In the novel Dragon's Egg, the human crew use one asteroid to swing other asteroids in place to counter the gravity of the neutron star. I understood that it was similar to a gravity sling shot, but I wasn't able to fully get how the crew were able to move the smaller asteroids in place using the big one. Can anyone explain that further?
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Please note that Dragon's egg is a "hard sci-fi" book, which in theory should be consistent with known laws of physics (so assume them), including GR. – Sklivvz Dec 11 '10 at 15:32
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2maybe whoever can answer this could also explain how six asteroids, orbiting their common center of gravity, while collectively orbiting a neutron star, could possibly be stable with the kind of tidal forces that would be present. – JustJeff Dec 11 '10 at 17:01
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1I've read the book's plot on the wikipage, and I just realize that a Star Trek Voyager episode, "Blink of an Eye", basically stole the plot from Dragon's Egg. – Raskolnikov Dec 11 '10 at 18:45
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1I do not know about the answer but if you ever come across a guy who can calculate such stuff, ask him to contact an authority with the solution of n-body problem. – Cem Dec 12 '10 at 01:04
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@Cemm: There are actually probes that "correct" the orbits keeping them stable... :-) – Sklivvz Dec 12 '10 at 04:23
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I think a picture of the neutron star and the orbits of the bodies (the six small asteroids, the two big asteroids, St. George and Dragon Slayer) would be very helpful. The technical appendix in the book doesn't have that picture. – Bala Dec 14 '10 at 04:40
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I know nothing of this book, but I do know a little about N-body gravitational interactions. When N >= 3, you can do just about anything you want with a little propulsion, but it may take a very long time. This has been proposed by NASA (good approximate for hard SF) as a way of sending probes to the outer solar system: http://en.wikipedia.org/wiki/Interplanetary_Transport_Network
As far as the stability questions, it is known that you can put 7 or 8 equal-mass objects on a stable circular orbit around their center of mass: http://adsabs.harvard.edu/abs/1988A%26A...205..309S
After introducing an external mass and its accompanying tidal forces, I imagine there is still some stability regime, which could be pretty close to the NS, especially if the "asteroids" have WD density, their constellation could be on the order of 1km, while the NS radius is probably closer to 15km.
Jeremy
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When a small spacecraft slingshots around a large moon or planet, it can greatly increase its speed at the expense of very slightly decreasing the speed of the larger body. Given a method by which to move a larger body, you could very simply cause it to move smaller bodies by traveling near enough to attract them. More complicated would be moving it in such a way that the smaller body was given a specific increase in absolute velocity, which would be the "reverse slingshot".
Sparr
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Not an answer but to give context:
There are eight asteroids: 2 large and 6 smaller ones. All the asteroids have been collapsed by injecting their core with magnetic monopoles that makes them collapse to white dwarf density. The large ones were originally 250km in diameter and collapsed to 100m.
The larger asteroids are on highly elliptical orbits.
http://books.google.co.uk/books?id=YataAAAAMAAJ&dq=isbn:034543529X&q=six+asteroids#search_anchor
Sklivvz
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Funny; you just said it should be consistent with the known laws of physics. Magnetic monopoles certainly aren't... though some might like them to be. – Noldorin Dec 11 '10 at 20:40
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4@Noldorin: AFAIK magnetic monopoles are consistent with the known laws of physics. And some GUT theories (most of them?) actually predict them. So let's wait and see :-) – Marek Dec 11 '10 at 22:28
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@Marek: They're not, per se. One needs to be careful with their choice of words. One of Maxwell's four equations (Gauss's law for magnetism) specifically states $\nabla \cdot B = 0$. So it is not consistent with the laws of physics at present. (The laws could however be adapted quite easily!) – Noldorin Dec 11 '10 at 22:39
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@Noldorin: I thought you may point this out but may I also point out that Maxwell equations hold only in the classical limit? :-) Monopoles have to do more with quantum world and if you really take the actual present known laws of physics, i.e. Standard Model and QFT they should be fully consistent with them. – Marek Dec 11 '10 at 22:44
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5@Noldorin: The zero RHS of Gauss's law is not a requirement in the theory, but simply an expression of the non-observation of magnetic monopoles to-date (notably various textbooks I've read exhibit the "with magnetic monopoles" form of Maxwell's Equations in a footnote or appendix). IMO positing physics allowed but not yet observed is fair game in that kind of fiction. – dmckee --- ex-moderator kitten Dec 11 '10 at 23:11
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@dmckee: No, it's considered part of the theory. Just because the theory as original based on observation, it doesn't mean it's any less aparty of theory! – Noldorin Dec 11 '10 at 23:24
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@Marek: This is true; it is however a requirement of any quantum theory that it reduces to Maxwell's equations in the classical limit. I'm being picky here, I hope you see my point. We have to stick to what we no for sure at present. – Noldorin Dec 11 '10 at 23:25
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4@Noldorin♦: What a weird remark! So you suggest that there can't exist any macroscopic effect in nature predicted by QFT if it's not consistent with Maxwell's equations? I hold a completely opposite view: Maxwell's equations hold only in a very narrow parameter space. If you move outside of that space they don't need to (and they don't) hold anymore. Portion of that parameter space might be physics of monopoles. If they exist then Maxwell's equations simply don't describe our world completely (which we know anyway). – Marek Dec 11 '10 at 23:33
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1@Noldorin: Maxwellian electromagnetism does not rule out magnetic monopoles: observe one tomorrow and the theory adjusts smoothly to accommodate it. Heck, in this case the math is already worked out. – dmckee --- ex-moderator kitten Dec 11 '10 at 23:50
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@Marek: Can the bloody attitude, I'm getting sick of it. It's a simple fact that quantum theory should reduce to classical theory in the appropriate in limit. I'm amazed you don't understand this. – Noldorin Dec 12 '10 at 00:41
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@Noldorin: And I am amazed that you don't understand that not every limit is appropriate. And that you somehow consider quantum theory just a correction to a classical theory. While in reality it's the complete opposite ;-) Anyway, I don't want to continue this discussion. In the past few days you've already shown your huge ignorance for the basic parts of modern physics (biggest one being the CFT) ;-) – Marek Dec 12 '10 at 07:28
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@Noldorin: just to explain what I meant... The author does not make use of any "deus-ex-machina". The use of monopoles is based on the assumption of the standard extension of Maxwell - his interpretation of it is that the div of B is equal to the current of the monopoles... He doesn't introduce anything any new theory - just the existence of a particle which is not in contrast with our current understanding of the universe (we have never seen monopoles, but they are not forbidden by theory). – Sklivvz Dec 12 '10 at 09:43
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@Sklivvz: Fair enough. It's kind of subtle; it depends what exactly you call the theory. In any case, I think we both accept that it's very easy to add monopoles to Maxwell's equations. – Noldorin Dec 12 '10 at 16:42
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1Why would a monopole make an asteroid collapse down to white dwarf density? It might paramagnetically polarize the asteroid, but the monopole charge would have to be pretty large to destabilize the entire asteroid. And even if that happened, why would the exclusion pressure of electrons be enough to stabilize it, rather than causing the collapse to continue? – Zo the Relativist Dec 12 '10 at 18:22
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1@Jerry: Been too long since I read that book. Forward (the author) is a physicist, but it concerns intelligent life living on the surface of a neutron star so is necessarily highly speculative. The whole business with the collapsed asteroids is a tide balancing arrangement to allow the human characters to move into close orbit around the star without experiencing Neutron Star (the Niven short story) type problems. – dmckee --- ex-moderator kitten Dec 12 '10 at 19:12
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@Jerry Schirmer, here is what the book says: "The monopoles entered the asteroid and in their passage through the atoms interacted with the nuclei, displacing the outer electrons. A monopole didn't orbit the nucleons like an electron. Instead, it whirled in a ring, making and electric field that held the charged nucleus, while the nucleus whirled in a linked ring to make a magnetic field that held onto the magnetically charged monopole (cont.) – Sklivvz Dec 12 '10 at 20:52
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With the loss of the outer electrons that determined their size, the atoms became smaller, and the rock they made up became denser. As more and more monopoles where poured into the center of the asteroid, the material there changed from normal matter, which is bloated with light electrons, into dense monopolium. The original atomic nuclei were still there; but now with monopoles in linked orbits around them, the density increased to nearly that of a neutron star (cont.) – Sklivvz Dec 12 '10 at 20:54
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As the total amount of converted matter in the asteroid increased, the gravitational field from the condensed matter became higher and soon began to assist in the process, crushing the electron orbits about the atoms into nuclear dimensions after they had only been partially converted into monopolium. After the month-long process was complete, the 250-kilometer-diameter asteroid had been converted into a 100-meter-diameter sphere with a core of monopolium, a mantle of degenerate matter of white dwarf density, and a glowing crust of partially collapsed normal matter." – Sklivvz Dec 12 '10 at 20:58
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As a reference, the science in the book should hold up according to the author: http://www.space.com/sciencefiction/books/forward_egg_000328.html – Sklivvz Dec 12 '10 at 21:06