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Comment by gregmac | Hacker News Reader

If you haven't read the books or watched the show, The Expanse does a great job at working in a world with realistic physics.

A lot of combat takes place from extremely far distances, using telescopes, rail guns and missiles, and it can sometimes take days for a missile to reach the target (the TV series doesn't play this up as much though, for obvious reasons).

When there is closer combat battles, they are constrained by the G forces humans can tolerate, wear full helmets and de-pressurize their ships, so when the inevitable shots do go through (usually in and out the other side) it's not catastrophic.

There are no force shields, magic gravity generators, lasers (well, except used for point-to-point communications). I found it to be very refreshing mainstream sci-fi.

One warning: after watching The Expanse, the battles in things like Star Wars, Star Trek or Battlestar just look ridiculous by comparison - even with that technology they'd never fight like that.

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The Expanse gives the illusion of real physics, but the actual physics are pretty fake. None of the orbits of the planets or the space ships follow realistic path. One of the authors mentioned that they didn't calculate the positions of the planets or the trajectories between them. So the Expanse isn't as realistic as 2001 or the Martian where the orbital mechanics play a key role in the story.

This may seem like nitpicking, but if you want to discuss the politics of space, the distance between planets and the travel time between space polities becomes a critical aspect of politics.

The Expanse is in an uncanny valley of hard sci-fi. It's not as realistic as Kerbal Space Program but use technology as a magical way to explore speculative scenarios like Star Trek did.

And a core part of the first arc is a stealth ship which is not hard sci-fi at the Expanse's tech level: http://www.projectrho.com/public_html/rocket/spacewardetect....

Well said. I recently got into a "the Expanse is not hard sci-fi" discussion, and the politics of space (between distant planets) was an interesting angle we hadn't considered.

Why can there not be stealth in space? The link you gave doesn't seem to have any arguments. Not saying you're wrong, just curious.

Some simple arguments that it's not a ridiculous question:

There is stealth on earth - paint and geometry that reduces EM signature. Why couldn't there be something similar in space?

It's not like it's super easy to detect everything in space today either - we have big telescopes but regularly miss small asteroids?

Because your spaceship has to generate heat for various systems - at the very least, to keep its occupants alive. That heat, unlike on Earth, really stands out against the cold of space. You can hide the emissions of many vehicles on Earth, because Earth and its atmosphere is pretty warm compared to space in the first place. But in space all you need to do is look for heat signatures(infrared and others) and you will spot any vessel immediately.

Some Sci-Fi has tried addressing it with plausible ideas - like for instance dumping all heat into shielded internal tanks, which have to eventually be vented(so that stealth cannot be infinite because you run out of heat storage capacity on board).

Right - or maybe directionally radiating it away in a direction your adversary can't observe?

The James Webb space telescope cools it's observing side instruments to 10s of degrees above absolute zero, using active cooling. I wonder how hard it would be to detect from it's cool side, at a space combat distance?

Asteroids in the near solar system are normally 100-150 kelvin, apparently, and don't seem trivial to spot.

All that still seems to allow room for stealth in a hard sci-fi setting.

There's other problems. Even if you contain your heat or vent it from a different side, you still need to produce hot exhaust to accelerate. If you don't, your position can be inferred from your last known position/acceleration.

This and other points are discussed here: https://tvtropes.org/pmwiki/pmwiki.php/Analysis/StealthInSpa...

In theory, you could fix the hot exhaust problem with a mass driver.

In the realm of hard sci-fi the detectability of an exhaust plume depends on a variety of factors including the ionization, temperature, shape, velocity, and angle of the plume wrt adversary, and the local medium.

There are a number of methods one could theoretically use to minimize these factors. As an example, imagine the detectability of a small angle coherent neutron beam or the IR exhaust of a black hole starship.

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

My personal fantasy tech for subspace is that it functions as an absolute zero heat sink. So heat engines can use absolute zero as the ultimate heat sink to build a Carnot contradiction: They don't actually sink any heat at all, and can extract all of the heat energy as work. Quantum mechanics forces them to actually sink a tiny amount of heat proportional to process entropy. Those tiny fluctuations are the "subspace signatures" that get detected anyway.

Its still more fantasy than science, but hey, so are warp drive, hyperspace, and all that jazz :)

I imagine it'd still be like stealth on Earth, but at much greater distances. I can always see a stealthy spy plane when standing next to it in a hangar. But it's purpose is to fly high above the ground.

If heat signatures are a sure way to identify space ships, then the value of stealth comes in at the limit of detection range. If an enemy can only detect me at X units away, but I can detect him at X+1 units, I have an advantage.

If I'm spouting nonsense, I'd be legitimately delighted to hear why it can't work.

this is more or less consistent with everything I've read on the topic, but X can be a surprisingly long distance (billions of km under thrust, or millions of km in "cold running" mode). you would be detectable from pretty much anywhere in the solar system while burning, and still quite a long way away in "stealth mode". even if you had really effective stealth in cold mode, adversaries that observed you while burning could accurately predict your trajectory.

X+1 detection range isn't much of an advantage if X already includes most of the places you can actually be and/or X is already an order of magnitude past effective weapons range.

There is a relatively famous and long blog about this: http://www.projectrho.com/public_html/rocket/spacewardetect....

IIRC in the series they turn off everything to stop emitting anything that can be detected - mostly IR radiation, I would guess.

The rest of stealth in space is relying on the vastness of it. A spaceship a few kilometers away is already hard to see.

IIRC, they don't turn everything off. they turn off the reactor and run the ship's systems off of battery power. against a nearly 0 K thermal backdrop, there's no way an object like this could completely evade detection.

they do actually address this in the show. you can infer from the dialogue that they don't expect to be "invisible"; rather, they are trying to be indistinguishable from space junk.

I do think people are being a bit harsh in this thread. the show isn't perfect, but it's about as hard sci-fi as you can be while still getting renewed each season.

> None of the orbits of the planets or the space ships follow realistic path.

I noticed that, but it is obviously a necessity for the TV version so that the plotlines are not interrupted. I’ll allow it

The physics are fine. Once you have 1G Torchships, orbital mechanics becomes very irrelevant.

You basically take straight line paths between to points in space, constantly accelerating for half the journey then flipping over at the half way point to decelerate for the rest.

Earth to Mars at closest takes 46 hours at 1G. Earth to Jupiter at closest takes less than 6 days. With 1G torch ships, the entire solar systems becomes accessible in less than two weeks journey. And the Expanse has ships that have much more acceleration.

I can't describe the feelings I feel in the last couple of weeks. It really feels like we are living in a (dystopian) future. Elon Musk is working on a brain-chip, people are wearing robot-arms and legs, space-travel gets faster and easier year by year etc.

Just crazy, I am stoked to see what new technologies and possibilities will arise in the next few decades.

I watch with grim satisfaction how well the cyberpunk books predicted our present and near future from 30+ years ago. (Satisfaction, because what's coming looks familiar, we've been warned.)

I only started reading and watching Cyberpunk related things about a month ago, so it's interesting to see how far we already are.

The thing with the prosthetics is that I feel like there's hundreds, if not thousands of companies and individuals working on them, but there doesn't seem to be any convergence, or more importantly, mass production - meaning that prosthetics remain expensive and made-to-measure. This is a thing with a lot of things that look very high-tech or science fiction.

That said, video calling was the realm of exclusive, high-tech / demos for a long time, then without really being aware of, it's been available in everyone's pocket for a long time now. It went from near-sci-fi to mainstream in the blink of an eye.

Prosthetics haven't solved the bandwidth problem i.e. how do you connect them to the nervous system.

Well, Elon Musk's Neuralink is claiming they're close at least, but I could also imagine someone coming from the other direction entirely, and making a robot limb that was autonomous and worked with the wearer instead of being directly controlled (but the failure mode would be no fun...)

Sensory input is the other 50% of the game that remains unsolved. If Neuralink's robot works it will be a game changer for artificial eyes, limbs, everything.

Besides that a robot limb can't be autonomous in it's movement as most movements we do are asymmetrical and thus it needs input from the other limb as well.

It's fun to estimate how hot the exhaust of a torch ship must be, and how much mass should it expend on constant 1G sustained for days.

Rockets exchange impulse with the exhaust at a linear rate (mv), but put the kinetic energy into it at a quadratic rate (mv²/2), so increasing mass efficiency by increasing exhaust speed / temperature costs more and more. Realistic with fusion reactors, to the extent to which compact fusion reactors themselves are realistic. Or even with fission reactors, in the form of bombs (see project Orion).

Nothing like the current simple, low-tech chemical rockets.

There's an interesting speculation as to how an Epstein Drive could work in reality. The heat management problem is pretty central:

http://toughsf.blogspot.com/2019/10/the-expanses-epstein-dri...

The discussion at the bottom of the page has also a lot of interesting ideas.

It's a very interesting discussion, thanks!

I don't really mind the Epstein Drive and really appreciate the effort to portray a decent stab at realism. It's very refreshing.

The one thing that bugs me a bit is the way they conceptually represent the solar system in the show. If you follow the way the situation is represented you essentially have Earth on one side of the solar system, Mars in the middle and the asteroid field on the other side. That seems to be how the strategic situation is presented. To get from the asteroid field you essentially have to go past Mars, which is closer to the asteroids. If you think about the actual layout of the solar system this makes no sense, in reality Earth is about in the middle and any given asteroid is closer to the Earth at any given time than it is to half the rest of the belt.

Again, it's fine. There's no way to capture the real dynamics of the solar system in the story without hopelessly bogging everything down, and for the purposes of the show the simplified model works fine. I suppose you could argue that during the period of the show by chance most of the main asteroids and Mars happen to be lined up on roughly the same side of the solar system as Jupiter, and then it basically works. It's just a bit of a fudge.

The strategy representation could also be rationalized as the Mars actively positioning their forces near most efficient transfer routes between points of interest. There seems to be relatively few places in the Belt and outside where anyone would want to go to, and guarding empty space where you'd expect no one to pass wouldn't make much sense. So ships transiting from Earth to the Belt would be going through Mars' solar orbit doughnut-of-influence.

This of course ignores that with the Epstein drive magic, one could always trade travel time for going around ie. off-plane. There's plenty of, well, space for that. (Edit: according to the no-stealth-in-space link in a sibling post, one would be seen doing that, and likely get chased for it as being suspicious).

Ah, the Epstein Drive, invented by Solomon Epstein, the Zefram Cochrane of The Expanse - seems like an unfortunate naming in retrospect, but I guess the novels were published a few years before Jeffrey Epstein became notorious...

Not to take away from your post, because I largely agree, but the Epstein drive might as well be magic. In no realistic universe are there large battle cruisers zipping around the solar system under constant 1G (or even Mars’ third-G) acceleration for months at a time.

But aside from that and the whole aliens technology thing the plot revolves around, it is very realistic. They certainly get a lot closer than anybody else.

Physics is realistic, engineering not so much? :)

I am not sure how unreasonable is prolonged 1g. You can’t do it for months (since if it’s all in one direction you will start getting relativistic, one month of 1g gets you to almost 0.1c) but at medium speeds it isn’t too bad. Suppose you have a megaton ship moving at 100km/s with 10 m/s^2 acceleration

Power = m v a = 1e9 x 1e5 x 1e1 = 1e15 W

While petawatt is a lot, a megaton ship is also huge. Epstein drive is a fusion drive of sort, the easiest aneutronic reaction would be

B+p -> C + 16 MeV

So 2.5e-12 Joules per fusion

Which means you need to burn

1e15 J/s /(2.5e-12 J/atom)*(2e-26 kg/atom) = 8 kg/s

Suppose your engine runs at 10% efficiency, it means you are powering a megaton ship on less than a 100kg/s of material. So if your megaton ship starts as 10% fuel you have ~115 days of continuous thrust.

You're assuming way too high an efficiency. A real fusion rocket would fall far, far short of that. 2% efficiency would be generous. Googling fusion rocket mass efficiencies shows estimates of around 1 ton / MW, so you're not too far from the mark.

But that gets to what the Epstein drive basically is, in-universe: a 100% efficient fusion drive. No physical laws being broken, technically, just the nuclear rocket equivalent of a perpetual motion machine.

They do in fact go around at 1G (or 1/3 G) thrust for months at a time in the books and show, accelerating full speed for half the trip, then turn around and decelerate for the other half, for trips to/from the outer solar system. In the inner solar system it's only a few days to weeks to get anywhere. This lets them operate in artificial gravity without spinning the spacecraft, which is convenient for filming (although it's in the source novels too).

I think comparing future technologies with today's Googke searches is unrealistic.

Exploratory engineering does put firm limits on future capabilities. There are many possible steam engine designs, and I'm sure those we use today were unimagined in the 1820's. Yet none are more efficient than an ideal Carnot steam engine. The same principle applies here.

Yes, and for aneutronic fusion reactions the maximum drive efficiency is really high. Bremsstrahlung x-rays are still a problem in terms of heat dissipation but shouldn't cut into efficiency too much.

EDIT: Well, I should say that the propellant mass/delta-v is really high and that most of the fusion energy will can end up as kinetic energy of the propellant and ship. But the vast majority of that kinetic energy will be the propellant rather than the ship. To maximize ship kinetic energy over fuel energy you want the propellant to exit the ship at the same velocity the ship is traveling which is very far from what you see in an Epstein drive. But fusion provides tons of energy so that isn't going to be a figure of merit we care about.

True, I didn't consider aneutronic fusion reactions. I don't think the sources I'm thinking of do, either.

Here's one source that does: http://toughsf.blogspot.com/2019/10/the-expanses-epstein-dri...

I think I was underestimating how much energy could be lost to x-rays on some of those. D2-He3, the aneutronic fusion everyone likes to talk about is still over 80% efficient but H1-B looks really bad if you're not driving a heat engine with it.

Yes, though the H1-B reaction itself emits no electromagnetic radiation at all. The x-rays are from bremsstrahlung radiation. Some boron fusion projects think they have ways to avoid that (which is necessary to avoid the plasma cooling too fast to achieve net power).

My point is that even if it is 10% efficient, which is only 5 times more than your estimate， you have 4 months. Even if it is 2% efficient, you get ALMOST a month.

Re: accelerating for many months at 1G is definitely bad physics, not only are you relativistic, you start getting into the kind of speeds where a minor meteorites will wipeout your ship. 1 gram meteorite with a relative velocity of 0.5c has 1.4e13 J of kinetic energy = 3.5 kton of TNT, so a small nuke.

yeah the speed they travel through the inner solar system is pretty ludicrous, especially when you consider how much space junk is generated by all that activity.

> the Epstein drive might as well be magic

For me, good sci-fi does just that, ie limit the magic to one thing or technology, and use that to explore something interesting.

Same with Altered Carbon. What would happen to society if we could swap bodies but keep our minds? That one piece of technology ("the stack") is magic, but from it follows lots of interesting exploration of the possible impacts on society etc.

What I hate is when they try to explain the magic. Don't do that. It never works. Just assume it's there, someone discovered it, invented it whatever.

But you have to try and explain the magic. If you didn't, your universe would break. Depending on the postulated mechanisms of action, the principles behind the magical invention will have vastly different impact on societies.

For example, it matters how is it the ships can casually sustain a 1G burn for a week at a time. If it's efficient nuclear fusion, then the ships get bulky, energy is no longer a problem in the society at large, and the exhaust of said engine is a weapon of mass destruction. If it's subspace field shenanigans, then aforementioned impacts don't necessarily apply, but there is an open question of how small such subspace field combobulators can get, and what else could you do with them.

Technologies have second-order consequences, and societies shape to accommodate them. Half the fun of science fiction is in exploring those.

If you just call it magic and define it by a set of arbitrary rules, then your fictional universe will look shallow and inconsistent.

>But you have to try and explain the magic. If you didn't, your universe would break. Depending on the postulated mechanisms of action, the principles behind the magical invention will have vastly different impact on societies.

Star Trek goes into intricate detail trying to explain its warp drives and comes off looking far less grounded than the vast majority of sci-fi properties with FTL or jumpgates or whatnot that either don't explain anything, or just offer a few details. Whether it breaks your universe depends on the themes you're trying to explore, such as the nth order effects of radical technologies on society. Most of the time, the story isn't about the technology (in the same way that Westerns aren't always about animal husbandry and gun manufacturing), and it's sufficient that you push some buttons, pull a lever and spaceship go brrr.

> Most of the time, the story isn't about the technology (in the same way that Westerns aren't always about animal husbandry and gun manufacturing), and it's sufficient that you push some buttons, pull a lever and spaceship go brrr.

everyone is entitled to define their own categories, but to me a key element of sci-fi is that the science and its implications drive the story as much as the characters. if the science merely enables the story, it's space fantasy.

Would you not consider Babylon 5 to be science fiction simply because they never really explain how the jumpgates work, and the plot isn't specifically about the implications of the discovery of hyperspace and FTL travel on society?

The science in that case is necessary to bring the plot elements together and constrain the action under a set of known rules and behaviors regarding hyperspace and related technology, but little of it is explored in detail. It's just there for the same reason that boats have to be there for a story set in the Age of Sail.

I've never watched babylon 5, so I can't comment too much on it.

a common trope in sci-fi is that jumpgates (or something similar) were built by some ancient super-advanced civilization, while the current civilization understands them to the minimum extent necessary to actually use them. this is a good way to sidestep the issue, since any inconsistencies can plausibly be the result of missing knowledge in-universe.

as to "whether it's still sci-fi", imo it depends a bit more on the setting. if FTL has already existed for 100+ years, it makes sense for it to be a background detail. on the other hand, if you have a story where FTL was discovered recently and it doesn't profoundly impact the plot, I would consider that to be pretty bad sci-fi.

like I said, this is just how I personally categorize things. to me, sci-fi is not just lasers in space. it's a genre where science/technology drives the plot, rather than being made up after the fact to justify the plot you want to have. it can still be sci-fi if the tech is totally implausible from a modern physics perspective; it just needs to be consistent.

star wars is usually my go-to example of "not sci-fi". there are no consistent rules for how anything works in-universe. all the technology works in whatever way is necessary to justify the current scene and will likely contradict itself later for a different scene. it's still a very enjoyable franchise, just not what I would consider sci-fi. a show like the expanse is almost the opposite. there's some unrealistic stuff in there, but it's internally consistent. once you nail down what's possible given "the rules" in-universe, the plot almost writes itself.

There is a very sharp distinction drawn between hard science fiction, and the less-particular, broader genre of sci-fi.

For the record, Babylon 5 was explicitly designed to be The Lord of the Rings in space. It's fantasy in sci-fi dress-up. Similar things could be said for Star Trek, Star Wars, and other sci-fi staples.

Hard science fiction like The Expanse, or most of the collected works of Arthur C Clarke, Alistair Reynolds, Kim Stanley Robinson, and Robert Forward present entirely self-consistent universes set in the future with logical extrapolations of technical capability governed by physical law.

Some people don't care for the distinction, but there are also those of us that have very strong preferences for ONLY readying hard science fiction. I like to feel that I learned something, or that the book describes societies that might someday come into being, or alien cultures that might realistically exist.

> Star Trek goes into intricate detail trying to explain its warp drives and comes off looking far less grounded than the vast majority of sci-fi properties

As much as I love Star Trek, it isn't really explaining all that much (it would be, if the writers could be arsed to read the technical manuals created for the series, but even the manuals themselves say they aren't mandatory reading, so here we are). It's DDoSing your disbelief with technobabble to allow for whatever plot the episode's writer had in mind.

(Though it's not impossible to make something relatively consistent out of Star Trek's universe; /r/DaystromInstitute is a subreddit essentially dedicated to doing just that. It's an exercise in function interpolation, but having lots of data points does constrain possibilities.)

There's very little point in choosing sci-fi as your setting if your story is setting-independent. The westerns whose story is entirely interchangeable with action movies set in ancient times or present days don't tend to be very interesting movies (YMMV). Or at least I tend to avoid those, because I find them boring. With sci-fi, if the story is all about setting-independent human drama, I can have that with any other genre. I like my sci-fi when it extrapolates and elaborates on science and technology, and its impact on societies.

> I like my sci-fi when it extrapolates and elaborates on science and technology, and its impact on societies.

That's it for me too, but what I'm trying to get at is that I vastly prefer it without "higgs reactor", "tachyon transmitters" or similar nonsense.

I'm guessing I misunderstood what you meant by "explain the magic". You were probably thinking about explaining magic by introducing more magic; I was thinking in terms of making the magical bit maximally small and constrained.

Right, that's what I prefer as well. Minimize your magic, restrict it as much as possible, and build your world around the consequences of its existence.

But don't try to explain the inner workings of the magic by spouting nonsense. That just takes me out of the story. Take it for granted, aliens made it, something other than semi-randomly assembled sentences sprinkled with fashionable pop-sci terms.

What, no. This isn't consistant at all.

You need magic to describe shit that doesn't exist. Otherwise, then nothing makes sense? How do you have FTL when any possible explanation of how it works might as well be words chosen at random from a textbook because none of it will be plausible.

> If you just call it magic and define it by a set of arbitrary rules, then your fictional universe will look shallow and inconsistent.

Literally doesn't make any sense. All your complaints are second order issues. Good Sci-Fi places rules around its magic and is consistent about it. What "good" sci fi that relies on tech that doesn't exist is there that DOESN'T have its magic bounded by in universe rules.

Sci-Fi breaks down when you start to ignore or make up new rules around the magic, especially if its just "stupid"

In the limit, if you're describing something with no grounding in physics, you need magic. But you can make this magic very much constrained. There's a fundamental difference between "fusion reactors that are absurdly efficient for magical reasons" and "FTL drives that work by magic". A difference in the level of abstraction, if you like. The more lower-level your magic, the bigger and more interesting the network of potential consequences. And also the easier to keep universe self-consistent - because you have small-scale axiom, and large-scale rules are extrapolations using real-world physics and reasoning.

> But you have to try and explain the magic.

You don't always have to. Vernor Vinge explains it as, you are allowed one central conceit which frees you to deeply explore some core theme. That single wild assumption has the interesting property of being both inconsequential to the themes you wish to explore and the single thing that makes it all work. A lot of good hard science fiction has this property: there is internal consistency and broad agreement with real world physics but with at most a single unexamined wild assumption from which the consistently handled deviations flow from. The Zones in his Zones of thought or the Bobbles in Across Real-time are examples of this principle applied for excellent effect. Orthogonal by Egan is an interesting one when viewed in this lens, what with it being physically unrealistic but mathematically driven.

A lot of would be hard scifi authors, in the hopes of increased realism, mistakenly add too much unnecessary detail to the detriment of their story or core big idea. On the other hand, if you manage your central conceit well, you might sometimes be able to enlist the imagination of your readers (some of whom will have more technical expertise in the relevant area) to fill in the blanks.

The Expanse did a great job showing space battles on screen with a rather approriate distance. Star Wars and others, and I am a full blown Star Wars freak, did a good job with space battles. After the Expaense, it became little bit ridiculous, so. I resembles the problem films like Top Gun have with modern aerial combat. Already in that case, distances are so great that on-screen, the viewer is seeing nothing. For space battle, the enemy would maybe show as a small blip, if at all on screen. Especially with all the hyper advanced tech from Star Wars and Star Trek. This is also the reason why I igonre any stab at estimating weapon ranges based on on-screen scenes.

> even with that technology they'd never fight like that

This applies to just about all fictional combat, no matter the setting.

That's what I like about For All Mankind too, though it seems we have to wait for the second season for any real action to occur.

I found For All Mankind really refreshing and enjoyable. One of my favorites is the fact they didn't shy away from prejudices of the time and they didn't make a series where guys from 60s have fully modern views.

If you want a space opera with more realistic setups/politics/internal logic, Legend of the Galactic Heroes is top tier stuff

The Expanse is pretty bad at engagements or physics. They only look superficially well and that because other movies are completely unrealistic and space battles are there just to fill the screen with action and graphics effects, hopefully as background for the real narrative.

Also, if you are looking for any kind of realism in Star Trek battles you are completely missing what the series is about.

I like SF and I love Star Trek just as I love reading hard science fiction. Just don't mistake the two, because looking for realism in Star Trek would be like trying to do linguistic analysis on magical incantations in fantasy.

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