Letters to the Editor

Students are Confused by Meaning of Heat

While it may be acceptable in a column written for APS members, the way the term “heat” is used in the April “This Month in Physics History” can be quite misleading to students who have not yet mastered the first law. There is evidence that students find the concept of heat very confusing, convoluting it with thermal energy as a property of an object. This leads them to not be able to use the first law correctly. When something melts, the correct way to think about it, if you don’t already completely understand what is going on, is that it absorbs thermal energy. The amount of thermal energy that was transferred in the process is the term heat in the equation. Heat is never a property of a single object. You cannot say how much heat is in something, but you can measure its thermal energy. Although as physicists we all know what we mean, it does not hurt to remind ourselves that our students hear what we say, not what we mean. I know this publication is intended for those who do understand, but I love sharing some of the features with teachers and potential students. This one will require some careful framing!

Gay B. Stewart
Fayetteville, AR

Secretary of Energy is a Political Job

In his column “Can Science and Politics Coexist?” (April APS News), Michael Lubell complains that Secretary of Energy Chu, who is a scientist, has been criticized for his political positions. But the Secretary of Energy does not principally serve as a scientist; the Secretary is the administrator of the department that is charged with setting policy, which will among other things determine the energy future of the country.

A more balanced article would have had merit; instead, the assertion that science and politics should be insulated results in a politically motivated assault on Republicans and defense of the Obama Administration.

Peter Friedman
Dartmouth, MA

Verifying Casimir Effect Took a Long Time

I enjoyed reading the history column about Casimir in the May APS News. However, I feel I must correct two errors. 

First, Casimir worked with Polder on the so-called Casimir-Polder force between two atoms, and between an atom and a conducting plate, but it was only in the following year, 1948, that Casimir alone published his paper on the Casimir force between parallel conductors. This followed a conversation with Niels Bohr who suggested the CP force must have something to do with zero-point energy. And I believe that Casimir never attempted an experiment to verify his theoretical work. The first experiments may have been attempted by Derjaguin and Abrikosova in 1957, but these were inconclusive, and even Sparnaay’s experiment had 100% error. The Lifshitz theory was verified with good accuracy in 1973 by Sabisky and Anderson.

Kimball A. Milton
Norman, OK

Correcting an Omission in the Timeline of Fission

In an interesting letter in the March APS News commenting on the history column’s article on Fermi that was in the December 2011 issue, Fred Peet says, “Thus Meitner did not learn of fission as a result of Hahn’s and Strassmann’s manuscript as the APS News article implies, but rather she explained the observations (with her nephew) as a result of Hahn asking her if she could do so.” Although this is correct, it is nevertheless historically incomplete, because four years earlier, in 1934, following the publication of Fermi's work which suggested that the bombardment of the uranium by slow neutrons had resulted in the production of transuranic elements, a German woman chemist, Ida Noddack, published an article in Anwendte Chemie (Applied Chemistry) entitled, “Zur Element 93,” in which she pointed out that in chemically identifying the products of the bombardment, it was not sufficient to test chemically around uranium, but all the way down the periodic table, since the neutrons might have fractured the uranium nucleus. Although she wrote to Fermi about this possibility, he rejected her suggestion on theoretical grounds, as did everyone else working in nuclear science. She even sent her husband (who together with her and a colleague had earlier discovered the element rhenium) to speak to Hahn about her idea, but Hahn rejected it as impossible. Consequently, when Hahn wrote to Meitner on Dec. 19, 1938 ( as quoted in Peet’s letter), “Perhaps you can put forward some fantastic explanation,”as to how he and Strassmann had found barium, there is an obvious puzzle as to why Noddack’s proposal of 1934 wasn’t mentioned.

The failure to mention Noddack in many accounts, or to relegate her 1934 proposal to a footnote has had some unfortunate consequences in the public domain. This shows up in Michael Frayn’s play “Copenhagen,” in which the following dialogue occurs early in Act I:

“Bohr: Otto Hahn–he’s still there. He discovered fission, after all./ Margrethe: Hahn’s a chemist. I thought that what Hahn discovered..../ Bohr: ...was that Enrico Fermi had discovered it in Rome four years earlier. Yes–he just didn’t realize it was fission. It didn’t occur to anyone that the uranium atom might have split, and turned into an atom of barium and an atom of krypton.”

Obviously it did occur to Noddack that the uranium atom might have split, that is why she emphasized the importance of testing all the way down the periodic table, which is what Hahn and Strassmann did, and found barium.

However, this story about Ida Noddack has even greater ramifications, because it is clear that all the other nuclear scientists of the time, not just Fermi, were not following the scientific method on this matter, which stresses that one does not have scientific knowledge unless one has empirical foundations for that proposed knowledge. Instead, the nuclear scientists relied on theoretical arguments based on the limited understanding they had of nuclear physics at that time. If they had followed Noddack’s suggestion, they would have found that Fermi had indeed split the nucleus, and the recognition that followed in 1939 that there was the possibility of a uranium bomb (or atom bomb as it came to be known) would have arisen four years earlier in 1935. This would have had significant consequences for the subsequent development of world history, since it raises the question as to whether the WWI allies would have allowed Hitler's war machine to start to build it, and hence mobilized and taken military action if necessary. Thus, undoubtedly, the failure to follow the scientific method and experimentally test Ida Noddack’s suggestion in 1934 obviously had consequences best left to historians and novelists to envisage.

Frank R. Tangherlini
San Diego, CA

Joseph Black was not a Scot

I enjoyed the column in the April 2012 APS News, which celebrated the discovery of latent heat by Joseph Black, especially the author’s understated little joke about the historical importance of the careful lecture notes taken by Edinburgh students! However, one point should be corrected: Black was not a Scot. At his death in 1799, a newspaper in Belfast, in the north of Ireland, claimed he had been born in Belfast, but it seems he was actually born in Bordeaux, France, son of a Belfast man who was a wine factor there. Black returned to join the rest of his family in Belfast, where he attended school. He is one of many notable scientists whose careers are recorded in the Dictionary of Irish Biography, which was published in 2009 by the Royal Irish Academy and Cambridge University Press.

Linde Lunney
Dublin, Ireland

Physics PhD Data Clarified

In his Back Page article in the April APS News, Geoff Potvin raises a number of very important issues. However, he makes and repeats an incorrect statement. I feel the need to correct the record so that this might not become another myth about physics graduate education.

Potvin states that “the time needed to get a physics doctorate is getting longer on average” and that “doctorate completion times … have steadily risen in recent decades.” There are, of course, many ways to measure time to degree. However, one of the simplest is age at time of degree. According to the NSF annual Survey of Earned Doctorates (SED), the median age of new physics PhDs is unchanged since the class of 1990.

Potvin also states that the “number of PhDs awarded to US-born citizens has been stagnant or declining for some time.” I am happy to report that the number of US citizens who entered physics PhD programs has been larger than the number of foreign citizens for each of the last 6 years and thus we will soon see a dramatic increase in the number of US citizens earning physics PhDs.

Roman Czujko
College Park, MD

Ed. Note: The author is Director of the Statistical Research Center at the American Institute of Physics.

Geoff Potvin Replies:

Thanks to Roman Czujko for providing more clarity on two of the trends to which I obliquely referred. I particularly appreciate his broader message: that we should endeavor to be clear and precise in our discussions of graduate physics education, especially so that we do not discourage potential future physicists from considering a doctorate through the creation or transmission of myths.

He is correct to point out that the age of PhD recipients (a reasonable though imperfect proxy for doctoral completion time) is approximately the same as in 1990; however, the trend to which I referred was over several decades going back at least to the 1970s. In fact, the length of a physics PhD appears to have incrementally crept upwards for many years, reaching an all-time high in the late 1990s and has since come down somewhat to approximately the same level as 1990–but it is still longer on average than, say, the 1970s (a common trend in the physical sciences and many other fields). To be sure, these shifts from year-to-year are not monumental, but incremental. I did not intend to suggest that a PhD in physics has suddenly turned into a ten year ordeal for most students, but the historic trend is important to note.

Similarly, Czujko rightly noted that the number of US-born graduate students entering physics doctorates has been higher than foreign-born students for the past six years; however, the number of such students has only recently gotten back up to the level of an earlier peak around 1990, a peak which was transitory and was followed by a decade-long decline. Thus we should be cautiously optimistic on this point.

I would also like to take the opportunity to thank Czujko and the entire AIP Statistical Research Center staff for their invaluable efforts over many years to collect and publicize critical information on physics.