THE SCIENCE OF DOCTOR WHO

At 60 years old, Doctor Who, the BBC Show following the adventures of the regenerating Time Lord, continues to be highly enjoyable fiction. But it's science fiction. The Doctor's primary tool is a sonic screwdriver, not a magic wand. The Gallifreyan takes science seriously. And so should we...

- STEPHEN BAXTER

THE TARDIS

One of the most immediate, and paradoxical, scientific puzzles raised by Doctor Who is the nature of the TARDIS, the Doctor's vehicle, home and constant companion.

You know what comes next: 'It's bigger on the inside than the outside!' (Or, as one episode's smart alec putdown line had it, 'It's smaller on the outside!'). With the exterior of an antiquated British police telephone call box - the size of a large-ish wardrobe - it appears to contain at least the volume of a roomy house, if not more. Can this be plausible?

In fact, an imaginative extrapolation of Albert Einstein's theory of relativity might permit the possibility that something being bigger-on-the-inside doesn't have to be immediately ruled out as well as that old police box's main function, as a time machine. Because relativity is all about distortions of space and time. And as fans of Doctor Who know, TARDIS is an acronym of Time And Relative Dimension In Space.

Einstein's great breakthroughs in physics, starting in the early decades of the 20th century, came from imaginative, even playful, dreaming of which the Doctor would probably approve. What, Einstein wondered, would the Universe look like if you tried to catch up to a beam of light by travelling on a very fast starship? The problem is that the speed of light around 300,000km per second - is determined by other laws of physics, specifically the laws of electromagnetism. And physics doesn't care how fast you're travelling as you measure that speed; those laws don't change, no matter how fast you go. And so a given beam of light always has the same velocity relative to you - and to everybody else, no matter how they're moving.