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Neutrino Thruster/Sail vs Photon Thruster/Sail

by sanman
Tags: neutrino, photon, thruster or sail
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sanman
#1
Jun8-10, 12:56 PM
P: 656
So neutrinos, once thought to be completely massless and travel at the speed of light, have recently been deduced to have mass because of their ability to undergo flavor change. It's not known however exactly how much mass the neutrino has.

So what then would the characteristics of a neutrino thruster be, in comparison to a photon thruster? Likewise, what would the characteristics of a neutrino sail be, in comparison to a photon sail?

Neutrinos seem to then have a rest mass, while photons do not. Photons have more coupling interaction with matter through electromagnetism, while neutrons only seem to interact weakly through the weak force.

Does a neutrino thruster or sail offer any advantages over its photonic counterpart? If so, then under what circumstances?

Would the fact that the neutrino has rest mass (howsoever miniscule) then afford the neutrino thruster greater thrust than the photonic one?

Since neutrinos only interact weakly through the weak force, then it seems challenging to conceive of a sail that gathers momentum from them.

http://en.wikipedia.org/wiki/Neutrino
The neutrino has half-integer spin (\begin{matrix}\frac{1}{2}\hbar\end{matrix}) and is therefore a fermion. Neutrinos interact primarily through the weak force. The discovery of neutrino flavor oscillations implies that neutrinos have mass. The existence of a neutrino mass strongly suggests the existence of a tiny neutrino magnetic moment[8] of the order of 10−19 μB, allowing the possibility that neutrinos may interact electromagnetically as well.
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Cotton_Sorbet
#2
Jun10-10, 07:31 PM
P: 5
Vastly different; As you know, neutrinos practically don't interact with matter at all, which means you can't make a sail out of them, because a sail implies you'd catch the neutrino "wind" from, say, the Sun. They have collosal cavities of water underground for
detecting solar neutrinos - they expect for several of them per week to interact. Two or three neutrinos stopped per few hundred thousand tonnes of material per day does not give much thrust >.<

So you'd be making a neutrino thruster.
This means taking the nuclear reactions that produce them and doing something to make it more probable that the neutrinoes will go out of your nuclear sample in one direction than in another.

The fact that they have no mass doesn't really matter; the momentum they carry away (which is where the thrust comes from) is proportional to their energy - the correction to this that comes from their mass is vanishingly small.

Which means; E = p/c (Energy of neutrino or photon = momentum of photon/speed of light) for photons and neutrinos.
That means for every 300,000,000 joules of neutrinos you fire out the back of your ship, you can push 1 kilogram of mass from 0 to 1 meter per second. So a ship that weighs a thousand tonnes wanting to change its speed by 100,000 meters per second needs to emit neutrinos with a combined energy of ~3*10^19 joules (~7 gigatonnes TNT equivalent). Thats a lot of neutrinos.

Now, if you were very clever and you did manage to make a device that emits neutrinos preferentially in one direction, the advantage over solar sails would be that you could put the engines right at the front of the ship and be projecting neutrinos straight through all your living space or whatever and it wouldn't matter, because neutrinos just go through everything.

Unfortunately, the thrust from this would be very small indeed, so it would only be useful if you were in deep space and starlight was very weak indeed.
In reality, you'd still pick solar sails because you can get more thrust than from a neutrino thruster by painting the side of the sails towards the direction you want to go black, with the back reflective (the black side absorbs light going onto it, which gets re-emitted as heat rays by the front and back more or less evenly, because they stay at about the same temperature, whereas the reflective side absorbs and then bounces back light rays hitting it, which means you get a net force towards the side with the black paint).

I have a special interest in trying to influence the direction that products of particle decays will go to - basically my idea for it is as follows; the higher the energy released by a nuclear decay, the higher the reaction rate. So if you put a nuclear sample in a very strong magnetic field so that decaying in one direction results in less energy overall in the system than decaying the other way, the rate of reaction going one way will be more than all the other directions. No idea if that will work in real life though.
sanman
#3
Jun11-10, 09:02 AM
P: 656
I'm wondering if electronic participation in the beta decay reaction would allow for some influence over the emission products. For instance, the C60 buckyball has shown its ability to influence the rate of beta decay, by symmetrically concentrating electronic charge around the nucleus. What if it could be made asymmetric, so that more positive charge could be directed at the trapped nucleus from one side of the cage, while more negative charge could be directed at it from the other side of the cage? Would there be any directional bias to the beta decay emission products? I think it might be worth investigating.

Cotton_Sorbet
#4
Jun12-10, 08:29 AM
P: 5
Neutrino Thruster/Sail vs Photon Thruster/Sail

Ah, upon further thought, it wouldn't work anyway; even if you could influence the direction in which the decay products from a nuclear interaction go in the decay's wake, the neutrino is a third body, hence it would still only be confined to spherically symmetric possible paths.

For instance, if in beta decay you could make it more likely (by a small amount) that the electrons would come out in one particular direction, the neutrinos would then be more likely to emerge in a disk (very nearly) perpendicular to the path of the beta particle and daughter nucleus. A disk is spherically symmetric, so no overall thrust in any one direction.

You would instead just eject the electrons from the back of the engine, but this isn't worth it; electrons are Fermions, which means you can't have very many of them close together at any one time, so by neccesity the thrust must be vanishingly small or any increase in efficiency you might gain you could get just as well from a conventional ion engine, for much less trouble >.<

Alpha particles sound more promising; they are Bosons, so perhaps if you were exceptionally clever you could compress a group of unstable nuclei such that they might make spontanious emission on a large scale - an Alpha particle Laser so to speak (an Aaser? xD). However, this is like a gamma ray laser but much, much more difficult; they work on paper, but in practice the resonance effects are so delicate that the slight variations in speed of the nuclei emitting the gamma rays dilutes the probability of stimulated emission.

Basically, it seems pretty hopeless to try to get ordered behaviour from groups of these tiny and immensely compact and complex phenomena (nuclei), we may just have to stick with playing with electrons instead.

Also I'm descidedly reserved about whether the prefered direction of decay products can be influenced by any macroscopic phenomena in the lab (ie a powerful magnet). As such, I'm rather apprehensive towards the idea that electron densities over atomic distances (I pressume the wave functions of the electrons in the carbon atoms creep out and superpose with that of the central unstable nucleus or something, right?) could actually have an impact on nuclear decay rates.
Can you link me to the study that found these results?
sanman
#5
Jun12-10, 09:17 AM
P: 656
Hi,

Here are some links:

http://prl.aps.org/abstract/PRL/v98/i25/e252501

http://www.phys.ncku.edu.tw/mirrors/...cay_rates.html

http://www.hps.org/publicinformation/ate/q7843.html

I think that if you had some kind of ellipsoidal egg-shaped version of the buckyball, then this could be used to concentrate charge a little more asymmetrically, so that one side of a nucleus would be exposed to higher or lower electron density than the other.

Or what about NEET?

http://focus.aps.org/story/v6/st8
Cotton_Sorbet
#6
Jun12-10, 11:20 AM
P: 5
Ah, I see, I misunderstood; electron capture does involve an electron being absorbed into a nucleus, so it's less shocking to hear that it can be effected (if only very slightly) by external factors and indeed it's very interesting.
However, if it's only electron capture, it won't be of much use to us for a propulsion system; the neutrino is all that gets out and we can't influence it's direction of emission.

By that I mean that if you were to be able to make the electrons in your 7Be prefferentially be absorbed from one direction (this may be dubious, since I'm not sure absorbtion really works like that from a quantum mechanical perspective) at best, surely, you can only force the neutrino to be emitted (roughly) perpendicular to the straight line drawn out by the electron moving into the nucleus. Perpendicular means it can be emitted from any direction that's between the nucleus and the electron.

I reckon even this isn't possible because really the electron is being absorbed not by a smooth spherical nucleus by a specific proton in the nucleus, which is moving around very fast (the energy of a proton in a nucleus is in the MeV, whilst the energy of an atomic electron is in the tens/hundreds of eV) and changing its direction very frequently in a highly chaotic way (the reason nuclei are so tightly bound is that the nucleons interact so strongly with eachother, which means that exchanges of force carrying bosons (Pions in this case) happen very frequently).
A nucleus is therefor a mess of activity, so that a neutrino might pop out from electron capture in any direction.
sanman
#7
Jun12-10, 11:34 AM
P: 656
Well, is it possible that we could arrange for our target nucleus to be a suitable nuclear isomer, so that the nucleons in our target nucleus have spin characteristics that could influence the direction of the emission products?


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