We have built an amazingly powerful (fast) spaceship, which we have been using to traverse the great void. We have visited most of the galaxies in our supercluster and made mundane and awesome discoveries. Now we venture beyond the supercluster and approach a far distant galaxy, crossing a vast empty region. As it turns out, the galaxy we are headed toward is composed predominantly of antimatter. Will we be able to turn around (assume that is feasible) before annihilation?
IOW, as far as I can tell, a galaxy of antimatter would look exactly the same with the tools we currently have. Would there be some way to discern what we are looking at?
Not that I know of, however, if the hypothesis of baryogenesis is correct (and in fact, even if it isn’t) there’s every reason to believe that the universe on a large scale is homogenous, which means that matter is made up of the same quarks and atoms everywhere, and that it’s exceedingly unlikely that large clusters of antimatter could exist anywhere, unless some unknown process was creating it. Not really an answer, but just meant to suggest that the problem wouldn’t arise.
Yes; there would be annihilation radiation from its antimatter interstellar gas and dust interacting with the matter-based gas and dust of the universe surrounding it.
The only science fiction consideration of such a scenario is Larry Niven’s “Flatlander”, where the POV characters discover a protosun with one planet passing through the galactic plane.
The protosun is glowing much hotter than it should be at its stage of development, the planet’s surface appears to have been polished smooth, there are no other members of the system detectable (not even detectable amounts of dust), and the radiation levels surrounding the system are extremely high for no good reason.
The POV characters are saved from landing on the planet and, in the words of the narrator, “going up in pure light” by having their allegedly invulnerable General Products hull evaporate in a puff of dust. (Good thing they had their vacuum suits on and the helmets in reach.)
The hull was a single huge artificial molecule and traces of monantomic antimatter finally eroded the hull molecule until it lost integrity.
Space is big. Really big. You just won’t believe how vastly hugely mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist, but that’s just to space
Annihilation would be occuring many exameters outside the edges of the antimatter galaxy. Significant quantities of normal matter would not be getting anywhere near an antimatter galaxy. Our ship might in fact not be able to approach it due to the matter-antimatter barrier (which would be a cloud so huge and diffuse that we might not notice it).
Note that the most itinerant particles in the universe – neutrinos – do not have antiparticles.
This is a quantitative questions with a mix of ludicrously big numbers and ludicrously small numbers, some in relation to space and some in relation to time, so intuition won’t be a great help. In practice, annihilation signatures should be visible – and have not been detected. However, the constraints from direct searches for antimatter streaming through our backyard (particularly light anti-nuclei) might feel more direct anyway.
This is unknown. Neutrinos definitely have two states that are to good approximation distinct, but whether these are a neutrino and antineutrino or whether these are two spin states of a single particle remains an open question.
As for the original question, as your ship approaches the possible antigalaxy, your technology would surely be good enough to notice that the cosmic rays streaming out of the various astrophysical engines of said galaxy all had the wrong polarity, and you could slam on the brakes in time.
This is why I never trust my life to a technology that nobody understands and for which the ‘magic’ that drives it might fail at any time without warning. Giant electrostatically reinforced molecules, giant habitable rings made of a material with a tensile strength approaching the nuclear strong force, a ‘stasis’ field that violates every known law of physics, ‘reactionless’ thrusters that somehow pull a ship through space without distortion or momentum transfer? Give me a break; hokey religions and ancient weapons are not a good match for a blaster at your side, kid.
I’m sure there are many, many examples of large antimatter objects in sci-fi. Greg Egan had a time-reversed galaxy in The Hundred Light-Year Diary, which would be antimatter from our perspective. IIRC, it was so distant that there was no problem with normal matter interacting with it.
Well, I don’t know of any other treatments of “traveling to a big antimatter woobie and detecting what it is before becoming a Horrendous Space Kablooie.”
I’m not sure about the specific case of not knowing an object is antimatter until it’s too late, but Collision Orbit (1951) has antimatter asteroids in the solar system that pose a hazard to normal matter (but are also useful for their annihilation energy). I haven’t read the stories but I wouldn’t be surprised if there’s a point where the characters blow themselves up because they don’t realize the space thing is AM.
The terminology is a little confusing in this area, unfortunately. Neutrinos might or might not have corresponding antineutrinos. This remains unknown. But we still talk about “antineutrinos” because, even if an “antineutrino” turns out to be just a different spin state of a neutrino and not actually a distinct species, no experiment carried out to date can tell the difference between those two cases (which is why it remains unknown.) But there are experiments underway trying to distinguish between those cases.
To be clear, the two objects – neutrino and either antineutrino or “antineutrino” – act very differently in experiments regardless of the underlying ontology, so the term has significant practical value even if we may discover that the antineutrino is not actually an antiparticle of the neutrino.
Maybe the only one you can think of, but it’s a really, really common topic in science fiction.
To the OP, matter in the great intergalactic voids is extremely sparse, but even in the sparsest voids, there are still some atoms. And if some galaxies were antimatter, there would necessarily be some boundary region where half of the atoms were antimatter. There’d be enough distinctive radiation from such a region that we would be able to detect it, even from across the Universe. And we don’t. Therefore, no antimatter galaxies.
Most of the characters in the Known Space stories don’t understand General Products hulls or scrith, but humans (at least, those humans with a sufficient education) do understand the principles behind stasis fields and thrusters. We, the readers, don’t understand those principles, but then, that’s usually true of science fiction tech.
Strictly speaking, the antineutrino definitely is an antiparticle of the neutrino. Also strictly speaking, the photon definitely is an antiparticle of the photon. The question is just whether they’re distinct.
There have been experiments, for decades now, which do claim to be able to distinguish between the Dirac and Majorana cases, and which claim to come down extremely confidently on the Majorana side (i.e., that they aren’t distinct). But most physicists aren’t as confident on the matter as those guys running those experiments.
I gave a talk once on a topic that depended on the distinction, and seeing that there wasn’t consensus, I just calculated and reported both sets of numbers, for both cases. One audience member was annoyed that I had done this, because it had been conclusively absolutely proven that neutrinos were Majorana. I just told him “Well, then, you can just use that set of numbers”.
That’s what I meant, since I usually explicitly caveat pretty much everything I say with “AFAIK”. But if it’s a common topic in SF, I’m sure you’ll enlighten us with just more applicable example.
One of my favourite SF plots involving antimatter can be found in Greg Bear’s novel Anvil of Stars. One of the protagonists (and their ship) become converted into antimatter, and they gradually become sick, because ‘antimatter chemistry is slightly different to normal chemistry’. This is (I believe) a reference to CPT violation in antimatter, which suggests there is a real difference between the ways that matter and antimatter behave.
If antimatter really does have slightly different chemistry to matter, then it should be possible to tell the difference between a matter and an antimatter galaxy using spectroscopy. However (IIRC) recent experiments on antihydrogen have failed to find any gross differences, so detecting antimatter using spectography alone might be problematic.
That reminds me of another fictional example in the novels Life Probe & Procyon’s Promise. In the setting there’s just as much antimatter as matter, it’s just all in the form of stable micro black holes. The holes not decaying via Hawking radiation isn’t a physics mistake, it’s a longstanding mystery of physics in the setting.
It turns out the reasons are that antimatter does operate under different physics, that the old idea that antimatter can be considered time-revered matter is accurate, and that the universe is cyclic. Antimatter black holes are survivors of a previous universe that was younger than ours, which is why it’s all in the form of black holes; in the greater energy and density of that universe everything was crushed into black holes. And that in turn is because we are the antimatter; what we call “antimatter” is from a younger, more energetic universal cycle and we are the ones traveling into the past from a lower-energy future.
I wouldn’t be so prescriptive. As noted, the language in this area is far from clean.
You will find sources that say an antiparticle is just whatever you get when you apply the charge conjugation operator. This definition is mathematically tidy, but because the Standard Model violates parity symmetry so violently, it is phenomenologically unhelpful. Therefore, you will find other sources that say an antiparticle is whatever you get when you apply the charge conjugation and parity inversion operations together. In this case there’s decent alignment with what can be measured, at least for fundamental fermions. However, the Standard Model also violates time-reversal symmetry, so qualitatively this has the same problem as before. Therefore, you will find still other sources that say an antiparticle is whatever you get when you apply the above operators together with the time reversal operator. These choices certainly matter for the neutrino case if you want to “speak strictly”.
In terms of how the Standard Model’s math actually looks, some particles appear alongside distinct antiparticles and some don’t. Language that takes that to heart is used in suitable contexts, either implicitly or explicitly. You might say “a muon and an antimuon collided” but you would never say “a photon and an antiphoton collided”. If you did, people would ask what the heck you’re talking about. Similarly, if you said, “We measured an anti-\pi^0” or “an anti-Z boson”, eyebrows would raise because that’s not how the terms are actually used.
Given the points @eschereal and @OldGuy were making, it seems important to note the linguistic ambiguity.
This is grossly mischaracterized. There was one experiment from a small team a quarter century ago, and they never claimed a particularly high level of confidence. More to the point, though, a few members of the team released a null interpretation of the result, while a few other members of the team published something that, in a very literal sense, was LOL-inducing. The analysis was so obviously invalid that it was never taken seriously at any level by any serious physicist. There was a short period two decades ago when one had to begrudgingly put a footnote on related conference slides and the like, since someone who kinda maybe heard about it would inevitably bring it up. But that is long since past, and this “result” doesn’t feature at all in contemporary discussions. (Much more genuine experimental efforts have since soundly ruled out anything that that group could have been claiming anyway, as well.) I encourage you to dig up the original paper. You may very well literally LOL just upon glancing at the plots.