For those in need of a grade school refresher, a prime number is defined as a positive integer that is only divisible by one and by itself. According to New Scientist, Dr. Curtis Cooper of the University of Central Missouri recently discovered the largest prime number ever as part of the Great Internet Mersenne Prime Search (GIMPS). Known as M74207281, the number has 22,338,618 digits, which is five million more than the previous largest prime number (which Cooper also discovered back in 2013).
Curtis Cooper's discovery was announced earlier this month, but the computer found the number way back in September of 2015. A post on the Mersenne blog explains that a "bug prevented the email notification from being sent," so even though the new prime was found months ago, the discovery date reflects when a human first saw it. The number joins the exclusive ranks of only 48 other Mersenne primes—rare numbers (primes that are one less than a power of two) which are named after the French monk Marin Mersenne who studied them in the early 17th century. (There's a handful of helpful YouTube videos if you want a more in depth look at Mersenne primes.)
For the find, Cooper is eligible for a $3000 GIMPS research discovery award, even though the number itself doesn't really have any practical application. As explained in the Mersenne blog post: "While prime numbers are important for cryptography, this prime is too large to currently be of practical value."
That said, the search itself is useful as a means of testing computer hardware, so it's not just about the destination.
To see exactly what a 22,338,618-digit number looks like, head over to the Mersenne site, and check out the video below to hear an interview with Curtis about the discovery.