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In Dragon's Egg by Robert L. Forward, a neutron star gives birth to a unique type of life, based on "compounds" constructed of nuclei bound by the strong force.

This life evolves to sapient creatures, called "cheela". Eventually they manage to build civilization and design spacecraft.

The book states that:

We do not know the propulsion technique used by the cheela to lift their spacecraft off the surface of the neutron star (the escape velocity of Dragon's Egg is 39% the speed of light). ... they suspect that the cheela used some sort of antigravity catapult to get off the star

Are there any realistic ways for take off from a neutron star?

I suppose the best way would be take advantage of the magnetic field and produce lift with the Meissner effect ora diamagnetic "sail". Of course it would be hard to make a superconductor or diamagnetic material even work in neutron star conditions...

Are there any hard science based designs that show the possibility/impossibility of space launch for the cheela?

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    $\begingroup$ The problem is that gravity control is required (in the same way we would need, say, oxygen) for beings of neutron star material to travel in space. $\endgroup$
    – jdunlop
    Aug 2 at 2:00
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    $\begingroup$ The sequel novel, Starquake, describes what Robert Forward thought was a plausible method: they exploit the general-relativistic gravitomagnetic effects of a "rolling" toroid of nuclear matter to counteract the surface gravity at launch and landing sites. The titular starquake destroys those sites and strands cheela astronauts in space. $\endgroup$ Aug 2 at 4:36
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    $\begingroup$ Hello Vashu. It's really important to note that you're asking us to speculate on a "hard science" level to the possibility of a fictional propulsion system in a fictional application. More importantly, please note for future reference that asking about 3rd party/commercial worlds is off-topic, are you working on a fictional world of your own? $\endgroup$ Aug 2 at 7:02
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Are there any hard science based designs that show the possibility/impossibility of space launch for the cheela?

Given that Robert L Forward was a professional physicist who did a lot of work on things like space fountains (the method used by the Cheela to leave Egg the first time around) I'm inclined to think that the methods in the book are reasonably realistic... if reliant on some materials science that is completely beyond us. Since we're unlikely to survive anywhere close to an actual neutron star I don't imagine that we're too worried here about what humans are capable of.

The book states that:

We do not know the propulsion technique used by the cheela to lift their spacecraft off the surface of the neutron star (the escape velocity of Dragon's Egg is 39% the speed of light). ... they suspect that the cheela used some sort of antigravity catapult to get off the star

Actually, that's just the humans in the story not knowing what's going on down on the surface. The story itself goes into quite a bit of detail from the perspective of the Cheela.

There are two main technologies the Cheela use to escape from (and return to) Egg:

  • Space Fountain (Dragon's Egg)
  • Gravity Catapult (Starquake)

Space Fountain

In "Dragon's Egg" the Cheela develop a space fountain composed of stabilized magnetic monopole blocks fired nearly to orbit by a magnetic accelerator. They magnetically couple to the fountain to raise a platform at a sedate pace until the reach a height limit based on their need for a high gravity field to keep their body together. From there they can make it the rest of the way into orbit relatively cheaply.

The science behind this is reasonably sound, except perhaps for the magnetic monopoles. While these are theoretically plausible we have yet to find any examples of them in the real world. The energy requirements are fairly insane, but that's just an engineering problem. A species that developed on the surface of neutron star will probably have different limits than we do, so I'm happy to cut them some slack on the little details.

Space fountains are an alternative to a space elevator, and are better suited to a neutron star. A space elevator requires a high-tensile tether material even in relatively low-gravity environments like Earth. We'd need something about on par with scrith to do the job on a neutron star.

Gravity Catapult

In the second novel "Starquake" the space fountain is destroyed, leaving a number of Cheela stranded in space with no way to return to the surface of Egg. After several generations the space-going Cheela manage to recreate an older technology for manipulating gravity via counter-rotating charged singularity (black hole) dust. They land a small gravity catapult by throwing it at the surface and firing it downward at the right time to slow it enough to land safely, then reverse it to catch small ships. They build a larger version to launch the ships again, establishing two-way traffic between orbit and the surface.

Ignoring the material science requirements, the idea of twisting space into pretzels uses some of the more interesting ideas from general relativity. The effect of large rotating masses were predicted by general relativity, and was demonstrated when the effect turned out to perfectly explain the discrepancy in Mercury's orbit. The Cheela's gravity catapult could operate on a related set of principles. If you could make specific "shapes" in the curvature of spacetime - our best current guess as to the underlying mechanism of gravity - then it's conceivable that we could use gravity to move pretty much anything anywhere. Hell, with enough power you could open the event horizon of a black hole.


Other ideas (as I think of them)...

Magnetic Levitation

The magnetic levitation idea isn't impossible either, again dependent on some material science that we don't have access to here.

A big deal is made throughout both novels about the strength of the magnetic field on Egg, and its pervasive nature. It's strong enough that the locals are shaped by it, and build all of their stuff along the field lines. The field lines are 'frozen' into the star's structure, giving a nice strong and reliable field to push off.

Given that the Cheela have the ability to build things out of magnetic monopoles it could be possible to create a field rider that engages with the field lines and travels along them. If the magnetic field lines aren't too constrained you might make it high enough to reach orbit, or hopefully get close enough to make the rest of the way under power.

The downside to this idea is that a neutron star rotates very quickly. If the magnetic poles are aligned with the spin poles then Coriolis is going to rip you off the field shortly after you leave the vicinity of the pole. For this to work you're going to need a very specific type of neutron star where the magnetic poles are on the equator. On the plus side, you'd probably get some assistance from Eotvos forces. Still, it'd be a hell of a ride.

Warp Drive/Exotic Matter

Alcubierre drive, as Willk suggests, is an interesting option. We'll hand-wave the exotic matter requirements. It would need to be extremely powerful to warp space sufficiently to counter the existing warpage of the gravity well itself. The energy and the sheer amount of exotic matter required to effect that sort of field would be... tremendous.

If you think about it though, there's a more direct way to do it. The Alcubierre drive requires exotic matter with negative mass. Negative mass means the force of gravity is reversed. Instead of being crushed by millions of gravities the exotic matter will be forced upwards with force proportional to its mass. This could be used directly to provide lift without all that fooling around with spacial curvature.

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    $\begingroup$ Worth noting that the station the humans occupied use monopolium masses as the tidal compensators. They made theirs from a single particle captured more or less by accident, IIRC, and used to create more. $\endgroup$
    – Zeiss Ikon
    Aug 2 at 12:54
  • $\begingroup$ Nice, thank you. It looks like there were catapults before fountain "ancient gravity catapults now lying dormant on Egg, which had only to toss small spacecraft into the sky" $\endgroup$
    – Vashu
    Aug 2 at 23:15
  • $\begingroup$ @Vashu Yeah, I kind of skipped those since they only launched shuttles. The bigger ones in Starquake were able to do more I believe - it's been a while since I read the books :P $\endgroup$
    – Corey
    Aug 3 at 10:48
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Don't try to take off straight up. Get going along the surface, performing very many orbits of the star before you finally escape its gravity well. The first few orbits will be only millimeters above the surface, and it will be necessary to smooth a track around the equator into a perfect circle so these first few orbits can happen.

A neutron star at its equator rotates at a significant fraction of the speed of light already, so that will help overcome the gravity well if the ship begins take-off in the same direction as the star is rotating. The track may be lined with electromagnets that help speed the ship while getting it off the ground, like a maglev train. Ground-based lasers along the track will help accelerate the ship further, once it is out of reach of the electromagnets, by pushing on a light sail mounted on the ship.

An alternate method - perhaps more economical, if available - to get explorers off the star would be through radio communication with aliens that are already in space. Just beam up the mind-scans of your explorers, or the plans for a machine that can manufacture members of your race, and the aliens will create your astronauts for you, already in position.

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    $\begingroup$ Slowing down is no problem--the same gravity that you are trying to escape will naturally slow you down as you move away--to zero velocity at infinity, in fact (that's what escape velocity means). $\endgroup$ Aug 2 at 4:37
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    $\begingroup$ @LoganR.Kearsley Ah, yeah, thinking it over I suppose you are right. Once the ship is going fast enough for its very first true orbit less than a millimeter above the surface of the star, it will be going the fastest it ever will. As it continues boosting, it will actually start to go slower, and the energy of the boost will increase its altitude rather than its speed. Because orbital speed is sqrt(G M / R), which decreases with R. $\endgroup$
    – causative
    Aug 2 at 5:25
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    $\begingroup$ The magnetic field of a neutron star is measured in the millions of Gauss. If you cross the field lines at high speed... it's not going to be a good day for you. $\endgroup$
    – Corey
    Aug 2 at 12:54
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    $\begingroup$ Sure, the aliens will make your astronauts -- in a few thousand generations. Don't forget the time scale difference between humans and cheela. $\endgroup$
    – Zeiss Ikon
    Aug 2 at 12:56
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    $\begingroup$ @Goodies You clearly haven't read the story :P They have to work hard to move across the field lines, and doing it too fast is painful to them because of the way it screws up the alignment of their nuclei. A few million Gauss is nothing to sneeze at. $\endgroup$
    – Corey
    Aug 2 at 13:59
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Alcubierre drive.

Alcubierre Drive without FTL?

The Alcubierre drive works by distorting space around a bubble: expanding space behind it and contracting space in front of it. It's a nice way to get faster-than-light travel without, well, technically traveling faster than light. But the basic mechanism behind it can, it works out, work at any speed. Nothing in the equations forbids sub-light speed travel... answered May 11 '20 at 16:53 HDE 226868♦

Using an Alcubierre drive you are fake-accelerating by altering space. That means you can fake-accelerate your ship to cancel the acceleration caused by the gravity of your star. It is a great use for an Alcubierre drive and a different one from its use to effectively traverse spaces faster than light. Of course it requires negative mass but that should not be a problem for the cheela.

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  • $\begingroup$ I am pretty sure distorted space == gravity. If you change gravity on surface of neutron star it would cause giant waves at best, disintegration of neutronium at worst. It could easily sterilize neutron star anyway. $\endgroup$
    – Vashu
    Aug 2 at 4:21
  • $\begingroup$ @Vashu - I agree. You would want to keep your warp bubble clear of the star. $\endgroup$
    – Willk
    Aug 2 at 22:03
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There are some designs in sequel, Starquake.

Gravity catapult

The workhorse of early cheela space transportation was a gravity catapult. We are not sure exactly how it works, but again the Einstein General Theory of Relativity gives us a clue. It has been shown , that the Einstein theory of gravity has a number of similarities to the Maxwell theory of electromagnetism. In electromagnetism, the basic source of all the forces is the charge on the electron. The charge generates an electric field. If you move the charge to form an electric current, the current generates a magnetic field. It is also known that if you increase or decrease a magnetic field, that changing magnetic field in turn generates an electric field.

The same thing happens in gravity. The basic source of all the forces is the mass of whatever particles you are using. The mass generates a gravity field. If you move the particles to form a mass current, the current generates a new field that is the gravitational equivalent of the magnetic field. In Figure 10 we show a torus wrapped with tubing carrying a mass current T and generating the new field P called the protational or Lense-Thirring field. If you increase or decrease the protational field, it will generate a gravity field G at the center of the catapult that will push any object at the center of the ring in an upward direction. The cheela gravity catapults must work in somewhat the same manner, but it is also obvious that new physics must be involved. The Einstein theory would predict that a machine using neutron star density material could not make a strong enough gravity field to catapult a spacecraft off Egg.

As it assumes some physics beyond Theory of Relativity, it is not hard science.

Also space fountain

Cliff-Web had designed a tower 200 times taller than the diameter of Egg

It was humming with power from the constant stream of rings that were accelerated in long circular tunnels at the base of the mountain and shot upward in a fountain of metal.

The top of the Space Fountain stopped at 405,900 meters

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