Physics of the Dyson Sphere

The Silence and I · Post by **The Silence and I** » 2004-10-28 04:21pm

You may be right. However Winston Blake has offered evidence that neutronium actually has protons and can keep an electron cloud, this may allow a small enough molecule/tight enough mesh to electrically contain the material. However exceedingly low temperatures may be a necessity to avoid scattering, as I imagine the electron cloud about neutronium would be proportionally weaker than that of most any atom.

Kuroneko · Post by **Kuroneko** » 2004-10-29 01:23am

nasor wrote:I don’t think that the carbon cage could even really exert any pressure on the neutronium, since the neutrons would be about one hundred thousand times smaller than the holes between the carbon atoms.

You're very much correct. However, in a sci-fi setting where highly controlled and localized artificial gravity is possible, the situation is different. Neutrons at standard temperature (273.15K) have average velocity of over 2km/s. Containing them even with artificial gravity will be nigh impossible, unless of course they are cold. The neutron are so tiny that carbon does nothing to actually contain them, but it may be simply provide a structural backbone--something to affix the gravity control to.

Alright, that's a very large amount of rationalizing what was a silly idea to begin with, but that was sort of the point of this thread.

The Silence and I wrote:You may be right. However Winston Blake has offered evidence that neutronium actually has protons and can keep an electron cloud, this may allow a small enough molecule/tight enough mesh to electrically contain the material. However exceedingly low temperatures may be a necessity to avoid scattering, as I imagine the electron cloud about neutronium would be proportionally weaker than that of most any atom.

That's not a good idea. Introducing any substantial amount of protons into such a tiny space will produce extremely large Coloumb forces, causing them to fly apart almost instantaneously. One just can't expect the protons not to repel each other.

Winston Blake · Post by **Winston Blake** » 2004-10-29 07:34am

Kuroneko wrote:
The Silence and I wrote:You may be right. However Winston Blake has offered evidence that neutronium actually has protons and can keep an electron cloud, this may allow a small enough molecule/tight enough mesh to electrically contain the material. However exceedingly low temperatures may be a necessity to avoid scattering, as I imagine the electron cloud about neutronium would be proportionally weaker than that of most any atom.
That's not a good idea. Introducing any substantial amount of protons into such a tiny space will produce extremely large Coloumb forces, causing them to fly apart almost instantaneously. One just can't expect the protons not to repel each other.

I'm no expert, but wouldn't the strong nuclear force of all the neutrons adjacent to each proton overcome the coulomb repulsion between them? After all, isn't that how the protons in the tiny space of a nucleus manage not to fly apart?

You seem to know more than i do about this stuff, but i think i remember something about stray protons being in the neutronium of neutron stars.

Kuroneko · Post by **Kuroneko** » 2004-10-29 08:40am

Winston Blake wrote:I'm no expert, but wouldn't the strong nuclear force of all the neutrons adjacent to each proton overcome the coulomb repulsion between them? After all, isn't that how the protons in the tiny space of a nucleus manage not to fly apart?

The strong force will cause the formation of atomic nuclei, but how does this solve the problem of electromagnetic repulsion between those nuclei? Even if one could somehow keep them together despite this repulsion, they would attract electrons, causing the formation of full atoms, so it will no longer be possible to have the density that neutronium is famous for.

Winston Blake wrote:You seem to know more than i do about this stuff, but i think i remember something about stray protons being in the neutronium of neutron stars.

The key distinction here is the density of the charged particles. Any time there is a significant density of like charges, there will be tremendous Coloumb repulsion. All the strong force does is move the picture from lone protons repelling one another to higher atomic nuclei repelling each other.

Winston Blake · Post by **Winston Blake** » 2004-10-29 09:11am

Ok, i think i understand how that works now.

Could it work if at the centre of a neutronium ball, there was just a single multi-proton nucleus? E.g. Samarium-149 with a positive charge of 62 and half-life of 4E14 years (practically stable considering the estimated lifetime of the universe is about 13.7E9 years).

Kuroneko · Post by **Kuroneko** » 2004-10-29 08:17pm

Winston Blake wrote:Could it work if at the centre of a neutronium ball, there was just a single multi-proton nucleus? E.g. Samarium-149 with a positive charge of 62 and half-life of 4E14 years (practically stable considering the estimated lifetime of the universe is about 13.7E9 years).

In that case, this nucleus would not be able to keep immense amount of other neutrons there. The strong force has a truly tiny range, which is why super-heavy nuclei are extremely unstable.

Post by **Darth Wong** » 2004-10-30 12:35am

The question is the decay rate. Given two protons in a super-massive nucleus, it seems likely that one of them would be ejected, so a blob of neutronium would "evapourate" over time. The question is how quickly this would take place.

The Silence and I · Post by **The Silence and I** » 2004-10-30 03:24pm

That's a good point. I had forgotten neutrons decay if left to themselves.

[Bah! I know not enough about this. Does anyone have some relevant books to recommend? The internet just isn't the same thing...]

Winston Blake · Post by **Winston Blake** » 2004-10-31 08:53pm

Kuroneko wrote:
Winston Blake wrote:Could it work if at the centre of a neutronium ball, there was just a single multi-proton nucleus? E.g. Samarium-149 with a positive charge of 62 and half-life of 4E14 years (practically stable considering the estimated lifetime of the universe is about 13.7E9 years).
In that case, this nucleus would not be able to keep immense amount of other neutrons there. The strong force has a truly tiny range, which is why super-heavy nuclei are extremely unstable.

Ok, so my charged neutronium ideas are utterly impossible. Damn!

BTW, why don't neutron stars decay? Shouldn't they practically disappear in about a day, considering the decay of free neutrons? Do the neutrons have a stabilising effect on each other?

Kuroneko · Post by **Kuroneko** » 2004-10-31 11:38pm

Darth Wong wrote:The question is the decay rate. Given two protons in a super-massive nucleus, it seems likely that one of them would be ejected, so a blob of neutronium would "evapourate" over time. The question is how quickly this would take place.

That's a good observation. Neutrons have a half-life of around 10 minutes, which is very bad. On the macro-scale, the electromagnetic forces should cancel out (since the neutrons decay into protons and electrons), but nevertheless there would be a substantial losses for the protons near the surface of the 'neutron ball'. Another problem is mass discrepancy between the neutron and the proton+electron+neutriono, which is a substantial 2MeV/c², that goes into kinetic energy--in other words, the neutron mass will be self-heating.

The Silence and I wrote:That's a good point. I had forgotten neutrons decay if left to themselves.
[Bah! I know not enough about this. Does anyone have some relevant books to recommend? The internet just isn't the same thing...]

I would recommend getting acquainted with special relativity and basic quantum mechanics first. Once you've done that, I can recommend M. G. Bowler's Femtophysics as a very readable introductory text on particle physics. I'm not anywhere near qualified enough to judge the relative merits of the more involved books in this field.

Winston Blake wrote:
Kuroneko wrote:In that case, this nucleus would not be able to keep immense amount of other neutrons there. The strong force has a truly tiny range, which is why super-heavy nuclei are extremely unstable.
Ok, so my charged neutronium ideas are utterly impossible. Damn!

Yes. Coloumb and gravitational forces are inverse-square laws, which means their potential (electrostatic/gravitational, respectively) is inversely proportional to distance. On the other hand, the strong force is mediated by pions, which have a rest mass. By the uncertainty principle, ΔEΔt > h/4pi, so they are 'allowed' to travel only about a distance of cΔt, which at ΔE ~ mc² = 140MeV/c² gives it an effective range on the scale of a femtometer or so. However, things are not so simple, and the pions experience an effect analogous to quantum tunneling, and so the strong potential is actually exponentially decreasing with distance (in quantum tunneling, in which a particle can penetrate a classically forbidden potential barrier, the probability of such an event is also exponentially decaying). In any case, it is quite clear that the strong force has no chance in competing with either on the larger scales.

Winston Blake wrote:BTW, why don't neutron stars decay? Shouldn't they practically disappear in about a day, considering the decay of free neutrons? Do the neutrons have a stabilising effect on each other?

The neutrons undergo beta decay, to form a proton, electron (i.e., 'beta particle'), and an electron antineutrino. There is no stabilizing effect, nor is there need for one, because there is a reverse process, creatively called inverse beta decay, in which a proton and an electron form a neutron and an electron neutrino. Since the neutron star is overall electrically neutral, the Coloumb forces tend to cancel out each other, leaving gravity to dominate. Of course, this process is not self-perpetuating, and there are immense losses, but the inverse beta decay reaction slows down the destruction of the neutron star to the scale of millennia rather than a day.