First, Contact by Carl Sagan is one of my favorite science fiction books (slightly edged out by The Martian Chronicles), and perhaps one of my favorite books ever written. It tells the story of alien first contact in a believable scientific way, but my favorite part is the authenticity with which Sagan expresses the motivation and sentiments of scientists. They’re portrayed as reasonable people who sometimes disagree, and even the bad guy program director isn’t written to be a trope – he has legitimate concerns that aren’t boorish or at the convenience of the plot.

Of course Sagan was uniquely positioned to write the science well, too, and although he took licenses (as he should have, it’s a work of fiction), by and large as a reader you can learn a lot about both the science and the scientific process. Which is why I was suprised to come across this scene below, in which Eleanor Arroway (the protagonist), is explaining to Kenneth Her Deer (the President’s science advisor) for the first time how the aliens contacted Earth.

Her Deer: I may be the President’s science advisor, but I’m only a biologist. So please explain it to me slowly. I understand that if a radio source is twenty-six light-years away, then the message had to be sent twenty-six years ago. In the 1960’s, some funny-looking people with pointy ears thought we’d want to know that they like prime numbers. But prime numbers aren’t difficult. It’s not like they’re boasting. It’s more like they’re sending us remedial arithmetic. Maybe we should be insulted.

Arroway: No, look at it this way. This is a beacon. It’s an announcement signal. It’s designed to attract our attention. We get strange patterns of pulses from quasars and pulsars and radio galaxies and God-knows-what. But prime numbers are very specific. Very artificial. No even number is prime, for example.

Of course, this is incorrect. Two is an even number and is very much prime, since its only divisors are 1 and itself. I recall reading that exchange several times, startled, that Sagan would have had his main character (who he portrays to be hyper-competant in all other scientific dialogue) make such an egregious mathematical error. Perhaps a well-intentioned editor replaced Sagan’s original text which explains the usual mysticism of the primes as atoms for the integers, or perhaps Sagan just made a mistake.

In case you don’t believe me, Page 86 of the 1985 edition of the Simon and Schuster printing of Contact is below, in which Arroway goes on to explain n more detail what prime numbers are. In doing so, she even uses 2 as an example of a prime, contradicting the previous paragraph. And let me state again that this doesn’t take away my enjoyment of the book; it is amazingly written and a great story. This is just a diversion that is obviously an oversight.

Page 86 of the 1985 edition of the Simon & Schuster printing of Contact by Carl Sagan.

Interestingly, I don’t think this mistake remains in the movie version, which came out much later. Contact was actually always intended to be a movie way back in 1979, but because of the development hell that it found itself in, Sagan eventually just wrote a book; the movie came out in 1997, tragically just after his death in 1996. The movie is very good, but the book is better.