Something about the extrapolation of events millions of years into the future reminds me greatly of The Last Question by Isaac Asimov (not to mention probably my most favorite Imgur comic about it).
You know, I think it's about time we got this ball rolling.
FaceDeer: ChatGPT, how can the net amount of entropy of the universe be massively decreased?
ChatGPT: The net amount of entropy in the universe cannot be massively decreased according to our current understanding of the laws of thermodynamics. The second law of thermodynamics states that the entropy of an isolated system tends to increase over time. Entropy can be thought of as a measure of disorder or randomness in a system. In a closed system, such as the universe, the total entropy can only remain constant or increase.
While it is possible to reduce entropy in a localized region or system, it always comes at the expense of increasing entropy elsewhere. For example, we can decrease entropy in a living organism by expending energy to maintain order and structure, but in the process, we release heat and increase entropy in the surrounding environment.
The concept of decreasing the net amount of entropy in the entire universe on a massive scale is not supported by our current scientific understanding.
I've seen a lot of discussions on this kind of thing in the more scientifically-oriented science fiction subreddits, such as /r/IsaacArthur (currently blacked out as far as I'm aware). There are two main radiation-related concerns during long-duration sub-light travel. One is the damage done by cosmic rays and the potential radiation from whatever star drive or power source your vehicle is using. Long-duration trips will require some pretty hefty radiation shielding. Fortunately that's "just engineering", put a couple of meters of water in tanks around your important stuff and that's probably going to be good. For very long duration trips you might need to occasionally strip down and rebuild exposed parts since radiation will slowly degrade the materials they're made out of. That's more simple engineering.
The other problem is specific to "sleeper" ships, in which the passengers are frozen for the journey. The human body has trace amounts of potassium 40 and carbon 14 in it, those naturally occur in the environment. Normally this is no problem - they decay slowly and the body has self-repair mechanisms that deal with the tiny amount of damage they cause. But a frozen body isn't doing self-repair, so the damage accumulates over time. A human body will end up with a fatal amount of radiation damage after just a few thousand years frozen.
Granted, being frozen is going to be tremendously damaging to the human body as well. So it could be that by the time we can freeze and thaw people without killing them we'll have the tech to just fix that radiation damage as well. In a pinch, we can thaw the passengers every thousand years or so and give them some time to recover before re-freezing them. It'd be kind of a miserable experience subjectively since you'd only be aware of the time you spend recovering from radiation poisoning over and over, so perhaps best to keep the passenger sedated throughout that process. There's nothing else they'd need to be doing at those times anyway.
For journeys that could take 100k+ or millions of years though, I think no amount of preservation will allow any living organism to be reviveable at the destination, not even in embryonic form.
The only feasible way to establish continuation of life from Earth may be to store DNA and cellular molecular biology data in digital form, and artificially reconstruct fertilized eggs at the destination from raw materials. Even then, corruption of the data via cosmic rays would have to be mitigated, by having many copies of it and comparing frequently, with periodic refreshes of the storage medium.
We could digitize (or otherwise store) human consciousness, print new bodies at the destination, and install those consciousnesses in them. The idea is touched on in Exception. I'd guess the number of variables that come into play for this are pretty large, though.
I don't give a unit to that time as it doesn't matter.
Although listed in years for convenience, the numbers at this point are so vast that their digits would remain unchanged regardless of which conventional units they were listed in, be they nanoseconds or star lifespans.
Don't want to spoil anything for you if you're reading one a day so heads up
My favorite entry so far is that theoretically as the earth becomes totally uninhabitable due to increased size and temperature of the sun mars becomes more and more habitable
The great migration from Earth to Mars, similar to how we migrated from Eurasia to America. Hopefully an ice bridge also appears to facilitate the transfer
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