If you disbelieve in 30g fusion reactors and set a minimum viable weight of 500t for an efficient propulsion system (plus negligible weight for replicators) then you get an additional factor of 1e7.
Combining both for fusion at 0.8c would give you a factor of 5e12, which is significantly larger than the factor between “single solar system” and “entire galaxy”. These are totally pessimistic assumptions, though: Deceleration probably can be done cheaper, and with lower minimal mass for efficient propulsion systems. And you almost surely can cut off quite a bit of rocket-delta-v on acceleration (Stuart assumed you can cut 100% on acceleration and 0% on deceleration; the above numbers assumed you can cut 0% on acceleration and 0% on deceleration).
Also, as Stuart noted, you don’t need to aim at every reachable galaxy, you can aim at every cluster and spread from there.
So, I’m not arguing with Stuart’s greater claim (which is a really nice point!), I’m just arguing about local validity of his arguments and assumptions.
If you have to use the rocket equation twice, then you effectively double delta-v requirements and square the launch-mass / payload-mass factor.
Using Stuart’s numbers, this makes colonization more expensive by the following factors:
0.5 c: Antimatter 2.6 / fusion 660 / fission 1e6
0.8 c: Antimatter 7 / fusion 4.5e5 / fission 1e12
0.99c Antimatter 100 / fusion 4.3e12 / fission 1e29
If you disbelieve in 30g fusion reactors and set a minimum viable weight of 500t for an efficient propulsion system (plus negligible weight for replicators) then you get an additional factor of 1e7.
Combining both for fusion at 0.8c would give you a factor of 5e12, which is significantly larger than the factor between “single solar system” and “entire galaxy”. These are totally pessimistic assumptions, though: Deceleration probably can be done cheaper, and with lower minimal mass for efficient propulsion systems. And you almost surely can cut off quite a bit of rocket-delta-v on acceleration (Stuart assumed you can cut 100% on acceleration and 0% on deceleration; the above numbers assumed you can cut 0% on acceleration and 0% on deceleration).
Also, as Stuart noted, you don’t need to aim at every reachable galaxy, you can aim at every cluster and spread from there.
So, I’m not arguing with Stuart’s greater claim (which is a really nice point!), I’m just arguing about local validity of his arguments and assumptions.