John Barrow gives us an overview, from Aristotle's ideas to Cantor's never-ending tower of mathematical infinities, and from shock waves to black holes.

The holy grail for 21st century physics is to produce a unified theory of everything that can describe the world at every level, from the tiniest particles to the largest galaxies. Currently the strongest contender for such a theory is something called M-theory. So what is this supposed mother of all theories all about?

It's been nearly 18 months since the Large Hadron Collider at CERN started up and scientists are eagerly awaiting their first glimpse into the cosmic mysteries it was designed to explore. But when can we realistically expect the first ground-breaking discoveries to come through? Last week, John Ellis, outgoing leader of the theory division at CERN, addressed an audience of physicists at the University of Cambridge to update them on the current state of play. Plus went along and also managed to catch Ellis for a quick interview.

You're unlikely to ever run into a black hole, but here's what it "looks" and "sounds" like when two black holes run into each other. The movie shows a simulation of the gravitational waves generated when two black holes collide and form a third, and the sound file shows what these waves would sound like if you cold hear them.

We're all on a journey into the future, but can we travel into the past? Find out with Kip Thorne

And what are gravitational waves?

Find out with Martin Rees

Scientists crack black holes' balancing act

With online socialising and alternative realities like Second Life it may seem as if reality has become a whole lot bigger over the last few years. In one branch of theoretical physics, though, things seem to be going the other way. String theorists have been developing the idea that the space and time we inhabit, including ourselves, might be nothing more than an illusion, a hologram conjured up by a reality which lacks a crucial feature of the world as we perceive it: the third dimension. Plus talks to Juan Maldacena to find out more.

On May 19 2009 the Space Shuttle Atlantis released the Hubble Space Telescope back into orbit after a hugely successful servicing mission. To mark the occasion, Mario Livio, one of the scientists involved in the mission and intimately acquainted with Hubble, takes stock of its scientific legacy.

Using their X-ray vision, astronomers have for the first time found two black holes in the same galaxy, and taken a peek inside a neutron star.

This issue of Plus is a special, marking the occasion of Stephen Hawking's 60th birthday. Plus attended his Birthday Conference in Cambridge, where we interviewed some of the world's most influential mathematicians and physicists.