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It begins as a joke: A military agency goes to a scientist and asks him to build a superweapon capable of penetrating the strongest armor. He agrees, and proceeds to build... a neutrino laser.

Neutrinos are tiny, near-massless particles with no electric charge that can easily pass through miles of solid matter without interacting with it or indeed affecting it at all. While a neutrino laser can indeed "penetrate" any armor, it would also seem to be the most pointless weapon ever constructed... or is it?

Using relatively hard sci-fi (advanced technology is allowed, but nothing that defies physics as we currently understand it) what military applications would a neutrino weapon have? Think outside the box.

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  • $\begingroup$ I think the universe most feeble lifeform would eventually died of boredom before beginning to become neutrino cured at point blank... unless it don a black hole body armor(so strong that nothing can penetrate muhahahahahaha!) $\endgroup$
    – user6760
    Feb 28, 2016 at 13:12
  • $\begingroup$ No joke, Indigo. Particle weapons are in development, and are much desired to counteract vehicle armor that otherwise requires a lot of kinetic energy to get through. wait for it. $\endgroup$
    – Nahshon paz
    Feb 28, 2016 at 15:45
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    $\begingroup$ Obligatory XKCD (which may actually have your answer) what-if.xkcd.com/73 $\endgroup$
    – Cort Ammon
    Feb 28, 2016 at 17:10
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    $\begingroup$ And that, @user6760 , is how black holes are formed. Some alien is all like: "Nooo! Not with the neutrino laser! I better develop and put on some really resistant armor" $\endgroup$
    – Nahshon paz
    Feb 29, 2016 at 8:09
  • $\begingroup$ The irony of your question — and where you are not thinking enough inside the box — is that the very quality that lets neutrinos pass through armour is also that which makes them utterly harmless. As an important side-note: armour is not about making something become harmless in its effect, but rather for it to be harmful against something that does not give a damned (the armour itself), and thus away from that which does mind (the squishy flesh-heap that is you). $\endgroup$
    – MichaelK
    May 19, 2017 at 9:05

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Neutrinos have so little to do with the "hard" world that weaponising them would probably accidentally blow up the world as a side effect

Fortunately xkcd (or more particularly Andrew Karam has done the maths on Lethal Neutrinos. To be killed by a fatal dose of neutrinos, you would have to be hit something like $10^{35}$ of them.

You need energy to make Neutrinos. The Neutrinos released by beta emission are pretty energetic, about 1 MeV each, so our lethal dose requires about $10^{22}$ Joules, or the equivalent of 2000000 Megatons of TNT.

I wouldn't like to be the one holding the gun, if even a very very small proportion of that energy is released as waste heat.

So to create a lethal dose, we have left the realm of reasonable physics.

Whats left? well a neutrino pulse could be an unstoppable messaging system. If the bad guy has stopped you from communicating by radio, it may yet be possible to sent pulses of neutrinos to a distant detector. Since these can travel through a planet you can sent messages quicker than if they have to go around. If you can send pulses, you can communicate.

There is also some notion that neutrinos could catalyse some nuclear reactions. If this was the case you might be able to use a neutrino stream as a trigger to set off nuclear chain reaction.

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  • $\begingroup$ Good thinking. So making a lethal neutrino weapon would be practically pointless (unless you're dealing with highly advanced civilizations who might be using supernovas as weapons and planets as shields), but it might be able to disrupt some delicate but well-shielded technology. $\endgroup$
    – IndigoFenix
    Mar 1, 2016 at 10:03
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10^35 neutrinos sounds to be in the ballpark, and if we assume solar neutrinos with arround 1MeV of energy, a beam energy of arround 1.6x10^22 J, or 16 zettaJ. The neutrinos will be at near c, since they're close to massless. I'll take as given that beam generation is 100% efficient, so we don't have to deal with wast heat. Even a slight inefficiency is going to give us serious trouble.

So you've just fired a 180 tonne, lightspeed bullet at your enemy. Recoil is gonna be a bitch.

Lets say you can't quite do 16 ZJ (zettajoule) pulses, but you can do a 3 TW portable neutrino beamer. Irradiation only works out to about 0.3 microsieverts/sec I think, which the target probably won't notice in the heat of the moment. Average annual background dose at sea level is about 3 millisieverts, with a fatal dose being about 3-5 sieverts. You'll need to hold him still for a million seconds for a fatal dose to accumulate, about 12 days.

I can suggest a better way: find a one tonne rock, put it in a sling. Use your portable beam generator to lift the rock - a 3 TW photon rocket will suspend 1 tonne in a 1g gravity field. You can fire straight through the rock, these are neutrinos after all, not photons. That way stability while you hover the rock won't be an issue. The rock will get a little warmer at about 300 microwatts/kg. Just hover this over your target and drop it on him.

To an unlettered barbarian, it'll look like you have a reactionless drive. Of course, if you know what it is, you wouldn't want to stand under it. It would be like standing under an old style xray machine. A lead apron won't help though.

If the beam has a bit of divergence, like a regular rocket or even a torch, then it should cover a good many square kilometers when it emerges from the other side of the planet, it should be well below background radiation levels then.

Much nicer drive than a photon rocket, or even a regular chemical rocket.

If you've got a vacuum certified mobile home and can do 30 TW, then 100 kg of neutrino reaction mass will get you to mars in about 85 hours. That's straight line 1g all the way. You'll need another 100kg to get home again.

If you're considering neutrinos for reaction mass for a drive system of some kind, a few more useful facts:

Neutrino interaction cross section with nucleons goes up quadratically with neutrino energy.

If the neutrino energy is above a reaction specific threshold, it can cause nuclear transmutation. For example, a Cl37 to Ar37 neutrino mediated transmutation requires energy above 0.814MeV. This was used in the Homestake mine neutrino detector.

Interaction cross section with electrons goes up linearly with neutrino energy.

So use low energy neutrinos. Radiation damage to the environment for 3 TW of 1keV neutrinos would be about 1000 times less than for 1MeV neutrinos. Neutrinos are so close to massless that it won't impact drive efficiency much at all, they will still be moving at lightspeed for all intents and purposes.

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  • $\begingroup$ To answer the question in your edit suggestion: you can't delete something if you are unregistered. Look at your other question where I linked how to merge your accounts. Once you do that you can delete this post. $\endgroup$
    – Secespitus
    Oct 7, 2017 at 12:40
  • $\begingroup$ .3 Micro sieverts * 1 million seconds = 0.3 sieverts. $\endgroup$
    – blademan9999
    Jun 29 at 10:09
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While not a lethal weapon per se, New Scientist has an article which suggests that a neutrino beam of sufficient power could neutralize nuclear warheads and reactors by "scattering atomic nuclei". Suddenly shining a neutrino beam of that power through the planet at nuclear warheads or nuclear reactors would disarm the enemy and shut down aircraft carriers, nuclear submarines and the parts of the power grid energized by nuclear reactors, which would make prosecuting a war somewhat difficult, to say the least.

Of course, before bracing yourself for the surprise attack by a neutrino beam weapon, the scale of such a thing would make it pretty difficult to conceal, and shooting a beam through the planet to disable a reactor or warhead would be surprisingly difficult, given the small diameter of the beam:

But the “muon storage ring” generator needed to propose the neutrino beam would need to be 1000 kilometres wide. It would also require 50 gigaWatts of power to operate – the same as used by the entire UK – and would cost an estimated $100 billion to construct.

and

the first stage of a generator might be feasible within 10 to 20 years, but he reckons the main problem is that the neutrino beam produced would be just a few metres wide. This means a target would need to be very precisely located beforehand.

For disappointed weaponeers looking to lay waste to other targets, there is this "happy" thought:

the beam would produce dangerous alpha and neutron radiation in any living thing in its path.

https://www.newscientist.com/article/dn3734-neutrino-beam-could-neutralise-nuclear-bombs/

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  • $\begingroup$ Huh, so people are actually giving thought to this? Neutralizing nuclear weapons and technology with a beam that bypasses any shielding certainly has strategic value. $\endgroup$
    – IndigoFenix
    Mar 1, 2016 at 10:09
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You could send two streams of different types of neutrinos and coincide the crossing of the two beams with the exact position of the biological target.

The two neutrino beams will interact with each other and annihilate in the form of energetic gamma rays, thereby frying the target without needing huge energies.

A beam of Tau and Anti-Tau, for example.

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    $\begingroup$ Neutrinos have a head enough time hitting comparatively massive atoms how is getting them to collide any easier? $\endgroup$
    – Joe Kissling
    May 18, 2017 at 23:24
  • $\begingroup$ I meant to say hard $\endgroup$
    – Joe Kissling
    May 19, 2017 at 0:42
  • $\begingroup$ I think you mean a neutrino beam and an antineutrino beam intersecting and annihilating each other to produce gamma ray photons. There is one drawback. Neutrinos and antineutrinos don't annihilate each other efficiently. This will produce a low rate of gamma rays. It will require massive amounts of neutrinos & antineutrinos for a lethal gamma dose. I only recently found out about the low annihilation cross-section of neutrinos & antineutrinos. Otherwise I would have given a similar answer. Neutrinos are still cool! $\endgroup$
    – a4android
    May 19, 2017 at 2:08
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Ignoring the exact mechanics of neutrinos and focusing on the role of neutrino weapons in warfare:

If you threw a lot of neutrinos at someone you wanted dead, you could eventually give them cancer and they would die, along with anyone behind or between them and the projector.

On a larger scale, shooting a massive neutrino beam across a planet would let you sterilize the entire species (very slowly). Actually, used in this way, it could make an almost incredibly overpowered weapon, because it:

  • is completely undodgeable
  • goes through any and all armor and shielding
  • dooms the entirety of life on that planet to total extinction
  • leaves most resources unharmed
  • is hard to detect
  • can't be stopped
  • can't be reasoned with
  • doesn't feel pity or remorse

...wait.

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