Once we can see them, it’s too late(scottaaronson.com)
scottaaronson.com
Once we can see them, it’s too late
https://www.scottaaronson.com/blog/?p=5253
113 comments
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.
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.
They still need to decelerate. Even nanomachines will not survive if they hit planets at close to c.
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.
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.
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.
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.
You might want to read about matroska brain - star and then galaxy sized sentient computer.
Meaning a single individual would be and use the whole galaxy.
Meaning a single individual would be and use the whole galaxy.
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.
3D printing.
Deliver the printer, rest is just information.
Deliver the printer, rest is just information.
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.
In theory you can manipulate atoms and molecules at a distance with light.
So you can use a laser beam to construct a 3D printer at a distance.
Rinse and repeat, and this is asymptotically expanding at c.
So you can use a laser beam to construct a 3D printer at a distance.
Rinse and repeat, and this is asymptotically expanding at c.
OK, you're tossing out quantum mechanics, in particular the Heisenberg uncertainty principle.
We are firmly in the realm of science fantasy now.
We are firmly in the realm of science fantasy now.
The first few pages of Glory by Greg Egan describe something a bit like that.
https://bobson.ludost.net/hugoawardnominees/2008/GregEganGlo...
https://bobson.ludost.net/hugoawardnominees/2008/GregEganGlo...
I hadn't read that before; thanks :)
A laser beam expanding out over light years is not a laser anymore and definitely not atom sized.
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.
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.
If the errors are happening further out than your civilization has ever been, what's correcting them?
They are still quantum objects and still have diffraction and such.
An interesting paper that I don't see brought up often enough is "Dissolving the Fermi Paradox" by Anders Sandberg, Eric Drexler and Toby Ord[0]. They show that it isn't a surprise that we are alone in the universe if we consider the uncertainty inherent to parameters of the Drake equation. Their analysis estimate that there is (at least) a 39% chance that we are alone in the observable universe.
[0]: https://arxiv.org/abs/1806.02404
[0]: https://arxiv.org/abs/1806.02404
From a previous discussion of this paper I took away the following clarifying insight:
The Drake equation is usually turned into the Fermi paradox via some estimations that show the average (mean) number of alien civilizations we expect to see is large (>1).
However, just because the mean is large does not imply that seeing 0 is low probability.
More concretely, suppose there were 10 possible 'parallel' universes. In 9 of them humanity is alone and in the tenth there are tens of millions of alien civilizations. The probability of appearing to be alone is 90% even though the number of alien civilizations we'd expect to see is in the millions.
The Drake equation is usually turned into the Fermi paradox via some estimations that show the average (mean) number of alien civilizations we expect to see is large (>1).
However, just because the mean is large does not imply that seeing 0 is low probability.
More concretely, suppose there were 10 possible 'parallel' universes. In 9 of them humanity is alone and in the tenth there are tens of millions of alien civilizations. The probability of appearing to be alone is 90% even though the number of alien civilizations we'd expect to see is in the millions.
You don't even need to invoke parallel universes. Look at the Hubble Deep Field photo[0]. Our observable universe has hundreds of millions of galaxies. It's entirely feasible for tens of millions of alien civilizations to be spread about and not visible to us in the narrow window of space and time in which we've been looking.
[0]https://en.wikipedia.org/wiki/Hubble_Deep_Field
[0]https://en.wikipedia.org/wiki/Hubble_Deep_Field
Invoking parallel universes was solely for ease of explaining/emphasizing the math, not because I think it's a realistic model.
Talking about the Hubble Deep Field kind of misses the point. The original Drake equation[0] estimates the number of alien civilizations in our galaxy. I probably should have used galaxies in my example, but left it with parallel universes as that is the common phrase.
[0]: https://en.wikipedia.org/wiki/Drake_equation
Talking about the Hubble Deep Field kind of misses the point. The original Drake equation[0] estimates the number of alien civilizations in our galaxy. I probably should have used galaxies in my example, but left it with parallel universes as that is the common phrase.
[0]: https://en.wikipedia.org/wiki/Drake_equation
I really dislike these type of papers. Just like Drake equation, it is all guesswork with completely unknown boundaries for many of the parameters.
How will their paper work if we confirm life on Venus, especially if it independently evolved.
How will their paper work if we confirm life on Venus, especially if it independently evolved.
Another interesting idea I've been playing with is that there's no guarantee that the lifeform that replaces us will be the same level of complexity. What does it feel like to be a molecule? A mitochondria? A cell? All of these are or were once self-contained "things", but now they exist as part of larger systems. Our image of the beings that replace us tends to look like us and be roughly the same size - robots or little green men. But what if the thing that replaces us encompasses us, so that individual humans become cells in a giant superorganism.
There's some evidence that this is already happening - with 6 billion humans on earth, the global economy is already showing specialization (similar to how cells develop into organs in a human) and complex emergent behavior, with parts of it dying off without killing the whole.
What if the reason are experience looks like it does, with life a full 1/4 of the age of the universe, because at later points in the universe's evolution, life at this scale becomes meaningless and the primary sentient beings are planetary civilization interacting at the speed of light through the galaxy? Or galactic civilizations interacting throughout the universe?
There's some evidence that this is already happening - with 6 billion humans on earth, the global economy is already showing specialization (similar to how cells develop into organs in a human) and complex emergent behavior, with parts of it dying off without killing the whole.
What if the reason are experience looks like it does, with life a full 1/4 of the age of the universe, because at later points in the universe's evolution, life at this scale becomes meaningless and the primary sentient beings are planetary civilization interacting at the speed of light through the galaxy? Or galactic civilizations interacting throughout the universe?
The is an interesting take on a superorganism in Venor Vinge's A Fire Upon the Deep
It seems like the argument gets a lot less interesting if the most-expanding interstellar civilizations don't spread at anywhere near the speed of light?
It's easy to think of reasons why this might be the case. There is acceleration, deceleration, and replication time, which seems like it would be substantial for any civilization that doesn't want to put most of its resources into replicating as fast as possible.
It's easy to think of reasons why this might be the case. There is acceleration, deceleration, and replication time, which seems like it would be substantial for any civilization that doesn't want to put most of its resources into replicating as fast as possible.
Ah, the classic mistake in a game of "Risk"; expanding as fast as you possibly can, leaving too few resources (troops) at your border to prevent a rapid collapse. If a fast-as-you-can civilization meets a slowly-expanding civilization, it is not at all clear to me that the former would prevail.
In rsk everybody is at a roughly similar technological level. it really depends on things like technological level difference (ask the mayas and aztec) and philosophy, and if you're expanding as fast as you can, you can probably mobilize resources rather easily. Then you have philosophical difference: are you expanding to make friends, or just to gather resources for the hive queen?
I like to imagine that this is how things would look if we entered a Karadashev type III civilization's light cone:
One day, in a small quadrant of the galaxy, we'd notice starts "shutting off", simply going dark. After a few months, our astonomers would confirm their new model: the amount of stars going dark is increasing. Days later: the rate of increase itself, is increasing. One day we'd wake up, and a small portion of the night sky would be dark. A disk of darkness, right at the brightest part of the galactic plane. By then we would have figured it out of course: most stars in the galaxy were being turned into dyson spheres. They were spreading almost as fast as light, making it look almost instantaneous, even though the process had taken hundreds of thousands of years. In a few thousand years a large portion of our sky would be dark. And after that...
One day, in a small quadrant of the galaxy, we'd notice starts "shutting off", simply going dark. After a few months, our astonomers would confirm their new model: the amount of stars going dark is increasing. Days later: the rate of increase itself, is increasing. One day we'd wake up, and a small portion of the night sky would be dark. A disk of darkness, right at the brightest part of the galactic plane. By then we would have figured it out of course: most stars in the galaxy were being turned into dyson spheres. They were spreading almost as fast as light, making it look almost instantaneous, even though the process had taken hundreds of thousands of years. In a few thousand years a large portion of our sky would be dark. And after that...
Doesn't it seem kind of stupid that a K-III civilization would depend on collecting starlight for energy? Even the most extreme-primitive cultures, barely off their home planet, will have mastered controlled fusion.
Almost immediately, they have no more use for small rocky gravity-well planets, or even for stars. They live in Oort clouds where thermal noise is low and cold is plentiful.
Almost immediately, they have no more use for small rocky gravity-well planets, or even for stars. They live in Oort clouds where thermal noise is low and cold is plentiful.
I agree. It seems very shortsighted to assume a highly advanced civilization would act in this way. Not only would creating Dyson Spheres be extremely visible (Dark Forest comes to mind), but I doubt that these type of civilizations would be energy constrained. Simple fusion reactor solves most of the energy issues.
Maybe Dyson spheres as farms? Fusion wouldn't produce food.
Sure, but if you have abundant energy you can use hydroponics to achieve the same. Dyson spheres provide vast amount of space but if you have practically infinite power you can create habitats that provide the same functionality with added benefits of gravity (O'Neill cylinder) and not being visible at star distances.
This is approximately one of the opening ideas in the Commonwealth Saga by Peter Hamilton. Highly recommended!
The ideas discussed in his post, especially the idea of rapidly expanding spheres of civilizations consuming all resources in their path, were beautifully explored in Stephen Baxter's sci-fi book, Manifold: Space (a spin-off of his earlier book, Manifold: Time, which is also excellent). In his book, alien intelligences are common; once they become sufficiently advanced, their civilizations tend to rapidly expand and consume all available resources, often to the detriment of other civilizations in their path. This pattern leads to some interesting phenomena: first, while the night sky might seem quiet at first, once we do encounter aliens, we tend to see their signals across many star systems in rapid succession. The reason is pretty obvious: there is only a brief period of time when we are on the surface of a sphere - a few years after our first observations of aliens, we are engulfed within their sphere and observe their signals from all over our stellar neighborhood. Another idea he plays with is the idea of "refugee" species, who attempt to flee oncoming spheres by evacuating ahead of their path instead of being consumed.
Actually, he pushes this idea even further: in the book, our solar system was already engulfed in a few spheres millions of years ago. He suggests that this why Venus is such a hellscape: the aliens came, took the resources they wanted, and left behind a polluted mess. In the case of Venus, they left lots of greenhouse gases behind as the result of some chemical process used to extract resources; as a result, Venus quickly became the warmest planet in the solar system. It's a fun twist on the Fermi paradox: signs of aliens are actually all around us, we are just too dumb to notice them.
Another interesting idea he explores a bit is "ownership" of resources. Do the resource-rich asteroids in our solar system really belong to us? Or are they available to any alien race who happens to pass through? In the book, we first notice aliens by observing unexplainable infrared radiation from the asteroid belt (later revealed to be thermal emissions from their resource extraction). He suggests that these aliens will potentially crowd out humans; even if they are not overtly hostile, they could gobble up all the resources we would have used to expand our civilization.
Highly recommend this book.
Another interesting idea he explores a bit is "ownership" of resources. Do the resource-rich asteroids in our solar system really belong to us? Or are they available to any alien race who happens to pass through? In the book, we first notice aliens by observing unexplainable infrared radiation from the asteroid belt (later revealed to be thermal emissions from their resource extraction). He suggests that these aliens will potentially crowd out humans; even if they are not overtly hostile, they could gobble up all the resources we would have used to expand our civilization.
Highly recommend this book.
The site appears to be down. (504 Gateway Timeout) https://archive.is/ki5Vm
Notice that, in Robin’s scenario, the present epoch of the universe is extremely special: it’s when civilizations are just forming, when perhaps a few of them will achieve technological liftoff, but before one or more of the civilizations has remade the whole of creation for its own purposes. Now is the time when the early intelligent beings like us can still look out and see quadrillions of stars shining to no apparent purpose, just wasting all that nuclear fuel in a near-empty cosmos, waiting for someone to come along and put the energy to good use.
This presumes that The Most Technologically Advanced Civilization sees virgin nature as nothing but raw material waiting to become something useful. That's possible, but probable? I think that it's likely that diminishing marginal utility still holds even for TMTAC, and therefore they are disinclined to convert all the universe's visible matter and energy into Dyson swarms of Space Product.
My favorite (not particularly testable) solution to the Fermi paradox is that TMTAC originated shortly after the first heavy elements and planets formed. It became space faring and expanded throughout the visible universe before our solar system formed. Its agents have been lurking in our solar system since before life first appeared here. Having long ago achieved immortality and technological supremacy, there's no motivation for plundering or trading with terrestrial creatures. They silently observe like space faring bird watchers. They'll intervene if/when we start to approach the capabilities of TMTAC, particularly if we show destructive paperclip-maximizer inclinations toward converting the universe into Space Product.
To borrow some terminology from Nick Bostrom's Superintelligence book, it's possible that the universe has been colonized by a singleton civilization -- the first one to become star faring. But it's not particularly chatty or inclined to let potentially competing star faring civilizations expand.
This presumes that The Most Technologically Advanced Civilization sees virgin nature as nothing but raw material waiting to become something useful. That's possible, but probable? I think that it's likely that diminishing marginal utility still holds even for TMTAC, and therefore they are disinclined to convert all the universe's visible matter and energy into Dyson swarms of Space Product.
My favorite (not particularly testable) solution to the Fermi paradox is that TMTAC originated shortly after the first heavy elements and planets formed. It became space faring and expanded throughout the visible universe before our solar system formed. Its agents have been lurking in our solar system since before life first appeared here. Having long ago achieved immortality and technological supremacy, there's no motivation for plundering or trading with terrestrial creatures. They silently observe like space faring bird watchers. They'll intervene if/when we start to approach the capabilities of TMTAC, particularly if we show destructive paperclip-maximizer inclinations toward converting the universe into Space Product.
To borrow some terminology from Nick Bostrom's Superintelligence book, it's possible that the universe has been colonized by a singleton civilization -- the first one to become star faring. But it's not particularly chatty or inclined to let potentially competing star faring civilizations expand.
How a civilization would expand at the speed of light? Unless it lives directly on the fabric of space or something like that, matter is discrete in the universe. You settle on planets that are not everywhere, or in space stations built with Oort cloud or asteroid belts materials, but it takes time to settle and expand in each new ground you get. You are not talking about the speed of light anymore there.
Of course, here I'm trying to think like an alien civilization that is far ahead from us in technology and scientific knowledge, besides having an alien way of thinking, but the same goes for the article.
In any case, if that is like any disaster spreading through the universe at the speed of light (big rip?) not only we won't have time to notice, we won't be able to feel the effects neither.
Of course, here I'm trying to think like an alien civilization that is far ahead from us in technology and scientific knowledge, besides having an alien way of thinking, but the same goes for the article.
In any case, if that is like any disaster spreading through the universe at the speed of light (big rip?) not only we won't have time to notice, we won't be able to feel the effects neither.
Think “matter scanner/printer”.
Scan an object, at atomic & energy vector level. Transmit that information to a destination’s printer. Expansion is then indeed at speed of light, capped only by need to physically move a small “seed” printer (rapid transport may have enormous energy cost, but that’s all that needs moving; everything else is just data).
Having seen tech go from digital imaging to 3D organ printing in a fraction of my lifespan, seems plausible.
Scan an object, at atomic & energy vector level. Transmit that information to a destination’s printer. Expansion is then indeed at speed of light, capped only by need to physically move a small “seed” printer (rapid transport may have enormous energy cost, but that’s all that needs moving; everything else is just data).
Having seen tech go from digital imaging to 3D organ printing in a fraction of my lifespan, seems plausible.
But first you have to deliver the printer to the destination somehow.
Here's an idea.
Imagine you have a way to build von Neumann probes that can boost themselves to some high fraction of c (say, 0.9). You start pumping them out in your home system, and send them towards some stars - not necessarily the closest ones. You keep sending them. There's now a shell expanding at 0.9c that's shaped by your production capabilities. Of course, the probes will spend time accelerating and then decelerating towards their destinations. But even as they decelerate, other probes, going to further targets, pass ahead of them.
Any probe that lands in a system sets up a facility for building and provisioning more of itself, and sends those to further star systems, adding to the wave. Meanwhile, any technological advancement could be transmitted at the speed of light - so the probes receive improvement instructions as they fly, and the entire shell keeps upgrading itself as it expands.
What's the aggregate speed of that shell? Less than 0.9c, I think, but I'd say it's still a high fraction of it.
The main warning sign we could look for, assuming there are no crazy unknown physics allowing creation of silent drives, are deceleration plumes and associated thermal emissions. But then, civilizations like ours should be easy to spot from a distance. Don't bother with radio waves, just look at the spectral lines of the planets in a system; if you spot high concentrations of chemicals that don't have geological rationale to be there (such as oxygen in our case), this implies something is keeping the planet away from equilibrium - likely some form of life.
If we go by Dark Forest theory, the standard procedure upon detection of such "life signs" would be to send updated instructions to the von Neumann probe heading towards that system: "don't decelerate, ram the planet (or the star) at 0.9 c". If there's anything of use left there, another probe will come later and consume it.
So, by the time we spot the evidence of the von Neumann probes, we may just be about to get hit by a relativistic kill vehicle at the vanguard of the expanding shell.
(I'm tempted to build a toy model and play with the parameters to see how much of an advance warning we would have.)
Imagine you have a way to build von Neumann probes that can boost themselves to some high fraction of c (say, 0.9). You start pumping them out in your home system, and send them towards some stars - not necessarily the closest ones. You keep sending them. There's now a shell expanding at 0.9c that's shaped by your production capabilities. Of course, the probes will spend time accelerating and then decelerating towards their destinations. But even as they decelerate, other probes, going to further targets, pass ahead of them.
Any probe that lands in a system sets up a facility for building and provisioning more of itself, and sends those to further star systems, adding to the wave. Meanwhile, any technological advancement could be transmitted at the speed of light - so the probes receive improvement instructions as they fly, and the entire shell keeps upgrading itself as it expands.
What's the aggregate speed of that shell? Less than 0.9c, I think, but I'd say it's still a high fraction of it.
The main warning sign we could look for, assuming there are no crazy unknown physics allowing creation of silent drives, are deceleration plumes and associated thermal emissions. But then, civilizations like ours should be easy to spot from a distance. Don't bother with radio waves, just look at the spectral lines of the planets in a system; if you spot high concentrations of chemicals that don't have geological rationale to be there (such as oxygen in our case), this implies something is keeping the planet away from equilibrium - likely some form of life.
If we go by Dark Forest theory, the standard procedure upon detection of such "life signs" would be to send updated instructions to the von Neumann probe heading towards that system: "don't decelerate, ram the planet (or the star) at 0.9 c". If there's anything of use left there, another probe will come later and consume it.
So, by the time we spot the evidence of the von Neumann probes, we may just be about to get hit by a relativistic kill vehicle at the vanguard of the expanding shell.
(I'm tempted to build a toy model and play with the parameters to see how much of an advance warning we would have.)
Suppose that there is no need to have in the equation living beings. So you have your von Neumann probes assimilating all the matter in their path into new von Neumann probes, lets call them Borg von Neumann. And they are nodes of a collective /distributed "computing" scheme called Alien Civilization 3.0.
Still you need time to stop, make your conversion, pick next target, accelerate and deaccelerate. And you may be turning everything into nodes, i.e. suns, exoplanets, etc. As we find stars pretty far away and exoplanets in far away solar systems, we are safe for some million years at the very least, even with relativistic speeds.
And still, it is about what we think with the wrong mindset and a lot of unknown unknowns to properly understand the problem. Expanding may be obviously wrong for advanced enough... something.
Still you need time to stop, make your conversion, pick next target, accelerate and deaccelerate. And you may be turning everything into nodes, i.e. suns, exoplanets, etc. As we find stars pretty far away and exoplanets in far away solar systems, we are safe for some million years at the very least, even with relativistic speeds.
And still, it is about what we think with the wrong mindset and a lot of unknown unknowns to properly understand the problem. Expanding may be obviously wrong for advanced enough... something.
Would make a great toy, implemented along the lines of Universal Paperclips.
Yes, but a whole lot easier to deliver a printer than a functioning civilization, when you can just print a civilization.
This is extrapolation from an extremely small set of facts, and tremendously conjectural assumptions. It’s the kind of thing you might expect to see from a person living in a civilization that just discovered electrons 200 years ago. Come back to me in 100 years once the economy and population has reached some kind of steady state and then let’s revisit some of these claims.
A lot of what we think is rational discourse is actually ingrained, unspoken ideology floating to the surface and being coated with a patina of post hoc rationalization. That’s why it’s so hard to do science properly, especially social science- people don’t even know that they have biases, let alone what they are. But biases aren’t something you can point at directly and say, this belief is not true, it’s more that you might be overestimating the probability of truth of a whole bunch of little things by 10 percent and that creates a self reinforcing network of probable facts that adds up to weird beliefs when they are all put together.
The author of the article believes on some deep level that progress is inevitable and will go on indefinitely, combined with the unconscious imagery of a lifetime watching science fiction films, and a life of watching progressively more impressive gadgets appear on store shelves. It all adds up to essays like this.
A lot of what we think is rational discourse is actually ingrained, unspoken ideology floating to the surface and being coated with a patina of post hoc rationalization. That’s why it’s so hard to do science properly, especially social science- people don’t even know that they have biases, let alone what they are. But biases aren’t something you can point at directly and say, this belief is not true, it’s more that you might be overestimating the probability of truth of a whole bunch of little things by 10 percent and that creates a self reinforcing network of probable facts that adds up to weird beliefs when they are all put together.
The author of the article believes on some deep level that progress is inevitable and will go on indefinitely, combined with the unconscious imagery of a lifetime watching science fiction films, and a life of watching progressively more impressive gadgets appear on store shelves. It all adds up to essays like this.
Or... is it possible that we are not supposed to meet the "others?
Consider: If you wanted to build a "terrarium" for sentient evolving life forms how would you keep them from escaping?
1. Make the distances really really big 2. Put a speed limit in the environment
Rolled up with the Fermi paradox this simulation has "smelled fishy" to me for some time. :)
Consider: If you wanted to build a "terrarium" for sentient evolving life forms how would you keep them from escaping?
1. Make the distances really really big 2. Put a speed limit in the environment
Rolled up with the Fermi paradox this simulation has "smelled fishy" to me for some time. :)
> 1. Make the distances really really big 2. Put a speed limit in the environment
Or 3. Don't give humans capabilities to see them. Hell, I'd go even further, what if the "others" are already living on our planet, but we can't see them?
Or 3. Don't give humans capabilities to see them. Hell, I'd go even further, what if the "others" are already living on our planet, but we can't see them?
Below [1] Hanson explains some of the details of the argument in length.
https://www.youtube.com/watch?v=Rjm--7t8Llk&feature=youtu.be
https://www.youtube.com/watch?v=Rjm--7t8Llk&feature=youtu.be
Huh?
> Only when the sphere’s thin outer shell had reached the earth—perhaps carrying radio signals from the extraterrestrials’ early history, before their rapid expansion started. By that point, though, the expanding sphere itself would be nearly upon us!
Wouldn't the problematic portion of the sphere be, by definition, millions of years away from us? Given that radio waves travel at the speed of light, and the alien civilization travels at slightly less than the speed of light, it seems like we should have at least however many million years it took for those aliens to get from producing radio waves to "maxing out" travel.
> Only when the sphere’s thin outer shell had reached the earth—perhaps carrying radio signals from the extraterrestrials’ early history, before their rapid expansion started. By that point, though, the expanding sphere itself would be nearly upon us!
Wouldn't the problematic portion of the sphere be, by definition, millions of years away from us? Given that radio waves travel at the speed of light, and the alien civilization travels at slightly less than the speed of light, it seems like we should have at least however many million years it took for those aliens to get from producing radio waves to "maxing out" travel.
The first radio transmission by humans was made in 1895. It took another 74 years for humans to land on the moon. It would be disappointing if our augmented and/or artificial descendants really needed millions more years to reach beyond the solar system.
Right, but that was sparse and unlikely to be detected, and the author is talking about the portions of the sphere which travel at near speed of light catching up to earth. If human progress is measured on the scale of reaching near-speed-of-light travel, it is easy to imagine being thousands if not millions of years off.
Interstate idea. The title overstated too late though.
Following the premise that an expanding civilization is expanding near the speed of light and it's radio will reach us first.
We can expect hundreds of years between radio and anything physical.
1) We started producing a lot of radio around 100 years ago. And we are still a long way from expanding at near speed of light. Even with expodential growth in capacity.
2) expanding near speed of light is much less than speed of light. Say 99% of the speed of light. And expanding civilization is probably going to be coming from far far away, just because there is a lot more far far away than pretty close. So say they are comiy from 10,000 light-years away (still close by galactic standards, and incredibly Close by Universal standards). Then the radio they start transmitting when their grown begins has gained on their expansion ba lead of 100years by the time it reaches us
Sure on the million and billion year time-scales mostly talking about that 100+ years is very little but it is still generations.
1) We started producing a lot of radio around 100 years ago. And we are still a long way from expanding at near speed of light. Even with expodential growth in capacity.
2) expanding near speed of light is much less than speed of light. Say 99% of the speed of light. And expanding civilization is probably going to be coming from far far away, just because there is a lot more far far away than pretty close. So say they are comiy from 10,000 light-years away (still close by galactic standards, and incredibly Close by Universal standards). Then the radio they start transmitting when their grown begins has gained on their expansion ba lead of 100years by the time it reaches us
Sure on the million and billion year time-scales mostly talking about that 100+ years is very little but it is still generations.
This seems like a variant of a hypothesis I've heard before: "The universe is old enough, and the rate of expansion of a spacefaring civilization is fast enough (relative to the age of the universe, even at sub-light speeds) that either the aliens should already be here since long ago, or we're the first (or among the first)." ?
How do you determine who was "first" when simultaneity doesn't exist on a cosmic scale?
Well, at any given point of reference (e.g. a star or galaxy) there's a clear ordering of events, who is first there. And the relative velocities between different stars and galaxies aren't that large; if we're speaking also about cosmic time scales (e.g. not days or years, but millions of years) then there's no need to involve relativistic reasoning about who's first.
Fermi's paradox.
It assumes that technology will get to the point where it becomes trivial to explore space. That's not an easy assumption to grant.
It assumes that technology will get to the point where it becomes trivial to explore space. That's not an easy assumption to grant.
Trivial and possible are the same. If colonization is possible it will eventually be mastered and then you get geometric growth. Covid has given us all a quick refresher course on the nature of geometric growth.
On the timescale of the universe, geometric growth in colonization is functionally hyper rapid.
So either colonization (and thus, geometric growth) is impossible or there's nobody else out there.
On the timescale of the universe, geometric growth in colonization is functionally hyper rapid.
So either colonization (and thus, geometric growth) is impossible or there's nobody else out there.
Well, if establishing colonies and moving from one star system to the next is possible, but takes a few hundred million years each time, then it wouldn't be surprising in any way if even the 'first' civilization to arise has barely colonized a handful of star systems. Perhaps in a few hundred billion years the universe will indeed be full of colonies.
Sure, but my question was just if the article was making the same basic argument as the given hypothesis (along with the same basic assumptions).
What makes you think we'll be among the first?
No human being has ever set foot on a different planet.
No man-made object has ever reached a star system outside our own, let alone landed on a planet.
We may still be quite a long way away from accomplishing those things at scale.
No human being has ever set foot on a different planet.
No man-made object has ever reached a star system outside our own, let alone landed on a planet.
We may still be quite a long way away from accomplishing those things at scale.
I'm just asking if the article is making a similar argument to the stated hypothesis.
This may be beside the point, but I don't think there is any alien lift out there. I am going to make up number N_life_universes which is the number of universes needed in order for an instance of intelligent life to evolve. Many people seem to assume this much less than one. I think it is much greater than one, meaning most universes you look at will be void of life. I'd also say there are many universes, one source being the rules of quantum mechanics. But I know some people believe in wave function collapse, and if that is true I am not sure what this means for having mulitple universe.
There is a related number, N_hamlet_universes. This is the number of universes needed in order for monkeys to type Hamlet typing as fast as they can, covering every planet spaced 6' feet apart (in case one gets sick). I think this is also a very large number but I haven't figured it out.
There is a related number, N_hamlet_universes. This is the number of universes needed in order for monkeys to type Hamlet typing as fast as they can, covering every planet spaced 6' feet apart (in case one gets sick). I think this is also a very large number but I haven't figured it out.
For a long time I've figured that the Fermi Paradox is not much of a paradox.
Open question: Are we likely to be able to detect the difference between large asteroids and a small swarm of habitable environments in a star.
And the second question is: Is it worth spreading out? An advanced civilisation can defend against or modify (maybe even focus) the damage that an imploding star would cause, and an Issac Arthur video on the topic of the amount of space we have in the solar system states clearly that there's more than enough for trillions of trillions of trillions of habitats. There doesn't seem to be much reason for spreading a civilization out whatsoever, given the cost of communication and the assumed cultural and informational lag between the settlement and the source civilization.
As the article states, it's unlikely to be the case that we see an expansive civilization until it's too late.
Open question: Are we likely to be able to detect the difference between large asteroids and a small swarm of habitable environments in a star.
And the second question is: Is it worth spreading out? An advanced civilisation can defend against or modify (maybe even focus) the damage that an imploding star would cause, and an Issac Arthur video on the topic of the amount of space we have in the solar system states clearly that there's more than enough for trillions of trillions of trillions of habitats. There doesn't seem to be much reason for spreading a civilization out whatsoever, given the cost of communication and the assumed cultural and informational lag between the settlement and the source civilization.
As the article states, it's unlikely to be the case that we see an expansive civilization until it's too late.
I have not studied this area of science but the article reminded me of the fictional book series The Three-Body Problem by Liu Cixin.
I really don't want to say anything to spoil the books but the series were so good that I devoured them last summer which is something I rarely do.
I really don't want to say anything to spoil the books but the series were so good that I devoured them last summer which is something I rarely do.
We're just not looking hard enough. It might seem like a ton of money has been poured into it, but space tech and especially monitoring is so underfunded that it could be a joke when compared against anything else.
Any entertainment industry on its own has more funding than all of humanity's space pursuits. All the apps for tracking time, buying clothes, losing weight, scanning tomatoes, properly wiping, get more money than the ESA or JAXA (at least NASA has more than them heh).
All the speculation and dreams of a great space colonization are nice, but the reality is that most of us will die on this planet, and likely take half of the life on it with us.
Any entertainment industry on its own has more funding than all of humanity's space pursuits. All the apps for tracking time, buying clothes, losing weight, scanning tomatoes, properly wiping, get more money than the ESA or JAXA (at least NASA has more than them heh).
All the speculation and dreams of a great space colonization are nice, but the reality is that most of us will die on this planet, and likely take half of the life on it with us.
If you look carefully enough at the implications of our situation through the lens of the 2nd law of thermodynamics,
the Fermi paradox becomes a lot less paradoxical.
Edit: Apparently I have violated a HN taboo in even daring to mention that bothersome little law.
Understanding it and its implications and applying that knowledge to our IRL situation on earth might not be fun, however IS fundamental to making plausible statements about perpetual motion and related topics (i.e."von Neumann probes.")
Now, all that remains to be seen, this being HN, is: Will this comment be downvoted into oblivion without a good reason being proferred?
Edit: Apparently I have violated a HN taboo in even daring to mention that bothersome little law.
Understanding it and its implications and applying that knowledge to our IRL situation on earth might not be fun, however IS fundamental to making plausible statements about perpetual motion and related topics (i.e."von Neumann probes.")
Now, all that remains to be seen, this being HN, is: Will this comment be downvoted into oblivion without a good reason being proferred?
This is the most profound thing I've read in a while.
I'm tempted to whip up some numerical simulation to verify this to some extent...
> But here’s the interesting part: conditioned on all the steps having succeeded, we should find ourselves near the end of the useful lifetime of our planet’s star—simply because the more time is available on a given planet, the better the odds there. I.e., look around the universe and you should find that, on most of the planets where evolution achieves all the steps, it nearly runs out the planet’s clock in doing so.
I'm tempted to whip up some numerical simulation to verify this to some extent...
> But here’s the interesting part: conditioned on all the steps having succeeded, we should find ourselves near the end of the useful lifetime of our planet’s star—simply because the more time is available on a given planet, the better the odds there. I.e., look around the universe and you should find that, on most of the planets where evolution achieves all the steps, it nearly runs out the planet’s clock in doing so.
Want something scary?
The form of reasoning used is exactly that in the https://en.wikipedia.org/wiki/Doomsday_argument for why humanity is unlikely to last long, and almost certainly will not have a future where we break out and colonize most of the Solar System, let alone the stars.
If you believe both arguments, the most likely outcome is that there is one more step on the way to Robin's argument. And that outcome is the replacement of biological intelligence with mechanical. So indeed we give rise to an expanding wave of technological civilization. But we are close to peak population and will not ourselves see that civilization.
All hail our future AI overlords!
The form of reasoning used is exactly that in the https://en.wikipedia.org/wiki/Doomsday_argument for why humanity is unlikely to last long, and almost certainly will not have a future where we break out and colonize most of the Solar System, let alone the stars.
If you believe both arguments, the most likely outcome is that there is one more step on the way to Robin's argument. And that outcome is the replacement of biological intelligence with mechanical. So indeed we give rise to an expanding wave of technological civilization. But we are close to peak population and will not ourselves see that civilization.
All hail our future AI overlords!
We are within 500 years of creating millions of self sustaining spaceports. We are within 500 years of AGI. We are within 500 years of understanding quantum gravity. We are within 500 years of fixing biological death. The order we get to those and how they interplay and change the world matters so much that it doesn't make sense to worry about making predictions.
What about the climate crisis halting or slowing down technological progress?
Or if we find out that micro-plastics lead to a marked increased cancer rates, resulting in a precipitous drop in life expectancy.
Climate change is not a big deal as far as innovation goes. It is an enormous disaster for specific cities though
Only if civilization doesn't collapse.
But there is no reason to expect better. All the signs point there. Global climate disruption destroys crop yields in tropical and subtropical places, creating political instability. Millions, then tens of millions, then hundreds of millions of refugees flee toward higher latitudes. Nations at higher latitudes elect fascists to repel them. Fascists start wars, wars disrupt trade, economies collapse, nuclear weapons come out, boom.
It would be nice to be wrong about this, but it has all started already. 70M+ Americans voted for fascism. Brazil, Turkey, Hungary, India already got it. Germany, France, UK, Italy, Poland threaten. China and Russia fell long ago. Europe is already strained by the first few million refugees.
Sending the hundreds of millions of refugees on to Siberia and the Yukon could work, in principle.
But there is no reason to expect better. All the signs point there. Global climate disruption destroys crop yields in tropical and subtropical places, creating political instability. Millions, then tens of millions, then hundreds of millions of refugees flee toward higher latitudes. Nations at higher latitudes elect fascists to repel them. Fascists start wars, wars disrupt trade, economies collapse, nuclear weapons come out, boom.
It would be nice to be wrong about this, but it has all started already. 70M+ Americans voted for fascism. Brazil, Turkey, Hungary, India already got it. Germany, France, UK, Italy, Poland threaten. China and Russia fell long ago. Europe is already strained by the first few million refugees.
Sending the hundreds of millions of refugees on to Siberia and the Yukon could work, in principle.
Climate instability would unlock changes that one by one cause feedback to worsen the situation irreversibly. The bio diversity that prevents this, we actively destroy every day.
You state these "we are within 500 years" as if they are certainties, but you show no evidence for any of them. The only one I consider even slightly likely is that we are within 500 years of understanding quantum gravity.
So... evidence? Argument? Just stating baldly that it is so is not very convincing.
So... evidence? Argument? Just stating baldly that it is so is not very convincing.
I don't have a time machine so no I cannot prove what will happen in the future
Fine, but you seem to regard it as at least virtually certain. Why? What is your basis for regarding it as close to certain?
I've seen that article. I don't find it convincing. Start with the first screen: An exponential graph does not have a sharp kink in it just past where you are standing. The rest of the point is valid (an exponential curve means that the next bit if change will be faster than the last bit, and that's going to surprise you), but he's cheating to try to make the point more visually dramatic. It implies "there's a discontinuous first derivative coming almost immediately", when the argument is actually that there's a continuous increase in the first derivative.
So, yeah. Overselling the case, in the graph on the first screen.
So, yeah. Overselling the case, in the graph on the first screen.
It's just as likely that we all greet our future AI grandchildren, as they will be us.
Robin has done a fair bit of numerical simulation on his OP posts here https://www.overcomingbias.com/2020/12/how-far-aggressive-al..., and says he'll have an actual paper out in a week or two.
Thanks! Looking forward to reading the paper..!
We don't see aliens because we are looking wrong. They're probably right there.
http://cantrip.org/slow.pdf
http://cantrip.org/slow.pdf
So in any case I agree that the world needs to get its shit together and thinking about it: I have never really considered to give it a serious try to make/contribute to make that happen.
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Analogy: humans have existed for something around 100,000 years, yet we only explored the whole ball - and began affecting it to the point of worrying about making it uninhabitable - within the last few decades. Any lesser culture/species was dominated or destroyed before they could come to grips with the expansion ... not so much because of malice as of “bug vs windshield”.
This sounds like the grey goo problem [1], but at a cosmic scale and at (near) the speed of light.
Like other commenters have already said, this scenario assumes that near-speed-of-light travel is workable.
[1] https://en.wikipedia.org/wiki/Gray_goo
Like other commenters have already said, this scenario assumes that near-speed-of-light travel is workable.
[1] https://en.wikipedia.org/wiki/Gray_goo
Paper: https://arxiv.org/abs/2102.01522
HN discussion of paper: https://news.ycombinator.com/item?id=26045731
HN discussion of paper: https://news.ycombinator.com/item?id=26045731
The anthropic principle is about sampling. It is often counterintuitive to think about sampling problems.
Even if we do have young competitors like us, they could still be millions of years older than us- the blink of an eye, in a planet’s lifecycle.
But millions of years of technological evolution is plenty enough time to make grey goo (or light speed spheres of incoming civilization).
But millions of years of technological evolution is plenty enough time to make grey goo (or light speed spheres of incoming civilization).
Even in case #3, is it reasonable to assume that the aliens would expand in 3 dimensions at the speed of light?
And even of that is true, wouldn't we expect a significant fuzziness in that on the order of a millenium, where we see signs but haven't yet been engulfed?
And even of that is true, wouldn't we expect a significant fuzziness in that on the order of a millenium, where we see signs but haven't yet been engulfed?
But if a civilization has been expanding for a billon years the chances very small that you are this millennium of fuzziness.
Hubrisimus. Our current models are the most useful ones we have that fit our limited dataset, but we have no clue whether we understand the first thing about space and time and the age and size of the universe(s).
What's the alternative to working with the data you have?
There is no alternative to working with the data you have.
But believing that you understand the data when there are huge glaring inconsistencies in your understanding is... naive.
But believing that you understand the data when there are huge glaring inconsistencies in your understanding is... naive.
I guess I missed the part where this was proclaimed to be Truth.
Yeah fair enough I didn't specifically
point out the part I was responding to.
He says there are only 3 possibilities (the part starting with "then either" 1) ... 2) ... 3) ...
But that's ignoring category 4) which is that we have only a fraction of a fraction of a grain of sand of the real data, and might be a bit early to come to conclusions.
So not to say #3 is wrong, and heck it could very well be more probably than #1 and #2, but I wouldn't bet the house yet when we don't know if there's an option #4 - #4,000,000,000,000 because we haven't even observed data from dimensions we don't even know exist it.
He says there are only 3 possibilities (the part starting with "then either" 1) ... 2) ... 3) ...
But that's ignoring category 4) which is that we have only a fraction of a fraction of a grain of sand of the real data, and might be a bit early to come to conclusions.
So not to say #3 is wrong, and heck it could very well be more probably than #1 and #2, but I wouldn't bet the house yet when we don't know if there's an option #4 - #4,000,000,000,000 because we haven't even observed data from dimensions we don't even know exist it.
Archive link: https://archive.vn/ki5Vm
Robin Hanson is a pretty decent high-concept sf author, and his occasional forays into cosmic horror can be enjoyable too. It's just too bad he ended up in the wrong line of work.
What's this about? Fnords? After two paragraphs, i'm not sure if this is a real article or a diary entry
Wa there really such a big need for such a clickbait title?
Now I'll never know what it was about. Probably someone selling stuff.
Now I'll never know what it was about. Probably someone selling stuff.
You could either read the article or read the comments. (If there aren't any, wait until there are and read them.) But don't do what you did, which is make a completely uninformative comment. Even if the article is clickbait, and therefore a waste of everybody's time, don't add to the waste with comments like yours.
On the contrary. If the author sees my comment and the clickbait was unintentional, then they'll be informed of its grave consequences.
On the other hand, if it was intentional, then they'll be informed that it doesn't necessarily work as expected.
You might as well claim that "The Social Dilemma" was spam because you can just choose not to open facebook today.
On the other hand, if it was intentional, then they'll be informed that it doesn't necessarily work as expected.
You might as well claim that "The Social Dilemma" was spam because you can just choose not to open facebook today.
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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).