Today is March 14, otherwise known as Pi Day by math and science nerds, although a case can be made for July 22 being Pi Day since 22/7 is a better approximation for π than 3.14.

It is customary for science and math bloggers to acknowledge Pi Day in some way. I will do that by referring you to a nice post by Sean Carroll who explains why π appears in Einstein’s field equations for general relativity. This is particularly appropriate since March 14 also happens to be Einstein’s birthday.

To start with, note that Newton’s law of gravity F=GMm/r^{2} does not contain π.

But Einstein’s equations for gravity do. Einstein’s equations take the form:

R_{μν} – (1/2)Rg_{μν}=8πGT_{μν}

In this equation, the left hand side describes the curvature of space-time while the right hand side represents the energy-momentum. So the equation says that the energy and momentum in a region distorts space-time correspondingly and it is this distortion that spreads throughout and influences other bodies that are far away.

So why does π appear in the latter but not the former? Carroll explains that Newton’s law represents the force between two point-like particles at a distance with no explanation as to how that force is transmitted over space. When you generalize it, as Einstein’s did, to masses spread out over all space, the equation says that the force is due to a gravitational *field* that spreads out through all space. And it is the introduction of this field due to the spread out masses that causes π to appear.

## 9 comments

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## Menyambal

March 14, 2014 at 11:57 am (UTC -4) Link to this comment

Interesting. I had not seen that aspect of it before. Thanks.

## left0ver1under

March 14, 2014 at 1:14 pm (UTC -4) Link to this comment

How about May 13th (13/5)?

22/7 is a horrible approximation. My favourite is one I learnt a few years ago, which was discovered in China in the 5th century. It’s the most accurate fractional approximation with a denominator less than 30,000.

Repeat the first three odd digits twice: 1 1 3 3 5 5

Separate them with the long division symbol and solve: 355/113 = 3.1415929203

It’s accurate to within 0.0000000849 – not good enough for engineering, but probably good enough up to the second year of college.

## Félix Desrochers-Guérin

March 14, 2014 at 2:03 pm (UTC -4) Link to this comment

You could even say it’s the more

rationalchoice.## Mano Singham

March 14, 2014 at 4:00 pm (UTC -4) Link to this comment

Why are the readers of this blog so much better at gags like these than me?

## Rob Grigjanis

March 14, 2014 at 5:13 pm (UTC -4) Link to this comment

There’s always delicious pi, if you look hard enough. Newton’s law, translated by Gauss;

∇.g= -4πGρ## Rob Grigjanis

March 14, 2014 at 5:29 pm (UTC -4) Link to this comment

Actually, the equation in #5 is, pretty much, the static, weak field, low velocity limit of Einstein’s field equation.

## Rob Grigjanis

March 14, 2014 at 5:37 pm (UTC -4) Link to this comment

And of course, if I wasn’t such an ass, I’d have read Carroll’s post and seen that he said the same thing. *bray*.

## Kevin Alexander

March 14, 2014 at 6:32 pm (UTC -4) Link to this comment

I didn’t even know that today was pi day and yet I made beef pie for dinner!

BTW, pie are not square, lasagna are square, pie are round.

## Paul Jarc

March 17, 2014 at 6:04 pm (UTC -4) Link to this comment

I think you mean “half tau day”.