[Yizahi]

To send and then slow a device of meaningful size across ten or hundreds of light years would require an enormous amount of energy, like truly incomprehensible amount. Then a civilization would need to produce them in millions and send to every single rock in the galaxy sector, because nuclear fission blasts are undetectable outside of star system. And then these robots need to function for billions of years continuously without any failure, because who knows which rock and at which time may develop sentient life. And when detection fission decay, such a robot must produce an enormous amount of power, to send a coherent optical signal over the tens of light years of distance. Meaning it has a gigantic power generator and equally impressive emitter. Which means even more mass has to be accelerated and then decelerated initially. And his sentient robot has to stare at a rock for billions of years without degrading electronically and without going insane.

And all that galaxy construction level effort for what? To learn hundreds or thousands of years late, that at rock number 123ABCD a fission has happened? And do what exactly with that useful information? Send extermination fleet? Or a robot with flowers, to pay respects?

People for some reason refuse to comprehend just how hard is it to send a speck of dust over light years of distance, let alone anything meaningful which won't break down in the process.

[cryptonector]

The reason to do this is not to prevent other civilizations from destroying themselves but to colonize the galaxy. It would still require all that you said (fantastical technology and enormous amounts of time) and then some.

And since the amount of time we're talking about is so large -- larger than the amount of time the beings that create these robotic probes can possibly continue to be alive -- that the only way it could work is if those beings accept robots as acceptable replacements for themselves, or if the probes carry embryos and can terraform planets and raise those embryos to adults and bootstrap a civilization.

Plenty of sci-fi has been written along these likes, like Ursula K Leguin's books, where human-ish beings on any given planet (e.g., Winter) turn out to be sent there from other planets to bootstrap a civilization and they have no memory of it. Or Pushing Ice, by Alastair Reynolds, where there is a robotic probe thing going on, but rather than continue the originating species [redacted to avoid spoilers].

[Yizahi]

We just went from absurdly insanely hard task, to a task I'd guess a thousand times harder. A communicator and observer probe is almost impossible enough, but to additionally preserve biological tissue for thousands of years in space is even harder than sending pure robot. And then terraforming part is just orders of magnitude harder than that. Communicator probe would be a few thousand tons maybe and packing maybe a few megawatts of onboard power. Lets be generous and give them a thousand times more - a few gigawatts, provided they are magic aliens and stuff. What can you terraform on a few gigawatts? Raise a temperature in a ten meter circle by one degree? Produce a few cubic meters of something from atmosphere? To terraform one would need a giant fleet of giant vessels, all fine tuned for some processes, and then they will work for millennia to change planet a tiny bit. We would notice that kind of operation in the Solar system.

I love LeGuin, Reynolds, and other, sci-fi is practically 90% of what I'm reading. But come on, the whole interstellar stuff is always predicated on very very optimistic assumptions and eventually magic.

[cryptonector]

Sure, ok, but what would a communicator probe accomplish? It could not communicate back to the origin -- the origin would be long gone. It could only communicate with the civilizations it finds, but to what end?

If any civilization were to build such a thing they would make it perpetuate themselves.

[sliken]

Only if you are in a hurry, say an advanced civilization has been around for 1M years (0.07% faster than us). It might well be worth sending out millions of drones to the most promising areas at 1% of the speed of light, their advanced sensors and telescopes and science would likely be able to pick the most likely stars based on metal content, vicinity (i.e. stable of 1B years), water, temp, etc.

Not to mention they could send probes closer and further from the galactic center to take advantage of the slower and faster rotation rate to see new stars.

As for the nuclear fission blast I have my doubts. Ham radio folks brag about 1000 miles a watt, in a lossy atmosphere and multiple bounces that reflect less than 1% for each bounce. Using advanced things like tubes of transistors and a copper cable thrown over a tree branch.

Using the 1 watt per 1000 miles the largest nuclear explosion would be 22 light years, and clear line of sight through space is going to transmit quite a bit better than bouncing off the atmosphere then off the ground several times.

An advanced civilization could make say a square km array (which us lowly humans have managed) and would understand nuclear bombs enough to know their likely signature, decay rate, shape of the curve, etc. Much like how astronomers use supernovas as standard candles for distance, despite crazy different red shifts.

Seems quite reasonable for a civilization to keep track of anything going on in their fraction of the galaxy.

"People for some reason refuse to comprehend just how hard is it to send a speck of dust over light years of distance" It's only hard if you are in a hurry, in fact we have 3 rocks come through our solar system from well more than a light year away.