But what's the motivation to do that, so that the collapse is bigger? Beyond just because it could also many good reasons not to do that...
Life harnessing new resources is the most constructive way to compete.
Creatures that expand, expand exponentially.
Creatures that don’t, collapse exponentially.
It takes a minuscule percentage of a population to be expanders, for the whole population to quickly become expanders.
It would take an exceptional explanation to explain why creatures that could expand, don’t.
An exceptional explanation on par with explaining how a balloon could be popped in a vacuum, but the compressed air inside remain in place instead of dispersing.
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Also - starlight is going unused on a galactic scale. Actively managing energy production would enable a larger, longer, and more efficient future.
1/1,000, 1/1,000,000 ... it doesn't matter how small the fraction is to start.
Affluence in humans vastly increases consumption. By several orders of magnitude. Billionaires enjoy jets, yachts, and build companies that use vast resources to accomplish new things.
AI isn't going to be concerned about human lifetimes. 20 years to the nearest star for a laser sail ship smaller than a human takes physics we have now. For nano tech that would be a massive ship.
If you want a teleological argument, here is one: Life expands by colonizing and adapting to new environments and inventing new capabilities. Then expanding and adapting over and over again in response to each wave of change. Life expanding across the galaxy will produce beauty and complexity we can't imagine.
We will be sending entire civilizations of nanobots, or at least small artificial organisms.
At 5% of the speed of light, including travel, reproduction & relaunches, a civilization like that at the center of the galaxy will reach the rim in 100,000 years.
At the nano level time effective massively expands. Activity at that scale operates with speeds many magnitudes faster than gargantuan Ent-like beings like us, can easily imagine. :)
It isn’t a journey suitable for humans, and our position here is changing rapidly decade to decade. The galaxy was never going to belong to humans, but hopefully our progeny.
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Then there is hope from physicists, that harnessing stellar level energy will allow faster than light warp travel. That would let beings cross the vast spaces between galaxies.
Not at all depending on the timeframe you want to talk about in.
For example, one way to travel around the galaxy is to find some other object with a very fast orbit and colonize it. Then as it goes along you throw off seed ships. This is the whole idea behind 'von neumann probes'. Massive numbers of things working in parallel can get a lot of work done.
But were you saying "get started on this in 1000 years" instead of finish in 1000 years?
Once a civilization began harvesting the energy of a few star systems, it would have a good chance of colonizing their whole galaxy in a blink of an eye.
(100 thousand years, plus or minus an order of magnitude, as compared to universe time in billions of years.)
So we would be much more likely to see totally colonized galaxies, rather than partially colonized.
I would be curious about which instruments would detect infra-red galaxies, I.e. galaxy sized objects where all starlight was put to use, with only infra-red light as waste.
And would that be an object that would stand out if a major telescope came across it, or require specific initiative to search and find?
And speaking of optics, is there any reason to suspect an efficient civilization would leak lower frequency light than infra-red?