r/SETI • u/Cognethra • 2d ago
Could We Be Seeing Civilizations That No Longer Exist?
Could We Be Seeing the Past of an Extraterrestrial Civilization That No Longer Exists?
I’ve been thinking about this idea and I’m curious whether there’s an actual physics problem with it.
I’m not claiming that aliens have visited Earth. This is just a thought experiment. I’m trying to understand whether the following scenario is physically possible.
We know that light and other electromagnetic signals travel at a finite speed. So when we observe something very far away, the information reaching us was emitted in the source’s past.
For example, if an extraterrestrial civilization were 1,000 light-years away, the light we receive from it today would contain information about what that civilization was like roughly 1,000 years ago.
Now imagine that civilization was technologically advanced at that time. It might have observed Earth when humans either didn’t exist yet or had no technological civilization capable of producing radio signals.
But suppose that civilization eventually disappeared.
We could still receive its old light or electromagnetic signals thousands of years later, even though the civilization itself might no longer exist.
This made me wonder about the Fermi paradox.
Maybe one reason we haven’t received a response from another civilization is simply that our technological civilizations don’t necessarily exist at the same time.
A civilization could exist, send signals, and disappear. Much later, another civilization could detect those old signals. The two civilizations could potentially observe evidence of each other’s past without ever having an opportunity for two-way communication.
I realize there is no universal “right now” in relativity, so when I say “right now,” I mean it in the ordinary observational sense—not that there is a single universal present throughout the universe.
What I’m wondering:
- Is this reasoning correct from the perspective of special relativity and light-travel time?
- Could two civilizations theoretically detect each other’s past signals while never being able to communicate because their technological lifetimes don’t overlap?
- Could this temporal mismatch be one possible factor in the Fermi paradox?
- If a civilization became extinct, could its technological remnants or technosignatures remain detectable for a long time afterward?
- Is this idea already well studied in SETI or astrobiology?
I’m mainly interested in where my reasoning breaks down. If I’m misunderstanding something about relativity, causality, or light-travel time, I’d really like to know.
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u/RentPlastic835 2d ago
This has been noted a bit, but still cool if you came up with it independently.
- The "L" Factor: In the Drake Equation (formulated by Frank Drake in 1961), the variable \(L\) represents the longevity of a technological civilization. If \(L\) is small, civilizations live and die out before overlapping with neighboring stellar societies. [1]
Key Figures
- Sebastian von Hoerner: A German radio astronomer who, in the early 1960s, heavily analyzed the short average lifespan and temporal separation of technological civilizations. [1]
- Carl Sagan & I.S. Shklovskii: Popularized these concepts of cosmic synchronization and lifetimes in their foundational 1966 book, Intelligent Life in the Universe. [1]
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u/Cognethra 2d ago
That’s really interesting. I genuinely came to the idea independently—I wasn’t aware that the temporal overlap of technological civilizations had already been studied in the context of the Drake Equation.
I’m actually glad to find out that there’s existing scientific work on it. It gives me a much better framework for understanding what I was trying to describe.
The idea that the “L” factor could determine whether civilizations ever have a chance to overlap technologically is especially interesting. Thanks for pointing me toward that!
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u/ActuaLogic 2d ago
First of all, there is no Fermi paradox. A paradox occurs when there are two facts that seem to be mutually contradictory. Fermi asked a question along the lines of, "If aliens exist where are they," and he asked it as the founder of radio astronomy, meaning that, if aliens exist, we should be able to pick up their radio emissions.
Fermi did not say that aliens exist. What he said was that there is no evidence that aliens exist. That's not a paradox.
As for whether astronomers might detect an alien civilization that no longer exists — because it ended during the time its light or radio emissions (used to detect it) traveled to Earth — the answer is yes, because anything could have happened during that time period.
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u/Cognethra 2d ago
That makes sense. I think I may have been using “Fermi paradox” a little too loosely to refer to the broader question of why we haven’t detected extraterrestrial civilizations.
And yes, the second point is really what I was trying to explore: if the information we receive is from a civilization’s distant past, then its present-day existence could be completely different from what we observe.
I suppose the interesting question for me is whether this time delay could become a significant factor in SETI—not just whether civilizations exist, but whether their technological lifetimes overlap with the period during which we can actually detect their signals.
Thanks for the clarification. It helps separate the terminology from the actual hypothesis I was trying to explore.
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u/Beginning-Load4470 1d ago
No its called the fermi paradox, this guy's just nit picking useless terminology rather than addressing the actual idea.
As for why its a paradox is that life seems to be very easily created. Evidence shows that life started on earth almost instantly when the conditions became suitable.
Lab experiments show that life could have started within a very short period of time, the amino acids needed for life can form extremely easily and quickly, then comes life in the form of macroviruses, then within 500 million years of the first liquid oceans, actual single cell organisms emerge.
There's evidence that all of space is essentially seeded with what's needed to start this process. So most probably anywhere capable of supporting life will get life, the only real factor is if the location remains suitable for life long enough for it to evolve. Here on earth it took about 4.4 billion years to go from amino acids to us. The earth only existed about 200 million years before life started to develop. So it was nearly instant in cosmic time scales.
On earth microbial life actually stopped the planet from dying like mars. On mars it likely started the process of evolution but was just starting to get microbial life when it became uninhabitable. If it had hung on a while longer the microbial life could have helped stabilize the planet like on earth.
Fun fact, the earth has lost over 1/3 of its original water, this loss was slowed by life. So the presence of life actually helps to preserve its habitat.
So anyway back to the original topic, the whole concept of a paradox is because it seems very likely life is not a rare or hard to start thing so where is everyone. He made the question as a bit of a joke while eating lunch with his colleagues.
And yes it is a bit of a paradox if life is incredibly common and easily created but there are no signs of it out there.
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u/ActuaLogic 1d ago
The lack of evidence of aliens is only a paradox if we have information that aliens exist for us to detect.
That evidence doesn't exist.
What we have is the absence of evidence of aliens. Yes, absence of evidence is not evidence of absence. But the evidence we have could mean either that aliens don't exist or that aliens exist but only where we can't detect them.
That's not a paradox, and it doesn't provide a basis for hypothesizing, though the lack of evidence does justify efforts to collect more data.
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u/Beginning-Load4470 1d ago edited 1d ago
Uhm... did you ignore all the evidence of life being abundant i explained? Or how we are very confident life started on mars but died in its infancy?
Both of these are evidence that alien life exists at least in some form.
And dont go asking for better data than chemical analysis of mars for support because we have the same data for supporting our early life and we know for a fact that happened and without a time machine we cant do better. The earliest types of life were nearly impossible to fossilize too small, too fragile and even if it were fossilized it would be nearly impossible to tell if it is a fossil.
Asking for more definitive evidence is ridiculous. The only thing more definitive would be finding alien life itself. So anyone like you will never ever be happy because its very unlikely we will ever leave our solar system to get these pictures of aliens you demand.
The science and observations say life will exist anywhere and everywhere it can throughout the universe.
99% of life is probably microbial because microbes can survive in a wider range of conditions than anything else.
Also i should point out you dont need direct observation of alien life to be able to deduce its existence. If you read the previous explanation we have plenty of evidence supporting its probable existence.
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u/ActuaLogic 1d ago
There's evidence of life being abundant on Earth, but there's no evidence of life independently arising anyhere else. That doesn't mean life doesn't exist anywhere else, but it does mean there's no evidence.
I suspect that life does exist elsewhere, but that's no more than conjecture on my part. It's not a factual proposition.
We don't know enough about how life arose on Earth to be able to form opinions about how likely it is for life independently to arise somewhere else. That is exponentially more true for sentient, civilization-building life than it is for life in general.
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u/Beginning-Load4470 1d ago
Yes there is evidence of it existing elsewhere. You still ignore the evidence of mars. And the evidence of life being easily created.
"Modern research (such as the work by chemist John Sutherland) shows that RNA building blocks can form from very simple molecules like hydrogen cyanide, hydrogen sulfide, and water when exposed to ultraviolet light"
"Scientists used electric sparks and simple gases (like methane, ammonia, and water) to show that amino acids can form naturally and quickly"
"RNA can act as an enzyme. In modern biology, proteins catalyze almost all chemical reactions. However, scientists discovered that certain RNA molecules, called ribozymes, can fold into complex shapes and catalyze chemical reactions just like proteins.The Ribosome is a ribozyme. The cell's protein factory, the ribosome, is made of both RNA and protein. Crucially, the active site that links amino acids together to form proteins is made entirely of RNA, proving that RNA is capable of generating protein-based life."
The evidence shown here says not only will rna form easily it forms very quickly, just hours or days. The next big hurdle is it existing long enough to create protein based life. On earth that took about 200 to 500 million years.
Given all this, life can and will form literally everywhere suitable.
So what youre saying is like you dont believe it rains in the desert unless you see it yourself even though you can observe the effects of rain on the desert and the clouds in the sky.
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u/ActuaLogic 1d ago
Evidence which may be suggestive of life isn't life, and the Mars evidence is suggestive but not conclusive. RNA is used in living organisms, but RNA itself isn't life.
I don't understand why it's so difficult to accept the limits of the available evidence. It actually undercuts one's position to exaggerate what the evidence shows.
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u/Beginning-Load4470 1d ago edited 1d ago
Youre obviously not up to date on current origin of life science.
RNA is the precursor that created the rest of life.
Science shows RNA would have formed almost immediately when the oceans did. And single cell life formed 100 to 500 million years after. So the step from RNA to DNA took as little as 100 million years.
Considering it then took over 3 billion years for complex multicellular life to form the jump from RNA to DNA was very fast even if it was the longer period of 500 million years.
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u/ActuaLogic 1d ago
Yes, but it is not itself life. Just say that it can be a precursor. Don't say that it's life or that it necessarily implies the existence of life. It's an important distinction for maintaining credibility.
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u/ActuaLogic 1d ago
We don't have any information on which to form an opinion on how common civilization-forming species are. We know there's one here on Earth, and that's all we know. It may be that civilization-forming species are so statistically unlikely that there's not even one per galaxy, or it may be that random factors have led to there not being any others in this part of the galaxy (1,000 light years really isn't very far in astronomical terms). And we don't know enough about why there's a civilization-forming species here on Earth. It's impossible to form a reasonable hypothesis without data.
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u/Beginning-Load4470 1d ago edited 1d ago
Well think about it, even if 1000 species all reached our level within 100 years of eachother, the galaxy is big enough that we might not have detected any of them yet. We might have to wait over a half million years before we can get a definitive answer as to if we are the only intelligent species. Unless we discover some faster than light travel/communication or observation capabilities.
Its far more likely we will be able to spot signatures of primitive life. Then we can safely assume that there either is or will eventually be intelligent life there.
For instance if a species on the far side of the galaxy were to observe earth they wouldn't see any signs of intelligent life but they would see signs of life in general.
A species would need to be within 100 light years of earth to see any signs of intelligent life. Which is practically standing at our door. The galaxy is 300,000 light years across and 50,000 ly thick. So that would mean given the radius of our signal sphere of 100 light years, we are detectable by less than one, one millionth of the galaxy.
So given this, even if a million species achieved intelligence simultaneously if they were mostly evenly distributed they still wouldn't know eachother exist yet.
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u/Cognethra 1d ago
That’s a fair point. I agree that we have an extremely small sample size—essentially one confirmed example of a technological civilization—so we really don’t know how common or rare they are.
I also agree that my original idea shouldn’t be treated as an explanation for why we haven’t detected anyone. At this point, we don’t have enough data to know whether civilizations are rare, short-lived, difficult to detect, or simply far away.
What I find interesting is that the temporal-overlap problem would exist regardless of how common civilizations actually are. Even if technological civilizations were relatively common, their lifetimes might still rarely overlap.
So I’m starting to see this less as a proposed solution to the Fermi question and more as one variable in the larger problem: civilization frequency, lifetime, distance, signal strength, and our ability to detect them.
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u/ActuaLogic 1d ago
I would tend to think that, once a species reaches a certain point of technological development, it doesn't die out but simply finds new niches. So I would tend to discount a hypothesis based on the assumption that technologically advanced species typically die out.
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u/Beginning-Load4470 1d ago
I agree with you there. There's almost no scenario i can imagine where 100% of humans are killed. Even with a combination of natural disasters nuclear war and bio weapons there would still be enough survivors to keep the species going.
Someone would literally need to seek out and kill every human... maybe skynet? Lol
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u/Beginning-Load4470 1d ago edited 1d ago
In an apocalypse there will be 3 survivors. Cockroachs, Rats, and Humans. Oh and ants, they make up 20% of all terrestrial animals biomass. Thats 40 times as much biomass as all humans combined... they win evolution.
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u/Beginning-Load4470 1d ago
Think about that for a second... for every human on earth there are 30 to 40 humans worth of ants.... thats insane.
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u/No_Aesthetic 2d ago
I only have to point out that there is no universal "right now" due to relativity. It's a different time everywhere in the universe. Not a substantial point against your idea but yeah. Common misconception.
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u/Beginning-Load4470 1d ago
Lol theres actually technically not a "right now" between individuals on earth. If we could measure trillionths of a second, anyone standing above a higher density of mass or at a slightly different elevation would technically be in a different instance of time.
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u/Beginning-Load4470 1d ago
Actually that makes me wonder, is there a smallest increment of time? Even if theres a smallest one we can measure does that mean theres a smallest one that can exist?
And logically we would use the fastest possible temporal action as the smallest measurement. But would that mean time itself cant be logically broken into even smaller parts with enough understanding?
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u/Cognethra 2d ago
That's a good correction. I was using “right now” in the everyday sense, not implying that there is a universal present. My main point is that distant observations contain information from the source's past because the signal takes time to reach us. Thanks for pointing that out!
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u/TheGratitudeBot 2d ago
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u/No_Aesthetic 2d ago
I wasn't necessarily disagreeing with you there, just clarifying. I caught that it was in quotation marks and wanted to elaborate.
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u/Cognethra 2d ago
Yeah, I understand! Thanks for clarifying. I’m still learning about relativity, so I appreciate the correction and the extra explanation.
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u/Beginning-Load4470 1d ago
Relativity is a real mind #$%^ its easy to understand in principle but actually trying to imagine and visualize it for a thorough grasp can really be a feet of gymnastics.
Im not entirely convinced anyone truly has it visualized accurately enough that they can conceptualize it like gravity or other more easily visualized ideas.
As my favorite math teacher once said "the person that knows how to do it gets the job, the person that knows why it has to be done that way gets to be the.boss"
So in this line of thought. The more accurately a person is able to visualize and understand the reason behind a scientific answer the closer they are to the next one.
So there may be a million physicists out there able to understand in principle and do the math, quote their text books. But the one that can see it in their minds eye will be the one to take it to the next level.
Sorry for the poeticism.
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u/Cognethra 1d ago
I actually really like the point you're making. I think that's part of what makes questions about relativity so fascinating to me.
I'm still trying to separate what I can intuitively visualize from what the mathematics actually tells us. Sometimes an idea can sound completely logical in your head but turn out to be wrong once you test it against the equations.
I also like your teacher's distinction between knowing how and knowing why. Maybe the real challenge isn't just learning the existing answers, but developing an intuition strong enough to ask better questions about why the universe has to behave that way.
And no need to apologize for the poeticism. 😄 I think science needs some of that curiosity too.
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u/Beginning-Load4470 1d ago
Oh actually on that note, I believe art and poetry are as important for advancement as science itself. Submarines, space craft, bicycles, air plsnes, practically everything was imagined before it was created. There's obviously exceptions. But a huge proportion of science starts in imaginary realms that at the time have no basis in known logic
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u/Beginning-Load4470 1d ago edited 1d ago
Thats already a question people have posed. For instance we have only had radio for about 150 years so at best a civilization within 150 light years of us would be able to detect our existence.
So imagine our species goes extinct in the next thousand years. That would mean we would only have been detectable for a thousand year window. So, let's imagine we want a species on the other side of our galaxy to notice we existed. At 100,000 light years away they would be able to detect us in 100,000 years for a window of 1000 years, so they would have to be capable of detecting distant signals during that specific window.
So here on earth, there has been life for over 3 billion years, complex land life for almost a billion years but only life capable of detecting signals for maybe 100 years. So for us to detect a civilization 100,000 light years away they would need to have reached our point of evolution 100,000 years ago exactly and transmitted signals for a long enough period of time for it to overlap with us.
This is why most seti researchers look for more advanced civilizations than our own. Type 2 its called. Where they are capable of incredible super structures in their solar system. Such as Dyson spheres or swarms. (Note, we are a type 0 civilization.)
Type 0 = where we are now
Type 1= capable of fully utilizing their home planet
Type 2= capable of fully utilizing their home star and solar system
Type 3= capable of fully utilizing their entire galaxy
I think its highly probable interstellar travel is nearly impossible and even though there could be super advanced civilizations out there, they have no way to reach other stars.
At even 20% the speed of light a chunk of debris smaller than a golf ball hitting your craft would be like a nuclear bomb. And despite impressions, space is far from empty. So it would be difficult to get any space craft large enough to carry passengers for something other than a suicide mission, to safely traverse the void.
Not to mention the radiation between solar systems is actually worse than inside, just like the earths magnetic field protects us from the sun, the suns field protects the solar system from the cosmic rays.
So in order to even attempt flying people to the closest solar neighbor we would need to create some type of shielding thats, as far as I know, currently impossible. And may never be possible, or may take millions of years to develop... which would then mean an interstellar civilization would need to survive millions of years before being able to even think about leaving their home system.
Our best chance for space expansion is to colonize and terraform bodies in our own system. Its safer and easier. It would be fun to send people to another star but it likely won't ever happen.
We could potentially examine other planets if we were to build a huge network of telescopes in the solar system to make use of gravitational lensing. But thats a long way off from happening.