### The Most Striking Equation in Mathematics Leonhard Euler, one of the greatest mathematicians of all time. He also contributed to physics, astronomy, and the fashion of wearing boxer shorts on his head.

Nobody can write about the history of science and mathematics without eventually bringing up Leonhard Euler (1707-1783). (Most Americans end up pronouncing his name like “oiler”.) So many important findings in math and physics that it’s hard to list them all, so I won’t try. I don’t really want to write a biography of him anyway: I just want to focus on one profound equation he discovered, and follow where it leads into some other interesting math…and of course physics.

Without further ado, here’s the most famous version of Euler’s equation: The numbers π and -1 you should know; the exponential number e may be less familiar. It’s another geometrical number like π, and it has a value approximately equal to 2.71828 (but since it goes on forever without repeating, I’ll spare you any more digits). There are several ways to get at e, but we don’t need to worry about them for now. The main thing is that it’s standard, built into scientific calculators, and well-understood. The imaginary unit i also appears in the equation: recall that So this is where things get interesting: e and π are real irrational numbers, but including i should give us a complex number, but it doesn’t: it gives the negative integer -1. (Irrational means these numbers can’t be expressed as the ratio of integers; integers are the whole numbers 0, 1, -1, 2, -2, 3, -3, etc.) Somehow the combination of two irrational numbers and the imaginary unit produces -1.

In case you’re thinking I’m making a big deal out of nothing, type “exp(pi)” into Wolfram Alpha: you should get which is significantly larger than 1, and a positive number to boot. In fact, you won’t ever get a negative number by raising e to a real number. Try these in Wolfram Alpha: exp(-10), exp(-1), exp(0), exp(1), exp(10). Something weird and cool happens when you include an imaginary number in the exponent, and Euler realized after some careful computation what was going on. Illustration of the complex plane: the connection between complex numbers and points in two dimensions. Four points are plotted so you can see the correspondence between x and y coordinates and the real and imaginary parts of the complex numbers.

First of all, the number π is special: if you plug another number in its place (say 1), you won’t get a real number out: It’s a complex number: the sum of a real number with an imaginary number. (Refer to the figure on the right for a refresher on how to interpret complex number using coordinates.) However, watch what happens when we square the real part and the imaginary part and add them together: I’m rounding in all these cases; if you want to be more accurate, keep the full set of digits you get out of your calculator before squaring…and you’ll still get 1 as your answer. This will happen if you use any real number in place of π, and Euler determined that you can split the exponential into real and imaginary parts like this: where “cos” is the cosine function, “sin” is the sine function, and θ is a real number.

If you’re like me, you first learned about sine and cosine in the context of triangles, and that’s a pretty useful way to think of them here too. To start, let’s draw a circle with radius equal to 1. All points on the circle will be the same distance—1—from the center; after all, that’s really what a circle means. Take an arbitrary point on the circle and map its x– and y-coordinates; draw a line from the center of the circle to your point. Now complete the triangle by connecting your point with the x-axis and the center, as shown in the figure below: Relating a circle to complex numbers. The radius line (in blue) has a length of 1, and we'll use that as the hypotenuse of a triangle. Then the x- and y-coordinates of the end of the line are given by the cosine and sine of the angle θ as shown.

If we associate the x– and y-axes with the real and imaginary parts of a complex number as before, the angle between the blue line and the x-axis, marked by θ, is the same as in Euler’s formula. The x-coordinate is the cosine of the angle, and the y-coordinate is the sine of the angle. Properly, we need to write the angle in radians, not degrees: one circle (360°) is 2π radians, so a half-circle (180°) is π radians. Looking at the circle above, you can see that an angle of π corresponds to x = -1 and y = 0, exactly what Euler’s formula predicts!

I won’t prove Euler’s formula (since it really needs calculus to do properly), but with a little more work we can see how useful it is. By drawing a larger or smaller circle, we can represent any complex number using a variation on Euler’s formula: where r is a real number representing the radius of the new circle. Instead of writing the real and imaginary parts of the complex number like x– and y-coordinates, we can use r and θ, which are known as polar coordinates. The angle θ is also known as the phase of the complex number, and r is its magnitude. That’s a very simple formula, and makes doing many calculations with complex numbers quite easy. Rotating a square using complex numbers: write the coordinates for each corner as complex numbers. Use Euler's formula to write a complex number with the angle you want to rotate. Multiplying "a" by the rotation factor gives you "A", and so forth. Despite how it may look, each corner is the same distance from the axis of rotation before and after.

For example: if you want to mathematically represent rotations in two dimensions, you can do it using complex numbers. Take the coordinates of (say) the corners of a square, and write them as four complex numbers a, b, c, and d. To rotate the square by angle θ, multiply the number for each corner of the square by and you’ll get the coordinates of the rotated square: ## Euler and Quantum Mechanics

Quantum physics doesn’t just use complex numbers out of convenience (like we did to perform rotations): complex numbers are absolutely necessary, and Euler’s formula is more than useful. The wavefunction in quantum mechanics consists of the real probability amplitude and a phase: The phase is generally undetectable in experiments, but the difference in phase between two particles is measurable. The difference leads to interference, which is key to understanding the famous double-slit experiment and the Aharonov-Bohm effect.

## Euler’s Formula Using Quaternions

A few weeks ago, I wrote a post about a fancier version of complex numbers known as quaternions, discovered many years after Euler’s death. Instead of one imaginary number, quaternions have three (labeled i, j, and k), and we showed them to be useful for representing rotations in three dimensions. Therefore, as you might expect, there are three Euler-like formulas for quaternions, representing rotations around the x, y, and z axes:  NASA schematic for an airplane showing the three angles pitch, yaw, and roll.

In aerospace engineering, the axes are attached to the body of the airplane or spacecraft, and the angles θ, φ, and ξ are called pitch, yaw, and roll. The different controls aboard the plane are designed to control rotation in those directions: for example, during liftoff, you need to raise the pitch, but keep yaw to a minimum, while roll can be used to steer the plane in the right direction. (By the way, there’s another set of angles known as (guess what?) Euler angles, but these don’t correspond directly to the angles I’m using here.)

By now you may guess the way my twisted brain works, and know what our next logical step must be. If complex numbers generalize to quaternions, are there even higher-dimensional versions of complex numbers—with attendant rotations and Euler formulas to go with them? The answer of course is a resounding yes, though we have to introduce a new type of number to make everything work. However, by the time we’re done, we’ll have the math to handle relativity and particle physics, and you, Dear Reader, will arrive at the cutting edge of modern physics.

#### 10 Responses to “The Most Striking Equation in Mathematics”

1. 1 joaquinbarroso December 9, 2011 at 21:32

My high school math teacher told us about this equation that in it we could find historical numbers important in the development of conceptual mathematics: Negative numbers were created to answer a – b where b > a; imaginary numbers were created to answer sqrt(-a); pi was linked to Pythagoras and his school of thought derived from Plato’s; e was the answer to finding a curve which had at every point a slope equal to the function value at that same point.
Truly an elegant equation, exiting and as you put it, striking.

You have a nice blog. Keep it up

2. 2 Porlock Junior December 10, 2011 at 14:18

OT, but I contend that obiter dicta are always fair game for a response:

How would an Anglophone pronounce the name apart from

“oiler” (in the local accent of English)

full German pronunciaton (in accent of choice)

something more or less ludicrous (Cf. BBC announcers talking of jag-you-ars in Nick-a-rag-you-a)

?

With apologies for my nationalistic mood this morning. In all fairness, I think that BBC Standard may call for a reasonably realistic pronunciation of Vincent van Hokkh, so universally mangled by Americans as Van Go. And as for pronouncing Huyghens, forget it — can’t be done anyway.

Great treatment of Euler’s gem, by the way.

3. 3 Moisés C.D. Marcón Rosado December 12, 2011 at 10:53

Mind stretching read that I couldn`t completely understand but that it taught me about the `yaw` -thanks man. Keep `em coming!

1. 1 W. K. Clifford: The Geometry of Physics « Galileo's Pendulum Trackback on December 19, 2011 at 16:44
2. 2 Everything is Geometrical: Hermann Grassmann’s Algebra « Galileo's Pendulum Trackback on April 26, 2012 at 15:51
3. 3 A Brief Family Tree of Some Important Math | Whiskey…Tango…Foxtrot? Trackback on April 27, 2012 at 12:11
4. 4 Imaginary Numbers are Real « Galileo's Pendulum Trackback on June 9, 2012 at 11:43
5. 5 The Most Striking Equation in Mathematics « Galileo’s Pendulum « linkstream2 microblog Trackback on February 4, 2013 at 21:32
6. 6 Irish mathematics for St. Patrick’s Day | Galileo's Pendulum Trackback on March 17, 2013 at 11:31
7. 7 Talk mathy to me: what’s the square root of i? | Galileo's Pendulum Trackback on April 8, 2013 at 15:29