Marvel Comics has finally offered an explanation for why only the worthy can lift Thor's hammer, Mjolnir.
The explanation comes courtesy of Mark Waid and was printed in the back matter of Avengers #679. It was the culmination of a conversation between Waid and the other writers and editors of Avengers: No Surrender about whether Molyn the Metal Master could use his powers to turn Mjolnir back on its master.
Waid settled the argument with some "science" about Thor's magic hammer.
"The science of Thor's magic hammer, taken in part from all the research I did for that Thor/Hulk crossover I did a few years back with Walt Simonson: Uru metal was forged in fiery pits by dwarven blacksmiths," Waid explains. "Based upon its observed properties – that is is nigh-indestructible, cannot be lifted by anyone except if they be as worthy as Thor, and always returns to his hand – there can be only one explanation: "Uru metal" must actually be an exotic form of matter that can be induced to emit gravitons. Gravitons are particles (theoretically predicted but, unlike the Higgs boson, still not experimentally confirmed) that mediate the force of gravity, just like photons transmit the force of electromagnetism. While we are unable to forge Uru metal on Earth, the dwarven blacksmiths, being as advanced compared to us as we are to our early ancestors, could craft a hammer whose properties seem like magic to us."
Waid goes on to explain that Odin's enchantment compels the hammer to boost its gravitons if someone unworthy tries to lift the weapon.
"Being able to change the rate of emission and absorption of gravitons is equivalent to being able to change an object's mass and even shape it," he writes. "If a person whom the hammer has determined to be unworthy attempts to lift the hammer, thanks to Odin's enchantment, the Uru metal will dramatically increase the rate of graviton emission. This will result in an exponential increase int eh gravitational attractions between the Earth and the hammer, such that it cannot be budged. When Thor grips Mjolnir's handle, the 'identity recognition enchantment,' if you will, causes the graviton emission to cease, and the hammer returns to its normal weight."
So gravitons are responsible for keeping Mjolnir out of the hands of the unworthy. But what about Metal Master? Waid explains the gravitons would also affect the hammer's inertia. Metal Master may not be able to lift the hammer, per se, but he can manipulate the gravitons just enough to turn up the inertia, which would lock it into a specific point in space regardless of Earth's rotation. That rotation of the Earth would create the illusion that the hammer is being thrown when it fact its targets are riding the Earth on a collision course towards the hammer.
Avengers #679 is on sale now.