It’s not unreasonable if they still have direct support from Earth and maybe an assist from the moon / asteroids / Mars for raw supplies. But getting the energy to accelerate a space station to a significant fraction of the speed of light (and slow it down when we get where we’re going) and maintaining that in interstellar space for numerous generations (including dealing with high energy particles and radiation that we’re no longer shielded from by the sun) sounds extremely difficult, even with tech 100 or 1000 years more advanced than ours.
This seems overly pessimistic. The metallicity of stars has been increasing gradually over the last thirteen billion years, and many planets have been found which are much older than the Earth (for instance the various Trappist-1 planets, 51 Pegasi b, Kepler-452b, and so on). Perhaps planets were a little smaller in the past, but the Earth is one of the smaller planets currently known to science, so small may be good in this context.
There is no reason to expect that life was not possible until a few million years ago - the oldest biosphere in our galaxy (if any exist) could be many billions of years older than the one found on Earth.
- Keeping score, OP ruled out a galaxy brimming with advanced altruistic civilizations that want us to notice them. Because we’ve looked systematically (but not at all comprehensively), advanced civilizations would figure out what we would do and would have the ability to be noticed. That hasn’t happened.
What about inadvertent signals (TV, radio)? Apparently, Arecibo’s ability to pick them up (when it operated) was pretty weak: SETI at home focused on intentional signals beamed directly at us. And maybe Alaska-style radar to some extent.
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Post 2: I rule out Dyson galactic supercivilizations, which is a subset of galactic supercivilizations. Can’t be sure about the latter, but I’ll provisionally rule out the former. Based on a single paper. So it’s tentative, like everything else.
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Mijin rules out, “Planets are rare.” Nice. I’ll tack on, “Planets with liquid water are unique to Earth”, because we have 3 examples now (Earth, Europa, and Enceladus).
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Horatius rules out technological civilizations within 100 light years of us. I’m not certain about that (again, it’s my understanding that SEIT at home focused on intentional signals). I/we should search the SETI websites a little deeper.
If we accept all 4 of the above (big if), we’re still in the land of knowing, “almost nothing” (wolfpup) or “Even less than that”, (Exapno_Mapcase). I tend to agree with them.
We’re not done yet. I’d like to know more about the feasibility of Von Neuman replicators. An article covering the underlying ridiculousness of space opera in a systematic fashion would also be nice.
Stepping away from the OP, I hypothesize irresponsibly speculate that there are civilizations capable of interstellar onsite visits, but that after a certain point that approach is displaced by vastly cheaper technological solutions. Specifically remote viewing via quantum blah blah. Sounds ridiculous, but our electromagnetic civilization is only about 100 years old, and our high-growth civilization dates to about 1870. Cite. Another cite, same author. Read the book!. So yes, we know little about the sort of breakthroughs that a high-growth civilization is capable of, beyond the first 150 years or so. We have canals today, but not super high tech canals. Because there are better approaches to inland transportation.
Do I know that? Of course not. We don’t know enough about the relative sizes of the bottlenecks between no life and technological civilization. Though based on one planet’s history, the jump between cellular and multicellular appears to be sizable.
That’s at best outdated. Heavy elements existed in the early universe. JWST has been discovering galaxies that look remarkably like ours and have similar elemental characteristics, dating back to within roughly the first billion years since the big bang.
From the other thread:
Further discussion on this thread: What is the end game of SETI? - #40 by Sam_Stone
I’d like to pose some questions about von Newmann machines in the spirit of this thread.
If Von Newman machines are a) thermodynamically plausible at b) a reasonable price and c) devoted to the propagation of meatsacks, would we observe them given d) a few civilizations capable of such technology and with preferences governed by c)? I think the answer is yes. They would be in all the papers.
Those are weird preferences though. They are understandable and perhaps familiar, but nonetheless they are weird. Substitute in “Space exploration” or “Shits and giggles” for c) and you no longer are bound by meatsacks. Robots and AI would do the work. But in that case, you could have probes from 6 or 7 civilizations mining our asteroids and I’m not clear whether we would notice them. Especially if they preferred to do it quietly. They aren’t motivated by expansion within each solar system after all, just observation and construction of more interstellar probes.The Earth is interesting, but not that interesting.
But maybe we can rule out advanced Malthusian von Neumann civilizations in this galaxy c.100,000 years ago.
I’m in agreement with wolfpup. There may be intelligent life elsewhere but the universe is just too large.
It would be like a crab in the south Pacific wondering “are there any crabs in the North Atlantic?” There may be, but he could never travel that far to find out.
Supporting the wolfpup/Exapno_Mapcase/Velocity conjecture, “If an extraterrestrial civilization has a SETI project similar to our own, could they detect signals from Earth?” From SETI FAQ:
In general, no. Most earthly transmissions are too weak to be found by equipment similar to ours at the distance of even the nearest star. But there are some important exceptions. High-powered radars and the Arecibo broadcast of 1974 (which lasted for only three minutes) could be detected at distances of tens to hundreds of light-years with a setup similar to our best SETI experiments. Cite: FAQ
So an altruistic alien world with our technology would have a broadcast sphere of tens to hundreds light years in radius. Meanwhile, we are 26,000 light years away from the galactic center.
But wait, what if we simulate the Earth’s cell phone towers? Just this year we’ve studied exactly that and the answer is still no, even for Barnard’s star (6 light years away).
So if Barnard’s star has a civilization with c. 2020 technology, we wouldn’t necessarily know about it (though we might detect their radars). And that’s the 2nd closest star system to Sol. But if they were altruistic and intentionally beaming their recipe books at us, we would probably notice.
We know that von Neumann machines are possible because we are von Neumann machines ourselves. Admittedly we can only replicate within an environment where a vast array of resources are spread out in relatively accessible form, but this was not always the case, and our last universal common ancestor probably lived in an environment where resources were much harder to come by. Yet we survived.
On the other hand the journey from an organism which could utilise raw materials on a virgin Earth to a civilisation which can build rockets has taken a billion years or more; if each stage of replication is doomed to take millions or billions of years, then the colonisation of the Galaxy will take a long time.
I hope and suspect that given enough information wrangling, the replication time of such systems will be quite a bit shorter.
Perhaps Virtual reality is the great filter in a sense. They are all out there, but plugging into your own personal fantasy kingdom is way too enticing and every intelligent species goes that route well before developing interstellar travel.
The chances that any nearby civilization would have 2020-level technology is so low as to be almost zero, for reasons that should be obvious.
Ok, but there’s no reason to believe that 2020 + XXX, XXXX, or XX,XXX civilizations would have dramatically higher electromagnetic radiation leakage. They could very well have lower leakage. My cell phone tower link notes that we have fewer powerful TV and radio transmitters than we used to, FWIW, which is not much given the forecast horizons.
The point is about our ability to detect passive transmissions from alien worlds even if they exist (not much, with caveats) vs. intentional transmissions (maybe, but we haven’t found any so far).
As for what we can rule out, the only thing I can think of are Kardashev-3 civilizations, defined as those that modify entire galaxies to harness their energy. And we can’t rule them completely out, but we know that if they exist they would be very rare.
We have actually done a survey looking for ‘strange’ galaxies that don’t have typical spectrographic features or other characteristics of a modified galaxy, and found nothing. Kardashev-3 civilizations should cause entire galaxies to emit more in IR like a big Dyson Sphere.
This study looked at 4859 spiral galaxies, and found nothing.
The next level, Kardashev-2, can harness the energy of their star. In other words, Dyson spheres or swarms. We have not discovered any of them, but we only discovered a star that *could yave been a Dyson swarm a few years ago (Tabby’s star). Since then we have found other similar ‘dimming’ stars, and we haven’t characterized all of them.
The idea that we would have discovered Dyson swarms by now if they existed is just not true. We would have to be extraordinarily lucky to have spotted one by now. As it was, Tabby’s star was a bit of a fluke. It would never have been discovered had Kepler’s reaction wheels not failed, as it was part of the Kepler-B mission that had to look at different parts of the sky. And even then, the tools used to spot exoplanet dips did not find it, and it took citizen scientists pouring theough the data to discover it.
Then there’s the case of Przybylski’s Star. Before he died, Carl Sagan suggested that one almost foolproof way to find an alien signature would be to look for very short-lived elements in the star’s spectrum. He speculated that the most powerful way to signal other civilizations would be to dust a star with these elements, the signature of which wouod be elevated to the power of a star’s output and be visible across the galaxy. Such short-lived elements were thought to be impossible to form naturally and continuously, so an intelligence muat have done it.
Cue Przybylski’s star, which appears to have exactly those kinds of elements in its spectrum. So now we are trying to find natural explanations for it, but none have been particularly satisfying. Still, it’s never aliens until it’s aliens, so we leep looking for explanations that don’t involve ET dumping their nuclear garbage into their star or using their star like a big smoke signal.
Folks keep talking about radio and earthly transmissions as beacons to aliens. Or equivalently their transmissions as beacons detectable by us. IMO that’s bunk.
What we transmitted in the late 1880s and after = Marconi’s era, were pitifully inefficient in a Shannon sense. Which made them easy to identify if you were close enough to pick up their pitiful low power. They stood out as very organized and unnatural. But out past even Jupiter they were indistinguishably faint. They cannot be detected at Alpha Centauri by any possible tech no matter how advanced; at that range they’re fainter than the noise floor of the Universe itself.
Modern radio, like what makes your mobile phone work, is the polar opposite. It is a very mathematically sophisticated waveform that “sounds” almost exactly like noise because the information density is so high. We now have the tech to transmit vastly more powerful signals to space. But they “sound” like nothing but noise, even if your receiver is adjacent to the transmitter.
The window of opportunity where we or aliens are broadcasting high powered and easy-to-identify obviously unnatural signals is going to be a historically short interval of time. Unless someone is deliberately setting out a beacon to be found. Which seems culturally implausible / unwise.
Yeah, we are already in the process of going ‘radio dark’, as broadcast TV dies, radio goes spread spectrum, and communications move to cables or low earth satellites using low powered signals.
But any omnidirectional signal is going to be impossible to pick up because it follows the inverse square law, and degrades with the square of diatance.
For example, Voyager beams signals to us using a directional antenna with a gain of 48 db. It is then received by very large dish antennas of the Deep Space Network. If it wasn’t for those huge dishes, we’d never heard from it. Not even it it broadcast 23,000 watts instead of 23 watts.
Here’s a paper on trying to pick up Voyager’s signal using the Allen Telescope Array,
consisting of 42 6.1 meter dishes. Even so, its 23W of output power are reduced to about a billion billionth of a watt at the amplifier after being collected by all those huge antennas. That’s an absolute power of -161.53 dBm. To even hear this signal you need advanced, cooled receivers and amplifiers with a very low noise floor.
Detecting Voyager 1 with the ATA).
And remember, this signal was boosted 48 db by a directional antenna. Without that, we would have no chance of hearing voyager, even if it broadcat 2300 W instead of 23W. In fact, to make up for the gain of the antenna Voyager would have to broadcast at 2.3 million watts, if I did the math right, And that’s still using the Deep Space Network to do it. There is absolutely no chance of receiving it on an omnidirectional antenna, or even a small dish, even if the power was a million times stronger.
Voyager is about 159 AU away from us right now. The closest star to us, Proxima Centauri, is roughly 269,000 AU. And power drops off with the square of distance in an omnidirectional transmission.
Space is very, very big.
There’s quite a variety of plausible alternatives:
I’ve admitted that the idea that we’re among the first advanced technological species to evolve is extremely unlikely just in terms of pure statistics, but if we’re not, then we go back to the central question of the Fermi Paradox-- assuming there are many advanced technological races out there that are millions of years older than us, why haven’t we detected any yet?
I think it’s a combination of reasons: the vast distances involved, for one. Another is, as I mentioned, the idea that though life may be common, intelligent life may be rare. Even if there are technological civilizations much older than us, perhaps there aren’t too many, the great majority of planets with life having nothing more than, say, alien bacteria and lichens. The few advanced races that possibly do exist may still be too far away. If a civilization began their technological footprint a million years ago and they’re 1.2 million light years away, we still have awhile before we detect their presence (their former presence at least; they could have long ago wiped themselves out in a “Great Filter” event and we’re just detecting a ghostly relic of their distant past.).
I would love to say “Hey the fact we don’t see any evidence of ETs doesn’t mean much…we’re primitive, the universe is huge and we have no idea what technologies advanced ETs would use”.
But I don’t think it stands up to scrutiny.
Yes it’s huge, but we have a clear view across it…billions of light years. Aliens don’t need to come to us (though there has been time for them to do so, particularly with self-replicating probes), they just need to do anything with EM radiation on a large scale.
Which brings me to the other point. Yes, it’s probable that aliens will use hi-tech means of communication and transport that we would have no idea how to detect. That doesn’t mean that it is not significant that we don’t see anything in the EM spectrum.
An advanced species would have the ability to block, refract or emit EM radiation on vast scales for whatever purpose.
The fact that we don’t see even one instance of this is a significant data point. It doesn’t rule out advanced ETs of course, but certainly gives us more reason for doubt.