[HN Gopher] Loops Across Space
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Loops Across Space
Author : sohkamyung
Score : 122 points
Date : 2023-06-22 12:32 UTC (10 hours ago)
(HTM) web link (www.gregegan.net)
(TXT) w3m dump (www.gregegan.net)
| namanyayg wrote:
| Egan is one of my favorite sci-fi writers. It takes a while to
| understand some of the concepts he writes, but imo he's written
| some of the best works in hard sci-fi that also deal with a lot
| of philosophy.
|
| Great to see him active on his website (though I'll admit I gave
| up on understanding this around halfway)
|
| Editing to add: of his works, Permutation City is one of my
| favorites. Multiple mind blowing concepts. I'm sure it'll be up
| the alley of many other HN readers.
| BLKNSLVR wrote:
| Quarantine is my favourite. Permutation City was great, but
| difficult - I think because whilst I can mentally touch the
| concept I can't quite hold it.
| tremon wrote:
| I'm of two minds about Quarantine. I loved the overall
| concept of it and the mystery narrative of gradual discovery,
| but the end of the story felt rather flat and unsatisfying.
| Maybe he intended it that way, to contrast it with the
| grandiose events in that short window before (I could say
| more, but I don't want to spoil it). But it still felt kind
| of rushed to me.
| JadeNB wrote:
| > I loved the overall concept of it and the mystery
| narrative of gradual discovery, but the end of the story
| felt rather flat and unsatisfying. Maybe he intended it
| that way, to contrast it with the grandiose events in that
| short window before (I could say more, but I don't want to
| spoil it).
|
| I think at least part of it is just that he was very early
| in his writing career, and probably wasn't sure how to
| stick the landing yet. I love the book, but there's no
| denying that it has its rough spots.
| NoMoreNicksLeft wrote:
| Permutation City isn't that tough, is it?
|
| I'm not claiming that I have ever been clever enough to think
| up the idea myself, but upon reading it was almost intuitive.
| I'm half-convinced that our own universe works something like
| in that story.
| QuackingTheQ wrote:
| I would also go to bat for Diaspora, single-handedly re-
| invigorated my interest in science fiction
| ajmurmann wrote:
| I loved Permutation City and his short story collection,
| Axiomatic and really liked Distress. However, I stopped
| reading Diaspora 30% in. If it hasn't click yet, should I
| still keep reading?
| JadeNB wrote:
| > I loved Permutation City and his short story collection,
| Axiomatic and really liked Distress. However, I stopped
| reading Diaspora 30% in. If it hasn't click yet, should I
| still keep reading?
|
| It depends on what doesn't click. If you're waiting for it
| to get more down to earth, it doesn't (either literally or
| figuratively). But I do remember it as starting out very
| dry, and getting, while not less dry, considerably more
| absorbing as it went along.
| ajmurmann wrote:
| I am totally fine with dry. What usually pulls me in with
| Egan and hard scifi in general, is cool technological
| ideas and seeing how they play out societal (example:
| floating island state in Distress). Egan sticks out to me
| that he also will apply or combine existing concepts in
| mind-warping ways (example would be the infinite cellular
| automata in Permutation City). So far non of that has
| really happened for me in Diaspora. AI existing at a
| faster speed than the physical reality is cool, but feels
| like table stakes.
| QuackingTheQ wrote:
| I'm not sure how far into Diaspora you made it, but the
| further you go the more mathematical or physics-based it
| gets. Computability/complexity theory show up later,
| there's a large portion of the book dedicated to an
| alternative physics model, etc
| ajmurmann wrote:
| Oh, that sounds wonderful. None of that has really come
| up yet. Thank you! I'll give it another shot.
| mostlysimilar wrote:
| Diaspora is one of the most interesting and captivating
| things I have ever read. I love it dearly.
| gcr wrote:
| I loooove diaspora! Egan was inventing neopronouns 25 years
| ago and it totally works in the context of the story, reader
| doesn't even bat an eye seeing ve/ver/vis so consistently and
| casually after a few pages. Just one of the many forward-
| thinking aspects of that story.
| abecedarius wrote:
| When this book came out I'd previously seen ze/zir which
| sounds less jarring in legacy English. I think this was
| from a few people in the sf world in online discussions
| rather than in fiction, though I can't really remember
| anymore.
|
| Either way, it scales better than having everyone publish
| an individual pronoun policy and every else remember it,
| O(1) vs. O(N^2).
|
| _Diaspora_ is excellent.
| pc86 wrote:
| I went to a small rural school on the east coast and
| circa 2005 or so I recall getting a mass email from an
| acquaintance explaining their new pronouns of ze/zir.
| That was the first I had ever heard about someone
| preferring different pronouns, and it was probably close
| to a decade before I heard those particular ones in any
| other context. All that is to say that it makes sense if
| it had made its way out to a rural college in 2005 it was
| probably being used a bit more widely in the SF a few
| years prior.
| danbmil99 wrote:
| Greg Egan is a true genius, and I don't use that word lightly.
| He's a true polymath. In another era, I suspect he could have
| been one of the leading scientist/mathematician/philosopher of
| his time.
|
| One comparison I really like: Johannes Kepler -- he figured out
| planetary orbits _and_ wrote one of the fist (if not the first)
| SciFi novels. Oh and he saved his mother's life when she was
| accused of witchcraft -- a parallel to Egan's work regarding
| the indigenous Australian population.
|
| I sometimes wonder though whether people like Egan are properly
| appreciated in the modern age. So much noise and competition
| for attention...
| thatcherc wrote:
| I'll go in to plug the Clockwork Rocket series (Orthogonal and
| its sequels) and Incandescence, all of which have much more to
| do with physics and spacetime than Permutation City and short
| story collections like Instantiation (which I also love).
|
| I think Incandescence is my favorite book of the physics set,
| but the Orthogonal books and their corresponding web notes [1]
| are an absolute tour de force of deriving a whole universe of
| physics from a tiny modification to the rules we're familiar
| with. It was wild to read those as a physics undergrad and
| still very cool to think about today.
|
| [1] -https://www.gregegan.net/ORTHOGONAL/00/PM.html
| gpderetta wrote:
| Instantiation does contain a short story (The Gateway) about
| a space anomaly which, IIRC, had some Didicosm-like
| properties, like lack of mirroring.
| Filligree wrote:
| If you liked Clockwork Rocket, you'll also like Dichronauts.
|
| Whereas Clockwork is set in a +/+/+/+ universe, Dichronauts
| is +/+/-/-. Between that and the +/+/+/- of the ones set in
| our own, he's covered every possibility.
|
| (+/+/+/+ and -/-/-/- being mathematically equivalent.)
| 0cf8612b2e1e wrote:
| Unless it is a complete spoiler, could you elaborate on
| what those universe designations mean?
| gpderetta wrote:
| IANAP, but those +/+/+/+ are the signs in the distance
| function of your universe. Specifically ++++ is an
| Euclidean spacetime where the distance of any point in
| spacetime is the sum of the squares of the distances in
| each dimension (including time). Our universe is not
| Euclidean but Minkowski spacetime where the opposite sign
| is used for the time-like dimension (which gives origin
| to special relativity).
|
| Egan explores universes with two timelike dimensions
| (++--) and all spacelike dimensions (++++).
| tialaramex wrote:
| Incandescence is my favorite because it makes me cry.
| [deleted]
| marsten wrote:
| Interestingly there are no photos or video of Egan publicly
| available. All we have over the last 30 years is his/her/its
| textual output. I'm 50/50 on whether "Greg Egan" is in fact a
| LLM.
| andybak wrote:
| Can someone _please_ make a simulator of this! I 'd love to see a
| Geometry Wars style shooter or a puzzle game using these spaces.
| RogerL wrote:
| http://www.gregegan.net/DICHRONAUTS/02/Interactive.html
| bryan0 wrote:
| > The first homology group of the didicosm is Z4 x Z4.
|
| What about the other 5 "orientable platycosms"? I assume 3-Torus
| is another with Z x Z x Z (?)
| mydriasis wrote:
| Love Greg Egan. Especially loved Schild's Ladder. It was such a
| cool book, and it's very clear that the author has a serious
| background to boot, further evidenced by publications like this.
| Awesome!
| kleer001 wrote:
| Yay, doughnut space! I played with this kinda stuff as a kid back
| in the QBasic days.
|
| Like Asteroids style.
|
| Basically:
|
| If X>10 then X=0
|
| If Y>10 then Y=0
|
| And then again
|
| If X<0 then X=10
|
| If Y<0 then Y=10
| munificent wrote:
| You actually want: while x > 10 do x -= 10
| while x < 0 do x += 10 while y > 10 do y -= 10
| while y < 0 do y += 10
|
| What you have will discard some position information every time
| it wraps. Also, it won't correctly handle changes in position
| per frame that are larger than 10.
| notfish wrote:
| Or just x = x mod 10 y = y mod 10
| munificent wrote:
| That depends on how the programming language implements
| modulo. Some don't wrap around the way you would need here
| when the dividend is negative. See: https://en.wikipedia.or
| g/wiki/Modulo#Variants_of_the_definit...
| ithkuil wrote:
| Would this work in both flavours of the modulo operator?
| x = ((x mod 10) + 10) mod 10
| gpderetta wrote:
| The issue is that in 3d the mapping is less straightforward if
| you want avoid stange effects like being mirrored .
| jugg1es wrote:
| How can you have a 3d space with finite volume and no boundary?
| Sharlin wrote:
| In 2D the most intuitive way is the "Asteroids topology": when
| you exit from one edge you reappear from the opposite one. This
| space is flat, finite, and without a boundary. In mathematical
| terms it's what you get when you take a rectangle and
| _identify_ the left edge with the right and the top edge with
| the bottom one. It is also topologically equivalent to the
| (surface of) a torus, which is also flat and wraps around the
| same way. (Note that the _embedding_ of a toroidal surface in
| three dimensions is curved, but the 2D space itself is flat:
| the angles of every triangle add up to exactly 180deg.)
|
| In 3D, just generalize from the 2D case.
| pdonis wrote:
| _> the 2D space itself is flat_
|
| It's not that simple. "Flat" is not a topological property,
| it's a metrical property. The correct statement is not that
| "the" 2D torus is flat, but that it is possible to put a flat
| metric on the 2D topological space that is called a "torus".
| That's what the "Asteroids" space does.
|
| But it is also possible to put a curved metric on the same
| topological space--for example, just use the obvious metric
| derived from the embedding in 3D Euclidean space that we're
| all familiar with. The "torus" topological space in itself
| has no metric, and both the flat "Asteroids" metric and the
| curved "doughnut" metric are valid metrics on that
| topological space.
| Sharlin wrote:
| Yes, good point, I wanted to include the flatness property
| (which was the third condition mentioned in the article)
| but simplified a bit too much in the process.
| cubefox wrote:
| > It is also topologically equivalent to the (surface of) a
| torus
|
| Interesting, I thought it would be like the surface of a
| sphere. What's the difference?
| dllthomas wrote:
| Connectivity. When you go off the top, you come in on the
| bottom. When you go North on a globe, you come in elsewhere
| in the North.
| cubefox wrote:
| Ah, this makes sense. I'm thinking of the Mercator
| projection of the globe.
| Sharlin wrote:
| When you wrap a rectangle around a sphere, all points at
| the top edge are identified - thecedge is compressed into
| a single point, the "north pole", and similarly with the
| bottom edge. When you go off the top/bottom edge of
| Mercator at longitude N, you emerge at another point at
| the same edge, namely at longitude N+180 (mod 360).
|
| (Also, in Mercator it looks like you can approach the top
| or bottom edge diagonally, but this is an illusion, an
| artifact caused by the projection. You can only ever
| approach the north pole directly from the south, and once
| you cross it, you find yourself having "rotated" exactly
| 180deg and are now facing south, in addition to having
| jumped to the opposite longitude. And vice versa for the
| south pole.)
| roywiggins wrote:
| The poles don't get identified with any point in the
| plane at all, really - the Mercator projection is
| infinitely tall, less a rectangle than an infinite strip.
|
| The equirectangular projection does map the top and
| bottom edges to their respective poles, but Mercator just
| keeps going up (and down).
| [deleted]
| perihelions wrote:
| In the same way a sphere, like the Earth's surface, is a
| 2d-space with finite area and no boundary.
| alchemist1e9 wrote:
| Mobius strip:
|
| https://en.m.wikipedia.org/wiki/M%C3%B6bius_strip
| jugg1es wrote:
| What does boundary mean in this context then? Because the
| mobius strip has a boundary on the sides even if it is
| continuous. I guess boundary means an edge in all directions?
| simonh wrote:
| A mobius strip is an example of a space that is finite but
| unbounded in a single direction. A two dimensional plane
| that is finite but unbounded in all directions is called a
| Kline bottle, but you can't build them for real in 3D
| space, only distorted approximations.
|
| You don't have to do such geometric contortions though. The
| surface of a sphere is a two dimensional space that is
| finite but unbounded.
| ithkuil wrote:
| Not sure why you need a Mobius strip when an untwisted
| rectangle with two opposite edges joined (outside of a
| cylinder) has the same property. Unlike a sphere that
| would be a flat 2d space (but unlike a sphere it would be
| unbounded only in one direction)
| layer8 wrote:
| The same way the surface of the earth is a 2D space with finite
| area and no boundary.
| BLKNSLVR wrote:
| When you reach one of the boundaries you re-enter the space at
| a different boundary. It was one of the very few things I was
| able to understand.
|
| There's "continuity" between two points that, in the 3D
| rectangle representation, don't appear to have continuity. This
| continuity is possible because spacetime may be all warpy like
| that... You can't exit the space at the boundaries. The door
| out of the room walks you into the same room from a different
| door.
|
| eg. exit Face C at some point, and re-enter from Face A,
| because Faces C and A are aligned at that point.
|
| I'm lost beyond that point though.
| mike_hock wrote:
| TFA tells you how.
| prof-dr-ir wrote:
| in 2d: take a square and glue the two pairs of opposite sides
| together. If you do this with one pair you get a cylinder, and
| then gluing the other pair gets you a torus. No boundary is
| left.
|
| In 3d: take a solid cube and glue the three pairs of opposite
| sides together. Maybe a bit difficult to visualize, but the
| idea is that if you live in the cube and try to exit it on one
| side then you re-emerge into the cube from the opposing side.
| dylan604 wrote:
| Would this allow for Escher type spaces?
| Filligree wrote:
| Yes, though not with a didicosm. (Unless your standards for
| Escher-esque spaces are low.)
| kibwen wrote:
| Escher's most fantastical spaces would be classified as
| non-Euclidean, whereas the article here stresses that the
| spaces it describes are strictly Euclidean.
| falcor84 wrote:
| s/Euclidean/locally Euclidean/
| cubefox wrote:
| I think they are rather forms of parallel projection
| (where farther objects aren't smaller, unlike perspective
| projection) combined with forced perspective, where
| things look locally connected from a specific angle:
|
| https://en.wikipedia.org/wiki/3D_projection#Limitations_o
| f_p...
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