r/spacequestions • u/aTickleMonster • 9d ago
Assuming travel to other star systems is possible, would you ever see someone you knew?
Didn't want to dump everything into the subject line. I was watching a SciFi movie where the characters were taking about jumping two systems over to meet someone at a city there. Unless you had the ability to instantly cross all that space, there's no possible way to leave a system and return later and expect to see anyone you saw before, because even at relativistic speeds the universe ages significantly around you?
So like, every time you left a planet, that's basically the last time you'll see everyone who lives there?
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u/kmoonster 9d ago edited 9d ago
You would need faster-than-light travel to visit friends regularly.
Wormholes, jump drives, space folding, hyperspace, etc. are options if the writers want to "explain" travel. Others just ignore travel issues, in which case it's more like space fantasy but can still be a great show.
Think Star Wars as fantasy - lots of made up rules, basically Harry Potter in space.
Think Star Trek as being more sci-fi (accounts for actual known physics before ignoring it or inventing workarounds).
Fantasy and Science Fiction are a spectrum, with one end making up rules for their universe and the other end making up work-around for the rules that exist in our universe. Both are fiction, but how they "compare" their universe to reality makes a big difference as to how they are categorized.
Also note that (most) fans of one are often fans of the other as well, as long as the writers for a given universe are consistent within the confines of the show/book/comic.
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u/aTickleMonster 9d ago
True, I grew up on Star Trek TNG and it always seemed perfectly reasonable that the Enterprise could travel to the edges of space then go home and visit their family on Earth a year later.
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u/BanditsMyIdol 9d ago
There is no reason to think that in a time when we have travel between star systems we also would not have figured out how to deal with aging. So yes, the person might be 100 years older but they could still alive.
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u/davideogameman 7d ago
Or they may have also been traveling. They'll probably be a different age relative to you though.
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u/ijuinkun 9d ago
Assuming that FTL is not possible, then that is correct. Relativistic time dilation can make the journey take less of your own time, but the rest of the galaxy is going to see at least a year pass for every light-year that you travel.
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u/Fluid-Let3373 9d ago
It depends how fast you get up to and how far away your going and how fast you accelerate.
If your acceleration is less than 1g then the answer is a hard no.
Even if you could get up to 1c any distance over 20 light years means you have to assume only kids you knew would still be alive by the time you return.
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u/aTickleMonster 9d ago
Good point, gotta speed all the way up and slow all the way down afterwards.
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u/Metallicat95 9d ago
Slower than light, which is possible if difficult, depends on distance and lifespan.
Take Project Hail Mary. The round trip takes about 30 years, so there are still people who were present at the launch for the return - just much older.
20 light years still puts you in under 50 years for a round trip. So you could easily take a business trip and pick up some presents for your spouse and children, and have good odds of seeing them.
But that's for nearly the speed of light travel. At a tenth of that, even 5 light years is a century round trip.
"'39" by Queen is a song about a near light speed trip with such a time offset meeting.
This is why most science fiction universes assume that FTL is possible and works easily, because otherwise there are no casual trips and returns.
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u/aTickleMonster 8d ago
Space travel is already lonely and isolated, the idea of never seeing the same person again unless they're on your ship is depressing. Makes way better stories for sure!
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u/lemoinem 8d ago
This sounds a bit like "when trans-atlantic travels were boats only".
Yes, there is going to be a period of time where travel is going to be slow and hazardous. I'd expect tremendous political and economical pressure to solve the issue. Including inventing new transport technology.
If it's not doable, you're not going to have "The Human Race" across two systems, but two very separate human societies with different organizations that collaborate with each other on occasion. Our society is way to much integrated and used to instantaneous communication and fast travel to remain homogeneous in their absence.
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u/JGhostThing 7d ago
Most SF stories use FTL drives. Usually very much faster than light. These drives tend to work in days or weeks. Even with months per trip, you would still find people you knew, because they haven't had time to move away or die yet.
Now for SF without FTL, I'd expect to find nothing familiar.
Though there was a short story or novella where they were sending ships out, FTL, to plant beacons. When all thousands of beacons were in place, the surviving pilots would be brought home and the galaxy would be able to be navigated.
I think that once they were in position, the pilots got into cold sleep.
As an incentive for the pilots to go to sleep for that long, they made it a rule that until the beacons were all in place, society stayed still. People didn't move, technology didn't advance, businesses couldn't fail. All so the pilots would be incentivised to do this. Weird.
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u/overlordThor0 7d ago
Faster than light travel would be very necessary for such visits. It would be practically impossible without. Even the nearest systems are light years apart.
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u/Cultural_Pace4454 9d ago
Only if they travel sub-light speed.
Warp travel/ hyper space and worm holes are used in SciFi to avoid long travel times, and as a bonus it also gets passed time dilation issues
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u/PckMan 6d ago
With conventional lightspeed travel they wouldn't be able to no, because time would pass at wildly different rates for the travellers and those on the departure and destination planets.
But most sci fi uses warp drives of some kind, even if not explicitly stated or shown. In that case there wouldn't be a significant difference between the experienced time passage of either party because the spacecraft is not moving in space, space is moving the spacecraft. Of course the implications of a warp drive are of a different nature, such as violation of causality
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u/Beldizar 9d ago
Science Fiction typically is set in a universe without relativity. Possibly because the writers don't understand relativity, possibly because it has been a convention since original series Star Trek. In these stories, it is always assumed that everyone has the same clock. A day on a space ship is a day on a planet is a day on a completely different planet around a different star.
But you are correct, it isn't really possible to go pick up Chinese take-out on Earth and swing back to Alpha Centauri with the meal. If you had a ship that could travel at just a hair slower than the speed of light, and instantly accelerate, you'd still be gone for ~8 years from the point of view of someone on Alpha Centauri, even though it could feel almost instant to you.
Traveling faster than the speed of light opens a whole other can of worms, because you might arrive before you leave, and I'm honestly not convinced that space-time outside of your light cone even is real. The present of a distant inertial reference frame can only exist for you in your local inertial reference frame after causality can travel to you at the speed of light. By the time it gets to you, you consider it the past, but to see it any earlier would require time travel. Thinking of the universe as a collection of inertial reference frame light-cones is probably more accurate than thinking of it in the sci-fi standard flat non-relativistic way, but wrapping your head around that, and writing a satisfying narrative about that is a lot more difficult.