# Despite almost zero energy consumption why does my spaceship scheduled cryostasis in shifts?

In the distant future a breakthrough in cryostasis technology can put healthy people into frozen state so that all metabolism halts and cells never age a bit, the spaceship is carrying them across billions of light years hopping between star systems in search of newfound hospitable planet.

I am wondering if the journey spans several centuries what benefit would encourage people to wake from cryostasis state periodically?

The ship's AI had plotted a route in interstellar space to avoid crossing path with asteriods and high velocity giant molecular gas and clinical trials suggested that cryostasis is 99.99% safe to use.

We know that human being requires high maintenance so would it be optimal if everybody wakes at the destination instead of in batches enroute and wasting on the essential resource like oxygen and water.

• Sorry, but this is a pet annoyance of mine. It would not be a generation ship if the passengers are cryopreserved. It is only a generation ship if new generations are born on the ship over the course of the journey. The correct term for this idea is that it is a sleeper ship. Some people use generation ship as a catch all term for colony ship, but it really isn't. Apr 22, 2020 at 5:30
• Maybe it's a "generation ship" because, plot twist, the ship is secretly powered by killing and absorbing the passengers one by one over the eons..... Apr 22, 2020 at 13:24
• billions of lightyears, several centuries?
– ths
Apr 22, 2020 at 20:23
• @ths There is no issue with that, you can arrive arbitrarily quickly if you travel at a sufficient percentage of the speed of light (approaching zero time as you approach the speed of light), due to time dilation / length contraction. The "billions of lightyears" is only relevant to an observer at your starting point. Apr 23, 2020 at 4:24
• there is no way to test the effects of thousands of years of cryosleep before you use it, statistically undetectable processes built up. They may be thawing them out just to TEST the effectiveness of cryosleep, to look for unforseen effects, or cumulative effect that are too rare to spot in anything but a full scale test. they could be testing for several of the answers people have come up with.
– John
Apr 24, 2020 at 3:11

1. Ship maintenance. While automated, one still needs to mitigate the not null risk of unforeseen events that require prioritization of repairs and better-taken-by-humans kind of decisions.

2. cryostasis may be 99.99% safe, but the number of cells damaged by high energy particles grows over time. While active (not in stasis), the body eliminates the nonviable/dead cells and replaces them.

3. "cryostatis induced dementia" - unless the cryostasis happens at temperatures within fractions of K to zero absolute, the biochemical configuration defining the strength of neural synapses will drift over time. The biochemistry of the body will still define a viable organism, but the memory and intellectual capabilities of the human brains degrades (a bit of plausible handwaving on the line of "the whole greater than the sum of parts", where cryostasis fails to preserve the relations even when it preserves 99.99% of the parts)

As a note: current level of cryogenics can't even guarantee the quality of cryopreserved sperm over 20 years, so your 99.99% safety of cryostasis already requires handwaving.

• #2 is very nice. I haven't heard that one before. Nice idea! Apr 22, 2020 at 19:39
• RE: #1, I'd suggest that some people need to wake up to do ship maintenance, the rest of them wake up periodically so that they do not accumulate huge, uncomfortable age differences with their spouses, lovers, children, parents and friends. Apr 22, 2020 at 19:42
• I don't like #1, but I LOVE #2! Apr 23, 2020 at 16:05
• the damage to the occasional random brain cell would be the biggest issue, letting those accumulate could lead to serious brain damage, you don't have to kill many brain cells to cause problems, and no radiation shield is 100% perfect. we can plot particle tracks through volcanic glass over geologic time, a frozen person will have the same issue. without repair even extremely rare damage will build up.
– John
Apr 24, 2020 at 3:04
• @AdrianColomitchi legacy of heorot, and the sequel is Beowulf's children. although keep in mind it was written before we knew stems cells could replace neurons.
– John
Apr 24, 2020 at 4:32

Another possibility is that if you use grounded hibernation tech, you'd need to wake up so that your body could self repair every so often. Even if the system is perfectly shielded from the outside, the body has naturally occurring radioactive atoms that cause damage, which means you'd probably need to thaw out every fifty or so years. If this was done properly in shifts you could also use it to make sure the ship was running without problems as a bonus.

In terms of the AI side of things, Stuart Russel has a potentially interesting solution to the control problem that essentially says that AI should be constructed as a perfect altruist that never assumes that it knows better than a person, and will rely on people as its basis for what they want and need. Having an AI like that operate without people could potentially cause problems.

It would take hundreds of years to prove that the cryostasis works for hundreds of years. ("Our simulations say it will be perfectly safe for 5000 years" "They said that about the nuclear jet pack, and look what happened there. Simulation is not enough.")

I suspect the only way to keep your insurance valid will be to bring patients round after every X years (X being the guaranteed safety rating of the device) and check everything is fine, update them on current events etc. They only need to be awake for a day every few decades, so it won't have much effect on their journey overall. If anyone wants to volunteer for untested, unknown-safety hibernation longer than the warranty allows, that's up to them, but it's at their risk and no refunds if you end up permanently popsicled or messily thawed.

• – PTwr
Apr 22, 2020 at 21:22
• This answer easily passes the sniff test with no further handwaving required. Apr 24, 2020 at 0:33

Why do people spend months lying in beds despite being healthy?

Why do people spend months living in places like an Antarctic base or a cave hundreds of meters underground?

Why do people spend years of their life playing with radioactive substances that will later kill them through that radiation?

For the same, simple answer: intellectual curiosity, a.k.a. science.

You can automate a lot of functions, but you need a trained human mind to recognize interesting events and investigate them.

Observing the depths of space without the bother of a close-by star is a unique opportunity for many observations. It makes sense that scientists would want to take the chance of using such a once-in-a-lifetime laboratory.

• "Why do people spend months laying in beds despite being healthy?" Who does that? Apr 22, 2020 at 13:25
• "Why do people spend years of their life playing with substances that will later kill them through their radiation?" What sort of "radioactive substances" are you referring to here? Apr 22, 2020 at 13:25
• @AsteroidsWithWings, there are volunteers participating in study about microgravity who spend month in reclined bed to simulate the physiological effects of microgravity. And Marie Curie spent years refining radioactive materials to discover radium and polonium and to be later killed by radiation. Her notebooks are still so radioactive that are stored in lead boxes.
– L.Dutch
Apr 22, 2020 at 13:29
• Thanks. You may consider adding these examples to your answer. They are not common things that people do. Apr 22, 2020 at 13:58
• Would you rather spend your life looking at space with no star nearby, or exploring and colonizing a new planet? Apr 23, 2020 at 16:11

Wakeup time.

If it takes several hours to several days to wake people up from cryosleep, then even though the odds of something requiring human intervention are low, the fact that if something did happen everyone would be dead before a human could be woken to fix it would be a good incentive to keep emergency crews awake through the whole trip.

You'd rotate them through so that nobody gets too much older than everyone else.

"Earth Star Voyager" is a pretty good two-part serial with a similar setup if you can find a copy. They have an AI that can run the ship, but they run shifts of human crew to deal with unforeseen circumstances. (And it's a good thing they do because there's a plot afoot, obviously.)

Two possibilities come immediately to mind:

1) Hot bunking. If there are more people than cryopods on the ship, then someone has to be thawed in order for someone else to go into stasis. (One story possibility this presents is if the excess population is simply a small maintenance staff, they could become a hereditary caste of protectors/guides as they rotate in and out of stasis for the centuries of the journey, while the bulk of the population remains frozen for the entire trip. What happens when they arrive, everyone wakes up, and the proud caste of protectors expect to be honored for their generations of service, but the ever-sleepers still see them as nothing more than glorified janitors?)

2) Freezer burn. Long-term, uninterrupted cryostasis could have a number of negative side-effects which cause permanent damage to the body (or mind, especially if you take a dualist view in the story) and/or make it less likely that the sleeper can be successfully awakened.

• Yup. Freezer burn was the first thing that leapt to my mind. Apr 24, 2020 at 17:51

A couple points:

1. Cryopreservation of humans is not some far future distant dream. It's a modern reality, so you don't need distant future for that. (See: Alcor)
2. In many states, there are fully AI run trains. However, they require a human conductor anyway, as basically someone who can pull the 'stop' in case a situation unforeseen by the AI's designers comes about. It's basically a safety check.

So this is a story that could easily be set a few decades from now. So combine these two, everyone is specialists in different things, so people would be continually cycled through, every so often, so that someone can just be there to 'pull the emergency lever' if need be. On a massive ship of thousands, it could easily just be a couple people at a time awake, and everyone on the ship gets one shift during the thousands-years journey.

• I looked at Alcor and all they do is the easy part: "the freezing of human corpses and brains in liquid nitrogen after legal death, with hopes of resurrecting and restoring them to full health in the event some new technology can be developed in the future." Wake me up (from my cryostasis) only after the future invents a safe way to do it. Apr 23, 2020 at 10:20
• Just to emphasize a bit of that Alcor quote: "with hopes of resurrecting and restoring them to full health in the event some new technology can be developed in the future." i.e., "We aren't able to revive anyone yet, but we hope that, maybe, someday, we'll figure out how." Apr 23, 2020 at 10:38
• @AdrianColomitchi This is not entirely accurate. They have not resuscitated any humans because they are still perfecting the process. They have, already, resuscitated complex living organisms (specifically one of the researcher's pet dog). The dog was able to continue living after. However, it did incur several handicaps from the process. They won't thaw humans until they have addressed what causes those handicaps. But they could if they were so inclined (fortunately, they're not.) Apr 27, 2020 at 21:35
• @DaveSherohman Se my comment to Adrian. Apr 27, 2020 at 21:36
• They have, already, resuscitated complex living organisms (specifically one of the researcher's pet dog). That would be an interesting link in your answer. Apr 27, 2020 at 21:38

What does it actually mean if cryosleep is 99.99% save?

Here is a simple answer: if you put 10000 people to sleep, 99.99% of them wake up okay. This implies that one of them does not. How many people do you have on your ship? If it is more than 10000, you better have some extras.

A slightly more complex definition: If you cryo-sleep for 1 hour, you have a 99.99% chance to wake up okay. If you sleep for two hours, your chance to wake up okay is 0.9999*0.9999 = 0.9998, so, 99.98%. If you sleep for 5 hours, your chance to wake up okay already decreased to 0.9999^5 = 0.9995 (99.95%). If you sleep an entire day, it is 0.9999^24, so 99.76%.

Sleeping an entire week (7 days), means you are okay with a chance of 98.3%. Your chance after an entire year (365 days) is 41%. Less than a coin flip.

Question is what happens to the math if you wake up in regular intervals? When I do the calculation like this, it does not change anything. The probability to survive your first 365-day interval is 41%, your probability to survive two 365-day intervals is 16%, same as for a 730-day interval (two years!).

I would consider it reasonable to say that the 99.99% only apply, say, for the first month: Being in cryosleep for one month is 99.99% save. Being in cryosleep for two month is only 80% save. Now if you wake up in between, you have a probability of 99.98% to be okay overall. If you do not wake up in between, your chance is just 80%.

You can design the probabilities to fit your plot, and use the answers others gave to argue why longer periods of cryosleep are more dangerous.

# Lonely AI

The AI is afraid it will get lonely and has insisted that it have human company during the trip.

Alternatively, the designers of the AI are worried it will go insane over the years without human input to ground it.

Either way, you need some people awake to keep the AI happy and sane, and you need more people awake to keep the first people happy and sane.

And, as others have pointed out, you want everybody to age at approximately the same rate, so they take shifts.

It's common knowledge that all living beings have a soul. This is why, despite all technology, actual life is still a kind of magical mystery that we cannot reproduce. A living human and a dead human have the same amount of atoms. Despite this, we are, for some reason, unable to transform one into the other (well, it does work in one direction!).

The reason for all that is that your body is not just flesh, or biochemistry, but also an anchor for what we call, in lack of a better description, the soul. The body connects to the soul via a kind of elastic bond. We do not know how exactly it works, but we do know from observation that it is that way.

You have probably already had the chance to witness it yourself. When you have a bad hangover, you feel like you're standing besides yourself. You literally are! As your body is not functioning properly, it doesn't do a particularly good job at anchoring your soul.
Luckily, the elastic bond is quite durable and flexible, so no bad things happen even when you travel in an airplane with a bad hangover. The trip truly isn't going to help with the hangover, but eventually, after a good night's rest and a good meal, your body will pull its soul back into place.

Now, a starship is another story, it travels at stellar speed and distances are, well... on an astronomical scale, literally. If you stay in stasis indefinitely, you will eventually, after a few hundred million kilometers, reach a point where the bond ruptures. That's bad because your body will no longer be able to pull your soul into place once you are woken.

Thus, it is necessary to wake before that happens.

• I like the idea of cryosleep having an impact on the soul, but your scenario implies that the souls are somewhat anchored somewhere and that by moving too far away while unconnected you would rupture the link. Why not simply make the soul "rot" when it is uncoupled with the body for too long?
– Hoki
Apr 24, 2020 at 10:12
• @Hoki: That would probably work, too. But I deemed it a bit too simple, too profane. After all, soul rot is not more spectacular than dementia, is it? The idea of a functional body holding the soul in place, and that being a problem when you move too far away (millions of kilometers) struck me as innovative and funny. One might create additional plot twists around the length of stasis (and thus the time it takes the soul to be fully back in place), too. It might also give an upper limit within a particular time, but not an absolute limit with rest in between (as opposed to rotting). Apr 24, 2020 at 21:44
• It's also nice insofar as you can have things happen that are hard to explain or that are logical fallacies (or appear to be to fans, later), and you can wave them away with "We do not know why this happened, the soul is a thing we do not understand fully yet". And you know what, there's nothing one can object to that argument (other than for a perfectly good scientific explanation, which you can dispute and submit to logic checks). Apr 24, 2020 at 21:46
• Gotta be careful suggesting fantasy answers in sci-fi scenarios. Lots of sci-fi fans are hard sci-fi fans, and this wouldn't fly with them (especially since CERN tested and disproved the existence of souls a few years ago). However, an easy (and realistic) alernate is that the cryonics or radiation shielding isn't perfect in the setting, and cosmic radiation, although mostly blocked, still slowly degrades the body and mind, so an occasional wake-up so the body and mine can heal is needed. It'd have the same effect without the fantasy cliche'. Apr 27, 2020 at 21:55
• @liljoshu: True, though the Q isn't tagged as "hard scifi", so I'd say it's allowable. Also, I wouldn't necessarily place the concept of a "soul" necessarily in the fantasy realm, and incompatible with "scifi" (think of many Japanese scifi animes). I wouldn't necessarily say the concept of a "soul" is fantasy because if we're being honest, we have no idea what it is. Scientists, physicists in particular, tend to suffer from an unbearable hubris in which they believe that they know everything and that truth is what fits in their little model. What they don't realize is... Apr 28, 2020 at 9:32

Without nanotech-repairs you would need to wake up periodically anyways.

Just because you're frozen doesn't mean that the you are invulnerable. The cumulative radiation released from the radioactive decay of elements in your body, and the materials your ship are made of, and just anything that leaks past your shielding from the cosmos can be significant.

You need to awake periodically so the body's mechanisms can repair itself.

Legal reasons

In the year 2200 the media mogal and wealthiest person alive, Lucy Portabello, entered cryosleep to avoid dying from her incurable cancer.

Over 200 years later her decedents launched a legal challenge against her estate. They claimed that the cancer remained incurable, and probably would be for all time. Thus Lucy was essentially already dead and her wealth should be passed to her inheritors.

The legal battle was long and protracted, and ended only when Lucy's estate went bankrup. As a result a new law was passed by the Supreme Government of Humanity United (a totally non-fascist government for everyone everywhere and at all times - Praise the Leader!). This law stated that any person to have been in cryosleep for a continuous period of over 200 years is considered legally dead.

As a conseqeunce all colony ships revive their sleeping crews from stasis at least once every 200 years. The ships food/oxygen/water recyling can only keep up with part of the crew waking up at a time.