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Particle Physics and Pony Fiction Experimentalist

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How to build a neutrino gun · 12:47pm February 23rd

Continuing my discussion of the particle physics in Daring Do and the Inexplicable Artifact. If you haven’t read the story yet, do so now.

EDaring Do and the Inexplicable Artifact
Daring Do finds an artifact that does... something. She just doesn't know what.
GMBlackjack · 7.8k words  ·  141  4 · 1.4k views

As I said in my last post, the particle physics technology uncovered by Daring Do has the same goal as real world experiments like T2K and DUNE – creating a neutrino beam to shoot through the Earth.


If you want to make a gun able to fire a beam of neutrinos, how would you do it?

The simplest source of neutrinos, that you might find while exploring the ruins of a technologically advanced civilization, would be a lump of radioactive material. Get some strontium-90, or anything containing isotopes that undergo beta decay. It will kick out plenty of electron neutrinos (or anti-neutrinos). However you will need a lot of it to produce enough neutrinos to have a chance of detecting one, and then you will need to encase it in a shield to stop the beta particles from frying you. It will spray particles everywhere, and there is no way to turn it off…

Or you could implode a volume of enriched uranium or plutonium into supercriticality and create a nuclear explosion, resulting in a lot of neutrinos. But you wouldn’t want to clean up the mess. A-bombs are not recommended research tools…

A more controlled way to generate a large flux of neutrinos is with a nuclear reactor. See: Rock Farms and Nuclear Reactors for the Pinkie Pie explanation of how that works. While usually built to produce heat, they are useful neutrino sources. A nuclear reactor was the source of the first neutrinos ever detected. However it doesn’t seem likely the Daring Do’s artefact is a nuclear reactor, as it doesn’t produce heat, is easy to carry, and doesn’t emit neutrinos in all directions.

ERock Farms and Nuclear Reactors
Pinkie Pie shows Twilight Sparkle how to generate energy the earth pony way. Twilight has concerns about long term safety.
Pineta · 12k words  ·  805  18 · 8.4k views

What we really want is a beam of neutrinos, all going in one direction. This needs a different approach. As neutrinos pay no attention to electric fields, you can’t focus them into a beam. Instead, you need to make of beam of charged particles – pions are a good choice – and accelerate it in the direction you want. Then wait for the pions to decay into neutrinos. Conservation of momentum will keep them going the same way. This is the method used by the big long-baseline neutrino experiments. The full method involves multiple steps:

  • Use a standard particle accelerator to accelerate a beam of protons.
  • Send the protons into a target. Wham. This creates all sorts of new particles.
  • Use magnets to select charged pions out of the resulting particle debris. Focus them into a beam.
  • Pions decay in-flight to muons and muon-neutrinos, which keep moving in the same direction.
  • Let the muons smash into a beam dump. The neutrinos keep going straight through the Earth…

All explained in this cute animation:

All this is a pretty inefficient process, with most the energy producing other particles, which are absorbed by the beam dump and create a lot of heat. If this sort of thing is going on inside Daring’s artefact, there must also be some so of magic spell to absorb the heat. Still, I quite like the idea that she was carrying around an unrealistically small portable particle accelerator, not unlike the Ghostbusters' proton packs.

Next question: How do you detect neutrinos? Using magic crystals or other means…

Comments ( 9 )

Wow that is a really cute video. Neutrinos are the cool boys, apparently.
-GM, master of neutrinos.

Hopefully, further research and development will lead to understandingjust what neutrinos are and how various mechanisms create them, so that eventually maybe something along the lines of a reciculating pump storage naser, even ifit islinear, will be able to make them efficiently. The trick then becomesin startin the thing up without needing the Kamchatkya nuclear pumped particle beam system.

Another fun fact: Randall Munroe of XKCD fame did the math, and supernovas are so ungodly huge that the neutrino radiation would be enough to instantly kill you on its own. It obviously won't get the chance due to everything else going on, but it's still amusing to contemplate.

Out of curiosity, since we now have neutrino beams and detectors, has anyone tried using them for transmitting information? I mean, you could at the very least brute force a binary code (Morse or something more sophisticated with forward error correction) by sticking a shutter or deflecting electromagnet in front of the graphite target. Of course, the bandwidth might be absolutely atrocious if the beam or detector efficiencies are as low as I suspect they are.

I can see that first message sent by a couple of post-grads now…


If you're close enough. Just checked https://what-if.xkcd.com/73/ which says close enough is 2.3AU.

Yes. It has been done, but the bandwidth is very limited. Potentially neutrinos could be very slightly faster than optical fibres as they go straight through the Earth. The difference is so small as to be of no value to most people, but I heard some investors were interested in the idea as when sending buy/sell orders to stock markets, a tiny time delay can make a big difference to how much money you make or lose. The size of the neutrino detectors you would need mean it isn't yet a serious plan. If you could develop a more efficient neutrino detector, then it might be, and I have also heard of particle physicists managing to blag some funding from venture capitalists this way.

Obviously, the roadblocks to improving bandwidth sufficiently to make ultra-low latency direct market access trading feasible via neutrino narrowcasting are considerable. On the other hand, look how difficult it is to get broadband Internet at the South Pole! The Starlink and OneWeb satellite constellations are going to be hideously expensive to build out, but at least then the Pole will have a consistent, persistent connection.

Maybe the Ice Cube could be repurposed for broadband reception if neutrino gun efficiency were to be substantially improved…

There's only 1 AU between us and the sun on average, so that's enough to qualify for the values of "you" that most people would instinctively apply. Supernovae are insane, is my point.

Was kinda hoping for step by step instructions...

Ah well, this is extremely fascinating, and I'm gonna have to read that story now.

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