Summary/prognosis of the workshop Page: 3 of 19
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astrophysics of methodology from nuclear weapons codes, about which Henyey
learned at Lawrence Livermore Laboratory. But the die was already cast for
me; I h'ad decided to work on-stellar atmosphdres.:
I started my thesis work on O-star atmospheres in the autumn of 1961.
Caltech was then a hotbed of work on both stellar atmospheres, and the signif-
icance of the results to broader astrophysical questions. Greenstein's spectro-
scopic stellar abundance project was in full swing, and the data were ofrele-
vance to work on stellar nucleosynthesis by Fowler, Hoyle, and Goeff and MIar-
garet Burbidge. We had a constant stream of visitors; luminaries like Uns6ld
and Payne-Gaposchkin, and brilliant newcomers like Roger and Guisa. Cayrel,
Sargent, Searle, and Wallerstein. Oke "was leading the field'of photoelectric
spectrophotometry, and had completed a two-channel photoelectric scanner for
line-profile work at the 100" coude. At Mount Wilson, Deutsch was working
on peculiar A-stars, Merrill on the spectra of evolved giants, and Kraft on the
spectra of Cepheids. Also, Christy had begun his monumental computational
work on RR Lyr pulsations, using techniques he had mastered at Los Alamos
during the Manhattan Project.
2. Computational Tools
Ours is a computational science. The equations are nonlinear, with subtle cou-
pling on multiple scales which can differ by orders of magnitude. 'Asa result, the
radiation field at any one frequency and depth in the medium can depend on the
field at all other frequencies at, all other depths.' This complicated interlocking
results from scattering, which allows radiation at some depth and frequency to
propagate not only one mean-free-path, for a huge number of.mean-free-paths
until the scattered photons are finally destroyed in a collisional process while
interacting with the material, and their energy is thermalized. The earliest work
on these problems was analytical, and yielded important insights for highly sim-
plified problems. We began to appreciate the difficulties even better when we
began to try to solve the equations computationally.
My first experience with computers was at UCLA in 1957. They had a prim-
itive machine, called SWAC (an acronym for: "National Bureau of Standards
Western Automatic Computer"). It was a vacuum tube machine similar to the
one made by von Neumann at the Institute for Advanced Study. It occupied an
entire old bungalow. It had 64 (!) random-access memory cells (vidicons, each
recording 64 bits), a three-address command system, an input/output device (a
surplus teletype), and a "rapidly rotating" magnetic drum (with dreadfully long
latency and transmission times) for mass storage. Coding it in machine language
(overwriting already-used code with data, or vice versa, to minimize accesses to
the drum) was torture. By dogged persistence, the applied mathematicians had
been able to develop some useful linear algebra routines. It was not unusual to
find a large sign posted on the entry, saying "Do not slam door between 2 pm
and 4 pm. Large matrix inversion in progress". The machine was slow, and the
wiring so fragile that even a mild bump could introduce random bits throughout
the memory. The machines of today are to SWAC as an F-16 is to the fragile
kite the Wright brothers flew at Kitty Hawk. How did this come about? There
are three important elements in the change: hardware, software, and networks.
Here’s what’s next.
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Mihalas, Dimitri,. Summary/prognosis of the workshop, article, January 1, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc926066/m1/3/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.