Because to creatures such as us that have only been looking for a hundred years with limited equipment, a relatively ‘full’ galaxy would look no different from an empty one.
Consider the possibility that you have about 10,000 intelligent systems that can use radio-type effects in our galaxy (a number that I think would likely be a wild over-estimation given the BS numbers I occasionally half-jokingly calculate given what I know of the evolutionary history of life on Earth and cosmology and astronomy, but it’s just an example). That puts each one, on average, in an otherwise ‘empty’ cube 900 light years on a side that contains millions of stars. EDIT: if you up it to a million intelligent systems, the cube only goes down to about 200 light years wide with just under half a million stars, I just chose 10,000 because then the cube is about the thickness of the galaxy’s disc and the calculation was easy.
We would be unable to detect Earth’s own omnidirectional radio leaks less than a light year away according to figures I have seen, and since omnidirectional signals decrease with the square of distance even to be seen 10 light years away you would need hundreds of times as much. Seeing as omnidirectional radio has decreased with technological sophistication here, I find that doubtful. We probably just would never see omnidirectional signals, and furthermore given the proportionality to the square of distance i doubt anybody would see ours either.
That leaves you looking for directional transmissions. Those might not even be radio, as optical transmissions can be sent directionally and are rather better for very long distances due to diffraction issues, but I will ignore that possibility. That means that you need a directional transmitter, and over the distances we are talking about a directional receiver. Such a directional signal could just randomly be pointed towards us along line of sight to a spacecraft or along a horizon, or it could be specifically sent out to other star systems. What are the odds that two points that do not know of each other’s existence (given that human impacts on the atmosphere of earth are only two to three centuries old and radio even younger) happen to both have the transmitter point in the proper direction and the receiver look in the proper direction at the exact right moment?
In short, the only things we have really excluded reliably so far are truly huge engineering projects like dyson clouds (which you would see in the infrared), ridiculously powerful omnidirectional signals of a sort I find unlikely in our immediate neighborhood (tens of lightyears), or something that for some reason decides to spend a lot of time and effort pinging millions of nearby stars every few years or less for quite a large fraction of its history. We’ve sent out what, a dozen or two directional beams to other stars over half a century?
Factories can already self-replicate given human help. They’ll probably be able to do it on their own inside the next two decades. After that, we’re looking at self-replicating swarms of drones that tend to become smaller and smaller, and eventually they’ll fit on a spaceship and spread across the galaxy like a fungus, eating planets to make more drones.
That doesn’t strictly require AGI, but AGI would have no discernible reason not to do that, and this has evidently not happened because we’re here on this uneaten planet.
OK, but even if you are right we know it’s possible to send radio transmissions to other star systems. Why haven’t we detected any alien TV shows?
Because to creatures such as us that have only been looking for a hundred years with limited equipment, a relatively ‘full’ galaxy would look no different from an empty one.
Consider the possibility that you have about 10,000 intelligent systems that can use radio-type effects in our galaxy (a number that I think would likely be a wild over-estimation given the BS numbers I occasionally half-jokingly calculate given what I know of the evolutionary history of life on Earth and cosmology and astronomy, but it’s just an example). That puts each one, on average, in an otherwise ‘empty’ cube 900 light years on a side that contains millions of stars. EDIT: if you up it to a million intelligent systems, the cube only goes down to about 200 light years wide with just under half a million stars, I just chose 10,000 because then the cube is about the thickness of the galaxy’s disc and the calculation was easy.
We would be unable to detect Earth’s own omnidirectional radio leaks less than a light year away according to figures I have seen, and since omnidirectional signals decrease with the square of distance even to be seen 10 light years away you would need hundreds of times as much. Seeing as omnidirectional radio has decreased with technological sophistication here, I find that doubtful. We probably just would never see omnidirectional signals, and furthermore given the proportionality to the square of distance i doubt anybody would see ours either.
That leaves you looking for directional transmissions. Those might not even be radio, as optical transmissions can be sent directionally and are rather better for very long distances due to diffraction issues, but I will ignore that possibility. That means that you need a directional transmitter, and over the distances we are talking about a directional receiver. Such a directional signal could just randomly be pointed towards us along line of sight to a spacecraft or along a horizon, or it could be specifically sent out to other star systems. What are the odds that two points that do not know of each other’s existence (given that human impacts on the atmosphere of earth are only two to three centuries old and radio even younger) happen to both have the transmitter point in the proper direction and the receiver look in the proper direction at the exact right moment?
In short, the only things we have really excluded reliably so far are truly huge engineering projects like dyson clouds (which you would see in the infrared), ridiculously powerful omnidirectional signals of a sort I find unlikely in our immediate neighborhood (tens of lightyears), or something that for some reason decides to spend a lot of time and effort pinging millions of nearby stars every few years or less for quite a large fraction of its history. We’ve sent out what, a dozen or two directional beams to other stars over half a century?
You’re forgetting self-replicating colony ships.
Factories can already self-replicate given human help. They’ll probably be able to do it on their own inside the next two decades. After that, we’re looking at self-replicating swarms of drones that tend to become smaller and smaller, and eventually they’ll fit on a spaceship and spread across the galaxy like a fungus, eating planets to make more drones.
That doesn’t strictly require AGI, but AGI would have no discernible reason not to do that, and this has evidently not happened because we’re here on this uneaten planet.