As mentioned, I'm following the Copernican principle for this line of questioning. Yes I've considered and read many other possibilities, but this question is for Copernican-style answers ONLY. Your welcome to ask your own similar non-Copernican question in a separate question, and inform me and I'll give my support as well as my own best answer(s), or you can ask me to formulate my own non-Copernican version.

Estimations are based on various websites, feel free to adjust inaccurate numbers if they change a final formula by more than 5%.

Also, please no government-conspiracy type responses.

Summary: why considering these and other numbers, haven't we already found mainstream accepted proof of an ETI?

The numbers: quick search indicates that within 50 Light years (I'll abbreviate LR) of us, 133 stars are visible to the naked Eye, and 1400 stars are found and/or estimated to be in that 50 LR span. As I understand it (introduce some shaky logic with semi-arbitrary numbers and conjectures), we've been sending highly "unnatural" signals to space for about 50 years. Many of them I assume probably aren't strong enough to be detectable past a LR or two (pure conjecture) of space with our own detection instruments, but some I assume probably were strong enough to make it through the noise. I think perhaps our nuclear detonation may also have been able to be detectable by even an earth like civilization.

Also read that nearly all of them have (or probably have planets). And 25% have or probably have planets in the habitable zone.

That's 350 in the habitable zone within 50 LR of us. If 1% have life, that's 3.5 If 1% with life have intelligent life, that's .035... so maybe 1. The number of planets our signals will reach grows exponentially as time goes on. I'll arbitrarily say that in another 100 years we're at something like 160,000 stars reached, 40,000 have planets with life friendly conditions (assuming 25%) thus 400 with life (assuming 1%), thus 4 with intelligent civilizations (assuming 1%, which I will point out that if we applied the copernican principle to this portion of the formula this makes us "special" but I'll make an exception here) Recap: 160,000 stars in 150 years.

Us plus the other intelligent civilizations at a 25%/1%/1% formula makes 5 intelligent civilizations for every 160,000 stars...

Now the copernican principle.... if you took it very seriously, and if my math is kind-of accurate, then we should have inadvertently been sending unnatural signals, with various levels of detectability through space for 50 years. If we "imagine" that we are just an average form of life, and that the universe may be teeming with life (since here we are assuming we're not special, wether it's lone survivor, or short-bus special) then also if we are not special, the universe should be teeming with life; and surely a bunch of it would also use signals similar to ours, maybe not decipherable, but at least similar enough to differentiate between natural and unnatural. Yet per-Fermi's paradox, they have not been detected....

The question: why the **** aren't we noticeably detecting their signatures if we ourselves have been able to send stuff through space for 50 odd years???

Looking for fresh ideas. If you HAVE to, you can point out the great filter or some other already mainstream hypothesis. I'm not discrediting them. Yes many of those types of ideas have merit Imho.

Looking for fresh thoughts and discussion on this, while preferably being able to hold true to the Copernican principle. Again, I'm not saying they're like us because we're awesome, I'm saying if they're there, then too assume WE are like THEM since we may NOT be awesome nor behind the curve, nor on a separate playing field, just average.

Last words: I'm not saying we're not special. Yep, we might be. We might be the only ones or the only ones to make it through a filter(s). What are some other "non-special" possibilities, that might allow ETs and ETIs to exist, while keeping us from seeing them, and perhaps keeping them from seeing us, or each other as well

  • 2
    $\begingroup$ Oh, but we should be neither special in location - nor time! That would mean some life happened billions of years ago - and extrapolating our development time scale, should have spread to every corner of the galaxy and progressed far beyond our wildest imagination. So - completely disregarding our outgoing signals, why don't we see any external anomalous signals? -- This is called Fermi's Paradox, and if you look it up, there are some suggested hypotheses. $\endgroup$
    – SF.
    Mar 20, 2017 at 2:35
  • $\begingroup$ I mentioned drakes equation and the copernican principle. I assure you you I'm also familiar with fermi's paradox. This is at least an arguably refined and definitely biased form of the paradox. I'm closing the doors to a large number of hypothesis to see what I can find here in stack exchange in the minds of others that might offer explanations for the paradox based on the copernican principle. But thank you for the response! If anyone who ever reads this is unfamiliar with fermis paradiix I highly recommend checking it out $\endgroup$
    – Nomadyn
    Mar 20, 2017 at 2:37
  • $\begingroup$ We are particularly "special" - in the way that we're only aware of the universe for a couple hundred years - less than a blink of an eye on the cosmic scale. We're an extremely young civilization. And that may mean we're only looking for signals of type that is waaay obsolete everywhere else, and we're not even aware how to begin observing the ubiquitous signals in common use. $\endgroup$
    – SF.
    Mar 20, 2017 at 2:41
  • $\begingroup$ If you look at Earth, life appeared pretty much the instant it became plausible. Then it took a few billion years before it even became multicellular. Mars was once like early Earth and might have had life then too, but then it lost its atmosphere. My bet is that great filter is in complexity. Life isn't very picky about conditions, but complex life is. You need a reasonably stable environment for quite a long time, and it will probably help if lots of oxidizer can build up in the air or water. Those conditions may be quite hard to come by. $\endgroup$ Mar 20, 2017 at 2:44
  • $\begingroup$ @SF. Have definitely considered that myself. Any info on how far our own signals that we've sent thus far will travel through space before being 99.99% untrackeable? I wonder this frequently. A good answer for this question might provide some good food for thought to both of us. $\endgroup$
    – Nomadyn
    Mar 20, 2017 at 2:44

2 Answers 2


This answer will make some assumptions (which I will state explicitly as much as possible) and simplify things, but it should give a rough explanation as to why detecting extraterrestrial life is difficult.

Now, we can, of course, use the Drake equation (and all of the assumptions therein) to make a rough estimate of how many civilizations there might be in our galaxy. Answers to the Drake Equation can range from $N\lt \lt1$ to $N\gt 10^{8}$. Since we're concerned with the Fermi Paradox, let's go ahead and use the high end of the estimates and say that there should be ~1 million civilizations in the Milky Way.

Now let's assume that these civilizations are evenly spread throughout the Milky Way. In this case, we can model the Milky Way as a disk, calculate its volume, and calculate the density of civilizations. From this, we'll find the mean inter-particle distance to estimate the average distance between civilizations. Mathematically, this looks like:

$$ d = (\frac{\pi R_{MW}^2H_{disk}}{N})^{\frac{1}{3}} $$

Plugging in our numbers yields an average distance of 0.088 kpc or 287 light years. This alone is a very large distance. Assuming that we can't travel faster than the speed of light, this means that it would take a message from an alien civilization 287 years to reach us.

However, one problem with this is that it takes a lot of energy to transmit a message 287 light years. EM radiation intensity falls off with the square of the distance, meaning that signals at that distance will be extremely hard to detect if they're radiated in all directions. In fact, just about the only kind of signal we'd be able to see is one beamed directly at us. So there would need to be an alien civilization close enough to detect us in some way, they would need to beam a signal straight at us, and we would need to be watching that area of the sky when it arrived. All in all, the probability of that happening is extremely low.

Now that covers receiving messages from other civilizations. But what about indirect methods?

Well, the best method for indirect detection of aliens is called "transit absorption spectroscopy". If you want more information on how this works, you can check out the answer to this question. The problem with this method is that it needs a telescope sensitive enough to detect small changes in the spectrum of a star. Currently we can try to do this, but we're really waiting for the James Webb Space Telescope to launch in 2018 to give us better sensitivity and higher resolution when looking at exoplanets.

TLDR: Distances in space are very large and that makes it very hard to do detection of civilizations.


If we hypothesize that nothing about our state in spacetime is peculiar I would argue that the question has answered itself: we have not detected ET life because why would we?

Under that assumption, there is no basis to believe that our neighboring stars, or indeed our entire galaxy, are privileged in that regard. And even if they were, there is no reason to believe that they are now. And even if they were, there is no reason to believe that they could contact us in a manner we could understand. And even if they did, there is no reason to believe that they would like to contact us. I argue that under that principle there is no basis to the conjecture that we should have seen anything at all.

It sounds reasonable that life must exist elsewhere, and even be common. (I believe so myself.) But there is a big stretch between that conjecture and the expectation that we will ever find out, or that we somehow should find out. Our entire history is less than nothing when compared to the time and scale of the universe. Entire civilizations may have existed and may exist in between blinks; it may be that we are not special, but just so happened to live through a blink.

Primitive or even microbial life? Possibly (Europa! Mars!) Intelligent life? My guess is as good as any. It seems to me that the question is undecidable.

  • $\begingroup$ Also I have considered this, and I agree we can't necessarily assine there are 5 intelligent civilizations our our backyard, but if we just follow statistics, and were to be VERY generous and assume that we have already explored every crevice in our own galaxy, then the best statistic we would have left is there is 1 intelligent civilization per galaxy or so. (Since we, per our definition of intelligent, are an intelligent civilization, and occupy this civilization(. Question is still awaiting a hypothesis and/or literature about a hypothesis following a copernican/mediocrity style (continued) $\endgroup$
    – Nomadyn
    Mar 20, 2017 at 4:44
  • $\begingroup$ Theory as to why we haven't found life, considering we ourselves transmit data, thus assuming we are average others would do the same. This is not a bait question. Genuine question! Instead of assuming they're all dead or we can't interpret their signals, or some other theory that makes us "special", AND taking into account that our own short existence thus far has already sent signals to 1400 stars, so even a civilization far more advanced but like us (since we for this question only are assuming we're not special), what might be an explanation as to why we haven't sern the signals? $\endgroup$
    – Nomadyn
    Mar 20, 2017 at 4:48

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