The DoomFleet returns!

And this time, it's with a more "Conventional" weapon - That is, projectiles with an antimatter core. Ultra-simple explanation is they have a "small" amount of antimatter in them, maybe a few dozen grams, suspended by some "basic" EM field. Of course, being projectiles, when they hit something, the field generator is destroyed and the antimatter does the rest.

The question is not about the antimatter rounds. Those exist in a place that can be handwaved easily.

What I want to know is whether an Antimatter explosion would have a unique signature, detectable to astronomers elsewhere. And if so, how far away would it be noticeable? Part of my story has something like this being detectable a few hundred light years away, which may require some rather large AM rounds.


Yes, indeed. The gamma rays relase a specific amount of energy, both for the final annihiliation and the binding energy which has two different levels. Check out positronium for the case of electron-positron aniliation. You’ll have similar effects for the protons and neutrons too. With a detailed x-ray spectum, you could even tell which elements/antielements were used.

The pulse of x-rays also might provide a source for anaylzing the surroundings too — numerus instruments are based on various interactions between x-rays and matter.

The case of Positron-Electron Annihilation in the case where the spins are opposed can conserve total spin by producing two gamma rays, and to conserve momentum they must be equal energy and opposite in direction. So this is a clear spike at the rest mass of an electron (less a small binding energy), 511 keV.

If the spins are aligned, you need three photons emitted to conserve spin, and they can divide up the momentum in any manner. However, you can figure out a statistical distribution.

Annihilation of the protons and neutrons are messy, since they are made up of more elementary particles. This will be 2000 times greater in magnitude per particle, with the protons themselves equal to the number of electrons, plus however many neutrons you have.

But, certainly a statistical distribution of energy in the resulting spectrum, with several complex features (bumps and valleys in the graph). No doubt it can be recognised as such. You might ask on physics just what that is exactly; someone may have a graph handy.

  • 2
    $\begingroup$ I'll just add: Positron-Electron Annihilation for example produces gamma rays at 511 kev for example. To pick it out at a few hundred light years it would have to be quite a bit of anti-matter and it may be challenging to distinguish from natural process. $\endgroup$ – Joe Kissling Mar 23 '17 at 2:58
  • $\begingroup$ A singular burst might be hard to tell is artificial, but weapons exchanges over the course of a few days would probably be a lot easier. Especially if the observer can detect planets and roughly where the signatures come from. Also worth noting is "background" radio would also suddenly cease. $\endgroup$ – Andon Mar 23 '17 at 3:05
  • $\begingroup$ Currently, we can't detect radio or see planets at that distance so I think even repeated gamma ray detections would be hard to distinguish from some natural process. Probably just a special class of gamma-ray burst which we currently don't understand very much about. $\endgroup$ – Joe Kissling Mar 23 '17 at 3:23
  • $\begingroup$ We've detected planets at 27,710 light years, and possibly one in the Andromeda galaxy. We've also directly imaged a large number of planets, including this system, from >100LY, so I think aliens with few decades worth of improvements will have significant improvements on that. $\endgroup$ – Andon Mar 23 '17 at 4:26

As noted, the electron/positron reaction will have a very distinct gamma ray signature at 511 KEV, which is enough for any observer to immediately identify this as an antimatter reaction.

Protons and antiprotons release a violent stew of charged and uncharged particles. Most of these won't be detected directly, since their lifetimes are measured in milliseconds, but they too decay into energetic photons.

Here are the specifics of the reaction that produces the mesons. Protons consist of two up quarks and one down quark, while antiprotons consist of two anti-ups and an anti-down. Similarly, neutrons consist of two down quarks and an up quark, while antineutrons consist of two anti-downs and an anti-up. The strong nuclear force provides a strong attraction between quarks and antiquarks, so when a proton and antiproton approach to within a distance where this force is operative (less than 1 fm), the quarks tend to pair up with the antiquarks, forming three pions. The energy released in this reaction is substantial, as the rest mass of three pions is much less than the mass of a proton and an antiproton. Energy may also be released by the direct annihilation of a quark with an antiquark. The extra energy can go to the kinetic energy of the released pions, be radiated as gamma rays, or into down or strange quarks. The other flavors of quarks are too massive to be created in this reaction, unless the incident antiproton has kinetic energy far exceeding its rest mass, i.e. is moving close to the speed of light. The newly created quarks and antiquarks pair into mesons, producing additional pions and kaons. Reactions in which proton-antiproton annihilation produces as many as nine mesons have been observed, while production of thirteen mesons is theoretically possible. The generated mesons leave the site of the annihilation at moderate fractions of the speed of light, and decay with whatever lifetime is appropriate for their type of meson.[5]

This blaze of energy also has a very distinctive "spike". Much like nuclear reactions are @ 1000000 times faster than chemical reactions, matter/antimatter reactions happen orders of magnitude faster than nuclear reactions.

Warships, civilian radiation detectors and astronomical instruments tuned to these frequencies will see a very rapid spike of energy, with the distinctive 511KEV signature. If there was a strike on the target, monitoring of the area will reveal the infrared radiation of the spacecraft, asteroid or moon with a hot spot where the warhead struck. (t should be noted that antimatter weapons suffer the same limitation as nuclear weapons in space: there is no atmosphere to convert the radiant energy into shockwaves. The inverse square law suggests that you actually need to detonate the device very close to the target to cause damage.

Nuclear weapons can be used as compact energy sources to drive weapons effects, ranging from "shotgun" charges which can drive pellets at up to 100km/sec, nuclear shaped charges which can drive streams of liquid metal at up to .03 c and even devices which can drive spindles of plasma at .10 c. Using an antimatter reaction to drive these sorts of weapons will provide a much more compact and energetic device compared to a nuclear one.

  • $\begingroup$ I love your ideas for the antimatter-powered weaponry - In the end, it doesn't necessarily matter what sort of antimatter weapons are used because they're energetic enough and uniquely identifiable, and these are far more fun than simple AM Explosive shells. $\endgroup$ – Andon Mar 23 '17 at 4:18
  • $\begingroup$ Where’s the blockquote from? $\endgroup$ – JDługosz Mar 23 '17 at 6:45

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