(See parts 1, 2, and 3.)

Everybody literally sees the world from a different point of view. Each person is standing in a different location and looking out in a different direction from everyone else. But all viewpoints share certain similarities. If you and I are near one another, we'll see the same events happen in the same order, and although we may differ in our use of the words “right” and “left”, if we're watching something from opposite sides, we'll at least agree on the distances between things. If I see two people holding hands, you'll never see them on separate sides of the street at the same time, no matter where you're standing. All the different viewpoints *preserve* certain observed properties: distances, angles, durations, causality, and so on.

Mathematically, we can write this in two equations. For each of us, every event has a measurable position in space (`x`, `y`, `z`) and time (`t`). If we put my observations on the left-hand side and yours on the right, they will match.

We agree on distances: `x`^{2} + `y`^{2} + `z`^{2} = `x'`^{2} + `y'`^{2} + `z'`^{2}

We agree on durations: `t` = `t'`

Even if I'm in a car doing eighty and you're sitting on the sidewalk enjoying an ice cream cone, we'll agree on the distances between and durations of any events we both happen to witness as I zoom by.

...Or so everyone thought. Don't get me wrong, this is a lovely picture. Mathematically, it's your basic three-dimensional Euclidean geometry, plus a separate dimension for time. All our viewpoints are identical except for a bit of spacial displacement and rotation. There's only one problem. This isn't how the universe really behaves.

1887 was the year of the famous Michelson-Morley experiment, which blew this nice, simple Newtonian view all to hell. For twenty years, confusion reigned. By 1905, a mere eyeblink in academic terms, physics had righted itself, now with a totally new model of space and time.

The new theory was called special relativity. It was built on brilliant new insights from Hendrik Lorentz, Henri Poincaré, and Albert Einstein. And it went something like this: Two observers traveling at incredible velocities (relative to one another) actually do *not* agree on distances, angles, durations, or even the relative time-order of events. But they will agree on something even more fundamental: the basic laws of nature, including laws of motion, causality, and—in particular—the speed of light.

This had the advantage of being, you know, consistent with experiment. But geometrically, it was awfully weird. It wrecked the two equations above. Individual viewpoints were not simple spacial rotations and translations of one another. They were, uh, Lorentz transformations. Yeah. It was two more years before geometry caught up with physics.

In 1907, Hermann Minkowski discovered a kind of geometry (a four-dimensional manifold) that exactly describes the spacetime of special relativity. That is, Minkowski space is the actual geometry of the universe around us, according to relativity. Minkowski's geometry succeeded by treating space and time as interrelated. For example, in Minkowski space:

We may not agree on the spatial distance between two events: `x`^{2} + `y`^{2} + `z`^{2} ≠ `x'`^{2} + `y'`^{2} + `z'`^{2}

We may not agree on the passage of time: `t` ≠ `t'`

But we will agree on a particular mathematical combination of the two: `x`^{2} + `y`^{2} + `z`^{2} - `c``t`^{2} = `x'`^{2} + `y'`^{2} + `z'`^{2} - `c``t'`^{2}

(Here `c` is the speed of light.)

Now comes the controversial, beautiful part. Define a variable `w` as **i**`c``t`. We're going to use `w` as our time coordinate, instead of `t`. Then the last equation above becomes:

`x`^{2} + `y`^{2} + `z`^{2} + `w`^{2} = `x'`^{2} + `y'`^{2} + `z'`^{2} + `w'`^{2}

This looks a lot like our original equation for distance. And in fact this equation describes basic Euclidean geometry in four dimensions. Time becomes just another spacial dimension. All viewpoints are again simple rotations and translations of one another—not in three-dimensional space, but in four-dimensional spacetime.

Here the role of the complex numbers is to provide a new way of looking at the geometry of the universe.

But... what does it all mean? Is time really an imaginary dimension? What does it mean for three dimensions to be real numbers and one to be an imaginary number? These questions are, in a way, the same questions RT asked me months ago, the questions that got me interested in telling this story. What *are* the imaginary numbers? Do they exist? Do they appear in nature? I don't think anyone really knows. Einstein found the **i**`c``t` trick interesting at least (he mentions it twice in his short book *Relativity*, which by the way I enthusiastically recommend), but some physicists think it's a red herring. Maybe we're just dressing the universe up to look more comfortable and familiar.