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Comment by pavel_lishin | Hacker News Reader

I don't think you even need generation ships, if you can maintain a constant acceleration. And with constant acceleration, going to our nearest potentially habitable neighbor isn't that much further, subjective-time-wise, than it is to Andromeda.

Assuming acceleration of 1g, from here to Alpha Centauri, it would take 6 years objective, 3.5 subjective. To the center of the galaxy, it would take 27,000 years objective, but only 20 years subjective. Magellanic clouds? 162k years objective, but only 23 years subjective! 28 years to Andromeda.

Fuel may be in short supply, but relativity really works for you - ignoring abrasion and impacts, if you can get to one galaxy, you can get to most others in your neighborhood.

(All calculations derived from http://spacetravel.nathangeffen.webfactional.com/spacetravel..., because I'm too lazy to do it by hand.)

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And the last paragraph is the main problem. The ship would get torn apart from lone atoms and to actually stop yourself you need an exponential amount of fuel, because you need to accelerate ( much more than )half of it just to stop yourself.

Even if you protect yourself from relativistic atoms, they still create a drag. And that happens before you reach 0.99c.

Yeah, the energy issue is really, really tough. Stars are very, very, very freakingly far away.

I'm in California. If the whole Earth was this 2mm breadcrumb, then the Sun would be a soccer ball 25m away (across the street). Speed of light would be the speed of a running ant. Uranus would be a peanut 1/2km away.

And the nearest star would be another soccer ball somewhere in Greenland.

It. Just. Boggles. The. Mind.

http://florin.myip.org/blog/i-had-no-idea-just-how-big-solar...

I look up at the sky often. Those specks of light are so incredibly far away. We just need new physics, otherwise we'll never get there.

I'm an amateur astronomer and telescope maker, on a quest to see what's the biggest aperture that an amateur could build working alone. I can't go to the stars, but I can bring them a few hundred to a few thousand times closer to the eye.

I have to say I love the "speed of a running ant" analogy. It really emphasises the size of the universe.

But can an ant run 25m in 8 minutes? I honestly can't imagine how fast an ant runs. That would be a foot in 4.8 secs - that's a fast ant actually. But I guess close.

> It really emphasises the size of the universe.

Here's another comparo:

If your average galaxy was the size of a coin, the size of the observable universe would be on the order of a large town.

50mm/sec - yeah, it would have to be one of those bigger ants I saw sometimes outside in the summer, hiding in crevices in the sidewalk, while I was growing up in Eastern Europe.

http://www.asu.edu/clas/sirgtools/ecology-1991.pdf says that Pogonomyrmex rugosus travels at 0.1914 * T - 1.983 meters per minute, for ground temperatures T (in C) between 20 and 40. This is roughly 3.8 m/minute or 60 mm/sec.

The equation for Messor pergandei is 0.0878 * T - 0.1724 or roughly 40 mm/sec.

So 50 mm/sec seems quite possible.

I have just realised this conversation has become about the average speed of an unladen ant, European or Otherwise.

Both laden and unladen, in the paper I linked to. ;)

Just to point out a design assumption here, saying that an exponential amount of fuel is required is making 'the rocket assumption': that both the mass and energy required for momentum change are carried with the vehicle, and are expended upon reaction mass.

For travel between star systems, these assumptions do not need to be true. For deceleration, it makes more sense to transfer the original kinetic energy out of the vehicle's movement than to expend even more energy on accelerating reaction mass yet even faster in the original direction of travel. Remember that you have a whole star system of reaction mass at your destination. There may be engineering challenges, but there is no physics reason why you couldn't expend your original energy in "pushing off the sun", and then recover it by "pushing against the new sun" when you arrive. Depending upon efficiency, this could then leave you with the energy for another flight (perhaps home).

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.)

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? )

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.

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.

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

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.

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.

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.

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.

Why spend eons strapped to a seat, fixated on your destination, when you can play shuffleboard on the cruise ship while you wait? You are just re-describing the first of my two possibilities--the species so well-adapted to space travel that they never actually need to stop anywhere.

And if they don't need to, they probably won't. If you lived in the country, and wanted to visit the city, you might do so frequently if the trip cost you 15 minutes and $10. You might never do it at all if the trip took 50 years and $100billion.

Space travel is more like the latter than the former.

If, on the other hand, travel to anywhere on Earth cost you 1 second and $0.01, you might just visit every city. That's why I say that any non-galactic visitor to a galaxy is more likely to have a kick-ass travel technology. It's a purely time-based argument, and has nothing to do with any property of the species that has it. They would simply spend far more time at their intended destinations than traveling between them.

Yeap. Relativity and the speed of light are actually not a limitation for space travel, despite popular belief. Relativity actually helps you get there faster - in rocket time. It only takes longer for those left behind.

But the real problem is the stupendous amount of energy. Even with total mass conversion, you'd need a bazillion tons of mass to get to Andromeda. See the links posted below.

We need new physics.

You can't accelerate the mass of even a single human at 1g for 6 years using current physics and a feasible ship design. You'll need generation ships to get anywhere.

Though I do like some scifi stories I've read where the first humans went our on generation ships and in the meantime a new physics was developed that allowed those target stars to be colonized hundreds and even a thousand years before they arrive.

But developing a new physics presumes much.

My pet peeve: The colonists in those scifi stories are douchebags. If they had the technology to overtake the first explorers by a factor of thousands, why didn't they stop by to offer their ancestors a ride on the newer, faster ship?

You make it sound like constant acceleration is easy. Where's the energy to maintain 1g for 20 years going to come from?

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