Professor Jim Kakalios is the Taylor Distinguished Professor in the School of Physics and Astronomy at the University of Minnesota who wrote The Materials Science of the Avengers for Hollywood Chemistry, a collection of essays published in 2014 by the American Chemical Society, as to why Thor could pick up Mjolnir but no one else could.
For Wired, he expanded further, referring to a certain scene in Avengers: Age of Ultron:
When Tony Stark tries to lift Mjolnir using his Iron Man glove, he exerts a large upward force, greater than its weight, and yet the hammer remains at rest. So where does the additional downward force come from? One can only conclude that a unique property of uru metal is that, under the proper stimulus, it can emit large quantities of gravitons. On Earth, these fundamental particles have not been experimentally confirmed to exist, but as stipulated, the Asgardians are ahead of us scientifically. Gravitons are conjectured to transmit the gravitational force, and if an object emits additional gravitons, it is equivalent to increasing its mass. Thus, when an “unworthy” person applies an upward force, the uru metal increases the hammer’s weight to exactly cancel this lift, and the hammer remains unmoved. When Tony and Rhodey simultaneously exert a larger upward force, the emission rate of gravitons increases, to again neutralize their efforts. The greater weight will not damage the tabletop, as only enough gravitons are emitted to balance out all upward forces, to keep the hammer stationary. Once the lifting force is stopped, the excess graviton emission also ceases.
This then got picked up by the comics themselves, citing the professor. As he stated:
I shared this information with Mark Waid, the writer at the time of Marvel Comics’ The Indestructible Hulk, at the start of a storyline where the Hulk would have an adventure with Thor. In The Indestructible Hulk #8, the question of whether or not the Hulk could lift Mjolnir arose, and no less a scientific authority than physicist Bruce Banner (the Hulk’s alter ego) provides peer-review and approval of my “graviton emission proposal,” as shown here:
Well, in today’s Avengers #679, to explain how Metal Master in Avengers #678 could move Mjolnir, Waid has gone back to the well for why no none but Thor can lift the hammer. No mention of Professor Jim Kakalios this time, so I thought we’d provide it.
Okay. 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. Based upon its observed properties–that it 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. 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. 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 in the gravitational attraction 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 resumes its normal weight.
Though, of course, it’s her hand right now… for a few more months at least!