In a book like Light Years there are always going to be a few gaps. Aspects of the subject that slip through the net until it's too late.

Here's a surprise for many people who thought they knew all there was to know about light. Not only can beams of light differ in colour (or wavelength if you prefer), and in polarization (effectively which direction the ripple in the light's wave points in), light can also be given a twist.

In normal laser light, each ripple of light is moving in harmony. If you were to take a slice through the light beam, no matter which part of the beam you looked in, each wave would be at the same stage of its up and down motion. But take that same slice through a twisted light beam and as you progress around the circle of the the beam you will find that the each wave is slightly earlier (or later depending on whether the twist is left or right handed) in its progress.

Think of light as a sequence of photon, and twisted light's photons travel in tight, corkscrew spiral.

This strange phenomenon already has potential applications in astronomy - it could even be the answer to why we've never spotted alien transmissions out there in the universe, as they could send a signal by radio or other light wave not by modulating the frequency or amplitude as we do, but by modulating the twist. But whether or not it will ever do anything practical, it's one more strange fact to add to the picture of light.

If you aren't averse to technical stuff, take a look at this page from the optical physics group at Colgate University in Hamilton, New York. You'll see various examples, but also a very simple looking diffraction grating that can be used to give laser light a twist.

Brian Clegg is a popular science writer whose books include A Brief History of Infinity and Light Years.