In the Thor movies we have seen that only Thor can lift up his hammer Mjolnir. In The Avengers/Avengers Assemble The Hulk attempts to pick it up on the Helicarrier and cannot even move it.
What is the scientific reason behind the fact that only Thor can lift his hammer and not others?
From the perspective of the humans in the series, the hammer is magic. It has an enchantment on it that makes it impossible for anyone who's not "worthy" (presumably according to however Odin defines the word) to wield or move it.
The enchantment was put on the hammer directly by Odin, an Asgardian. To the Asgardian people, magic is just a normal part of their technology. If you asked one of them for the scientific explanation of the hammer's behavior they would just tell you "The All-Father enchanted it, of course."
To my knowledge, we have never been given any deeper explanation as to the inner workings of Odin's magic beyond "it works."
As pathetic mortals, we can only gaze in wonder at the extremely advanced technology that the alien race known as Asgardians take for granted. To us, it looks like magic.
All we really know about Mjolnir is that an inscription on the side of it states that "Whosoever holds this hammer, if he be worthy, shall possess the power of Thor", and that in Thor, we see Odin, father of Thor, revoke that power when he judges his son to be unworthy.
When unworthy in Odin's eyes, Thor is unable to lift Mjolnir and regain his former powers - it is only when he redeems himself that Mjolnir can then be lifted.
As far as we can tell, only people that Mjolnir (or possibly Odin) deems worthy can wield Mjolnir. This is demonstrated by a number of instances in the Marvel Cinematic Universe, such as when the members of The Avengers take turns attempting to pick it up in Avengers: Age of Ultron.
At the same time, we see in Thor: The Dark World that a coat hanger is enough to stop Mjolnir from reaching the ground, so we can conclude that it isn't extremely heavy. It seems that Mjolnir simply exerts an opposite force equal to the person who is trying to pick it up unless they are worthy, which is why we see The Hulk's feet begin to crumble the ground beneath him when he attempts to lift it in The Avengers/Avengers Assemble.
