How did Thanos (or anyone) know the Infinity Stones would give him this power?

Question

In Avengers: Infinity War, Thanos wants the Infinity Gems because

having all of them allows him to destroy half the population of the universe with a snap of his fingers.

At least a few other characters know about this, and also that his plan is to do exactly that.

Although having all six gems understandably grants a person an unbelievable amount of power, this incredible ability might have just been hyperbole. But at the end,

Thanos does exactly this and kills half the universe.

How did he, or anyone, know that he would have this specific capability?

Answer 1

One supposes that Thanos spent a lot of time bragging to Gamora about it. Don’t forget, she was his daughter, and until relatively recently in the MCU timeline, was at least somewhat on board with his plans of conquest. In fact, I believe she was the one who said he could destroy half of the universe with a snap of his fingers.

As to how Thanos himself would know this, it seems that the principles of operation of the Infinity Stones are somewhat understood in the MCU, whether by small-scale experiments (how we understand large phenomena in real life) or by the revelation of powerful, ancient entities such as Death or the One Above All (how some people believe we also understand large phenomena in real life). How else could someone make a gauntlet to control them (or possibly more than one)? More than that, the construction of their various containers suggests a certain understanding of how to contain and use their power by various individuals, not just Thanos.

Finally, even if Thanos didn’t understand exactly how the Stones worked, if he had the idea that they would give him “ultimate power,” he might logically have extrapolated that this included destroying half of all life in the universe. And from there, his specific plan (doing it with a snap of his fingers) might be mere personal preference, and of course he would have shared it with his daughter....

Answer 2

How do you (or, anyone) know that shooting a neutron to the nucleus of an unstable radioactive atom sitting in a gram of material can blow out a city?
Answer: By extensive study and research, obviously.

In MCU, Infinity Stones are legends and they have been studied and researched for billions of years by different alien races at different points of time. e.g.

• Tesseract was originally on Asgard. Later, it came to Earth and humans even researched it and tried to make use of it but they didn't succeed more than making some advanced guns. But, some alien races know better (who taught Loki how to use Tesseract to open portal).

• Eye of Agamotto was with humans for thousands of years and it was researched by humans heavily. There were full texts available to Doctor Strange using which he could make full use of it.

Also, advanced races like The Collector, Asgardians, Earth Wizards know better about Big Bang/ The Creation.

So, when you have lots of knowledge and better theories, you can piece lots of things together to know collective destructive power of all six Infinity Stones. Also, you can build a weapon to harness powers of all six Infinity Stones.

Answer 3

There may not have been any hard evidence that he'd be able to do it, but there was enough evidence of the power of the stones that they should not be allowed to fall into the hand of a guy who plans to do such a thing with them.

Even if he couldn't literally snap his fingers and do what he did, he may still have been able to engage in massive galactic, even universe-wide actions. And as his intent was well-known, he'd likely not suddenly change his mind and choose instead to give everyone in the universe a puppy.