How does mass conservation work in the MCU Ant-Man movie?

Question

In Ant-Man, it's (briefly) explained that the Pym particle reduces the distance between molecules, allowing a person to shrink while retaining his mass and strength.

Much of the movie is consistent with this explanation. For example:

• While ant-sized, Ant-Man falls from the edge of bathtub onto a tile floor, cracking the tile as though with the weight of a full sized human.
• He falls from a rooftop onto a car, denting the roof.
• He punches full sized humans, and they respond as though hit by a real punch.
• Another character, while tiny, is hit by a speeding toy train. The train is knocked aside as though it hit a full sized human.

But then there are things that appear totally inconsistent with "mass stays the same":

• Ant-Man is constantly running and climbing around on surfaces (air vents, ant tunnels, etc.) that couldn't possibly support the weight of a 200 lb man on a footprint the size of an ant.
• The toy train, when expanded to the size of a real train, is heavy enough to rip through a wall and crush a police car.

• Hank Pym carries a shrunken tank in his pocket on a keychain. It obviously doesn't weigh as much as a tank, but it does when expanded to full size.

• Another character, while tiny, gets swatted with a (normal sized) table tennis paddle, and goes flying like an insect.

Obviously the out-of-universe explanation is that mass behaves in whatever way is most convenient at the moment because it's just for fun.

But I'm curious: is there any in-universe explanation, either hinted at in the film itself or in the comic books, as to why mass is conserved sometimes but not always?

Answer 1

TL;DR - It doesn't.

The basic idea for the way that the Ant-Man suit works out of universe is a bastardisation of the Sqaure-Cube Law, which is the idea that as a shape grows in size, its volume and mass grows quicker and as such bones and muscles have to be exponentially stronger (and therefore larger and heavier) in order to survive the significantly higher loads that they are dealing with. This law also means that smaller animals are comparatively stronger when compared to larger animals, which makes for a fairly cool superpower premise.

But it's probably worth pointing out that even in the examples you give as the basic conservation of mass idea working in Ant-Man, it doesn't actually work. You say that "He falls from a rooftop onto a car, denting the roof", but I'm fairly certain that something as dense as Ant-Man when shrunk would have gone straight through the roof of the car - let's remembers that this is effectively just a 200lb object that is only an inch or so long and it's largest, probably making Ant-Man one of the densest objects on Earth (and certainly unable to ride on a flying ant named Anthony).

Towards the end of the film, we see;

Scott throw a weapon that enlarges objects, which accidentally hits an ant, making it the size of a large dog. At this stage, the ant either takes on an appropriate mass (meaning it would be crushed to death under it's own weight thanks to its basic design not being sturdy enough at larger sizes/weights) or it remains the same weight as a normal ant and gets blown away in the first mild breeze.

In short, the given in universe explanation (that the distance between atoms is reduced, meaning objects get smaller but retain the same mass) is pure nonsense from the demonstrations of it that we have seen.

However, this maybe true even in universe, as we see Scott;

shrink down into the "quantum realm", smaller than the size of molecules and atoms

which shouldn't be possible if the only change is the distance between atoms, so there is the potential that Hank simply lied to Scott (or is incorrect) about how the Pym Particles work.

Answer 2

To add a little to the other answers, according to the Marvel Cinematic Universe wiki, Pym particles work by transferring matter to another dimension:

The Particle's ability to warp mass and size can transport said mass, and also objects and living beings, into what is known as the Quantum Realm, a subatomic dimension of space, that holds any mass of those who have shrunken with the use of the Particles. The mass, while still part of the shrunken subject, is shunted inside the Quantum Realm until returned. 

This is similar to what the Marvel wiki says, and of course, has no citations, as there is no source other than the film. As far as I can tell, it is simply using data taken from other universes (as pointed out in the comments, my original answer pertained to the wrong universe), as it isn't covered to this level of detail in Earth-199999 yet.

This of course doesn't really explain any other behaviour observed in the MCU, or previous Ant-man canon.

We can either therefore assume that Pym lied, simplified for the audience/in-film characters, or that it simply doesn't hold up.

Answer 3

He loses his mass, but is as strong as a human

I think the comic explanation is, that the Pym Particles make his mass vanish (to another dimension or something) so he really only weighs as much as an ant. This explains all the feats he has interacting with ants.

How can he punch people around? The suit somehow makes it possible for him to retain his human muscle power. This is explicitly addressed in the movie when he is training with Hank Pyms daughter. She says "The force of a human punch on the area of a bullet" So his miniaturized muscles can still exert the full force of a grown human. The only hand-wavy thing is conservation of momentum - if he is practically weightless, he should more punch himself back than the other one forward. But if we accept the fact that he can somehow release his original mass if he uses his muscles and attacks or hits something, then this would work.

And this could also explain the cracks in the floor/dent in the car. Ant-Man seems to have a solid background in parcour/climbing and falling. So he will naturally have reflexes to hit the floor with his arms and dissipate energy on hitting the ground like most martial artists (Just watch videos how judo people fall and hit the ground with their free arm when thrown). If Ant-Man has this reflex and hits the ground when falling he will punch a solid dent into the floor - not from his mass, but from using his muscles to hit the floor from old reflexes.