The mystery of how Father Christmas can deliver presents to 700 million children in one night, fit down the chimney and arrive without being seen or heard has been explained by a UK physicist using Einstein's special relativity theory.
Santa and his reindeer zoom around the world at such speed that - according to relativity theory - they would shrink, enabling Father Christmas and a huge sack of presents to fit down chimneys, according to Katy Sheen, a physicist at the University of Exeter in the UK.
Sheen has also found a scientific explanation for why Santa is not heard arriving by children, and why they rarely catch a glimpse of him on Christmas eve.
Santa's stealth delivery is partly explained by special relativity theory devised by Albert Einstein, she said.
Relativity theory explains how Father Christmas can fit down the chimney. At the speeds he needs to travel to deliver presents to every child, Father Christmas shrinks - or gets thinner - in the direction he is travelling.
Sheen said that relativity also explains why Father Christmas appears not to have aged throughout the ages, because relativity can slow down clocks.
The physicist has calculated that Santa and his reindeer would have to travel at about 10 million kilometres per hour to deliver presents to every child expected to celebrate Christmas in 31 hours (taking into account world time zones).
If millions of children have been good, and deserve bigger stockings, he may need to travel even faster. Such speed would make him change from red to green and, at greater speeds, he would disappear, she said.
Children would not be able to recognise him as he would appear as a rainbow-coloured blur, eventually disappearing to the human eye, according to the physicist.
Travelling at more than 200,000 times faster than Usain Bolt, the world's fastest man, the laws of physics explain why Santa is rarely seen by children while delivering presents.
The Doppler effect would make Santa change colour because the light waves he releases would get squashed at such a high speed, she said.
The Doppler effect also explains why children cannot hear Father Christmas arrive. As Santa and his sleigh approach, the sound of bells and his deep 'ho, ho, ho' would get higher and higher and then become completely silent, because he would move beyond human hearing range.
She said if children hear a bang on Christmas night, it may not be the sound of Santa dropping his presents, landing on their roof, or sliding down the chimney with a plop. Santa's reindeer could have broken the speed of sound, resulting in a 'sonic boom.'
Sheen calculated how fast Father Christmas would have to travel by working out the number of households likely to be celebrating Christmas around the world, along with the number of children likely to be in them.
She hopes her explanation for Santa's stealth delivery system - and therefore his very existence - will inspire children to take a greater interest in physics.
