This kind of assumes that a “maxed out” civilization can expand at speed of light, but I don’t think it is given. It may very well be that meaningfully massive transport cannot exceed even a small fraction of c. After all, propulsion has to make energetic sense.
1. There is energy to get to speed
2. Mass of fuel to decelerate (and you have to accelerate that fuel in the beginning!)
3. Highly blue shifted CMB (that alone limits how close to c you can get before becoming plasma)
4. Collision with micrometeorites (so the “ship” has to be microscopic or probability of collision quickly goes to 1, and there is no way to survive even micro collisions while moving at c).
5. Limits on how fast you can give impulse (because your ship is made of matter with finite strength). Eg You cant reasonably railgun something to close to c without converting it to plasma.
There is also all the things you do when you get somewhere before sending more probes to expand, presumably it’s not a single location that seeds everything else (and if it is then the energy budget of that is also complicated).
Now, even at 0.1 c and with many pauses to replicate on the new worlds it would still take only a few million years to span the milky way, but if someone was mid-expansion we would see them way before the bubble hits. Being limited to 0.1c also means that those not in our galaxy will likely be unable to expand into ours (and vice versa).
If you can seed enough intelligence with self replicating "nano-machines", which seems reasonable, then you can accelerate clouds of that material at quite close to the speed of light with high power lasers, much like diffuse solar sails. No need to carry large payloads, reaction mass, or energy... those are at the endpoint where you find them (and convert them into grey-goo?).
It's more of a viral mode of expansion where (virus) seeds carry the operational commands to new worlds (cells) which host them until they are consumed by the continued expansion. The intelligence and power consumption are concentrated where it's available, while passive catalytic action and stored knowledge is transported at speed.
That's a good point, although I don't think you could target a planet in any case. You're looking at solar systems, and properly designed a single surviving nano-machine skipping off the atmosphere of a gas giant would be sufficient to start exponential replication.
If we think about the limits of the physical laws, one possibility is having a civilization that maximizes the available computation, and I suspect that this looks like some kind of black hole expanding with the speed of light.
Given the nature of 3D expansion and the size of galaxy vs the age of the universe 0.1c vs 1C isn't really that big of a difference.
However I do wonder why the fermi folks get all bent out of shape about not seeing other civilizations. How far do you think you could see a fusion or antimatter ship accelerating or deaccelerating from 0.1c? Unless they are zipping around our solar system (Expanse style) doesn't seem likely the we'd notice them.
If the aliens were curious about us and had detected our expanding RF emissions they might well be close by listening.
Seems far from clear that civilizations would bother messing with stars (that would be detectable from a decent distance) instead of just propagating to other solar systems. People are already talking about sending probes to our nearest star via laser propulsion.
Sometimes I wonder if a prerequisite to becoming the kind of civilization that could span a large volume of space is becoming the kind of civilization that has solved the problems that might drive a civilization to need that kind of expansion.
Consider Earth. If we set T=0 at one year ago, and assume that there really is no way around the speed of light, then it follows that by year N, every human alive must be within a sphere of radius N light years.
The average human has a volume of about 0.06 m^3, according to a few sites I found on the net. If you divide the volume of a sphere of radius N light years by 0.06 m^3, you get an upper limit on the number of humans that can be alive in year N.
Given a fixed population growth rate, you can figure out at what year the population hits the upper limit. It is startling faster than you might guess.
At 1% annual growth, we hit it by year 12059, when we'd have a population of 1.04 x 10^62 needing to fit in a volume of 6.22 x 10^60 m^3.
At 0.1% annual growth, we hit it by year 127168, with a population of 1.27 x 10^65 needing to fit in 7.26 x 10^23 m^3.
At 0.01% annual growth, we hit it by 1341545, with 1.45 x 10^68 people needing to fit in 8.56 x 10^66 m^3.
It is clear then, barring faster than light travel (or expansion into other dimensions or parallel universes or something like that), that growth rates for an growing civilization must in the long run go down, and keep going down until growth is very very low if that civilization survives long term.
Keep in mind that my numbers above are the time to hit a ridiculously high limit (packing all space that could be reached at the speed of light starting last year with over 16 humans per m^3). If you use a limit not as ridiculous, such as the still ridiculous limit of all mass within that volume is used to make humans, you'd get a much shorter time to the limit.
I think this might be a candidate for the Great Filter. Lowering growth rates means getting some people not to reproduce, but millions of years of evolution have selected for reproduction. Maybe civilization hit a point where they must slow down growth more than their citizens are willing to do, and that leads to internal conflict that if it does not destroy them at least stops them from further advancing.
The result may be that there may be a lot of civilizations out there, but they get stuck at the first size limit that they hit where (1) they don't have the technology to get past that limit but (2) they have the technology to make war over that limit devastating.
Civilizations that avoid this have figured out how to thrive indefinitely without depending on growth. Such a civilization might not feel the need to expand into a large volume of space. They may be content to stay within their solar system.
> Sometimes I wonder if a prerequisite to becoming the kind of civilization that could span a large volume of space is becoming the kind of civilization that has solved the problems that might drive a civilization to need that kind of expansion.
Or we may just discover new risks to avoid. Our ancestors created new villages because the river dried up, we might want other planets due to extinction-level asteroids, and our descendants may need to sidestep The Interstellar Ooze or whatever.
I understood the "close to the speed of light" really meant something much much smaller than 0.1c, since on cosmological timescales the difference between the first radio signals and domination from an alien civilization could be millions of years, and that'd still be an instant compared to the time before and after the event. So I'm assuming for the fermi paradox to work out here, we'd not need speeds anywhere near 0.1c
If you move slower than 0.1c then you can’t really cross between galaxies (or at least you are limited to very nearby ones). So we only have to worry about grabby aliens in our galaxy AND we can be fairly certain that there are currently none! Since we ourselves are probably only thousands of years from being grabby this implies we don’t have advanced competitors .This is a very different picture from what Hansen discusses.
0.1c isn't slow enough to solve the empirical puzzle that we don't now see alien volumes. That is the fact that pushes for higher speeds in our analysis.
Sure, but printers have mass/volume and require energy. I think in all Van Neumann probe scenarios (which this is) the fact that you bring a micro—replicator that makes other micro-replicators is a given.
Right, a bacterium is proof-of-concept for a literally microscopic printer. Maybe the seed to grow into a radio dish out of asteroid materials and sunlight would necessarily be bigger than that, but I'd be surprised if it really had to be bigger than, say... a seed.
You are assuming present day lasers. An advanced civilization will individually control the photons.
You also don't really need atom resolution anyway. You can build mechanical computers driven by rolling stones by abblating matter with powerful beams - lithography on continent size scale, etc.
Even if you individually control the photons, I'm not sure that's possible. Matter still behaves like a wave at quantum levels. I don't know enough to say with certainty. This is all highly speculative.
You don't need all your photons to get to the destination. Only some of them. And you can use error correcting methods to ensure they have a useful pattern when they arrive.
1. There is energy to get to speed
2. Mass of fuel to decelerate (and you have to accelerate that fuel in the beginning!)
3. Highly blue shifted CMB (that alone limits how close to c you can get before becoming plasma)
4. Collision with micrometeorites (so the “ship” has to be microscopic or probability of collision quickly goes to 1, and there is no way to survive even micro collisions while moving at c).
5. Limits on how fast you can give impulse (because your ship is made of matter with finite strength). Eg You cant reasonably railgun something to close to c without converting it to plasma.
There is also all the things you do when you get somewhere before sending more probes to expand, presumably it’s not a single location that seeds everything else (and if it is then the energy budget of that is also complicated).
Now, even at 0.1 c and with many pauses to replicate on the new worlds it would still take only a few million years to span the milky way, but if someone was mid-expansion we would see them way before the bubble hits. Being limited to 0.1c also means that those not in our galaxy will likely be unable to expand into ours (and vice versa).