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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