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No. 109: Jan-Feb 1997

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Indeterminacy In Computers

In SF#108, it was remarked that a chessplaying computer will sometimes make different moves when faced with identical boards. R.G. Everit responds that this is not really mysterious. The better chess-playing computers are actually designed to behave unpredictably when confronted by several moves of roughly equal promise. This feature makes it more interesting for human players.

Everit also sets us straight in the matter of computer determinacy.

"However, contrary to your basic assumption, most computers (even a home PC) can be forced to behave truly unpredictably. This cannot be done using the random-number generators supplied with the software, as these depend upon some mathematical formula and so are determined in advance, even if they appear to show no pattern. But if the machine has an internal clock readable by the programmer, he can determine the machine's choice depending upon the time required for some complex calculation, which will vary according to such factors as minute voltage variations and the aging of the machine's components. For example, the CDC 3600, on which I learned to program in 1975, had an accessible microsecond clock, and my program to calculate the first five perfect numbers* required about 15 minutes of run time; the last few digits of the exact number of microseconds required to run this program each time varied quite unpredictably. In other words, it was a random number, except perhaps from the standpoint of philosophical determinism, which claims that every event in the entire universe has been determined from the beginning."

(Everit, Richard G.; personal communication, November 2, 1996)

*A perfect number is equal to the sum of its divisors. The first two are 6 and 28; the others being difficult to find with just pencil and paper!

Comment. Computer unpredictability? There's something human in those chips! Of course, K. Capek knew this would be the case with any complex machine, as he predicted in his 1921 drama R.U.R. (Rossum's Universal Robots).

From Science Frontiers #109, JAN-FEB 1997. 1997-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987