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From a mathematical standpoint (CPT symmetry) it is most probable that antihydrogen has the same spectra (absorption and emission) as hydrogen. The CERN confirmed this hypothesis to a high accuracy for the 1S-2S ray: ALPHA CERN 1S-2S antihydrogen spectrum.

Within this hypothesis, how can we state that a far (in a neighbor galaxy) cloud or star is made either of hydrogen or antihydrogen?


NB: this question is a small step along a path to better understand the history of matter and antimatter in the universe, one of the components of this excellent question How would we tell antimatter galaxies apart?.

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    $\begingroup$ Kind of related: physics.stackexchange.com/q/26397/123208 $\endgroup$ – PM 2Ring Oct 28 at 10:06
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    $\begingroup$ @PM 2Ring: thanks, highly related 👏🏻! My Q is an atomic focused version of this previous Q. $\endgroup$ – dan Oct 28 at 10:20
  • $\begingroup$ I asked this question because I am not convinced by the argument ( the missing annihilation huge γ and neutrinos bursts ) explaining a possible dissymmetry between matter and anti-matter in our observable universe. For example: what about anti-matter galaxies inside BH? $\endgroup$ – dan Oct 28 at 11:27
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    $\begingroup$ What do you mean by "anti-matter galaxies inside BH" ? Yes, black holes can consume antimatter, just like they consume matter. OTOH, matter (or antimatter) structures cannot survive for long inside a BH: everything reaches the core of the BH rather quickly. We don't know exactly what happens there, we need a Quantum Gravity Theory to address such questions. But it's very unlikely that even a particle as large as an atom can survive there. $\endgroup$ – PM 2Ring Oct 28 at 11:36
  • $\begingroup$ “What do you mean by "anti-matter galaxies inside BH" ?” I was considering very large BH with a low density which can home stars, galaxies, BH, and as a few astro-physicists think a full universe. $\endgroup$ – dan Oct 28 at 11:49
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As you've already noted, we cannot detect cosmic antimatter from its spectrum.

As the answers at How would we tell antimatter galaxies apart? indicate, there are two ways we could detect cosmic antimatter.

Firstly, we would see the tell-tale 511 keV gamma ray signature of electron + positron annihilation reactions coming from the border of the antimatter region where it contacts normal matter. Antiprotons & antineutrons will (of course) also annihilate on contact with normal matter. Such annihilation reactions are rather messy, and can emit gamma rays of various energies. They also emit other particles, eg pions, but they decay quickly, and the long-distance signature of these reactions is fairly similar to that of the electron + positron annihilation. I give more details on annihilation reactions in my answer to What happens to the quantum information of a particle and an antiparticle when they annihilate?

Annihilation reactions are intense. After all, all the mass involved gets converted to photons, whereas even the most powerful nuclear fusion reactions only convert a small percentage of rest mass to photons & kinetic energy. But you can't compare annihilation reactions at the border of a space gas cloud with the power of a supernova.

Space gas tends to be extremely tenuous, with fewer particles per cubic metre than the hardest vacuums that we can produce on Earth. A cloud of antihydrogen reacting with the surrounding hydrogen would give off a lot of gamma, heating the gas, but it doesn't create any kind of chain reaction, and the cloud will take some time to completely annihilate.

According to Hydrogen-Antihydrogen Collisions [P. Froelich, S. Jonsell, A. Saenz, B. Zygelman, and A. Dalgarno Phys. Rev. Lett. 84, 4577 – Published 15 May 2000]

Surprisingly, starting with $n_H = n_{\bar H} = 10^7 \text{cm}^{-3}$ and energy <10 K, it takes a whole 17 minutes for the mixture of equal amounts of hydrogen and antihydrogen to lose half of all atoms.

$n_H$ and $n_{\bar H}$ are the particle number densities of hydrogen and antihydrogen, respectively, in a cold homogeneous mixture of hydrogen and antihydrogen. Note that particle densities in the interstellar medium are typically much smaller, ranging from $20 \text{cm}^{-3}$ to $10^{-4} \text{cm}^{-3}$. The mean density in molecular clouds, where stars are formed, can be much higher, eg $10^2 – 10^6 \text{cm}^{-3}$, but even the dense end of that range is 1/10 the density mentioned in that quote from Froelich, Jonsell et al.

OTOH, the collision of a normal star with an antimatter star would be extremely spectacular. ;)

The other option mentioned at the linked question is that if there are whole stars made of antimatter we might be lucky enough to detect the antineutrinos from an antimatter core collapse supernova. As explained on Wikipedia, core collapse also produces large amounts of thermal neutrinos & antineutrinos in equal quantities, and they outnumber the electron capture neutrinos by several times. So we'd need to detect both neutrinos and antineutrinos and measure their ratio carefully to distinguish between a regular supernova and an antimatter one.

Neutrino / antineutrino detection is hard. The best detectors using current technology can only detect neutrinos with kinetic energy around 300,000 times their rest mass. And even then, billions of neutrinos pass straight through the detector undetected for each neutrino that is detected. We did detect a handful of neutrinos from the supernova SN 1987A in the Large Magellanic Cloud). Hopefully, modern detectors could detect a few more, if the supernova were close enough. But I don't know if we could catch enough of them to make a sufficiently accurate determination of their ratio.

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  • $\begingroup$ Re "even the most powerful nuclear fusion reactions only convert a small percentage of rest mass to photons & kinetic energy": Black holes can be more efficient in converting mass to photons. "With rotating black holes ... releasing more of their energy--up to 42%." (Yes, I know, but there are other sources.) $\endgroup$ – Peter Mortensen Oct 28 at 23:48
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    $\begingroup$ FWIW, I did some supernova energy calculations here: physics.stackexchange.com/a/455549/123208 $\endgroup$ – PM 2Ring Oct 29 at 6:43
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    $\begingroup$ $e^+e^-$ annihilation (at rest, interstellar gas will have at best thermal energies) produces a strong $511\,\textrm{keV}$ line from parapositronium decay. OTOH $p\bar p$ annihilation at rest mostly produces three to five pions with momenta up to a few hundred MeV. $\pi^\pm$ decay in the continuous spectrum, the $\pi^0\to\gamma\gamma$ produces two hard gammas, but the momentum is washed out because the $\pi^0$ momentum is $O($rest mass$)$. Also hard gammas will pair-convert on their way through the universe. So isn't the only signal we can actually observe the $e^+e^-$ annihilation signal? $\endgroup$ – tobi_s Oct 29 at 10:47
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    $\begingroup$ ps $p\bar p\to \gamma\gamma$ would also lead to monochromatic photons, but is a very rare occurence (no surprise there, my first google hit calculates annihilation branching fractions, compares them to measured ones and doesn't even bother to mention this channel core.ac.uk/download/pdf/25179099.pdf). Additionally, $500$MeV photons wouldn't get very far: they'd convert to $e^+e^-$ at the first nucleus they encounter on their way through the universe, so again it's an unobservable signature. $\endgroup$ – tobi_s Oct 29 at 10:56
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    $\begingroup$ Sure, but these photons are not coming from matter-antimatter annihilation. I should add one ps: a 511 keV photon can Compton scatter, and there are lots of electrons on the way through the universe. So the rare high-energy photon would be a freak occurence for positron-electron annihilation even more so than for pion decays. The signature to look for will therefore be rare high-energy photons (up to half a pion mass in energy, but redshifted as the sources are likely very far away) forming an excess over the expectation for a universe where matter-antimatter annihilation is absent. $\endgroup$ – tobi_s Oct 30 at 6:02
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Antihydrogen atoms have been created in the lab and their basic spectral characteristics confirmed as identical to hydrogen. So we cannot tell by directly observing an object.

But we infer it from the fact that no gross interactions between matter and antimatter have been observed. A full answer is given at How would we tell antimatter galaxies apart?, but here is a short version.

Interstellar space, even intergalactic space, is not entirely empty. Matter may be vastly attenuated, down to a handful of atoms per cubic metre or whatever, but it is still there. If an object were made of antimatter, some of its atoms would dissipate into space and would eventually meet ordinary matter. Some matter in space is ionised. Electrons and anti-electrons (positrons) are oppositely charged. Lab experiments have shown that they will briefly bond to form a positronium atom before mutually annihilating themselves in a flash of radiation. Something similar must occur between a proton and an antiproton, though I do not know if this has been lab-tested. Other, charge-neutral annihilation interactions will also occur.

So if antimatter were out there, there would be a constant trickle of the characteristic radiation from the buffer zone, with occasional mega-bursts as material and anti-material objects collided. These radiation emissions would be readily detectable with modern instruments, but they are simply not there in the sky.

Quite why there is no antimatter out there is one of life's little mysteries; something somewhen broke a symmetry (conservation law), but we have no idea what, when or how.

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    $\begingroup$ @dan These are really ones for the question I linked to, with its more detailed answers. No doubt the whole thing would be messy, but a large enough mass/antimass cloud/cluster would ignite sporadically and could be held together by gravity for just long enough. Either way, the radiation would be intense and is just not there. $\endgroup$ – Guy Inchbald Oct 28 at 10:46
  • $\begingroup$ Thank you for this useful piece of advice. I migrated my comments. $\endgroup$ – dan Oct 28 at 11:21

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