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I’ve noticed that in some topics people mention something in the line of: “Oh, my universe is purely hard sci-fi, there’s no FTL, people use cryo-sleep to travel long distances between the stars”.

That makes no sense, how is cryogenics or any other method for “freezing” someone or making him/her sleep over decades without aging “hard” sci-fi? There’s no known mechanism on how this can be done in any conceivable way today by any medic or scientific standard. There’s no known way how to keep someone from aging medically or technologically over long periods of time, nor there’s any known way to keep someone sleep for long periods of time without all kinds of health issues including calcium loss in the bones, skin sores and possible brain damage.

People in a coma, for example, are moved every day by nurses in order to let their skin rest, do you remember that scene in The Walking Dead with Rick waking from a coma? Well, he won’t be able to walk and his back would be covered in open, maybe bleeding sores. Now imagine that having someone into a machine not moving for decades or even centuries?

It baffles me because I heard people saying that a movie like Passengers is “hard sci-fi” because it doesn’t have FTL, but they do show a magical machine that can keep people sleeping and frozen in their age for more than a century. Yeah, very hard!

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    $\begingroup$ @L.Dutch, the question is actually the title. The rest of his post explains why he is asking the question. Daniel, it is better to restate the question in the post itself. $\endgroup$
    – ShadoCat
    Commented Aug 23, 2018 at 17:13
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    $\begingroup$ Daniel, I'm just letting you know that this post borders on being opinion based. It is, however answerable. It might be better to present future questions along the lines of: What makes cryo harder SF than FTL? It's subtle by "why" questions are often opinion questions. $\endgroup$
    – ShadoCat
    Commented Aug 23, 2018 at 17:18
  • $\begingroup$ Well actually I asked the same in the worldbuildging forum of Reddit an recieved a lot of pretty clarifiying answers about why scientifically cry-sleep may exists. But thanks anyway and sorry if there was any inconvenient. PD: Although here there were a lot of good answers too $\endgroup$
    – Daniel
    Commented Aug 23, 2018 at 18:20
  • $\begingroup$ @Daniel. Take a look at this article: sometimes people hibernate by accident (when runaway hiding in the landing gear of a plane). CNN article here: cnn.com/2014/04/21/us/hawaii-plane-stowaway/index.html $\endgroup$
    – Carlos Zamora
    Commented Aug 23, 2018 at 22:49
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    $\begingroup$ Right now, we know of organisms that can be frozen solid and then unfrozen with no damage done; we don't know of anything that can travel faster than light (other than space time itself). $\endgroup$
    – alexgbelov
    Commented Aug 23, 2018 at 23:31

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The main reason is that space-time is very simple compared to biology, so we understand it much better.

We have excellent reasons to think that we have a very good understanding of space-time, and that understanding -- a simple, elegant, theory that has survived every test generations of clever physicists and astronomers have been able to throw at it -- says "No FTL, no how...or at least not at all easily...it's really, really hard...Nah, don't plan on it."

Biology, OTOH, is complex and we know that we don't know a lot that we'd like to know. While we currently have no way to put someone in suspended animation and then later take them out of it, unaged and unimpaired, we have no theory that says we won't someday be able to do it. All we know is that it's well beyond our current technology.

So if you're trying for the hardest SF which hews as closely as possible to what we know, we cold sleep is a lot more plausible way to get to the stars than an FTL drive.

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    $\begingroup$ It's also definitely possible to put other lifeforms (famously, goldfish are easy to do this to) into suspended animation for long periods of time. We just don't know how to do it to humans yet. $\endgroup$
    – jdunlop
    Commented Aug 23, 2018 at 18:34
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    $\begingroup$ I think it's also important to note that there is a difficult suspension of disbelief for cryo-sleep. Most people's philosophy regarding who they are is at odds with what can be done if they could be suspended. We don't like to think about the philosophical consequences of being suspended. $\endgroup$
    – Cort Ammon
    Commented Aug 23, 2018 at 20:53
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    $\begingroup$ @Cort Ammon I think you'll find that this is a place where YMMV is in full force. Many people see cold sleep as a special form of sleep and thus have no trouble with it. Others, perhaps seeing it as death followed by a revival from death would doubtless see it otherwise. $\endgroup$
    – Mark Olson
    Commented Aug 23, 2018 at 20:58
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    $\begingroup$ @jdunlop that's because a brain damaged goldfish still swims and looks like a goldfish. So we can say we unfroze it successfully. Where as, we can quickly tell by the drooling lips and far off gaze that we were unsuccessful in unfreezing a human. $\endgroup$
    – Reactgular
    Commented Aug 23, 2018 at 21:14
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    $\begingroup$ See also: frogs and water bears. $\endgroup$
    – Draco18s no longer trusts SE
    Commented Aug 24, 2018 at 4:51
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Cryo sleep is just an engineering problem. We know that water in the cells freeze and burst the cells. That gives us a single problem that we can solve that doesn't force us to rewrite the laws of the known universe. As others have pointed out here, we even have some clues about how to do it since some animals manage to do it.

FTL, on the other hand, requires us to rewrite the known laws of the universe. That is allowed since in our history, every time we've known how the universe worked, we were dead wrong (James Burke, How the Universe Changed). But it is squishier SF since it does require breaking the rules as we currently know them.

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    $\begingroup$ I think you're dead before any water freezing ruptures cells, so, it's not quite “a single problem that we can solve”. But yeah, it's for sure a problem that seems not as hopeless as FTL travel. $\endgroup$
    – leftaroundabout
    Commented Aug 23, 2018 at 17:55
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    $\begingroup$ That's not true. You die either from trauma, or from when you can no longer find resources for your metabolic processes to consume. When frozen completely (and most hard scifi uses close to 0K as the temperature) your metabolic processes are themselves stopped or almost entirely stopped. You effectively hit the slow-mo button on life. The major problem we currently have is the ice crystals tearing the cell walls, causing trauma. Some animals replace their fluids with a natural antifreeze to keep ice from forming, and they thaw out perfectly alive and kicking. ShadoCat is spot on here. $\endgroup$
    – Stephan
    Commented Aug 23, 2018 at 19:16
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    $\begingroup$ @leftaroundabout, there's "dead" and then there is "really dead." Really dead is when you can't bring someone back again. People "die" frequently in operating rooms (often on purpose) and are brought back again after the procedure is over. $\endgroup$
    – ShadoCat
    Commented Aug 23, 2018 at 22:30
  • $\begingroup$ Yeah, but from what I've read the coldest anyone has been and could be brought back was +5°C, and usually it's already no return when you go below something like +15°C. @Stephan even if you'd get around the metabolism problem if you could just freeze quickly enough – how? Even if you jump right in a vat full with with liquid nitrogen, your body core temperature doesn't instantaneously plunge to -192°C. You'd need to first chop up the body in nice small steaks to make it so fast; I don't think that simplifies the waking up... $\endgroup$
    – leftaroundabout
    Commented Aug 23, 2018 at 22:44
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    $\begingroup$ All these comments mirror exactly why Mark Olson's answer is correct. There is a very vast amount of knowledge about biology that we haven't even explored. Due to all these unknown, it is easier assume something will work, vs actual breaking a known law of the universe we have tested over centuries. $\endgroup$
    – Nelson
    Commented Aug 24, 2018 at 0:58
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Freezing people is likely possible, since nature already does it. http://www.latimes.com/science/sciencenow/la-sci-sn-alaskan-frozen-frogs-20140723-story.html

We just need to figure out how. FTL so far, we haven't found any examples in the universe where it happens. We almost did, but turned out to be faulty measuring tools.

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    $\begingroup$ What Trevor said. People routinely freeze human-derived cell lines and thaw them out. There are organisms that can freeze solid, thaw and live. . $\endgroup$
    – Willk
    Commented Aug 23, 2018 at 17:31
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    $\begingroup$ It's not just that we haven't found any examples, it's that all current theory (very well tested theories) points at it being impossible as under current theories it should lead to logical paradoxes. $\endgroup$
    – Shufflepants
    Commented Aug 23, 2018 at 18:15
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FTL breaks causality

I haven't seen anyone explain why FTL is impossible. I don't quite understand the nuts and bolts of the proof, but FTL is a time machine. Causality is the notion that "the universe makes sense": that effects never precede the things that cause them, that things cannot cause themselves to happen (or cause themselves to not happen, which is where the trouble really starts). Causality means that paradoxes are impossible, because a paradox requires a circular cause-and-effect circle; one of the links in this chain must cause some effect backwards in time, and causality says you can't do that.

FTL throws causality out the window, because going fast alters the rate at which time flows for you (relativity). This hits an asymptote (infinity) at the speed of light - meaning that going FTL involves crossing that threshhold.

Genres heavily reliant on FTL usually ignore relativity altogether

It's hard for readers to wrap their heads around the idea. It's hard for most writers to wrap their heads around the idea. And it dramatically complicates most space opera.

Battles in which different parts of the battle happen at different speeds are a hard thing to convey sensibly to the audience.

If the protagonists pop over to a neighboring star system, and stay there for a week, it's much more convenient to simply have that take a week. In reality, time would progress slightly differently in each star system (because they have a relative velocity to each other), and going back and forth would be really wonky. Worse, these rates are all related, so if you have five star systems in your story being repeatedly visited, and you wanted to account for time dilation in a way that actually worked, you'd have to actually somewhat understand the math, otherwise people will point out that your dates don't work. (This is the internet, folks ;)

It's much easier to simply slap 'stardates' or 'universal galactic time' measurements on things and be done with it. In most space opera, that's exactly what the writers do. There's nothing wrong with this; action movies don't usually track how much ammunition anybody's using. But it's not hard science.

TL;DR

FTL directly contradicts the current state of the art in astrophysics, and it's commonly used as an enabler for stories that ignore time dilation altogether and have time flow at the same rate everywhere always.

In contrast, cryo-sleep is an excellent choice for hard sci-fi future tech: Given that we know it's possible for some creatures here on earth today, it is quite believable that hundreds or thousands of years of technological development could make it happen.

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  • $\begingroup$ For some more details on exactly why FTL necessarily is a time machine: io9.gizmodo.com/5846519/… $\endgroup$
    – Ton Day
    Commented Aug 24, 2018 at 6:31
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Cryosleep is already possible, we just don't know how to wake them up again. The general concept of freezing something to preserve it is well-known and it stands to reason that it might be very much possible in the future to wake up frozen humans. That doesn't mean it's certain, but it's very much a realistic extension of our current technology.

FTL isn't an issue of technology. According to our understanding of the most fundamental rules of the universe, mass cannot travel at the speed of light, and nothing can be faster. Now there is always the possibility that our understanding is wrong, but so far, the model seems to hold up and is extremely consistent within itself and with many other observations. If it turned out that things could travel faster than light, everything we know about time, space and relativity is pretty much completely wrong and we have to start at zero. Not to mention the things we do know about time dilation and such would obviously still be true, and it is very hard to imagine how these mechanics would scale to FTL.

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    $\begingroup$ There is no evidence that we can wake up people who have already been frozen and quite a bit of evidence that current methods just kill the subject, i.e. destroy the mind. Worse, we won't know that we've found a valid cryo-sleep method until we can wake someone up and verify that we successfully stored their mind as well as their body. Of course, FTL is still worse than that, as the proof there is that it is impossible to do in this universe. $\endgroup$
    – Brythan
    Commented Aug 23, 2018 at 18:58
  • $\begingroup$ I can't really agree with the claim that cryosleep is already possible (by humans). For starters, if it were possible then it wouldn't be science fiction, but science fact. Also, you can't really claim it's possible until someone has woken up from it. Right now, as far as we know, we only have cryo-preservation of corpses. $\endgroup$
    – TylerH
    Commented Aug 23, 2018 at 20:29
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    $\begingroup$ @Brythan that current tech is not functional for successfully freezing living humans today is not an indication that technology cannot ever advance. There are other living animals which can freeze solid, store for prolonged periods, and then be revived - applying those features to humans is a plausibly solvable problem. Just because technology in 2018 has not achieved it yet does not mean it is impossible (we are not currently at the peak of all possible technological development). $\endgroup$
    – pluckedkiwi
    Commented Aug 23, 2018 at 21:14
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    $\begingroup$ There is a trap in long term cryosleep even if we could make it work biologically: Radiation! 100m Sv/Year is not a deal for a worker in an inherently dangerous profession, but after 50 years you have accumulated 5 Sv worth of damage, instantly in biological terms and that is going to be a major problem. Go 100 years at that dose rate and you have a 10Sv dose at revival, and that is not survivable. Don't forget that the body contains meaningful amounts of Potassium and Calcium both of which have radioactive isotopes, so while you wont get 100mSv/year that way there is a inherent limit. $\endgroup$
    – Dan Mills
    Commented Aug 23, 2018 at 21:28
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    $\begingroup$ @Brythan, there actually is some evidence: drownings. There's an old saw that you don't declare someone dead of drowning until they're warm and dead. People have survived after spending ridiculously long periods (estimated in some cases to be up to 30 minutes) in cold water and were revived with no neurological damage. I personally helped in the rescue of a boy who went into Arctic water and was underwater for more time than you'd think would cause irreparable brain damage due to lack of oxygen; he ended up fine. So there's at least a hint it might be possible, totally unlike FTL. $\endgroup$
    – Keith Morrison
    Commented Aug 23, 2018 at 22:39
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We're already taking our first baby steps towards "cryo sleep": https://www.space.com/27348-nasa-mars-crew-deep-sleep.html

"Therapeutic torpor has been around in theory since the 1980s and really since 2003 has been a staple for critical care trauma patients in hospitals," aerospace engineer Mark Schaffer, with SpaceWorks Enterprises in Atlanta, said at the International Astronomical Congress in Toronto this week. "Protocols exist in most major medical centers for inducing therapeutic hypothermia on patients to essentially keep them alive until they can get the kind of treatment that they need."

...

Coupled with intravenous feeding, a crew could be put in hibernation for the transit time to Mars, which under the best-case scenario would take 180 days one-way.

...

SpaceWorks' study, which was funded by NASA, shows a five-fold reduction in the amount of pressurized volume need for a hibernating crew and a three-fold reduction in the total amount of mass required, including consumables like food and water.

Overall, putting a crew in stasis cuts the baseline mission requirements from about 400 tons to about 220 tons.

Extending on this to complete stasis that would halt aging is merely an engineering/medical problem. The biggest problem is repairing cell damage from a complete freezing, but "medical nanobots" requires much less handwavium than FTL travel. Therefore, it is "harder" sci-fi.

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Cryosleep provides exactly one thing to a fictional universe: the raw possibility of interstellar travel. But it provides it at a dramatic cost: time. Travelling to even a nearby star system takes years, even if the traveller doesn't experience any time passing. This puts the fictional universe in a position similar to the Age of Discovery, where travel to other continents is possible, but so slow and costly that it is effectively one-way.

FTL, on the other hand, doesn't have this cost. (It may have other costs, depending on the author's choices, but it is -- by definition -- not slow.) FTL travel makes it possible for people to travel back and forth between star systems without time-debt, making casual travel and tourism feasible. It also means that fast communications and trade are possible -- in short, it puts star systems in a position more comparable to modern nations.

What makes cryosleep considered a "harder" SF element than FTL is less the differences in technology than the differences in what they enable. Since cryosleep is much more limited, it is considered less of a compromise.

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    $\begingroup$ Whilst this is true as an analysis of the different tropes and storytelling you can do with FTL vs Cryosleep, the last paragraph doesn't make sense to me. "Hard" sci-fi is about hewing to known scientific laws and guessing realistic (for some value of it according to authors and fans) advancements. Think "hard" as in "a strong drink without mixers". It is not about how difficult it makes things for protagonists in the story - which seems to be what the last paragraph is saying. With that in mind, although a nice analysis, this answer does not really address the OP's question. $\endgroup$
    – Neil Slater
    Commented Aug 23, 2018 at 21:08
  • $\begingroup$ @NeilSlater If we're going to compare the "hardness" of a fictional universe with Technology X vs. one with Technology Y, we need some metric to compare the impact they have on the universe (and, by extension, on the story they enable). This is necessarily a subjective comparison! The metric I'm using is, essentially, "what applications does this technology enable". Fewer = harder, both in terms of SF "hardness" and difficulty in storytelling. $\endgroup$
    – user25661
    Commented Aug 23, 2018 at 21:14
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    $\begingroup$ @duskwuff that is not any kind of common definition of hard sci-fi. The terms have nothing to do with how difficult you want plot points to be. It is a term purely indicating whether or not it conforms to known science or if it incorporates fantasy elements (like ignoring the laws of physics). While there is some uncertainty, in that science has not yet fully defined the universe with perfect fidelity, there is little subjective about violating what humanity currently understands about the laws of physics. $\endgroup$
    – pluckedkiwi
    Commented Aug 23, 2018 at 21:19
  • $\begingroup$ @duskwuff: I think the problem is then that this answer attempts to re-define "hard sci-fi", whilst it already has a definition. The OP is asking in the context of the consensus definition. A change to the last paragraph would fix the discrepancy. $\endgroup$
    – Neil Slater
    Commented Aug 24, 2018 at 7:33

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