20 July, 2005

Physics and philosophy

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?

* 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.


Yup, quantum mechanics leads to a lot of conclusions that contradict our intuition.

1. A better way to look at things isn't true/false/undecided, but rather that every outcome (e.g. path of a photon) has an amplitude associated with it, much as a classical event might have a probability associated with it. Amplitudes and probabilities are related - amplitudes are complex numbers and the square of their absolute value is the probability of observing that event if we make a measurement - but definitely not the same.

Intuitively, it's not that terrible, unless you're really committed to the truth/falsehood thing:

2. Quantum computing can be thought of as a field that tries to make sense of this "quantum weirdness" by making it operational. In particular, branches of a computation (on a yet-unbuilt quantum computer) can interfere the way that photons do, and in some cases this can be quite productive. 

Posted by aram harrow

Hmm. As someone who once seriously contemplated becoming an atomic physicist, I have to say: I can't actually remember the moment when I transitioned from being awed and amazed by this and being nonchalant about it, and even now that I don't spend much time thinking about it, I do rather take it for granted. It's still cool and amazing and weird. . .but not AUGGH. But I don't have any pithy wisdom for "well, if you think about it like blank, then all will be well," and I've never seen a consistent version of that. So I think it's like the flu. . .repeat exposure makes you immune.

If you're SUPER interested in Foundations, you might want to take a look at my buddy Robin's thesis on his site , or this book. But really, the back of Griffith's QM is quite nice. And it's got a dead cat on the cover. It only looks  like it's sleeping. . .

You might also be interested in the Quantum Pontiff.  

Posted by Saheli

The duality of particle and wave is especially clear here. In the first experiment, photons are acting like waves, which can easily be in 2 places at once.I realize waves need to propagate through a medium, and that in current thinking there is no medium in the universe, but to hell with it -- they act like waves. It's just a metaphor. In the second experiment, I don't quite understand why anyone would think a momentum detector would be nonintrusive. If it's taking up space in the slit, it's going to affect the photons' behavior. (I just need to learn more about what a momentum detector is.)

In any case, this whole situation doesn't blow my mind because I am not a linear thinker. I routinely have multiple colliding thoughts in my head. That's why I have done some of my best writing while sick or under the influence of depressants -- they allow the thoughts to come one at a time. Alas, I don't like sickness or depression, so I instead have gotten used to a multiferous universe, in which a single stimulous can lead to multiple simultaneous and sometimes contradictory outcomes.

Yesterday I interviewed a lawyer who answered my first question by saying something like, "Our conversation will have three sections. First I will explain the four major purposes of the federal law. Then I will explain how changing the law will change consumer risk. Then, last, we can discuss whether this is good or bad." The man's brain was so linear and organized that to me, he might have belonged to another species. And sure enough, he did -- he was human, and I am hedgehog.

I think the challenge people have in accepting quantum physics is because of a reverse-Genesis metaphor. Just as people like to think we are formed in God's image, they like to think that the universe will somehow follow the same rules our thoughts follow. At times this works -- planetary physics are the music of the spheres. Euclidean mental paradigms of triangle and square, instilled in us through erector sets and legos, are reified and amplified in our European urban planning.

I suspect that people like Nils Bohr and Albert Einstein are not necessarily genii, but rather they are people whose minds work differently enough that their internal models conceive of the world differently, permitting them novel perceptions and preventing conventional blindness. And they happen to live at the right time to be able to express it in new language and reveal it in new experiments -- very lucky lunatics!


Posted by hedgehog

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