Santa's Science

Nick D Kim - more cartoons here Christmas Lunch

When a sleigh, or indeed any object, exceeds the speed of sound, there will be at least one sonic boom. This is a shock wave sent out when the sleigh catches up with pressure waves it generates while moving, explains Nigel Weatherill of the University of Wales, Swansea, who helped the Thrust Supersonic Car break the sound barrier in 1997.

Santa, however, does not generate any sonic booms on Christmas Eve. In his book Unweaving the Rainbow, Richard Dawkins says he has used this fact to disprove the existence of Santa to a six-year-old child. To a biologist this may indeed seem persuasive but, to an aerodynamics engineer, it suggests that Santa has found a way to suppress sonic booms. For example, says Nigel Weatherill, perhaps Santa cancels the peaks and troughs in the shock wave with troughs and peaks of 'antisound' generated by a specialised speaker on his sleigh.

The speed of light is absolute and cannot be exceeded, so we should check that Santa is not breaking cosmic law. The usual figure for the speed of light is 300 million meters per second (984 million feet) which, given that there are 1,000 metres per kilometre (5,280 feet per mile), works out to be 300,000 km (186,000 miles) per second. Santa is comfortably within this limit, travelling at around one-145th of the speed of light - too slow to worry about the implications of Einstein's theory of relativity. This assumes, however, that Santa throws the presents down the chimney while passing overhead. In fact, he stops at each house so that he has to achieve double the speed calculated above (form a standing start, he has to travel the distance between each house in two-10,000ths of a second). That means going from 0 to 4,116 km (2,558 miles) per second in two-10,000ths of a second, an acceleration of 20.5 million kilometres (12.79 million miles) per second per second, or 20.5 billion metres (67.3 billion feet) per second per second.

The acceleration due to gravity is a mere 9.8 metres (32ft) per second per second, so the acceleration of Santa's sleigh is equivalent to about two billion times that caused by the gravitational tug of the Earth. Given that Santa is excessively overweight, say around 200kg (30 stone), the force he will feel is his mass times his acceleration: around 4,000 billion newtons. Even fighter pilots can't cope with accelerations more than a few times that of gravity: they have to use special breathing and so called g-suits to keep the blood in their head. Santa would have to cope with around 2 billion times this acceleration. As the physics professor Lawrence Krauss put it, that would reduce our fat friend to 'chunky salsa'.