Here’s the clinching argument against all those who don’t believe in Santa Claus: Albert Einstein. Over a century ago, Einstein’s Special Theory of Relativity had laid the foundation for dismissing some of the pointless arguments raised by sceptics. This Christmas season, a British scientist has gone back to the theory to explain how Santa does all the things the sceptics said he couldn’t.
Santa couldn’t possibly deliver presents to all the world’s children in one night, they said. Even if he did, how could he go unnoticed when he came? Besides, they said, he’s so fat he would never fit into the chimney.
Dr Katy Sheen, 33, a geophysicist at the University of Exeter, has calculated that all Santa needs to do is travel at 10 million kilometres per hour to deliver presents to 700 million children in a night of 31 hours, taking various time zones in consideration. To explain why Santa goes unseen and unheard, Dr Sheen refers to what is known as the Doppler effect in physics: Santa’s high speed would affect light and sound waves from the perspective of an observer, making Santa invisible and taking him — and his reindeer — beyond hearing range.
The chimney is where the Special Theory of Relativity comes in. High speed would make Santa shrink, Dr Sheen explains, though she does caution him against stopping for a mince pie in the chimney, for that could make him grow back to full size.
The Special Theory of Relativity, proposed in 1905, has two postulates; the second one states that the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.
“Imagine you throw a ball in an aeroplane,” Dr Sheen wrote in an email to The Indian Express. “To you, moving with the plane, the ball will appear to move at the same speed as if you had thrown it standing on the Earth’s surface. However, to someone watching the plane from the Earth’s surface, the ball will appear to be moving incredibly quickly. So motion appears different to different observers.”
If you replaced the ball with a photon of light, however, Einstein’s theory would mean that it would have to travel at the same speed (in a vacuum), no matter where you watched it from. “For this to be true, something else has to change between the two reference frames — and that has to be time or space,” Dr Sheen explained. “So essentially, time slows down, and space contracts for moving objects.”
The speed of 10 million kilometres per hour, however, is only a tenth the speed of light — although it’s 200,000 times as fast as Usain Bolt. “I suspect that Santa Claus can actually travel much closer to the speed of light as that would allow the thinning (length contraction) and time slowdown to have an observable effect,” Dr Sheen wrote. “For example, for Santa to thin by 50%, he would need to reach about 87% the speed of light… which is why he has to be careful he doesn’t get stuck in the chimney!”
Time contraction, meanwhile, explains another mystery about Santa, one that had puzzled Dr Sheen as a child, when she had written to Santa to ask him why he never aged. The answer she got was that it was “all magic”. She has since figured that it must be Special Relativity that slows down the clock.
Dr Sheen now works on the physics of the atmosphere and oceans. According to the University of Exeter web site, she thinks Einstein has a resemblance to Santa. Having connected the two, she stressed in her email, “This is a bit of fun really, to be taken in the festive spirit — I am not at the stage of writing it up as a research paper. It’s a great way to get children engaged in science and thinking about some difficult, but fascinating, physics!”