"Dragons egg" by Robert L Forward

[k!rl]

The book is about humans coming into contact with intelligent life on a neutron star, who due to their homeworlds massive gravity evolve incredibly fast relative to the human time frame in higher orbit. It's written by Robert L Forward, a physicist, however shouldn't higher gravity cause time to slow down relative to the outside observers?

This puzzles me to no end

 

[mfb]

Time dilation on neutron stars is a small effect compared to other factors.

Bacteria evolve (for some suitable metric) thousands to millions of times faster than humans, for example.

 

[SteamKing]

The book is about humans coming into contact with intelligent life on a neutron star, who due to their homeworlds massive gravity evolve incredibly fast relative to the human time frame in higher orbit. It's written by Robert L Forward, a physicist, however shouldn't higher gravity cause time to slow down relative to the outside observers?

This puzzles me to no end

I think Forward's explanation was that the creatures on the neutron star, the Cheela, functioned at a high rate due to their bodies being composed of nuclear, rather than biological, material. That is, their metabolism functioned at the rate of a nuclear, rather than a chemical, reaction.

https://en.wikipedia.org/wiki/Dragon's_Egg

 

[k!rl]

Thanks for the explanations, I can't believe I've read this far while misunderstanding this point.

 

[Czcibor]

The book is about humans coming into contact with intelligent life on a neutron star, who due to their homeworlds massive gravity evolve incredibly fast relative to the human time frame in higher orbit. It's written by Robert L Forward, a physicist, however shouldn't higher gravity cause time to slow down relative to the outside observers?

This puzzles me to no end

I'm not a physicist, but a while after reading your post I had one afterthought. Shouldn't on a neutron star the time actually slow down a bit? So any hypothetical star neutron beings shall actually evolve slower?

The holy book says that:

Gravitational time dilation is a form of time dilation, an actual difference of elapsed time between two events as measured by observers situated at varying distances from a gravitating mass. The stronger the gravitational potential (the closer the clock is to the source of gravitation), the slower time passes.

https://en.wikipedia.org/wiki/Gravitational_time_dilation

 

[SteamKing]

I'm not a physicist, but a while after reading your post I had one afterthought. Shouldn't on a neutron star the time actually slow down a bit? So any hypothetical star neutron beings shall actually evolve slower?

The holy book says that:

https://en.wikipedia.org/wiki/Gravitational_time_dilation

Yes, there is some time dilation on a neutron star, but not to the extent that the imaginary creatures experienced in the story. The lifespan of one of the Cheela was only about 40 minutes in proper time, which is a dilation factor much higher than suggested by the size of the neutron star which these creatures inhabit.

Forward was a physicist and an aerospace engineer before he turned to writing science fiction. The novel Dragon's Egg contained a scientific appendix which explained many of the strange physical effects one would experience on a typical neutron star. However, due to the strange biochemistry of the Cheela, their metabolism supposedly was greatly accelerated over that typically experienced by creatures which exist through conventional chemical reactions.

While seemingly plausible, one must remember that this work is a novel, not a scientific treatise, and that the author has undoubtedly taken certain liberties in order to spin a more compelling tale.

 

[Vanadium 50]

It's written by Robert L Forward, a physicist, however shouldn't higher gravity cause time to slow down relative to the outside observers?

Did you read the book? Forward explains all this in it.

 

[Ryan_m_b]

While seemingly plausible, one must remember that this work is a novel, not a scientific treatise, and that the author has undoubtedly taken certain liberties in order to spin a more compelling tale.

Exactly this point. I found Dragon's Egg a very interesting book on the basis of the time difference between the Cheela and humans. It was fascinating to consider any sort of relationship between species that are separated by their perception of time. I found the Cheela a bit too human-like but beyond that quite interesting.

 

[Loren]

I gave up on the book when he started talking about black holes inside suns.

 

[Ryan_m_b]

I gave up on the book when he started talking about black holes inside suns.

I don't remember that bit, it has been several years since I read it. What was the context and why did it bother you?

 

[Loren]

I don't remember that bit, it has been several years since I read it. What was the context and why did it bother you?

It's been a little while, but I think the Sun had at least one black hole in it, which I could not fathom how a small black hole could exist in a star without soon gabbling up the star.

The mass of a star determines its type, so a significant part of that mass would need to be part of the black hole, which leaves insufficient mass for a G-type main-sequence star. That assumes that a black hole in the center of a star could even exist in equilibrium.

While it's been proposed that a micro black hole might be created under the conditions of the big bang, the observable data for those conditions have pretty much ruled out that possibility. That leaves the minimum mass for an observed black hole as 3.8 solar masses, which is much more massive than our sun.

 

[SteamKing]

I gave up on the book when he started talking about black holes inside suns.

I just scanned a copy of Dragon's Egg, and I find no mention of any black holes inside the sun. I think you have this novel confused with something else ...

 

[Loren]

I just scanned a copy of Dragon's Egg, and I find no mention of any black holes inside the sun. I think you have this novel confused with something else ...

I was just thinking that you might be right when I reviewed the book on my iBook and found the passage where it states in the chapter Pulsar from the book Dragon's Egg, " ...the Sun had a case of indigestion. It had eaten too many black holes." Try searching on that string. The following paragraphs describe four dense masses (probably primordial black holes) circling inside our Sun.

 

[mfb]

While it's been proposed that a micro black hole might be created under the conditions of the big bang, the observable data for those conditions have pretty much ruled out that possibility. That leaves the minimum mass for an observed black hole as 3.8 solar masses, which is much more massive than our sun.

Observations have ruled out small black holes (or any other compact massive objects) as primary source of dark matter for a large range of masses. They could still exist if they contribute only a small fraction to the total matter density.
That doesn't mean I would trust any "black hole in a star" model without proper simulations to estimate their lifetime.

 

[ogg]

We have to be careful when thinking of black holes "sucking", "vacuuming up", "gobbling", etc. Who was the sci-fi author (maybe there were several?) who used black holes as weapons? Some aliens dropped one on Earth (Or did we do it to ourselves? IDK) and it slowly, over decades, swallowed the matter of Earth. Its orbit was BELOW the surface of Earth, but being so massive, that didn't slow it down much. The question there would be: as it orbits the barycenter, how much matter flows into it? The viscosity of the matter (solid or molten) in the near-by (extreme) g field, along with its orbital velocity, would determine the inflow. Similarily, for a black hole inside a star, clearly the matter in its direct path would be (mostly) swallowed (I'd guess, I'd have to do the dynamics calculations to verify this, and I don't have the competence to do them) but near-by plasma would be limited to the regular laws of dynamics (in a general relativistic context). As an estimated lower bound, consider two disks, one the radius of the Sun, R, the other of radius of the BH, r. Then assume that each orbit removes the matter equivalent to πr². So the number of orbits required is R²/r². For a BH with an event horizon of 30 km (10X the Sun's mass, a large stellar sized BH) and given Sun's R=700,000 we have 7E5²/900 ~ 600,000 orbits. Assuming velocity is ~√(11GM/r) we can estimate that one orbit takes about 10 years at an orbital r = ½R, or it would take 6 million years for the Sun to be consumed. This is an extremely crude estimate, but it suggests that there's no reason that a bh couldn't exist temporarily inside a star. (Of course, the nasty (radiation) effects would be observable much sooner.)

 

[SteamKing]

I was just thinking that you might be right when I reviewed the book on my iBook and found the passage where it states in the chapter Pulsar from the book Dragon's Egg, " ...the Sun had a case of indigestion. It had eaten too many black holes." Try searching on that string. The following paragraphs describe four dense masses (probably primordial black holes) circling inside our Sun.

I checked out that chapter. I don't know how I missed it the first time.

Forward used the "black holes" as a plot device which led to the discovery of the Dragon's Egg pulsar. The human scientists were tasked with making observations of the sun, and their data showed a periodic "scruff" which appeared but couldn't be accounted for by normal stellar astrophysics. Eventually, they found that this "scruff" was evidence that a previously unknown pulsar was passing close to the solar system, and an expedition was mounted to observe this object. It was while their observatory was orbiting the Dragon's Egg that the alien Cheela were discovered living on the surface of the neutron star.

 

[Loren]

I checked out that chapter. I don't know how I missed it the first time.

Forward used the "black holes" as a plot device which led to the discovery of the Dragon's Egg pulsar. The human scientists were tasked with making observations of the sun, and their data showed a periodic "scruff" which appeared but couldn't be accounted for by normal stellar astrophysics. Eventually, they found that this "scruff" was evidence that a previously unknown pulsar was passing close to the solar system, and an expedition was mounted to observe this object. It was while their observatory was orbiting the Dragon's Egg that the alien Cheela were discovered living on the surface of the neutron star.

Yes, that rings a bell. I got as far as about 33% through the book when I stopped and started reading a number of novels by Reynolds. Eventually I will get back to it, but I have one novel by Reynolds that I am finishing now, plus one of my own I am still writing, plus my regular day job. :)

 

[mfb]

@ogg: How did you get an orbit of 10 years? The Earth revolves at 1 AU around a 1 solar mass object and needs 1 year. Clearly the sun revolving around a 10 solar mass object at less than .01 AU would need less than a day.

The matter would be heavily influenced by the black hole over the whole volume of the star - the net gravitational force is always towards the black hole. To pass a free 10 solar mass BH with ~30 km/s initial velocity, you need an impact parameter of the same order of magnitude of the solar radius. Intense radiation from the accretion disk is the only thing that can prevent the star from collapsing within very short timescales.

The Eddington limit still applies: the system should reach a (quasi-)equilibrium where radiation pressure cancels gravitational pressure. That gives a timescale of $\frac{\dot{M}}{M} \approx 0.5 Gy$ (from here). If (!) this equilibrium is reached the black hole cannot consume the star (at 1/10 its own mass) faster than ~50 million years.

 

[Hornbein]

I looked into this once. Time dilation on the surface is small. I would guess less than 1%. At the center of the most massive neutron stars it would be between 30 to 90 percent. (The calculation depends sensitively on the radius of the star, and this is not known.)

This interested me because it seems to create a dilemma. The interior of a neutron star is a superfluid. Essentially this means that all of the neutrons share a single wave function. But if some of the neutrons have such a large time dilation, how can they share of the wave function of the non-dilated neutrons?

I imagine the answer is that they share the function locally. What is happening a kilometer away doesn't matter.

By the way, I think Anders Sandberg's idea that the aliens are living in the superfluid interior of the star is much more realistic. The surface of a neutron star seems to me to be to be the most unfavorable possible environment outside of a black hole. Among other things, there tend to be regular starwide thermonuclear cataclysms.

 

[mfb]

But if some of the neutrons have such a large time dilation, how can they share of the wave function of the non-dilated neutrons?

In a superfluid, there are no "outer" and "inner" neutrons. All neutrons are the same.

 

[Hornbein]

no reason that a bh couldn't exist temporarily inside a star.

Back in 2011 there was proposed a survey that searched for stellar oscillations caused by small primordial black holes passing through stars. There has been no news since, so perhaps it never occurred.

I don't know what the effect of a small black hole passing through the Earth would be, but I suspect that the energy released would cause a cataclysm.

 

[Hornbein]

In a superfluid, there are no "outer" and "inner" neutrons. All neutrons are the same.

OK, then what is the effect of time dilation on the wave function? Let's say that there is 50% dilation at the center...

 

[mfb]

It will certainly influence the wave functions, but QFT in curved spacetime is ... challenging, and I don't know how it works.

 

[Dr Wu]

Warning, spoiler ahead: there were actually five black holes inside the Sun at the start of 'Dragon's Egg' - not four. The cheela discovered and neutralised it (if that's the right word) along with the other four. I just thought I should set the record straight, that's all.

 

[livelar]

I just read this book and really enjoyed it, and also found this thread really interesting! - I would love to hear what you think about the science of orbiting that fast around a neutron star, what would be the major obstacles to overcome in actually doing this. I'm drafting on a hard scifi novel with a similar event ((orbiting extremely close to a small black hole) and I would love to get this bit to feel as realistic as possible!

 

[Dr Wu]

Well, it appears that at a mean distance of 100,000 km, the St George Interstellar Ark - the human expeditionary spacecraft in 'Dragon's Egg' - would take just 14 minutes to complete one orbit round the neutron star. Matters would be even more extreme for the so-called 'Dragon Slayer' surveyer probe, which descends to a mere 400km altitude above the star's surface. Here at this geo (astro?) stationary orbit, it and its human crew would be rocketing around Dragon's Egg at a very respectable 45 million km/h, completing one orbit every fifth of a second. That makes it between three and four percent light-speed. One of the many striking features about the novel is the ingenious means Robert Forward deployed to protect the crew of Dragon Slayer from the locally huge gravitational tidal pull they would otherwise experience while orbiting the neutron star at this low altitude. Personally, I believe Dragon's Egg could be a highly original (and entertaining) SF movie, given a great director and a reasonable budget. But please not Hollywood. . . anything but that!

 

[Dr Wu]

Just remembered: two issues to bear in mind concerning potential obstacles, with regards to neutron stars and stellar-mass black holes: one threat would be their magnetic fields -100 million tesla in the case of Dragon's Egg. Also, their extent must be borne in mind, or at least as far as it is possible to determine this, given our present state of unknowingness. Another major life-threatening issue would be micrometeors, or even plain old grit-sized meteors. I don't have the maths to back this up, but in the vincinity of any ultra-compact body - black holes included - I should imagine their orbital velocities to be insanely brisk. A bit like finding yourself at the business-end of the Large Hadron Collider, I wouldn't mind betting - but even worse.

 

[livelar]

Well, it appears that at a mean distance of 100,000 km, the St George Interstellar Ark - the human expeditionary spacecraft in 'Dragon's Egg' - would take just 14 minutes to complete one orbit round the neutron star. Matters would be even more extreme for the so-called 'Dragon Slayer' surveyer probe, which descends to a mere 400km altitude above the star's surface. Here at this geo (astro?) stationary orbit, it and its human crew would be rocketing around Dragon's Egg at a very respectable 45 million km/h, completing one orbit every fifth of a second. That makes it between three and four percent light-speed. One of the many striking features about the novel is the ingenious means Robert Forward deployed to protect the crew of Dragon Slayer from the locally huge gravitational tidal pull they would otherwise experience while orbiting the neutron star at this low altitude. Personally, I believe Dragon's Egg could be a highly original (and entertaining) SF movie, given a great director and a reasonable budget. But please not Hollywood. . . anything but that!

Yes I found the environment of such a place very exotic, orbiting a body 5 times a second. Entering such an orbit must be a nauseating experience if the probe allows you to have a visual of the exterior environment. Looking up away from the star and all you see is a blur. I imagine it would require some kind of optical technology to let the crew have a clear view of the spinning universe above. I agree that the lenghts Forward goes into explaining how they survive the tidal pull is what makes this such a good hard scifi novel. Also agree that it could make a good film. My day job is in visual effects for film, and every book I read I tend to imagine it as a film. The only thing I had a hard time picturing, is how to make life on the star surface look good in film, since everything is so squeezed it's basically flatland!

Btw, wouldn't there be relativistic effects to consider between the people on earth, the arc and the probe?

 

[Dr Wu]

I should think the relativistic effects would still be exceedingly small, even under those conditions. The crew of Dragon Slayer might need their watches resetting afterwards. . . if even that?

I wonder if the surface of Dragon's Egg would be entirely analogous to Flatland? It surely would from a human perspective, of course. The cheela might not agree, however, when confronting their 20mm high mountains - dizzying Alpine peaks in their straitened world. And this 'worms-eye' viewpoint is precisely the strategy a film needs to adopt if it is to make a visually convincing case of life on a neutron star. . . as seen through the eyes of the cheela. Still, there are other issues. How, for instance, would any prospective film-maker deal with the intense magnetosphere and the ways this affects the star's multifarious life forms? Nonetheless, not explaining everything can be its own reward. There are a handful of SF movies that make a virtue out of not explaining everything - or anything much at all in one or two celebrated cases. Again, sometimes it's better to be baffled than bored.