# How long to overpopulate the galaxy?



## skeptical (Feb 1, 2010)

Imagine humanity a thousand years in the future.   We have just built the first interstellar ship, which is about to head off to Alpha Centauri.  Technology permits a cruising speed of 0.2 c, since we are not breaking the laws of physics.

How long do you think it will take before humanity has expanded throughout the galaxy?

My own best guess is a million years.  Not totally a guess.   I calculated it, but there are a number of basic assumptions, any of which might be wrong.


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## Nik (Feb 2, 2010)

IMHO, a lot depends on three pinch-points and one 'gotcha'...

If Skylon and its ilk fly in the next decade, we can get to Moon, Mars and asteroid belt without bustin' a gut. Phobos is probably a good target due to orbital and escape velocity considerations. IIRC, Phobos' rotation is synchronous. Also, the regolith material seems ideal for excavation of flare/meteoroid shelters.

Slightly OT, Phobos would be a good candidate for a whip aka open-ended tether-- Not down to Mars, but for docking 'cycling' craft without the worries of dust...

With practicable fusion, the solar system lies open to us. I suspect the winner will look more polywell than tokomac, but it ain't over until break-even.

What is out beyond Neptune ?? If there's something Mars-sized or above, something with modest gravity and lots of available ices yet no Gas-giant radiation belts, it may act as a base in the Kuiper belt. From there to the edge of the Oort cloud takes a modest delta-v. From there, any 'nearby', to-be-discovered Brown Dwarf would be a natural target for fusion driven probes. And then the nearby stars beckon...

IMHO, a lot of the outward urge will depend on future generations' symbiosis with computers & AI. One 'gotcha' would be 'life storage', where your entire experience from birth could be captured for posterity. That would lend itself to cloning & down-loads which, together, mean the grim void between the stars need not be endured in real-time...


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## chrispenycate (Feb 2, 2010)

A couple of centuries of solar system doesn't make much difference in a million years. Assuming, of course, that humanity survives them.

But I don't think humanity will ever _over_populate the galaxy; there is just too much of it. Oh, habitable planets might be few and far between (very far in human terms) but inhabiting unsuitable environments has become a human speciality. Before leaving the solar system, mankind will have become adept at surviving on airless lumps of rock, in the cometary belts, on moons and floating in the atmospheres of gas giants. These little bubbles of life in a hostile universe might get overcrowded but, as long as there is matter and energy unused, the galaxy cannot be said to be overpopulated.

The machines and modified humans adapted for this expansion might well reproduce faster than their modern human equivalents, but, even assuming we don't achieve species extinction in the meantime (not a safe assumption, by any means), there is too much potential, too much space, for any reasonable exponent to fill it in exponential expansion.

Exploring it all might be possible (although I would expect closer to ten million years than one) but stretching its resources, overcrowding its space, with transit times in the thousands of years sounds beyond even our fecundity.


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## skeptical (Feb 2, 2010)

chris

Re overpopulating.

The human species increased its numbers in the 20th century by 6-fold. (600%). For purposes of argument, let us imagine that, over the next 10,000 years, the increase is only a third of that. ie. Humanity doubles its numbers per century.

Also for purposes of argument, let us imagine that the number required to 'overpopulate' a solar system is one trillion. Estimated number of solar systems in the galaxy is 100 billion. So total number of people to 'overpopulate' the galaxy is E23 (100,000,000,000,000,000,000,000).

At the rate of population growth of a doubling each century, the population increases 1000 fold each 1000 years.   (note that I am rounding numbers down here.) Thus, to reach a population size of E23 people would take less than 10,000 years.

Clearly, potential rate of population growth is not a limiting factor.

I calculate that the major limiting factor is speed of travel between stars. If we take the furthest distance from Earth that is still within the Milky Way, we are talking about a straight line distance of 70,000 light years. At 0.2 C, a space vessel going directly from Earth to that point would take 350,000 years to get there. That is a sizeable percentage of the one million years I originally suggested as the time taken to overpopulate the galaxy.

My own view is that, within a few hundred years, humanity will have computers and robots capable of doing anything a person can do. Such machines will not have the time limitations of humans. So, if we truly wanted to colonise the opposite side of the galaxy, we could send a vessel carrying frozen embryos, and with computers and robots capable of thawing them, incubating them, and rasing the resultant babies/children to adulthood, using teaching programs from Earth. Meaning the resulting adults would essentially be Earth human.

So does this make it seem more likely that humans can colonise the galaxy to 'overpopulation' within 1 million years?


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## Scifi fan (Feb 2, 2010)

But that's like saying lemmings will overpopulate the Earth in a few years - nature will always put a stop to that.


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## skeptical (Feb 2, 2010)

scifi fan

My point is simply that population growth is not a limiting factor.  

To fully colonise the galaxy will take, according to my calculations, 1 million years.  Plus or minus quite a chunk depending on what assumptions we use.    The calculation I presented on population growth shows simply that that will not be a limit.

As I said, I suspect that the speed of travel between stars will be the prime limiting factor, slowing humanity's spread.


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## fruit (Aug 20, 2010)

saying lemmings over populate it


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## Harpo (Mar 25, 2013)

How are we doing so far?

http://www.worldometers.info/world-population/​


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## Mirannan (Mar 25, 2013)

Absolute limit to spread to every corner of the Galaxy (the disc portion, anyway) would be around 100,000 years; this assumes travelling at very close to the speed of light and not stopping for very long in any one place.

More realistically, reduce average speed while travelling to maybe 0.5c and (which is far more important) allow a couple of centuries for the colonists to breed like rabbits after arriving at each new colony system, and build new ships during that time. This works out to 20-30 million years; still an eyeblink in the life of the Galaxy.


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## Harpo (Mar 26, 2013)

skeptical said:


> Estimated number of solar systems in the galaxy is 100 billion.


 
Assuming we only send one interstellar spacecraft to each of those solar systems, what are the chances of us being able to construct 100 billion craft in a million years?  That's (on average) 273.9 interstellar spacecraft being built every day for a million years. (excluding sundays and Bank Holidays, of course)


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## Mirannan (Mar 26, 2013)

Harpo said:


> Assuming we only send one interstellar spacecraft to each of those solar systems, what are the chances of us being able to construct 100 billion craft in a million years?  That's (on average) 273.9 interstellar spacecraft being built every day for a million years. (excluding sundays and Bank Holidays, of course)



That depends on how you define "us". It's more of a problem of exponential growth, with the steps taking a few decades or a couple of centuries perhaps.

To make it clearer, within a few hundred years the vast majority of the ships would be built on colony worlds. Or in colony systems, anyway.

I think the limit on this is human biology - assuming we stay biological indefinitely, which is far from certain.


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## Gramm838 (Mar 26, 2013)

As any SF fan should know, the population of the Universe is zero...


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## Harpo (Mar 26, 2013)

or as near to nothing as makes no odds....


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## K. Riehl (Mar 30, 2013)

The argument against this theory is why hasn't some other life form already accomplished this? Beings from another part of the galaxy may have had a billion years to do this already and we haven't heard a peep via any of our listening devices. Of course they could have seeded the stars long ago and become....us?


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## Harpo (Mar 31, 2013)

K. Riehl said:


> The argument against this theory is why hasn't some other life form already accomplished this? Beings from another part of the galaxy may have had a billion years to do this already and we haven't heard a peep via any of our listening devices. Of course they could have seeded the stars long ago and become....us?


 


You're assuming that our 'goldilocks zone' would be the same as something _else_'s goldilocks zone.  Maybe they colonised the galaxy and in our local area they prefer Mercury or Venus or Jupiter or the Kuiper Belt?

I think it's about the population - let's assume that one person in a million actually _wants_ to go to another planet and gets to go.  (I'm basing this on the fact that only a tiny proportion of the population actually want to become an air force pilot, and that only a tiny proportion of air force pilots actually want to become an astronaut, and that only a tiny proportion of wannabe astronauts actually get to go into space)
So, with one person in a million going off to colonise other planets, how long will it take until we have a large enough crew to populate one colony ship?  A thousand colony ships?  A hundred billion colony ships?


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