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Could the cosmic radiation of heavy ions which hit us be the exhaust of advanced electric propulsion used by alien space travelers? While supernovae seem to cause part of them, much of it is far from explained AFAIK. Our SEP probes spew out noble gas ions. Fission-fragment propulsion would spew out heavier ions at even higher speed. Almost any ion could be used for propulsion. Is there any physical argument against alien civilization origin of some of the cosmic radiation (ions from outside of the Solar System)?

More generally, what kind of SETI could be done today by studying cosmic rays, heavy ions rather than photons?

Plots like the one below really make cosmic rays look like a natural astrophysical phenomena. But there are error bars on the very exponential graphs like the one below. And the neat distribution function maybe just reflects what cosmic magnetic fields do to ions after a billion years regardless of origin. Does the spectrum of high energy ions represent cosmic alien market shares for different propulsion brands? (Hillas 2006, preprint arXiv:astro-ph/0607109 v2) (Hillas 2006) Aside, why is there an E² factor on the vertical axis here?

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    $\begingroup$ This can only be answered by speculation. The answer by asdfex is worthwhile as a good overview of the phenomenon and presents well how they in no way depart from the statistical distributions predicted by our knowledge of the cosmos. However, this is a classic issue that in this case more than most a negative can't be proven. I consider this a candidate for closure, but leave it to the judgement of the community. $\endgroup$
    – kim holder
    Commented Aug 9, 2016 at 13:50
  • $\begingroup$ To produce fluxes this large, you'd need alien civilizations that remained very large and technologically super-advanced for very long time periods, and they would probably have to be in the business of moving entire solar systems around at relativistic speeds, not just little rocket ships. $\endgroup$
    – user687
    Commented Aug 9, 2016 at 15:06
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    $\begingroup$ For a relativistic rocket, the ratio of impulse to mass-energy is $p/E=(m\gamma v)/(m\gamma c^2)=v/c^2$, where $v$ is the exhaust velocity. As the exhaust velocity approaches $c$, we have $p/E\rightarrow 1/c$, so it's hard to see the motivation for using ultrarelativistic exhaust velocities. Once you get $v$ up to 0.9 or 0.95 c, you're not getting much better performance by increasing it further. $\endgroup$
    – user687
    Commented Aug 9, 2016 at 15:09
  • $\begingroup$ @BenCrowell there was I time when I could do that, but not any more. I think it's an excellent answer to this question and worth posting! $\endgroup$
    – uhoh
    Commented Aug 10, 2016 at 1:52

2 Answers 2

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I can't give a precise answer to your primary question besides "Extremely unlikely", but here are some facts on cosmic rays that might help coming to a conclusive answer:

Current models are able to describe the distribution of energies and ion masses rather well. What we do not know precisely is the source of this radiation. There are plenty possible sources, but none of them is able to describe the full spectrum of cosmic rays. I would like to point you to a summary of our knowledge about cosmic rays at http://pdg.lbl.gov/2015/reviews/rpp2015-rev-cosmic-rays.pdf .

First of all, the total energy of cosmic rays in our neighborhood is huge. Don't forget that Earth is just a tiny "pale blue dot" in space and the fraction of cosmic rays actually hitting us (and our detectors) is vanishingly small. Secondly, the spatial distribution of cosmic rays is very homogeneous, with fluctuations on the level of 0.001 only. These two facts make it rather unlikely that cosmic rays are produced by technology of aliens. Required power sources must be extremely huge and we would expect more localized sources.

In fact, we do observe localized sources for extremely high energy rays only (because they need to come from sources near-by and can not travel too far). However, these high energies are rather unlikely to be used in an engine because they are hard to generate. For comparison: Todays ion thrusters can accelerate Xenon up to 1 keV, 6 orders of magnitude to the left of the diagram above. Using the best known particle acceleration technologies known today (about 100 MeV/m) these extremely high energy rays needed an accelerator with a length comparable to the radius of the solar system (70 AU). Lower energy emissions are much more likely to come from an extraterrestrial space ship, but are also much harder to detect because of the huge background.

Further notes on cosmic rays

You also commented about the large error bars near the right end of the plot. This is purely due to statistics, not due to some not understood factors. In this region we expect about 1 particle of this energy per square kilometer and year which makes them very hard to measure. For us it is rather fortunate that these events are rather seldom - they deliver the energy needed to lift an apple by ten meters concentrated in a single atomic nucleus!

The factor of E^2 is used in this plot is to give us convenient units. In many cases this plot is made with a factor of E^2.6, which makes the first part of the plot flat and more easy to see the various kinks and bends in the spectrum. Compare your plot to figure 28.8 in the PDF linked above.

These various changes in slope of the curve seem to be caused by solar and galactic magnetic fields which can trap particles to some extend within our galaxy while very high energy particles are reaching us directly from extragalactic sources. The steep drop-off at the end of the spectrum is not unexpected as well: Faster particles start to interact with the cosmic microwave background and are slowed down.

The PDF also shows relative numbers for various types of nuclei (figure and table 28.1). E.g. iron is about 1/4000 of the number of protons (hydrogen nuclei). As you can see, the shape of the curve is rather similar for all measured ions. The plot shows this distribution only for relatively low energies because it is hard to distinguish masses at very high energies - the mass of the heaviest nuclei is just 250 GeV, compared to a million GeV kinetic energy. There are also no numbers for really heavy ions beyond iron, because count rates are getting lower and lower and error bars are getting bigger.

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  • $\begingroup$ Great explanation of a big subject! Can you clarify how "...total energy... in our neighborhood is huge." and "...the amount of cosmic rays actually hitting us (and our detectors) is vanishingly small..." are both true at the same time? I'm sure these are both true, but the distinction isn't clear with the present wording. Also, is the only defense of your "Extremely unlikely" that there exist models that can fit the data? Do these models have arbitrarily adjusted parameters (e.g. overall flux) in order to agree? Are there other points to add, like the angular distribution (or lack thereof)? $\endgroup$
    – uhoh
    Commented Aug 9, 2016 at 9:58
  • $\begingroup$ Great answer! If the frequency of cosmic rays were plotted against the mass of the nucleus, instead of against its (kinetic?) energy, would the curve change character substantially, or is energy and nucleus size closely correlated? Xenon doesn't happen to be over represented, indicating that our current SEP has an eternal future? ;-) $\endgroup$
    – LocalFluff
    Commented Aug 9, 2016 at 10:36
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    $\begingroup$ LocalFluff: I added a paragraph on ions at the end. @uhoh The models is not an argument, you can fit almost anything with a model. My main arguments are isotropy and total amount of cosmic rays in the galaxy. Both require that there is an extremely huge amount of engines almost everywhere in the galaxy. Your first question: The usual thing about "space is huge" - if we can measure dozens of particles per square meter and second there are huge numbers around in the galaxy. $\endgroup$
    – asdfex
    Commented Aug 9, 2016 at 11:03
  • $\begingroup$ This is a good overview and argument, but I would take issue a bit with the 'extremely unlikely' conclusion. If one goes looking for how indications of artificial production could be masked by the data, there are probably good arguments for that. I'd tend more towards that there is insufficient information to give a meaningful answer, but no indication of artificial production. $\endgroup$
    – kim holder
    Commented Aug 9, 2016 at 13:58
  • $\begingroup$ But the isotropy and maybe the energy distribution could be the result of interstellar and intergalactic magnetic fields. Regardless of the origin of the ions. The quantities are daunting, that's convincing. Rocketry competing with supernova, we'd notice that in other ways. But if a particular ion, such as a noble gas one, is over represented... $\endgroup$
    – LocalFluff
    Commented Aug 9, 2016 at 17:15
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Asdfex did an absolutely superlative job explaining why it's probably just background stellar emissions we're seeing, but let me ask you this: Who said a sun can't be an engine?

If you're willing to posit the existence of Megastructures (that we can't directly detect) then I offer you the Shkadov Thruster:

Well ain't that a monster

It's basically an immense photon-sail pushed by nearly half the output of a star. Its size allows it to gravity-tractor its powering star (and solar system) along with it, turning it into a literal starship.

Note: I don't think this is even remotely possible, I'm just giving you an edge-case where ion emissions can be the result of alien propulsion.

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    $\begingroup$ Nice concept of a "starship". Stars like the Sun are not very good in accelerating ions. The protons emitted by the Sun are almost entirely below 1GeV, outside the bounds of the diagram in the original post. $\endgroup$
    – asdfex
    Commented Aug 9, 2016 at 18:02
  • $\begingroup$ @asdfex Fair point. I suppose our alien friends could build Shkadov thursters around higher-output shorter-lived stars, and when it nears the end of its life transfer to another, but like I said I'm just floating this as a kind of wish-fulfillment. $\endgroup$
    – UIDAlexD
    Commented Aug 9, 2016 at 18:23
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    $\begingroup$ I see... How does this not collide with the star again? Are they orbiting a common center of mass and just spinning in place? Or is the radiation pressure actually supposed to be enough to prevent them from falling onto one another? (A far cry from far fetched) $\endgroup$
    – toniedzwiedz
    Commented Aug 9, 2016 at 22:52
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    $\begingroup$ @toniedzwiedz Right on the money with the radiation pressure. The sail sails away as fast as it falls towards the star. $\endgroup$
    – UIDAlexD
    Commented Aug 9, 2016 at 23:23
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    $\begingroup$ @uhoh Maybe it isn't one star, but almost all the stars. But I think we'd notice more of their existence than the exhaust fumes from their obsessive interstellar Nascar races. Seriously, since ions are electrically charged, they follow magnetic fields. Their origin cannot be traced, unless we get a map of magnetic fields between us and its origin since its origin. Difficult. $\endgroup$
    – LocalFluff
    Commented Aug 10, 2016 at 5:25

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