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Black Hole Propulsion as Technosignature (centauri-dreams.org)
235 points by elorant 11 days ago | hide | past | web | favorite | 119 comments





For a captivating exploration of these same ideas in contemporary fiction, try Liu Cixin's The Three Body Problem. As a tip, I enjoyed it much better having not even read the blurb on the back of the book. It spoils events that don't truly unfold until far into the story.

The Three Body Problem is officially the most overrated book on Hacker News. I am trying to understand why. It has no soul, the "science" is garbage, not a single character is memorable, it's way, way too long for what it is.

It's a sci-fi book with an impressive sense of scale. Every hundred pages or so the story zooms out; from one man's life, to a country's struggle, to a global calamity, to an interplanetary and then galactic crisis, and finally to even higher dimensions.

It does this while threading characters from each stage of "zoom" into future stages, creating for a lot of interesting cross-over plots and sub-plots.

As other commentator's have said, it also wove together Chinese culture with science fiction and history with drama very fluidly.

It was, for me personally, a very "fun" read. Kept me excited at each chapter, and I genuinely didn't know what to expect next. I highly recommend to any sci-fi fans out there, even those like me, who initially didn't think they could stomach around 1,000 pages of a translated-from-Chinese sci-fi novel.


> It's a sci-fi book with an impressive sense of scale

Which is not really new. The three books could be condensed a lot. It felt like reading the Dune prequels and sequels: useless details not advancing any plot just to make the book bigger.

It may come from the difference between western and Chinese ways to tell a story. Or the translator job. But they were snooze-fest for 300 pages then 10 pages of something happening then back to boring.


I would mostly agree, but having lived in China and studied Chinese for several years the element of Chinese culture and history intermingled with sci-fi made it a lot more intriguing to me. Also, it was a great way to practice reading Chinese.

Eh? It reads like a book translated into Chinese. It's probably one of the most not-Chinese Chinese books you can read.

Which makes it an easy first book for a beginner.

I mostly agree but I still enjoyed the story, at least until book 2, the ending of book 3 was horrendous. For the "soul" I gave it the benefit of the doubt, maybe Chinese writing different from western writing or something get lost in translation. For the characters I don't know if it's them or me dealing with Chinese names, I get lost playing games like shogun total war because of this.

I'm reading Shuǐ hǔ Zhuàn right now, there are 108 heroes along the ~2000 pages but somehow the most important remain memorable enough: Gao Chai, Song Jiang, Li Kui, Wu Gong...

See, I can even remember some names :)


Extremely pleased to find I'm not the only person to think so.

Ditto. I found the trilogy to be incredibly over-hyped. The "science" was essentially just inventing ridiculous things to make the story work. The plot started off alright, but by the end of the 2nd / beginning of the 3rd book, it really went downhill.

I thought that policeman was memorable

>It has no soul

Non-arguments like this make me actually want to check it out.


Probably because of the dark forest idea which gives a satisfying answer to the Fermi Paradox.

That "idea" has been around since the 1980s.

Seeing as we're discussing sci-fi, the super predator theory (https://en.wikipedia.org/wiki/Fermi_paradox#It_is_the_nature...) is explored in the excellent Revelation Space trilogy. This series takes a lot less liberties with known physics as well.

Edit: I see you linked to the wrong sub-section there. However, overall the contrast between super-predator theory and (self-)annihilation is a bit more complex than one would think. They're not entirely binary.

On one hand, the self-annihilation idea implies that super-predator civilizations, being extremely rate to exist, would not have strong motivations for exterminating competitors.

What would possibly be the need, given the vastness of time and space, and the thermodynamic arguments given? Indeed, following this logic, the extraordinarily rare galactic survivor civilizations are almost guaranteed a long, peaceful existence.

On the other hand, it all develops on the curve of the galactic gaussian justifying possible pre-emptive defense investments. There's a difference of 3-4 or n sigmas statistics likelihood. In the former case, the likelihood of a rare rival emerging is high enough that pre-emptive defense strategies will be considered, and rudimentary systems of deterrence put in place. In the (high) n theory, it probably would not. How a civilization in the early stages of hegemony determined this fact, would be an interesting and possible civilization-saving (or destroying) feat....

Side note, haven't heard of the Revelation Space trilogy. Does it offer a workable theory for the origins of the predator dynamic?


> Side note, haven't heard of the Revelation Space trilogy. Does it offer a workable theory for the origins of the predator dynamic?

Not really, it's doesn't get any philosophical treatment and is more of a plot device. Without giving away spoilers it's more a bit like iRobot (the movie) with the predator having noble but extremely long term goals of what's best for life itself, I don't think why it does this really holds up either but it works as a plot device. It's happy with intelligent life as long as it doesn't expand into the galaxy, it doesn't act preemptively.

> On the other hand, it all develops on the curve of the galactic gaussian justifying possible pre-emptive defense investments. There's a difference of 3-4 or n sigmas statistics likelihood.

The other factor is the cost to put down potential rivals, in the dark forest universe this cost is incredibly cheap. If the cost were a lot higher then the outcome would be very different.


One thing that especially irked me about Liu was how contingent so many plot points were on specifics, and how little attention was paid to that.

E.g. If there were no FTL travel, deterrence would look very different. Especially if the average time for a civilization to become multi-planetary < the average distance to the nearest annihilation capable civilization


When I saw that URL I thought you were linking to this:

https://en.wikipedia.org/wiki/Fermi_paradox#It_is_the_nature...

which I think is the most memorable and haunting section title of an encyclopedia article ever written.


> Liu Cixin's The Three Body Problem

His vision of the future is bleak in the extreme. His characters are naively constructed and occasionally make incomprehensible choices.

The worst sin is he's unsure whether he wants to write hard sci-fi or just mostly "fi" with a thin veneer of "sci". Occasionally he seems like he wants to stick to solid science with a few extra assumptions (in the best tradition of hard sci-fi); but then he veers off into some cartoonish "science" that's quite jarring. In the end, you get the impression he just tries to brag about how much science he "understands".

In the end I was reading it the way some folks watch a bad movie - to make fun of it.

If you want cosmic scale hard sci-fi, try Stephen Baxter. He can be a bit dry at times, but that's his only fault.


Personally, I actually liked the ideas, but everything else was a complete disaster.

The book is written in a very simple, juvenile style, at the linguistic level of a YA novel. There's zero depth to any characters. By modern standards, it's all "tell", no "show". I've read several reviews that suggest this is actually how most modern Chinese novels are written. That doesn't make it more readable, unfortunately.

The style seemed rather unintentionally surreal. My sense after reading was that I had just been reading Vonnegut's "The Sirens of Titan" but without any of the humour. Many of Vonnegut's books are breezy, somewhat outrageous fables that don't go very deep into their characters, of course, but they work because they're funny, colourful, inventive, well written, and are full of great ideas. I think Lui Cixin's book would have worked much better as a Vonnegut-style satire.

If you want a similarly dark look at the future of humanity's role in the universe, take a look at Peter Watts' Blindsight [1]. While it certainly won't win any prizes for literature, and it has some odd quirks, it's considerably better written, and might especially appeal to fans of Neal Stephenson's loose, ragged style of writing. Moreover, it's probably the creepiest sci-fi novel I've read. On the surface it's a somewhat straightforward story about a team of transhuman specialists who are dispatched to investigate an apparently alien artifact. But it goes deep into some fairly cynical ideaas about human consciousness that are somewhat existentially unnerving.

[1] https://www.amazon.com/Blindsight-Firefall-Book-Peter-Watts-...


> The book is written in a very simple, juvenile style, at the linguistic level of a YA novel.

> The style seemed rather unintentionally surreal.

This might be an artifact of translation, combined with whatever cultural differences there are between Chinese literature and the "Western" literature (scare quotes because the category is loosely defined). The style actually seemed somewhat poetic to me. I'm just not knowledgeable enough in this neighborhood to pass judgment.

But I agree that the style doesn't connect with the substance of the book, mostly because the substance is disjointed and so uneven.

Blindsight is fairly bleak as well. At least Stephenson is unabashedly optimistic. Peter Watts is echoing there some very important debates in philosophy and neuroscience these days - Thomas Nagel's "what is it like to be something", David Chalmers' hard problem of consciousness, p-zombies, the skepticism of Dan Dennett, etc. The problem of consciousness in general. Blindsight is great as a sneaky introduction to these concepts, even though it's quite cynical, as you've said.

I've never connected well with Vonnegut, actually. But then I didn't really connect either with hugely popular authors such as Douglas Adams or Terry Pratchett. Maybe there's a pattern there.


Vonnegut is definitely in the same camp as Adams and Pratchett. I like to think that Adams was influenced by Vonnegut in his own writing, as Sirens could easily have passed for a Douglas Adams novel if it'd had more jokes and a bit more Britishness.

One thing Adams and Vonnegut have in common is their almost nihilistic sense of cosmic absurdity and the futility of purpose. Without spoiling anything, the whole story in The Sirens of Titan turns out to be meaningless. A bit like how, in Hitchhiker's Guide to the Galaxy, we eventually realize that Earth was originally created by little white, superintelligent mice to compute, over billions of years, the meaning of life, the universe and everything. (The answer isn't... terribly useful.) It's absurd, and funny, and sad at the same time.

Vonnegut got more eccentric in his writing over time, but Sirens is actually very straightforward, and it's a masterpiece. Give it a shot. Cat's Cradle is also excellent. I never liked Slaughterhouse Five, which everyone recommends, as much.


> almost nihilistic sense of cosmic absurdity and the futility of purpose

You nailed it. That's the part I could not swallow.

But I'll give Sirens another try. Thank you.


Second the recommendation for Blindsight. It's competently written (I don't mean that as faint praise) and has a darkness to it that's substantial enough to stick with you for a good long while.

You're right that the prose is a little Stephenson-ish, but to avoid giving anyone the wrong impression I'd add that the pace, story, structure, and—how to put this?—evenness of quality aren't.


Agreed, the pacing was great, and he really landed the ending, which is something of a rare feat in sci-fi. The only thing I didn't particularly enjoy was the flashbacks to the protagonist's past on Earth, which seemed a bit trite and repetitive.

> he really landed the ending, which is something of a rare feat in sci-fi

The only persistent offender here I can think of is Stephenson. Who else are you thinking of?

BTW, many years ago I used to hate Stephenson's dry style. But for some reason I kept reading his books because I like his ideas. Eventually I figured what he's really like - just a different attitude - and at that point I started to enjoy his writing quite a lot. Anathem is one of my favorite books in all literature, ever.

> The only thing I didn't particularly enjoy was the flashbacks to the protagonist's past on Earth, which seemed a bit trite and repetitive.

Yeah, that was boring.


Hm, Philip K. Dick, Stephen King, and Terry Pratchett come to mind.

William Gibson, although I liked Neuromancer's ending, and every novel since that has followed the same template (mystery bigwig hires someone to find McGuffin, but is really manipulating/being manipulated about something else, the end).

Maybe I was being harsh. I'll see if I can think of more examples.

At the other end of the scale, Iain M. Banks never wrote a badly paced book that didn't have a perfect ending, as far as I'm concerned. The guy was a master.


Gibson's books clicked for me where I read an interview in which he claimed Idoru was his favorite. (Though this was before his newer stuff)

I finally realized his enjoyment is in Type B writing. It's the detail next to the main plot, rather than the motion itself.

Which parallels what I like about film noir -- it's not about finding the murderer: it's about watching the characters find the murderer.

Which is why I maintain the Lost ending was hilarious. JJ Abrams essentially said "Look, you didn't really care about the answers: you cared about watching these characters look for answers. And in the end, it's the characters who mattered." Which is probably not something you should ever point out to a TV geek if you want to keep your head intact. ;)


Gibson is incredible at conjuring up his environments. On the other hand, his prose is also second to none, usually.

All that said, just imagine how fantastic his novels could be with better plotting! He's not terrible at characters, it's the plot that ultimately disappoint. I think his imagination peaked with Neuromancer, unfortunately; his later novels didn't bring much new to the table beyond what he's laid out in that book, and I find his later obsession with art (like the locational VR art in the Blue Ant trilogy) and video games tiresome. His newest one didn't click for me at all.

Lost, ugh. Such a promising start completely wasted by lack of planning.


> The only persistent offender here I can think of is Stephenson. Who else are you thinking of?

Peter F. Hamilton.

He has some awesome universes but his endings are usually a lot fo Deus Ex Machina. But the thoughts of MorningLightMountain analyzing a human is a good example of how he can get some alien concepts done.


+1 for Anathem. I re-read it about every 18 months. The dry humor is fantastic. Also, monks in space!

I guess I'm the only one who thought Blindsight's prose was delicious from front to back, then? I can see why people would think it's a little overdone, but I like that about it as well.

Oh, no, the prose was fine. Did my post come off like I don't like the prose in Blindsight? I just didn't want anyone to think anything else about the book was similar to Stephenson's work at all—for one thing, Blindsight has an entire story that's fully plotted beginning to end, not just a bunch of amazing ideas and great first third that proceeds to go nowhere.

[EDIT] the action writing in Blindsight is particularly good, in fact. Some of the best I've read.


wow, love that author biography on amazon! I like his chutzpah.

> This is awkward and a little creepy. They tell me I have to do it for promotional purposes, but I've already got a blog. I've already got a website. Being told that setting up an author page on fcuking Amazon is essential to success? A company that treats us all like such goddamn children it doesn't even allow us to correctly spell an epithet with a venerable history going back 900 years or more? That just sucks the one-eyed purple trouser eel.

> Still, here I am. But if you're really all that interested, go check out my actual blog/website. Google is not your friend (any more than Amazon is), but at least it'll point you in the right direction.


Peter Watts is great. I've very much enjoyed all of his books.

I _love_ Blindsight.


You actually hit on the point when you say it was 'unintentionally surreal' - you can probably lose the 'unintentionally'. It has a report-like 'stranger than fiction' quality that works very well for the novel I think, as if it is a declassified communist era file. It never lets up, never once winks at you. Contrast say "Use of Weapons" by Ian Banks - I just read this before "Death's End". This is a good book, lauded as one of the best Culture novels. Reading it you can sit back and think oh that's a nice metaphor, and I like how allegory is used here, and the pastiche of advanced technology and primitive human war is very interesting, and this unreliable double narrative structure running backwards and forwards in time is extremely clever. But when you read it you can sense it is being written for you, it lacks sufficient strangeness to induce a sense of wonder. I was no where near as riveted or interested in what was going on as I was with the 3 body books, I genuinely wanted to know what was going to happen next.

I certainly enjoy literary books, but I think describing the style and tone of Liu's books as simple and juvenile is missing the point a bit.


No, I do think it's accidental. Or at least not intended the way I read it.

It's surreal in the sense that the writer asks you accept certain developments at face value, just as it asks its characters to. If it had been a satire, then it would have perhaps been able to reach the heights of Vonnegut, Lem or Adams. But everything seems like it is meant sincerely, without irony. The wooden main characters run around like chess pieces with little or no inner life, just to fulfill the necessities of the next scene. The book lacks realism -- not in the sense of being realistic against our world, but in the sense that it's a fable, told like a children's story, yet seemingly marketed to adults. You kind of have to brace yourself a bit, as a reader, because it's told in the way an adult might speak to a slow child, which feels condescending. I don't want to be told how a character thinks or feels.

There's nothing wrong with the story, per se (although I have issues with some things, like the nanotech at the end of the book, which comes as a huge deus ex machina -- it asks the reader to accept too much in terms of technological plausibility). The story is interesting. It's just told in a completely ridiculous way.

Regarding Use of Weapons, I don't completely disagree. Unlike a lot of people, I don't think it's Banks' best work. I do think it's the superior book by far. I can forgive Banks for being playful about narrative forms even though it's a gimmick that doesn't elevate the story in any way (at least for me).


> it's told in the way an adult might speak to a slow child, which feels condescending. I don't want to be told how a character thinks or feels.

OMG, this. So much this.

> If it had been a satire, then it would have perhaps been able to reach the heights of Vonnegut, Lem or Adams.

Unsurprisingly given my stated preferences, the only book by Lem I truly enjoyed was Solaris. And when I say enjoyed I mean it was a huge, near-mystical experience (well, I was a teenager).

The faux-encyclopedic style, I'm pretty sure he doesn't mean it in an ironic sense, does he? Seems more similar to Borges in a way. I re-read it recently and again it seemed a completely sincere book. He just opened up the floodgates for everything that mattered to him, affairs of the heart and of the mind also.


Solaris is somewhat satiric, but it's subtle. After that long, encyclopedic summary of solarisist research that spans decades (or is it a whole century? I forget), the protagonist tells us everyone has developed their own theory about Solaris, and every single theory is wrong, because it ultimately reflects the mind of the theorizer. Then, ironically, the protagonist tells us his theory. Lem is deeply cynical here about humankind's ability to understand anything outside themselves. It's a pretty overt critique of modern academia, perhaps especially the soft sciences like psychology and psychiatry. It's not meant to be a very emotional novel, I think. Lem hated the movies, which misinterpreted his intentions and honed in on the love story aspect.

Did you read Lem's His Master's Voice? It goes even beyond Solaris in that way. It's about the infighting, political maneuvering etc. that surround an attempt to decipher what may or may not be an extraterrestrial signal. It's perhaps a bit drier than Solaris, being written as a memoir by one of the scientists involved.

I'd say Lem had two modes of writing. He wrote comic adventure stories like the Pirx stories and The Cyberiad. And he wrote ironic, somewhat cynical or pessimistic criticisms of science, such as Solaris, HMV and The Futurological Congress. The latter is great -- during a scientific conference on the future of human civilization, the drinking water is poisoned with psychedelics drugs, and the main character a future dystopia. It's a short, fun book. Cynical, but fun.

His other novels, such as Eden and Fiasco, reflect this view. He's never struck me as a fan of humankind.

I'm not sure I see much of a Borges connection, other than the fictitious history of Solaris.

Have you read any of the Strugatskys' books? They're like anti-Lem's -- lot more optimistic and unabashedly humanist, I think. Roadside Picnic is a great start, but they wrote a bunch of great novels. Beetle in the Anthill is one of my favorites.


Hm, I still don't see Solaris that way - satiric, I mean. Quite aware of the limits of our knowledge, definitely. A dash of transcendental, a la The Kid by the Strugatsky's (the English version of the title, 'Space Mowgli', is an abomination), or Childhood's End by A.C. Clarke. I feel he says, look at all this flood of mind stuff, and what do we get for it? Finally he's standing on the shore, at the end of knowledge, and he's touching the vast unknown ocean in front of him - very symbolic.

The love story is definitely not the main thread, but I felt it was very personal. Not sure why. Kelvin is Lem, no doubt about it, and he poured into the book some measure of his soul. I didn't catch that when I was 16, but I felt it quite powerfully a few months ago when I re-read it.

The contemplative atmosphere of the movie by Soderbergh seemed to capture some of the spirit of the book, but it would take a heck of a lot of text to explain why. But that was it, it didn't capture much else.

I didn't read much else by Lem - the Cyberiad and some Pirx stuff, but that's about it and it was long ago and I don't remember much. Nothing felt as meaningful as Solaris to me.

I didn't read many books by the Strugatskys, but those I've read are awesome.


I'm completely with you. It baffles me how many people loved Liu Cixin's book. It's written like a 70's era 'ideas, no execution' sci-fi novel, but the ideas are terrible. I mean, the central 'xeno-anthropology' thing, that's treated as like a deeply profound maths proof, is kinda a brain fart.

If you read a writer like Peter Tchaikovsky, you get actually interesting ideas, and really good execution. And I'd say PT is pretty mid-range for contemporary sci-fi authors. People like Yoon Ha Lee or China Mieville are really pushing the genre forward. Lui Cixin's work on the other hand reads like one of those 70's books about uplifted dolphins and sexy spacewomen, all robotic characterization and uninspired writing.


I think you mean Adrian Tchaikovsky, right? Peter was a great composer of music, but not SF.

Where would you recommend someone start with Adrian Tchaikovsky?


Yeah - oh man, I'm terrible with names. Adrian Tchaikovsky's a really good execution guy - fantastic standalone novels, not with anything especially out there, but really well done. I was really impressed by The Freeze Frame Revolution, and Children of Time was also good. His fantasy stuff is pretty uninspired, though.

> It baffles me how many people loved Liu Cixin's book.

If this is the first sci-fi book you read, and especially if this happens at an early-ish age, I imagine it may seem impressive.


How funny, a friend recommended it to me last week and I just started reading it yesterday. I'm only in the beginning, but I must say I really like it so far.

It starts with a horrifying dive into '60s Revolutionary China, then veers off into an extremely surreal and nerdy anime plot, and proceeds on a very cool and foreboding sci-fi arc (I think; I don't remember how much of the sci-fi it has vs. how much of it is in the two sequels).

That's a good way to put it. The novel series manages to have 1) brilliant and highly imaginative set-piece sci-fi events 2) ruminations on the possibly dark nature of humanity in space 3) amateurish characters - save two or three 4) universal-scale scope 5) awareness of just 6 countries in the world : all at the same time.

And a lame ending compared to the rest of the trilogy. It's like in the last 50 pages he run out of ideas.

In his defense, he explored quite a bit.

I really had kind of hoped to get a glimpse into what he'd have come up with for if the universe restarted.


Ending is hard, and when it comes to sci-fi series it empirically looks impossible: I can't remember a single one whose end wasn't disappointing.

I was going to say something similar about the Xeelee stories (I also love the The Three Body Problem) but remembered that humanity in those stories only manages to have weaponized neutron stars in a feeble attempt to fight the Xeelee.

> events that don't truly unfold until far into the story

Aka "the last 100 pages, where something actually happens."


I really enjoyed the trilogy, some clever and original stuff there. I didn't enjoy the third book so much, but don't want to spoil anything.

I love Centauri Dreams. It's hopeful, makes me feel perspective, and realize how small and primitive we are.

We fret every day about consumption, waste, and energy, and here are some people discussing methods of moving planet-sized objects. It's a kind of optimism that's so rare it almost feels outdated.


If you do fret about those, the article was inspiring, free, and creates little waste.

Unlike most of what is for sale in this world, which happens to be uninspiring garbage.


> We fret every day about consumption, waste, and energy,...

I'm afraid we have to keep fretting. In the Numberphile video about the number of particles in the Universe (https://youtu.be/lpj0E0a0mlU?t=378), they take an interesting diversion to ask "How long until every particle in the Universe is inside a human being?" In other words, when will we reach the ultimate limit to growth? Well, at the current annualized population growth, it's less than 9000 years! And that scenario involves every human being at that future time floating around naked in the vacuum of space. There is no food to eat, water to drink, nor air to breathe left over.

Practically, this means a couple of things that are less than obvious. First, the best case is that we will reach a point where the growth in volume of the space we occupy will exceed the speed of light. In other words, "long" before 9000 years from now, we will either establish some sort of equilibrium, or we will run up against a hard Malthusian limit. We will absolutely be concerned about "consumption, waste, and energy" in a way that is far more focused than it is now. (The only economic models that are permitted to work are zero-growth models, practically speaking.) Second, to give you an idea of how soon this is coming, the Universe is much larger than 9000 light-years across. That is to say, even if we could manage the resource usage of our 1.011% growth rate optimally, we could never actually fill the Universe in that time frame, as it would take about 15 billion years or so if we were all flinging ourselves outward at very near the speed of light. (n retrospect, these two are restatements of the same idea.)

Finally, and most dismally, the analogous calculation for the Solar System (an infinitely more reasonable calculation to perform) must be abysmally worse and monumentally more urgent. To give you an idea, the current doubling time of human population is something like 60 years. That means, for example, that if we were to anticipate that we'd run out of meaningful room on Earth in 60 years' time, we'd have to start working on our "second home" now, to have it ready. Once it was ready, we'd have to be working on two more. Then four, and so on. Good luck with that!

(Note: After we have consumed the Earth and converted it to habitats, we will have to consume Jupiter in a similar way in about 500 years. This gets out of hand very quickly.)

EDIT: El-oh-el at the few people who's fee-fees were hurt enough by this back-of-the-envelope analysis of growth rates and resource use to downvote it. Keep fighting the good fight, no matter how lonely!


Of course, "current annualized population growth" is a hilariously bad assumption to make for the next 9,000 years. I would expect population controls long before having our "second home" prepared that would limit the rate of our population growth to the speed we can realistically expand.

I also don't think humanity will last this long, but as an academic problem it's fun to think about.


It's not like humans are the only thing that grows exponentially for a limited period of time. If the yeast in a loaf of bread continued growing, it too would fill the universe very quickly. The saying is "trends that can't continue, won't".

>Of course, "current annualized population growth" is a hilariously bad assumption to make for the next 9,000 years. I would expect population controls long before having our "second home" prepared that would limit the rate of our population growth to the speed we can realistically expand.

Funny I would have expected that to happen when we started consuming more than the primary production of the planet: https://www.nationalobserver.com/2017/08/09/opinion/heres-ho...

We didn't.


This is not a planet-scale phenomenon, though. Any student of geography would know that as nations become more mature their birth rates stabilize to more or less match the death rate. It is the less-developed countries that would suffer first in a world where our planet can no longer support our population, in which case I would expect that either they would go to war with each other over whatever resources can be taken from each other (and also reducing their populations in the process) or else face starvation (again, reducing population). Either way there's a large difference between starvation and saturation. The human race is not endangered by the fact that we'll soon saturate our planet, and those that do starve as a result would simply join a long line of people who have already met the same fate.

Yesterday I ate a bagel. Today I ate two bagels. Based on the current growth rate, in 1 year my daily bagel-intake will surpass the mass of the planet.

The pattern I infer is that you will be eating 365 bagels--without invalidating your argument.

Although this article doesn't make it clear, the underlying concepts (like the light slingshot / "halo drive") are explored in more detail in a previous article:

https://www.centauri-dreams.org/2019/03/05/investigating-the...


The problem with firing lasers at a black hole light years away is beam spread. Using the biggest best telescopes we currently have, at our admittedly miserable technology level, the tightest beams we can project on as close a target as the moon are kilometres across. Even if we were to create a mirror as wide as a planet to create a beam, it would be many, many times that wide by the time it reached the binary system. It would be further diverged by the slingshot round the black hole (or neutron star), and diverge further again on it’s way back to the spacecraft. I don't see any way such a system could be made practical.

This is part of a short series on Centauri Dreams, starting with this article: https://www.centauri-dreams.org/2019/03/04/pondering-the-dys...

As it says in that article,

> Such concepts obviously imply an interstellar civilization capable of reaching the objects in the first place. But once there, the energies to be exploited would be spectacular.

The paper discussed in these articles is not proposing that we puny current Earthlings fire lasers at the closest binary black hole systems. It's exploring the possibility of getting close enough to such a system to make this technique useful.


If you have the technology to reach the nearest binary blackhole ("you just have to pay the one-time fee of the cost to reach the nearest binary black hole system", at 16:12 min in this video: https://youtu.be/rFqL9CkNxXw?t=972 ), you can most likely deal with the mirror/energy harvesting problem as well.

You would have to be fairly close to the black hole in question to do this. A distance of r=3 MG/c² according to Stuckey '93 [1], which is referred to in the paper referenced [2] in the article. Skimming the articles, I can't see any reference to how sensitive to error in aiming this would be.

[1] The Schwarzschild black hole as a gravitational mirror https://sci-hub.se/https://doi.org/10.1119/1.17434 [2] http://coolworlds.astro.columbia.edu/halodrive_preprint.pdf


A civilization using this method might not use optical systems to focus the beam like we do, but might individually fire precisely aimed photons.

You'll have a similar problem due to the Uncertainty Principle. Even a single photons will experience the same kind of diffraction as a beam of photons, limiting the degree to which you can aim it.

assuming that we got that right and didn’t just misunderstand what we are seeing (or assuming that there’s no “side channel” to know how to correct for it)

we also can’t get to a black hole, so for this civilisation it’s safe to assume they have a significantly better understanding of physics than us. we don’t know what we don’t know


The reason that makes it imposible to avoid the dispersion even of a single photon is not technological, it's a theoretical reason. You can't solve it with more advanced technology, you must change the laws of Physics.

It is sure that the current laws of Physics are incomplete, and some may even be wrong, but this effect is so fundamental and the proof is so simple that I doubt it will change.


why is our technology miserable? do you know of a better benchmark out there? if not I would assume this is the most advanced technology <ever>

I work with physicists and engineers. The physicists think the equipment is miserable because it doesn't fit their idealized models. The engineers think that their tech is the best because nobody is doing it better. As a mathematician, I can see that both groups are consistent in the application of their assumptions, and both sets of assumptions have utilitarian foundations.

This is literally a discussion about detecting interstellar civilisations using black holes for near light speed travel, and similar technologies we might develop in the future. I am posting in that context.

me too since we developed a framework to think about ways to detect more advanced civilizations than us that may not exist. My context is that we are very advanced. But you seem convinced that we are miserable and I guess our difference is pessimistic vs optimistic rather than objective.

At every point in human history, current Human technology has been the best technology ever developed. It's a completely useless perspective because it always gives you the same answer under every circumstance that has ever existed.

The discussion we are having now gives an eternal objective reference point - the technology required to exploit the gravitational energy of binary black holes. How advanced would you say our current technology is, compared to that reference point?


I don't find this fruitful. Compared to infinite possible intelligence that creatures could conceivably reach either here or other worlds or machines, Einstein was extremely stupid. What good of a convo does it make?

Interesting concept. Why couldn't we do this with matter?

Instead of firing light off at black holes, why not fire some material around a gravity well and allow it to strike the spacecraft in a way that it A) imparts the inertia it has gathered from whatever body you shot it around and B) is captured to be fired again at a new angle according to your new position?

I know there are a ton of logistics to wade through with this, but it should be at least hypothetically possible. It's the same concept as in the article, save for the fact that we are now firing matter around objects we actually can reach in our lifetimes rather than light around black holes that would take an exceedingly long time to propagate.


You're proposing to fire some material around a planet in such a way that it comes back to a mothership with more momentum than it started with. Now imagine that instead of going "there and back", you find a trajectory that sends it from planet A, to planet B, then back to the mothership. You're coming back even faster now! Why stop at two planets? Find a trajectory so you can do planet A, planet B, planet C, perhaps planet A again... and so on.

Now just delete the mothership from this picture; the 'material' is the spacecraft itself.

This is a gravity assist, and by stacking enough of them you can basically pick up as much extra delta-V as you want (at least until you reach Solar escape velocity).

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

The main drawback is that it can take decades of flying a convoluted series of gravity assists before you get to where you need to go, and that's all within one system.

(The two-body idea you proposed has the same problem. The mothership has to sit there and wait while the matter flies back and forth on multi-year long orbits. Also now you have to figure out how to 'catch' a hypervelocity projectile...)


This idea was to try to get AROUND some of the limitations of a gravity assist. For instance, a gravity assist around a large body might require a solar powered spacecraft to be in the shadow of the object for quite some time, rendering the solar panels useless and requiring a radio blackout. Assuming that the method of firing off the projectile and catching it can move gimballed from the rest of the craft, you could allow the ship as a whole to resume the same attitude/trajectory, save for the acceleration of the returning projectiles.

Another thought is that if the projectile were small and aerodynamic enough, you could dip closer into a body's gravity well than the spacecraft could due to atmospheric drag, grabbing more momentum than the spacecraft itself could. The lower periapsis might yeild more delta-v due to the more pronounced Oberth effect than the spacecraft could provide. That's all assuming you don't go TOO far into the atmosphere with the projectile.


Or in the case of slingshot around a star, your projectile might be able to take more of a beating than the ship. Something like tungsten balls or something.

So: shoot ourselves, catch the bullet, shoot again?

I think the logistic issues of catching a bullet are probably the limiting factor here. Light at least _wants_ to be absorbed. Matter just wants to go through you.


>Remember the methodology: A spacecraft emits a beam of energy at a black hole that is moving towards it, choosing the angles so that the beam returns to the spacecraft (along the so-called ‘boomerang geodesic’). With the beam making the gravitational flyby rather than the spacecraft, the vehicle can nonetheless exploit the kinetic energy of the black hole for acceleration.

> Light acts the same way, but light cannot return faster than the speed of light. Instead, in gaining momentum from the black hole, the light blueshifts.

How do you get propulsion from blueshifted light?


Blueshifted light has more energy (and momentum) than it did before the blueshift. If it returns to your spacecraft blueshifted, you can get more energy and momentum out of it than you used in sending it. The change in momentum can be used for propulsion.

But what is the nuts and bolts of how it is done? What do you capture the light with? How are you propelled? Are we talking solar panels or sails or some other technology?

Maybe depends on what you want. Do you want momentum transfer (thrust)? A sail, because you get more by reflecting than by absorbing. Or do you want energy (more than just kinetic)? A solar panel, because you only gain kinetic energy from a sail. If you want both energy and momentum, tune the solar panel/sail ratio to get what you want.

Lightsail. You carry a laser and a lightsail. You shoot the laser at the black hole and the laser light gravitationally slingshots around the black hole, returning to you with more energy than you gave the laser. The returning laser light pushes your lightsail. The black hole needs to be moving for this to work, although it doesn't really matter what direction the black hole is moving.

It does very much matter which direction the black hole is moving; it needs to be moving toward you or you won't get any blueshifting effect, which is the entire reason the energy budget works out.

The idea in the paper is to put up a big solar panel. Simply by absorbing the light, you are propelled. And you use the energy in the light, which is more than the energy you used to send it, to send more light.

I think you might gain momentum simply from absorbing the light.

You gain even more momentum from reflecting the light back into the black hole! When you absorb a photon, you gain its momentum. However, when you reflect a photon, you gain twice its momentum to account for the momentum possessed by the photon now flying away from you.

Isn't that a three-fer?

You first emit the photon towards the black hole, momentum gain. Absorbing the photon on the return would be a two-fer, and reflecting a three-fer.


Absorbing the photon would be a two-fer. Reflecting it is just starting the next two-fer. It's two-fers all the way down.

Ooh. Even better, it's like compound interest.

You emit photon 1. It has momentum m. It comes back blueshifted, so you get momentum km, with k > 1. You reflect that back, and you get momentum k^2m...

It's not that simple, because as you start to move away, you redshift the photons you reflect. And the alignment issues are going to be pretty severe. And if you get too much compound interest, the photons may be blue-shifted enough to kill you or damage your ship. Still, the idea is pretty slick.


Also in all slingshot maneuvers you get more the closer you are, so you wanna pour in the most energy the closest you can get.

I can see how you gain energy but you’re also putting yourself deeper into a gravitational well that you’d have to expend energy to get out of.

Wouldn’t it be a wash?


We do this fairly regularly with space probes already.

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

You steal energy from the object you're slingshotting off. The black holes are slowing (infinitesimally) down every time you do it, gifting their energy to the accelerated object.

Hence the article talking about potentially detecting this sort of behavior by monitoring for prematurely merging and overly eccentric binary black holes.


The paper says that the maximum velocity obtainable would be 2x the rotational speed of the body used. Some such bodies rotate at appreciable portions of the speed of light.

Obviously you cannot exceed the speed of light but it doesn't seem like it would be a wash.


At some level, that's also what glider pilots do to take off from mountains.

Emit low energy photons, expend energy X, receive higher energy photos with energy Y (Y> X)

Maybe this way?


Yes I want my free energy device too.

This isn't a free energy device because you're extracting energy from the rotation of a black hole (this process only works on rotating black holes). After a long enough time, you could extract all of the angular momentum from the black hole using this method.

So if I shoot a beam of, let's say, a movie or live feed of myself typing this comment around a black hole some 50 light years away precisely enough to have it land back here on earth 100 years in the future, would an observer see back into the past 100 years? ...in the same thread if I observe precisely enough a position near a black hole some N light years away could I see what was happening on the earth N*2 years in the past?

Yes, but there's an easier way: use a mirror. Depending on how far you place it, you're looking at yourself in the past.

Based on other comments, and my own very limited knowledge of the relevant physics, there's no way to do this from Earth; all of the black holes (and binary block hole systems) are too far away for us to reliably aim a laser beam to slingshot around the black hole as is being described.

But the general form of what you're describing isn't any more outlandish than a time capsule, i.e. 'transmitting information into the future'.

But no, sorry, you can't see the past with the paucity of black holes near the Earth (and ignoring all of the other impediments to such a thing happening).


Yes, this is just a special case of the fact that when you look at something N light years away, you're seeing the object as it was N years ago. You're always seeing the past, never the present! Most of the time in everyday life it's infinitesimally close to the present, though.

A question for a physicist from a layman. All objects in the universe are connected to each other through gravity, no matter how distant. So what happens to the energy or mass of an object (or total energy/mass) in the expanding universe? As space inflates, the distances are greater between objects, so where the excess of gravitational radiation went to? Can that account for the dark energy?

Gravity is not radiation. The expansion of the universe does work against gravity but there are limits. Gravity decreases with distance. So if the universe is expanding fast enough (it is) it will overcome gravity. Imagine setting a bomb off between two asteroids. Use a small bomb and they move apart but eventually drift back. Gravity wins. But use a big enough bomb and they both hit escape velocity. Gravity looses. The objects are moving away from each other fast enough that the force of gravity falls away so fast that it will never turn them around let alone bring them back together.

Dark energy is the concept that while the universe is expanding, gravity wins / everything is at escape velocity, it shouldn't be expanding faster. Dark energy is the fact that the two asteroids separated by the bomb (the big bang) aren't just at escape velocity, something is causing them to accelerate.


> Gravity is not radiation.

'Gravitational radiation' is, apparently, a thing; gravity waves basically. It's not _electromagnetic_ radiation tho, which is what I'm guessing what you meant.

I don't think it's crazy to think that gravity is propagated at the speed of light but that we can't, and likely won't ever be able to, measure any gravity waves smaller than those generated by extremely large-gravity systems.


Non-flat spacetime doesn't necessarily conserve energy (Noether's theorem).

> All objects in the universe are connected to each other through gravity, no matter how distant.

I don't believe this is correct, as gravitational force/bending of spacetime moves at the speed of light.


Why not have an actual black hole on the ship and use the Hawking radiation to gain energy? More [here](https://www.youtube.com/watch?v=EzZGPCyrpSU&feature=youtu.be...).

> No fuel is spent aboard the spacecraft which, using stored energy from the beam, continues to accelerate up to terminal velocity

Wouldn't fuel need to be expended on the ship to generate the beam? Or is the beam reflected back to the black hole for re-energisation?


You reabsorb the beam which now has even more energy because it's been blue-shifted, which means you can actually charge your batteries as well as accelerate.

A mirror should do it.

If you think the title of this is pure porn, check out those images!

Related: [Black Hole Bomb](https://www.youtube.com/watch?v=ulCdoCfw-bY).



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