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Three Worlds Collide

4.20  ·  Rating details ·  1,228 ratings  ·  84 reviews
Three Worlds Collide is a story I wrote to illustrate some points on naturalistic metaethics and diverse other issues of rational conduct. It grew, as such things do, into a small novella. On publication, it proved widely popular and widely criticized. Be warned that the story, as it wrote itself, ended up containing some profanity and PG-13 content.
ebook, 56 pages
Published January 30th 2009
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Saulius Šimčikas yeah, it's all freely available so there's no need to buy it. There is also an audiobook version which can be found here…moreyeah, it's all freely available so there's no need to buy it. There is also an audiobook version which can be found here
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 ·  1,228 ratings  ·  84 reviews

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Hussain Elius
Jul 10, 2012 rated it it was amazing  ·  review of another edition
Shelves: sci-fi, short-story
I wish Eliezer Yudkowsky was a full time writer. So many good ideas!

This novella was about humans encountering the alien race of Babyeaters - who are civilized but eat their babies, and the humans, as soon as they learn of this, are morally shocked about the custom that is taken to be natural. Most of them want to wage war to save the babies (so cliche) and relieve them of their pain! But then, another alien race arrives, the Maximum Fun-Fun Ultra Super Happy People. One that thinks even having
Nov 02, 2019 rated it did not like it  ·  review of another edition
This story takes place in a utopian future, where “humanity got its shit together”.
Let’s address the elephant in the room, the author wants us to know that rape has been legalized in this bright future.

Is it important for the story, does it advance the plot? No. Why is it added in the first place? Apparently it’s to illustrate how quirky and weird the future is, and that an old character that remembers present time (where rape is bad) can’t make decisions in that bright future, because well he
Sep 02, 2013 rated it really liked it  ·  review of another edition
An interesting science fiction long short story (100 pages). It reminded me of I, Robot, except instead of quaint 1950s charm ("positronic brains") it has awkward references to internet culture ("Any suggestions get reddited up from the rest of the crew?"). I made an EPUB version for ereader users. ...more
Shayan Kh
Sep 26, 2015 rated it really liked it  ·  review of another edition
Yudkowsky is officially one of my favorite thinkers. Too bad he doesn't write more often. I liked the way he shaped hpmor, although I didn't like the whole story. But this time around, he presented a very disturbing version of prisoner's dilemma, in a sci-fi about different kinds of sentient beings, and how their morals might be radically different from ours. It is very thought provoking and I really enjoyed this piece of work.
I hope He writes more stories. I'm definitely gonna read his work.
Apr 20, 2018 rated it did not like it  ·  review of another edition
Shelves: scifi
CW: rape mention, murder mention, weird anime shit

OKAY I read that whole HP Rationality book aaaages ago, back when it was still being serialized, and at the time it seemed pretty good! (Now that I'm not an impressionable teen, it... really doesn't.) But for some sadomasochistic reason, I decided to read one of his other, SHORTER stories. IT'S TERRIBLE.

All quotes are in spoiler brackets so that you can easily avoid them :)

(view spoiler)
Sep 04, 2016 rated it it was amazing  ·  review of another edition
Maybe the best short story I ever read. If you do not get HPMOR, you should anyway try this.

This is like Star Trek but orders of magnitude better. These 100? pages, contain the intellectual value of 2000 pages of traditional sci-fi, and without the nonsense.

If Yudkowsky is not canon in 2100, then canon is mostly random. He is Borges and Bertrand Russel in one.
Quinn Dougherty
Nov 16, 2019 rated it really liked it  ·  review of another edition
Highly recommended. Haunting, I continue to think about it almost every day even months after I read it. The most sober reasoning about alienness that you're likely to find. One of its conclusions/provocations, that if you look at the distribution of possible space-faring civilizations, it's probable that if they're legible/recognizable at all then they're morally monstrous. At the same time of course a future, space-faring humanity may also be monstrous. Do explore this idea by reading the whol ...more
Zach Toad
Jan 25, 2015 rated it really liked it  ·  review of another edition
Insanely thought-provoking, but didn't quite get all the references to game theory other than talk about the prisoner's dilemma. I wish I had a college level game theory course under my belt before reading this. The play with ideas in metaethics was very well-done, however. Relevant, too, to all our political conflict over whether to intervene or not in foreign affairs.
Not sure that it brought me any closer to answers!
Sep 24, 2018 rated it it was amazing  ·  review of another edition
Shelves: repeat-authors
The writing felt a bit cringey at times, but I’m glad I got past that to the actual concept, which was really interesting. (view spoiler) ...more
Gustav Tonér
Sep 04, 2017 rated it really liked it  ·  review of another edition
Hilarious and frighteningly plausible
Yevgen Antymyrov
Reminded me of The Three Body Problem, Dark Forest, and Harry Potter and Methods of Rationality (probably because of audiobook narration by same people). It's a short novella, I wish there was more. ...more
Anton Hammarstedt
This does a very good job of presenting an intuitively graspable version of the prisoner's dilemma vis a vis a paperclip maximizer (in the general sense, a foreign intelligence whose goal function shares no resemblance to our own). Recommended follow-up reading: Superintelligence and Friendship is Optimal.

Setting presentation, design and originality (how cool is the setting?): 5
Setting verisimillitude and detail (how much sense does the setting make?): 5
Plot design, presentation and originality
Sep 02, 2015 rated it it was ok  ·  review of another edition
I find the characters impossible to relate to. You might say that's beside the point for this particular story, but I think it exactly is the point for a case of first contact with an alien (in all senses) culture. Here, I felt that all the species were alien.

The story describes certain alien behaviors that produce visceral emotional reactions in the characters (and for good reason, probably). And yet I had no emotional reaction myself, which I don't think was the reader's fault.

The author can b
Jun 28, 2015 rated it really liked it  ·  review of another edition
Shelves: 2015, sci-fi
This review has been hidden because it contains spoilers. To view it, click here.
Jan 26, 2019 rated it really liked it  ·  review of another edition
Short, gripping and stunningly intuition-wracking. The story of three different species with three different evolutionary histories meeting reveals much about our own ethical intuitions as a species as of now (~early 21st century). Especially tasty is the way that the story unveils some (possibly hidden) preferences in the reader (humans should win/prevail etc.) and this brings the themes of the story into the real world for further discussion.

This short story is brief enough that it can be read
Simona Vesela
Oct 02, 2016 rated it it was amazing  ·  review of another edition
I loved it. If playing the classic prisoner's dilemma, you cooperate with a single individual. Almost always. You defect only in fear. But what if you really-really want to defect and you are playing against aliens for whom you are morally disguising, and you feel the same about them.
Amazing story, well written, strange, thought-provoking. Nice to accompany it with the original reader comments on Less Wrong.
I recommend it
Andy McKenzie
Some good portions and does challenge your assumptions. He's done better elsewhere I think. ...more
Hamish Seamus
Very, very interesting. Plus, it's quite short (extreme unnecessary length is a common feature of EY books which puts me off), which has made it one of the pieces of EY writing which I have most enjoyed.

Interesting questions raised: if you encounter a baby-eating alien species, do you impose your own moral standards onto them? Or do you perhaps aim to add some of their values to your own and some of your to theirs in such a way which satisfies everyone? If you encounter some super-fun-happy-peop
I read (and listened to the audiobook version of) this because I loved Yudkowsky's HPMOR.

Unlike HPMOR, this book cannot be categorized as "fanfiction" because it is not based off of an existing story (although it has a bit of a Star Trek/Star Wars vibe simply because it takes place in space). Unlike HPMOR, this is obviously something that is very unpolished and not even close to being ready for publication (by traditional means). Although the concepts are interesting, and I can appreciate how Yu
Dessislava Ivanova
OK, that mini book (or novella) was a real emotional rollercoaster for me - I felt hopeful and also super sad. It was a scary and a happy reading. And my moral compass was spinning like crazy because I was so confused! We think we know what's good and bad and what to enforce our "truth" to everyone but what if some alien race is exactly like that but also with a totally different moral? The reasoning in this story is so good - the writer is a thinker on a whole new level and was tinkering with m ...more
Jun 23, 2019 rated it liked it  ·  review of another edition
Three Worlds Collide is an excellent moral and ethical dilemma presented through the vessels of mediocre and shallow characters. After reading this, I couldn't tell you how many distinct characters there were, nor their feelings, goals, or much else about them. They blur together after serving their purposes as authorial representatives of discrete ideas. However, Three Worlds Collide presents multiple intriguing ethical questions and introduced me to new viewpoints on the nature of life outside ...more
Feb 12, 2021 rated it it was amazing  ·  review of another edition
This book was not at all what I expected, but an extremely good quick read nonetheless. Makes several important points about the Prisoner's Dilemma, metaethics, and the orthogonality thesis. Also raises some fun questions about what aliens would really be like, what we want the future of our civilization to look like, and what we ourselves would do when faced with huge decisions. Does contain some material that could be triggering, without content warnings, where adding them really wouldn't detr ...more
Jesper Lindholm
Masterfully written first chapters. 5/5.Then the writer advances the story beyond his capabilities as a writer and original thinker. This decreases the credibility of the fiction.

It is still an interesting story. Quite hard to follow all dialogue in the later chapters.

There are some dilemmas with obvious solutions left untested, which raises questions of false dilemmas, as far as rationality goes. (This is a rationalist fiction). Also some disproved scientific details, but they do not affect the
Dec 05, 2018 rated it really liked it  ·  review of another edition
Shelves: finished-in-2018
Thought provoking. Pessimistic. A lot of important factors and options were ignored, but I guess it isn't possible to cover everything. His depiction of emotion is often hyperbolic to a degree that interferes with immersiveness and believability, but that isn't the worst writing crime. He makes some quite strange assumptions about the preferences of the human race as a whole. There is some extremely heavy content that probably should have been treated with more sensitivity and a clear content wa ...more
Gianluca Truda
Dec 10, 2018 rated it really liked it  ·  review of another edition
Thought-provoking, gripping, and deeply unsettling.

This short rationalist story should not be the first avenue for those unfamiliar with Yudkowsky’s writing, but nevertheless offers a mind-bending commentary on human psychology, meta-ethics, and the troubles of interstellar game theory.

It lost a star for less-than-ideal execution and an unsatisfying ending (logically and emotionally). Perhaps that was the point, but this is generally not up to the same exacting standard as Harry Potter and the
Piet van Dongen
I like that there were 2 endings, that's the upside of an internet-published thing. I also liked the thought experiment as a whole and the way the story was used to explain metaethics and all that. But to me, there was an undertone of sexism, of some incel-like values and some kind of libertarian idealism which I did not like at all. You could argue that they were used to drive home the fact that these humans are far removed from our humans, but still, I didn't like the image. ...more
Lukáš Kubíček
Aug 07, 2017 rated it it was amazing  ·  review of another edition
Shelves: fun, worlds, scifi, visions
The best "first contact" scifi I have ever read. You know like everytime in scifi genre when aliens are introduced they look like humans just with silly eyes or ears? The lack of imagination and human-centrism always disappoints me. Three Worlds Collide is the opposite of that, the aliens created by Eliezer are completely different, yet believable. And to book is fun, you will laugh out loud. ...more
Apr 10, 2018 rated it really liked it  ·  review of another edition
Shelves: 2018, type-ebook
"Humans exploring the universe encounter alien species" could be a completely cliché plot, but Yudkowsky adds some interesting sociological exploration when the humans encounter a species that literally eat their young. I listened to the audio version from and recommend that version for anyone who likes audiobooks. ...more
Ryan Dash
Mar 31, 2019 rated it liked it  ·  review of another edition
Shelves: novella, sf
3.5 stars. A very interesting thought experiment presented in novella format, largely through dialogue. The philosophical implications are certainly worth pondering, in an era with technology close to capable of making them practical. It would have been nice to have more showing and less telling, though.
Apr 22, 2018 rated it really liked it  ·  review of another edition
Surreal existentialism meets sci-fi. Don't read it for the story, which is also good, by the way, but read it for the impossible-possible dilemma of a theoretical future human race it presents. I love this author.

I liked the second ending better.
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From Wikipedia:

Eliezer Shlomo Yudkowsky is an American artificial intelligence researcher concerned with the singularity and an advocate of friendly artificial intelligence, living in Redwood City, California.

Yudkowsky did not attend high school and is an autodidact with no formal education in artificial intelligence. He co-founded the nonprofit Singularity Institute for Artificial Intelligence (

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