I finished reading the book with the given title, penned by Herr Werner Heisenberg, a few days back. I don't really know much about physics at all, but I've always been impressed by the fact that physicists don't run around tearing their hair out and screaming "AAUUUGGHH!!" all the time. I dated an atomic physicist for many years (she might even be reading this), and as far as I can tell she never ran around screaming "AUUGHH" (except on account of my not having done the dishes).

The reason why I expect such behavior of physicists is probably familiar if you've explored quantum physics in even cursory detail. H. Heisenberg's preferred example is the famous double-slit diffraction experiment first performed by Thomas Young, which suffices to establish the key confundments in a simple fashion.

The setup is simple: you have a screen which has two narrow vertical slits in it, a monochromatic light source on one side and a detector screen on the other. Light strikes the first screen and is scattered^* as it passes through the slits; the two waves of light interfere with each other (reinforce when their peaks coincide, cancel when they are in opposition), producing a characteristic "diffraction" pattern on the detector screen, with bright peaks where the waves reinforced and darkness where they canceled.

It is important to note that this reinforcement pattern is a definite result of the interference of two waves. That is, if we were to only have a single slit, we would get a different pattern on the screen, and the pattern produced from a double-slit diffraction experiment is very different from a simple overlay of the patterns produced by two single-slit experiments.

Well and good, so far. Now comes the bizarre part.

Herr Heisenberg points out that the interaction of light with the detector screen is a quantum phenomenon - that is, it involves a single photon interacting with an atom. It implies fixing precisely the position of the photon in space. In fact, we can decrease the intensity of our light source to the point where we can actually observe single photons striking the screen (if, say, the screen is actually a CCD camera). Now, a single photon must be traveling through one slit or the other. If we send photons through the apparatus one at a time, then, it must behave just the same as it would in a single-slit experiment. We should thus expect to see NO interference pattern, but instead the aforementioned overlay of two single-slit patterns.

Not so: observe the results to the right obtained by a Princeton group that performed exactly this experiment. Despite the ability to watch the progress of individual photons striking the detector, the interference pattern STILL emerges. That is, the photon passes through both slits and interferes with itself.

If this doesn't raise the hairs on the back of your neck, consider this experiment instead: we can place a detector on either the receiving screen (as above), or we can place detectors along the slits themselves. This can be done by simply recording momentum transfers as photons pass through the slit, so that it need not disrupt the process. In the first instance (we learned above), we see a diffraction pattern. In the second instance, we see none. WE SEE NONE! The simple act of observation affects the behavior of the photon. (At this point you should flip out and run around screaming.)

Herr Heisenberg would have us believe that a fundamental epistemological principle that we all accept, i.e. contradiction, is simply not true at the quantum mechanical level. That is, there are actually THREE conditions: true, false, and undecided. For quantum phenomena, there are instances where the impenetrable mystery, the unknowable, cannot be resolved. We cannot say which slit the photon passed through after it has struck the receiving screen, because it was not decided at that point. There was a fundamental uncertainty, and so long as it was not resolved, both mutually contradicting conditions were equally true.

And, vexingly, the act of observation plays an inseparable role in this process. UNTIL we observe, the contradiction exists. But as soon as we do, it vanishes. Thus the totally bizarre result in the final experiment. By simply observing which slit the photon passes through, we decide its mode of behavior. I.e., we establish the truth by observing it.

I think this should be enough to fray anyone's mental fiber and keep them up at night, sweating furiously. But I don't know what to make of it beyond that; so, existence is bizarre and runs against our deepest expectations. Is some other truth hiding behind that quantum mechanical uncertainty?

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^* This scattering is itself a result of the uncertainty principle, actually, which says that we can never know both the position and velocity of a particle to an arbitrary degree of precision; there is a fundamental limit (based on Planck's constant), below which we must sacrifice one for the other. Since we know that the light has passed through the slit, its position becomes known precisely, and its velocity can therefore take a broader range - it might pass in any direction at all.