[pavel_lishin]

edit: Nevermind - you're totally right, and I'm totally wrong - I didn't think it through, and didn't realize that since relativity helps you less and less as you decelerate, the amount of fuel required does increase hugely if you actually want to stop at your destination.

Unedited original post follows:

> to actually stop yourself you need an exponential amount of fuel, because you need to accelerate half of it just to stop yourself

Hang on, I don't understand this part. First, you're right, I didn't think about deceleration - but that only doubles the trip length at most, since you have to accelerate halfway, then decelerate halfway. And probably not even exactly that if you're carrying your fuel, since deceleration will be a little bit easier - you've burned some gas, so there's less mass to push around.

And you're right, I didn't think about the drag - but that actually works out better for extragalactic visitors! There isn't as much stuff there to stop them when they're taking off, and once they hit a galaxy, it actually helps them decelerate. (Again, my dreamy eyes are ignoring the practical hazards of this "help" which might just turn them into a fast moving gas cloud.)

[readerrrr]

I'm not talking about the time it takes, but the mass of the fuel. Which is more than just doubled.

That is what equations tell you: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.h... ( scroll to How much fuel is needed? )

[pavel_lishin]

Argh, thank you, you're right, I edited my post to reflect that.

I wonder if you could accelerate the whole way in a giant ship, and only slow down a tiny capsule at your 'destination' - maybe just a tiny self-replicating robot factory and some data storage. Decelerate that, land it, have it build you a new body, some tools, etc., etc.

[logfromblammo]

Essentially, any civilization that developed outside of galaxies would need to invent one of two technologies.

The first is the technology to support their culture for a very long time without the support of a nearby star. In that case, there is no particular reason for them to visit galaxies at all. They simply pick a direction and go. Galaxies would be of no particular interest to a civilization that doesn't even need one star, nor would they need to travel at particularly high accelerations.

The second is a propulsion technology that does not require reaction mass (or reaction energy, as with laser propulsion). You would have to devise a way to move something without throwing something in the opposite direction. Thanks to the equations involved, trying to get a long distance away with decent speed using only chemical rockets basically means your ship will start the trip as more than 99.9% fuel by mass.

That is what prompts the imagining of alternate propulsion technologies, such as the Orion nuclear rocket, solar/laser sails, Bussard ramjets, slingshot orbits, and magnetic braking loops. Accelerating the fuel that you will later need to decelerate is a huge problem just for inter-system travel; you can't even bother with it for inter-galactic travel.

[comrade1]

But they have a star. These are stars outside of a galaxy. Probably with planets.

[logfromblammo]

If they want to travel anywhere beyond their own isolated system, they would either have to figure out how to leave their star behind or to bring it along with them.

One of these problems is much more easily solved than the other. Either way, that civilization would then have no particular use for galaxies as a travel destination.

[kijin]

Couldn't a galaxy be an attractive travel destination anyway, just like a large city can be an attractive travel destination for country folks? Not necessarily as a place to harvest resources, but as a place to go sightseeing and interact with foreigners.

[logfromblammo]

The short answer is no. Space is really, really, really, really big.

Tourism usually requires that you be alive when you finally get there. At that scale, if you chose to visit even the closest galaxy to ours, not only will you be long dead and thoroughly recycled when your vessel arrives, but the passengers that disembark to snap a group photo might not even be considered Homo sapiens any more.

That kind of commitment can only come from existential necessity. Any visitors to a galaxy that came from outside of one would undoubtedly have a technology that allows travel without actually traversing the intervening distance.

[kijin]

You're assuming that those lurking planets harbor forms of life that even remotely resemble us.

Imagine an alien lifeform with an average lifespan of several million years, perhaps the size of a mouse (not much mass) and with extremely slow metabolism (not much supplies needed for travel). For them, traveling to a nearby galaxy at relativistic speeds (only a few years from the traveler's perspective) might be seen as little more than a nice long vacation. Sure, a dozen lifespans might have passed by the time they get home, but maybe they don't care because they don't have children like we do and their civilization doesn't change much. "They released the Galaxy S9 already? That's crazy! Three new models in a billennium!"

You don't even need FTL transportation if you can afford to spend a few eons strapped to a seat.