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Uncertainty (D. Lindley)

2008-12-01 6 min read Books marco

There is no topic in elementary physics as odd as quantum mechanics. Once you learn the formalism, it is easy to apply (although the mathematics required can be daunting). The experimenters say, on the other hand, that the results you get from quantum mechanical computations are accurate within the limits of measurement.

Problem is, there is no reason for that. We are all a little surprised by how accurately QM models the world. It’s as if God in his or her infinite wisdom had decided to choose QM as the infinitesimal model of the world on a whim.

The problem, you see, is that there is absolutely nothing necessary about QM. It certainly suffices the requirement to make sense in the everyday realm, but in the microscopic it sounds and feels just like one of many possible descriptions of physical reality.

No surprise, then, that QM was a more controversial topic than most other ones in physics. D. Lundley’s book is a fascinating account of the personality clashes involved in the hashing out of details in the fateful years, almost a century ago, when experiment and theory converged.

The problem, you see, is that classical mechanics cannot explain the atom. Physics knew about electrons, neutrons, and protons. It had been determined that electrons had negative charge and that they were tiny (compared to the other two) and were all over the place, as if orbiting some nucleus. Neutrons and protons, on the other hand, made up that very nucleus – tightly packed, heavy, and immobile.

Now, why would the positive charges want to sit all together in one place, instead of trying to be as far apart as possible? Why would the electron, screeching through space around the nucleus, not emanate the radiation that all electrons send off when they change direction?

Enter three sages: Einstein, Bohr, Heisenberg.

You know the first one. By the time of QM, Einstein was already the most famous physicist in the world. He had, on his own, pushed physics forward in a number of directions, of whom the theory of relativity is only the most commonly known. A towering giant of a scientist, Einstein was a polymath that had already given pretty much all he had to give to the science he so loved.

Bohr, on the other hand, was not as well-known, nor is he today. He ran an institute of physics in his native Copenhagen, not much of an Alexandria of knowledge back in the days. Bohr was an odd man, fond of meandering and unclear statements out of which interpretation had to be evinced, in a way like nature itself makes statements that physics interprets as laws.

The third one, Heisenberg, is my hero in the fable. The young upstart, a plodding but deeply original person, became Bohr’s assistant for a while and came up with the main idea behind QM: that particles were not really particles, but waves. That the waves could be described as functions, and that the functions could be operated upon using mathematical operations that would, henceforth, be given physical meaning.

To find the speed of a particle, for instance, you apply the “speed” operator to the particle’s wave function. To find the energy, you apply the “energy” operator.

Now the weird thing: if you do this, there are a few things that don’t make any sense to us. No sense at all. For instance, if you apply the speed and then the location operators, you get a different result than if you apply the location and then the speed operators. That means you cannot measure location and speed at the same time, since you have to decide which comes first, and then you lose the second.

QM is full of these things. It’s completely unclear, e.g., what these wave functions actually mean. Sure, you can ask them whatever you want, and they will give you an incredibly accurate result, but what’s the function itself about? Bohr thought it was a representation of the probability of measuring the particle in any location, but that’s more a consequence of the location operator than of the wave function.

Heisenberg didn’t care. We went along, all happy that he could explain things nobody else could explain. The young kid he was (26), he left it to Bohr to explain in bumbling terms the philosophy behind his findings, while he merrily explained everything away. You have to imagine: you just play around with your math, and something makes sense that never made sense before, and suddenly everybody thinks you are the god of physics. What do you care about the why?

Bohr and Einstein did, a lot. Bohr was a strong ally for Heisenberg and the other quantum mechanicist. With his insights into the explanation of things he could defend the nascent branch of physics from detractor. People like Einstein, who thought the whole thing was absurd.

Einstein never reconciled his thinking with QM. That’s particularly odd, since his contributions on Brownian motion were one of the corner stones on which QM was built, and because he was the iconoclast of his time. But Einstein had always been strongly deterministic and in his old age got a streak of theism in his thinking that seemed to cloud his judgment.

In the end, the inevitable happened: Einstein came up with ever more clever ways to test QM, and every single time he was either proven wrong by experiment or refined arguments. Mind you: there is no necessity for him to have been wrong, and his experiments were valid tests of a theory that seemed to offer paradoxical results. But there you go, the world seemed to be made of quanta. Einstein was proven wrong by the same god who, as the only divinity of physics liked to say, didn’t throw dice.

Lundley’s book tells the story of these men in coherent fashion. It’s a shame that, like most authors of science non-fiction, he is clearly not a physicist. Sometimes the grasping for a clarification that might make things easier to understand for the reader reveals that the author is a little unclear about the concepts, himself.

The people of this world, though, come across very vivid and clear. Einstein’s stubbornness, Bohr’s rambling, Heisenberg’s bouts with self-doubt: it’s all in one book, and a well-written and engaging one.

One item of criticism: Lundley’s German is not up to the task. He tries to grasp for working translations of the German words used in the context of QM, and tries to pass himself off as an expert. Considering how many of the German words cited are misspelled, though, the claimcannot be confirmed.