A whiff of heresy is in the air. There are very few beliefs in science that have the force of doctrine, but among them is the conviction that the speed of light is fixed. It forms the rock upon which Einstein built one of the 20th century's most enduring monuments, the General Theory of Relativity. But in science nothing is sacred, and several leading theoretical physicists are now toying with the delicious notion that the speed of light may not be constant after all - but may actually have varied during the 15 billion years the Universe has existed. To some, it remains anathema. "It took us a year to get the first paper published in Physica