Surely “unvisited” is insignificant. There’s no current science suggesting any means of faster-than-light travel. So, if you assume that extraterrestrial life would have lifespans grossly similar to terrestrial lifespans, we ought to remain unvisited.
“Saturated in the Great Silence” seems like a far more significant point.
So, if you assume that extraterrestrial life would have lifespans grossly similar to terrestrial lifespans, we ought to remain unvisited.
Human beings spread all over the globe on foot 75000-15000 years ago, despite the fact that no single human probably walked all the way from Africa to Australia. It’s a fairly trivial assumption that an expanding interstellar civilization would not be limited by the lifespan of its inhabitants.
The galaxy may be big, but it is very small compared to the time-scales involved here. At walking pace (~5 km/h), you could travel 61 light years during the time from when the milky way formed up to now. At speeds easily achievable using chemical propulsion (~15 km/s), you could travel the circumference of the milky way—twice!
You didn’t actually do the math on that. According to this paper by the Future of Humanity Institute (Nick Bostrom’s group), if life evolved to the point of interstellar travel 3 billion years ago and could travel at 50% of c, then you would expect it to travel not just to this galaxy, but the nearest million. If you go back five billion years and assume travel speeds of 99% of c, it could reach a billion galaxies. 75% of stars in the Milky Way that could support life are older than our Sun. It really is an enigma.
So, if you assume that extraterrestrial life would have lifespans grossly similar to terrestrial lifespans, we ought to remain unvisited.
Are talking about civilization/life lifespan or individual organism lifespan?
Civilizations can send out long lived probes, and individual lifespans are somewhat irrelevant, especially for post-biological civilizations.
If life is as plentiful as it appears to be, then due to the enormous numbers we should expect to have been visited in our history unless there is alot of future filtering somewhere in num civs avg civ ‘active’ lifespan fraction of civs that explore.
FTL travel isn’t necessary at all. The natural easy way to travel around the solar system is to use gravitational assists, which allows for travel at speeds on order of the orbital speeds. The sun orbits the galaxy at a respectable speed of around 251 km/s or 0.1c, and some stars such as Schol’z star travel in the opposite direction. So it should only take about a million years for even a slow expanding civ to expand out 1,000 lyrs. And very small scout probes could more easily travel at faster speeds.
Basically we should expect the galaxy to be at least fully visited, if not colonized, by at most one galactic year after the birth of the first elder space civ. Earth is only about 18 gyrs old, whereas the galaxy is 54 gyrs old.
Surely “unvisited” is insignificant. There’s no current science suggesting any means of faster-than-light travel. So, if you assume that extraterrestrial life would have lifespans grossly similar to terrestrial lifespans, we ought to remain unvisited.
“Saturated in the Great Silence” seems like a far more significant point.
Human beings spread all over the globe on foot 75000-15000 years ago, despite the fact that no single human probably walked all the way from Africa to Australia. It’s a fairly trivial assumption that an expanding interstellar civilization would not be limited by the lifespan of its inhabitants.
The galaxy may be big, but it is very small compared to the time-scales involved here. At walking pace (~5 km/h), you could travel 61 light years during the time from when the milky way formed up to now. At speeds easily achievable using chemical propulsion (~15 km/s), you could travel the circumference of the milky way—twice!
http://www.fhi.ox.ac.uk/intergalactic-spreading.pdf
You didn’t actually do the math on that. According to this paper by the Future of Humanity Institute (Nick Bostrom’s group), if life evolved to the point of interstellar travel 3 billion years ago and could travel at 50% of c, then you would expect it to travel not just to this galaxy, but the nearest million. If you go back five billion years and assume travel speeds of 99% of c, it could reach a billion galaxies. 75% of stars in the Milky Way that could support life are older than our Sun. It really is an enigma.
Are talking about civilization/life lifespan or individual organism lifespan?
Civilizations can send out long lived probes, and individual lifespans are somewhat irrelevant, especially for post-biological civilizations.
If life is as plentiful as it appears to be, then due to the enormous numbers we should expect to have been visited in our history unless there is alot of future filtering somewhere in num civs avg civ ‘active’ lifespan fraction of civs that explore.
FTL travel isn’t necessary at all. The natural easy way to travel around the solar system is to use gravitational assists, which allows for travel at speeds on order of the orbital speeds. The sun orbits the galaxy at a respectable speed of around 251 km/s or 0.1c, and some stars such as Schol’z star travel in the opposite direction. So it should only take about a million years for even a slow expanding civ to expand out 1,000 lyrs. And very small scout probes could more easily travel at faster speeds.
Basically we should expect the galaxy to be at least fully visited, if not colonized, by at most one galactic year after the birth of the first elder space civ. Earth is only about 18 gyrs old, whereas the galaxy is 54 gyrs old.