How many dimensions are there? In the latest online poll of our *Science fiction, science fact* project you told us that you'd like an answer to this question. So we asked theoretical physicist David Berman to find out more. We also bring you a range of other *Plus* articles exploring the question, as well as two articles from FQXi who are our partners on this project. Happy reading!

And don't forget to vote for the next question you'd like to have answered!

Kaluza, Klein and their story of a fifth dimension — David Berman explains that, although we might not initially think of it this way, we are actually used to living in a curved, multidimensional universe. And a mathematical argument might just explain how those higher dimensions are hidden from view.

The ten dimensions of string theory — String theory has one very unique consequence that no other theory of physics before has had: it predicts the number of dimensions of space-time. David Berman explains where these other dimensions might be hiding and how we might observe them.

How many dimensions are there? – the podcast — You can listen to an interview with David Berman as he tells us how Kaluza, Klein and their fifth dimension might help us understand the ten dimensions of string theory.

Hidden dimensions — That geometry should be relevant to physics is no surprise — after all, space is the arena in which physics happens. What is surprising, though, is the extent to which the geometry of space actually determines physics and just how exotic the geometric structure of our Universe appears to be. We talked to mathematician Shing-Tung Yau to find out more.

Exotic spheres, or why 4-dimensional space is a crazy place — For years, scientists and science fiction writers have contemplated the possibilities of higher dimensional spaces. What would a 4- or 5-dimensional universe look like? Or might it even be true that we already inhabit such a space, that our 3-dimensional home is no more than a slice through a higher dimensional realm, just as a slice through a 3-dimensional cube produces a 2-dimensional square?

String theory: From Newton to Einstein and beyond — Over the last few years the words *string theory* have nudged their way into public consciousness. It's a theory of everything in which everything's made of strings — or something like that. But why strings? What do they do? Where did the idea come from and why do we need such a theory? David Berman has an equation-free introduction for beginners.

Taking on a 10-D Universe with 8-D maths — A bizarre set of of 8-dimensional numbers could explain how to handle string-theory's extra dimensions, why elementary particles come in families of three... and maybe even how spacetime emerges in four dimensions. This article is from the FQXi community website.

The quest to simplify — String theory predicts there are more than the familiar four dimensions of space-time. But where do those extra dimensions come from? Eva Silverstein is looking for the answer. This article is from the FQXi community website.

## Additional dimensions

How can there be additional dimensions when the dimensions we are familiar with do not actually exist in nature? Dimensions are man-made conceptual constructs that allow us to place objects in space and in time. In this context, "additional" dimensions are nonsensical.