Fermi Paradox
Fermi ParadoxCalculations suggest that if intelligent life is somewhat common, then we should have been visited or contacted by now. So, why haven't aliens openly visited or contacted us yet?
See http://www.geocities.com/CapitolHill/Lobby/7049/fermi.htm
Summary of possibilities:
Many people think, when first confronted with this paradox, that a little bit of CommonSense logic can solve it. Of course aliens haven't landed here: perhaps there aren't any, or it's too difficult to get here, since it turns out that StarTrek's convenient warp drive cannot really exist and that in fact nothing can travel faster than the speed of light (except for bad news, of course).
Re: "Of course aliens haven't landed here". We don't know that. They may be stealthy. We might belong to an empire that "protects" us. I think this is called the "Zoo hypothesis".
Let's look at the second point first: aliens cannot get here. After all, perhaps they come from the other side of the galaxy, or even from another galaxy. Well, Earth is about 32,000 LY removed from the galactic center, so 64,000 LY would be the distance to the other side of the galaxy. That takes 64,000 years when travelling with the speed of light.
OK, let's suppose that one alien civilization actually manages to build a colonization ship; not one like in StarDreck?, but something like we might build ourselves in a couple of centuries: it goes veeery slow compared to the speed of light, so it arrives only at its destination after about 1,000 years. OK, so now we are there, and the next question is: when will the new civilization build its next vessel? Well, we give them time to fall back to stone age a couple of times, so let's say: 9000 years, which has the advantage to add up with the previous number to 10,000 years. OK, so I pull these number from my *ss, but I hope that I'm somewhat convincing that if a civilization is capable of starfaring at all, 10,000 years might be a reasonable time from launch of ship to planet A to launch on A from ship to B. In other words, 10,000 years would be a rough estimate for the time it takes to double the total number of colonized planets. We all know that exponential growth is too optimistic; after a negligible amount of time, all close-by systems have been colonized and expansion happens only at the surface of the sphere containing the colonized star systems. You then launch to a very-close-by system (of course), say 10LY away. So it takes 10,000 years to cover 10LY, in other words: expansion goes at one thousandth of the speed of light. That still means that you can be here from the other side of the galaxy in a mere 64 million years. That means that if the dinosaurs had spacecraft, they would be at the other end of the galaxy by now. If someone else had spacecraft at the time of the dinosaurs, they would be here by now.
So this strongly suggests that we are really alone in the galaxy. Well, it's ours then, isn't it? Just let's see what we have. A run-of-the-mill galaxy like ours is 100,000LY in diameter and has a center of 10,000LY. It contains mostly ordinary stars (about 10e11 or 100 (American) billion stars). In the center is probably a black hole of about a million solar masses. So that's ours! That's 20 star systems for every person in the world. And then we still have the two Clouds of Magellan: they are a mere 169,000 respectively 205,000 LY away, so sufficiently close for a visit. But wait! What about those nasty Andromedians? Andromeda (M31) is slightly more than 2 million LY away: If there were star-faring Andromedians back when the Earth was trying to evolve algae, they would be here by now. So Andromeda is probably either only "recently" colonized or still virgin terrain, so we could take that, too. And the rest of the Local Group: the whole cluster containing the Milky Way, Andromeda, 7 other big spiral galaxies like ours and some 30 small elliptic ones. Total radius: 6.5 million LY. Peanuts for any sufficiently advanced society.
Of course, the Local Group is, unfortunately, one of the tiniest clusters around. Our closest neighbor, the Virgo cluster, is some 60 million LY away, and it contains thousands of galaxies. Definitely worth going there. Another one is in Coma Berenice, and lies 450 million LY away. It contains about 800 galaxies. The Local Group and Virgo, together with some 100 smaller clusters, form a so-called supercluster, with a total radius of 300 million LY.
Galaxies are densely packed in the universe. The typical distance between two galaxies is a million LY, whereas the typical diameter of a galaxy is 100,000 LY. Galaxies collide "all the time", i.e. every few billion years. A typical (finite) model of the universe gives a total number of galaxies of thousand American or one British billion; this leads to about 1e23 total star systems, of which a large fraction will be like our sun (the sun is a rather mundane type of star). That's 20,000 billion star systems, spread over the whole universe, for every person in the world. So next time the bank asks you what you have to back your mortgage, you know what to tell them! By now, you should get the impression that the universe is indeed a rather big place.
Isn't it nice to know that it is all ours? If you still believe we're alone, that is. What would CommonSense suggest?
If you take out a gram of gold from the bank at 6% cumulative interest then you would owe double in about a decade. Let's make it a century because I don't want to bother with precision. So in 10 million years, there would be 100,000 doublings and you would end up owing 2^100,000 grams of gold. That's approximately 10^30,000. Regrettably, you can't take out the Local Group as collateral on your mortgage. You couldn't even take out the universe as collateral on a measly ten thousand year mortgage.
That means that if the dinosaurs had spacecraft, they would be at the other end of the galaxy by now. If someone else had spacecraft at the time of the dinosaurs, they would be here by now.
This part of the argument isn't as tight as the rest. If dinosaurs had developed spacecraft, and decided to explore as much as possible, they could have reached various places 64 million light-years away by now. But it would take an incredible number of such vessels to reach every such point, so chances are they wouldn't have ended up near any given star system unless there was a special reason to be interested in it. Our sun doesn't look special from very far away - slightly above average size, with planets if you look closely, broadcasting a radio bubble on the order of a hundred light-years across. Volume gets larger must faster than distance. Even if someone sent out a billion ships from 64 million light-years away, they'd still probably have missed us.
If at least some are like us, they'd want to colonize. Even at a slow pace, a civilization a billion years old would have settled our entire galaxy by now.
[Keep in mind that the galaxy, let alone the universe, is really big. I mean, really, really big. It's entirely conceivable that a space-faring race would have thoroughly colonised all nearby worlds (for some reasonably-vast value of "nearby") and still not have come anywhere near us. Or, have not found any good reason to come near us... For a sufficiently-advanced star-faring civilisation, yet another barely-industrial "intelligence" might be as interesting to them as the average anthill is to us.]
That's pretty much the "rare" option above. If ET is rare, then the average separation between different ET's is so wide as to make encounters unlikely.
Metals
Suppose that intelligent life was rare enough to only evolve in roughly one out of 100 galaxies. This may be rare from our perspective, but from a universe-wide perspective it is not.
Second, 1st- and 2nd-generation star systems may not contain enough metals and carbon to support the evolution of intelligence. (Our sun is roughly 3rd-generation.) Metals and carbon increase over time as the left-overs of the death of stars. Third-generation star systems are roughly no more than about 5 to 10 billion years old. This would suggest that the oldest civilizations are no older than 5 billion years old, assuming a similar planetary and life evolutionary path as ours. (If their sun formed 10 bil years ago, then they'd be ahead of us by 5 bil years.)
So if the frequency of civilizations is 1 per 100 galaxies, and the oldest are about 5 billion years old, assuming we are in a "typical" neighborhood, would they "be here" by now? Further, those qualifying with enough metal/carbon may have been relatively rare at the start. It may have started at a rate of about 1 per 500 galaxies, for example.
This is roughly one civilization per galactic cluster. The nearest cluster to us, per above, is 60 million light years. Colonizing would have to take place at roughly 1 percent or more of the speed of light in order for them to be here by now. This is not unrealistic for scout ships, but it is probably for colonization, per above. This would suggest we have been scouted but don't yet have ET neighbors. They may know about us, but haven't got around to doing anything about it yet. We are in a relatively small galactic cluster; and this may suggest something. More on this below. -t
CommonSense might suggest SiTi
Existence of IntraTerrestrialIntelligence? doesn't explain non-existence of ExtraTerrestrialIntelligence?. Why would the GodGoo allow only a single civilization (us) to exist?
It would have no such restriction. It would merely regard us as we regard clouds.
Imagine we discovered the clouds in the sky are actually great, slow intelligences. That they operate on scales 100,000 times larger, slower, and less diverse than our scale. That everything they can understand and manipulate, everything they can conceive and are concerned with, is just other clouds, their degrees of fluffiness, and a few very rough approximations of flow and distribution. And that two of them one day consider that, just maybe, they will one day interact with some very tiny hypothetical puffs of mist and fog.
What would we have to say to these cloud-people? What meaningful interaction could we possibly obtain? -- PeterMerel
We'd blow them up. There isn't enough room in the sky for both of us. That's your meaningful interaction right there. Seriously, clouds are an untapped resource and if we had the technology to use them, we would. Same with human bodies, any sufficiently advanced civilization would consider us to be a waste of precious resources. -- RichardKulisz
On the contrary: we can't "blow up" the clouds - we depend on them for useful things like rain. We could perhaps arrange clouds more conveniently - end droughts, floods, and drizzly days - but such tinkering might not suit us as well as we imagine.
As for advanced civilizations, it seems pretty plain that any civilization that is not in danger of devolution to GreyGoo must maintain some kind of PrimeDirective of non-interference. Indeed, given all the unused scales of structure we see available for our own engineering, there's plainly plenty of room in the sky to spare.
As for ExtraTerrestrialIntelligence?, it seems fair to think it would take less than 1,000 years for any civilization of our ilk to go from radio to uploading. After they upload, their signals become noise to our scale. This thousand-year window dramatically reduces the odds that we'd ever detect anyone by radio.
In short, there's no FermiParadox to worry about. Aliens have landed, and they live in your nose. -- PeterMerel
Gesundheit Hypothesis? This would explain an increase in allergies and asthma.
Maybe all sufficiently evolved technological societies also evolve to be governed by CorporateCapitalism. They haven't visited here not because they don't have the time, or the means, but just because it would not be profitable for them to do so. See also: LamentForTwoThousandOne?
I think that "sufficiently evolved" and "governed by CorporateCapitalism" contradict each other.
Maybe the one prevents the other, and that's why sufficiently evolved tech societies don't exist. If nothing changes for a few centuries, then I will update this with an example.
They'd probably sell "exotic vacations" to Earth, but are barred from exposing themselves as outsiders because they'd spoil the setting of being among a clueless culture. Thus, they visit as avatars and blend in as awkward tourists. Because they are cheapskates, just like our CorporateCapitalism, they hire the cheapest pilots, pay them shit, and spread them too thin such that they sometimes accidentally leave some of the ship lights on or poorly manage the cloaking system, causing the occasional UFO sighting.
Do you believe there are beings of superior intelligence?
Yes.
Then why haven't they visited us?
Because of their superior intelligence.
(Quoted not quite verbatim from B.C.)
Why haven't we openly visited or contacted aliens?
Most of us are a bit too far away, or in [untranslatable], for you humans to reach. The few of us who are here would rather you didn't "contact" us.
Is Lady Gaga one of yours?
No, but probably not human, either.
Please take her anyhow.
The fact that we are in a small galactic cluster, per typical cluster, suggests its small size has protected us from being visited or invaded. If we had evolved in a medium or large cluster, the most likely case otherwise due to density, then perhaps we'd have encountered ET's by now. ET's are less likely to visit & colonize sparse clusters because it's too far to travel for too few resources.
Copernican Principle and Anthropic Principle would suggest that some factor is involved to "keep us out" of denser clusters, where probability would otherwise place us. The boondocks are protecting us. Nobody is bothering us because we are stellar rednecks hidden in the difficult-to-reach woods. -t
Rednecks in Space
See also: AnthropicPrinciple
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