I've got my own take on this. A little way into the future, we may well have the capability to

• Colonize the entire galaxy within at most a few million years
• Create self replicating machines and other hardware with the capability to reshape entire planets and solar systems - either to make Dyson spheres - or just as art installations or trivial things such as to make the entire galaxy into paperclips if the whim so took us.
• Create Stanford Toruses and O'Neil Cylinders from the debris in solar systems - and even in the Oort clouds of stars - using fusion power as energy source, and self replicating machines to create the habitats with almost no effort on our own part - so that - even without demolishing planets - almost any solar system could house trillions of humans in free space.
• Create new forms of replicating life  - both based on our own DNA, on whatever new types of life we find elsewhere in the galaxy - and on artificial XNA and biochemistry - and again - spread this throughout the galaxy in a very short timescale
• Create cyborgs, mixture of living and machine.

WHY THESE ARE REASONABLY LIKELY THINGS FOR US TO ACHIEVE AFTER AT MOST A FEW CENTURIES OF TECHNOLOGICAL DEVELOPMENT

Note - I'm not making any controversial assumptions here at all. These are all reasonable projects for future technology.

For instance - there is a big debate about whether programmed machines can be self aware. Personally I think a programmed machine will never be self aware, persuaded by Penrose's arguments on the matter.

But - whatever you think about that - all we need here for our self replicators is that they are machines that can make exact copies of themselves - apart from that don't need to be especially sophisticated. So for instance - if you want to build enough solar panels to provide all the energy we need on the Earth from solar power - just build one self replicating machine with a solar panel as the main element of its design - and then task it to make copies of itself using materials in the NEOs. Process will be slow to start with, maybe just one solar panel the first year, two the next, four the next - but after surprisingly few years you have made all the solar panels in space that you could ever need.

We are just a few technological steps away from doing that, and surely will have the ability, say, by the end of this century. Basically just need 3D printers that work at the nanoscale level able to replicate computer chips - and then there would be few other things to sort out except details.

The other things in the list are also straightforward extrapolations from things we can do already.

Except perhaps fusion power - but - though it's taking far longer than expected - we are making definite progress also - as well as many alternative ideas of ways of doing it such as the Polywell - might get it within a decade from those ideas - or the other way -  maybe instead of 50 years - if we are being over optimistic there - it takes 100 years or 200 years - but surely some time in less than a thousand years we will have it. That's only a blink of time for the timescales we are talking about here.

And - we might be able to do all those things over as short a timescale as a few decades if technology moves forward particularly quickly.

HUMANS FILLING THE GALAXY - EACH WITH THE POWER TO RESHAPE IT WITH SELF REPLICATORS

Now - add to that - that if we colonize the galaxy - then that means - you would have three hundred billion stars each with many trillions of colonists.

So, within a timescale that is just a blink in geological time, you have hundreds of billions of trillions of colonists - each of those with the power to reshape the entire galaxy by unleashing self replicating machines to fulfill their purposes. Many evolved to new species by then also - especially if they use genetic adaptation and accelerated evolution.

And - that - go a few generations down from the initial settlers - and their purposes may be totally obscure,

• they may have reshaped themselves so they are not even recognizably human,
• some of them may be colonies of machines or cyborgs
• some star systems may just be "colonized" by trillions of mindless machines programmed to make paperclips or whatever their programmer or artist tasked them to do, together with a few "rogue machines" that have somehow replicated incorrectly..
• Their philosophies and religions and ideologies would probably span at least the full range we have on Earth - but also far more that we haven't yet thought of yet.
• Their objectives and aims may have become completely obscure to us
• Some may be far more intelligent - but maybe not wiser
• Some may be far less intelligent, than us, maybe similar in intelligence to chimpanzees, but with ability to control immensely powerful machines which they don't understand - and do silly childish things, or just press buttons for fun not knowing what they do - because they have long ago lost any need to act to ensure their own survival - machines do everything for them.

FACED WITH SELF DESTRUCTION

An ET, or we ourselves, can look at this, and as you think though the implications

• Unintelligent descendants with power to reshape the galaxy with self replicators
• Cyborgs
• Newly created biological entities
• Beings who might look approximately human but have aims and ideas as different from ours as the imagined monsters of our alien invasion movies - they might pause

And - then you realise - that in the worst case - the ET destroys itself as well as all the other ETs if it colonizes the galaxy. because the colonizers, a short way down the line, are no longer their species, ae cyborgs, or created species, or rapidly evolved, or machines created by them that run amok, or beings that will become the alien monsters for them of their invasion thrillers

COMMUNICATION DELAYS

And also add - that it can take a hundred thousands years to send a message from one side of the galaxy to another. So if, for instance - a spreading wave of different ideas for reshaping the galaxy causes problems when they interact with each other - then - the galaxy has probably already gone belly-up before the originators get a chance to talk to each other.

To take an example, if someone has set off a "turn galaxy into paperclips" wave of self replicators - then ETs who encounter this at once side of the galaxy will not be able to learn the lessons learnt by the ETs who encountered another frontier on the other side of the galaxy for tens of thousands of years. It may be next to impossible to take coordinated action to deal with some threat to the galaxy, either for a single species, or many species or colonists working together.

IS THIS A SAFE GALAXY TO LIVE IN - FOR US OR OTHER ETS?

Looking into that possible future - you might then think - is this a safe and a good future for the galaxy? Is it going to be a good direction to take for other ETs and lifeforms in the galaxy? Is it going to be a good future for ourselves?

Would these future humans and human creations and species evolved from humans all have peaceful and beneficial intentions, and if they did - would they have the wisdom needed as well - and would they co-exist in harmony, would they come to a shared understanding of how to shape the galaxy?

You might well hesitate at that point.

TIPPING POINT COMES SURPRISINGLY EARLY ON

And - there is a tipping point here, which comes really early on - not thousands of years into the future when someone tries to establish a galactic empire.

As soon as you start up a single independent colony that is not dependent on our solar system - e.g. an Oort cloud colony that relies on fusion power instead of our sun - or a colony around another star - then - unless you have already taken careful precuations to prevent it -  it is almost impossible to prevent future colonization of the entire galaxy.

Even a relatively benign independent colony in the Oort cloud - just spreading out using more and more comets - on the timescale of years and decades seems nothing is happening much. But Oort clouds of neighbouring stars mix. And - it is a case of initially unlimited and approximately exponential growth, however slow the exponential, and doubling time - even if it is decades or a century or two - still - it will have one.

You'd have a few decades perhaps where there is a chance of doing something about it - but not easy to get the political will and the unified decision making to carry it through.

Then - once they get to other star systems - and light speed delays - hardly likely you could get things organized to stop the process - even getting representatives of all the different political groups together to discuss things might be practically impossible.

UNSTOPPABLE WAVE

So, over timescales of millions of years, the unstoppable wave of human occupied habitats constructed from resources mined from comets would fill the entire galaxy.

Even selfish ETs out to get the galaxy for themselves - unless also very short sighted - will not want to destroy themselves in this way and create something that will come back and destroy them some day down the road.

Also, ETs don't have to be selfish. Unlike animals, we are thinking creatures and can make decisions e.g. quarantine rules etc.

So, what would ETs do, once they stop and think through these questions?

SENSIBLE, FORWARD LOOKING ETS

Quite possibly - in my view -most ETs who are sensible and forward looking, look at this situation - and decide, that they are not sure that their progeny and their creations would be, on the whole, beneficial for the galaxy or themselves, in the long term.

They, obviously, won't have colonized the galaxy.

ETS WHO ACT BEFORE THEY THINK, AND SEE IT AS A WONDERFUL OPPORTUNITY TO TAKE OVER THE GALAXY

And - the ETs who look at this situation and just think "great, nobody seems to have colonized the galaxy - this is our chance, let's sieze the moment and not think about consequences" - anyone reckless enough to think like that has probably destroyed themselves long before they got to the stage where they start off creating their first interstellar colony. At least - most likely they have destroyed all their space colonies before then.

FRAGILE STEPS TOWARDS SPACE COLONIES - THAT COULD DESTROY RECKLESS ETS LONG BEFORE THEY CAN ATTEMPT GALAXY COLONIZATION

I say that because - there are many fragile steps between here and then. Some we have already passed through such as nuclear weapons, and biological warfare (which could create biological agents able to make us extinct), and widespread extinction of species (we have taken many measures to reduce the rate of extinction and safeguard important crops etc) and so on.

There are likely to be many future issues like this for space colonies.

For instance - that space colonies would be immensely powerful but also immensely fragile, ability to destroy each other just by diverting the path of an asteroid or a few tons of mining materials at the kms per second of interplanetary flight.

Also economically - if it is just a free for all out there - then chances are that we would suffer back here on Earth - long before the colonies could get independent of Earth - and the resulting economic crisis long term would destroy economies on Earth and halt colonization in its tracks.

And - many things we could develop that are dangerous. For instance - self replicating nano technology - that by itself could easily be dangerous enough to destroy space colonies. Or - ability to change the path of asteroids so that they can't harm the Earth - in the wrong hands - could become a weapon that means nobody is safe.

PROMISING SIGNS THAT WE JUST POSSIBLY MAY MAKE IT

Now we've already come through a few - such as atomic warfare, biological weapons, chemical weapons, ozone layer, DDT, the prospect of mass famine solved by the green revolution etc etc. So - I think that proves that - as a species, if not individually - we are at least moderately wise and far seeing, if not hugely so.

Somehow - with many mistakes, and not in a particularly planned way - we do seem to muddle through many disasters - neither doing particularly well - making many mistakes - but also somehow, so far, avoiding the very worst of the disasters - at least to some extent.

So - I think we do have a chance of not destroying ourselves.

IF SO - WE MAY NUMBER AMONGST THE NON SELF DESTRUCTIVE - FORWARD THINKING SENSIBLE ETS

But if so that probably puts us into the second category of ETs that would decide, eventually - that on balance - the galaxy may be better off without them, or at least - I mean - not without any humans at all - but - with most of the humans in our original solar system - and a few adventurous explorers and scientists - and unmanned probes - exploring the rest of the galaxy. 

And that might become something that is so widely accepted that it becomes unthinkable to start up a colony outside of the solar system.

You would only do that, perhaps, if the solar system itself becomes uninhabitable - which with the level of technology talking about here - probably won't happen at all, not for trillions of years - could survive easily right through red giant and into white dwarf phase just using free space habitats.

Or perhaps we might move solar system when the sun goes red giant - but just to one new solar system not spread through the galaxy, but move everyone, leaving the old one empty - like nomads moving their tent encampments.

HOW OUR ATTITUDES MIGHT CHANGE SO THIS HAPPENS

How that happens - I don't know for sure. But might just be through talking through the ideas - raising this very question, just as I'm doing here - and nobody comes up with answers to it, and after some decades or centuries it just becomes accepted that there is no answer. 

Or - it might be that as our telescopes get better we can see other galaxies and find some distant galaxy where this went wrong, an object lesson that has been taken to heart by all ETs in our galaxy.

Or might be that there is an ET in our galaxy - billions of years old - that has set up automated listening posts and warns us if we start to attempt to colonize the galaxy.

WHAT IF WE ARE FIRST ET IN OUR GALAXY

Or might be that we are the very first ET to evolve in our galaxy. There are some quite reasonable arguments that lead to that conclusion - it's not as unlikely as some might think. For instance - if each of the steps of evolution is far harder than we think - we might, by chance - be the only ET that has gone through all of them by now. In that situation - apparently - if you work through the probabilities - you find out - that you will see the various phases of evolution roughly evenly spaced - not because they are easy - but because the only way to have got this far in evolution so quickly is if by chance you happen to live on a planet where all of those steps went through quickly.

If so we have a huge responsibility. If lucky might be able to learn from ETs in other more distant galaxies as our technology improves - or might just be on our own and have to work through all these issues for ourselves, for the first time.

It could be that we are the "ur ancestor" ETs of our galaxy - the ones that younger ETs billions of years from now will discover as ancient cities and relics and barely understood machines littering the galaxy. If so, we have a huge responsibility to the future of - ourselves and all other ETs in the galaxy.

CARE NEEDED BEFORE STEPPING OUT TO OTHER STARS

Either way - I think we need to take great care before we set up any habitats in space that have the capability of either interstellar flight to set up colonies around other stars - or capability of sustaining themselves in free space without use of our sun as an energy source.

Other ETs may all come ot the same conclusion.

And the few that may be able to colonize the galaxy safely even so - if they exist - peaceful,  powerful, wise, probably ultra long lived races by then - with stable populations and no population growth pressure and all their purely technological problems solved - they may well choose not to do so anyway, at least feel under no pressure to do so - no particular surprise that they are not here yet.

We might be due a visit from them some time in the next few millennia when they notice our activities - that is if they exist at all.

Or - perhaps after a few more millennia or a million years or so of development and evolution or whatever,  - or maybe more quickly - we might become those ETs ourselves.

See also my science blog posts:

• Why ET Populations Can’t Continue To Expand For More Than A Few Millennia
• Why Only Very Young ET Civilizations Will Have Expanding Populations - Opinion Piece
• Self Replicating Robots - Safer For Galaxy (and Earth) Than Human Colonists - Is This Why ETs Didn’t Colonize Earth?