An English physicist has come up with the answers to many mysteries surrounding the man in red.

Dr Katy Sheen from the University of Exeter says in order to deliver presents to children all over the world in one night, Santa would have to travel about six million miles per hour.

She told Rachel Smalley according to Einstein's Theory of Relativity, your time slows down when you start travelling incredibly quickly.

"Santa - his ageing actually slows down, which explains why he's been around for hundreds of years delivering presents to all our ancestors, and he never seems to get any older.

"And the other thing which Einstein worked out was that you actually get thinner when you travel this quickly, so Santa Claus has actually been getting squished, which could be why he could fit down all those chimneys."

Dr Sheen has also come up with an explanation as to why we can't see Santa - an object that's moving quickly towards you goes through all the colours then becomes invisible to the human eye.

LISTEN ABOVE AS DR KATY SHEEN SPEAKS WITH RACHEL SMALLEY