## Silly Question: Neutrino Rocket

Moderators: gmalivuk, Moderators General, Prelates

Mechatherium
Posts: 11
Joined: Thu Jul 03, 2014 1:44 am UTC

### Silly Question: Neutrino Rocket

I've been working (sorta) on a SF story of mine. One of the goals with this project is to include well-thought out, kinda-hard technology.

Instead of the traditional antigrav for floating spaceships and such, I want to use constant 1-g propulsion to hold "floating" spaceships in the air. One crazy idea I've had is to use a neutrino rocket; being unreactive neutrinos, the exhaust should be both invisible and harmless.

Then I read the "what if" about how close you'd have to be to a supernova for the neutrino flux to be lethal (point blank!) and thought, maybe these guys can tell me if the exhaust from a hovering neutrino rocket (say, 4000 kilos weight) would actually be harmless to a human standing right underneath it.

So, if I stood under a neutrino rocket hovering overhead, could I survive the neutrino radiation?

Xanthir
My HERO!!!
Posts: 5400
Joined: Tue Feb 20, 2007 12:49 am UTC
Contact:

### Re: Silly Question: Neutrino Rocket

If it's throwing out enough neutrinos to thrust with, it's throwing out enough neutrinos to kill with. A neutrino rocket shouldn't be much different than a fusion rocket, insofar as it only avoids being classified as a weapon by the same loophole that keeps cars from being classified as weapons.

Thrust is thrust; energy is energy. You can't get a free lunch here.
(defun fibs (n &optional (a 1) (b 1)) (take n (unfold '+ a b)))

peregrine_crow
Posts: 180
Joined: Mon Apr 07, 2014 7:20 am UTC

### Re: Silly Question: Neutrino Rocket

Xanthir wrote:If it's throwing out enough neutrinos to thrust with, it's throwing out enough neutrinos to kill with. A neutrino rocket shouldn't be much different than a fusion rocket, insofar as it only avoids being classified as a weapon by the same loophole that keeps cars from being classified as weapons.

Thrust is thrust; energy is energy. You can't get a free lunch here.

I don't think this holds entirely. Yes, both a neutrino rocket and a conventional rocket put out particles with the same total kinetic energy, but much more of the particles coming from the conventional rocket will transfer this kinetic energy to your body. Sure, the neutrinos will eventual hit something to transfer their energy to, but considering that a beam of neutrinos can pass through the earth without losing much energy the impact will be so ridiculously spread out as to be meaningless.

As for the feasibility of this idea: I haven't got the faintest idea how to calculate it, but I'm guessing it isn't happening for the same reason that we don't built shapeships that use photons as reaction mass (other than maybe solar sails, but you won't be using those to stay floating).
Ignorance killed the cat, curiosity was framed.

speising
Posts: 2341
Joined: Mon Sep 03, 2012 4:54 pm UTC
Location: wien

### Re: Silly Question: Neutrino Rocket

at least we know how to generate photons.

Xenomortis
Not actually a special flower.
Posts: 1445
Joined: Thu Oct 11, 2012 8:47 am UTC

### Re: Silly Question: Neutrino Rocket

We know how to generate lots of neutrinos too.
Although sticking a nuclear reactor onto a rocket for its neutrinos would be inefficient.

Nicias
Posts: 166
Joined: Tue Aug 13, 2013 4:22 pm UTC

### Re: Silly Question: Neutrino Rocket

Well, we don't know how to generate neutrinos going in a particular direction. In chemical an nuclear rockets that fuel is in some sort of bottle where it is under high pressure, with a hole on one end. The unbalanced pressure opposite the hole (and the similar force on the nozzle) push the rocket forward. So even if we put a reactor on board, the neutrino flux would have no net effect because it would be isotropic. It is the same as how a glowing spaceship wouldn't be the same as a photon rocket. If we could produce a neutrino laser or somesuch, I think it would be safer than an equal thrust photon drive. It might be the one exception to the Kzinti Lesson

scarecrovv
It's pronounced 'double u'
Posts: 674
Joined: Wed Jul 30, 2008 4:09 pm UTC
Location: California

### Re: Silly Question: Neutrino Rocket

Nicias wrote:Well, we don't know how to generate neutrinos going in a particular direction. In chemical an nuclear rockets that fuel is in some sort of bottle where it is under high pressure, with a hole on one end. The unbalanced pressure opposite the hole (and the similar force on the nozzle) push the rocket forward. So even if we put a reactor on board, the neutrino flux would have no net effect because it would be isotropic. It is the same as how a glowing spaceship wouldn't be the same as a photon rocket. If we could produce a neutrino laser or somesuch, I think it would be safer than an equal thrust photon drive. It might be the one exception to the Kzinti Lesson

Actually we do know how to make a directional neutrino emitter. The basic idea is that you make some particles that decay into neutrinos, but aren't neutrinos yet, and then accelerate them in the direction you want. Then some time later they turn into neutrinos, and then you put some mass in the way to absorb the non-neutrino leftovers from the process.

gmalivuk
GNU Terry Pratchett
Posts: 26679
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

### Re: Silly Question: Neutrino Rocket

peregrine_crow wrote:Yes, both a neutrino rocket and a conventional rocket put out particles with the same total kinetic energy
This is only true if the exhaust velocity is similar in each case, but I assume the neutrinos would be going much faster than conventional rocket exhaust. All rockets require at least ve/2 W/N (plus anything lost as heat or exhaust going in the wrong direction or whatever). If neutrinos are traveling at almost the speed of light, you'd need 150MW of power per Newton of thrust, compared to a kilowatt or two for more conventional rockets.

It's still safer, because even that much energy in the form of neutrinos isn't going to deposit much in any given piece of matter. But it ties into your other point about why we don't use photon rockets: Even though the mass efficiency goes up with exhaust velocity, power requirements go down. If you don't have a way to generate that much power, you can't get the thrust you want out of a high-specific-impulse rocket.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

Nicias
Posts: 166
Joined: Tue Aug 13, 2013 4:22 pm UTC

### Re: Silly Question: Neutrino Rocket

scarecrovv wrote:
Nicias wrote:Well, we don't know how to generate neutrinos going in a particular direction. In chemical an nuclear rockets that fuel is in some sort of bottle where it is under high pressure, with a hole on one end. The unbalanced pressure opposite the hole (and the similar force on the nozzle) push the rocket forward. So even if we put a reactor on board, the neutrino flux would have no net effect because it would be isotropic. It is the same as how a glowing spaceship wouldn't be the same as a photon rocket. If we could produce a neutrino laser or somesuch, I think it would be safer than an equal thrust photon drive. It might be the one exception to the Kzinti Lesson

Actually we do know how to make a directional neutrino emitter. The basic idea is that you make some particles that decay into neutrinos, but aren't neutrinos yet, and then accelerate them in the direction you want. Then some time later they turn into neutrinos, and then you put some mass in the way to absorb the non-neutrino leftovers from the process.

Hmm, I suppose that would work, but probably wouldn't be helpful as a rocket. Since you could just let the "pre-neutrinos" out as exhaust and that would be better.

peregrine_crow
Posts: 180
Joined: Mon Apr 07, 2014 7:20 am UTC

### Re: Silly Question: Neutrino Rocket

gmalivuk wrote:
peregrine_crow wrote:Yes, both a neutrino rocket and a conventional rocket put out particles with the same total kinetic energy
This is only true if the exhaust velocity is similar in each case, but I assume the neutrinos would be going much faster than conventional rocket exhaust. All rockets require at least ve/2 W/N (plus anything lost as heat or exhaust going in the wrong direction or whatever). If neutrinos are traveling at almost the speed of light, you'd need 150MW of power per Newton of thrust, compared to a kilowatt or two for more conventional rockets.

Hmm, weird. I assumed that because the momentum is the same on both ends (rocket and exhaust) the total kinetic energy would be the same as well. Is this because momentum scales linearly with velocity while kinetic energy scales quadraticly?

Nicias wrote:Hmm, I suppose that would work, but probably wouldn't be helpful as a rocket. Since you could just let the "pre-neutrinos" out as exhaust and that would be better.

Well obviously, but at that point you just have a conventional rocket. The goal here was to generate thrust without having a dangerous exhaust.
Ignorance killed the cat, curiosity was framed.

gmalivuk
GNU Terry Pratchett
Posts: 26679
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

### Re: Silly Question: Neutrino Rocket

peregrine_crow wrote:Hmm, weird. I assumed that because the momentum is the same on both ends (rocket and exhaust) the total kinetic energy would be the same as well. Is this because momentum scales linearly with velocity while kinetic energy scales quadraticly?
Yes. If you have 1/X as much mass going X times as fast, it's the same amount of momentum but X times the kinetic energy.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

Tass
Posts: 1909
Joined: Tue Nov 11, 2008 2:21 pm UTC
Location: Niels Bohr Institute, Copenhagen.

### Re: Silly Question: Neutrino Rocket

Most posts in the thread discuss the infeasibility of a neutrino rocket, rather than addressing the question of radiation, so let us assume a perfect magical matter-to-neutrino-thrust rocket, and see whether it would be harmful.

Randalls references for the lethal neutrinos what if is a good place to start.

Let us assume neutrinos of some MeV, like those from a supernova. Regardless of whether they have a rest mass of some eV's or not they will be moving at very close to the speed of light. So the impulse they carry will be basically E/c. That makes your magical rocket as efficient as a photon drive, just with more ghostly exhaust. This is good, it will be able to hover for about a year before needing refueling (Isp=3*107s ≈ 1 year).

A supernova releases a few times 10^45J in neutrinos (1057*15MeV) divided by c that gives an impulse of 8*1036 kg m/s. At one parsec this was calculated to give a radiation dose of 0.5nSv, by dividing with the area of a one parsec sphere, we find that one square meter gets an impulse of 670 Ns/m2 at a cost of 0.5nSv of radiation dose. That is 1.3*1012 Ns/m2Sv. This is the same as the impulse that every square meter of neutrino rocket could deliver while delivering 1Sv of radiation to anyone staying in the beam path. So how much is this? What can you lift for how much damage? Well if you want light levitating signs etc., that people might occasionally walk under then you are fine. A flying gadget covering a square meter and weighing 50 kg will over the course of a minute deliver 20nSv or the equivalent of eating one fifth banana. If you want levitating infrastructure like houses and bridges that people might live under more or less permanently, then you are out of luck. At ten tonnes per square meter (ten meters of water or several stories of building), the yearly dose of living underneath is 2 Sieverts which will give you severely increased cancer rates.

Of course neutrinos of other energies might be even more benign. Interaction cross sections and energy transferred in an interaction both varies with energy, the paper already found a significant difference in the effect of super nova neutrinos depending on 5MeV or 15MeV being assumed.

In any case this is already impressively benign, considering that you are burning 300MW of energy into a focused radiation beam for every measly 100 grams levitated and you could still stroll under a building riding on this beam with little effect. Had you tried it with a photon rocket it would have been the equivalent of the last instance of this.

So congratulations on finding another example of the "knock me over with a feather" thing, where two extremes combine to produce moderate effect.

Edit: Broken links put here for possible future editing.
Spoiler:
Last edited by Tass on Fri Jul 11, 2014 11:37 am UTC, edited 2 times in total.

Mechatherium
Posts: 11
Joined: Thu Jul 03, 2014 1:44 am UTC

### Re: Silly Question: Neutrino Rocket

OK. IOW: back to the drawing board. Thank you, everyone who answered!

Tass, I'm thinking you were using some code that was supposed to render math. What should I use to read it?

peregrine_crow
Posts: 180
Joined: Mon Apr 07, 2014 7:20 am UTC

### Re: Silly Question: Neutrino Rocket

Mechatherium wrote:OK. IOW: back to the drawing board. Thank you, everyone who answered!

Why would you go back to the drawing board? Tass's post sounded kind of promising to me, sure you'd need ludicrous amounts of power for a fairly small amount of volume and mass, but even in fairly hard sci fi that should be a reasonable handwave. Light crafts for maybe three or so people should be fine so long as they don't hover directly over anyone's house for extended periods of time.
Ignorance killed the cat, curiosity was framed.

gmalivuk
GNU Terry Pratchett
Posts: 26679
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

### Re: Silly Question: Neutrino Rocket

Mechatherium wrote:OK. IOW: back to the drawing board. Thank you, everyone who answered!

Tass, I'm thinking you were using some code that was supposed to render math. What should I use to read it?
They're just urls that the forum software didn't parse correctly. Paste it into your address bar.

peregrine_crow wrote:
Mechatherium wrote:OK. IOW: back to the drawing board. Thank you, everyone who answered!

Why would you go back to the drawing board? Tass's post sounded kind of promising to me, sure you'd need ludicrous amounts of power for a fairly small amount of volume and mass, but even in fairly hard sci fi that should be a reasonable handwave. Light crafts for maybe three or so people should be fine so long as they don't hover directly over anyone's house for extended periods of time.
Personally I would go for some kind of exotic (i.e. handwavy) material that, when hit with just such-and-such frequency of laser, decomposed completely into fast-moving neutrinos. The ship could use a full photon drive for better mass efficiency when away from inhavited areas, and switch to neutrinos when the exhaust could potentially cause damage.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

Tass
Posts: 1909
Joined: Tue Nov 11, 2008 2:21 pm UTC
Location: Niels Bohr Institute, Copenhagen.

### Re: Silly Question: Neutrino Rocket

gmalivuk wrote:
Mechatherium wrote:Tass, I'm thinking you were using some code that was supposed to render math. What should I use to read it?
They're just urls that the forum software didn't parse correctly. Paste it into your address bar.

Whoops, yeah, that doesn't look too nice. Any idea how to fix it? Maybe I should just remove the google calculator links.

gmalivuk wrote:The ship could use a full photon drive for better mass efficiency when away from inhavited areas, and switch to neutrinos when the exhaust could potentially cause damage.

With an exhaust velocity of about 0.9999999999999c, the difference in efficiency between the neutrino rocket and a true photon rocket is negligible.
Last edited by Tass on Fri Jul 11, 2014 12:44 pm UTC, edited 1 time in total.

gmalivuk
GNU Terry Pratchett
Posts: 26679
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

### Re: Silly Question: Neutrino Rocket

Well I wasn't necessarily supposing that the photons were quite energetic enough to knock neutrinos out at that speed.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

Tass
Posts: 1909
Joined: Tue Nov 11, 2008 2:21 pm UTC
Location: Niels Bohr Institute, Copenhagen.

### Re: Silly Question: Neutrino Rocket

gmalivuk wrote:Well I wasn't necessarily supposing that the photons were quite energetic enough to knock neutrinos out at that speed.

Oh, right. I was just assuming MeV neutrinos since they are the only ones we have good data on. Of course if the energy is lower then the exhaust velocity drops too, although it has to be very low for the neutrinos to get down to non-relativistic speeds.

Also I was assuming that the exotic matter fuel was disintegrating to neutrinos, in other words that the energy comes from the mass carried on ship when tickled with some sort of radiation. Were you proposing a regular matter-antimatter photon rocket, with the option of converting the photons to neutrinos with some exotic matter "filter"?

In any case if the speed is lower then we know nothing of the radiation damage it would do since no one has observed cold neutrinos.

drachefly
Posts: 196
Joined: Thu Apr 23, 2009 3:25 pm UTC

### Re: Silly Question: Neutrino Rocket

Tass wrote:With an exhaust velocity of about 0.9999999999999c, the difference in efficiency between the neutrino rocket and a true photon rocket is negligible.

Photon rockets are far far more efficient if you can aim them at mirrors aimed at mirrors mounted on your spaceship so your photons bounce back and forth multiple times.

Tass
Posts: 1909
Joined: Tue Nov 11, 2008 2:21 pm UTC
Location: Niels Bohr Institute, Copenhagen.

### Re: Silly Question: Neutrino Rocket

How is that at all relevant for the discussion?

Anyway, the same is true for a regular rocket, if you bounce the exhaust back an forth between the rocket and a planet. It is then not really a rocket anymore, but some sort of exotic propulsion scheme.

Copper Bezel
Posts: 2426
Joined: Wed Oct 12, 2011 6:35 am UTC
Location: Web exclusive!

### Re: Silly Question: Neutrino Rocket

How is that at all relevant for the discussion?

One of the side questions you were considering was how much efficiency is lost over using a photon rocket, right? There's no equivalent to the mirror trick for neutrinos.
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

she / her / her

Neil_Boekend
Posts: 3220
Joined: Fri Mar 01, 2013 6:35 am UTC
Location: Yes.

### Re: Silly Question: Neutrino Rocket

Copper Bezel wrote:
How is that at all relevant for the discussion?

One of the side questions you were considering was how much efficiency is lost over using a photon rocket, right? There's no equivalent to the mirror trick for neutrinos.

Besides, if the neutrino's are photon generated with less then 100% efficiency it's wise to use the photons directly if you don't harm anyone with it. And how could it be 100% efficient? At least you're generating heat that you have to shed somehow.
Mikeski wrote:A "What If" update is never late. Nor is it early. It is posted precisely when it should be.

patzer's signature wrote:
flicky1991 wrote:I'm being quoted too much!

he/him/his

gmalivuk
GNU Terry Pratchett
Posts: 26679
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

### Re: Silly Question: Neutrino Rocket

Yeah, if you've got the mirror setup, it means you're already making sure the path between that and the ship is clear of obstacles, so just go the photon route. Hell, generate them on the ground instead and you don't even have to worry about the waste heat on your ship.

The neutrino suggestion was as a way to make a high-specific-impulse drive safe when the exhause isn't pointed at a launch installation's mirrors.
---
Incidentally, what is the power-to-thrust ratio of a neutrino rocket, anyway? I know photons require c W/N, and nonrelativistic exhaust requires v/2, but don't the equations get a bit messy for relativistic massive particles?
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

Tass
Posts: 1909
Joined: Tue Nov 11, 2008 2:21 pm UTC
Location: Niels Bohr Institute, Copenhagen.

### Re: Silly Question: Neutrino Rocket

gmalivuk wrote:Yeah, if you've got the mirror setup, it means you're already making sure the path between that and the ship is clear of obstacles, so just go the photon route. Hell, generate them on the ground instead and you don't even have to worry about the waste heat on your ship.

The neutrino suggestion was as a way to make a high-specific-impulse drive safe when the exhause isn't pointed at a launch installation's mirrors.
---
Incidentally, what is the power-to-thrust ratio of a neutrino rocket, anyway? I know photons require c W/N, and nonrelativistic exhaust requires v/2, but don't the equations get a bit messy for relativistic massive particles?

For hyper-relativistic particles, like neutrinos usually are, it is just c. If they are just regular relativistic it probably gets a little bit messy. But I can tell you that you'll get slightly lower specific impulse and slightly higher thrust-to-power.

Also the pedant in me wants to correct your "c W/N". c already comes with units. In SI those units are m/s which is the same as W/N.