Tuesday, January 08, 2008

Quantum giants humanized

kw: book reviews, nonfiction, quantum theory, scientists

A "simple" thought experiment: Suppose the Universe consisted of four items, two moving particles, an edge, and a sensitive screen. The first particle moves past the edge and strikes the screen. The second particle, moving in exactly the same path as the first as it approaches the edge, moves past the edge and strikes the screen.

The salient question: Does the second particle strike the screen at the same spot as the first? This question cannot be answered. Suppose both particles do strike the same spot on the screen. We must first ask, How can we know whether the two particles actually followed the same path?

The concept known as Heisenberg's Uncertainty Principle states that we cannot measure both the position q and the momentum p (including direction) of any object with infinite precision. Heisenberg quantified his insight: pq≥ћ, where ћ is Planck's Constant, a very small quantity that has popped up everywhere since Planck first derived it to describe the quantization of "blackbody" radiation.

The philosophical question: does the attempt to measure either q or p by itself affect both quantities so that we can't be sure of their values, or do these quantities have a built-in "fuzziness" that attempts at measurement simply unveil?

In 1926 and 1927 Neils Bohr and a number of the giants of theoretical physics wrestled with the implications of quantum mechanics and quantum theory, and produced the Copenhagen Interpretation, which decided in favor of the latter, and furthermore, requires the presence of an "observer" to discern the result.

My own dissent against the Copenhagen Interpretation is this: In the thought experiment above, the Edge is "observer" enough. Its presence affects the path of the particle. No matter how far the particle "misses" the edge, its path will be changed, it will not continue as if there were no edge.

There is also some finite probability that the screen may record nothing. That is, the equation that describes diffraction near an edge has a sinusoidal shape of decreasing magnitude, but while that magnitude is zero for a series of angles, it is nonzero at all points except that specific series of angles, even near 180º.

Well, I may rant about these things further some day. These are some thoughts that arose after I finished reading Faust in Copenhagen: A Struggle for the Soul of Physics by Gino Segré. This book actually says little about such things. Segré presents to us the persons involved.

It is a combined biography, first of seven giants of quantum physics (Niels Bohr, Paul Ehrenfest, Lise Meitner, Werner Heisenberg, Wolfgang Pauli, Paul Dirac, and Max Delbrück), then to a lesser extent, of their collaborators, including Albert Einstein, Max Planck, and Enrico Fermi.

I'd always thought Bohr something of a jerk. I just knew the stories of his notorious tendency to argue until his opponent collapsed. Indeed, once when Heisenberg fled to his bed, Bohr sat at his bedside and argued him into insensibility. I never wondered why they tolerated it and came back for more. They loved him...no, they adored him. They knew he loved them even more.

The author, nephew of Nobel laureate Emilio Segré, a lesser giant among these titans, shows Bohr as a family man (he and Margrethe had six children, including a son who won a Nobel of his own), one whose "family" included dozens of physics students. Niels Bohr is the prototype of the scientist who needs others to do his science. He simply had to discuss in order to think. So if you went skiing with him, you could be sure of plenty of physics with your evening toddy.

Even the more, this collective biography shows that genius isn't the unique possession of any personality type. Paul Dirac was the nearest thing to a Vulcan, Paul Ehrenfest was insecure and eventually suicided, and Wolfgang Pauli could be as caustic as Jackie Leonard, yet was almost as beloved as Bohr.

The human side of science is clearly shown in books written in 1929-30 by Heisenberg, Pauli, and Dirac. Their subject and goal is the same, and their understanding of the subject was equivalent, but only Dirac's The Principles of Quantum Mechanics is still in print and in use. What they say is much the same, but the way they each connect with the reader differs. It is paradoxical that Dirac, the least personable, has the clearest writing style.

And what of the book's title? The last major meeting that the seven were supposed to attend (Pauli had to miss it) was Bohr's Copenhagen conference of April 1932. Those attending wrote and produced a skit, as they had before. This one was based on Goethe's Faust, in which the Lord (Bohr) and Mephistopheles (Pauli) dispute the fate of Faust (Ehrenfest). Quotes from Goethe's play are found throughout the book.

By 1932 the Copenhagen Interpretation was considered settled truth by most (Einstein was a notable exception). The skit was a loving look back at the wrangling that produced it, a chance to blow off some steam and mend fences.

World War II scattered the attendees, and many of them became developers of the nuclear bomb, which made their choice of Faust for their last revel all the more striking.

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