Hans Bethe, 1906-2005

Hans Bethe died on Sunday at the age of 98.  Bethe has been an ancient, eminent figure in physics since I was a kid, and, given his great longevity and his continued scientific activity, it always seemed like he would hang around forever.  So for this professionally-trained astrophysicist and lifetime science enthusiast, it is a sad moment.  I never met him personally, but I did have the good fortune to hear him give a colloquium presentation at Caltech back in the early 1990s when I was a graduate student.  It was like watching a living legend.

Obituaries are all over the papers, but this one in the Los Angeles Times is particularly good.

His scientific and societal contributions were immense, but the one that stands out to me is that this is the guy who figured out the details of how the Sun shines.  The Sun, source of all the energy that warms our planet and fosters the multitude of life forms it contains.  Scientific problems do not come more basic and fundamental than that.

Bethe was introduced to the problem at the 1938 Washington Conference on Theoretical Physics, where astronomers speculated that perhaps nuclear reactions were involved.

He went back to his office at Cornell University after the meeting and within six weeks had solved the problem. Given the known temperature of the core of the sun — about 20 million degrees Celsius (about 36 million degrees Fahrenheit) — and the proportions of carbon, hydrogen and other elements in its interior, he deduced which nuclear reactions could be responsible for its radioactive fire.

He ultimately developed a six-step cycle in which carbon and nitrogen atoms act as catalysts for the conversion of four hydrogen nuclei (protons) into one helium nucleus. The very slight mass lost in the process is converted into large amounts of energy, according to Albert Einstein's famous equation, E = mc2.

Bethe later remarked with typical modesty that he found the answer by "looking through the periodic table [of elements] step by step. So you see, this was a discovery by persistence, not by brains."

One oft-told story about Bethe recounts the evening after he discovered the secrets of starlight. During a late-night stroll, his fiancee, Rose — daughter of theoretical physicist Paul Ewald — remarked on how beautiful the stars looked. He responded: "Yes, darling, and I'm the only one on Earth who knows how they do it."

Yet as brilliant as Bethe was, and as encyclopedic as his knowledge of nuclear physics was, this was not a problem he solved on his own.  And, as such, it is a great illustration of how real science actually works the vast majority of the time. 

The stage was set for him to come in and work it out, but it took a lot of work to get that stage in order.  As far as I know, Bethe was not a crucial figure in the development of the theory of quantum mechanics.  That was accomplished by many other brilliant minds, including those of Max Planck, Niels Bohr, Werner Heisenberg, Wolfgang Pauli, Enrico Fermi, and Paul Dirac.  Bethe was not the fellow who surmised that nuclear reactions were the power source for the Sun.  That was accomplished by Sir Arthur Eddington in 1920, based on experimental work by F. W. Aston and theoretical work of Albert Einstein.  Bethe also was not the first to quantitatively study nuclear reactions with quantum mechanics.  That was accomplished by George Gamov and others, just as Hans Bethe was starting his long career in physics.  Bethe didn't even discover one of the crucial nuclear reaction cycles that powers some stars, the CNO cycle.  That was done by C. F. von Weizsäcker.

What Bethe did do was use all of this previous work to meticulously, quantitatively demonstrate for the first time which specific nuclear reactions occur in the centers of stars, and how the rates of these reactions vary with the mass and temperature of the star.  As theoretical astrophysicist John Bahcall writes in an excellent article on the history of how the problem of the Sun's energy source was solved:

By April 1938, it was almost as if the scientific stage had been intentionally set for the entry of Hans Bethe, the acknowledged master of nuclear physics. Professor Bethe had just completed a classic set of three papers in which he reviewed and analyzed all that was then known about nuclear physics. These works were known among his colleagues as "Bethe's bible.'' Gamow assembled a small conference of physicists and astrophysicists in Washington, D. C. to discuss the state of knowledge, and the unsolved problems, concerning the internal constitution of the stars.

In the course of the next six months or so, Bethe worked out the basic nuclear processes by which hydrogen is burned (fused) into helium in stellar interiors. Hydrogen is the most abundant constituent of the sun and similar stars, and indeed the most abundant element in the universe.

Bethe described the results of his calculations in a paper entitled "Energy Production in Stars,'' which is awesome to read. He authoritatively analyzed the different possibilities for reactions that burn nuclei and selected as most important the two processes that we now believe are responsible for sunshine. One process, the so-called p-p chain, builds helium out of hydrogen and is the dominant energy source in stars like the sun and less massive stars.

The CNO cycle, the second process which was also considered by von Weizsäcker, is most important in stars that are more massive than the sun. Bethe used his results to estimate the central temperature of the sun and obtained a value that is within 20% of what we currently believe is the correct value (16 million degrees Kelvin) . Moreover, he showed that his calculations led  to a relation between stellar mass and stellar luminosity that was in satisfactory agreement with the available astronomical observations.

Yet even after Bethe's 1938 work, which won him the Nobel Prize for Physics in 1967, the problem of how stars shine was not completely solved.  For one thing, Bethe apparently missed a couple of the reactions involved.  And also, he was even completely wrong on one point.  In the abstract to his seminal paper that Bahcall refers to above, Bethe wrote:

It is shown further that no elements heavier than He-4 can be built up in ordinary stars. ......  The heavier elements found in stars must therefore have existed already when the star was formed.

But in fact, it was later learned that the "triple alpha" process (three helium nuclei colliding virtually simultaneously to fuse into a carbon nucleus), contrary to Bethe's initial claim, can occur, if only in the late stages of a star's life, when it is a red giant and the central temperature climbs from the tens of millions of degrees (Celsius) into the hundred of millions of degrees.  So elements heavier than helium are indeed built up in ordinary stars.  It is a good thing, too, because otherwise, there would be no carbon, oxygen, iron, etc, and what would we be made out of then?

Hans Bethe, a scientific giant, standing on the shoulders of other scientific giants, will be missed.

Whether or not their governments respond to their advice, scientists have an obligation to speak out publicly when they feel there are dangers ahead.
                                                                 —Hans Bethe

Comments

Dear Fellow Bereaved,

Please note the spelling of Niels Henrik Bohr's first name.
(On the complimentary side, it stands to hope that the departed
left us a sound sample pronouncing "Hans Albrecht Bethe" ... &)

Best regards,                                                 Frank W ~@) R

[Peter]

Thanks for the correction. I've fixed that.