Today is a big day for both math and science nerds. It’s Pi Day, the day we celebrate the magical number 3.141592…, but it also happens to be Albert Einstein’s Birthday.

So for today’s edition of Sciency Words, we have a term that involves both circles (and the number pi) as well as Einstein’s physics. Today’s Sciency Word is:

**Rotating Disk**

The rotating disk thought experiment is one of the strangest ideas to come out of Einstein’s general theory of relativity. Part of relativity is the concept of length contraction. If I were to somehow accelerate a yardstick to a speed close to the speed of light, the yardstick’s length would contract or shorten noticeably. The same thing would happen in an extremely powerful gravitational field, such as the gravitational field of a black hole.

But what happens if a rotating disk is accelerated close to the speed of light? What happens to the circumference of the disk? Well, according to Einstein’s thought experiment, the circumference would also contract, just like the straight yardstick. However, the radius of the disk would not change, because the radius is always perpendicular to the disk’s rotation.

The math can get complicated, as it often does in general relativity, because we are no longer dealing with the Euclidean geometry we all suffered through in high school. Instead, we are now entering the strange world of non-Euclidean geometry, a world in which parallel lines *can* intersect, the angles of triangles do not necessarily add up to 180 degrees, and the ratio of a circle’s radius and circumference does not always equal pi.

To put that in a simpler way, pi does not always equal pi. This is not some weird fluke of mathematics but a real phenomenon, one which was predicted by Einstein and later demonstrated through scientific experiments. Do we live in a strange universe or what?

Happy Pi Day, everyone, and happy birthday to Albert Einstein!

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