I think - that if they are there - most likely reason is that they just haven't got around to it yet. Probably don't have Faster than Light travel - and can't be that many of them - obviously haven't decided to colonize the galaxy or they would be here already.
So - even if there were a million "star trek" type ET ships full of ETs roving the galaxy, we'd spot them rarely, with a hundred billion stars for them to visit.
Some of them probably have lifetimes of millions or hundreds of millions of years - so they don't actually need FTL travel to tour the galaxy, but can tour it several times in a lifetime even at sublight speed.
And would be in no hurry. Perhaps one of them came by here a million years ago and noticed that the primates seemed promising, and marked us down for another visit in a million years or so. May even have left some kind of a listening post, to send out a message if we leave the planet or develop radio waves - if so they may be a thousand light years away by now and may get the message some time in the next millennia.
Or maybe listening posts in every star system in the galaxy - even then - could be quite a while before we get noticed by whoever monitors them.
Or - if they found us more recently - again probably not in any great hurry to contact us, especially if contacts with civilizations in an early stage of development have caused problems in the past.
I think the ants analogy used here and often in the literature on this - just doesn't work for me. Let me explain why.
Or else - if they aren't like that - then they can learn from those few of us who are or people like that from our past also.
So - not saying that they need our religions - but just that, as thinking aware beings they'd have a lot that they could share with us and talk to us about.
There's also the issue of incompatible DNA / XNA. If life comes in many different forms, as seems likely, with different backbone, different basis - then just as synthetic XNA is thought to need extreme care on the Earth - then you might get extinction of entire planets by introduction of a single microbe with XNA if it turns out to be "fitter" than the local DNA.
If so then ETs might be extremely careful about landing on a planet in case they make all life on the planet extinct, they may have examples in the past when this happened.
That might be another reason for not traveling in person to visit other planets, especially in early stages.
If so then our first contact may be with machines sent by ETs carefully sterilized.
Or could be, with sentient plants since seeds can be sterilized - or some kind of ET that doesn't have microbes for whatever reason.
ETs CAN'T BE THE EXPANSIONIST COLONIZING TYPE
I think that if there are ETs and they have been in our galaxy for billions of years, they can't be colonizing and expansionist or they would have colonized the entire galaxy easily.
I think also that if you do colonize the galaxy in an uncontrolled way, then chances are that some of your descendants might do something stupid and hazardous like let loose an unrestricted Von Neumann machine - or they may turn into creatures that forget their heritage and come back and destroy your planet a few thousand generations down the way - or many other things may go wrong if you fill the galaxy with countless trillions of your descendants.
And is a little hard to see how you could control it to the extent that you are sure that won't happen.
So sensible ETs may be explorers but not colonizers. And reckless ones probably destroy their civilization before they get far, since space colonies and spaceships, in early stages anyway, must be extremely fragile.
So that makes it more likely that there will be only a few of them in the galaxy - and the nearest ones likely to be quite a bit away and - probably simply don't know what has happened here on Earth yet.
Many ETS have probably solved issues of short lives and may have lifetimes of thousands, or more likely millions of years or more.
They may carry out leisurely conversations with each other, and occasionally visit each other, but on timescales so long, and with optimized communication methods e.g. directed laser beams, or even some FTL communication method if such is possible.
They might live long enough to see many spins of our galaxy
and the most long lived ETs might expect to still be around to see the collision of the Andromeda Galaxy with the Milky Way. That's likely to give you a different perspective on colonization and exploration of the galaxy
I'm not sure about FTL myself because it usually leads to relativity paradoxes, would mean they are also time travelers, can change their own past, and I'm not sure if a universe with time travelers is feasible, it might be that anyone who invents time travel then ends up finding themselves in such a mess that the only way out is to go back and uninvent it, for instance, somehow, by changing their own past timeline so the invention never happens.Or may be some physical feedback or something that prevents it happening.
But - if long lived, they could get here very easily just through normal travel. And we might well be due a visit some time in the next few thousand or million years.
See also Projects To Get To Space As Easily As We Cross Oceans - A Million Flights A Year Perhaps - Will We Be Ready?
So - even if there were a million "star trek" type ET ships full of ETs roving the galaxy, we'd spot them rarely, with a hundred billion stars for them to visit.
Some of them probably have lifetimes of millions or hundreds of millions of years - so they don't actually need FTL travel to tour the galaxy, but can tour it several times in a lifetime even at sublight speed.
And would be in no hurry. Perhaps one of them came by here a million years ago and noticed that the primates seemed promising, and marked us down for another visit in a million years or so. May even have left some kind of a listening post, to send out a message if we leave the planet or develop radio waves - if so they may be a thousand light years away by now and may get the message some time in the next millennia.
Or maybe listening posts in every star system in the galaxy - even then - could be quite a while before we get noticed by whoever monitors them.
Or - if they found us more recently - again probably not in any great hurry to contact us, especially if contacts with civilizations in an early stage of development have caused problems in the past.
ETS AND ANTS
I think the ants analogy used here and often in the literature on this - just doesn't work for me. Let me explain why.
- Ants aren't intelligent, can't reason and think about consequences. If there were ETs doing mega-engineering and traveling in spaceships we'd notice them even if they weren't bothered about us - and even if their actions didn't make much sense to us - still we'd notice that they are not just a natural phenomonon.
- Also technologically advanced doesn't have to correlate with intelligent. They might even become less intelligent and forget how to make machines, as in many sci fi. stories if the tech gets 100% reliable, and very advanced.
- Also intelligent doesn't have to correlate with responsible, sympathetic, compassionate, loving etc - and if they have lost those capabilities they might have a lot to learn from us - though also lots of bad things also they could learn from us of course
Or else - if they aren't like that - then they can learn from those few of us who are or people like that from our past also.
So - not saying that they need our religions - but just that, as thinking aware beings they'd have a lot that they could share with us and talk to us about.
INCOMPATIBLE DNA
There's also the issue of incompatible DNA / XNA. If life comes in many different forms, as seems likely, with different backbone, different basis - then just as synthetic XNA is thought to need extreme care on the Earth - then you might get extinction of entire planets by introduction of a single microbe with XNA if it turns out to be "fitter" than the local DNA.
If so then ETs might be extremely careful about landing on a planet in case they make all life on the planet extinct, they may have examples in the past when this happened.
That might be another reason for not traveling in person to visit other planets, especially in early stages.
If so then our first contact may be with machines sent by ETs carefully sterilized.
Or could be, with sentient plants since seeds can be sterilized - or some kind of ET that doesn't have microbes for whatever reason.
ETs CAN'T BE THE EXPANSIONIST COLONIZING TYPE
I think that if there are ETs and they have been in our galaxy for billions of years, they can't be colonizing and expansionist or they would have colonized the entire galaxy easily.
I think also that if you do colonize the galaxy in an uncontrolled way, then chances are that some of your descendants might do something stupid and hazardous like let loose an unrestricted Von Neumann machine - or they may turn into creatures that forget their heritage and come back and destroy your planet a few thousand generations down the way - or many other things may go wrong if you fill the galaxy with countless trillions of your descendants.
And is a little hard to see how you could control it to the extent that you are sure that won't happen.
So sensible ETs may be explorers but not colonizers. And reckless ones probably destroy their civilization before they get far, since space colonies and spaceships, in early stages anyway, must be extremely fragile.
So that makes it more likely that there will be only a few of them in the galaxy - and the nearest ones likely to be quite a bit away and - probably simply don't know what has happened here on Earth yet.
LEISURELY TIMESCALE OF LONG LIVED ETS
Many ETS have probably solved issues of short lives and may have lifetimes of thousands, or more likely millions of years or more.
They may carry out leisurely conversations with each other, and occasionally visit each other, but on timescales so long, and with optimized communication methods e.g. directed laser beams, or even some FTL communication method if such is possible.
They might live long enough to see many spins of our galaxy
and the most long lived ETs might expect to still be around to see the collision of the Andromeda Galaxy with the Milky Way. That's likely to give you a different perspective on colonization and exploration of the galaxy
I'm not sure about FTL myself because it usually leads to relativity paradoxes, would mean they are also time travelers, can change their own past, and I'm not sure if a universe with time travelers is feasible, it might be that anyone who invents time travel then ends up finding themselves in such a mess that the only way out is to go back and uninvent it, for instance, somehow, by changing their own past timeline so the invention never happens.Or may be some physical feedback or something that prevents it happening.
But - if long lived, they could get here very easily just through normal travel. And we might well be due a visit some time in the next few thousand or million years.
See also Projects To Get To Space As Easily As We Cross Oceans - A Million Flights A Year Perhaps - Will We Be Ready?
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
4.8m answer views110.4k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0