Not sure why it was downvoted. Many people have looked into this. https://gtocx.jpl.nasa.gov/gtocx/competition/ Even entire competitions to find optimal colonization methods. Could take 90M years with measly future human tech. Age of Milky Way is 13.5 Billion so even conservatively you could indeed colonize the galaxy many many times over in that time.

The Milky Way lacked rocky planets early on, making that 12.5 Billion year age largely meaningless.

Which is just one of many bad assumptions in how people do these calculations. At a minimum you don’t get a straight path from A to B. Assuming any kind of limitation in distance traveled and your at the mercy of the distribution of whatever resource you need. Aka need rocky planets with liquid water, that’s likely going to be an very indirect path. Even just non binary star systems is significant. Further, assuming every trip is successful is again unlikely. How long you need to wait before the next trip is again a major qualifier.

For example, what happens if they first send a probe to verify habitability? Suddenly travel times more than double.

I have seen plenty of optimistic calculations that still add up to over a billion years before total colonization and sometimes much much higher than that. And that’s a billion years where expansion is a major priority at the frontier. Look at successful colonizations that don’t expand rapidly and again things keep slowing down.

One effect not considered in early considerations was the differential radial speed. So even the ability to seed planets 10 light years closer or further from the galactic core over a few 100M years results in expansion much more quickly than earlier models.

If you’re talking 10 light year colonization jumps that takes very deliberate and resource intensive expansion efforts. If you instead model things assuming generation ships as harvesting kepler Belt objects, moving between stars only incidentally or as local resources are exhausted. And only occasionally reproducing. The relative motion between stars becomes the primary driver of net expansion, and that’s quite slow.

> For example, what happens if they first send a probe to verify habitability? Suddenly travel times more than double.

This doesn't seem like a good idea if their method of traveling is generation ships. What would be the point?

One obvious consideration is to see if anyone is living there. Even just the basic economic consideration of spending a low fraction of the cost of a habitable ship to collect more data. Generation ships also implies different people will be arriving, however having a X thousands of year lifespan is another possibility. Which is kind of the thing, sharing physics is a given but biology and especially culture could be very different.

If you used the Von Neumann machine approach wouldn’t it not really matter if the planet was habitable or not?

Let’s assume your sending a Von Neumann probe to prep the way for your ship. Now it shows up and does whatever it needs to and then sends an everything is ready signal before you send your generation ship.

I mean sure you could in theory send out an endlessly replicating probe that keeps going to other star systems, but that’s even more risky.

What’s risky about it? The risk seems in them overtaking the entire galaxy more than anything, no?

Building something with zero possibility of change across millions of years is difficult. Something as simple as reproducing endlessly means they end up wasting enormous amounts of resources. But, you also risk things like probes that don’t slow down and hit one of your planets.

Sure, but we’re now beginning to talk about infinite scale and infinite resources. Who cares if 1 planet doesn’t produce probes as fast or at all? Who cares if 1 probe slams through a planet? That being said, I think the latter case might be more controllable by a civilization capable of creating the machines?

The fear is that you destroy one or more of your inhabited planet not just a lifeless ball. Which I would hope you and most people would consider a significant downside. That risk is balanced vs the benefit of sending self replicating probes to every star system which just doesn’t seem that useful.

Now is that a large or small risk? I don’t have the technology but it’s something that could reasonably concern a civilization.

There are problems of resiliency and energy storage that could be more or less fundamental problems. It could take millions if years of technological civilization to produce a machine capable of sustaining life for the hundreds of thousands of years necessary for interstellar travel, and civilizations may tend to collapse for various reasons before reaching such technological peaks.

It's also entirely possible that we are the first, or one of the first, technological civilizations in the galaxy. Given how little we understand about the appearance of life, the emergence of multicellular life from single cell life, and the emergence of intelligence from multicellular life, there's not way to put an estimate on these probabilities.

Think about the fact that in 3.5 billion years there hasn't been any new abiogenesis on the only planet in the universe we know for sure can sustain life, and in this billion years a single life form has ever evolved from a single-cell to multi-cellular life, aren't the priors pretty decent that life is an extremely rare phenomenon? Given the fact that the Milky Way is somewhere around 13 billion years old, how often can we expect a once in 3.5 billion years event have happened?

>Think about the fact that in 3.5 billion years there hasn't been any new abiogenesis on the only planet in the universe we know for sure can sustain life, and in this billion years a single life form has ever evolved from a single-cell to multi-cellular life, aren't the priors pretty decent that life is an extremely rare phenomenon?

Any freshly-evolved life would have to compete with organisms that have already been honing their survival strategies for literal billions of years. There's no reason to think such a thing is possible, and the lack of it doesn't speak one way or another to the difficulty of it happening in a virgin environment.

> It could take millions if years of technological civilization to produce a machine capable of sustaining life for the hundreds of thousands of years necessary for interstellar travel.

For a generation ship, are you referring to all the machinery that can maintain a balanced ecosystem of plants and animals and humans in order to provide food for the people during the trip to the new planet, and to make all the things needed to keep those people healthy during the trip such as medicine and drugs?

If so, there may be a way around that. Send all your colonists as frozen embryos or frozen sperm and eggs. Your ship only needs life support then to keep alive the people who run the ship. For food, don't grow it. Take it with you.

At first taking your food with you seems absurd, but if you had a food with the same caloric density as rice, enough of it to provide 2000 calories/day to one human for 100k years would fit in a sphere with a radius of 24.4 meters. So once your technological civilization figures out a way to preserve food such that they can make a rice ball equivalent with a 100k year storage life, a generation ship with a small crew becomes a whole lot more feasible.

For replacement crew throughout the journey, you can do a mix of using whatever kids the crew produces the old fashioned way and using kids produced from some of the frozen embryos/eggs/sperm.

I think technological civilizations will reach the point of being able to do this well before they are millions of years old. We aren't too far from being able to do it ourselves. We are probably farther out on the propulsion for the ship itself.

I'm always fascinated in the assumptions underlying some of these proposals, and how little thought they've given to the problem. For example, the complete disregard for the lives or autonomy of the "replacement crew", who are effectively treated as a consumable resource starting from childhood.

Science fiction set on generation ships tends to be extremely gloomy for good reason.

The issue of crew as consumable resource is interesting.

It's particularly interesting I think because now that you've brought it up, it seems to me there is a similar issue with much Earthly colonization or frontier expansion, and I've never seen it mentioned.

For example, anyone who left England to start or join a New World colony early on, when settlements were far apart and it was a struggle for each to survive, was entering an environment where their children would have a much harder, much more constrained life than they could have had back in England.

Were there debates back then about the morality of moving somewhere where you descendants, possibly for generations, would have harder lives than if you stayed where you are?

17th century American settlers were fleeing an environment where the child mortality rate was at least 1/3rd even for the upper classes, nearly all land and wealth was controlled by inbred gangsters, and the penalties for petty theft or incorrect religious worship included public hanging. From that perspective, the risks associated with colonization may have seemed much more attractive than the best-case scenario of spending several more decades of poverty in the same village. At least once you were in America, voting with your feet and moving west was always an option, something that was already recognized by 19th-century European socialists as a unique factor in the development of the US economy.

Of course I'm also assuming that the majority of the colonists were not only dirt poor, but also ignorant and uneducated and shamelessly lied to by the promoters of colonization.

And the early colonies had their problems too. IIRC at least one colony enacted capital punishment for anyone caught abandoning the colony to join the native American tribes.

I hadn't heard that one, but the freedom-loving Pilgrims executed Quakers for proselytizing in Massachusetts. Not really an environment founded on respect for individual autonomy or other ethical considerations.

Aren't all humans consumable resources for today's society?

Seems a bit silly, you can buy a glass sphere with small closed ecosystem that can last years with indirect sunlight. Surely we can manage something that recycles astronaut poop/pee into something vaguely edible with the help of bacteria, fungus, plants, and algae. Sure some genetic editing (of humans, bacteria, fungus, plants, and algae) might be necessary. Even just some minor tweak like happened in the last 1000s of years to make humans more compatible with cows milk.

That is taking energy from the sun, and it is relying on the Earth to maintain certain temperature parameters. It won't last 10k years anyway. If nothing else, the glass will probably degrade over a long enough period of time. There's also a constant chance of a freak mutation destroying the equilibrium of the system.

In outer space, cosmic rays will make this process faster. Also, in outer space, it will need a constant source of light for biological reactions, and also just to keep from slowly cooling down to 0k through heat emission.

> a sphere with a radius of 24.4 meters

A more human-sized comparison is "100 average single-family homes in the US, all filled to the brim with rice". Or 60 ISSes, all filled completely with rice. Per person. And this is volume, not taking into account the weight of rice.

Minimum sustainable population size seems to be about 100 people, so consider how large a ship you'd need just to fit all that rice storage.

It sounds ridiculously large when you put it as 100 average single family homes filled to the brim (I get about 50, BTW, using 2623 sq ft with 9 ft ceilings for the average single family home).

On the other hand, it is under two Mount Palomar observatory domes full.

I'm assuming anyone building a generation ship is going to build it in space, not build it on a planet's surface and then try to launch it, so size is not really going to be an issue.

The big engineering issue will be propulsion. We don't have anything now that could drive such a ship. My guess is that this is either something we'll have within the next couple hundred years or we'll never have it.

I'm assuming that by the time you want to build this kind of ship, you've gotten raising people from frozen embryos or frozen eggs and sperm perfected, and that is where most replacement crew would come from. That should greatly reduce the crew size needed.

I think we'll be able to handle the biological side of this within a hundred years, with a good chance of it being quite a bit sooner than that.

I am referring to all of the machinery on the ship in general - whether it's life support systems, engines, monitoring/navigation systems etc. Even if you could take a ship to outer space, freeze it to ~absolute 0, and launch it on its trajectory, you still need a computer to allow it to warm back up and land at its destination.

Keeping a computer running in outer space for a few thousand or hundred thousand years is a gigantic engineering task, far beyond anything we could achieve today.

And of course, in reality its unlikely you could rely on a single computer system and on 0 propulsion. It's far more likely that you'll need life support, engines, complex medical equipment, cooling systems, lights, all sorts of mechanical parts that will need power, replacements, and that degrade in time, especially at such huge scales.

I don't know why this got down-voted. The milky way is 100K light years across. It's 13.5 billion years old. It's entirely possible to blanket the whole galaxy with probes, without even getting into self replicating probes.

You have to make an assumption about speed - it certainly won't be more than a tiny fraction light speed. Eg voyager appear to travel now at less than 0.00006 c (16 km/s). Than turns 100k light years into ballpark 150 M year journey (no it doesn't its 1.8 bn years as mentioned in reply) . Still possible to cross 10.000 times - but that would be a straight line. I suspect the distances will grow somewhat fractally if you want to swing by any stars? (ed: so, not really very likely at all)

To traverse 100k light-years at 16km/s would take approximately 1.8 billion years. FWIW.

Thanks, not sure how I got to just 150 mill years.

To imagine that voyager technology will define the maximum velocity of interstellar travel, a thousand years from now, ten thousand, a million, etc is short sighted. Maybe we never can exceed light speed, but we could get up to a reasonable percentage of it.

Sure, but it puts things into perspective. If we assume 1% of c is feasible - it's still a 10 Million year journey to cross 100k light years. That's much better - it gives 1 000 chances in 10 bn years.

There's current projects on earth seriously working on getting probes up to 1% of the speed of light. Just a small/light weight probe, a solar sail, and a large (but feasible) array of lasers.

> Than turns 100k light years into ballpark 150 year journey.

Forgive my confusion.

If 100K light years is 100K years at c, how can

> 0.00006 c

yield 0.00150K (150) years?

It can't (unless you mistype into your plain android calculator app). Ed: Fwiw there was a typo there, too - it was supposed to say 150 million - but it's more like 1.5 billion.

It could be that none of them have seen any point in doing so.

Generally on Earth colonization has happened in order to provide a tangible economic benefit to those supporting the colonization effort. With someplace that you can only travel to and from by slow generation ship its hard to find any such economic benefit in having a colony there.

I'm fairly confident we'll eventually find that our form of life is the result of intentional or unintentional colonization of Earth.

If you look at how quickly life began after the oceans settled out, then look at precisely how complex even the most primitive forms of life are (in terms of likelihood of stochastic construction to the point of sophistication to support evolutionary mechanisms) it seems incredibly unlikely that we originated here.

This is an interesting idea. Look at how religion explains the origin of life: A lifeform not from this planet, far more advanced than us, gave "the breath of life", and calls us their "children".

The vastness of this universe leads me to believe that it's impossible to dismiss the idea that there is an intelligence/lifeform sufficiently advanced as to appear to be a God to us.

Totally agree. Joscha Bach laid out a few interesting ideas around this at the end of a podcast.

If you have a few minutes start here for a broader context (starts of wholly unrelated but with some great concepts):

https://youtu.be/P-2P3MSZrBM?t=10084

If not, he drills into it here:

https://youtu.be/P-2P3MSZrBM?t=10400

That doesn't explain where the said intelligence /lifeform would come from itself, so what the point of such a theory?

I read a short story that explained the explosion of life on earth during a very short period on some alien that didn't follow protocol and dumped some poop.

Possible, but far reaching. There are other explanations:

* We are a computer simulation (hence there is no colonization, we are in an experiment)

* There are like 1,000,000,000,000,000,000 habitable planets in the universe. Just because something is "extremely unlikely" doesn't mean that it didn't happen to us. I.e. whomever this happened to, would ask himself that question. We may just be the ones it happened to.

Also: Why would they settle species from millions of years in their own past. If they wanted to colonize, they could start with their own life-forms.

Could be several things as well:

* We are a genesis project and they will simply come back later and settle in what we left behind (i.e. they let the ecosystem build for a few million years before trying out this settlement)

* They don't want to create competitors

* We are a population bomb, i.e. they just sent out billions of probes trying to populate any given habitable planet, and they couldn't just spawn themselves there and instead had to start from somewhere where a sustainable ecosystem would derive from

> * We are a genesis project and they will simply come back later and settle in what we left behind (i.e. they let the ecosystem build for a few million years before trying out this settlement)

Depressing fiction idea: our progenitors return, look at what we've done with the world, shake their heads and leave us to suffer.

Keep in mind that molecular evolution happens at a massive parallel scale. If there's some simple recipe for basic evolving life it is very unlikely that is not being hit in a few million years by a large number of molecules and then primitive cells.

And it's unlikely that there is only a single recipe. Most likely there are many (just on Earth all the others died out)

Yes, I know the paradox. It's simple. It's too simple.

It has huge assumptions. Namely the assumption that the current problems to colonization can be solved in a cost-effective manner.

The longest continuous length of time spent in what limited space flight we have is just under 440 days. Not to mention, all of it was spent in low Earth orbit. Once we go beyond that, we lose all protection from the Earth's magnetosphere.

Then there's also the loss of bone density due to lack of gravity. 1% per month is nothing to sneeze at.

We are not built for space travel. We are built to inhabit this planet. We have evolved to the conditions here.

And before you say that the generation ships will just fake gravity and have appropriate shielding. Where do you get that assumption? We can't fake gravity. We have a space station where they take 10 times the radiation as here on Earth because we can't even realistically shield that.

And then we get to "autonomous probes", the great handwave of people who think they can cleverly sidestep the "humans aren't built for space" issue.

Who builds them? For what purpose? What's our expected return? How can you justify the investment in time and resources to shoot into space, with absolutely no hope of even knowing there's a chance for a return on that investment until generations later, a probe that can at best return data.

And then there's the issue of it being an autonomous probe. Two words that hide a lot of other problems. A lot of other assumptions. One being that we'll have created an intelligence capable of running this thing.

In other words, the Fermi Paradox just assumes that all of these incredibly hard problems can and will be solved and that the solutions are logistically viable.

What if they're not?

You talk like this is all expected to be done by 21st century humans when in reality if there were civilizations out there capable of doing this colonization they would likely be millions of years our elders and its incredibly foolish to try to guess their intentions or the capabilities of their technology. Your view is painfully anthropocentric.

Just to add, there are plenty of valid criticisms or plausible solutions to the Fermi paradox but I think you've failed to identify a single one.

You talk like this is all a forgone conclusion. Technology is not magic. Time is not a salve that solves all issues.

You say that my view is "painfully anthropocentric", but how?

It seems your only response is "but aliens". That is not good enough.

You want to say that it is foolish of me to try and guess their intentions and capabilities, but that's what the paradox does as well. It assumes their intentions are to colonize and their capabilities allow it. With no proof. The only life we have to base anything off of is right here.

And if you think I haven't identified a single valid criticism of the Fermi paradox, then I'm just going to assume you aren't aware of the criticisms.

I've basically touched on this one https://en.wikipedia.org/wiki/Fermi_paradox#Economic_explana...

So no, I'm not saying that just because it can't be done now, it can't be done ever. I'm saying the Fermi Paradox is basically saying "Once you solve these incredibly hard problems, this becomes easy."

I think the Fermi Paradox is phrased wrong. It should state that if it were possible to colonize the galaxy, it would have been done by now.

> You say that my view is "painfully anthropocentric", but how?

I'm not OP but the idea of thinking in terms of "return on investment" strikes me as very 20th century humanity.

Who's to say a future society wouldn't consider finding another civilisation to be a massive ROI? Or that a post-scarcity society living under Fully Automated Luxury Gay Space Communism would even care about a return, instead of just doing things because they care to do them?

> We can't fake gravity.

Yes we can, it’s a standard part of fairground rides.

(I’ll agree that the unknowns of space colonisation, and of the automation we’d need to be able to afford the infrastructure to even launch a serious effort, may prevent such colony efforts from happening)

Centrifugal force does mimic gravity but it's not really something we can do at scale or even in space. When we're talking about a generation ship, we're talking about something a little larger than the Gravitron.

Not to mention, they work within Earth's gravity. Take a tube and spin it around you in space, it does nothing to you because there's no other forces working on you. Nothing putting you in the frame of the spinning tube.

Once you lose the Earth, it becomes a lot trickier to tie you to a frame of reference. Nothing we can make has the mass necessary.

The best we could likely do is accelerate a ship at 1G. But that has problems, because about halfway through your journey, you have to start decelerating. And there's also the issue of turning.

> Take a tube and spin it around you in space, it does nothing to you because there's no other forces working on you.

Yes, it does — if you are in contact with the structure, the force you feel is the outside acting against your inertia to keep you in uniform circular motion. There is a layer of air in contact with the structure at any moment, so it ends up co-rotating, so anywhere inside except the axis of rotation itself will feel a force proportional to the distance from the axis.

> When we're talking about a generation ship, we're talking about something a little larger than the Gravitron.

Naturally. You can still spin them along and axis: https://en.m.wikipedia.org/wiki/O%27Neill_cylinder

We are a long way from been able to build such structures, and I have doubts about the suitability of human political psychology given the travel times involved and how long countries last for on average, but the physics of spin-gravity is fine, even though there may be noticeable Coriolis effects depending on scale.