Wednesday, August 28, 2013

The Divine Random in Physics

A place where gently blowing semi-trees
Stand constant in the changing sands,
And pretty birds flying flawless paths
Sing palindromic melodies
A while back, I sat in a conference room with a group of smart software engineers discussing randomness. Our new distributed software product relied very heavily on being able to generate truly random numbers on generic digital computers, a surprisingly hard problem as it turns out. The question had arisen, how sure were we that the numbers we generated were really, truly random? And by the way, what the heck does that actually mean?

Star System Arp 194 (Courtesy NASA)
The Question Mark Galaxy

Someone posited that a truly random number might be one that is universally unpredictable, a definition I quite liked. But instead of saying I liked it, I flippantly suggested there might be no such thing, that given enough information and a big enough computer, we might possibly predict the outcome of any conceivable physical process. Two members of my team (defensively, I will point out they both hold PhDs and very large brains) immediately spoke up to point out how wrong I was and exclaimed in unison, "Bell's Theorem!" I was simultaneously mortified that I had made such a mistake and proud that not one but two members of my team knew enough about quantum metaphysics to correct me. In hindsight, it isn't surprising that it was the concept of randomness that took us so quickly from a mundane discussion of Bug 1356 to the esoteric nuances of life, the universe, and everything. Here's how we got there.

Randomness, as a natural phenomenon, was discovered in 1927 by a young German physicist named Werner Heisenberg. Prior to that time, the universe was completely predictable, or at least that's how we viewed it. Physicists from Newton to Einstein all believed if we just looked hard enough we would find the rulebook of the universe, the answer to everything. And even if that answer were more complicated than, say, 42, it would still be possible to predict the outcome of any physical process, as I had suggested in the engineering meeting. The most brilliant minds the human race had produced all rejected the idea that the universe might be a giant game of chance. Albert Einstein famously rejected it explicitly. Then came the Heisenberg Uncertainty Principle.

The Uncertainty Principle basically says, certain pairs of properties of physical objects — simple things like where it is and how fast it's going — cannot be simultaneously measured with perfect precision. The more carefully you measure the position of, say, an electron, the less certain you can be about its velocity at that same moment. If you are very, very, very careful measuring the position, then whatever number you observe for the velocity is essentially meaningless; it is random beyond a certain number of decimal places. Now this limit on how accurate one can be with these combined measurements is quite negligible for larger objects like bowling balls or BBs, but for small things like electrons and photons it makes a difference. The combined limit on our accuracy of measurement is determined by the reduced Plank constant which is about 35 decimal places of accuracy. Beyond that, physical properties are universally un-measurable. This can be understood by thinking about how one's measurements affect the object being measured. Measuring the position of an electron involves shining a light on it, and a more accurate measurement requires shorter bandwidth, higher energy photons. When the electron is impacted by the high-energy photon, its velocity is affected, thus introducing randomness.

And that is the way it was presented and talked about at first, as a limit on experimental accuracy. The Quantum Physics textbook I used in college, the 1974 edition of Quantum Physics, by Eisberg and Resnick, explained the Uncertainty Principle by saying, "our precision of measurement is inherently limited by the measurement process itself [...]." Albert Einstein, and many other prominent contemporaries of Heisenberg, believed there must still be an underlying set of "hidden variables" that control the universe and provide precise, deterministic answers to any question, even if we were forever limited in our ability to experimentally verify those answers due to the Uncertainty Principle.

Einstein, together with his colleagues Boris Podolsky and Nathan Rosen, even wrote a famous paper in which they, almost mockingly, proved that Quantum Mechanics must be wrong, or else the world as we know it would be a truly strange place. To do this, they assumed only two seemingly obvious things about the world. First, that objects have intrinsic properties like position and velocity, even when no one is measuring them. This they called "reality." And second, that measurements of reality in one place and time cannot instantaneously affect other, far away realities, a property they called "locality." Einstein, Podolsky and Rosen basically said, who would want to live in a world where reality and locality did not hold. In other words, they believed our friendly, orderly universe could not possibly be intrinsically random.

But they were wrong.

In 1964, Professor John Stewart Bell proved a result that some have called, "the most profound discovery of science." The unassuming title of his brilliant paper, On the Einstein Podolsky Rosen Paradox, referred back to the "paradox" outlined by Einstein and his pals. Bell proved that the universe is in fact fundamentally, inherently, inescapably random. More precisely, he showed that no deterministic theory based on hidden variables could possibly explain all the observed results of Quantum Mechanics. And if that means there is no such thing as reality or locality, then so be it. Either the principle of reality or the principle of locality (or both) does not apply in our universe! A strange place indeed.

And so my brilliant colleagues were right. Heisenberg's Uncertainty Principle is not just a limit on how accurately we can measure things. It's a limit on what we are allowed to know about the universe in which we live. There are physical quantities that are universally unpredictable. At the very foundation of our familiar physical world, lies the Divine Random.

3 comments:

  1. The tension actually is between all of three properties: reality and locality as you've described, and the third being causality, in the sense that there is no traveling into the past. It has been decades since I studied this, so it may be Bell just assumed that last in his paper. Since it seems so reasonable.

    But, so do reality and locality.

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  2. You may be right that causality was another hidden assumption. I've read that Bell believed he had blown up the idea of locality, so "spooky action at a distance," as Einstein called it, really does happen. Some modern interpretations of QM eschew reality. Schrödinger's cat is both alive and dead until someone looks in the box. Even spookier IMO.

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  3. Interestingly, Professor Bell never knew he was to be nominated for a Nobel prize. Just before the committee notified him, he died suddenly of a cerebral hemorrhage. What a random way to go!

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