# And I think it's gonna be a long long time... (relativity question)

So I've got a bunch of freeze-dried colonists traveling in torpor to a solar system 55 light years away at 92% c. From an outsider's perspective it takes them sixty years to get there, but how long would it feel like for the people onboard the ship? How long would they be in suspended animation, and how many countermeasures would need to be put in place to regenerate lost muscle and generally keep them from looking and feeling sixty years older once they reach their destination?

I'm not sure of the actual numbers, so I thought I'd come looking for help here :)

• I should down vote you for getting that song stuck in my head... Jul 25, 2016 at 21:20
• youtube.com/watch?v=A0qm8nq8RcA Jul 25, 2016 at 21:21

According to this calculator if the ship were traveling at 92% speed of light and an observer on earth said 55 years had passed, then on the ship it would seem like 21 years had passed and if it's closer to 60 years (rounded from King of Snakes' answer) then on the ship it would be like 23 years. Roughly...

• Wait, aren't they asleep? People will be kept in a coma on the trip, so it'll be very less time biologically (a few years at most). The robots will automatically wake them up at the end of the trip, so 21 years is somewhat wrong Jul 25, 2016 at 15:53
• Whether they are sleeping or awake they time inside the ship will be 21 years. Whatever is made up for cryonics is none of my concern. Jul 25, 2016 at 16:28
• @KingofSnakes: If nothing else, the robots and the cryonics systems have to be designed to run themselves without human intervention for 23 years. How many devices do we have today that you can leave alone for two decades and expect them to run perfectly? Jul 25, 2016 at 17:44
• @MichaelSeifert : Thats true, but in this question they can travel at .92c, while for us the fastest is 0.067% of speed of light (google search, might be wrong) . Meaning they are WAY more technologically advanced. Making machines last shouldn't be a problem for them. Especially when human lives depend on it. Jul 26, 2016 at 4:38
• Actually, its not even true now. Industry makes products that have Built-in Obsolescence so that they can sell more, but if we wanted to we could make just about everything we use last virtually forever, failing that, at least as long as our lives. Jul 26, 2016 at 4:54

You are asking multiple questions (how long will they be in suspended animation, how much time passes for them and countermeasures against muscle atrophy).

Technically if they are 'freeze-dried' they won't feel the passage of time. They will go to sleep, and wake up. If they are travelling at .92c and have to travel 55 light-years - then it will take 59.782 years for them to reach (according to others). For them, they won't know about it because they are asleep.

If they have the technology to travel at .92, then preventing muscle atrophy should be easy. Also since they are in a coma they age more slowly. Though I would consider using face surgery :)

• I'm not sure of the scientific basis, but normally in science fiction "freeze-dried" astronauts are in suspended animation where from their point of view--mentally and biologically--no time passes. This would negate the need of your suggested cosmetic surgery. Again, I don't know how scientifically reasonable this is, though. Jul 25, 2016 at 15:10
• @NexTerren : You could freeze them, but its easier to put them in a coma or something (where some time will pass) Jul 25, 2016 at 15:11
• That's kind of what I was asking. I wanted my hibernation to be more realistic and the kind of hypothermic torpor we're currently playing around with would (I assume) not do much to halt the aging process. The solution I'm currently fiddling with is preserving their semi-frozen bodies in some sort of regenerative fluid or extracellular matrix that promotes muscle growth and tissue regeneration while they're unconscious to keep them looking and feeling younger. Jul 25, 2016 at 15:14
• @Z.Schroeder : All I know is that they can travel at .92c. Not much info sorry. Jul 25, 2016 at 15:17

First, you have to figure out how much time passes in the reference frame of the "outside observer", presumably one who's in the frame where the ship started out at rest before turning on the engines. As @King of Snakes calculated, this time is just the distance in this frame divided by the ship's velocity in this frame, or (55 light-years)/(0.92c) = 59.7826 years. Then to find the time in the ship's frame you must apply the time dilation formula $t_0 = t * \sqrt{1 - (v/c)^2}$, where $t$ is the time in the observer's frame, $t_0$ is the time in the ship's own rest frame, and $v$ is the ship's velocity in the observer's frame. This gives $59.7826 * \sqrt{1 - 0.92^2}$ which works out to 23.43 years as measured by a clock on board the ship. So if you have a suspended animation system which can keep someone suspended for at least 23.43 years on Earth, then since the laws of nature are the same in the ship's reference frame as the Earth's, it should work to keep people suspended for the duration of the voyage too (leaving aside external dangers their hibernating bodies might encounter in space but not on Earth like cosmic rays).

As for preventing muscular degeneration while in suspended animation, that seems like it should be a separate question since it's unrelated to the question about relativity, the answer would be the same regardless of whether the suspended animation chamber was operating on Earth or on a relativistic ship.