It goes by many names: the hypercube, the 8-cell, or the octochoron. It is represented by many shapes; a small cube inside a larger cube, two cubes connected by a bridge, a cube with slightly skewed angles. All the tesseract's names and shapes are attempts to visualize what we can never understand. What does the fourth dimension look like?

Most people's first encounter with the word 'tesseract' was in the dimension-hopping parts of *A Wrinkle in Time*. Madeleine L'Engle's characters hopped through vast reaches of space by 'folding' space into higher dimensions, bringing the edges of the three dimensional universe together the way someone might fold the edges of a towel together. This practice was called 'tesseracting,' and it worked very well, until the characters were nearly killed by accidently getting into a two-dimensional universe. L'Engle wasn't making the term up. She used a term coined by mathematician Charles Howard Hinton in the late 1800s. It refers to a cube, or what a four-dimensional being playing with four dimensional objects would consider a cube.

**Making a Mental Tesseract**

Picturing things in four dimensions is not physically possible for provincial three-dimensional consciousnesses such as ours. It's possible to envision, however, a building process that lets us understand the general idea of a tesseract, or hypercube. A point, a one dimensional object, has one 'corner' and nothing else. A line segment has two corners, and length, but not width. A square has four corners and length and width, but no volume. A cube has eight corners, length, width, and volume. Building on this, we understand that a hypercube, or tesseract, has to have sixteen corners (as the number of corners doubles each time), and length, width, volume, and some new quality.

If we want to visualize the making of a tesseract, we would again start with a point. If that point were made of malleable putty, and someone put their finger on it and drew it out in one direction, it would be a line segment. If that same person put their fingers on the corners of the line segment, and pulled them out, perpendicular to the line segment, the putty would form a square. If they were to take the four corners of the square and draw them out, perpendicular to the edges of the square, the putty would stretch into a cube. To form a tesseract, these magic fingers would have to swoop in again, take hold of each of the corners of the cube, and pull them out in a direction perpendicular to all the edges of the cube.

In three dimensions, of course, that last part is impossible. But add another dimension, and it could happen. This is why so many pictures of tesseracts show a smaller cube inside a larger one. That is the best visual approximation we have of a cube being pulled outwards. But it's not an actual tesseract. Just like each of the faces of a cube is a square, each of the faces of a tesseract is a cube.

**Playing With Four Dimensions**

The tesseract has tantalized people since its conception. Most children are pleased when they find that they can draw simple cubes on a piece of paper by connecting two squares with straight lines. It's a mental breakthrough, making three dimensional 'shapes' out of two dimensional lines. The tesseract seems to require nothing more than the same mental breakthrough. Just take those cubes and connect them . . . somehow.

Hinton himself, after writing a book about tesseracts and the fourth dimension, marketed 'magic cubes' that he claimed could help people see, or understand, the fourth dimension. This being the great age of Victorian spiritualism, they were used during seances to allow people to see ghosts, who, it was supposed, were as likely to hang out in the fourth dimension as anywhere else.