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The physics of space travel are harsh, and the distances involved even harsher. Plus, unless you want to go screaming by on a fly-by mission, you have to burn all that delta-V you generated to get there in order to slow down again.

The only real solution will be some kind of essentially magical faster than light technology - never say never, but it doesn't seem likely without some staggering breakthroughs in areas of physics we don't even have an inkling about.

Maybe it's the American exceptionalism and internalization of Manifest Destiny as the natural order of things, but the idea of being effectively stuck, as a species, on this one rock for eternity is pretty depressing. Especially if that is the fate, not just of ourselves, but of all life intelligent enough to look up at the stars and imagine walking among them. If the best we can hope for is a Motey-like[1] Malthusian cycle, it's not much to look forward to.

[1]http://amzn.to/2b5I0Ea

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It is possible to leave this rock, but we'll need to think in terms of millennia, not lifetimes. Long term thinking that's also kind of unAmerican.

https://en.wikipedia.org/wiki/Generation_ship

While we wait for research to get there we can play with the other rocks in the vicinity. All those climate engineering ideas could be tried on Venus, for example.

If we could change that atmosphere (engineered bacteria and collision with an ice asteroid to lose some gas and gain some water?) and cool it down a little.. It's not that far out of reach.

> we'll need to think in terms of millennia, not lifetimes

I don't think it's safe to assume the maximum human lifetime is a constant in these discussions.

> The only real solution will be some kind of essentially magical faster than light technology

Doesn't have to be faster than light, if you don't care about everyone you ever knew being dead for decades (or centuries, or millennia) by the time you reach your destination: thanks to time dilation/length contraction, you can travel arbitrarily far distances in your lifetime by going at a large enough fraction of c.

Assuming you've got the technology, of course.

I don't think that's true, I think you're still limited to the range of < c * lifetime, about 390 trillion miles. If you think this is wrong, can you post some links?

Edit: nevermind? The equation here does indicate that length contracts to approach zero as velocity approaches c

https://en.wikipedia.org/wiki/Length_contraction

This page has a nice table for time dilation:

https://www.fourmilab.ch/cship/timedial.html

If you can instantaneously accelerate up to 0.99 c, you end up with a range of ~500-1000 light years before you die (dilation at that speed is 7:1, you have ~80 years of ship time).

So the distance you can travel does depend a great deal on how fast you can actually go.

Isn't it more useful to think in terms of how fast you can accelerate? Doesn't help if you arrive as jelly.

Yeah, that will give a lower, better estimate.

My point was just to clarify that the arbitrary range from time dilation doesn't really start to kick in until you approach c. The simpler, coarser calculation still shows that quite well.

A constant 1g acceleration gives a good general basis for extrapolation. It's survivable (on an acceleration basis). If perhaps difficult to attain.

And if you could attain (and sustain) it, it would bring you close to c within a year (shipboard time).

I should have clarified: the case is sufficiently obvious that there are a bunch of illustrations of how long (subjectively) it would take to travel various distances at 1g constant acceleration.

Ah, there's a Wikipedia article: https://en.m.wikipedia.org/wiki/Space_travel_using_constant_...

And a chart of subjective and observer times to various landmarks: https://upload.wikimedia.org/wikipedia/commons/f/f5/Roundtri...

It takes a while to get going. For round-trip subjective travel times, Prox. Centauri is about 7 years. Galactic center about 42. Andromeda Galaxy, 50 years. Edge of the observable universe, 100 years.

Mind, the folks you left behind would be up nights waiting slightly longer. About 15 years to Prox. Centauri, rather longer to some of the remoter destinations.

But is it meaningful to talk of spending a year or more of subjective time accelerating at 1G?

According to the Wikipedia page you link to, it does not become harder to accelerate a spaceship the closer it gets to c, at least from the point of view of its occupants.

So you've been accelerating for ~300 days (shipboard time), and you're now nearly at c. In fact, you're only 10m/s below it, so you should reach light speed in a bit more than a second.

But we know that's never going to happen, because it can't.

I'm assuming (I don't know, I'm a layman) that that final second-and-a-bit will stretch like taffy, in time-dilation terms. You're already travelling at 0.999999966c, at which for every second of shipboard time more than an hour will pass on Earth. Dilation will only increase the closer you edge to c: at some point, every yoctosecond of shipboard time will equate to many times the predicted lifespan of the universe outside.

It's hard to imagine something terminal wouldn't happen to the ship in that final second, assuming there's anywhere left for it to happen in.

> Plus, unless you want to go screaming by on a fly-by mission

In space, no one can hear you go screaming by.

In any case, terraforming some of the rocks we can reach (Mars, Venus, et. al), while an enormous undertaking, might be a more solvable problem than FTL travel (because the latter is most likely impossible, while the former is merely insanely difficult and impractical). The problem is terraforming projects would likely be multi-century efforts requiring some level of international cooperation, so... yea, we're stuck here.

Basically wormholes/hyperspace/teleportation, or bust.

But I'd think living in orbital or interplanetary space right here in our solar system can be a happy medium, and sustain us for a loooong time to come.

Or just build a big ship and send it with settlers. If it takes 1000 years that's fine

Isn't "a ship where we can live for 1000 years" just the same as a planet?

Yeah, maybe the next big thing isn't travelling to a far away planet but just creating a ship big enough. I had a pretty cool theory about that when I was a kid, funny to see that it may be what actually happens.

Or just move the entire solar system closer.

I mean - crap - if you're doing going to do interstellar engineering, you might as well do it properly.

Supposing that next year we get viable coldfusion and that em drive thing turns out to be actually real somehow it would seem feasible to move the planets, but how would you move the sun? You can't very well put an engine on the sun. Maybe a Dyson sphere, but that requires a lot of material, probably more than is available in our solar system, so we'd need to start significant extrasolar mining operations before we could move it.

It only has to be thick enough to reflect light.

Oh wow thanks, good to know there's a name for this idea with some amount of thought put into it. Might have to binge on cosmic megastructures reading.

How many settlers? 1,000? 10,000? 100,000?

Would you launch adults (average weight about 70 kg) or children?

A founder population of 1,000 wouldn't be unprecendented.

https://en.wikipedia.org/wiki/Population_bottleneck#Humans

The real problem is that an individual can only learn so much and 1,000 can't support the skills needed to run something like a modern civilization.

The first and possibly only settlers will be AIs.

The scientific breakthroughs needed to make an interstellar spaceship are so far away that we must assume they'll be post-singularity tech.

A friend of mine was doing a Masters in Zoology a few years ago, and I briefly discussed the idea of generation ships with her. She explained to me that a foetus can't properly develop in microgravity, and they abort in the early stages. (I can't really remember because it's going back 12-13 years now.)

Accelerating such a ship at close to 1G for half the journey, then flipping it over and decelerating at the same for the other half of the journey would approximate Earth gravity for the occupants. Since acceleration acts by pushing against you, rather than acting against your entire body as gravity does, I'm unsure as to whether it would be an adequate substitute for gravity.

Perhaps someone here knows.

The other option (from the science fiction, anyway) would be to revolve part of the spacecraft enough to generate sufficient centripetal force to approximate gravity. It's become almost a trope since 2001, but I'd imagine there are some significant engineering challenges to actually implement such a thing. How do you keep the drum rotating, without spinning the rest of the craft? How do you engineer bearings that can handle the force, while at the same time dealing with vacuum and extremes of temperature? There's a lot of moving parts, and moving parts wear, seize, and can fail catastrophically...

If the bearings prove difficult, just rotate the whole thing.

You can just spin the rest of the craft as well. No moving parts required.

That increases mass and engineering considerations.

Designing a satellite or space probe is (structurally) pretty simple: you need a central truss or platform which is going to be subjected to very little stress or torque.

Pressurised compartments up the complexity a lot. Leaks are bad. Big, fast leaks are really bad. Even with out gravity, thermal gradients (sunlit side, dark side) present significant stress.

Pressurised compartments under constant 1g strain are even more complex. You've got stresses and strains, mass, things get dropped and bumped inside, etc.

Mass costs money. About $1k to $40k per kg in LEO, though the lower end of that's becoming more viable. Elon Musk and others would like to get down to $200 kg, though I find that unlikely.

Acceleration and gravity are entirely interchangeable. The feeling you get due to gravity is also because of something "pushing against you" - namely, the ground. If you didn't have the ground to push against, nor air to drag against you, you'd be in free fall, which is the same as being in zero gravity.

> Since acceleration acts by pushing against you, rather than acting against your entire body as gravity does, I'm unsure as to whether it would be an adequate substitute for gravity.

I'm far from an expert, but I thought it was a basic tenet of relativity that, without external information, there is no way of telling whether your spaceship is accelerating at 1G or standing on the Earth's surface.

I'm pretty sure steady 1G acceleration brings the subjective travel time within a lifetime for a lot of journeys. I haven't done the math myself, though, just heard it from others. Of course, the technology to apply such acceleration to any reasonable-sized ship is rather fanciful at this stage.

> She explained to me that a foetus can't properly develop in microgravity

How on (off?) earth can anyone know that?

Presumably animal models. Fish, frogs, mice, etc.

https://scholar.google.com/scholar?q=fetus%20development%20i...

That ship would have to be megatonnes of shielding and habitat. Consider that the article describes the entire energy output of North America being used to launch a dinky little probe like New Horizons. We're talking about a dozen orders of magnitude in energy technology needs. This is beyond "cold fusion" and into "cheap matter-energy conversion" like a cheap way to synthesize antimatter or something.

Or go part of the way and stop.

Interstellar space is riddled with bodies. Sol's Oort cloud extends out to a light-year or so, but the fringes of it mix with Alpha Centauri's. It's mainly composed of cometary nuclei and ejected rocky masses, so it's rich in low-mass elements: hydrogen, carbon, oxygen, nitrogen... all the elements of life. The total mass is on the order of tens of times the mass of the Earth, except unlike Earth, all that mass is accessible.

Finding estimates of the separation between objects is pretty hard, but the ones I've seen very from ~5 light seconds up depending on where you are and how big a body you're interested in. About half a light year out, it seems that there's likely to be a decent sized body (10s of kilometres across) within 30 light seconds. Larger objects are less common, but here's an estimate of there being about 10^5 moon-sized objects per star: http://arxiv.org/abs/1201.2687

It's surprisingly hospitable out there, too. The environment never changes, you don't really need to worry about radiation (except for the occasional supernova), and cooling is easy. Travel is easy, too: space is nearly flat, so you don't need to worry about transfer orbits: boost in a straight line, then decelerate and stop. Travel time is solely a function of how much energy you want to put in. Most objects are moving pretty slowly, too, way under a kilometre per second.

You need to be able to fuse hydrogen, of course. Or engineer yourself to use less energy.

It would be entirely feasible to build a ship, travel six months to a year, and then settle on a decent sized body and found a colony there. A few generations later, your children head off again. Eventually your child civilisation has hopped rocks all the way to Alpha Centauri, but of course by then the idea of living near a star would be utterly alien to them.

Personally, I think that when we discover proper alien intelligences, it'll be here: probably in the form of computational nets of bacteria-like organisms, living very slowly and efficiently, spreading via spores from rock to rock in the darkness. So much safer and more reliable than the fast, hot, dangerous worlds down on the edges of the solar fire...

My uncle was awarded a PhD in the late 1970's from Arizona for the technical papers on generations ship written for NASA, so I have some insight to this stuff [0].

The TLDR; for the project was that assholes suck. Many many things are hard to work out for a generation ship, from the correct number of light bulbs to bring with you, to the plumbing, to the risks of permanent 0g. In all, the engineering was remotely possible for the tech at the time, but extreme. A generation ship, ala a giant metal can with people and pipes in it, was engineeringly feasible, but the human considerations made it impossible.

The main issue was that the ship's inhabitants were to be born, essentially, in Alcatraz; a prison they could never escape. As time went on, the crew was expected to become less and less aware of the danger of the vacuum and an accident was exponentially likely to occur. Most highly isolated peoples, like on Rapa Nui or Naval ships, were brutally strict. As teenagers are wont to be boundaries testers, it was expected they would be the most risk to the ship. Add in deteriorating conditions and failure of parts, and you end up with the Stanford Prison Experiment[2] for generations. The project was a human rights nightmare. For thousands of years, potentially, you breed and die in what may eventually amount to a Nazi death camp.

The simplest solution was to make a giant Stanford torus [1] with the land area of between California or Maryland, stick an artificial sun in the center for light and heat, and launch it at your target [3]. Remove any computers or guidance of the Tube, just let gravity somehow guide you into the right place. Oh and make it big enough that some wacko death cult that may arise can't break it apart or kill everyone. Essentially, you launch a mini-Earth Tube, and let them fend for themselves. When the Tube gets into position, you hope they get curious enough to figure out a way to jump out.

Obviously, the mass of the thing alone is too much to begin to think about, let alone the time it would take to create, launch, fly out, orbitally park and decelerate, and finally disembark. Even back in the 1970's, they knew that the rate of tech increase would just make it so that you launch a robot with some zygotes and grow the people when you get there.

In the end, the possibility of a generation ship was deemed too remote to devote more funds towards. Our better natures think that we could live in harmony in the stars, but history tells us that our darker selves always arise. The issue with a generation ship of humans is not the ship, but the crew, as a lot of the time people can be nothing more than assholes.

[0] By awarded, I mean he published the papers, and someone at UA forced him to take a single class for the requirements, so impressed they were at the scholarship.

[1]https://en.wikipedia.org/wiki/Stanford_torus

[2]http://www.prisonexp.org/

[3]https://en.wikipedia.org/wiki/The_Book_of_the_Long_Sun

That's a very cynical view of humanity. For most of human history, people lived in small tribes of no more than 200 people and got along fine. The risk of sabotage is hugely over rated. Large cities survive on Earth every day because most people arent motivated enough to dump poison in the water supply or destroy critical infrastructure. The vast majority of people are not genocidal, and the few that are are terrible at it.

The zygote idea may be possible in the future. We don't have artificial wombs yet, not to mention baby raising robots. Perhaps cryopreservation could work. I believe a rabbit kidney has been successfully frozen and revived intact. But brains are more delicate.

For most of human history, people lived in small tribes and waged incessant tribal warfare over women, land, and resources. Trying to cast prehistoric humanity as some golden age of peace and harmony is disingenuous. And we know for a fact that in the historical era, mankind has been at war virtually every year since the Sumerians started pressing glyphs into clay or the Chinese scratched runes on ox bones.

I don't think there is a lot of evidence to support that. Ive read that war was mostly a product of agriculture, which led to famines and stores of food easy to steal.

But regardless, when hunter gatherers did go to war, they fought other tribes. Internal conflict is much less common. In a group where everyone knows each other and depends on each other for survival, war makes no sense. Groups of humans that didnt cooperate within a tribe would have died out long ago.

Such an environment will hopefully have plenty of resources. At least of food and water and other necessities. Which also reduces the motivation for war and conflict.

There's very little evidence of war (defined loosely as an organized conflict between two polities) before the development of agriculture and civilization. It's worth noting that, in ancient Sumer, whereas most advancements start from the south and go north, defensive walls originate at the north and go south. The best evidence for Paleolithic warfare--which is to say pretty much the only--is a single apparent cemetery (at the tail end of the Paleolithic) where about half the people died with signs of violent trauma.

Or a time stasis device, as described by Niven, Vinge, and others.

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