Stephen Hawking is Afraid of Aliens.

[Iosef Cross]

I'm sure the assorted indigenous peoples on the recieving end of European imperialism and the White Man's Burden will be more than happy to tell you what happens when a far more advanced culture comes into contact with a less-advanced one. There used to be a dizzying variety of cultures, languages, and historical traditions in the Americas. Now . . . indigenous languages are dying out, indigenous culture has been almost completely obliterated by church and later McCulture, and indigenous people suffer staggering rates of alcoholism and diabetes.

That's the natural result of integration. The long run tendency for our planet is for total cultural unification, there will be only one culture in the world by ~2200. Of course, if that's bad or good, well, that's a question of preference.

You are a "cultural environmentalist": Someone that values the preservation of the cultural "ecosystem".

Once again, you fail to grasp the orders of magnitude of difference between us and an interstellar civilization. The difference between an interstellar civilization and us is of similar magnitude to the difference between us and elephants or chimpanzees. Are we integrating their civilization with ours? No. We're eating the chimpanzees and shooting the elephants because they object to their land being turned into farmland. Even in a relatively benign example, like our interaction with dolphins . . . we're poisoning their environment and slowly killing them off as a side-effect of our exploitation of the oceans.

I would say that the differences between our civilization and an alien civilization would be smaller than the differences between our civilization and monkeys.

The thing that we would have in common to the aliens would be the existence of certain macro social processes that are like 10 orders of magnitude greater than anything that monkeys have done. We, as individuals, are a few orders of magnitude smarter than monkeys. Your social orders are themselves millions of times smarter than us.

Even if the difference between individual aliens and us are of several orders of magnitude, if the aliens discover us, our civilization would be the second most complex thing in the know universe, after their's.

Wow. No-limits fallacy much? For that matter, the astonishing idiocy of your statement only grows with each re-reading. Independent from natural resources? So, where do the hydrocarbons to make plastics and fertilizer come from? The Magical Free-Market Fairy?

You are so proud of your ignorance...

And you are one dumb dipshit.

The capacity to extract useful goods from natural resources grows with the advancement of civilization. By the time that they reach us, they would be able to convert dirt into a cup of coffee.

If you can't grasp that, then you're even more retarded than I first thought.

You are imprisoned into your preconceptions. Even them, if you think that they would convert Earth into space habitats, them, you should have understood that you are wrong about that:

The only resources that would be valuable would be mass and energy. And that, the universe has 99.9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999% of it outside Earth.

To the enormous regret of the Humans who used to live on the scattered Zeta Reticuliian orbital habitats formerly known as Earth . . .

Why they would travel to the solar system to convert Earth into habitats? There will be at least millions of stars nearer to their's.

If they travel here, it could happen for two reasons: They would be converting the entire galaxy into habitats and we just happen to be on the path. That would be fun: We would see in the period of hundreds/thousands of years your nearest stars disappear into Dyson spheres while we await for our certain destruction.

But, being more realistic, by the time they reach us, we would have turned our system into a Dyson sphere as well. Hence, for them to get your "mass and energy", they would have to go to war with something much more powerful than what we are right now. We could bombard any fleet of VN machines with a cloud of billions of teraton level relativistic missiles.

That would mean that getting our solar system's mass and energy would be at probably at a greater cost than the benefits.

While the gains of integrating your 7 billion sentients into a interestelar economic system are positive.

Seven billion sentients barely more advanced than their chimpanzee cousins. Seven billion sentients with likely incompatible biochemistries (so less to trade) and with none of the industrial base required to produce any goods of value to an interstellar civilization at any rate sufficient to satisfy any theoretical demand.

1- That's a case of an irrational process. To us that would be like an natural process, i.e. like our sol going into a supernova.

What the fuck does this even mean?[/quote]

It means that they wouldn't be consciously destroying us, because we would be so insignificant that they will be passing over us like we walk over amoeba.

2- An advanced alien civilization wouldn't be interested in destroying accidentally other civilizations.

Says who? You're counting on the fact that a human life will be more meaningful to an interstellar alien than the lives of chimpanzees, elephants, cetaceans and corvids are to us.

No, I am counting on that fact that human life and would be interesting than what the rest of the universe is: A bunch of rocks.

If we are the first alien civilization that the aliens discover, they would be just as impressed as we are.

Earth has only a insignificant fraction of the mass in the solar system with has an insignificant fraction of the mass of all the stars that are nearer to us than the nearest star with may have intelligent life. The only reason why they would come to our planet is because of us.

[Samuel]

That's the natural result of integration. The long run tendency for our planet is for total cultural unification, there will be only one culture in the world by ~2200.

Not really. Spain has been integrated for a while and their are still regional cultural differences, enough for the government to have to increase local autonomy. England has has Scotland for 300 years- heck, China has been united for over a thousand on and off and there is still cultural differences between internal groups.

The thing that we would have in common to the aliens would be the existence of certain macro social processes that are like 10 orders of magnitude greater than anything that monkeys have done.

Yes, but we don't know how things will change as they scale up.

The capacity to extract useful goods from natural resources grows with the advancement of civilization. By the time that they reach us, they would be able to convert dirt into a cup of coffee.

Only because wages should be so high that it is cheaper to do that than farm

The only resources that would be valuable would be mass and energy.

I don't think transmutation of elements is something we can pull off. We could probably use fusion to make some of the lighter ones, but heavier elements are going to be rarer and hence valuable.

[Iosef Cross]

Note that I didn't say it was impossible that alien species that come to Earth might want to destroy us. But only that it is improbable, like it is improbable that your neighbor wants to kill you, while it isn't impossible it is not likely.

[Oni Koneko Damien]

Note that I didn't say it was impossible that alien species that come to Earth might want to destroy us. But only that it is improbable, like it is improbable that your neighbor wants to kill you, while it isn't impossible it is not likely.

The difference is that if our neighbor wants to kill us, there are all sorts of things we can do to prevent it, ranging from locking the door and calling the cops to learning self-defense and/or keeping a firearm in a nearby (safe) location, to simply moving away. If an alien civilization that is magnitudes more powerful than us wants to kill us... well, we're shit out of luck there. So why go out of our way to gamble humanity's existence on those admittedly unknown odds by going out of our way to attract attention to ourselves?

[Bakustra]

What is the power density of the CMBR? My crude, crude approximation (universe is a 46 billion light year-radius cavity surrounded by and in thermal equilibrium with a 2.7 K blackbody) gives about 2E-32 W/m^3.

What are you using? I decided to ask an astronomy professor and he pointed me to the Stefan-Boltzmann law, which gives an intensity, for a 2.73K blackbody as the CMBR, of 3.15e-6 W/m^2. However, he kindly pointed out that this does not actually mean that this is the threshold of detectability, as, indeed, we would not have been able to receive messages from the Sojourner Rover if that was the case, for the intensity of its signals is 4.56e-16 W/m^2, about one billion times under the CMBR. Indeed, the majority of radio traffic on Earth is weaker than the background noise in general, but we can pick out frequencies even when they are overpowered by the CMBR and other sources of noise. So, we cannot use that as a limit. Mr. Degan, I must inform you that you are wrong, but that it is an easy misconception, as I thought so too.

[Big Orange]

Judging from this thread alien invasion or contact seems improbable, due to the abundance of basic resources on tap to any space faring culture or entity (433 Eros, a local asteroid, has 20 Trillion dollars worth of metal ore alone) and the truly mind boggling distances between inhabited systems (even if the light barrier could be broken). However don't count out banal curiosity if in the slim chance the aliens wanted to head our way (rather than be driven by a irrational need to harvast, abuse, and destroy. Hopefully.) and what's stopping them?

Consider how far out we can see today with our telescopes in significant detail, when we haven't been to fecking Venus and Mars yet, though aliens with vastly superior telescopes negates them having to go right up to us in person (or in nano-cloud, most probably). And being a immortal machine is necessary to being a true space traveller, an AI is not affected by vast periods of time and vast distances, and they're far removed from flawed, spiteful organic beings. The danger of alien contact most likely is the unintended indirect effects that could have on us (like spark a religious war etc), rather than the aliens being space Nazis (a very old, passive, and patient space computer would not be driven by the same impulses as a human megalomaniac).

Here's an interesting essay that counters Hawking's claims:

Alien Invasion: Why Stephen Hawking is Wrong

When British cosmologist Stephen Hawking warned against contact with extraterrestrials in a new Discovery Channel documentary, he was repeating a well-worn argument. “If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans.”

But Hawking’s reasoning is flawed on a number of counts. First, we can ask why the aliens would come here with guns blazing. What could they possibly want? Hawking suggests that Earth’s resources might be a reason. “I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach.”

It is a chilling image, reinforced by science fiction stories from “War of the Worlds” to “Independence Day,” but the argument doesn’t really wash. A super-civilization capable of making starships would certainly have the means to observe Earth in detail from many light years away, and they would have known all about our planet’s resources for as long as they had possessed advanced technology.

Here we hit another common misconception. Earth is about 4.5 billion years old, and there were stars and planets around long before the solar system even existed. Assuming intelligent life is likely, as Hawking suggests, then some alien communities would have emerged a very long time in the past. If resources are the motivating factor, then at least one group of aliens would surely have spotted Earth as a desirable destination millions of years ago, and come here when they could have had the planet for the asking, without pesky humans to complicate the takeover.

Another problem with Hawking’s picture is the sheer distances involved. The galaxy is huge by human standards. The nearest star is over four light years away -– about 25 trillion miles. Within the scientific community, even the optimists believe the nearest civilization could well be hundreds of light years away. Because nothing can travel faster than light, the Hollywood image of aliens plying the vast interstellar voids in star fleets is absurd. It’s far more likely that alien civilizations would limit contact to radio communication rather than engage in the sort of close encounters favored by movie makers.

But suppose by some fluke aliens did come to visit Earth in the near future, then comparisons with Columbus are in any case wide of the mark, and reflect the rampant anthropocentrism that pervades much speculation about alien life. Just because we go around wiping out our competitors doesn’t mean aliens would do the same. A civilization that has endured for millions of years would have overcome any aggressive tendencies, and may well have genetically engineered its species for harmonious living. Any truly bellicose alien species would either have wiped itself out long ago, or already taken over the galaxy.

By comparison, humans would quite likely be considered dangerous warmongers, posing a possible menace to our galactic neighbors in centuries to come. If so, then ET may act to eliminate the threat if we didn’t mend our violent ways. Ironically, the greatest danger from an alien encounter may be ourselves.

Paul Davies is author of “The Eerie Silence: Renewing Our Search for Alien Intelligence.” He also chairs the SETI Post-Detection Taskgroup, and is a Professor at Arizona State University.

Speakeasy

[Patrick Degan]

Time permits me to address only two points in this exchange:

Well, Mr. Degan, I can see that you have quietly abandoned your assertion that my proposals violate the Theory of Relativity. Well, that wasn't defensible in any way, so I will grant you that freely. However, I see that integrity is not on the menu at the Degan Diner today.

Oh, I'll quite readily withdraw that challenge, since it's apparent that you weren't proposing a physics-breaking "principle" so much as setting up yet another of your stupid strawmen to knock down. Which rather disqualifies you to comment on the subject of integrity, child.

You mean, except for the part where you decided to ignore Hawking's actual scenario, right? Because you presumed no actual effort to contact aliens, which is what Hawking is warning against. You also provided a mix of qualitative and quantitative analysis that led to deeply amusing conclusions. But now you have been gracious enough to provide corrections where my analysis was flawed.

I've ignored nothing. We've gotten sidetracked along this particular tangent as well by our mutual hostility. However, to return to the kernel of this subject: even given the deliberate effort to contact aliens, Hawking's fears are unwarranted for several basic reasons: a tight-beam transmission may indeed reach out to far longer ranges than omnidirectional radio signals but it also suffers the limitation that it's strictly line-of-sight. Unless the beam manages to actually, by random chance, intersect a point in space with a receiver apparatus at the other end, the signal won't be heard by anyone. And there are many ways the beam can be thrown off target, such as the rotational and orbital motions of the Earth carrying the transmitter off the intended trajectory, or the beam could be scattered by interstellar dust clouds. The signal might never make it to a receiver at the other end if the planet it's on is occluded by another planet at that moment, or by material in the target system's own Oort Cloud or Kuipier Belt, thus blocking the beam. If the calculation for aiming at a target system is off even by a very small fraction Earthside, that's an error that will pile up significantly across the lightyears and the signal will go off into empty space instead. Long and short of it: sending a message out into galactic space by tight-beam radio transmission (or laser) is a crapshoot with far worse odds than what you'll have to deal with at any gaming table here on Earth. It will be a pure accident if a contact is made at all.

And if, by pure chance, the signal actually does strike a receiving array somewhere out there, the sheer scale of interstellar distances makes sending an invasion fleet a very impracticable operation with STL spaceflight. While it is theoretically possible to construct a craft that might get up to better than .9c (and desirable in order to take full advantage of time dilation for flights across hundreds of light years), such starships so far have required obscene amounts of fuel by calculation to make such a journey possible. The .9+c starship may not be impossible as the FTL starship is, but it may be unrealisable if no way to solve the mass penalty of the requisite fuel is found i.e. a practical means of manufacturing and storing antimatter (and even then, a few thousand tons of the stuff will be required along with matter for reaction) being discovered.

This means, for practical purposes, starships which at best may get up to ~.33c, but that makes any journey beyond a 20 ly radius a very long, slow slog (decades/centuries instead of years/decades). And how long can a space vessel actually be expected to last? How many decades can it manage to remain in service before critical systems suffer irreversible breakdown, or before structural fatigue sets in? On those terms, a hypothetical hostile alien civilisation from, say, 500 lightyears away, that notices us may want to do something about the Earth, but if they can't actually get here in anything resembling their expected lifetime or within the practical service lifetime of any ship they can send out, Earth may as well be as far away as the outmarches of the Virgo Supercluster. They're not coming.

As far as the scenario of a ship (or migration fleet) in flight intercepting a radio beam from Earth is concerned, that also is something that would only happen by pure accident at best for many of the same reasons outlined above, only the situation is far more complicated by the fact that the "target" is moving at a velocity far greater than a planet's motion through the galaxy. However, to be generous, let's assume that a tightbeam transmission from Earth happens to be propagating along the ideal trajectory to intersect with the course of a passing starship tooling along at .11c and there's a clear line of sight at the point in space where the two shall meet —oops, too late: the ship has already passed through the intersection in a millionth of a second. Far too swiftly for anything to have even registered on the detectors. We actually made contact but they never noticed us at all. Again, they're not coming.

This is why SETI is very likely to fail to achieve anything other than enhancing our ability to map the known universe, why Hawking's fears are unwarranted, and demonstrates just how brutally and efficiently science murders the beloved tropes of science fiction.

BTW, Bakustra, sorry for the dustup.

[Big Orange]

Here's an interesting video on the possibility of advanced alien life, the flaw of SETI and the theory of the Multiverse by Michio Kaku:

[Superman]

Can I ask why so many people assume that an advanced spacefaring civilization must surely have evolved beyond the capacity for aggression and violence? Some people seem to treat this idea like it's a natural law; as if the evolution of a sentient species must ultimately lead to a race of peace loving pacifists. Why couldn't a sentient species survive and flourish because of a tendency for aggression and violence? Couldn't this too serve as a mechanism which might ensure the survival of a particular species? Personally, I'd hope that evolution universally does lead to peace loving civilizations, regardless of their own unique evolutionary histories. I'm just not sure this is an assumption we should make.

I'm not saying I agree with Hawking and his essay on why we should be a planet of fearful xenophobes, either. Hawking is definitely an expert with an opinion that matters, but people aren't always objective on matters of speculation.

[Sarevok]

Furthmore lets not forget the berserker threat I cited earlier. A star faring civilization will probably be dominated by AIs or at least use AIs as pilots for their starships. This is much cheaper, simpler than freezing a living crew or creating generation ships and probably only way to explore interstellar space. Immortal self replicating machines are not same as the species that created them. The AIs will not bear any similarities to organic species that built them. Whatever evolutionary pressure drove the creators species to sentience will not apply to them.

[Bakustra]

Mr. Degan, I accept your apology and offer one of my own. However, I still disagree somewhat. Look at it as though it were a sphere of transmissions. While the sphere currently is not that large, if we make a concerted effort to find aliens we can extend it to a far greater radius. Then we will have a greater chance that an alien starship, should such a thing be practical, pass through the sphere, and take note of our existence. If they move quickly, then perhaps they decide to send a probe later, or perhaps an expedition if they take a close look at Sol, or if they move slowly, they might decide to change course or make us their next destination after arrival.

While your point about durability is well-taken, I think that it depends on several important factors: firstly, the density of the interstellar medium, which is something we are not likely to have a good grasp of unless the Voyager probes or New Horizons are still functional after passing through the heliopause. This is the least important though. The second factor is what the starship is, exactly. In theory, it could be entirely crewed by computers and uploaded organics, and devote most of its space to industrial equipment for the arrival. On the other end, it could be a mobile habitat. To be honest, if we can build a habitat that can remain self-sufficient for a few centuries, then we can build a starship that can do so. If they take the habit of hopping from metal-rich system to metal-rich system in their travels, then they need not last much longer, as they essentially have become somewhat like the Age of Sail: solar systems are islands where one can make repairs and "dock", while interstellar space is the open sea. However, this analogy breaks down remarkably quickly beyond that, but in theory, they can last a very long time indeed. Now, if their onboard mining equipment breaks down, then we have, if I may digress for a moment, the materials for an interesting sci-fi story: an alien starship falls into our solar system, requesting refined metals, and we face the challenge of communicating with them and getting the metals to them.

Hawking's point, though, is that we should not attempt to draw attention to ourselves, because though the chances are, I agree, minimal, we still would be increasing them. In other words, until we have the ability to travel between the stars ourselves, anybody coming to visit will be well beyond us, and we will essentially have all our eggs in one basket. Even if they mean no harm, consider the Dawes Act of American history, and the "Indian schools", neither of which were ill-intentioned, but resulted in significant harm.

[Bakustra]

Furthmore lets not forget the berserker threat I cited earlier. A star faring civilization will probably be dominated by AIs or at least use AIs as pilots for their starships. This is much cheaper, simpler than freezing a living crew or creating generation ships and probably only way to explore interstellar space. Immortal self replicating machines are not same as the species that created them. The AIs will not bear any similarities to organic species that built them. Whatever evolutionary pressure drove the creators species to sentience will not apply to them.

Why is it simpler than creating a working generation ship? Why is it necessary that a starfaring civilization be dominated by AIs. Even if you can show that this will be the case for humanity, it does not follow that an alien species would have adopted artificial general intelligences, nor that doing so is actually cheaper or simpler than a working generation ship.

[Sarevok]

Why is it simpler than creating a working generation ship?

Think about how big a generation ship has to be.

Now think about if you had an AI running on a computer that could fit into a spacecraft the size of Pioneer 10.

Which is simpler to make and launch ? We could already send probes to other stars. Someday we could put AIs on those probes. But not even the wildest optimist can say when we can send a baseline human to another star system.

Why is it necessary that a starfaring civilization be dominated by AIs

They may or may not be dominated by their AIs. But it is likely they would be surpassed by their AI creations. Human or otherwise naturally evolved organic life is inferior to purpose designed intellects running on superior equipment.

Even if you can show that this will be the case for humanity, it does not follow that an alien species would have adopted artificial general intelligences, nor that doing so is actually cheaper or simpler than a working generation ship.

It is quite to simple to see really. You need far bigger or much faster ship to support an organic crew than a computer with an AI. Any rational alien species is going to come to this conclusion.

[Bakustra]

Why is it simpler than creating a working generation ship?

Think about how big a generation ship has to be.

Now think about if you had an AI running on a computer that could fit into a spacecraft the size of Pioneer 10.

Which is simpler to make and launch ? We could already send probes to other stars. Someday we could put AIs on those probes. But not even the wildest optimist can say when we can send a baseline human to another star system.

You have to show that a functional AI is easier to make than a generation ship. You have not done so.

Why is it necessary that a starfaring civilization be dominated by AIs

They may or may not be dominated by their AIs. But it is likely they would be surpassed by their AI creations. Human or otherwise naturally evolved organic life is inferior to purpose designed intellects running on superior equipment.

But why would they create AGI, necessarily?

Even if you can show that this will be the case for humanity, it does not follow that an alien species would have adopted artificial general intelligences, nor that doing so is actually cheaper or simpler than a working generation ship.

It is quite to simple to see really. You need far bigger or much faster ship to support an organic crew than a computer with an AI. Any rational alien species is going to come to this conclusion.

Prove that this outweighs any of the difficulties with AGI production. In other words, do not make authoritative statements unless you can back them up.

[Big Orange]

You have to show that a functional AI is easier to make than a generation ship. You have not done so.

Our biggest space station is little tin cans and tinfoil sheets tied together, and we haven't properly built a reliable engine yet. While neuroscientists backed by the Pentagon are making the first tentative steps towards replicating the functions of a cat's brain. It's still too early to gauge which technology is going to be more successful first.

I can imagine these long haul generational ships that is built by a alien culture we could comprehend would be like mobile worlds, cigar shaped, with a big ion engine at the back and thousands/millions of beings residing inside the cigar's interior.

[Sarevok]

Starglider would beat me if I made dumb statements here so I will refrain from making any technical statements. But consider this. Computer technology is improving far faster than spacecraft technology. Not much difference in rocket engines of 1980s and today in performence. Yet unbelievable progress has been made with computers. It stands to reason that AI technology will be perfected long before generation ships. I do not know what truly constitutes AI but fully autonomous spacecraft with possible self replication capability would very much likely arrive sooner than generation ships. Infact in order to economically build behemoth sized space based constructs like generation ships you may require such technology. Tireless automated factories run by computers and munching on asteroids could produce a generation ship much more quickly and for less cost.

[Bakustra]

Starglider would beat me if I made dumb statements here so I will refrain from making any technical statements. But consider this. Computer technology is improving far faster than spacecraft technology. Not much difference in rocket engines of 1980s and today in performence. Yet unbelievable progress has been made with computers. It stands to reason that AI technology will be perfected long before generation ships. I do not know what truly constitutes AI but fully autonomous spacecraft with possible self replication capability would very much likely arrive sooner than generation ships. Infact in order to economically build behemoth sized space based constructs like generation ships you may require such technology. Tireless automated factories run by computers and munching on asteroids could produce a generation ship much more quickly and for less cost.

"It stands to reason" isn't really a valid justification. In other words, try not to make authoritative statements if you can't back them up. Saying "unbelievable progress has been made with computers" does not, even if we accept it as factual, mean that AGIs are inherently easier to build than a generation ship. Consider how much effort has gone into computational technology vis-a-vis rocketry from the 1980s till now, and you will see why I don't find your argument convincing for aliens nor necessarily for humanity.

[Guardsman Bass]

If they take the habit of hopping from metal-rich system to metal-rich system in their travels, then they need not last much longer, as they essentially have become somewhat like the Age of Sail: solar systems are islands where one can make repairs and "dock", while interstellar space is the open sea.

How long do you think it takes to set up mining equipment, find material in the form of asteroids and the like, and get the proper facilities either built or up and running on the ship in order to repair and re-fuel? It could be a while, and that actually reinforces Degan's point to some extent - it adds additional time on to the transit of a potentially hostile civilization's attack, and time is important (not to mention that if they stop in a system relatively near Earth to re-fuel on their way, the civilization at Earth might very well detect the presence of their ships once the light from that system gets here).

Take Degan's 500-light-years away hostile civilization. By the time they've detected anything from Earth, the light is going to be 500 years old. If they somehow immediately do a response, and have super-anti-matter manufacturing facilities, maybe they'll get there in 600-700 years. That's more than a thousand years since the point of detection, and while that's not a long time for a civilization that has lived for hundreds of millenia (or even millions of years), it is a long time for a newly starting space-faring civilization. By the time the attack force gets there, the destination civilization might have already spread out to the stars while going full-upload or AI, making the task of doing something about them exponentially more difficult and time-consuming.

[Surlethe]

What is the power density of the CMBR? My crude, crude approximation (universe is a 46 billion light year-radius cavity surrounded by and in thermal equilibrium with a 2.7 K blackbody) gives about 2E-32 W/m^3.

What are you using? I decided to ask an astronomy professor and he pointed me to the Stefan-Boltzmann law, which gives an intensity, for a 2.73K blackbody as the CMBR, of 3.15e-6 W/m^2. However, he kindly pointed out that this does not actually mean that this is the threshold of detectability, as, indeed, we would not have been able to receive messages from the Sojourner Rover if that was the case, for the intensity of its signals is 4.56e-16 W/m^2, about one billion times under the CMBR. Indeed, the majority of radio traffic on Earth is weaker than the background noise in general, but we can pick out frequencies even when they are overpowered by the CMBR and other sources of noise. So, we cannot use that as a limit. Mr. Degan, I must inform you that you are wrong, but that it is an easy misconception, as I thought so too.

I was using the Stefan-Boltzmann law, which, yes, gives 3E-6 W/m^2, and then dividing that over the radius of the universe (note the units). I was thinking of the universe as a hollow cavity inside, and in thermal equilibrium with, a 2.7 K blackbody. But the point about the Sojourner is valid: what you can do is take the signal you're receiving and check the spectrum. If we're transmitting to aliens, even at very low intensities, it will be distinguishable from the CBR if we use wavelengths drastically longer or shorter than the background.

[Bakustra]

If they take the habit of hopping from metal-rich system to metal-rich system in their travels, then they need not last much longer, as they essentially have become somewhat like the Age of Sail: solar systems are islands where one can make repairs and "dock", while interstellar space is the open sea.

How long do you think it takes to set up mining equipment, find material in the form of asteroids and the like, and get the proper facilities either built or up and running on the ship in order to repair and re-fuel? It could be a while, and that actually reinforces Degan's point to some extent - it adds additional time on to the transit of a potentially hostile civilization's attack, and time is important (not to mention that if they stop in a system relatively near Earth to re-fuel on their way, the civilization at Earth might very well detect the presence of their ships once the light from that system gets here).

I would wager that it would take somewhat less than the hundreds of years being spent in transit already. You presume that they don't, for example, pack the mining and factory equipment specifically so that they can warm it up within days or hours of arrival. Also, fueling is not a difficult endeavor. Enter the solar system in its Oort Cloud or Kuiper Disk. Melt ice for hydrogen. Much, much easier than diving into a gas giant. Similarly, the third most common type of asteroid is the predominantly iron-nickel M-type. Now, M-type is a spectral type, which refers to the appearance of the asteroid. I find it likely that most hypothetical civilizations will know what to look for. Furthermore, the civilization need not be necessarily hostile, as many people have said, and as many others have ignored. This is not automatically an invasion scenario, and there is no need to make it so.

Take Degan's 500-light-years away hostile civilization. By the time they've detected anything from Earth, the light is going to be 500 years old. If they somehow immediately do a response, and have super-anti-matter manufacturing facilities, maybe they'll get there in 600-700 years. That's more than a thousand years since the point of detection, and while that's not a long time for a civilization that has lived for hundreds of millenia (or even millions of years), it is a long time for a newly starting space-faring civilization. By the time the attack force gets there, the destination civilization might have already spread out to the stars while going full-upload or AI, making the task of doing something about them exponentially more difficult and time-consuming.

Why are we assuming that they are trying to attack Earth?

What are you using? I decided to ask an astronomy professor and he pointed me to the Stefan-Boltzmann law, which gives an intensity, for a 2.73K blackbody as the CMBR, of 3.15e-6 W/m^2. However, he kindly pointed out that this does not actually mean that this is the threshold of detectability, as, indeed, we would not have been able to receive messages from the Sojourner Rover if that was the case, for the intensity of its signals is 4.56e-16 W/m^2, about one billion times under the CMBR. Indeed, the majority of radio traffic on Earth is weaker than the background noise in general, but we can pick out frequencies even when they are overpowered by the CMBR and other sources of noise. So, we cannot use that as a limit. Mr. Degan, I must inform you that you are wrong, but that it is an easy misconception, as I thought so too.

I was using the Stefan-Boltzmann law, which, yes, gives 3E-6 W/m^2, and then dividing that over the radius of the universe (note the units). I was thinking of the universe as a hollow cavity inside, and in thermal equilibrium with, a 2.7 K blackbody. But the point about the Sojourner is valid: what you can do is take the signal you're receiving and check the spectrum. If we're transmitting to aliens, even at very low intensities, it will be distinguishable from the CBR if we use wavelengths drastically longer or shorter than the background.

I realized that about thirty seconds after posting it and spotting the 3 over the m. Of course, we still ultimately have the problem of where the signal becomes indistinguishable, but that may well be a matter of Doppler shifting into a noise-filled frequency. In any case, the CMBR appears to have wavelengths primarily between 1 mm and 10 mm, with an average of about 1.5 mm. By comparison, most uses of radio outside of radio astronomy and high-frequency relays have significantly larger wavelengths; for example, cell phones use bands between 100 mm and 1 m. TV signals range from 100 mm to 10 m in wavelength.

[Iosef Cross]

Note that I didn't say it was impossible that alien species that come to Earth might want to destroy us. But only that it is improbable, like it is improbable that your neighbor wants to kill you, while it isn't impossible it is not likely.

The difference is that if our neighbor wants to kill us, there are all sorts of things we can do to prevent it, ranging from locking the door and calling the cops to learning self-defense and/or keeping a firearm in a nearby (safe) location, to simply moving away. If an alien civilization that is magnitudes more powerful than us wants to kill us... well, we're shit out of luck there. So why go out of our way to gamble humanity's existence on those admittedly unknown odds by going out of our way to attract attention to ourselves?

If your neighbor really wants to kill you, you will be killed. Just get a machine gun and shot at you while you sleep. Assume that he moved into the neighborhood yesterday, so you don't have any kind of expectations about him.

The fact is that people are hardwired to not kill each other. I believe that advanced alien civilizations will be hardwired too, the problem is: would they be hardwired to not kill other sentient species?

[Iosef Cross]

"It stands to reason" isn't really a valid justification. In other words, try not to make authoritative statements if you can't back them up. Saying "unbelievable progress has been made with computers" does not, even if we accept it as factual, mean that AGIs are inherently easier to build than a generation ship. Consider how much effort has gone into computational technology vis-a-vis rocketry from the 1980s till now, and you will see why I don't find your argument convincing for aliens nor necessarily for humanity.

Maybe more effort was put into computation technology than rocketry because the marginal gains from developing computational technology in 1980 were greater. I mean, a rocket is limited by it's fuel, there isn't much room for development of chemical rockets, and nuclear rockets are a bit dangerous/politically incorrect.

[Sarevok]

A lot of modern science and engineering depends on high speed computers and advanced software. The B2 bomber, designed decades ago owed its shape to computer aided design, I frankly dont see how a species without much higher computer technology than ours can even dream of making interstellar space craft.

[Oni Koneko Damien]

If your neighbor really wants to kill you, you will be killed. Just get a machine gun and shot at you while you sleep. Assume that he moved into the neighborhood yesterday, so you don't have any kind of expectations about him.

Where the hell did he get this machine gun? Does he have the law after him for owning likely a very illegal and tracked weapon? Just where is this neighborhood? Somalia? What is this about him moving into the neighborhood yesterday? Do you need some help moving those goalposts? Might as well change it to "If you're naked and tied down in the middle of the street and a man driving a tank wants to kill you, you will be killed" to make it more accurate.

The fact is that people are hardwired to not kill each other. I believe that advanced alien civilizations will be hardwired too, the problem is: would they be hardwired to not kill other sentient species?

Which, as has been pointed out far too many times in this thread, we have no way of knowing. From what little we can infer from ourselves and the basic requirements to form a civilization, they are unlikely to be, and no one here is denying that, but it is impossible to predict just how unlikely with any accuracy.

[Iosef Cross]

A lot of modern science and engineering depends on high speed computers and advanced software. The B2 bomber, designed decades ago owed its shape to computer aided design, I frankly dont see how a species without much higher computer technology than ours can even dream of making interstellar space craft.

We can make interstellar space craft with your current computers. I mean, it is not "rocket science ": Using your current physics, it would be powered by antimatter and it would have a simple design.