There’s a controversial little interpretation of Einstein’s theory of special relativity that could affect what happens to masses moving at a really high speeds: they appear to get heavier.

The effect isn’t huge until something nudges right up to the speed of light, when its mass seems to shoot up to infinity. This happens because you can’t break the speed limit set by light speed. As an object approaches that limit, it requires more and more energy to accelerate it, until eventually, it seems like you’re trying to push an infinitely massive thing, which would require infinite energy.

But that got me thinking. Santa probably has to travel at speeds close to the speed of light if he’s hoping to hit every house and still be home to spend some time with Mrs. Claus on Christmas Eve. Does that mean that Santa’s annual present-delivering jaunt would cause him to pack on extra pounds?

Before we try to answer that question, some important caveats. One, Santa’s mass would only change with respect to a kid on the ground, and Santa would still measure himself at his jolly 400 or so pounds. Two, a lot of physicists think this so-called relativistic mass is meaningless. I’ll talk about that after we do some fun math.

First off, how fast would Santa have to go to hit every house on Christmas eve? Let’s do a quick back-of-the-envelope calculation:

There are around 2.2 billion Christians on earth, and maybe around 4 people per household, on average, making 550 million total households. Sure, lots of homes are really close together and others are spread out over long distances, but there’s a huge amount of vacant space where no one lives on Earth, so let’s average it all out to around a mile in between each household (this is most certainly too high, but work with me here). Jolly Saint Nick probably has to travel that long, coiled-up zig zag in 24 hours, so that will be around 23 million miles per hour.

Einstein’s special relativity came about to help describe how things moving at different velocities relate to one another. Special relativity causes strange behaviors to arise in objects traveling at high speeds, those approaching the speed of light.

One of the theory’s most important concepts is the Lorentz factor, which explains some of those behaviors, and is represented by gamma, or γ:

If you plug Santa’s velocity (v) and the speed of light (c) into gamma, and then multiply that by his usual 400-or-so pound mass, you arrive at his relativistic mass, 400.2 pounds, as observed by a child on the ground, thanks to Santa’s high speed. But 0.2 extra pounds is boring.