Here’s how Santa Claus can visit 700 million children on Christmas Eve — according to science

A British researcher says she's solved the mystery of how Santa Claus can slide down chimneys without a sound and deliver presents to 700 million children -- all in one night.

Dr. Katy Sheen, a physicist at the University of Exeter, said the magic of Christmas was at least partially explained by Albert Einstein's theory of relativity, reported Phys.org.

Sheen, who delivered her findings Wednesday to an audience of children at the "Science of Christmas" festival, said Santa Claus travels so fast to reach every home around the world that he would shrink and grow thinner in the direction he was traveling.

That's why he can fit his ample girth down chimneys and escape detection from curious children staying up past their bedtime, the physicist said.

Sheen said she wrote a letter to Santa Claus when she was 7 years old asking why he never aged, and the "jolly old elf" wrote a reply explaining that it was "all magic."

That didn't satisfy Sheen, who sought a rational explanation 26 years later as a scientist.

Sheen calculated that Santa's reindeer would need to travel at about 6.2 million mph in 31 hours -- taking time zones into account -- to deliver presents to every child celebrating Christmas around the world.

The Doppler effect would cause Father Christmas to turn from red to green as light waves he gave off were distorted by his incredibly high speed, Sheen theorized -- and he would eventually turn into a rainbow-colored blur and then become invisible.

Such speeds would also render Santa Claus and his sleigh bells silent as he moved beyond human range, she found.

Sheen plans to present her research in a peer-reviewed journal -- although with a festive spirit in mind -- to help spark an interest in children for science and physics, but she was unable to refute the claim made by "Santa Claus" in that long-ago response.

"How does Santa manage to reach these phenomenal speeds?" Sheen said. "Well, that's magic."