“Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us.”
Calvin & Hobbes
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Of the many (interesting) theories as to why we haven’t been contacted yet, I think this one is particularly intriguing, and plausible, and quite sobering also:
TLDR - salient points:
" The Dark Forest solution explains why we haven’t heard from aliens by positing that they are purposefully keeping quiet.
The reasoning is laid out best in the science fiction novel The Dark Forest, by Liu Cixin. The plot of the book, the second in a series, concerns questions of how to best interact with potentially hostile alien life.
In the novel, the argument is laid out like this:
- All life desires to stay alive.
- There is no way to know if other lifeforms can or will destroy you if given a chance.
- Lacking assurances, the safest option for any species is to annihilate other life forms before they have a chance to do the same.
Since all other lifeforms in the novel are risk-averse and willing to do anything to save themselves, contact of any kind is dangerous, as it almost assuredly would lead to the contacted race wiping out whoever was foolish enough to give away their location. This leads to all civilizations attempting to hide in radio silence."
and from the mentioned novel:
" The universe is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound. Even breathing is done with care. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him. If he finds another life—another hunter, angel, or a demon, a delicate infant to tottering old man, a fairy or demigod—there’s only one thing he can do: open fire and eliminate them."
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There was an old anthology in the 60s titled First Contact, edited by Damon Knight, which had a number of stories about our first contact with ET. There were a number of takes on ET, including the title story, First Contact, in which we met another species in space, and neither would leave because both were afraid the other was hellbent on annihilation. (they decided all was safe after they got drunk together and told dirty jokes). An old theme but a good one.
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The Dark Forest theory has quite a significant issue at its basis. It first postulates the notion that is only possible through anthropomorphizing all the fauna of the universe, and then it ignores our own technical development timeline. Our radio bubble, Voyagers, Arecibo message. The Dark Forrest theory is extremely incoherent.
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Haha, I knew I should have stayed out of this… 
Just because we already stepped on every dry branch on the ground in that forest doesn’t mean the underlying premise is wrong… But, hey, it’s just a theory, an idea, a backdrop for a sci-fi story or two…
And yes, a huge percentage of aliens imagined (at least by Hollywood) are somewhat humanoid-ish, because truly alien lifeforms don’t lend themselves so well to story-telling where interaction with humans is part of the plot (I guess). Stephen Baxter has quite a few (short) stories on very exotic life forms, like convection cells in a neutron star (and stuff like that).
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These are vanishingly insignificant, though. Our radio bubble would not be detectable beyond 10-15LY (and we would only recently (~50y) detect it beyond 1); we beamed the Arecibo message in a tight cone at exactly one star; both voyagers will soon be dead and are only transmitting in a tight cone back to Earth.
The only omnidirectional signal on earth we have ever made that would have detectable range beyond 10-15ly would be bursts from the nukes, which would go out to 20-25, but are on the order of nanoseconds.
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Jeez Howard you are a walking encyclopedia 
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Well, I would take this as a viable argument if things like LIGO didn’t exist. Even though gravitational waves don’t suffer from inverse square law diminishment and scattering as radio waves do, the sensitivity of these laser interferometers is orders of magnitude higher than even our biggest telescope arrays created. And I hear your argument “But radio waves and gravity waves are two completely different things.”, but it doesn’t change the fact that detection of what would seem like a “vanishingly insignificant” signal 30 years ago is a weekly occurrence nowadays. And AFAIK even an extremely diminished electromagnetic signal is way above the energy level of the universe’s background noise. (Don’t talk about CMB here.)
This is just from my college major; lots of work with spectrometers and other signal processing. Plus a general interest in space as an amateur astronomer for a few years.
Yeah, interferometry background too. Though it’s been a while 
And yes, radio detection limits and gravitational detection limits are in fact different things. But this is true within a certain range:
Yes for sure, out to the range where the inverse square law reduces the signal strength to that point. And that’s the absolute detection range limit (which can be pretty easily calculated but I am lazy this morning). However we do have to posit some technological level for the detector here. I was using us right now as a baseline. (well, technically, us in the recent past because AFAIK we have just slightly fallen off of our radio detection peak.) You can’t use a future civ as a baseline, because the capabilities are unknown; using the past seems to be artificially limiting. Do you have a better idea to model this?
Some of the other stories in that anthology included a sentient colony of Ants which were taking over the world, and one called The Goldfish Bowl by Robert Heinlein, were a body was recovered bearing the message “creation took 8 days”, and the underlying message never stated was that we can’t perceive ETI anymore than goldfish perceived our civilization. Turned me on to Heinlein for life.
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I actually think that LIGO is a viable basis for the model of our potential detection abilities if the abilities in question aren’t “Could we detect radiowaves with our available radio tech?”, but rather “What is the current threshold of our technology, that creates a physical/technical limit for a detectable signal strength against the noise floor in the system and the environment?”
Wish I had the internet as a kid.
The Goldfish Bowl, by Robert Heinlein
The problem with using LIGO is you are comparing detecting one signal with inverse square decay to one that (we think) does not decay that way. You can’t do that, it’s equivalent to saying something like “they will just find us using The Force.” It’s positing magic-like capabilities that as far as we know do not exist. It’s fun speculation, but it’s science fiction, not science.
The fact is, based on our current understanding and detection theory, once the signal degrades to the point of the cosmic background noise, you are either done or are getting very close to the actual physical detection limit. Physics bats last.
Hey @Wombat-metal
Was it Heinlein who wrote Stranger in a Strange Land? I read that years ago and really liked it.
This thread has me itching to read some good sci-Fi. Haha
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2023 U.S. House, in general:
I don’t think I compared detecting various signals at all. It’s the main purpose of this modeling basis. It eludes the problem of variability in power and frequency of the source signal (I am quite sure that some of the military tech used in the last 70 years blasted radio pings of such power and frequency that would be detectable with our current tech even a couple hundred light years from Earth.) and focuses on the level of resolution achievable at our current tech level.
Military radar (or Arecibo broadcasts) would indeed have longer range than a spherical broadcast, but they are still 1/r^2, just with a tighter coefficient due to the focusing. It’s still an expanding wavefront. As I mentioned above, one Arecibo broadcasting directly at another one would be detectable out to around 1kly (I think).
Another factor is that the gravity waves LIGO are detecting are caused by such ridiculously catastrophic events that they would kill anyone not only in our solar system but also likely others close by around us, even if we could pull them off.
Yes it was. And Starship Troopers
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I understand your reservations and frankly, I am on board with the notion that radio waves are indeed a bad medium for interstellar detection and communication for a civilization at our level at the Kardashev scale (0,7?). However, I will stand behind my notion that we’re nowhere near the limit of hf radio-wave emissions detection and emission itself. We’re also nowhere near the energy availability even for a civilization that is fully at level 1. (Not to mention level 2 civilization which could aspire to build radio beacon fueled by a Dyson sphere.)
My first response was to The Dark Forrest theory which assumes that the reason why we don’t see anyone is because nobody is broadcasting. Which I oppose with the argument that 1st) we actually have no clue if nobody is broadcasting. (Our modus operandi in radio astronomy and radio technology isn’t technology bound. It’s economy bound.) and 2nd) The only available data we have are simply in opposition to that notion. The only advanced species we know indeed isn’t quiet at all. The only advanced species that we know, started to scream “Here we’re!” as soon as they could.
I personally believe that the reason why we don’t see anyone is that Great Filter is a bitch and on the grand scale, civilizations which are at our level or higher are extremely rare, and the overlap between civilizations being able and WILLING (Willing to spend resources.) to broadcast and detect radio waves on the level suitable for these purposes rarely happens. IMHO, civilizations follow all similar scientific trajectories and the so-called Great Filter is reliant on the ability of the species to overcome its biology via spaceflight and saturation of Overview effect amongst its populus. I think it’s impossible for a civilization to survive without getting on the technological level at which radio waves simply aren’t the most economical and effective way how to let others know about its presence.
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