Avengers: Endgame - How Thanos' Snap ACTUALLY Worked

1. The Reality Stone

If the Time and Space Stones determine how Thanos can interact with all manner and make an "event" happen, the Reality Stone governs how he changes that matter. It is the Stone responsible for that grim but not entirely uncool fading effect that had such an impact when his Snap victims died.

As is established in Infinity War, the Reality Stone can change matter to reflect false realities. It is a means by which the wielder may rearrange matter, like turning the Guardians into ribbons so they cannot fight him or turning laser bolts into bubbles. The Time and Space Stones allow Thanos to grip the universe's very matter, but the Reality Stone allows him to arrange it all as he sees fit.

Obviously without the Mind and Soul Stone, Thanos manipulation of matter would include "decimating" all the other types of matter that is linked to conscious existence. He wouldn't simply wipe out half of life, he'd wipe out half of all matter, which would not be ideal. And without the Power Stone he couldn't hope to do any of it on a scale as large as he needs for the Decimation.

Each Stone is a cog in a powerful engine and in order for Thanos to enact his very specific plan, he needs them all to do very specific jobs. It may look like he's snapping his fingers - a simple act - but the scientific theory beneath it suggests an incredibly complex event. Which is precisely why Thanos is accurate in saying that it cost him everything and why the Russos were so keen to reiterate that the snap fundamentally broke him.

All of this suggests that the only way to undo the Snap is to recreate exactly the circumstances of the first event or to bring in anomalous science that overrides all of that. For either of those things to be true, we have to turn to the realm where science doesn't matter (or at least not in conventional terms): the Quantum Realm.

For there to be time travel, the laws of Time have to be suspended. For there to be interdimensional travel, the laws of Space have to be suspended. For there to be a resurrection, the laws of finite consciousness have to be suspended. For there to be an event of equal magnitude to the Decimation (in order to reverse it), the laws of power have to be suspended. All that's left is Quantum Theory, Quantum Energy and the Quantum Realm.

Well, they did say that it would be key...