Science

# Relativity Theory Explains How Santa Fits Down Your Chimney

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When physicist Katy Sheen was a little girl, she craved a rational explanation for Santa’s magic. Now a professor at the University of Exeter, she’s holding symposiums to explain to children that scientific concepts like the Doppler effect and the special relativity theory can explain how Santa can deliver presents to 700 million children in one night.

Mixing science with the magic of Christmas is a tricky game, but Sheen thinks the miraculous things Santa does can be explained rationally. In her spare time, Sheen did the math and discovered that Albert Einstein’s special relativity theory can explain how Santa fits down a chimney. This theory states that all physical laws are the same regardless of what constant velocity an object is moving at and maintains that the speed of light is also constant. Crucially, Einstein also suggested that objects in motion shrink along their direction of travel.

“At the speeds he needs to travel to deliver presents to every child, Father Christmas shrinks — or gets thinner — in the direction he is traveling,” a statement on her work explained. “And he has to be careful to not stop for mince pie in a chimney, or he could grow back to full size!”

Sheen also says that the special relativity theory explains why Santa doesn’t appear to have visibly aged, because relativity can slow down clocks. According to the “time dilation” part of the theory, it is possible for there to be a difference in the time elapsed between events, depending on the position of the observer. Traveling at the speed of light theoretically allows Santa to travel into the future, while the rest of us, moving at normal speeds, perceive only regular time.

To get presents to every child in a single night, Santa would definitely have to travel at the speed of light — somewhere about 10 million kilometers per hour, Sheen calculates. This means that he would have to move 200,000 times faster than the world’s fastest man, Usain Bolt.

Another theory called the Doppler effect explains why children can’t hear Santa arrive, Sheen says. The Doppler effect is a scientific theory that says the frequency of sound increases as the source moves toward the listener and a decrease in the pitch as it moves away. In the same way there is a sudden change in pitch when a ringing ambulance passes by, the “sound of bells and [Santa’s] deep ‘ho, ho, ho’” get higher in pitch as he approaches and then becomes silent when he moves beyond the human range of hearing.

Sheen doesn’t plan on releasing a paper on her Santa science theories, but she does encourage everyone to think about the possible mathematics behind Santa’s magic — and to leave Santa a little glass of sherry on the big night.

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