On Hydrogen (& Physics Humor)

Posted by jns on 23 October 2008

I recently finished reading the book Hydrogen : The Essential Element, by John S. Rigden (Cambridge, MA : Harvard University Press, 2002. vii + 280 pages). Here’s my book note. It’s a book I can recommend.

As I mentioned in the book note, the “hydrogen” of this book is the physicist’s “hydrogen”,^* the simple atom of electron + proton (with some isotopic variations) that is the simple test case for all physical theories that deal with things atomic: if it doesn’t work for hydrogen, it’s not going to work.

Hydrogen is overwhelmingly the most abundant atomic species in the universe, making up about 74% (by weight) of the matter we can see. It is the predominant fuel that stars burn through fusion (to make helium nuclei). Hydrogen is the earliest element in the cosmos, protons condensing from a universe of quarks when the temperature finally became low enough, in the period (the “hadron epoch”) between one microsecond to one second after the big bang.^† It was some time longer before the universe cooled enough (some 380,000 years!) for the protons to capture and hold onto electrons, thus becoming actual atoms of hydrogen (Of course, there had to be electrons to capture; they condensed around one second ABB.^#)

Anyway, the history of our modern understanding of the hydrogen atom, and the efforts to gain that understanding, is virtually identical to the history of “modern physics”, by which we loosely mean all that physics stuff from the early twentieth century: quantum mechanics and its friends. Lots of other interesting things get thrown in, too, from all the attention the hydrogen atom got. A couple of the more interesting: the development of the hydrogen maser and very high precision time keeping (i.e., “atomic clocks”, leading to the GPS), and the invention of a technique known to physicists as NMR (nuclear magnetic resonance), which in recent decades developed into the familiar MRI (magnetic-resonance imaging^‡).

Anyway, that’s book-note stuff. What we’re all about here is a couple of leftover quotations from the book that go under the heading: “Physicist’s and their Strange Sense of Humor”. The first two quotations reveal things that physicists find almost knee-slappingly funny but may remain inscrutable to nonscientists (and I wouldn’t worry about that either, if I were you–you’re not missing all that much).

Paul Dirac was a[n] unusual person. Perhaps because Dirac’s father demanded that his young son use French rather than his native English to converse with him, the young Dirac adopted the habit of silence during his childhood simply because he could not express his thoughts in French. Whatever the reason, the adult Paul Dirac was a a man of silence. Dirac’s silence was so intense that it inspired a little levity among physicists. In physics, the units given to physical quantities like time or length are important. Physicists, clearly in jest [!], have defined the unit of silence as the dirac. [p. 89]

For this second joke, I might mention that it was Ed Purcell who pioneered the NMR technique, and that the technique uses magnetic properties of the hydrogen atom, which moves much like a gyroscope when magnetically disturbed (hence the reference to “precessing”^**).

I remember, in the winter of our first experiments, just seven years ago, looking at snow…around my doorstep–great heaps of protons quietly precessing in the earth’s magnetic field.
–Edward M. Purcell [quoted on p. 137]

Finally, this one goes into that file where we put really bad predictions of what the future might hold.

In 1952, neither Purcell nor Bloch could have predicted the ways their discovery would advance understanding of solids, of the structure of chemical molecules, and even more. In fact, a representative from Dupont Chemical Company visited Purcell soon after the paper announcing the discovery was published. The Dupont scientist asked Purcell what the practical applications of NMR might be. Purcell responded that he could see no practical applications. In this, Purcell was very wrong. [p. 147]

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^* Rather than, say, a chemist’s “hydrogen” with discussions of interesting molecules and acids and reducing reactions and carbohydrates, etc. Nor is it an engineer’s “hydrogen”, nor a politician’s “hydrogen” (as in “hydrogen economy”). They’re all stories for another book for someone else to write. What a publishing opportunity!

^† I just read this the other day about the big bang and the origin of the cosmos (and now I forget who gets the attribution): “In the beginning there was nothing, then it exploded.”

^# We could just say “it happened at one second”, since the current understanding has it that time (whatever it is besides a whole other story) began with the big bang.

^‡ I’m sure I’ve expressed my peevishness before about how the perfectly good word “nuclear” had to be expunged before MRI could be a commercial success.

^** When some body, like the Earth or a hydrogen nucleus, rotates about an axis, and that axis is tilted relative to some other axis about which the tilted axis itself executes a (generally much slower) rotation (a kind of wobble), that latter motion is referred to as “precession”. The precession of the Earth’s axis takes about 26,000 years. Hydrogen atoms do it at about 500 megahertz (or 500,000,000 times each second).