Isn't 4 equivalent to, 'no civilization will invent casual interstellar travel/spread to the stars'?
And isn't that hypothesis extremely likely, given what we now know about the costs of _interplanetary_ travel and its relative ease compared to interstellar travel?
That's another assumption, that it's _possible_ to colonize a galaxy from a planetary base. We don't even know that it's possible to colonize a nearby planet. If we were confident that we could, we'd be filling up Siberia, Ellesmere Island, and Greenland first.
There's also another assumption missing from your list- that technological civilizations can last long enough to colonize the galaxy. I'm also surprised that there isn't any discussion on the site or here of Great Filters. If the average technological civilization wipes itself out within a few hundred years of developing technologies that enable space travel or even radio, then all discussion of "filling the galaxy" is castles in the air.
Isn't 4 equivalent to, 'no civilization will invent casual interstellar travel/spread to the stars'?
And isn't that hypothesis extremely likely, given what we now know about the costs of _interplanetary_ travel and its relative ease compared to interstellar travel?
That's another assumption, that it's _possible_ to colonize a galaxy from a planetary base. We don't even know that it's possible to colonize a nearby planet. If we were confident that we could, we'd be filling up Siberia, Ellesmere Island, and Greenland first.
There's also another assumption missing from your list- that technological civilizations can last long enough to colonize the galaxy. I'm also surprised that there isn't any discussion on the site or here of Great Filters. If the average technological civilization wipes itself out within a few hundred years of developing technologies that enable space travel or even radio, then all discussion of "filling the galaxy" is castles in the air.