This article from *Scientific American*, April 14, 2003

By Max Tegmark (tegmark@mit.edu)

THE SIMPLEST TYPE of parallel universe is simply a region of space that is too far away for us to have seen yet. The farthest that we can observe is currently about 4X10^{26} meters, or 42 billion light-years--the distance that light has been able to travel since the big bang began. (The distance is greater than 14 billion light-years because cosmic expansion has lengthened distances.) Each of the Level I parallel universes is basically the same as ours. All the differences stem from variations in the initial arrangement of matter.

A SOMEWHAT MORE ELABORATE type of parallel universe emerges from the theory of cosmological inflation. The idea is that our Level I multiverse--namely, our universe and contiguous regions of space--is a bubble embedded in an even vaster but mostly empty volume. Other bubbles exist out there, disconnected from ours. They nucleate like raindrops in a cloud. During nucleation, variations in quantum fields endow each bubble with properties that distinguish it from other bubbles.

This graph shows the relative strength of the electromagnetic force (how much electrons and other charged particles attract one another, for example) and of the strong nuclear force (what holds the nucleus of an atom together). Notice that the scales of the axes are logarithmic, not linear. This means that the small black area where we sit is even smaller than it looks on this graph! The point is that the range of possible values for the two forces is quite small – small enough to allow some people to speculate that a greater force must have set the values “just so” to allow a hospitable environment in which humans could evolve.

**Quantum Dice**

IMAGINE AN IDEAL DIE whose randomness is purely quantum. When you roll it, the die appears to land on a certain value at random. Quantum mechanics, however, predicts that it lands on all values at once. One way to reconcile these contradictory views is to conclude that the die lands on different values in different universes. In one sixth of the universes, it lands on 1; in one sixth, on 2, and so on. Trapped within one universe, we can perceive only a fraction of the full quantum reality.

**Ergodicity **

ACCORDING TO THE PRINCIPLE of ergodicity, quantum parallel universes are equivalent to more prosaic types of parallel universes. A quantum universe splits over time into multiple universes (*left*). Yet those new universes are no different from parallel universes that already exist somewhere else in space---in, for example, other Level I universes (*right*). The key idea is that parallel universes, of whatever type, embody different ways that events could have unfolded.

**The Nature of Time**

MOST PEOPLE THINK of time as a way to describe change. At one moment, matter has a certain arrangement; a moment later, it has another (*left*). The concept of multiverses suggests an alternative view. If parallel universes contain all possible arrangements of matter (*right*), then time is simply a way to put those universes into a sequence. The universes themselves are static; change is an illusion, albeit an interesting one.