Space Game Theory and Space Stations

[Ender]

Also, in case anyone is interested, the formula for how much time you have after detecting the launch of an RKV aimed in your direction is d(1/v - 1), where v is the fractional speed of light of the RKV (e.g., if it's traveling at 0.95c, v = 0.95) and d is the distance to the aggressors in light-years. So if you detect someone 350 ly away shooting an RKV at you at 0.98c, you have all of seven years plus change to get ready.

Yes, but will you be able to see it? The problem is the sheer area of space you have to sweep and the resolution of your telescopes. Further out you place it, the more time you have to respond, but the lower the probability detecting it. Its a losing probability, since the time to see it increases linearly and the area to cover increases exponentially.

[Darth Wong]

Ender, you don't need to detect it in flight. The goddamned thing will require extensive infrastructure and many years to prepare. If you are aware of this other civilization, you would surely be monitoring them well enough to know they are preparing this little surprise long before they actually launch it.

"Hey guys, what do you think that giant ten thousand kilometre long accelerator system pointed at us might mean?"

"Uh gee, I dunno. They've only been building it for the last 15 years. What do you say we ignore it and randomly sweep space in every direction looking for threats?"

[Ariphaos]

What, to disrupt the energy/gravity balance in their star? I would think it would be to diffuse for anything else.

If you can resolve a planet with enough accuracy at a given distance to strike it, you can fry it. The size of the lens required to fire said beam is far, far smaller than those required to track planetary motions accurately enough to strike them. If the civilization uses large enough statites, they won't even need to build specialized equipment for it.

Yes, but will you be able to see it? The problem is the sheer area of space you have to sweep and the resolution of your telescopes. Further out you place it, the more time you have to respond, but the lower the probability detecting it. Its a losing probability, since the time to see it increases linearly and the area to cover increases exponentially.

The further away you are, the more accurate your measurements need to be in order to strike a planet. It is not sufficient to see Earth in order to strike it - if you're close, you also need to see Venus and Mars. If you're far, Mercury, Jupiter's moons and Kuiper belt objects start throwing your aim off.

The problem is, the defense does not require much preparation time for a Type II civilization. As I mentioned, a 1 light-year sphere of influence gives 44 hours of defense time.

You basically need to throw a small moon at the target planet in order to overwhelm sunshine and happiness.

RKVs are absolutely ridiculous weapons, plain and simple.

[Ariphaos]

"Hey guys, what do you think that giant ten trillion kilometre long accelerator system pointed at us might mean?"

Fixed.

"Uh gee, I dunno. They've only been building it for the last 15 years. What do you say we ignore it and randomly sweep space in every direction looking for threats?"

"How about we just point our Dyson swarm at theirs for a few minutes so they get the hint?"

[Surlethe]

"Hey guys, what do you think that giant ten trillion kilometre long accelerator system pointed at us might mean?"

Fixed.

Can you put some numbers out to back that up, please?

[Ender]

Ender, you don't need to detect it in flight. The goddamned thing will require extensive infrastructure and many years to prepare. If you are aware of this other civilization, you would surely be monitoring them well enough to know they are preparing this little surprise long before they actually launch it.

"Hey guys, what do you think that giant ten thousand kilometre long accelerator system pointed at us might mean?"

"Uh gee, I dunno. They've only been building it for the last 15 years. What do you say we ignore it and randomly sweep space in every direction looking for threats?"

Which would be even harder to detect. IR and Gammas from AM annihilation are very distinctive and easily determined from the background. Structures are going to be much harder to detect because they aren't actively emitting anything, or if they are it is very weak. You might get some radio to key you in to look at something, or if you have neutrino detection fusion sources might turn up (depending on how good it is) but otherwise you are looking for something rather small at a very long distance. To just see the 10 thousand km accelerator as a blob you would need an angular resolution of 1.7*10^-10 arcseconds. At visible light wavelengths that means your array needs a baseline 45 light-minutes. And again, that means you need to look at the right part of space to see the blob - the area of space you need to look at for anything still goes up exponentially with the distance. And since it is just a blog, you wouldn't be able to tell what it is. By comparison, seeing the exhaust plume is easy

[Ender]

What, to disrupt the energy/gravity balance in their star? I would think it would be to diffuse for anything else.

If you can resolve a planet with enough accuracy at a given distance to strike it, you can fry it. The size of the lens required to fire said beam is far, far smaller than those required to track planetary motions accurately enough to strike them. If the civilization uses large enough statites, they won't even need to build specialized equipment for it.

Do you have any numbers on this "fry the planet" thing? I'm not certain what level of devastation you are talking about.

The further away you are, the more accurate your measurements need to be in order to strike a planet. It is not sufficient to see Earth in order to strike it - if you're close, you also need to see Venus and Mars. If you're far, Mercury, Jupiter's moons and Kuiper belt objects start throwing your aim off.

Yes, I believe the estimates in rec.arts.sf.science was that >75 ly you were safe for this reason.

The problem is, the defense does not require much preparation time for a Type II civilization. As I mentioned, a 1 light-year sphere of influence gives 44 hours of defense time.

You basically need to throw a small moon at the target planet in order to overwhelm sunshine and happiness.

RKVs are absolutely ridiculous weapons, plain and simple.

If you are a type 2 R-bombs aren't an extinction threat anyways - your civilization has moved far beyond the planets and has significant space presence. R-bombs are only useful against weaker civilizations. And frankly for those you are better off with a small probe with onboard AI. You land on an asteroid in system and fire off a few jets to give it a nudge to send it into the planet. Looks more natural, and is a lot easier.

[Seggybop]

Regarding vaporizing the RKV when it nears the target, what would happen with the resulting gas cloud? Wouldn't a huge blob of hot gas traveling at .99c still be rather threatening? Or is the plan to only burn off a small portion to blow the missile off course rather than destroying it totally?

[Ariphaos]

Can you put some numbers out to back that up, please?

Figuring the feasibility of RKVs was part of the reason for this thread:

http://bbs.stardestroyer.net/viewtopic.php?t=122476

I still haven't had time to puzzle through the equations properly, but that sets a pretty stark limit on how short you can make a coilgun.

Let's make some assumptions.

1: We have a highly permeable magical material that has a magnetic saturation point at 2.5 Tesla and acts as if it is far from magnetic saturation at said point.
2: We can tightly control a magnetic field well enough that the field strength drops perfectly from 2.5 Tesla at the front of the projectile to 0 at the end, and maintain this perfectly and with no variation throughout the firing sequence.
3: The projectile will not lose ferromagnetic sensitivity as temperature rises.

All of these are unrealistic assumptions in favor of the RKV team. In part because I haven't been able to take the time to work out Kuroneko's refined bound yet, sadly.

The third assumption is not important for sufficiently low-speed RKVs or mass drivers, of course.

Anyway, the result of this is that you basically have an engine that uses no reaction mass (it's all external to the projectile) generating about 5 meganewtons of force per square meter of 'facing area' (the part that your target will 'see').

By choosing an appropriate density, area and length for the projectile, you can come up with the length of coil required. For example, a one cubic meter projectile (one square meter of surface area, one meter long) massing 7 tonnes (roughly iron).

Total acceleration: 715 meters/second

Time to .86 of c ignoring relativity: 360,000 seconds
Distance traveled getting to target velocity ignoring relativity: 46 billion kilometers.

Compared to the .99 c projectile people like to toss around like it's easy cheese, the .86c version is not spending much time at serious relativistic speeds. Still, the distance is going to be noticeably longer even for the .86c version.

So yeah.

RKVs are for morons.

[Ariphaos]

Do you have any numbers on this "fry the planet" thing? I'm not certain what level of devastation you are talking about.[/quote]

This is hard to specify, not because the math is hard for any given scenario - it's not - but because the type and style of attacks a pre-Type II civilization can perform with its host star are many and varied. So far the most effective one I have been able to think of is doing a rapid sweep of the target system's Dyson swarm.

But if you want to replace the idiocy in The Killing Star, turning the surface of a planet to slag is a trivial exercise for any world whose location you can predict with sufficient accuracy and is within a hundred light-years.

Yes, I believe the estimates in rec.arts.sf.science was that >75 ly you were safe for this reason.

Or anyone with a gravity tug.

There is, however, a serious problem if you piss a lot of your neighbors off, but this isn't that exercise.

If you are a type 2 R-bombs aren't an extinction threat anyways - your civilization has moved far beyond the planets and has significant space presence. R-bombs are only useful against weaker civilizations.

Even a Type I civilization can take pretty good precautions, and once a civilization starts placing solar statites it's only a matter of time.

And frankly for those you are better off with a small probe with onboard AI. You land on an asteroid in system and fire off a few jets to give it a nudge to send it into the planet. Looks more natural, and is a lot easier.

Are you kidding? At ~.7, we're already watching the sky and tracking hundreds of thousands of objects with some pretty impressive zeal. At 2, a race is going to have such a thorough stock of its local resources that there is no attacking them through such means.

[Darth Wong]

Which would be even harder to detect. IR and Gammas from AM annihilation are very distinctive and easily determined from the background. Structures are going to be much harder to detect because they aren't actively emitting anything, or if they are it is very weak. You might get some radio to key you in to look at something, or if you have neutrino detection fusion sources might turn up (depending on how good it is) but otherwise you are looking for something rather small at a very long distance. To just see the 10 thousand km accelerator as a blob you would need an angular resolution of 1.7*10^-10 arcseconds. At visible light wavelengths that means your array needs a baseline 45 light-minutes. And again, that means you need to look at the right part of space to see the blob - the area of space you need to look at for anything still goes up exponentially with the distance. And since it is just a blog, you wouldn't be able to tell what it is. By comparison, seeing the exhaust plume is easy

Why are you assuming that we would be trying to detect it with telescopes from here? Why wouldn't we have been sending a steady stream of probes to the area for years, before declaring that it is a high-tech society which must be aware of us?

[Junghalli]

I've given some thought to this question for my own uni. If two sublight civilizations want to attack each other with kinetic projectiles, just sending one big missile is a pretty dumb way to do it, for the reasons stated in the OP. It's only good for one target, which is a horribly inefficient way of going about things if your enemy has any real space presence whatsoever.

It'd be a much better idea for the relativistic ship to be a carrier that you fill up with little missiles. The little missiles need just enough delta V to nudge themselves onto a path that intercepts the various enemy space stations/habitats/facilities; they'd probably use low-thrust ion rockets (minimal chance of detection). You have a flyby probe go maybe a light year or two ahead of the attack force and send targetting data back via tightbeam or laser to guide the missiles home.

[Sikon]

The theory is that if two civilisations are aware of each other and able to send large craft at relativistic speeds to one anothers stars then the logical choice for each civilisation is to send a ship to the others home planet which doesn't slow down when it gets there. In other words a kinetic kill missile, or however many missiles it takes to take out the planet.

Now, putting the horrifying immorality of this prospect aside for a second, I have to wonder if it is really practical for advanced civilisations. See, when one is dealing with primitives like us, jammed as we are on to one planet, a single well placed salvo could probably finish us off forever. But when dealing with a more advanced culture, the kind which could build an interstellar missile, I think it's quite possible, probable even, that most of them wouldn't be on one planet, many of them could be dispersed around their solar system on space habitats of various kinds and sizes.

An advanced enough civilization would very early in its existence be able to see which star system(s) within its entire galaxy had a habitable planet developing life, and chances are extremely high that it would have such capability millions of years ago. The likelihood of any pair of civilizations arising without one developing millions of years before the other would be very low, considering the billions-of-years age of the universe.

If predisposed to wipe out potential competitors, it wouldn't need a relativistic kill vehicle of a rather questionable goal like 0.99c. With no urgency due to detection long before the development of a technological species on the target, a relatively non-relativistic moderate fraction of lightspeed works just as well to deliver destruction or a monitoring/control outpost to anywhere in that civilization's galaxy.

In contrast, the gaps between galaxies are great enough that, if there is no FTL, it is plausible that one civilization might develop before another civilization knew about it if they were in different respective galaxies. But just a planet-killing RKV would be pretty useless in that scenario against an opponent which could be spread across quadrillions of quadrillions times greater volume then a planet, so, if there was conflict, it could have to occur on a far greater scale and by different means than the RKV-type scenario.

In geological terms, there's a large length of time where a planet can be seen to be developing life (such as earth a billion years ago had life-produced oxygen in its atmospheric spectra) up to the development of intelligent life, then relatively a blink of an eye in which the civilization once formed can go from pre-industrial to become a colonization sphere expanding at a large fraction of the speed of light to encompass countless star systems.

********************

Actually, the closest to a scenario for RKVs would be a situation like human interstellar colonies fighting each other, since, unlike separate alien civilizations, they could be at comparative levels of tech and capabilities in the same century. However, space habitats are obtained before enough space infrastructure for interstellar flight, with real-world future interstellar colonization needing to not be planet-dependent anyway when there aren't a bunch of earths a few light-years away.

Plus the defenders have an enormous mass advantage with the enormous amount of industry involved in sending a comparatively small mass to another star. (Among various applications of this, while the interstellar medium is such a near totally perfect vacuum as to permit fractional-lightspeed travel, the defenders could with comparative ease create thin dust fields millions of kilometer in diameter lethal to incoming enemy craft at such speeds).

And the energy capability to send out thousands of colonization starships at a moderate portion of lightspeed is obtained before the capability to launch a single planet-destroying RKV, let alone to send out an armada big enough to theoretically exterminate a developed space civilization. E.g. your target colonized so-and-so new star systems during the years your armada was traveling enroute to one of their older colonies.

Anything is conceivable, but it is rather hard for interstellar war to be practical or desired by anyone in such a situation.

********************

When considering what is realistic in the real universe, there's of course the classic Fermi Paradox. If a significant number of the hundreds of billions of star systems in the galaxy developed life leading to an intelligent technological species, it would seem at least one should have already colonized the galaxy millions of years ago.

An explanation seems unlikely to just be alien civilizations socially evolving towards being uninterested in expansion or in travel which would take many years to progress from one star system to the next. That could be a workable explanation if just explaining away a single civilization not colonizing the galaxy. However, if there was so much as a civilization per million stars, it stretches belief that every last one of the resulting hundreds of thousands of civilizations in the galaxy would not be interested in expansion.

Any advanced civilization could technically colonize the entire galaxy in under a million years, a geological instant.

Some common answers in popular imagination are poor explanations. For example, interstellar travel being impossible can not be an explanation, as it is 100% known to be possible by means such as nuclear propulsion at a few percent the speed of light (speeds for which the interstellar medium isn't even close to a show-stopping issue), among other methods. Enrico Fermi came up with the Fermi Paradox when he observed what nuclear energy entailed for the possibility of interstellar travel.

Civilizations tending to die out doesn't make much sense either as an universal explanation, since there is so little that can even potentially kill a civilization once it is spreading between stars, and, even if one did nevertheless somehow die out, any large number of civilizations should include at least some surviving indefinitely.

Some potential answers:

(1) Life and/or the progression of it up to multicellular organisms and all the steps for the rise of an intelligent technological species is extremely uncommon. Perhaps there tends not to be orders of magnitude more than a single incident per galaxy in billions of years if even that.

Intuitively, 400 billion stars in the galaxy superficially seems like so many as for anything like the rise of civilizations to occur multiple times, but that's no real guarantee. As a loose analogy, there are far more molecules in a glass of water than stars in the galaxy, yet a glass of water does not contain every known chemical species.

To use random arbitrary example figures, it might be the case that only one in a hundred, a thousand, or some other large number of star systems has a relatively habitable planet for potential formation of life. (For example, those seen so far by current planet-finding efforts don't count as such, and the right conditions may involve a lot more than rocks and liquid water alone).

Although life arising early in earth's history is conventionally assumed to suggest it is a likely event within a short length of geological time under the right conditions, that's a guess rather than proof, with little to no ability to quantitatively model the probability directly today. Then the progression to multicellular life did not occur for a very long time in earth's history, perhaps an event with only a moderate or low chance per billion years and only under the right circumstances, and so on.

With enough factors like that for a chain of improbabilities multiplying each other, it is conceivable to reach some figure like eleven orders of magnitude more lifeless star systems than ones with a technological species, in which case the Fermi Paradox could be explained as being alone in the galaxy despite it having 4E11 stars ... e.g. a probability turning out to be something like 0.01 * 0.001 * 0.0001 * 0.01 etc.

or

(2) Something unclear under present knowledge prevents almost any civilization from continuing expansion as we know it.

As previously described, that can't be physical impossibility since starships are technically possible, nor are social factors potentially varying between different civilizations an easy explanation for them *all* not doing it.

The factor could be something hard to conceive like if civilizations eventually always obtain some technology rendering the resources of other star systems in the galaxy irrelevant, unneeded, and not desirable even to 1 in 1000 civilizations. A random example would be the ability to create new universes somehow and transit to them or something far out like that, although that's rather doubtful within the context of real-world physics.

Some have wondered what if the reason other civilizations are not seen is instead because fledging species are wiped out by something, such as killer self-replicating robot swarms seeded throughout the galaxy as the legacy of a first ancient civilization hostile towards potential competition. However, anything imagined like that should have already known of life on earth for a long time and should have already acted if it existed, though.

********************

Overall, #1 such as the rare earth hypothesis seems to be a particular possibility. For there to not exist a bunch of civilizations nearby but rather for the nearest alien civilization if existent to likely not even be in this galaxy would explain why everything in the sky seems to be only natural.

It would be saying that people are "lucky" insofar as the sequence of events leading to human civilization was extremely unlikely, but, really, no matter whether there are a million or a trillion total planets per planet with intelligent life, there's no way we could happen to be on a planet without such ... just like we are not lucky to be standing on patch of matter on earth's crust rather than in the center of a star since there's no possible way it could be otherwise with us here to talk about it.

Carter's Weak Anthropic Principle: "We must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers."

********************

But another part of what is noteworthy in such considerations is that the common sci-fi depiction of multiple alien civilizations nearby each other at coincidentally about the same level of technology and scale isn't consistent with anything likely.

The rare earth hypothesis would further suggest that civilizations are likely to arise not in each other's nearby stellar neighborhood but rather be far rarer and farther apart.

Scenarios like two technological civilizations arising a few light-years or tens of light-years apart in the same millenium then shooting RKVs at each other just aren't realistic. A scenario like an alien civilization per few thousand stars is patently implausible when that would suggest millions in the galaxy of 400 billion stars, and, in that case, some of those civilizations should have arisen many millions of years ago and already taken over the galaxy long ago if interested in such expansion.

[phongn]

I've heard another explanation - civilization tends to reach the point where it is capable of developing the means to destroy itself (whether runaway climate change, biological or nuclear warfare, etc.) and then does so. I think Stuart's mentioned that one before.

[Sikon]

Something like the rare earth hypothesis is required anyway though. Otherwise, if, for example, there was as much as one civilization per million stars, it would be hard to imagine every last one of the resulting hundreds of thousands of civilizations in the Milky Way of 400 billion star systems doing that, 99.999+%.

For them all to happen to destroy themselves before spreading becomes a little more conceivable if only there was instead only one or a handful of past civilizations in the galaxy of 4E11 stars. But if an intelligent species only occurs once per billions of star systems anyway, the main factor involved is the rarity of its original development ... #1 in the prior post. Such might even just be so uncommon that humans might be the only civilization ever arisen in the galaxy since there's only a relatively subtle difference when multiplying all the terms in the rare earth hypothesis needed to get a result of 1 in E11 or something versus a result like 1 in E10.

[Admiral Valdemar]

Actually, Sikon, Reynolds' explanation for your second scenario not being apparent to us yet is, as mentioned earlier, down to a limited number of such machines being spread around, with certain artifacts used as lures. For example, an unnatural object orbiting a star that wouldn't normally occur.

Any space faring race that finds such an interstellar oddity investigates, is remotely assessed and triggers an awakening in the nearest cluster of machines who then initiate a growth phase to tackle the emerging species threat. So long as you stay in your own home system, you're left alone.

Additionally, the human factions in his novels only use RKVs for tactical attacks given the tech level for defence. And they're the nuke pumped disposable ones at that.

[Lord of the Abyss]

Some potential answers:

(1) Life and/or the progression of it up to multicellular organisms and all the steps for the rise of an intelligent technological species is extremely uncommon. Perhaps there tends not to be orders of magnitude more than a single incident per galaxy in billions of years if even that.

Intuitively, 400 billion stars in the galaxy superficially seems like so many as for anything like the rise of civilizations to occur multiple times, but that's no real guarantee. As a loose analogy, there are far more molecules in a glass of water than stars in the galaxy, yet a glass of water does not contain every known chemical species.

To use random arbitrary example figures, it might be the case that only one in a hundred, a thousand, or some other large number of star systems has a relatively habitable planet for potential formation of life. (For example, those seen so far by current planet-finding efforts don't count as such, and the right conditions may involve a lot more than rocks and liquid water alone).

Although life arising early in earth's history is conventionally assumed to suggest it is a likely event within a short length of geological time under the right conditions, that's a guess rather than proof, with little to no ability to quantitatively model the probability directly today. Then the progression to multicellular life did not occur for a very long time in earth's history, perhaps an event with only a moderate or low chance per billion years and only under the right circumstances, and so on.

With enough factors like that for a chain of improbabilities multiplying each other, it is conceivable to reach some figure like eleven orders of magnitude more lifeless star systems than ones with a technological species, in which case the Fermi Paradox could be explained as being alone in the galaxy despite it having 4E11 stars ... e.g. a probability turning out to be something like 0.01 * 0.001 * 0.0001 * 0.01 etc.

One interesting bit of speculation I've heard in this vein is that perhaps the vast majority of life bearing worlds are water worlds, with no land for a technological species to develop on. It wouldn't take too much more water relative to the mass of a planet to do that; perhaps Earth is just freakishly dry for a life bearing planet

[Sikon]

Actually, Sikon, Reynolds' explanation for your second scenario not being apparent to us yet is, as mentioned earlier, down to a limited number of such machines being spread around, with certain artifacts used as lures. For example, an unnatural object orbiting a star that wouldn't normally occur.

Any space faring race that finds such an interstellar oddity investigates, is remotely assessed and triggers an awakening in the nearest cluster of machines who then initiate a growth phase to tackle the emerging species threat. So long as you stay in your own home system, you're left alone.

Although Beserker-probe type entities like the Inhibitors of those stories shouldn't really wait until a civilization is spacefaring before observing and attacking when it is simplest to wipe out fully, since self-replication should allow enough for observation of all relevant planets, hypothetical possibilities like that lead to a curious conclusion. If there was ever strong evidence against the Rare Earth Hypothesis discovered someday, that would be scary in a way, since it would raise the specter of a potential Great Filter in the future that 99.99+% of civilizations didn't pass, whether Beserker-type entities not attacking yet for some reason or something else.

(To take another random example, another rather simple albeit unpleasant answer to the Fermi Paradox would be if there tends to not be more than one advancing technological species around at a time because about 100% of such sooner or later build a multi-light-year accelerator to reach Planck Energy, and unknown physical results cause the end of the area then).

In contrast, it's a good thing that the Rare Earth Hypothesis appears likely with nothing disproving such so far at least.

WHERE ARE THEY?

WHY I HOPE THE SEARCH FOR EXTRATERRESTRIAL LIFE FINDS NOTHING

What could be more fascinating than discovering life that had evolved entirely independently of life here on Earth? Many people would also find it heartening to learn that we are not entirely alone in this vast cold cosmos.

But I hope that our Mars probes will discover nothing. It would be good news if we find Mars to be completely sterile. Dead rocks and lifeless sands would lift my spirit.

Conversely, if we discovered traces of some simple extinct life form—some bacteria, some algae—it would be bad news. If we found fossils of something more advanced, perhaps something looking like the remnants of a trilobite or even the skeleton of a small mammal, it would be very bad news. The more complex the life we found, the more depressing the news of its existence would be. Scientifically interesting, certainly, but a bad omen for the future of the human race.

How do I arrive at this conclusion? [...]

You start with billions and billions of potential germination points for life, and you end up with a sum total of zero extraterrestrial civilizations that we can observe. The Great Filter must therefore be powerful enough—which is to say, the critical steps must be improbable enough—that even with many billions rolls of the dice, one ends up with nothing: no aliens, no spacecraft, no signals, at least none that we can detect in our neck of the woods. [...]

Now, an important question for us is, just where might this Great Filter be located? There are two basic possibilities: It might be behind us, somewhere in our distant past. Or it might be ahead of us, somewhere in the millennia or decades to come. [...]

So one possibility is that the Great Filter is behind us. This would explain the absence of observable aliens. Why? Because if the rise of intelligent life on any one planet is sufficiently improbable, then it follows that we are most likely the only such civilization in our galaxy or even in the entire observable universe. [...]

The other possibility is that the Great Filter is after us, in our future. This would mean that there is some great improbability that prevents almost all technological civilizations at our current human stage of development from progressing to the point where they engage in large-scale space colonization and make their presence known to other technological civilizations. [...]

I shall say a few words about another theoretical possibility: that the extraterrestrials are out there, in abundance but hidden from our view. I think this is unlikely, because if extraterrestrials do exist in any numbers, it’s reasonable to think at least one species would have already expanded throughout the galaxy, or beyond. Yet we have met no one. [...]

If we discovered some very simple life forms on Mars in its soil or under the ice at the polar caps, it would show that the Great Filter must exist somewhere after that period in evolution. This would be disturbing, but we might still hope that the Great Filter was located in our past.

If we discovered a more advanced lifeform, such as some kind of multicellular organism, that would eliminate a much larger stretch of potential locations where the Great Filter could be. The effect would be to shift the probability more strongly to the hypothesis that the Great Filter is ahead of us, not behind us. And if we discovered the fossils of some very complex life form, such as of some vertebrate-like creature, we would have to conclude that the probability is very great that the bulk of the Great Filter is ahead of us. Such a discovery would be a crushing blow. It would be by far the worst news ever printed on a newspaper cover.

Yet most people reading the about the discovery would be thrilled. They would not understand the implications. If the Great Filter is not behind us, it is ahead of us.

So this is why I’m hoping that our space probes will discover dead rocks and lifeless sands on Mars, on Jupiter’s moon Europa, and everywhere else our astronomers look. It would keep alive the hope for a great future for humanity. [...]

None of this means that we ought to cancel our plans to have a closer look at Mars. If the red planet ever harbored life, we might as well find out about it. It might be bad news, but it would tell us something about our place in the universe, our future technological prospects, the existential risks confronting us, the possibilities for human transformation: issues of considerable importance.

From here.

One interesting bit of speculation I've heard in this vein is that perhaps the vast majority of life bearing worlds are water worlds, with no land for a technological species to develop on. It wouldn't take too much more water relative to the mass of a planet to do that; perhaps Earth is just freakishly dry for a life bearing planet

Interesting idea.

Possibly there could be quite a combination of factors. Some have also suggested that a large moon may have been needed to stabilize earth's axis for the evolution of complex life; that without a massive planet like Jupiter the flux of cometary impacts on earth would have been 1000 times higher causing too frequent mass extinctions; and so on.

Within the galaxy's habitable zone, one form of a Rare Earth Equation can be written as the product of the number of metal-rich planets in a star's habitable zone, times the fraction where life arises, times the fraction of those where that evolves to multicellular life, with evolution of complex organisms potentially dependent on the limited portion with a large moon, and so on for a number of factors. Multiply out enough factors of 0.1 or 0.01 or the like, and definitely an astronomically tiny end result seems quite a possibility.

[Surlethe]

Regarding vaporizing the RKV when it nears the target, what would happen with the resulting gas cloud? Wouldn't a huge blob of hot gas traveling at .99c still be rather threatening? Or is the plan to only burn off a small portion to blow the missile off course rather than destroying it totally?

Burning a small portion to blow the missile off course would work, I suppose - one would need to run numbers on it. The point of vaporization is to create a gas cloud, which expands; you just need to vaporize it far enough away that the gas cloud expands significantly before it impacts, or else you're still getting all of that kinetic energy delivered to the target.

[Admiral Valdemar]

I don't know which is worse: being all alone because no other species evolved to sapience and space faring status, or that there was once a prosperous galaxy that got silenced by a malevolent, mysterious force.

Still, the Inhibitors weren't originally made as Beserkers, but more helpful in keeping the Milky Way organised until the Andromeda collision is over. Of course, that the machines deteriorated over the aeons to become Beserkers isn't a good sign either. The discussions on life out there have always fascinated me, especially with respect to potential hostility.

[Illuminatus Primus]

What if you're not dealing with a logical foe? There's plenty of room for religious zealotry in space, and the idea of some species going out of its way to make sure no one rises to attack their status is interesting (although any elder species would probably erase or at least amend any such civilisation if in their locale).

Many instances of this scenario assume a species approaching a threat level, while the other one is already at least Level II on the Kardashev scale. In that case, it's fair to assume, given a more practical range from the target civilisation, that taking out the potential threat is much more enticing when they're unable to get off their planet en masse, never mind mount a counter-attack. Relativity factoring for time lag would be an issue, but no one goes half arsed with these kind of attacks. You either wipe them out to stone age level and send some probes to mop up later, or you face a very angry civilisation, united in their rage against you.

The basic assumption of the RKV scenario is it is THE ONLY LOGICAL alternative. Not a very well-thought out, theory, to be sure.