Turns out, only 46 - until recently, when the 47th was discovered by a "prime hunter" in Norway. (This one's for you, Bente, since I failed you in the thunder article.)
Bear with me, even if you aren't a math wizard, as the story is a quirky testament to the power of the media and the ability of anyone to contribute to science, amateur or expert.
NPR reporter Joe Palca said that this sought-after number represents the 47th Mersenne prime discovered since ancient Greek mathematicians first uncovered them.
These primes are called the "jewels" of number theory, and it takes a huge computing system about two or three weeks to test a single number to see if it could lead to a Mersenne prime. For those of you for whom basic math is a distant memory, a reminder: Primes are numbers that are divisible by only the number 1 and themselves. So 2 is prime; so are 3, 5, 7 and so on. The year 2003 was a prime year, and 2011 will be as well.
Ok, so we've brushed up on prime numbers, but what is so special about a Mersenne prime number? They are a special class of prime numbers, named after the French monk Marin Mersenne (1588-1648), with a very particular formula. N is a prime, and the result is a prime. What makes the Mersenne primes so interesting, Palca says, is how rare they are - and their gargantuan size.
Ancient Greek mathematicians were the first to describe Mersenne primes, calculating the first four by the 3rd century B.C.E. After a long lag, three were discovered in the 15th and 16th centuries, two in the 18th and 19th centuries, and the remaining 37 after that. The largest Mersenne prime discovered to date, the 45th (in August 2008), has more than 12.97 million digits.
Thanks to the GIMPS, or Great Internet Mersenne Prime Search, the search for these special primes has definitely sped up. GIMPS participants have found 13 Mersenne primes in 13 years - no triskadeskaphobia here.
The prime bet
NPR's Palca, reporting on these rare numbers back on April 10, told NPR listeners to download the GIMPS software and help the project along (which you can do, for free, here). He made a bet with GIMPS founder George Woltman that the next Mersenne prime would be found before 2012, thanks to help from GIMPS participants and NPR listeners.
By total coincidence, two days later an IT professional in Norway named Odd Magnar Strindmo found the 47th prime, more than 12.8 million digits long! Unfortunately, nobody noticed it until June 4, at which point the prime was independently verified on June 12. (Want a Mersenne prime number poster? Click here; see below!)
She should look a lot happier than this, considering she's in her prime.
Palca said the bet is still on - he actually meant the next largest prime, so it would have to beat the 45th Mersenne prime. "I'm willing to carry on the bet until the largest one is found. After all, we didn't bet any money, so I can afford to be magnanimous."
For a little history, some theorems and unsolved mathematical questions related to these primes, see this page.