[vikingerik]

This isn't likely. The Milky Way is 100k light years in diameter. That's tiny relative to the age of life. Animals comparable to humans in complexity have existed for something like 250 million years on Earth, and could have on planets of stars that formed billions of years before our sun. The possible timespan for signal-emitting life to have developed is many multiples of the timespan that signals take to cross the galaxy.

It's far more likely that signal-emitting life is so rare (or short-lived) so that they are separated by distances where their signals weaken to undetectability, than that we are the one fantastically lucky star to be the first among a hundred billion.

Star Trek showing all the rival civilizations exploring the galaxy at the same time makes no sense at all. It's far more likely that civilizations would arise separated by time of millions or billions of years, than that they would all be concentrated within a few centuries. (Trek does hint strongly that the explanation is panspermia, that life was seeded everywhere at the same time to account for the time-concentrated development.)

What we don't know is what makes signal-emitting civilizations cease to do so. But (if we aren't the fantastically unlikely first one) either something must, or they're so far apart that signals can't be detected between them.

[thomassmith65]

The radius of the observable universe is 45.7 billion light-years.

The universe is approximately 13.8 billion years old. Primates evolved only six million years ago. So let's be charitable and say it took only 13 billion years for a lifeform capable of transmitting radio waves to evolve.

Unless I'm missing something, if we're listening for ETs, we would only have visibility into a sphere with a radius around 1/45th the radius of what is out there?

My point is, if the Universe is such that it takes a minimum of, say, 10 billion years for planetary conditions to make evolution possible and evolution to wind up at intelligent life, then there could be hundreds or thousands of civilisations out there, at this very moment, whose earliest signals won't reach us for millions of years.

[inkyoto]

The article calls out that «the probability of life emerging based on known factors peaked about 13,000 light-years from the galactic center and 8 billion years after the galaxy formed».

A hypothetical civilisation may well have emerged with a head start of five billion years – a span of time sufficiently vast to allow for profound advancement, assuming favourable conditions and uninterrupted development.

However, I find the oft-repeated assertion – that alien civilisations have likely annihilated themselves due to traits as tediously predictable as short-sightedness, internal strife or some anthropocentric parody of self-sabotage – to be both intellectually lazy and philosophically barren. such a narrative reveals far more about the limitations of the human imagination than it does about the potential trajectories of alien life.

A more measured and plausible explanation lies not in self-inflicted extinction, but in the nature of the galaxy itself. Five or more billion years ago, the Milky Way could have been a significantly less hospitable environment – more unstable, more violent, and subject to higher stellar activity. One must also consider the precariousness of location: the galactic core, unlike the relative quietude of our outer spiral arm, is a congested and perilous stellar thoroughfare, far less conducive to the emergence and persistence of complex life.

To default to the notion of civilisational self-destruction is to betray a lack of imagination – or worse, a projection of our own inadequacies onto the cosmos.

[stOneskull]

i can't help thinking the galaxy would be symmetrical with earths, like petals round a flower, being at the same stage of development as each other.

[vikingerik]

Primates evolved six million years ago here. There are plenty of stars billions of years older than our sun (and still sufficiently enriched with heavy elements from prior stellar generations) where they could have, so that their signals would have reached us by now. The past light cone is everything you see in the sky. Even a million light-years from neighboring galaxies is small compared to the time scale of possibilities on the order of multiple billions of years.

[thomassmith65]

  Primates evolved six million years ago here. There are plenty of stars [...] older than our sun [...] where they could have

Fair enough. If we're certain a star billions of years older then ours could have the same conditions that would support evolution, then my comment is unlikely.

I, embarrassingly, have a crackpot theory where G isn't a constant. I'll spare HN; it would make me sound like a moron.

[JoeyJoJoJr]

This is tangential to what you are saying, but does it blow any else’s mind that we just so happen to exist right at the start of the universe’s existence? If the universe is going to have a habitable window of however many quadrillion years, what are the odds that it just so happens that our existence falls within the first blink of that timeframe. If life is abundant in the universe it would stand to reason that it will continue to be abundant for a very, very long time. Further, if I am to spawn in the universe as a self conscious being, wouldn’t it be likely I would spawn at a random point on (almost) the total timeline of the habitable universe? So then the odds of existing (basically) right at the beginning of the timeline is exceptionally small, unless there is some factor that influences where on the timeline we must exist.

[Sil_E_Goose]

Mr. Shabadoo, you might be interested in the doomsday argument if you are not already familiar.

https://en.wikipedia.org/wiki/Doomsday_argument

[JoeyJoJoJr]

Oh cool, this is interesting!

[ayaros]

It is panspermia. See TNG season 6 episode 20, The Chase.