When Albert Einstein’s theory of general relativity began to come to public attention in the 1920s, many people speculated about the “fourth dimension” that Einstein had allegedly invoked. What might be in there? A hidden universe, maybe? This was nonsense. Einstein was not proposing a new dimension. What he was saying was that time is a dimension, similar to the three dimensions of space. All four are woven into a single fabric called space-time, which matter distorts to produce gravity.

Even so, other physicists were already starting to speculate about genuinely new dimensions in space.

The first intimation of hidden dimensions began with the work of the theoretical physicist Theodor Kaluza. In a 1921 paper Kaluza showed that, by adding an extra dimension to the equations of Einstein’s theory of general relativity, he could obtain an extra equation that seemed to predict the existence of light.

In 1926 the Swedish physicist Oskar Klein expressed the idea that perhaps the fifth dimension was curled up into an unimaginably small distance: about a billion-trillion-trillionth of a centimeter.

The idea of a dimension being curled may seem strange, but it is actually a familiar phenomenon. A garden hose is a three-dimensional object, but from far enough away it looks like a one-dimensional line, because the other two dimensions are so small. Similarly, it takes so little time to cross Klein’s extra dimension that we do not notice it.

Physicists have since taken Kaluza and Klein’s ideas much further in string theory. String theory has emerged as the most promising candidate for a microscopic theory of gravity. This seeks to explain fundamental particles as the vibrations of even smaller entities called strings. And it is infinitely more ambitious than that: it attempts to provide a complete, unified, and consistent description of the fundamental structure of our universe. (For this reason, it is sometimes, called a ‘Theory of Everything’). The essential idea behind string theory is this: all of the different ‘fundamental ‘ particles of the Standard Model are really just different manifestations of one basic object: a string. How can that be? Well, we would ordinarily picture an electron, for instance, as a point with no internal structure. A point cannot do anything but move. But, if string theory is correct, then under an extremely powerful ‘microscope’ we would realize that the electron is not really a point, but a tiny loop of string. A string can do something aside from moving— it can oscillate in different ways. If it oscillates a certain way, then from a distance, unable to tell it is really a string, we see an electron. But if it oscillates some other way, well, then we call it a photon, or a quark, or a… So, if string theory is correct, the entire world is made of strings!

When string theory was developed in the 1980s, it turned out that it could only work if there were extra dimensions. In the modern version of string theory, known as M-theory, there are up to seven hidden dimensions.

What’s more, these dimensions need not be compact after all. They can be extended regions called branes (short for “membranes”), which may be multi-dimensional.

A brane might be a perfectly adequate hiding place for an entire universe. M-theory postulates a multiverse of branes of various dimensions, coexisting rather like a stack of papers.

These brane worlds should remain quite distinct and separate from each other because forces like gravity do not pass between them. But if branes collide, the results could be monumental. Conceivably, such a collision could have triggered our own Big Bang.

It has also been proposed that gravity, uniquely among the fundamental forces, might “leak” between branes. This leakage could explain why gravity is so weak compared to the other fundamental forces.

As Lisa Randall of Harvard University puts it: “if gravity is spread out over large extra dimensions, its force would be diluted.”

In 1999, Randall and her colleague Raman Sundrum suggested that the branes do not just carry gravity, they produce it by curving space. In effect, this means that a brane “concentrates” gravity, so that it looks weak in a second brane nearby.

This could also explain why we could live on a brane with infinite extra dimensions without noticing them. If their idea is true, there is an awful lot of space out there for other universes.

As Nikola Tesla supposedly said:“The day science begins to study non-physical phenomena, it will make more progress in one decade than in all the previous centuries of its existence.”