If you are a fan of Marvel Comics, you will surely remember the following scene:
We inform you that after a long wait, finally Marvel Comics gave a real explanation of why only Thor can lift the Mjolnir hammer, this explanation was provided by Mark Waid which was accompanied to the Avengers No. 679.
Derived from the conversation between Waid and other writers and editors of Avengers: No Surreder, they established their argument with some "science" by mentioning the following:
“The science of Thor's magic hammer, derived in part from all the research I did for that Thor and Hulk crossover a few years ago with Walt Simonson. The Uru metal was forged in fiery pits by dwarven blacksmiths. Based on the observed properties (which is almost indestructible, that can not be raised by anyone except if it is as worthy as Thor, and that always returns to your hand), there can only be one explanation: Metal Uru must be an exotic form of material that can be induced to emit gravitons. Gravitons are particles (theoretically predicted but, unlike the Higgs boson, have not yet been confirmed experimentally) that mediate the force of gravity, just as photons transmit the force of electromagnetism. Although we are not able to forge metal Uru on Earth, the dwarven blacksmiths, being as advanced with us as we are with respect to our ancestors, could forge a hammer whose properties seem magical to us.”
Later Waid continues explaining that the spell of Odin forces the hammer to increase its gravitons if someone unworthy tries to raise the weapon:
“Being able to change the amount of emission and absorption of gravitons is equivalent to being able to change
the mass of an object and even to shape it. If a person that the hammer has determined to be unworthy attempts to lift it, thanks to Odin's spell, the metal Uru will dramatically increase the amount of graviton emission. This will result in an exponential increase in gravitational attractions between Earth and the hammer, so much that it can not be moved. When Thor picks up the Mjolnir, the identity recognition spell, if you want to call it, causes the cessation of graviton emission, and the hammer returns to its normal weight.”