I have been working my way through Volume III of Feynman’s lectures, the one on quantum mechanics. A few months ago I watched his Quantum Electrodynamics lectures for the lay public and I was fascinated by the beauty and simplicity of the presentation. Now I want to dig deeper.
The basic idea is summarized in the quote (can’t find its source, probably Feynman though :-): “Everything that can happen, does. Physics is then reduced to the problem of finding out what can happen.” This is not philosophical many-worlds garbage postulating the existence of infinitely many alternative universes (I will get to that), but instead the interpretation of the Lagrangian form: if you want to find the probability amplitude of some event, you just add up the amplitudes for all the different ways it could happen. The generality of the principle is astounding, and making only very weak additional assumptions it is possible to completely derive the workings of electrons and photons (except for the mass of the electron, which is still a mystery). The rule is not just for electrons and photons though; those are just the easiest kinds of particles to get at. The entire universe works this way: the amplitude of an event is the sum of all the ways (including classically absurd ones) it could happen.
In the beginning of my studies, I was constantly tripped up by my conception of time. In the double slit experiment, a photon interferes with a version of itself leaving the excited atom at a different time. It was very hard to picture this when I was still attached to my idea of time and causality. This is the logic of the universe, not the dynamics. That is, we aren’t really computing the amplitude of an event to happen so much as the amplitude that, given some assumptions are true, some other thing about the universe will be true. We phrase the double slit experiment like this: given that this atom is excited at t0, what is the amplitude that this other atom is exited at t1? There is no notion of happening or the flowing of time, it’s just a connection between statements about the universe. Realizing this was an important step in my understanding. Of course, the way that these two atoms are connected does involve time — that manifests itself in the different “ways it could happen” and thus affects the amplitude.
Ok, so we have this logic which connects facts about the universe together as amplitudes, which are complex numbers. How do we take these amplitudes and get some information we can use? The rule is: the probability of an event, er I mean, a fact, is proportional to the absolute square of the amplitude. Simple enough. So you set up an experiment and calculate the amplitudes for all the different ways it could come out (you have to calculate all the ways, because the probability is only proportional, so you need to normalize them so they sum to one — I find this unsatisfying). Then you do the experiment, and what actually happens at the end of the experiment is one of those ways, proportional to the absolute square of the amplitude for that way.
This is extremely unsatisfying to me. Almost all of the resources I have used for learning QM have described it this way and left it at that. I’m pretty sure it’s because nobody really knows the answer to the next question: when, exactly, do you take the absolute square? If you take it too soon, e.g. before “the experiment” is over, then you will lose the interference effects and do not get an accurate answer. But you can’t just delay taking it forever, because then you only ever have amplitudes, not probabilities. There is this arbitrary barrier between the “quantum” world and the “real” world, and that’s when you take the absolute square. This is intentionally ignoring the idea that your experiment apparatus, your measuring devices, etc. are all governed by the quantum logic above as well, because that is too hard to think about. This is the piece I am determined to understand; I am interested in QM philosophically, not practically, so it is not at all satisfying to me to say “it works in practice, get used to it.”
The theory of quantum decoherence provides half of the answer. It shows how this interpretation of the barrier is equivalent to the state of the experimental apparatus (including the state of you, the scientist performing the experiment) becoming entangled with what happened in the experiment. Eventually the whole universe gets entangled with the result of the experiment and that’s what “really happened”. God got a bunch of amplitudes for the way the universe could be; he took their absolute squares, rolled the dice, and picked one. Now the arbitrary boundary has been pushed out as far as it can go — to the edges of spacetime — instead of being between experiment and apparatus. Quantum decoherence shows a sort of compositionality of this quantum logic. This is getting more satisfying.
I love it because it is right on the edge of my ability to conceptualize. All the “decisions” in the entire universe could go this way or that, and if they both lead to the same thing and have opposite amplitudes, they could interfere with each other and make that thing impossible. It is because the universe is a chaotic system, that small changes give rise to large changes, that we can’t observe quantum interference on large scales. These little decisions are very unlikely to lead to the same state. Entropy gives rise to the classical world.
When I get really deep into philosophizing, I explode into the annoying considerations of consciousness. Perhaps God did not pick a universe at random, but our consciousness did. Our memory must conceive of time linearly, it would violate entanglement not to, and that’s why we think there is a single “chosen” universe instead of the explosion of all possibilities. But whether all possibilities exist or there is a single universe chosen at random is likely not an observable distinction, so it is merely fodder for pipe dreams.
If there were some device that could measure some things about the universe, without disturbance, set up in such a way as to negatively interfere with itself when its measurements were “undesirable”, it could potentially control the way the universe would go. Now you see where the title of this post comes from. I have not been able to sketch this device as a black box, nor fully understand why it should be impossible. I suspect it has something to do with the uncertainty principle, the derivation of which I have yet to completely understand.
Quantum Mechanics is fascinating to me, and I am trying to keep my mind open to the deep, philosophical, passionate curiosity it invokes without descending into the insanity of a quantum crackpot. It is a challenge.