[ezmpildy] Intragalactic versus intergalactic

The distance between galaxies seems not that much greater than the size of galaxies.  Milky Way diameter 100,000 ly.  Distance to Andromeda 2.5 million ly, so scale factor of 25.  This means (maybe) that after becoming a galactic civilization, it's not that much more difficult to then become an intergalactic civilization.  Star Trek (and others) are unrealistic in limiting themselves to just one galaxy.

Compare this with other cosmic distances:

Half-circumference of earth (farthest possible distance between any two points on earth): 20,000 km.  Minimum distance to Mars: 55 million km.  Scale factor 2750.  Going from being a planetary civilization to being an interplanetary civilization is difficult.

Size of solar system, defined by semimajor axis of Neptune's orbit: 4.5e9 km.  Distance to Alpha Centauri: 4e13 km.  Scale factor 9000.  Going from being a solar-system spanning civilization to being an interstellar civilization is difficult.

These distance ratios don't take into account the difficulty of escaping gravity wells.  Don't know if it matters.

[gzhcqvfb] Black hole jet as Death Star superlaser

Black holes emit jets (through an as yet poorly understood mechanism) as seen in active galactic nuclei, e.g. Hercules A.  From this, it seems straightforward, even trivial, to build a Death Star: point a black hole in the right direction (moving it first if necessary) (building it first if necessary), then dump a bunch of stuff into it (assuming that's the mechanism that causes jets).

How large a black hole is needed?  Does it need to be a supermassive black hole or do stellar mass ones also produce useful jets (microquasars)?  How powerful / deadly is the jet -- how close does one need to move the black hole?  How much stuff needs to be dumped in to power the jet?  I suspect black holes are very efficient at E=mc^2.

Some galaxies might be so unlucky as to not need astroengineering.  Their SMB jets might have somehow (perhaps after merger) gotten naturally aligned with the disc of the galaxy (the black hole is "on its side", like Uranus), so as stars orbit the central black hole, its jets irradiate every star system in the disc.  Tell a story of a civilization which sees a jet approaching.

How do you move and point a (possibly supermassive) black hole?  If only stellar mass black holes are needed, then it might be easier just to find one that is already pointing the right direction.

Moving and pointing black holes seems so difficult that it's almost certainly easier just to move the target, perhaps into its sun.  (Though maybe the target has defenses against that, so a ranged weapon is desirable.)  However, sterilizing an entire galaxy as described above does seem an appropriate use case (assuming it even works: life often finds a way).

A K3 civilization might also use jets not just for warfare but also as a power source.

[bgyjrlrh] K3 villages are not very important

In a Star Wars-like universe in which planet-killing weapons are being frequently used, at first it doesn't seem so bad that the worst thing a character has done (as we explore the character's backstory) is that the character destroyed merely a village once, and accordingly, the character is set up as at most a minor villain, or maybe even the protagonist with some minor flaws.  Then, we learn that the concept of "village" in this universe means a planet with low population density like that of Earth, in contrast to "cities" which are planets that are ecumenopolises like Coruscant.  (In contrast to destroying villages, destroying a city does look bad on your permanent record.)

Then, we discover that the "village" that the character had destroyed was not just like Earth: it actually was Earth, our Earth.  This character had been mankind's greatest enemy and had won by destroying the planet and killing everyone.  But that act isn't even considered noteworthy in this universe.

[drdwhngj] If meteors were far away

It's probably not too difficult to use parallax to determine that meteors are atmospheric phenomena (actually to verify, since they had been assumed so, hence their name).  (Who first made this measurement?)

However, suppose this measurement, in an alternate universe, revealed that meteors -- the meteor streaks themselves -- were astronomical phenomena, shining from deep space far beyond the atmosphere.

We can roughly estimate the velocity of meteors (in our universe) by modeling a meteor grain as stationary in space and the Earth hitting it at Earth's orbital speed around the sun.  Assuming the visual appearance of meteors in the alternate universe is the same as in ours, I think meteors would be determined to be moving faster than the speed of light if they are more than 10000 au = 0.16 light year from the sun.  For the purposes of fiction, let's imagine alternate-universe astronomers struggling to explain the observation of no parallax even from widely separated space probes in the solar system observing the same meteor.  No parallax despite such a wide baseline implies their distance must be at least ten thousand light years.

Perhaps they are gashes in the fabric of spacetime.  But who's on the outside doing the scratching?

[yaomwkup] Just the right amount of water

It's a weird coincidence that we not only have water on earth (hypothesized to have been delivered by many comet impacts), but also not so much that the surface would be completely covered deeply.

It's hypothesized that it was in shallow tide pools, on the border between water and land, that life first evolved.  To have shallowness seems to have required fantastic luck.  Additionally, tides, if they were necessary, required a massive moon.

This possibly explains the Fermi paradox.  Someday we may explore many star systems, consistently finding no life because of either too much water or too little.

[qawcnoee] Moon target practice

The moon has much more maria on its near side, e.g., Oceanus Procellarum, than on its far side.  Explain this with a story that ancient technological creatures shot at the moon from earth with powerful devices that caused volcanism or liquified its surface.

Maybe for entertainment.

Were they merely visitors who brought their lawn chairs to earth to watch the show, then left?  Or did an ancient technological civilization evolve on earth then completely disappear?