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Current SETI efforts seem to focus on the electromagnetic spectrum in the "water hole," or EM waves between 1420 MHz and 1720 MHz since that's the resonant frequency of hydrogen atoms up to water molecules. The rationale being if an alien civilization were to attempt to contact us, it would do so using a frequency somehow associated with the building blocks of life.

I feel like this assumes that: 1) An alien civilization would attempt to contact us at all 2) Alien life has a similar chemistry to ours

If we were to instead look for the neutrino emissions from their nuclear reactors and atomic weapons, couldn't we circumvent these two assumptions? Neutrinos interact incredibly weakly and can travel MUCH farther than light can. So why aren't we focusing our SETI efforts there?

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    $\begingroup$ This question assumes that intelligent life uses nuclear reactors and atomic weapons. $\endgroup$ – cdlvcdlv Jun 13 '18 at 11:47
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    $\begingroup$ The question also makes some weird assumptions, such as that neutrinos can travel much farther than light. We are studying light which was left over from the end of the big bang, so even if it's true, it's irrelevant. $\endgroup$ – JohnEye Jun 13 '18 at 14:34
  • $\begingroup$ @cdlvcdlv I'd say that's a safe assumption. xkcd.com/1162 $\endgroup$ – UIDAlexD Jun 13 '18 at 15:19
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    $\begingroup$ @UIDAlexD Well I wouldn't. Apart from that graph only lists energy sources discovered by us, Randall Munroe unfairly compares chemical energy with fission energy. Organic stuff also carries nuclear energy (but much difficult to obtain AFAWK). How much chemical energy can you get per g of U? And unfairly leaves fusion energy out (also known by us and which uses hydrogen). It sure works for a joke (and this is not one of the best of xkcd), just that. But, finally... nuclear weapons? WTF? $\endgroup$ – cdlvcdlv Jun 13 '18 at 19:34
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    $\begingroup$ @cdlvcdlv You see weapons, I see Orion pulsed-propulsion units. As for energy sources, Fission is the best we have right now. Solar and wind are pretty damaging to the environment when you factor in mining and the requirement for toxic batteries, green fuels don't scale well, and I think we can both agree on the insanity that is "Clean Coal." Fission products aren't pretty, but they're miniscule in volume compared to the gigatons of carbon emissions that are the alternative. Do I want fusion? Hell yeah. We all do, but let's not delude ourselves. Better sources might not be in the cards for us. $\endgroup$ – UIDAlexD Jun 13 '18 at 19:48
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Neutrinos are difficult to detect. Much of neutrino physics starts with a gazillion neutrinos and hopes to detect a few interactions. These guys published a rough map of terrestrial emissions from all sources with their 2015 paper. This guy proposes a gigaton detector for identifying reactors here on earth. Deriving location information would take on the order of a year. Given that neutrino flux would fall off according to the inverse square law, the detector size and runtime involved for detecting and localizing reactors lightyears distant would be prohibitive.

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    $\begingroup$ In might be worth adding that Super-K has imaged the sun in neutrinos, but the poor angular resolution of the image suggests another problem with long range detection. Also that we doubtless detected the neutrino pulse from sn1987a, and that there is an on-going effort to detect supernova pulses in time to directed optical telescope to the right part of the sky in time to catch the first light. $\endgroup$ – dmckee Jun 12 '18 at 19:52
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    $\begingroup$ @dmckee, we detected the neutrino pulse from SN 1987A a few hours before the visible light arrived. And to give an idea of how hard spotting neutrinos is, that "pulse" consisted of 25 neutrinos distributed between three detectors. $\endgroup$ – Mark Jun 12 '18 at 20:37
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    $\begingroup$ @Mark But in 1987 we didn't have the infrastructure to get the direction data from the neutrino experiments to the traditional astronomy community in time for them to re-point any big scopes. We've got that now. (I was part of KamLAND for ~5 years.) $\endgroup$ – dmckee Jun 12 '18 at 20:53
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    $\begingroup$ A guy is searching for his car keys under a streetlight. A second guy comes to help but after a while he asks: "are you sure this is where you lost your keys?". The first guy replies "no, i lost them over there, but the light is better here". I feel like this question deserves a better answer than "we can detect EM waves easier than neutrinos". $\endgroup$ – Tibos Jun 13 '18 at 8:08
  • $\begingroup$ I wonder if knowing the neutrinos current state along with approimations for the decay of neutrinos and state transitions over time could yield results. Sort of like how we study the decay of lead to identify the age of objects. $\endgroup$ – Magic Octopus Urn Aug 18 '18 at 4:54
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Neutrino emissions are very hard to detect. There are a few neutrino detectors, but they can only detect massive events (supernova explosions etc). We haven't made any that are sensitive enough to detect emissions from individual stars (yes, except our sun), let alone nuclear reactors on exoplanets.

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    $\begingroup$ We can detect individual nuclear reactors (slowly) at a range fo a few meters, but we can't detect them from space. $\endgroup$ – Steve Linton Jun 12 '18 at 14:28
  • $\begingroup$ There are low energy neutrino detectors too measuring solar neutrinos as well as neutrinos from CERN, see Borexino. Not as massive as a supernova. $\endgroup$ – Uwe Jun 12 '18 at 18:28
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    $\begingroup$ I've made my answer more specific to exclude detections where the neutrino detector is right next to the reactor. $\endgroup$ – Hobbes Jun 12 '18 at 19:54
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    $\begingroup$ "We haven't made any that are sensitive enough to detect emissions from individual stars" should exclude the sun. I can list at least four experiments that worked on solar neutrino just off the top of my head. $\endgroup$ – dmckee Jun 12 '18 at 19:55
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    $\begingroup$ "We can detect individual nuclear reactors (slowly) at a range fo a few meters," - which just highlights the uselessness in an amusing way, because we can also do that without a neutrino detector. $\endgroup$ – immibis Jun 13 '18 at 0:51
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Stars emit neutrinos. Even if we could detect them easily (hard because as you point out they're very weakly interacting, see other answer), neutrino emissions are hardly a "clear channel" where nuclear technology is likely to be the main source of signals.

(Possibly viable if you can filter by neutrino energy, though, but maybe still not.)

Even on earth where our nuclear reactors are much closer than the Sun: (wikipedia)

The majority of neutrinos in the vicinity of the Earth are from nuclear reactions in the Sun. In the vicinity of the Earth, about 65 billion (6.5×1010) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun

To "outshine" our sun, a reactor has to be right next to the detector. (Sorry I don't have quantitative numbers on this).

From another solar system, we'd have the same problem as with visible light: the sun outshines the planets and is very close relative to the distance from them to us. Even a nuclear explosion is peanuts compared to what stars do continuously.


Like a photon or a cosmic ray, a neutrino can have any amount of energy.

Current detection methods have a minimum energy threshold, and can't detect the low-energy neutrinos left over from the Big Bang, or the lower-energy neutrinos from reactors. Wikipedia's main article is pretty short on numbers, and I haven't taken the time to dig further because this idea appears to be so far away from being plausible with current technology.

Supernovae produce very energetic neutrino / antineutrino pairs simply from being so hot (~100 billion Kelvins at the neutron core so there's enough free energy for pair-production).

I'm not sure if anything would distinguish neutrinos produced by fusion in a star from sources like a fission reactor that is more likely to be technological. (Or anti-matter annihilation, if you're looking for Star Trek technology levels.)

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In addition to the extreme difficulty of detecting neutrinos at all, and of getting directional information from the few you do detect, how do you separate the tiny number produced by the civilization from the larger flux of their star?

This is also making an assumption that the civilization uses nuclear reactors and nuclear weapons. Judging from our own history, there was only a period of a couple of decades in which nuclear explosions took place. (Plus a few small ones from North Korea, recently.)

A further point is that if you did detect a large flux of neutrinos from an alien civilization using nuclear weapons, there's probably not going to be much of a civilization around afterwards.

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Also neutrinos are emitted equally in all directions. Neutrinos can't be aimed like electromagnetic radiation (radio, lasers). So, power of neutrino signal will decay back proportionally to square of distance. You will need stellar-scale energies to produce enough neutrinos to be detected at another star.

So, if ever a device for neutrino communication would be possible, it would be uneconomic compared with electromagnetic communications.


EDIT

After BobT's comment I found that I was wrong. Neutrino beams have been created since 1961. Link with nice video. Official link to Fermilab's Long-Baseline Neutrino Facility (LBNF).

I wonder about angular width of neutrino beam, and could it be done narrow as laser. Here a 58 mrad width is reported, it's too wide (= 3.3 degrees) for effective communication.

Quote from the first link:

In the future, scientists hope to make better neutrino beams by using muons instead of pions. The muon is a heavy cousin of the electron. When it decays, it produces both a muon neutrino and an electron anti-neutrino. A proposed project, called nuSTORM, aims to manufacture a neutrino beam from these muon decays. Since muons live about 100 times longer than pions, they are easier to accelerate and focus, but they also travel a longer distance before they decay. The challenge is to produce and collect enough muons, propel them and store them in an accelerator ring until that decay occurs.

Also from the first link about communication:

Some scientists are already contemplating ways to use neutrino science for other applications. Perhaps neutrinos could become a future communication tool for places that radio waves can’t reach, such as submarines deep under water or satellites traveling across the far side of the Moon. This would require even better neutrino beams and supersensitive neutrino detectors.

Earlier this year [2012], a group of scientists showed just what would be required to make this possible. They used a neutrino beam at Fermilab to send a short, encoded message through 240 meters of rock. Using the MINERvA neutrino detector, the scientists detected and deciphered the message, which read “neutrino.” Sending this simple message 240 meters required the world’s most powerful neutrino beam and took about 90 minutes.

So, as other answers mentioned, the main problem is to catch neutrinos. If no more effective ways of neutrino registration could be found, neutrino communications are "uneconomic".

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  • $\begingroup$ Good point that SETI is hoping to detect directed EM from civilizations hoping to contact us, not so much omnidirectional broadcasts. The inverse-square argument applies to pretty much any directed-energy communication vs. power leaking out in all directions. $\endgroup$ – Peter Cordes Jun 13 '18 at 8:13
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    $\begingroup$ Hmmm... Neutrinos are not always "emitted equally in all directions", The Deep Underground Neutrino Experiment (DUNE) and other experiments like it create a neutrino 'beam' that points towards detectors hundreds or thousands of miles away. If you're only talking about reactors and stars, then yes... $\endgroup$ – BobT Jun 14 '18 at 1:48

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