Posted by fuidani 4/17/2025
If life, even of a very primitive sort, were found, it would stand to reason that it had done so in the past and that other civilizations, possibly even many of them, had formed in our huge galaxy long ago, giving them time to develop enough to be detectable even to us, so then, where are they?
Then again of course, there are probably many, many known unknowns and unknown unknowns lurking amidst all of the above supposition.
A few years ago some researchers at the Future of Humanity institute explored what happens if you take distributions into account. It turns out that this makes a surprising difference and results in a substantial probability of no other intelligent life. No need for speculation about great filters, predatory civilisations, etc... . The paper is freely available on ArXiv: https://arxiv.org/pdf/1806.02404
If that time is a few hundred years, then very few happen to be functioning _now_ (in relativistic meaning) and very far away to have meaningful contact.
That being said, I agree. I read in a similar thread yesterday someone confused how this would be bad news rather than good news—that there are many other intelligent species indicates that such a filter either doesn't exist or is very easy to pass. But, like your point does, I think it's important to recognize that such a "good news" position is predicated on the notion that we as a species are already past the Great Filter, rather than that we're still behind it and the others are ahead.
The older I get and the more I appreciate Just How Lucky we are to exist at all on our planet here, the more I favour the above thinking.
Failing that, you’d need thousands of optical interferometers larger than the Hubble spread across a distance wider than the Earth.
My math is below.
Note: I'm not an astronomer.
----
The angular resolution limit for a telescope is roughly the wavelength of the light it's sensitive to over the diameter.
If we want to sense things 10m across, with light at the shorter end of the visible spectrum (400 nm), we'd need a telescope with a diameter of about 1/4th of an AU (i.e. the distance from the earth to the sun), around 40 million kilometers.
More practically we could use a telescope array with this diameter, which could conveniently be in lot of orbits about 1 AU out. But the area is still a problem: assuming this 100m^2 object is as bright as it would be on earth under midday sun, it's going to be reflecting around 100 kw of energy. One of these photons has an energy of around 3 eV, so we're getting 2e23 of them a second. Unfortunately these spread out over a sphere with a surface area of 1e31 km^2 by the time they reach earth, meaning we see one every second if we have a telescope array with an area of 50 million square km.
Ok, so let's go kind of sci-fi and say we can build a 30 km diameter space telescope. It would be impressive (and unprecedented) but since it's floating in space and could be made of thin material you might be able to imagine it with today's technology and a lot of coordination. That gets us around 1000 square km! Now we just do it 50,000 more times.
Great, now we have 1 Hz of photons coming from each 100 m^2 patch of Alien Manhattan! I'm sure in the process of building 50k mega-projects we'll figure out a way to filter out the noise, and with a few years of integration we'll have a nice snapshot!