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mkline55
2013-Jun-28, 04:10 PM
It's Friday. The mind wanders.

You've probably seen the argument that galactic expansion would be exponential. Two colonies become four. Four become eight. Eight become sixteen. Sixteen become 32. And so forth, and in very short time, the entire galaxy becomes overcrowded. Actually, about 44 successive generations would do the trick to reach 1 billion planets. The trouble is, it simply cannot work that way.

Suppose some life form set out to conquer the galaxy. Call it "conquer", "explore", "populate", "terraform", or whatever. I'll stick with "conquer".

We'll need some assumptions. First, assume the technology exists to build a colonizing starship that can survive up to 200 years of interstellar travel. Just for arguments sake, assume the ship can accelerate to 10% of light speed within a relatively short period of time, coast for most of the journey toward its destination, and decellerate to match velocity with whatever target. Vary these numbers to suit your own speculation.

How the passengers are managed, although likely of personal importance to themselves, is inconsequential to the discussion, provided the ship can safely put the passengers onto a livable planet/moon, and that the population is sufficiently sized and trained/trainable to rapidly expand industry and population. Assume each population is capable of doubling every twenty years after arrival, and that the entire industry will be capable of building and populating another generation of starships from the ground up in just ten generations, or two hundred years.

With the entire galaxy to choose from, where would you go first? Well, you have a maximum range of twenty light years. I'd expect travel to proceed first to the nearest extra-solar likely-livable planets.

Let's say there were forty such planets within twenty light years. Also assume that the distribution of planets throughout the majority of the galaxy is somewhat similar. That seems really, really generous, but bear with me. I'll swing the pendulum the other way for comparison.

So you start from day zero and send out forty ships. Maybe you only send one a year for forty years, and maybe there's just a one-day super launch. After that initial launches, there could be follow-up launches to the same destinations, but there just are no additional likely targets. Two hundred years later, those first ships have all landed and began to establish colonies A1 through A40. Assume 100% success. They grow all of the necessary industries for building new starships such as mining, refining, energy management, farming, etc. etc. etc. Over the next two hundred years the population is roughly one thousand times larger than what initially arrived, and has sufficient resources to send a new generation of starships abroad.

So it's been four hundred years, some more, some less, since the first starships left, and now these forty first-generation colonies decide to launch their own colony ships. Where do they go?

For each of the first-generation colonies, about fifteen of the potential forty targets are already populated by similar first-generation endeavors. For those nearest the original star, they would likely find that most of their potential destinations are already populated. Let's say that leaves twenty-five uninhabited targets for each colony. Most of those twenty-five targets are also within range of one or more other colonies. At best, each colony has only a couple undisputed targets. The rest are more or less up for grabs. So that's where the trouble starts. It's where the term "conquer" comes into play.

The most rapid expansion possible would be somewhat spherical. After only a few generations, each additional expansion would only be to a few more destinations. Even ignoring the disk shape of the galaxy, growth practically levels off. One becomes 41, becomes 60, becomes 71, becomes 79, becomes 89. No exponential expansion. At ten additional planets every 400 years, it takes 40 billion years to populate 1 billion planets. Ouch!

Don't like that? Okay. Let's say there are just two viable targets within range of any colony. Ships leave for both targets at year zero. By year 400 both are ready to expand again. Each has only two potential destinations. One is somewhere far away. The other is right back where you started from. So, by generation two, you already have at best a linear growth. One becomes 3. Three becomes 5, becomes 7, becomes 9, etc. Again, no exponential growth. At two new colonies every 400 years, it takes 200 billion years to populate 1 billion planets.

You could make other assumptions. Suppose colonies could grow to full maturity in just ten years. Suppose travel could occur at 1/2 light speed and for as long as 1000 years. Or suppose the destination planets were not all that livable, and it takes 1000 years to reach maturity. You could assume anything. This is just speculation after all. Plus, it's Friday.

Cougar
2013-Jun-28, 07:35 PM
Let's say there were forty such planets within twenty light years.

Actually, we already have a pretty good idea on this. As far as is currently known, there are only 4 candidates within 20 lightyears (http://en.wikipedia.org/wiki/List_of_nearest_terrestrial_exoplanet_candidates), and only 7 candidates within 50 lightyears.

But a sphere centered on an exoplanet 20 lightyears away will have more than half of its volume that is greater than 20 lightyears away from the original launch point. Nice little math problem there!

neilzero
2013-Jun-28, 08:17 PM
There is also a communications delay problem unless the first million colonies mostly exchange data directly with a million other colonies = one trillion 2 way data links. Interstellar space craft will be sent to planets that were colonized successfully many centuries earlier = not necessarily bad, but slows the conquering of the galaxy. Worse unless the pre evaluating of potential colonies is fabulous, many of the colonies will not thrive sufficiently to ever send out another interstellar craft. Then there is Murphy's Law which probably means less than one percent of the galaxies habitable planets will have live colonies after a billion years even with more than one thousand interstellar space craft in route for the entire billion years. With very advanced technology, our galaxy likely has one trillion habitable planets.
The communication net is likely satisfactory if each colony and intersteller craft receives news and data from 1 or 2 colonies and relays to 1 or two colonies and intersteller craft in approximately the opposite direction. This keeps the data links much shorter, but the volume of data is huge from more than a million nodes. Neil

mkline55
2013-Jun-28, 08:37 PM
Don't assume a human species. Still, language would most likely rapidly diverge. Without some stabilizing factor, evolution might be significantly different after only a few million generations. Given a billion plus generations, who knows? Plus, in that time everything the original species knew about the distribution of habitable locations in the galaxy might as well be thrown out the porthole.

eburacum45
2013-Jun-28, 10:58 PM
Don't assume a human species. Still, language would most likely rapidly diverge. Without some stabilizing factor, evolution might be significantly different after only a few million generations. Given a billion plus generations, who knows? Plus, in that time everything the original species knew about the distribution of habitable locations in the galaxy might as well be thrown out the porthole.
This paper by Geoffrey Landis discusses colonisation, but makes quite different assumptions;
http://www.geoffreylandis.com/percolation.htp
not every colony succeeds, and not every target planet is reached. The end result is widespread colonisation, but there are plenty of holes in the galactic empire.


There are many possible models for colonisation, some of which result in an almost geometric growth over time. One possibility is that 'likely-livable planets' are not the prime targets. They may be so rare that none can be found among the local stellar community. By the time humans or their descendants are ready to colonise other stars, they will probably have found ways to exploit the other, non-Earth-like planets of our own solar system; the same strategies will be useful in other systems, and there might be entirely new environments (such as waterworlds) that require entirely new strategies.

If a colonsation effort emerges that can exploit all kinds of solar system, even ones with no planets at all, then colonisation will eventually occur on a broadly spherical wavefront - each new colony will only send missions towards stars that are directly opposite the point of origin, within a wedge of sky that decreases in size the further you get from the origin star. Eventually you will get so far from the origin star that each colony will only have a few stars that it can colonise; but at that point there will be so many colonising efforts that the sphere will expand at nearly 10%c on every side. Instead of colonising stars that are nearby, the ships will travel futher before decelerating; if the outermost colony isn't ready to launch any ships yet, there will always be one a bit further back which is.

Another constraint on this expansion is the thickness of the galactic disk. A colony a thousand lightyears from Sol towards galactic north or south would find that the stars further out are few and far between; colonies two or three thousand lightyears away in these directions will be surronded by large volumes of empty space.

SkepticJ
2013-Jun-28, 11:24 PM
Another possibility would be that intelligent space-colonizing beings could refrain from reproducing exponentially and spreading across a galaxy like bacteria on a petri dish. Mindless bacteria have no conception of the future, but beings bright enough to travel between the stars could perhaps reign in their reproductive imperative in favor for conserving resources for the long term. Entropy, after all, means that there is no truly renewable energy source--not nuclear fusion, not black holes, not anything.

Perhaps galactic colonization would be more of a diffusion than growth? Some growth, of course, but nowhere near exponential, or even linear.

TooMany
2013-Jun-28, 11:35 PM
One becomes 3. Three becomes 5, becomes 7, becomes 9, etc. Again, no exponential growth. At two new colonies every 400 years, it takes 200 billion years to populate 1 billion planets.

The conclusion is not correct. The expansion will grow on an expanding front whose area increases over time. The entire galaxy can be populated at the average speed of growth. Say that speed is 1/20 c, then our 100,000 ly galaxy can be populated within 20 * 100,000 or 2 million years, not 200 billion years.

That's why it's likely that it's already been explored.

I agree that the growth cannot be exponential. It really only depends of average speed and size (to a rough estimate ignoring other factors).

Jens
2013-Jun-29, 12:29 AM
One question: why do you assume that the colonists will want to keep expanding rather than just staying on their new home without building starships of their own?

eburacum45
2013-Jun-29, 01:08 AM
One question: why do you assume that the colonists will want to keep expanding rather than just staying on their new home without building starships of their own?
A good question; not every colony will want to build colony ships, so that could slow down the rate of expansion. On the other hand, if only one colony develops a culture of compulsive expansion, that culture will spread more than the non-expansive ones. A single expansive colony could send daughter colonies in all directions, including back towards the original home planet and beyond.

Martyn Fogg calls expansive cultures 'empires', and non-expansive cultures 'communities'. If it is random whether a colony becomes an 'empire' or a 'community', the expansion could be much more random, and resemble Landis' "percolation" model more closely.
http://www.users.globalnet.co.uk/~mfogg/fogg1987.pdf

But I don't think that the percolation model will apply if the various colonies remain in contact with each other. Assuming that the colonies can all communicate with each other over interstellar distances, the expansive empires will be aware of uncolonised systems and move to exploit them, without needing to disturb the development of non-expansive colonies. Communications between colonies will travel much faster than colony ships, so in a galaxy that has been patchily settled by the percolation method, any colony with working communication equipment will know about uncolonised and unexploited systems nearby, and move to exploit them if they feel moved to do so.

Jens
2013-Jun-29, 03:21 AM
, and move to exploit them if they feel moved to do so.

Using the term "colonize" I can understand, but why "exploit"? How could you possibly exploit something outside our solar system?

eburacum45
2013-Jun-29, 03:45 AM
Obviously the colonists will benefit from the exploitation of the new system. But would the old system gain anything from the new colonies? Very little. Almost nothing would be valuable enough to be exported from one star to another; no matter, not even precious antimatter, would be cheaper to transport from star to star instead of manufacturing it in situ.

The only thing that could be valuable enough to export from star to star is data; colonies could exchange information about scientific research they have carried out, technology they have developed, genetically engineered organisms they have created (in code form), art and other entertainments they have devised; they might be able to transmit entire sentient programs from star to star, allowing them to exchange smart tech and artificially intelligent entities (if such things can exist) at the speed of light, in code form.

I also like to think that they could transmit human-equivalent minds in this way, but this might not be possible for a number of reasons.

eburacum45
2013-Jun-29, 03:54 AM
That is the optimistic view, of course. Pessimistically the colonies could all become suspicious of each other, withhold their best data and technology from each other, and become paranoid about the other colonies. Separated by light-years of time and space, they could never be able to obtain real-time information what the other colonies are doing, and suspect them of developing weapons that could cause mayhem at interstellar distances. Surprisingly enough, it is possible to wage war on an interstellar scale, but it is more difficult to stop such a war when it is in progress, because of the scale of the battlefield. A kinetic weapon bombardment or a Nicoll-Dyson laser is difficult to recall, once fired.

mkline55
2013-Jul-01, 12:49 PM
The conclusion is not correct. The expansion will grow on an expanding front whose area increases over time. The entire galaxy can be populated at the average speed of growth. Say that speed is 1/20 c, then our 100,000 ly galaxy can be populated within 20 * 100,000 or 2 million years, not 200 billion years.
You have to allow time for each colony to grow large enough to want/require another expansion. I was being overly optimistic in assuming 200 years for growth and 100% success. Upon arrival, a colony ship would have to build an entire civilization almost from the ground up. You don't have a mining industry. You don't have a farming industry. You don't have plastics. You don't have roads or railways or airplanes or river barges. You don't have wood. You don't even have tillable soil. You don't have readily available anything except rock and water. You probably have to work for many years just to make the colony livable, then many more years to have a population large enough to build space ships.

TooMany
2013-Jul-01, 06:11 PM
You have to allow time for each colony to grow large enough to want/require another expansion. I was being overly optimistic in assuming 200 years for growth and 100% success. Upon arrival, a colony ship would have to build an entire civilization almost from the ground up. You don't have a mining industry. You don't have a farming industry. You don't have plastics. You don't have roads or railways or airplanes or river barges. You don't have wood. You don't even have tillable soil. You don't have readily available anything except rock and water. You probably have to work for many years just to make the colony livable, then many more years to have a population large enough to build space ships.

Such limitations may not apply at all for an advanced space-faring species. I would presume that with centuries more of advancements in power and robotics, raw materials could be directly transformed into whatever is needed. You are imagining that 24th century human technology would be the same as 21st century, except for interstellar spaceships.

I very much doubt the whole speculation of an interest in biological colonization. Colonization is a pursuit by biological beings who have needs for resources that cannot be satisfied where they are largely due to competition with others. That will become an obsolete problem prior to the capability of interstellar travel. There are political motivations as well, but those are also likely to disappear in a very advanced society.

mkline55
2013-Jul-01, 07:29 PM
Using your earlier 2 million year estimate for galactic domination, why has not even one advanced space-faring species has populated the entire galaxy in the past 5 billion years? The likely reasons I can come up with are 1) no such species, 2) no such technology, 3) no such motivation.

Our sample set is too small to make any conclusions about #1 above. While it is always nice to pretend technology is unlimited, there are practical limitations. Any "advanced space-faring species" has those same physical limits. And while various space programs have managed great things, the propulsion is essentially just an improvement on centuries-old technology. That leaves motivation. But without the technology, motivation will not get any species to even the first star in any reasonable time frame. Perhaps with enough motivation, a species could power a small moon-sized colony ship toward the next nearest star and arrive ten thousand years later, but then there goes the whole timeline again.

Could you explain how the need for resources would become obsolete? Is that because the species most likely dies off?

neilzero
2013-Jul-01, 09:09 PM
Some spectacular advances are needed to get humans to the closest star in less than 10,000 years, even with a huge investment, which present humans seem unwilling to make. We may however make huge investments to turn sand into platinum as platinum has properties which bring short term profits, but interstellar travel may not turn a profit even centuries later. If we can transmute elements, there will always be at least modest supplies of so called non-renewable resources.
I picture a fusion reactor that you fuel with unsorted trash and it sprays out a fan of hundreds of rather pure isotopes sort of like fractional distillation, with lots of energy as a by product. Neil

Jens
2013-Jul-01, 11:04 PM
The issue of feasibility is an interesting one, but the motivation seems key. I can't really see any practical motivation, except perhaps the romantic notion that "our species may be alive somewhere." As Neil pointed out, we never really destroy resources, just transform them, so I can't imagine it would ever be more feasible to bring back protons and electrons from other star systems than to mess around with what we have.

mkline55
2013-Jul-02, 01:39 PM
As the forum is about life anywhere but Earth, I'll leave out human factors and even Earth/Sun factors. Motivation might involve some unavoidable pending catastrophe, like an approaching solar-size mass, or determining that a nearby star is close to going nova. Something like that, though, might motivate a migration, but not a full galactic conquest.

eburacum45
2013-Jul-02, 08:20 PM
If your motivation is merely survival then it is only necessary to colonise one, nearby star; preferably a red dwarf, which will be stable for about a trillion years. How many interstellar species have only travelled one step to a safer star and stayed there for the rest of their species existence?

JustAFriend
2013-Jul-02, 09:07 PM
Expansion might stop dead in it's track the first time you run across a Klingon or Kzinti race....

eburacum45
2013-Jul-02, 10:11 PM
I would refer you to post #12 to see what is likely to happen to a Klingon or Kzinti race.

Jens
2013-Jul-03, 01:54 AM
Expansion might stop dead in it's track the first time you run across a Klingon or Kzinti race....

Those are just SF stories. In practice you won't run into them, because just like us, they'll be isolated colonies with essentially no contact with other members of their original species. So on each habitable planet you might have wars to see who dominates, but it wouldn't go beyond that. Suppose, for example, that humanity has managed to colonize a planet near Alpha Centauri. And suddenly a big ship appears there and sends a landing party that exterminates the members of the colony. They will send us a message of warning, but the message will take four years to reach us, and even if we want to send help we'd have to build a new ship and launch it and then several hundred years later it will reach them. But we know they're coming, and in a century or so we might have visitors, so we just wait and prepare a real big laser to blast their ship out of the sky when it arrives. There's no point in sending out a ship to intercept them, because chances are we'd never find them in the enormity of space, and even if we did it's improbable we'd have enough fuel to maneuver to actually intercept them.

JustAFriend
2013-Jul-03, 01:01 PM
So on each habitable planet you might have wars to see who dominates, but it wouldn't go beyond that.

Even human history doesn't hold up that.

Back in the days of the great sailing armadas when it took months or years to sail to a campaign,
the enormity of the oceans did not stop countries from carrying on decades or centuries of emnity towards each other.

Jens
2013-Jul-03, 01:50 PM
Even human history doesn't hold up that.

Back in the days of the great sailing armadas when it took months or years to sail to a campaign,
the enormity of the oceans did not stop countries from carrying on decades or centuries of emnity towards each other.

There is no "enormity" of the oceans. They are insignificantly small. You can cross them in an open boat, and humans did cross them in open boats.

molesworth
2013-Jul-04, 03:41 PM
There is no "enormity" of the oceans. They are insignificantly small. You can cross them in an open boat, and humans did cross them in open boats.
Have you been out there?? They can seem pretty daunting even in modern, well-equipped boats. I have nothing but admiration for the early explorers who crossed them with only basic craft and the simplest equipment to navigate by.

Much like I have enormous admiration for anyone brave enough to sit on top of a stack of high explosive and launch themselves out of the atmosphere :)

kzb
2013-Jul-04, 05:04 PM
I'm sure there must be something wrong with mkline55 reasoning somewhere along the line. This problem has been modelled several times I think, and no-one has ever come up with an answer near 40 billion years.

I think the fault lies with this:

At ten additional planets every 400 years...

What you really should say is 20 light-years per 400 years (or whatever your colony generation time is). Then you end up with 2 million years to reach 100,000 light years, (very approximately the opposite edge of the galactic disk). At that point the human race has populated essentially all habitable planets in the galaxy, however many that is.

The growth is exponential only in the early phase where you have a spherical colonisation front. As the occupied area fills up the shape of the galactic disk, firstly running out of stars above and below the disk plane, running up to edge of the disk (and also having to avoid the central few kpc), it ceases to be exponential. Near the end it will be quite slow.

JustAFriend
2013-Jul-04, 05:55 PM
There is no "enormity" of the oceans. They are insignificantly small. You can cross them in an open boat, and humans did cross them in open boats.

Go read some history on the extremely long voyages of the Spanish armadas to the New World or the Magellan expedition
that took years to circle the globe. Even Columbus' voyages took a year round-trip. It's a perfect corollary to expanding galactic
empires where the travel times are long, arduous voyages.

Your 'insignificantly small' oceans are a function of your lifespan of five-hour flights. 500 years ago a trip across the ocean was a significant portion of someone's lifespan.

I'm sitting on the Treasure Coast of Florida, where a hundred Spanish ships transporting gold and artifacts lay wrecked on the
bottom and thousands of men lost their lives in pursuit of just such an expansionism. The history is there....

Jens
2013-Jul-05, 04:39 AM
Go read some history on the extremely long voyages of the Spanish armadas to the New World or the Magellan expedition
that took years to circle the globe. Even Columbus' voyages took a year round-trip. It's a perfect corollary to expanding galactic
empires where the travel times are long, arduous voyages.

It is not a perfect corollary, IMHO. The distance from SF to Tokyo is about 8,000 km, so if you walked 30 km a day you could walk the distance in less than a year. I'm not saying ocean voyages are easy, and just because you seem to assume I'm a total ignoramus I'll point out that I have sailed sailing boats on the ocean, and I am quite aware of how difficult it can be. I'm not saying it's easy in the least, but I know, and you obviously know as well, that even several hundred years ago, when technology was much less developed, people were actually doing ocean voyages. I think this is a clear fact. I don't know if I could successfully cross an ocean, but I could sure give it a try.

What you're comparing it to is voyages that would take hundreds or thousands of years, with no possibility of stocking up on anything en route. No fish, no air, no nothing really. I mean, shipwrecks are serious business but people have survived them. Nobody has ever survived a spacewreck, and it's unlikely that you ever would.

I was going to start by complaining that your message sounded condescending, but I suppose I could just make my own point instead. You think that I am ignorant about sea voyages, and I think that you are believing that space travel is like in Star Wars. So I guess my point could be seen as condescending as well. :)

kzb
2013-Jul-06, 04:30 PM
A better way of modelling this is onion layers. Start with a sphere of r=20LY, its volume is 4/3(pi)*20^3. Next layer out is another 20LY, so r=40LY, voume of onion layer is 4/3(pi)*40^3 minus the previous volume.

It is obvious that onion layers always have a bigger volume than the previous one. Assuming a constant space density of habitable planets, the number per layer must increase with layer number. It can't be true that the planet number approaches a constant per generation. It constantly increases.

This is assuming a constant space density of course, which is not really the case. It thins out quite quickly as you head out the disk plane.

mkline55
2013-Jul-08, 02:54 PM
Unless a species is bent on galactic domination, has unlimited resources, and breeds like bacteria, 40 billion years is not invalidated. The motivation cannot be simply population management. Using the one technological species as a model, if half of the population could be shipped elsewhere immediately, the remaining population would rapidly grow to replace it in just one or two generations. The motivation might be that the species leaves every existing planet in an unlivable state, but it's hard to imagine a space-faring species being unable to manage its own environment. Perhaps a technology that increased the radioactivity until it was unlivable.

Suppose a species arrived at an unoccupied planet. My earlier estimates for growth were wildly exaggerated intentionally. The atmosphere would most likely be poisonous. The soil/sand/rock would be sterile. The water, if open water is even available, is certainly poisonous. The initial colony would find life very, very difficult. Almost certainly, it would take many years to establish a viable system. You cannot just go out and plant crops. You cannot just mine coal. You don't have trees to burn for fuel and heat. You don't have refineries iron, copper, aluminum, lead, potassium, and uranium. You don't even know where those things might be found. Did the colony ship include heavy-duty mining equipment? It's more likely to take ten thousand years or more to make the planet livable. So, that's one layer of the onion in ten thousand years. You only have 5000 layers to go. I still think that's a way too optimistic estimate.

kzb
2013-Jul-08, 06:11 PM
mkline55 wrote: It's more likely to take ten thousand years or more to make the planet livable

The generation time isn't relevant to the structure of the model. What I was questioning was the constant number of new planets per generation. It was that assertion that does not seem right to me.

If you make the generation time 10,000 years instead of 400 years, you then get 50 million years to reach the far side of the galaxy instead of 2 million years. I've no argument with that.

40 billion years implies an average generation time of about 8 million years. Seems a bit long to me.

mkline55
2013-Jul-08, 08:30 PM
Yes 8 million years seems like a long time. Then again, use ourselves as a representative sample, and use Mars as a representative destination. How long would it take to build a self-sufficient thriving population of 100 million there beginning with up to 1000 initial representatives of the species, and limited initial resources besides what exists on Mars today?

Oddly, no one has questioned whether 10% of light speed is even reasonable. Also unquestioned is whether or not any automated stasis or colony ship could continue to survive such a trip.

Expansion would only be spherical until the edge of the galactic disk was reached. Nevertheless, at 100,000 LY diameter, a species starting in the galactic center and traveling up to 20 LY per jump, has just 2500 jumps to make. Starting at one edge, full expansion takes 5000 jumps. So the timing is a matter of where you start, how long it takes to travel 20 LY and how long it takes to build a new generation of space-faring civilization on a sterile planet.

John Mendenhall
2013-Jul-08, 09:15 PM
I think you are underestimating the driving nature of technology and human curiosity.

No? Then turn on your smart phone and watch what the rovers are doing on Mars today.

Think about it.

You couldn't do any of that 20 years ago.

Regards, John M.

neilzero
2013-Jul-08, 09:55 PM
I hate being like the wags a century ago, who thought everything that can be invented has been, but there are numerous mature technologies where advances will be few, small, rare and not very important. The fastest advances are often play toys that are little help getting humans even to Mars. Neil

John Mendenhall
2013-Jul-08, 10:32 PM
I hate being like the wags a century ago, who thought everything that can be invented has been, but there are numerous mature technologies where advances will be few, small, rare and not very important. The fastest advances are often play toys that are little help getting humans even to Mars. Neil

Oh? Like superconducting power lines? Like 14" amateur catadioptrics that resolve the volcanoes on Mars? Like nuc subs good to over 3000 ft down? Like . . . whoops, wait a minute, the men in black are pounding on the door again . . . now, what was I saying? I can't seem to remember . . .

We think we have done everything. We haven't even scratched the surface. Talk about exponential!

Regards, . . . (Gotta get the M.I.B. to turn that memory eraser down. Forgot my name!)

molesworth
2013-Jul-09, 09:52 AM
Suppose a species arrived at an unoccupied planet. My earlier estimates for growth were wildly exaggerated intentionally. The atmosphere would most likely be poisonous. The soil/sand/rock would be sterile. The water, if open water is even available, is certainly poisonous. The initial colony would find life very, very difficult. Almost certainly, it would take many years to establish a viable system. You cannot just go out and plant crops. You cannot just mine coal. You don't have trees to burn for fuel and heat. You don't have refineries iron, copper, aluminum, lead, potassium, and uranium. You don't even know where those things might be found. Did the colony ship include heavy-duty mining equipment? It's more likely to take ten thousand years or more to make the planet livable. So, that's one layer of the onion in ten thousand years. You only have 5000 layers to go. I still think that's a way too optimistic estimate.
My assumption would be that "difficult" planets would be avoided, or possibly prepared for future colonisation by "seeder ships".

We're already close to having technology that will let us analyse exoplanet atmospheres, and I expect within another decade or two we'll be able to say, with reasonably high confidence, which planets are "human habitable", and those would be the prime targets for colonisation. Given the time scales involved, and the likelihood of it being a one-way trip (probably on a "generation ship"), it would make sense to choose the targets carefully, and possibly send unmanned probes to check the actual local conditions first. (Aside : I enjoyed Larry Niven's use of this idea, where the probes were a bit dumb, and would signal a single small high plateau or deep canyon as "habitable", leading to interesting problems for the colonists.) Even waiting a few decades, or even centuries for confirmation before setting off, expansion would be fairly rapid.

As techniques and technologies for identifying suitable planets improved, that information could be shared between colonies, increasing the expansion rate further.

Or at least that's how I'd see it happening... :)

Jens
2013-Jul-09, 10:27 AM
Is there any real likelihood that we would discover planets that would be fit for human habitation?

kzb
2013-Jul-09, 12:04 PM
Yes 8 million years seems like a long time. Then again, use ourselves as a representative sample, and use Mars as a representative destination. How long would it take to build a self-sufficient thriving population of 100 million there beginning with up to 1000 initial representatives of the species, and limited initial resources besides what exists on Mars today?

Oddly, no one has questioned whether 10% of light speed is even reasonable. Also unquestioned is whether or not any automated stasis or colony ship could continue to survive such a trip.

Expansion would only be spherical until the edge of the galactic disk was reached. Nevertheless, at 100,000 LY diameter, a species starting in the galactic center and traveling up to 20 LY per jump, has just 2500 jumps to make. Starting at one edge, full expansion takes 5000 jumps. So the timing is a matter of where you start, how long it takes to travel 20 LY and how long it takes to build a new generation of space-faring civilization on a sterile planet.

I agree completely with your last paragraph, that is the only point I was trying to make. With your other paragraphs you are widening the scope of the debate hugely.

What the generation time actually would be I've no real idea. I think it is fair to say that after the first generation things will speed up, because the knowledge to colonise a planet then exists.

Jens wrote:
Is there any real likelihood that we would discover planets that would be fit for human habitation?

This is the biggest question mark in my mind about this as well. Back when Asimov and Sagan wrote their book, the only planetary system available for study was our own. The logic was that the solar system would be typical. Terrestrial planets in the habitable zone would be common, and life would develop on those worlds. An ecosystem that produced oxygen within the correct range would come about almost inevitably.

These days, when you realise that the solar system is not typical, and you look at the diversity of planetary systems, this view seems a bit idealised.

mkline55
2013-Jul-09, 01:30 PM
Is there any real likelihood that we would discover planets that would be fit for human habitation?
How fit is fit? Do you ready-to-eat plants? How close to 78% nitrogen and 21% oxygen is fit? Is ready-to-drink water a requirement? How about just open water? How much can you rely on technology to make it fit? Do you modify the whole environment, put people in closed environments, or genetically modify the people to fit the environment?

There is one known fit planet. Unfortunately, planets don't breed like roaches, so finding one does not mean you have many. It means you have one. But you have theories about what it took for that one to occur, and can look for the most similar circumstances.

molesworth
2013-Jul-09, 01:56 PM
Is there any real likelihood that we would discover planets that would be fit for human habitation?

With the latest results from planet finders, and current theories, it looks like there are a very (very!) large number of Earth-sized-plus-or-minus-a-bit planets in the galaxy.

If we go with the generally accepted position that we're on an average planet orbiting an average star, then statistically there are likely to be a lot of habitable (plus-or-minus-a-bit :) ) planets out there. Some may be more suitable than others, and some may be downright hard to live on, but chances are there are more than enough for any likely human expansion out across the vasty deeps of space...

Jens
2013-Jul-09, 02:22 PM
The logic was that the solar system would be typical. Terrestrial planets in the habitable zone would be common, and life would develop on those worlds. An ecosystem that produced oxygen within the correct range would come about almost inevitably.
.

That's really a key issue, isn't it. There are surely many planets of the right size, but we don't really know if our atmospheric composition is one that generally happens or if it is just one of many possibly equilibria.

mkline55
2013-Jul-09, 02:38 PM
As photosynthesis is believed to be responsible for most of the free oxygen in the atmosphere, unless a destination has had some similar process in place for a long time, that's a major problem. It took natural processes something like a billion years to convert the atmosphere from its early state to one which would be breathable. As Jens indicates, finding a planet the right size is just a small part of the equation. Even assuming life is abundant in the galaxy, you still have to find the right planet at the right time.

kzb
2013-Jul-09, 04:39 PM
I'm coming round to the idea truly Earth-like planets will be (unfortunately) pretty rare. Even if we find one, is it ethical to transplant ourselves there, along with our crops, animals and micro-organisms?

eburacum45
2013-Jul-11, 07:49 AM
I'm coming round to the idea truly Earth-like planets will be (unfortunately) pretty rare. Even if we find one, is it ethical to transplant ourselves there, along with our crops, animals and micro-organisms?

It seems likely to me that planets with an oxygen-producing biosphere will be rare and precious locations. They will probably fall into two categorioes; ones where the biosphere would be compatible with an introduced Terran biosphere and with agriculture, and ones that are not compatible. Non-compatible biospheres might be toxic, allogenic, voracious or infectious enough to cause difficulties to humans, animals or crops that might be introduced as part of the colonisation effort.

It would be necessary to eradicate much or all of an incompatible biosphere before humans could live there.

Compatible biospheres might be the rarest of all, and one wonders if it is ethical to overwhelm such a rare ecology with introduced species, as it might have the potential to develop into an even more complex Earth-like system in the future. But I doubt that would be a real consideration, in the unlikely event that the planet is a target for human colonisation.

ravens_cry
2013-Jul-14, 10:37 AM
Maybe it never happens. Not all species are going to have a large, repetitively nearby planetary body, like our moon, to inspire exploration, and look where we've gotten. Technology, yes.
Technology to let us become a fully fledge spacefaring species, or even the beginnings of the infrastructure to help make that happen? Not so much. What frustrates me to know end is it is physically possible.

neilzero
2013-Jul-14, 11:41 AM
I suspect space craft collision destruction will be frequent even at 1% of c. Remedies will add greatly to the cost, and reduce the practicality of faster. At 1% of c a grain of sand that slipped though the defenses can total a large space craft. If grains of sand are typically one per light year (in the space craft's cross sectional area) then we need to deal with 4 or 5 of them on the way to the Centarii system. Neil

publiusr
2013-Jul-14, 08:38 PM
I hate being like the wags a century ago, who thought everything that can be invented has been, but there are numerous mature technologies where advances will be few, small, rare and not very important. The fastest advances are often play toys that are little help getting humans even to Mars. Neil

That is true by and large. Many advances are not always visible. Plastics become a little more long lasting, etc. I think there is this idea that Moore's Law applies to everything. It doesn't. I think it was Bob Lutz who took a dim view of certain dot.com wunderkinds thinking they could do GMs job better than they could.

Elon Musk vs Don Draper and the flannel suit you might say.


Like nuc subs good to over 3000 ft down?)

Well, that is still a turbine after all. What Rickover did was to change the institutional inertia away from oil burners to nukes. He pushed and pushed and pushed. That in some ways is more impressive than nuclear power itself.

There as a book called Big Science. This is the model we need. SSC, the Large Handron consolation prize, SLS--there will be advances, but one will also need to respect the big science/brute force approach too.

neilzero
2013-Jul-25, 02:59 AM
Let's assume the 20 th century remains the greatest advance toward interstellar travel anywhere and any time but future advancement is almost as spectacular on future Earth and in 100 civilizations of our galaxy, then my many starcraft spike/steeple strategy is most sensible as it allows interstellar spacecraft to help each other at least rarely. It also means that the spike will continue until it reaches the edge of the galaxy or is well into the hub where colonization is possibly much more difficult. This leaves 90%, possibly 99% of the galaxy uncolonised until perhaps rarely, some civilization starts a spike in a new direction. Even the dead starships are a source of fresher raw material. Communications is also much easier as all the interstellar craft are in just two directions instead of all possible directions. It also allows data to be relayed much reducing communications power that is needed.
If two spikes intersect their velocities = speed plus direction will be too different to make other than information exchanges. Possibly a spike is heading our way and will arrive soon to colonize a hundred locations in our solar system. Neil

Jens
2013-Jul-25, 03:05 AM
The "spike strategy" seems to assume that human beings are mathematical constructs that will blindly follow an algorithm. Even if the colonists remember what the strategy was, why will they go along with it?

neilzero
2013-Jul-25, 01:59 PM
A spike that has been operating for a million years has help, beamed energy, resources and new technology details in both directions, after the colony starships aproximately match the average speed of the spike. A new spike better be completely self sufficient or the travelers may be dead in a century. Also the craft that went before have partially cleared the path of grains of sand, assuming the spike is moving at an average of 1 percent of c with respect to near by objects. High star ship density in the center of the spike means help is available more often, but star craft collisions are possible. If that is a concern the joining craft can travel perhaps a million kilometers from the densest part until they need help. My guess is collision avoidance is easy for starships, but hard for grains of sand. Neil