To study Santa Claus under the light of common sense might not be sufficient to understand this Christmas phenomenon. Relativity and quantum mechanics have broken our understanding on how the world works. These theories have helped us understand the very large and the very small. In the same manner, they may be the keys to unlocking the real truth behind the dual-nature of Santa: a very large man (especially compared to the kids) traveling as a very small vehicle (compared to the earth). Spacetime Gravitation is a natural phenomenon by which objects with mass attract one another. With Santa's large figure (in which, he gains 2950.7 tons during the night) and the load of the sleigh of 321,300 tons (as mentioned in "The Truth about Santa"), the whole entity travelling during Christmas eve is truly massive! In this massiveness, we also get a significant amount of gravity. Santa would bend the dynamic spacetime and travel around the world with relative ease! Santa's initial mass would be really significant given the number of presents he has to bring. Along the way, he drops off these gifts in exchange for the milk and cookies (so don't forget to leave some milk and cookies!). As he builds up momentum during the night, the mass needed to bend spacetime decreases. Santa would probably explode if he had to exchange all 321,300 tons of gifts in body mass! The Truth of Uncertainty With Santa being a relatively small entity traveling around the world, one cannot be absolutely certain of both his position and velocity at a certain time.
"In quantum physics, the Heisenberg uncertainty principle states that the values of certain pairs of conjugate variables (position and momentum, for instance) cannot both be known with arbitrary precision. That is, the more precisely one variable is known, the less precisely the other is known. This is not a statement about the limitations of a researcher's ability to measure particular quantities of a system, but rather about the nature of the system itself."
Given the different timezones around the world we are certain to an extent that Santa would visit during these times. With nature of Santa's visits however, we are not certain to exactly where he is. Clincher: Does Santa Exist? Let's review the thought experiment on Schrödinger's cat:
"Schrödinger's Cat: A cat, along with a flask containing a poison, is placed in a sealed box shielded against environmentally induced quantum decoherence. If a Geiger counter detects radiation then the flask is shattered, releasing the poison which kills the cat. Quantum mechanics suggests that after a while the cat is simultaneously alive and dead. Yet, when we look in the box, we see the cat either alive or dead, not a mixture of alive and dead."
In uncertainty, Santa is simultaneously in existence and non-existence. When that uncertainty is broken, he will now either exist or not. For the believers, they are certain that he exists with the gifts he brings. For the non-believer, they are certain that he doesn't exist because they don't get gifts. This is the same reason why Peter Pan asks those who believe in fairies to clap their hands. It must be noted though that this certainty is not permanent and might change (typically during parenthood). Shun the non-believers! -- *Note: I did not major in physics and I might have used these terms and theories loosely and incorrectly. I do, however, find much interest in these theories as they try to explain everything around us.