We have known about the ordinary three dimensions of space — length, width and height (or to the mathematically minded, x, y, z) for ages. At least, suggests the University of Cambridge, since the days of the pyramids — the ancient Egyptians could not have designed them without an understanding of three-dimensional shapes.

A few thousand years after the pyramids, Albert Einstein demonstrated that time is the fourth dimension, which only goes in one direction. No doubling back for a redo. But what is the fifth dimension that we keep hearing about?

**Dimensions Galore**

Before we get lost in a multidimensional freeway interchange, Sciencing gives us a little road map into this whole dimension business. The first three were taken for granted. Hardly anyone but pure mathematicians thought much about additional dimensions until Einstein came along. He showed that treating time as a fourth dimension was the key to understanding why the speed of light is the same for every observer, no matter how fast you are going.

The result was Einstein’s theory of special relativity. And Einstein’s success at explaining the relationship between time and space, electricity and magnetism, also triggered a dimension chase in the theoretical physics community. Gravity still had to be brought into the fold, along with the strong and weak nuclear forces.

Two theorists, Theodor Kaluza from Germany and Oskar Klein from Sweden, independently proposed mathematical theories in which a fifth dimension would elegantly tie together electromagnetism and gravity. And today, string theory, a prominent school of thought in physics, argues that 10 dimensions are needed to pull everything together.

**Pill Bugs **

But if all these dimensions really exist, where are they? Oskar Klein had an explanation. As Sciencing colorfully puts it, he “came up with the idea that the fifth dimension is invisible to the human eye, as it is minuscule and curls up on itself like a pill bug rolls up under threat.”

Out on the frontiers of science, however, where scientists actually do their work, science does not speak with one voice. Scientists disagree with each other all the time. When Gizmodo asked physicists about their opinions on the fifth dimension, their answers varied widely. And that is before we even get to mathematical theories about the sixth, seventh, or eighth dimensions, on up to (depending on who you ask) 10 dimensions, or perhaps even an infinite number of them.

**Doors to Imagination — and Beyond**

At least the physicists more or less agree on what they disagree about. But in pop culture, the subject of “what is the fifth dimension?” has gained additional dimensions beyond just theoretical physics. When we speak of doors to interdimensional travel, in London or elsewhere, the worlds these doors are envisioned as opening onto seem only loosely related to the formal, mathematical dimensions that the physicists and mathematicians play with.

On the frontier, not only do scientists often disagree; the lines between science, imagination and metaphor can all get slightly blurred. A little bit of ambiguity that helps keep all of us on our toes.