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Comment by tlogan | Hacker News Reader

Question: would antiproton with antielectron orbiting around it be stable as normal hydrogen atom?

Did somebody made an experiment like that?

AFAIK, anti-hydrogen is very stable on its own and only gets annihilated because it quickly comes in contact with matter.

> In November 2010, the ALPHA collaboration announced that they had trapped 38 antihydrogen atoms for a sixth of a second, the first confinement of neutral antimatter. In June 2011, they trapped 309 antihydrogen atoms, up to 3 simultaneously, for up to 1,000 seconds.

https://en.wikipedia.org/wiki/Antihydrogen

Yes. It should be indefinitely stable as long as it does not come into contact with regular matter.

https://en.m.wikipedia.org/wiki/Antihydrogen

If antimatter was created during the Big Bang (and it was, we actually just don't know why there is more matter than antimatter [1]) it could exist in issolated-enough patches of the Universe. Photons, gravitational waves or neutrinos coming from those regions would not differ in any way, so we would not be able to identify those regions, up until they merge with normal-matter-filled ones (which is unlikely due to accelerated expansion of the Universe [2]).

1. https://home.cern/science/physics/matter-antimatter-asymmetr...

2. https://en.m.wikipedia.org/wiki/Accelerating_expansion_of_th...

Edit: as a matter of fact experiments, like the one described in the posted paper, allows to shed some light on the properties of antimatter and why there is more matter in the Universe (matter-antimatter anisotropy).

What a bummer - to send an intergalactic probe to a far-off supercluster only to find it’s made of antimatter and you cannot interact with it (if you survive the close encounter long enough to find this out).

If you are paying attention, and on a trip of that magnitude I think you would be, you would notice the halo of gamma rays as the antimatter supercluster interacts with normal matter around it.

That said, the Larry Niven short story “Flatlander” is pretty funny. You should read it.

> it could exist in issolated-enough patches of the Universe. Photons, gravitational waves or neutrinos coming from those regions would not differ in any way, so we would not be able to identify those regions, up until they merge with normal-matter-filled ones

Interesting. So how do we know there actually is more matter than antimatter in the universe? Couldn't there be a roughly equal number of sufficiently isolated pockets of each?

It is a well-known hypothesis, but space is not empty enough for it to be consistent with observations at least within our observable universe. If there really were antigalaxies or anticlusters out there, we would expect to observe characteristic gamma ray photons from the matter–antimatter boundaries where the extremely sparse but still existing intergalactic medium would interact and annihilate.

People have looked for evidence such annihilations, but the observations we have are not able to rule them out.

Antimatter superclusters (of clusters of antimatter galaxies) remain very possible. Nobody wants to talk about them because they don't want to be ostracized for speculating on what cannot be settled by evidence. But people do check, now and then, when new information comes up.

... It must be said, too, that nobody has identified any way for the antimatter to have become so far separated from the regular matter. All the processes we know of that make matter like to make both (exactly) equally.

Yes and yes.

As far as we know anti-atoms (antihydrogen in this case) are as stable as normal atoms.

To the point where it creates interesting questions -- if antiatoms are exactly as normal atoms, why we have abundance of normal matter but not antimatter?

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