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Comment by Aardwolf | Hacker News Reader

Is it possible that if they were, they would annihilate each other, so you couldn't live in an area where both are present, and so we are necessarily born in a place where there's much more of only one type? The antimatter might be outside of the observable universe, safely away from interacting with us for the foreseeable future.

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Yup, this reasoning even has a name: the anthropic principle. However we still would like to know the underlying mechanism that led to the asymmetry.

The principle is broader. In this specific example its application is actually reasonable.

Anthropic principle by itself suggests a mechanism. We know that with probability one there is life that can ask questions about antimatter. Yet the only known explanation for the lack of antimatter is an extremely unlikely fluctuation. But then to solve this probability mismatch one can assume that there many universes separated either in time or space or coexisting via, say, some wonder of quantum mechanics. Only in very few universes the matter dominates just by random accident. Which in turn suggests to look for signs of those other universes.

The problem there is that stuff in space is very evenly distributed, so that sort of super-clumpiness is hard to sell.

There would need to be a mechanism for why, in our observable universe, there happened to be no antimatter. If we saw other cosmic-scale clumpiness it might be a clue, but the differences are all tiny.

Afaik stuff in space is not evenly distributed. If anything scientists don't know why there are big voids in places.

There was a short story about an antimatter alien who visited Earth. It shook hands with a human and both were annihilated.

Wouldn't it already be annihilated by air particles though :)

It was very early science fiction. And yes it would.

Wouldn't it already be annihilated by photons from the sun?

It was by either Isaac Asimov or Stanislav Lem. I'm trying to find it...

EDIT: Also, as I recall it was about how an astronaut might determine whether a planet they are visiting is made of matter or anti-matter prior to trying to land there.

The short story Neutron Star by Larry Niven explores this question in a more plausible way (if you can ignore the FTL drives etc)

There was a story written in 1949, apparently using the concept of antimatter albeit not the word, about humans building the first starship and discovering that everything outside our solar system annihilated on contact. Thus the first attempt at landing on an alien planet created a catastrophic explosion. The humans of a follow-up mission gradually realize that they are not at war, but they are quarantined - there is a whole galaxy of life and civilizations that they can never participate in.

wlesieutre posted in this comment (https://news.ycombinator.com/item?id=18112501) a link that explain why be believe that everywhere there is more matter than antimatter https://www.scientificamerican.com/article/how-do-we-know-th...

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