The largest prime number ever discovered is 23 million digits long
Numbers might not sound like they need discovering, but a crowd-sourced project has now identified the largest prime number known. The number was discovered by the Great Internet Mersenne Prime Search (GIMPS), an online project of citizen scientists who spot and verify prime numbers using specialty software. It's been dubbed M77232917 – and if you're wondering why it needs a codename, well, we'd be here all day typing out the 23 million digits that make it up.
If it's been a while since high school math class, here's a quick refresher: Prime numbers are those that are only divisible by 1 and themselves. Small primes are fairly easy to identify through trial and error – 6 can be divided by 2 and 3, so it isn't prime, but 7 is – but as you look at larger and larger numbers it gets less obvious. It might take you a while to figure out, for example, that 11,319,033 isn't prime because it can be divided by 213 and 53,141. Finding the really large primes is a task best left up to computers running software like GIMPS.
As the MP in its acronym suggests, GIMPS is specifically searching for a rare class of prime numbers called Mersenne Primes. These are numbers that are one less than a power of 2, expressed as Mn = 2n - 1. That means that the newcomer M77232917 is calculated through a chain of 77,232,917 twos, and then subtracting 1. It's only the 50th known Mersenne Prime ever identified, and it's made up of 23,249,425 digits.
It was discovered on December 26, 2017 by electrical engineer Jonathan Pace, and it initially took six days of non-stop number crunching to show that it was indeed a prime number. Pace was using a consumer-level PC running an Intel i5-6600 processor, and after it was identified it was then independently verified by other users, with a range of other programs and hardware setups.
So what can we use this new number for? Not much, really. It mostly seems to be a curiosity for amateur mathematicians hoping for "the thrill of possibly discovering a record-setting, rare, and historic new Mersenne Prime." But then again, we didn't really have much use for prime numbers at all until relatively recently, and the Mersenne project points out that now they're the basis of cryptography algorithms.
If M77232917 isn't specific enough for you, you can download the number itself as a plain text file – a 23 MB plain text file, we might add. Grab a coffee, because you'll be scrolling through it for a while.