decades. What’s really amusing here is that while majority of physicists seem to have been easily fooled by how the world operates on quantum level, any contemporary half-decent software engineer, faced with problems of very similar nature, typically doesn’t exhibit folly of this magnitude.

We are not uncovering the Grand Scheme of Things every day, of course; what I’m saying is that we seem to be much less likely to come up with certain extremely bad answers to all the why? questions we encounter constantly in our work. Even the really hard ones (“Why-oh-why it doesn’t work?!”) are rarely different in this regard.

Thus I dare to say that we would not be so easily tricked by some “bizarre” phenomena that have fooled many of the early QM researchers. In fact, they turn out to be perfectly reasonable (and rather simple) if we look at them with programmer’s mindset. The hard part, of course, is to discover that such a perspective applies here, instead of quickly jumping to “intuitive” but wrong conclusions.

To see how tempting that jump can be, we should now look at one simple experiment with light and mirrors, and try to decipher its puzzling results.

A story of shy photons

The setup is not very complicated. We have one light source, two detectors and two pairs of mirrors. One pair consists of standard, fully reflective mirrors. Second pair has half-silvered ones; they reflect only half of the light, letting the other half through without changing its direction.
We arrange this equipment as shown in the following picture. Here, the yellow lines depict path the light is taking after being emitted from the source, somewhere beyond the left edge.

Source of this and subsequent images

But in this experiment, we are not letting out a continuous ray of light. Instead, we send out individual photons. We know (from some previous observations) that half-silvered mirrors are still behaving correctly in this scenario: they just reflect a photon about 50% of the time. Normal mirrors, obviously, are always reflecting all the photons.

Knowing this, we would expect both detectors to go off with roughly similar frequency. What we find out in practice is that only detector 2 is ever registering any photons, and no particle whatsoever reaches detector 1, at any time. (This is illustrated by a dashed line).

At this point we might want to perform a sanity check, to see whether we are really dealing with individual particles (rather than waves that can interfere and thus cancel themselves out). So, we block out one of the paths:

and now both detectors are going off, but not simultaneously. This indicates that our photons are indeed localized particles, as they appear to be only in one place at a time. Yet, for some weird inexplicable reason, they don’t show at detector 1 if we remove the barrier.

There are all sorts of peculiar conclusions we could come up with already, including the mere possibility of photon going both ways to have an effect on results we observe. Let’s try not to be crazy just yet, though. Surely we can establish which one of the two paths is actually being taken; it’s just a matter of putting an additional sensor:

So we do just that, and we turn on the machinery again. What we find out, however, is far from definite answer. Actually, it’s totally opposite: both detectors are going off now, just like in the previous setup – but we haven’t blocked anything this time! We just wanted to take a sneak peak and learn about the actual paths that our photons are taking.

But as it turns out, we are now preventing the phenomenon from occurring at all… What the hell?!