The result shows that the edged sampling mirror and the edged vacuum window can be treated as a pair of prisms, which generates negative group velocity dispersion.

Three weeks ago Capello exploited a power vacuum at the Football Association to remove a clause in his contract, which gave both sides a two-week window to sever their deal.

But experience has taught the physicists never to open a window because, when they do, all the air rushes out and leaves the room a vacuum.