Why We Haven't Met Aliens Yet

[Delvo]

So you reckon that a vast (I am talking several galaxies-wide, or more) civ that has FTL and can go anywhere and do just about anything would not be visible in some way?

They might be. They might not be. For it to happen, they would need to not only be able to do something we happened to be able to detect, but also have some reason to want to. And it would need to happen at the right time, when we happened to be looking in the right direction. And if the process took time, it would need to happen quickly enough for us to catch whatever was changing, or we'd need to keep watching long enough (even though we wouldn't know what to watch for or where to watch). And on top of all of that, we'd need to know how to interpret what we were looking at. What would tell us that it wasn't just a natural phenomenon, whether or not we had come up with a natural explanation for it yet?

Even if my (dumb) ideas of Dyson spheres

Dyson's dumb idea.

what signs do you suppose there could be - given that this civ exists?

I suppose that there is no way for me to imagine what they might be. If they're already doing one thing that's impossible as far as I know, then I don't know what is possible/impossible for them, nevermind what they have any interest in doing.

And on the subject of Earth's despoilation - there are other posts here that postulate that aliens might be watching Earth, how do you suppose they could do so if they are further than the moon?

If they can get anywhere near as close to Earth as the moon is, then they can get closer if they want. But in any case, whatever distance they feel like stopping at, they'd watch us the same way anybody watches anybody: with a variety of sensors tuned to different wavelengths of the electromagnetic spectrum. The closer they get or the longer their sensor ranges are, the more detail they can observe.

Watch the composition of the atmosphere over time, for one thing.

Okay! Nice. So it's possible to see signs of a civ's affect on its ecosystem (from afar.)

That depends on what you mean by "afar". We can watch for small, subtle changes in our atmosphere using satellites that are barely outside it. We might barely be able to manage a simpler, less precise form of monitoring from some other points within the inner solar system (the orbits of Mercury through Mars, which is a small fraction of the total solar system because they spread farther apart after that), but when we want detail and precision about the atmospheres of Mars or Venus, we still send probes. Compared to that range, the nearest other star system is hundreds of thousands of times farther away. (We're about 93 million miles away from the sun, and just one light year is about 5.9 million million miles.)

And another thing it depends on is knowing how much of whatever substance you're looking at means "industrial pollution" and how much is a natural level, even though different planets might have different natural levels. (And the same planet could have different natural levels at different times. Ours almost certainly had more of several different substances we would now call pollution a few hundred million years ago, during times of more volcanic activity and before the formation of fossil fuels. The Dimetrodons didn't have a big polluting industry.) Worse yet for this kind of civilization monitoring, some pollutants can actually reverse course as the civilization gets more advanced and gets better at avoiding doing things like that. We actually have lower levels of some pollutants now than we did for a while before. But exactly which ones will increase or decrease at what times is something you can't predict unless you already know there's a civilization there to watch and something about their behavior and choices... which you probably already know anyway, if you were close enough to measure such tiny changes in such a tiny fraction of the total atmosphere.

Thus, the reverse should be possible; we should be able to see signs of any large FTL civ.

Non sequitor.

 

[Dinwar]

In order to use the composition of the atmosphere to detect civilizations, you first have to know what atmospheres on planets lacking civilizations look like, so you can see if there's a difference and what that difference is. The issue is that there are a HUGE number of variables. Take CO2, the standard go-to "industrial" chemical. There were pre-industrial portions of Earth's history when CO2 was much higher than what we consider normal--and by "pre-industrial", I mean "pre-Quaternary" (we know this several ways, including isotopic data and C3/C4 plant ratios). Being able to differentiate between normal planetary variance and what happens when industrialization occurs requires a pretty good dataset of non-civilized planets.

It's not enough to see; you have to correctly interpret what you're seeing. And we currently don't have enough data to correctly interpret it. Aliens watching us from afar, on the other hand, certainly would (they'll have been to at least one other planet).

 

[Donn]

They might be. They might not be.

Right, I follow your gist. You seem to know more detail than I do. I was spitballing based on a brief idea that I got from the Lensman series. In those books the humans (at least) were throwing planets at their enemies, and using suns as weapons! So I thought, well heck, that's gotta show!

Dyson

's dumb idea.

His idea, my dumb second-hand derivative suggestion.
Seems there has been a search for it though. Um, lemme google:
http://home.fnal.gov/~carrigan/infrared_astronomy/Fermilab_search.htm

Nothing found yet, I gather

I suppose that there is no way for me to imagine what they might be. If they're already doing one thing that's impossible as far as I know, then I don't know what is possible/impossible for them, nevermind what they have any interest in doing.

Alright. I was thinking of what gross effects hard use of galaxies (say for resources, or huge scienctific experiments) would look like.
I see a lot of pics online of galaxies, fields of them. Of gravity lenses. Of presumed dark matter interactions. Of galaxies colliding. Of massive clouds of dust. There's also SETI. All the data shows 'pristine' defaults, so to speak: what we would expect to see, even with all the surprises.
For example: if in an image of a dust-cloud we saw a geometric denuding of the dust - like a fractal pacman was eating it from one side - that might indicate harvesting of the matter in the cloud. It would be out of place. That kind of thing.

And another thing it depends on is knowing how much of whatever substance you're looking at means "industrial pollution" and how much is a natural level, ..

I see. All this is kind of making the case for these aliens rather slight.

Dimetrodons

That's Dimetrodonns

Non sequitor.

Probably. I meant that we can go from the small to the large, if the small is possible. Surely that follows?

 

[Hellbound]

Not sure there's enough data to support this. I'll grant you intelligent life on Earth tends to be aggressive, but it's not universally so. Octopi, for example, are highly intelligent, without being known for agressive tendancies.

Only if you aren't a crab . But then to coutnerpoint this, octopi are not dominant. That rule should have read (I got it wrong the first time):

The aliens will be intelligent, alert, aggressive, and ruthless when necessary. The idea being that we're talking about a technological species on another planet. I don't think it's feasible to assume they wouldn't be the dominant lifeform (unless you posit multiple intellgient races), as they'd need the resources of a planet to support space travel on this scale. And you don't become the dominant species wihtou beign aggressive. Is that abetter statement?

Other species are necessary for the survival of any intelligent non-photosynthetic or -chemosynthetic organism, so I'm going to disagree with this. From an outside perspective we spend a lot of time caring for cows, pigs, and chickens.

I think you misunderstood. It doesn't say that they'll kill off everythign else just because, but that if it comes down to a choice, they'll choose themselves. For example, if you, a cow, and two friends are trapped on a mountain with no food...who gets killed first? The point being, that if it comes down to a chance we kill them off or they can kill us off, they won't sit back and say "Okay, you're right, your species deserves to live...kill us all off!"

I don't see how the two are related. If we fired off every nuke on Earth at the same place at the same time, it'd result in less than half the destructive power of the Chixilub Impact, and while the K/Pg represents a mass die-off it really wasn't an overly influential mass extinction. We tend to think of it as more than it was, because the dinosaurs died out--but when you look at invertebrates, we're finding that it had less of an impact to the biosphere than we thought. The two biggest ones--the end-Ediacaran and the End-Permian--didn't even come close to wiping out the biosphere. They came close to wiping out ANIMALS, sure, but the biosphere as a whole was fine. Simply put, beyond liquifying the planet we really don't know what it'd take to wipe out life on a planet. I mean, let's say they nuke us from orbit. Let's say the do it in such a way that every inch of the surface is inhospitable to even viruses and bacteria. You still have the lithophiles to deal with--2 km of rock provides pretty good shielding from radiation.

They could probably wipe US out, yeah. Any engine that can travel the stars will certainly be powerful enough to be useful as a weapon. But even if they do, they're going to face a biosphere that's as alien to them as they are to us, and as likely to kill them.

I'm not talking about nukes. I'm taling about impacts, and at the speeds possible they'd be a lot bigger than Chixilub.

Do the physics calculations to see the equivalent explosive force of a 10,000 lb. object hitting Earth at, say, .25c. And that's a relatively small size (about a bus or an empty tractor-trailer).

Nukes and asteroids are popcorn.

All they have to do is send the ship to impact the planet (or better yet, two ships, 12 hours apart, to hit both hemispheres).


In any case, as I stated, I don't think it's necessarily likely myself. It does imply a cold logic that disregards a lot of other factors. Still, though, something else to through intot he mix (and it always generates comments in these discussions ).

 

[Dinwar]

But then to coutnerpoint this, octopi are not dominant.

Neither are we. Bacteria are.

This isn't a symantics issue, either. How we determine what's dominant is going to be critical during future exploration of other planets.

The point being, that if it comes down to a chance we kill them off or they can kill us off, they won't sit back and say "Okay, you're right, your species deserves to live...kill us all off!"

Ah. I see. A fair point. Though I still think that cooperation and resource partitioning is vastly more likely (seriously, what are the odds we'll be able to even enhabit the same environment?).

Do the physics calculations to see the equivalent explosive force of a 10,000 lb. object hitting Earth at, say, .25c. And that's a relatively small size (about a bus or an empty tractor-trailer).

I've got my doubts about this. I mean, the angle of attack is also a factor, as is where they hit (hit a sulfite deposite and bad things happen with much lower forces; hit granite and it takes a bigger force to do damage). There's also the fact that Chixilub was 10 km across--10,000 lbs. isn't that much, when you hit us with something the size of a city.

Then you have the geophysics. It's one thing to say there's enough energy to liquify the planet--you have to TRANSFER that energy, however. An arrow can go through a pillow pretty easily, without causing much damage, despite having sufficient energy to kill a person.

It's not that I don't get your point--it's just that I've found that people have a weird view of what impacts do. I did myself--only reason I know more is because I spent a few years researching it. To sterilize a planet would take vastly more resources than negotiating with the inhabitants, and once you sterilize it you've got a dead rock. Unless you're already at war, I really don't see the point--it's akin to droping a nuke on a city because you don't like your old neighbors.

 

[WhatRoughBeast]

I've got my doubts about this. I mean, the angle of attack is also a factor, as is where they hit (hit a sulfite deposite and bad things happen with much lower forces; hit granite and it takes a bigger force to do damage). There's also the fact that Chixilub was 10 km across--10,000 lbs. isn't that much, when you hit us with something the size of a city.

I share your doubts, but the numbers matter. The post you replied to specified a 5000 kg mass at .25 c. Kinetic energy is about 2 x 10^18 joules. Chixlub was about 4 x 10^23 joules (per wiki). 5 orders of magnitude count. In this case the original energy is about 500 megatons - really sucky at ground zero, but not a dinosaur killer.

Now try it with, let's say, .9999 c.

 

[Dinwar]

Numbers only matter if they do anything. If the rock misses, or glances off our atmosphere, it doesn't matter how big it is, the devistation will be minimal. No matter how big or how fast the thing is, without something intelligent piloting it it's going to be like trying to thread a needle with a sniper rifle in a huricane.

Science 101: First prove that there's something to discuss, then discuss that that thing.

 

[WhatRoughBeast]

Science 101: First prove that there's something to discuss, then discuss that that thing.

It is a little-known fact that dinos had developed space travel, and were considered a threat to the Galactic Federation.

Chicxlub was the result. Worked like a charm.

"One shot, one kill" - Motto of the 117th Planetary Bombardment Group

 

[Roboramma]

Numbers only matter if they do anything. If the rock misses, or glances off our atmosphere, it doesn't matter how big it is, the devistation will be minimal. No matter how big or how fast the thing is, without something intelligent piloting it it's going to be like trying to thread a needle with a sniper rifle in a huricane.

Science 101: First prove that there's something to discuss, then discuss that that thing.

I don't think something going at .9999c is going to glance off our atmosphere. It may explode on impact, but it's trajectory is unlikely to be affected much. And if we're positing intelligent aliens capable of interstellar travel, capable of firing something at .9999c, I think we can assume they'll have mastered navigation: ie. they won't miss.

On the other hand, I think .9999c is somewhat outrageous. How are we positing them achieving that sort of velocity? .9c, on the other hand, while still extreme for a mass that size, is considerably more reasonable. I'm not even sure something traveling that fast could survive an interstellar journey (particles of gas and dust in interstellar space would be impacting it at .9999c after all).

But I agree with what you said, it makes more sense to cooperate with us: after all we've already built all this infrastructure in this solar system, why destroy it only to rebuild your own when they could simply trade with us?

Moreover, worrying that we would try to destroy them is sort of crazy: unless they only just developed interstellar travel (and what are the chances of two civilizations emerging at pretty much exactly the same time considering the astronomical timescales involved?) they'll have settled other worlds and perhaps other stars. In that case, even if we did want to destroy them, we'd only be inviting our own destruction at their hands. It'd be like India deciding to fund a rocket program to build an ICBM (right now I think they've only got much shorter range rockets) so that they could fire a nuke at the US. Why? Sure, they'd do some damage, but they wouldn't take out american infrastructure, they'd be inviting a counter-attack that could wipe them out completely, and have nothing to gain in doing so anyway.
So, now we're somehow positing that the US is so worried about an Indian nuclear strike that it decides to do a first strike on them and wipe them out completely just in case? Instead, I think they can simply be confident that the Indians have nothing to gain and leave it at that.

 

[Roboramma]

In all this talk of aggressive aliens watching us (to make their first move) how is the watching done?

I don't know the physics. Could they be watching us from light years away, some kind of super telescope that's playing a movie (years in the past, viz light years distance) which shows zoomable detail? Perhaps it has sound and EM-band stuff?

Sagan has a bit on how aliens with technology similar to ours could look for life on earth in Pale Blue Dot which you might find interesting:

He first discusses a somewhat distant view of the earth, characterizing our atmosphere's composition:

YOU'RE AN ALIEN EXPLORER entering the Solar System after a long journey through the blackness of interstellar space. You examine the planets of this humdrum star from afar—a pretty handful, some gray, some blue, some red, some yellow. You're interested in what kinds of worlds these are, whether their environments are static or changing, and especially whether there are life and intelligence. You have no prior knowledge of the Earth. You've just discovered its existence.
There's a galactic ethic, let's imagine: Look but don't touch. You can fly by these worlds; you can orbit them; but you are strictly forbidden to land. Under such constraints, could you figure out what the Earth's environment is like and whether anyone lives there?

Another possibility is that ordinary visible light, which the Sun pours out in vast amounts, is used on Earth to break water apart—except that there's no known way to do this without life. There would have to be plants, life-forms colored by a pigment that strongly absorbs visible light, that knows how to split a water molecule by saving up the energy of two photons of light, that retains the H and excretes the O, and that uses the hydrogen thus liberated to synthesize organic molecules. The plants would have to be spread over much of the planet. All this is asking a lot. If you're a good skeptical scientist, so much Oz would not be proof of life. But it certainly might be cause for suspicion.

The other reveals something unusual: a material, covering vast areas, that strongly absorbs red light. (The Sun, of course, shines in light of all colors, with a peak in the yellow.) This pigment might be just the agent needed if ordinary visible light is being used to break water apart and account for the oxygen in the air.

Methane and oxygen together in the same atmosphere is peculiar. The laws of chemistry are very clear: In an excess of O2, CH4 should be entirely converted into H2O and CO2, The process is so efficient that not a single molecule in all the Earth's atmosphere should be methane. Instead, you find that one out of every million molecules is methane, ail immense discrepancy. What could it mean?

Skipping ahead a little, he looks at broad surface features:

Some of the lights, though, are not due to cities. In North Africa, the Middle East, and Siberia, for example, there are very bright lights in a comparatively barren landscape due, it turns out, to burnoff in oil and natural gas wells. In the Sea of Japan on the day you first look, there is a strange, triangular-shaped area of light. In daytime it corresponds to open ocean. This is no city. What could it be? It is in fact the Japanese squid fishing fleet, using brilliant illumination to attract schools of squid up to their deaths. On other days, this pattern of light wanders all over the Pacific Ocean, seeking prey. In effect, what you have discovered here is sushi.

 

[Roboramma]

200 years per light year is significantly faster than anything we've made yet. The fastest thing we've made has taken over 34 years (298044 hours) to get about 16 light hours away. That's about 18629 years per light year. For the 5-light-year distance you propose, that's over 93144 years. But that distance is much too short for what we're talking about.

We also weren't trying to travel to another star, we were trying to get to the outer planets.
If we were to build an interstellar spacecraft with current technology, it wouldn't look like Voyager. Similarly, before sputnik, the fastest thing ever built was (I'm guessing) the V-2. But, since it wasn't designed for orbital flight, the fact that it was incapable of orbital fight doesn't really tell us anything about the limits of someone who is attempting orbital flight.

There's only one other star system within that range. It doesn't have planets. Most stars don't.

I don't see how the presence of absence of planets is particularly important: as long as there are smaller bodies (comets, asteroids), there are materials to be used for development and construction of infrastructure.
In fact, given a civilization that has already learned to work in space such may be preferable to planetary surfaces, given that once you've gone down a gravity well it takes so much energy to get back up.

The nearest with any confirmed planets is only the 44th closest to us, and its planets are not suitable for a human colony. For that matter, none of the hundreds of others we've observed so far are either, even the ones that might have some life of their own, and there are only a few of those in the whole list anyway. The rest that are farther away don't get any better, just farther away, up to 23807 light years; average distance to exoplanets so far (even though they're all worthless to us) is somewhere in the hundreds. The 97 star systems with planets within 100 light years have at least 132 planets, all of which are about as suitable for human colonization as the other planets in our own solar system are. So whatever the fraction of star systems out there with planets is, it's small, and whatever the fraction of planets out there that we could colonize is, it's small too, so the distance between those has to be big.

I'll agree with your last sentence (referring our current technological level), but will point out that the rest doesn't really mean much: our current methods of finding planets biases the results toward larger, more massive planets. Earth-like planets are on the very far end of what we are theoretically capable of finding, so it's not particularly surprising that we're not finding many.

The closest to us that we could reasonably even imagine there might be an inhabitable/colonizable planet, that we've somehow missed so far, would take well into the hundreds of thousands of years to reach, at least, at the best speeds we can muster now. Millions is more likely. And that's with relatively small light probes; colonizing that soon would call for similar speed from a much larger spaceship, which is much more challenging. Maybe greater speeds less than c can be done, but we have no way to tell how soon we'll be able to build such a thing, especially at the size and mass that would be required.

You seem to assume that colonizing requires sending biological entities. I find it far more likely that civilizations will send machines to do the development and construction of infrastructure around other stars. In which case space-craft sizes don't need to be nearly as large.

Also, what's possible is one thing, and what anybody's going to actually have any incentive to bother with is another. Nobody's going to want to invest the relatively large fraction of a planet's resources that would be required to undertake such a journey, knowing it will take hundreds or thousands of times longer than their civilization has even existed yet.

In another thread on this subject someone pointed something out that I found interesting: on astronomical time scales stars pass by relatively close to each other. If we being to colonize other planets, asteroids, comets, within our solar system, some of those may be quite easy to divert toward a passing star and then carried along with it. Colonization of it's comets, asteroids, etc. then begins. After some time (hundreds of thousands of years?, Millions?) it passes by another star close enough that some of these can relatively easily be picked up by this other star's gravity, and now colonization of that star system begins. Etc.

Such a process is extremely slow by our time-scales, but will (if I'm remembering another poster's calculations correctly) nevertheless fill up the galaxy on the order of hundreds of millions of years. Maybe later I'll see if I can run the numbers.

The former are a stupid idea that there's just not the slightest reason at all to think will ever happen anywhere anyway, and even if they did, by definition, they'd be something we couldn't see.

Why exactly is it a stupid idea? And they are not "by definition something we couldn't see", they would radiate in the infrared in rather a specific way.

 

[Dinwar]

I don't think something going at .9999c is going to glance off our atmosphere. It may explode on impact, but it's trajectory is unlikely to be affected much. And if we're positing intelligent aliens capable of interstellar travel, capable of firing something at .9999c, I think we can assume they'll have mastered navigation: ie. they won't miss.

That's one problem. The others still remain, however. If it punches through, it's merely going to result in basalt flows. Nasty, yes--but not something that will sterilize a planet. Perhaps enough to be useful in warfare, the same way nukes are useful, but certainly not something that's likely to be used very often.

Of course, we can't discount dogmatic zealots getting their hands on the tech. That'd get nasty quick.

 

[Roboramma]

Hm, I suppose I did discount the idea of the through-shot. Interesting possibility.

 

[Dinwar]

The world is like an egg--hard outer shell, squishy center. Of course, I'm probably downplaying the devistation that would cause--all I mean is that if it did go through, it probably wouldn't destroy ALL life. We'd certainly be wiped out, as would most things that are bigger than an amoeba.

 

[Donn]

It is a little-known fact that dinos had developed space travel, and were considered a threat to the Galactic Federation.

Chicxlub was the result. Worked like a charm.

"One shot, one kill" - Motto of the 117th Planetary Bombardment Group

^^ This! Oh, so this.

 

[Donn]

Sagan has a bit on how aliens with technology similar to ours could look for life on earth in Pale Blue Dot which you might find interesting:

Thanks for quoting that. It certainly covers the close-approach situations. It reads like (if not better than) a lot of sci-fi ship-comes-into-new-system plots.

I like the calamari bit.

on astronomical time scales stars pass by relatively close to each other.

Nice idea. A 'drift' colonization. I can't help but think that another Sun coming close enough to attract anything would spell the doom of this system and its own. Sounds rough.

 

[Roboramma]

Nice idea. A 'drift' colonization. I can't help but think that another Sun coming close enough to attract anything would spell the doom of this system and its own. Sounds rough.

Well, we'd still have to travel out beyond the outer planets of course, but even the Oort cloud is a damn sight closer than 4 light years away.

 

[BravesFan]

I think that one thing nobody has considered (well in this thread) is that IF there happened to be an alien lifeform that had the ability to fold space and come to earth AND they knew we where here. Why the blazes would they want to?

Seriously, aside from the raiding of resources, the scientific enlightenment or practical joking, why would any alien species want to come have a sit down with a rather un-evolved bunch of goobers like us earthlings? We bring nothing to the table really, not yet anyway. In a few hundred thousand years we should be great to hang out with, but right now we would be about as interesting to them as ants. (well I guess there might be some alien entemologists who would be interested in us)

As far as resources go, I would think there would be plenty of unoccupied planets closer to home they could raid for all the minerals and water they could get their hands on. All without the trouble and expense of dealing with angry natives.

So unless the visitors from space are here to study us for their alien zoo and/or college classes. I find no motivating factor to be preparing for the kind and just rule of our new overlords quite yet!

 

[Roboramma]

Again, the proposition is not that they are living on a planet orbiting some far off star and then decide to come to the earth. The proposition is this: on some distant planet life began. Over time, intelligent life evolved. Over a little more time it developed a techological civilization. Over some more time it's technology progressed to the point at which it was capable of interstellar travel. By this time, or perhaps after considerably more time, it had already tapped the resources of it's home system to the point that it made sense to go elsewhere.

Some sort of interstellar craft was launched to that other star which was capable of beginning the development of that system that would, over time, put it on the course to attain a level of development similar to that of the previous system.
Wash, rinse, repeat.

Now, if this happened, it's unlikely that the first steps happened exactly as they did on earth: ie, if we aren't the first technological civilization, whatever other such civilizations there are would have come about much earlier than us.
The later steps are faster than the earlier steps: that is, once you've got an intelligent species (biologically similar to us in it's capacity to develop a technological civilization), it shouldn't take more than several millions of years for such civilization to get underway.
Once such is underway, it shouldn't take more than tens of thousands of years for it to reach our current level of development.
Once it has reached this level, if interstellar travel is possible it shouldn't take more than thousands of years to get there.
Once they have gotten to another star, developing it shouldn't take more than thousands of years, because all the technological innovations have already been made.
Thus, you get an expanding sphere of colonization moving outward from the home star at some fraction of c.

Why hasn't that sphere reached earth yet? It's not good to say "they can use the resources near home" they've already used them all.

 

[Aepervius]

I have had this discussion a number of times before and there are always a number of elements and possibilities that often get ignored.

1. Communications.

We have been looking for radio waves mostly as a sign of civilization. However, even our own civilization is starting to use other means of communication rather than sending powerful radio waves into space. Efficient communication means that only the intended recipient receives the message and only enough energy to get that message to the recipient is used. For example, early tv shows were broadcast by radio waves that went into space but now most are transmitted via cable. For us, there will be a period of about 60 years or so that you could detect us via radio waves. After that, you could not.

We are not detectable by radio wave unless you happen to be in our VERY short neighborhood, at worst 1 light year (more like much less than that). The reason is extremely simple : the signal energy is fix, but with the distance the wave front increase significantly (in d^2) making the energy of the signal drop rapidely, and for all signal sent on earth which were radio or TV, a quick calculation shows they did not get very far until they are so low as to be virtually undetectable above interstellar noise level for those frequencies.

You know which radio signal somebody *may* have heard ? The 2* 1 minute signal sent with a powerful emitter in a *directed* fashion toward stars many thousands of LY away. Yes you read that correctly. 2 time 1 minutes about.

That's it.

So unless Alien are bathing us with a very directed narrow radio signals and a lot of energy, they could as well live in the solar system next to us and never be detected...

2. Spreading through the galaxy.

The assumption is that a civilization would spread throughout the galaxy. Again, efficiency would tend to dictate that such a process would be done with an eye to minimizing the cost. Planets would be observed first, likely remotely as we do now, and then scouted. The biggest argument I see on that subject concerns the use of Von Neumann probes. In my opinion, there is not enough discussion on the vast technological problems involved in creating Von Neumann probes. Even if it were possible to do so, Von Neumann probes would not be more efficient than living organisms since they would have to find materials to reproduce, which would be difficult, time consuming and energy intensive. I think the time scale involved in settling the galaxy is likely enormous, even if faster than light drives were possible.

I think the explanation is much more simple : look at the energy needed to go out of space, compare to energy available in various form (nuclear/fossile fuel/coal etc...).
We had barely 100 maybe 200 years of plentiful energy, and we will not have 200 more with the same exponential increase. Space is simply too costly, unless we can derive better energy source. It does not matter if we have theoretical auto reproducing probe or arch in holed comete as generation ship, if we don't have the tech and energy to make it go out of the solar system.

Lastly, I don't think arguments such as "It is the nature of intelligent life to destroy itself", "It is the nature of intelligent life to destroy others" or "Life is periodically destroyed by naturally occurring events" are likely solutions. A galaxy is a huge place. To make the assumption that every potential civilization is going to follow a similar path seems unlikely.

The biggest problem in this is that we don't know exactly what is required for life to arise. Once we solve that, we can make an better estimate of the chances of life in the universe.

IMHO it is not a matter of life destroying itself, but more like the only civilisation we knwos of, consuming its energy, its cake, at a frightening rate. And once the cake has been eaten, it will be stuck forevermore in late middle age on the energy production side without any better energy production.

Effectively the universe could be teeming from life, with thousands of civilisation in our neighborhood, but if our civ is a typical one, then none of them ever get enough energy to go beyond their immediate planet neighborhood in their solar system before falling to low tech low energy.