Could there be more than three?

A possible way of resolving the greatest problem of physics.

Open door #1 and scroll through our changing understanding of gravity...

To celebrate the recent centenary of the first major test of Einstein's general theory of relativity, we look back over how our picture of gravity has changed.

Does light have weight? Newton thought so. His laws predicted that gravity would bend light, two centuries before Einstein's revolution.

Most of us think that we have the capacity to act freely. Our sense of morality, our legal system, our whole culture is based on the idea that there is such a thing as free will. It's embarrassing then that classical physics seems to tell a different story. And what does quantum theory have to say about free will?

And what are gravitational waves?

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.

Plus 1000 — Mathematical lives

In the last issue Lewis Dartnell explained how chaos on the brain is not only unavoidable but also beneficial. Now he tells us why the same is true for our solar system and sends us on a journey that has been travelled by comets and spacecraft.

The geometry says that now is the right time for a mission to Mars.