I agree with many of the other answers, that the most likely reason any ET would want to do something about us would be if we set out to colonize the galaxy in an imperialist way. Even if our society has moved on hugely and we no longer have wars, a kind of Star Trek type future society. Even then, in a Star Trek future at least without warp drive, an ancient billions of years old civilization would take the long view and wonder what would happen once we have filled the galaxy. It would take as little as a million years at average speed of a tenth of the speed of light - every single star and Oort cloud choc-a-bloc with humans.

I’m assuming here that they can colonize Oort clouds. It wouldn't take many centuries of technology - the key would be fusion power, and self replicating machines. The combination of the two of those would make it easy to mine the Oort cloud, make habitats, and to set up home there with fusion "mini suns" or similar to warm up the habitats. This also means that once we start to spread out to the Oort cloud - since clouds of neighbouring stars probably intermingle, then there is no need to colonize other star systems to fill the galaxy - just hopping from one comet to another we could colonize the galaxy through the Oort clouds. We’d soon be unstoppable.

Many of the other answers here mentioned the idea that they’d be scared of us filling the galaxy as an emperialistic colonizers - well I don’t agree that they would be scared of us. They would be way ahead of us technologically as the chance of simultaneously reaching the same level of technology would be tiny. Even if we had another planet with life and independently evolved extra terrestrials in our own solar system, the chance that they evolve to technology within a few centuries of us is minute.

But they would possibly be scared of their own people doing this, which is just as bad and harder for them to stop since their own people would have the same level of technology as themselves. At some point they must have worked through this in their past and found a way to prevent their own emperialistic galactic colonization. So, surely they would be ethically advanced, peaceful civilizations, and they can’t be imperialist any more - otherwise they would have already had a go at converting the galaxy into an empire themselves and would be here already.

Here I’m assuming that faster than light travel is impossible. If so, a galactic civilization is also impossible unless the members of that civilization have immensely long lives, long enough to travel from one side of the galaxy and back again within a single lifetime, and have children at a late age too, maybe you have your first child at age a million. Long lived beings like that might see the entire galaxy as about as small as our solar system seems to us.

So anyway - either they are a galactic civilization with immensely long lives. Or they inhabit only one star system or a few star systems and the rest of the galaxy they have sentinel robots as in “2001 a Space Odyssey”. Their sentinels could be detectable to us as in that film, or they could be undetectable using nanotechnology or whatever, that would just depend on what they wanted. Or they have some other solution to the problem of having a galactic civilization instead of a galactic chaos. I think any sensible civilization will find a solution to that conundrum.

The Sentinel (short story) This was the origin of the idea that became the sentinel in 2001 a Space Odyssey - originally it was a pyramid.

My solution to Fermi’s paradox combines several ideas. Looking around intelligent Earth species nearly as intelligent as us you see ruminants like goats, you see lions, you see elephants, dogs, octopuses, grey parrots and crows amongst others. Plus the great apes and whales. Possibly also termites. So I think that the temperaments of extra terrestrials, assuming there are many of them, probably vary at least as much as the variation between a lion, goat, parrot, and blue whale, and maybe even a termite.

So it’s not likely that the same future fate applies to them all - again assuming intelligent species in our galaxy are numerous. So I think that basically there are probably two types. First the short sighted aggressive ones. More short sighted and more aggressive than us. So - as soon as they get into space, at the time of the start of our space race, they start having wars in space, shoot down each others satellites. Like the US responding to Sputnik by shooting it down if they could, or if not the Russians sabotaging Apollo in some way. So then they just never get anywhere in space because spacecraft and habitats are so vulnerable. We don’t have forcefields. We couldn’t shield the ISS against a determined attack by some nation with similar level of technology, especially if they were deceptive and pretended to be on a friendly mission and then rammed it.

The space around their planet would soon be filled with orbital debris and they would be knocked back and lose their space capability over and over until they find a different way forward.

Then the long sighted more peaceful ones. If they are able to see forward enough to get through the early stages of the space race, through to being able to set up colonies - and to deal with what may be other hazards too, e.g. making synthetic lifeforms or returning life from another planet safely - then I think they have to have the capability of co-operating with each other and taking the long view. These ones then would also be the ones that would not set out on an imperialist colonization of a galaxy because they would see how it would impact and harm them back again thousands of years later. They would wait until they find a safe way to do it.

So now look at the situation of our very ancient ET that has worked through this long ago. They don’t need to police the entire galaxy stopping any ETs from getting into space. Most of them will either stop themselves through over aggression, or will be wise enough to make the decision to stop. And they don’t risk the galaxy being overwhelmed by imperialist humans or other ETs except for a few ETs that might be on just the cusp between aggressive stupidity and co-operation and long range thinking.

Also they don’t want to fill the galaxy with their own kind as that causes the very problem they want to stop. They may have explorers criss-crossing the galaxy but not colonies around every star.

So instead they have robotic sentinels around every star (if such an ancient species does exist). And they normally won’t do anything. Let an ET evolve all the way to space colonization capabilities. Even if they colonize their own solar system out to the Oort cloud, no problem. They have technology a billion years or several billion years ahead of us, most ancient first evolved extra terrestrials. But if we do become a problem - well they can just sterilize us. That’s humane. Maybe just sterilize anyone who travels outside the solar system. Some kind of nanobes, or undetectable technology that makes everyone sterile. Or some other method. We can already reproduce by cloning, and may be able to synthesize the entire genome and may have “maturation chambers” in the future - well if so they have to be able to interfere with those also in some way to prevent them from working.

They won’t want to kill us - that would be inhumane. They would probably try to reason with us, maybe leave us some archive to find and read to educate us. Maybe tell us about some object lesson of other extra terrestrials that went wrong. But if we just won’t get the message, then that would be their final resort.

No need to make us extinct I don’t think. No need to physically contain us either. What harm would a few million or billion sterile explorers do spreading through the galaxy? Just make it impossible for us to reproduce. And they don’t even need to do that to all of us. Maybe they make sure that some of us stay fertile so we don’t go extinct but few enough so that there are never more than a few million of us in the galaxy or whatever they think is a safe number. Or that you find you are sterile if you try to reproduce outside your home planet or outside your home solar system.

As to how they would do that, no idea. But you are talking about technology billions of yeas ahead of us. It would be like us sterilizing humans with stone=age technology. We’d find a way and might be able to do it in such a way they can’t even detect what happened.

Or - maybe they make our spacecraft so that they don’t work. The engines mysteriously fail whenever they try to travel beyond our solar system. Just glitches in the electronics. Things stop working. Or they disassemble. Key components disappear whenever we try to travel beyond our solar system…

I’m probably not being imaginative enough here. But some way or another I expect it would be a trivial job for them to keep us in control. On the hypothesis that such creatures exist at all.

They don’t need to be especially clever by the way. I see no reason why ETs have to be brilliant. I think it is unlikely that they would be so clever that we seem like ants to them. Indeed if ants were able to talk to us and had a civilization I think they wouldn’t seem like ants to us either.

If cleverness in the sense of being able to do long chains of reasoning for instance was a survival trait - or in the sense of being able to do lightning calculations for antoher example - we would all have those capabilities. There has to be some downside and one can think of some rather easily. So the same I think is likely to apply to ETs.

They may be clever, but they might also not be especially clever, not hugely more so than us. But just a very very ancient civilization and as a result they have invented everything we could think of billions of years ago. Not likely to surprise them technologically, so they would not be scared of us.

As for the idea that they would need us as slaves, that makes no sense to me. If they wanted slaves, why leave Earth to evolve for as long as it did - why not seed it with whatever species are the most useful as slaves long ago? And anyway what need would they have for slaves with such advanced technology? Machines could do everything for them if they wanted it. And also if they have reached this stage they must have a reasonable level of ethics plus as well they also have everything they want at least in material terms already.

You can make up science fiction scenarios. E.g. curios. That you get a lot of social cachet from having a genuine human from Earth - not a clone, but the real thing, in your family. Or genuine non cloned plants or animals from Earth. A bit like the way rhinos are hunted though it is easy to create fake rhino horn indistinguishable from the real thing.

But I don’t find those scenarios very plausible. I think a billions of years old civilization is likely to have gone beyond such things. Not totally impossible. A bit like say holing all the balls on a snooker table in one go after the break, not just one after another but in a single shot. Unlikely but probably not totally impossible in some situation with a very freaky shot fired really hard.

So, I find all the scenarios unlikely, no reason why they would want to invade or enslave us. But if we start colonizing other stars, and if we do it in an imperialistic way - just expanding exponentially without thinking of the consequences and they think we are headed for a galactic chaos instead of a galactic civilization, they would have to step in at that point to make sure that we don’t - for the safety of themselves, other ETs in the galaxy and indeed ourselves.

If those ETs don’t exist then we have the responsibility to make sure we don’t do that ourselves. The universe is still very young and if we did turn the galaxy into a galactic chaos, that would be irreversible. No way you could stop that once you have humans colonizing other stars say a hundred light years away. Not without warp drive anyway. Then how could they, or at least their creations such as cyborgs, uplifted creatures etc ever go extinct? If humans go extinct around one star - there would be others where they are not yet extinct and those would be vigorous and expand to fill the stars again, and the ones that reach maturity in the least time, say by age five, have huge families, travel fastest and are most aggressive are the ones that would refill the galaxy most quickly. If they can master cloning, or self replicating machines, they can fill it even faster. So that seems a recipe for endless chaos some time down the road. All of this assuming they don’t have warp drive as that changes everything.

So, if we were to unleash this on our galaxy, there is no way you can put the genie back in the bottle and our galaxy would be like that for trillions of years into the future, or until some creature evolves able to damage it so much it becomes uninhabitable to any form of intelligence.

This we must avoid. I think the safest future is to send robots as our emissaries first lots of tiny robots around nearby stars and then once we master safe self replication, self replicating robots around every star in the galaxy. And then for humans, keep colonization to our solar system for now, and just keep a careful weather eye on the long range future and hopefully we can develop enough sense to explore and perhaps colonize parts of the galaxy responsibly in a way that leads to a civilization, not just over timescales of centuries, also millions of years, rather than a galactic chaos.

If we can do that, then also we would be no threat to other ETs in the galaxy. It’s possible that demonstrating that we can do this would be our condition of membership of the galactic community of civilizations. Or perhaps it is just a matter of time, that they travel throughout the galaxy but have immensely long lives and one is due to come our way some time in the next few hundred thousand years, which seems a short time to them.

Incidentally I don’t think advanced civilizations have to be technological. They might be ancient but live, for instance, in an ice covered ocean, with almost no possibility of developing technology. We might even have such a billions of years old civilization in our own solar system. I’ve no idea how you assess the probability, and perhaps it is very low but I think it’s not impossible that Europa could have a billions of years old civilization. If they can have fish, squid or similar as some think is possible, then why not intelligent fish, or squid? And if so, well why not a civilization as in having language, probably maths, maybe music, art etc? But no technology at all. Such a civilization would be impossible for us to detect with current technology, not unless we drill into the ocean (which would need to be done with great care to avoid introducing Earth life to their world) and they would have no way of even getting through the ice to the surface so not only no way of knowing about us, but no way of knowing even about the solar system never mind the stars.

Europa (moon) moon of Jupiter, perhaps the closest place to Earth where there just possibly could be a billions of years old extraterrestrial civilization living in the ocean 100 kilometers beneath its icy crust. Hard to assess the probability but some think it could just possibly have the likes of squid, or fish, if so, then it could have intelligent fish so surely a civilization is not totally ruled out. Without any possibility of fire (probably) and only simple tools, they would perhaps be unlikely to develop technology and indeed might not even know of the existence of the rest of the solar system, never mind the galaxy, all they would know of would be their global ocean.

There is of course no risk of being invaded by non technological civilizations. But they would be at a huge risk from any technological civilization invading them as they would be helpless, not even have ever imagined such a thing as technology - or maybe they work out the ideas of how machines would work as mathematical ideas but never imagined they could exist in reality. Indeed they would know nothing outside their ice covered ocean and a machine penetrating into it from outside would be a huge cultural shock to them.

On Earth of all the species of animals that have high levels of intelligence, only a few seem likely to have a chance of developing tool based technology. Perhaps parrots (though their machines would start very small, sticks, and leaves and so on), indeed termites perhaps, octopuses, apes of course. Octopuses would be rather handicapped by living in the sea though maybe they would adapt to find ways to travel on land once they are intelligent enough to become curious enough to want to do so, over long enough periods of time?

While the likes of lions, blue whales, dolphins, goats, dogs, not very likely. As a result I expect non technological civilizations to out number the technological ones. Indeed it’s a bit surprising that the first civilization on Earth should be technological. Is it possible that it isn’t? Grey parrots have quite small brains yet are amongst the closest in intelligence to us. Suppose that some species of parrot developed a civilization in the past, millions of years ago. What trace would there be of it now? Probably a few sticks in interesting patterns laid out on the ground as art works or mathematical theorems long blown away? Maybe nothing left at all. Could our Earth have had non technological civilizations in the past already?