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The Fermi paradox is basically that:

  1. There are billions of stars, life is probably abundant throughout the universe.

  2. Even though it may take tens of thousands or millions of years for life on other planets to reach us, the universe is old enough that they should have reached us by now. (eg. a civilisation that developed space travel 100 million years ago could have explored the entire Milky Way.)

But is it possible that, yes aliens have reached earth, but they reached earth perhaps a million years ago, documented the life currently existing, and haven't bothered coming back?

What's wrong with this explanation?

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    $\begingroup$ Why would other civilizations want to make contact with us or explore the universe? What might be rare is the desire to explore the universe. $\endgroup$
    – GdD
    Commented Apr 8, 2015 at 9:20
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    $\begingroup$ I prefer this solution from smbc-comics $\endgroup$
    – neelsg
    Commented Apr 9, 2015 at 12:47
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    $\begingroup$ What if intelligence at some point turns into an intensive rather then an extensive development - and so colonization of any sort is just simply not interesting to it. An example: If a human's consciousness will be possible to migrate into a fully digital world (as suggested by some scientists it will be possible within 100 years) - like Flatline's mind in Neuromancer - and also we continue the development in the nanotechnology - then at some point maybe the whole humanity will not take up more space then a walnut. so why colonize... $\endgroup$
    – Peter Aron Zentai
    Commented Apr 9, 2015 at 22:28
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    $\begingroup$ @PeterAronZentai Another smbc-comic for this. $\endgroup$
    – user11153
    Commented Apr 10, 2015 at 8:03
  • $\begingroup$ Related XKCD $\endgroup$
    – Geza Kerecsenyi
    Commented Aug 7, 2019 at 16:34

4 Answers 4

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A couple of issues with the Fermi paradox:

  1. The Fermi paradox is closely tied to the Drake equation. This equation is the best method we currently have for thinking about the probability of intelligent life in our galaxy. The problem is that there are so many assumptions you have to make in that equation, that the results vary drastically. If you plug some of the more negative assumptions into the formula, you get as few as only 2 civilizations currently in our galaxy.
  2. To say that an advanced civilization would even bother to colonize the rest of the galaxy is a big assumption. Our forefathers colonized every place they could. Today, we have wildlife reserves where we specifically try to limit human interference. We take great care not to spread Earth life to other planets when we send science probes. Maybe a sufficiently advanced civilization will simply choose not to colonize everything.
  3. The more advanced a civilization becomes, the easier it is for them to wipe themselves out entirely. This is the view Carl Sagan took to explaining why we have not yet made contact with other intelligent life. If some rulers decided to wage war even 100 years ago, they could not wipe out all humanity even if they tried. If we detonate every nuclear bomb we have in stockpile today, the odds of even 1 human surviving is about zero. We are doing science experiments like trying to create black holes, creating catastrophic climate change and breeding ever more deadly viruses. The odds of us wiping ourselves out in the future seem to be increasing every year.
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    $\begingroup$ 2 "To say that an advanced civilization would even bother to colonize the rest of the galaxy is a big assumption." To say that NO civilization ever has colonized the galaxy is an even bigger assumption. It is a claim about how all of them always have been without any exception. The motive of human space flight, the cold war politics of the Sputnik and Apollo, would be pretty difficult to explain to an alien. We shouldn't expect to understand why they travel, but since they can, as we can, some of them will do so some of the time. $\endgroup$
    – LocalFluff
    Commented Apr 8, 2015 at 10:22
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    $\begingroup$ 3 I disagree. Once a civilization, or a branch of a tree of life, goes interstellar, it becomes immortal and will exist for ever after. No comet or supernova or war can wipe it out. It is as reversible as boiling an egg. Civilizations that colonize will be the most common type of civilizations, with time more common than microbial life, purely for evolutionary reasons. And please note that human population grew every year even during the second world war. Don't overestimate our ability to destroy ourself. $\endgroup$
    – LocalFluff
    Commented Apr 8, 2015 at 10:26
  • $\begingroup$ @LocalFluff Errr.... didn't they lately discovered a method to restore protein shape/order back exactly on the example of boiled egg? livescience.com/49610-scientists-unboil-egg.html. I still agree with the point, but the example is kinda missed... or maybe it does indicate something. $\endgroup$
    – luk32
    Commented Apr 8, 2015 at 10:41
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    $\begingroup$ Re If you plug some of the more negative assumptions into the formula, you get as few as only 2 civilizations currently in our galaxy -- It's easy to come up with much more negative assumptions than that, yielding as few as 2 civilizations in our galaxy group. And that might be the answer. Escaping star systems with sufficient resources and capabilities to colonize a galaxy (or parts thereof) is hard. Escaping galaxies to colonize a galaxy cluster is orders of magnitude harder. $\endgroup$
    – David Hammen
    Commented Apr 8, 2015 at 13:52
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    $\begingroup$ If we detonate every nuclear bomb we have in stockpile today, the odds of even 1 human surviving is about zero -- Nah. Civilization would likely collapse horrifically, and billions might die, but the species would almost certainly survive. $\endgroup$
    – Russell Borogove
    Commented Apr 8, 2015 at 17:49
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I think the answer to your question is "yes". The combination of "space is big" and "humans haven't been around long" is a valid (as opposed to good) explanation for the Fermi paradox.

I replaced "good" with "valid" because "good" is subjective and can be colored by one's own opinions, and desires, about the possibility of alien life. But as logical explanations go, yes, I think this is perfectly valid reasoning.

After all, we don't really know how hard it may be to break the FTL barrier. Maybe it really is impossible for all practical reasons, and since space is so big, alien species don't come around very often, and certainly not for no good reason. As far as colonization is concerned, you're going to pick the low-hanging fruit first - that is, the compatible systems that are closest will be the first to be colonized. And, even if you are aware, through remote detection, that other compatible systems are out there, you're not going to need to go until you're pretty well established with the closer ones.

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  • $\begingroup$ I don't really buy your 'lowest hanging fruit' explanation, as the time scales involved would suggest that, unless we live on the very corner of the universe, that those closest systems would have already been populated and we'd be discovered. $\endgroup$
    – dwjohnston
    Commented Apr 10, 2015 at 1:59
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If anyone anywhere anytime did space travel, they should have been here by now. The Milky Way is small compared to its age and travel times. The Sun has made about twenty orbits around it. The dinosaurs actually lived on Earth when it was on the other side of the galaxy. Either we are alone as a space faring society, or we have company very close by. The most unlikely scenario is that "they" exist but are far between. As if a part of the ocean had no fish.

Interstellar distances and the constant speed of light ensures that space faring civilizations, even colonies with the same origin, cannot be coordinated. When the communication time lag is thousands of years then technology, culture, policy will evolve completely differently. So if anything is out there, then everything is out there. If Stephen Hawkings is afraid of a dangerous type of aliens, he can rest assured that they, if anyone, do exist and that they are nearby. There are nice guys out there too, everyone is out there, they just cannot be similar to each other.

But that logic has no support at all from observations. All life on Earth is related to each other and had the same origin. You and I have some genes in common with every living cell be it a dog, an insect, a fungus or a disease. We are all family since over three billion years. Nothing ever came here to change that. And the ancient surfaces of the Moon and Mars, where we now can find our own small spacecrafts from orbit, show no signs at all of any artificial manipulation. We are definitely alone as a space faring society in the solar system, and we always have been. This in spite of the fact that the Sun has been all around the galaxy many times and has had many different neighbors.

And SETI has showed us that if they exist then they don't like to use radio very much. Nor do they rebuild galaxies or build Dyson spheres around stars. If we do have neighbors, then thankfully they aren't very noisy. Thinking about it, none of them ever was, which is a bit creepy. Like being all alone in a gigantic hotel, like in the movie The Shining. Your mind begins to make up ghosts as company.

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    $\begingroup$ We cannot be sure that no ET life has ever arrived. Maybe we are ET! We'll never know for sure. $\endgroup$
    – user8406
    Commented Apr 8, 2015 at 7:07
  • $\begingroup$ @andy256 We can't prove a negative. But I think that the untouched surface of the Moon is an especially convincing argument against any "visitors" during the last billions of years. We have ourselves already left traces there. Just as a byproduct of political prestige. $\endgroup$
    – LocalFluff
    Commented Apr 8, 2015 at 7:35
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    $\begingroup$ Agreed. Even if we produce life in a test tube, the "spontaneous ignition" theory is still unproven. The strongest evidence one way or the other would be finding life beyond Mars, and see if it's related to us or not. $\endgroup$
    – user8406
    Commented Apr 8, 2015 at 9:13
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    $\begingroup$ Re The most unlikely scenario is that "they" exist but are far between. Why? To me, that is the most likely scenario, and it is the easiest answer to the Fermi paradox. $\endgroup$
    – David Hammen
    Commented Apr 8, 2015 at 13:54
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    $\begingroup$ 2) We can't see radio waves. Sure good point, but radio noise from earth peaked after what 75 years? It's plausible that aliens are using wired in communication... $\endgroup$
    – dwjohnston
    Commented Apr 9, 2015 at 2:59
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I think you have to extend that a bit, to "(Technological) humans haven't been around long, and on present evidence aren't going to be around all that much longer". If the average lifespan of a technological civilization, from the point at which it can produce radio waves (or other emissions) that could conceivably be detected from another star to the point at which it destroys itself through ecologial degradation, averages about two centuries, what's the probability that two will exist simultaneously, close enough for one to detect the other?

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  • $\begingroup$ This is my parent site. $\endgroup$
    – Muze
    Commented Apr 27, 2019 at 3:37

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