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Answer by Ruby140

I wrote a post which targets the "and how do we know that?" part of this question.

Full post here which has elaboration and examples for each of the types. Headings for the argument/evidence types:

1. Our understanding of the laws of physics says it should be possible. (Argument from Physics/Basic Science)
2. Nature has done this, so reasonably we as intelligent beings in nature should eventually be able to too. (Argument from Nature)
3. We have a proof of concept. (Argument from POC)
4. We've done it already. (Argument from Accomplishment)

The post also includes a couple of paragraphs on where these arguments fall short and how they're stronger in the case of long-term space colonization.


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Arguments for the value of the long-term future tend to make the assumption that we will colonize space. What can we definitely accomplish in terms of space colonization? Why think that we can definitely do those things?

The FHI paper, Eternity in Six Hours, is very optimistic about what can be done:

In this paper, we extend the Fermi paradox to not only life in this galaxy, but to other galaxies as well. We do this by demonstrating that traveling between galaxies – indeed even launching a colonisation project for the entire reachable universe – is a relatively simple task for a star-spanning civilization, requiring modest amounts of energy and resources. We start by demonstrating that humanity itself could likely accomplish such a colonisation project in the foreseeable future, should we want to, and then demonstrate that there are millions of galaxies that could have reached us by now, using similar methods.

Is this paper reasonable? Which parts of its assertions are most likely to be mistaken?

This question was inspired by a conversation with Nick Beckstead.

Space Exploration & Colonization
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Space colonization: what can we definitely do and how do we know that?
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Arenamontanus

190

Looking back at our paper, I think the weakest points are (1) we handwave the accelerator a bit too much (I now think laser launching is the way to go), and (2) we also handwave the retro-rockets (it is hard to scale down nuclear rockets; I think a detachable laser retro-rocket is better now). I am less concerned about planetary disassembly and building destination infrastructure: this is standard extrapolation of automation, robotics and APM.

However, our paper mostly deals with sending a civilization's seeds everywhere, it does not deal with near term space settlement. That requires a slightly different intellectual approach.

What I am doing in my book is trying to look at a "minimum viable product" - not a nice project worth doing (a la O'Neill/Bezos) but the crudest approach that can show a lower bound. Basically, we know humans can survive for years on something like the ISS. If we can show that an ISS-like system can (1) produce food and other necessities for life, (2) allow crew to mine space resources, (3) turn them into more habitat and life support material, (4) crew can thrive well enough to reproduce, and (5) this system can build more copies of itself with new crew at a faster rate than the system fails - then we have a pretty firm proof of space settlement feasibility. I suspect (1) is close to demonstration, (2) and (3) needs more work, (4) is likely a long term question that must be tested empirically, and (5) will be hard to strictly prove at present but can be made fairly plausible.

If the above minimal system is doable (and I am pretty firmly convinced it is - the hairy engineering problems are just messy engineering, rather than pushing against any limits of physics) then we can settle the solar system. Interstellar settlement requires either self-sufficient habitats that can last very long (and perhaps spread by hopping from Oort-object to Oort-object), AI-run mini-probes as in our paper, or extremely large amounts of energy for fast transport (I suspect having a Dyson sphere is a good start).

Ruby

140

I wrote a post which targets the "and how do we know that?" part of this question.

Full post here which has elaboration and examples for each of the types. Headings for the argument/evidence types:

1. Our understanding of the laws of physics says it should be possible. (Argument from Physics/Basic Science)
2. Nature has done this, so reasonably we as intelligent beings in nature should eventually be able to too. (Argument from Nature)
3. We have a proof of concept. (Argument from POC)
4. We've done it already. (Argument from Accomplishment)

The post also includes a couple of paragraphs on where these arguments fall short and how they're stronger in the case of long-term space colonization.


Kevin Kohler

130

Hi Ruby, since I've actually given this topic some thought I'm gonna delurk for once. The issue I have with FHI's space papers is that they basically pay no attention to questions of governance. Being able to reach things and being able to coordinate/control things are two very different things. Space is way too big for central control. Even in the Milky Way you'd probably need something in the ballpark of 100'000c for a central government to make sense. Consequently, we probably overestimate the degree to which space settlement is controllable and the constant usage of the term "colonization" is confusing, if not plainly wrong. I wrote a more comprehensive version (ca 45 minutes reading time) of this argument here: https://medium.com/@KevinKohlerFM/cosmic-anarchy-and-its-consequences-b1a557b1a2e3

[-]Ruby50

That's interesting. I agree that given that consideration the term "colonization" is possibly misleading. I have been using it more in the sense of "you have human civilization over there" rather than "the colonies of the kingdom of Britain." I think I don't mind if the different "colonies" are autonomous.

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Ruby

70

The answer to this question likely depends heavily on what we consider to be adequate colonization:

1) Running computation in others star systems, i.e. Running digital minds on computers or other computational processes. (this is what Eternity in Six Hours assumes)

or

2) Having actual, ordinary biological humans colonize the stars.

There are challenges common and separate to each.

Lasting the Journey

In either case, you must be able to create a probe (to use the language of Eternity in Six Hours) which can last the duration for a trip which lasts thousands to millions of years. Is it at all feasible to have humans last long in some form? (Perhaps only as embryos which can be "grown" upon arrival, but even then, can we safely preserve biological material for millenia?) Could cryonics somehow be a solution? Even if you were only sending computers/robots, can we build electrical and mechanical devices which won't break down after such extremely long time periods?

Challenges for Humans

Nick Beckstead's prelimenary notes mention microgravity, cosmic radiation, health and reproduction in space, and genetic diversity as considerations which come into play when sending live humans through space.

Challenges for Computers

Can we build machines (assume non-AGI) we can solve all the problems they will encounter in different systems?

Ruby

70

An attempt from Nick Beckstead on almost this question:

Will we eventually be able to colonize other stars? Notes from a preliminary review (June 2014)

Summary
I investigated this question because of its potential relevance to existential risk and the long-term future more generally. There are a limited number of books and scientific papers on the topic and the core questions are generally not regarded as resolved, but the people who seem most informed about the issue generally believe that space colonization will eventually be possible. I found no books or scientific papers arguing for in-principle infeasibility, and believe I would have found important ones if they existed. The blog posts and journalistic pieces arguing for the infeasibility of space colonization are largely unconvincing due to lack of depth and failure to engage with relevant counterarguments.
The potential obstacles to space colonization include: very large energy requirements, health and reproductive challenges from microgravity and cosmic radiation, short human lifespans in comparison with great distances for interstellar travel, maintaining a minimal level of genetic diversity, finding a hospitable target, substantial scale requirements for building another civilization, economic challenges due to large costs and delayed returns, and potential political resistance. Each of these obstacles has various proposed solutions and/or arguments that the problem is not insurmountable. Many of these obstacles would be easier to overcome given potential advances in AI, robotics, manufacturing, and propulsion technology.
Deeper investigation of this topic could address the feasibility of the relevant advances in AI, robotics, manufacturing, and propulsion technology. My intuition is that such investigation would lend further support to the conclusion that interstellar colonization will eventually be possible.
Note: This investigation relied significantly on interviews and Wikipedia articles because I’m unfamiliar with the area, there are not many very authoritative sources, and I was trying to review this question quickly.

8 Related Questions

Answer by Ruby130

One of the things the paper does is make the overall determination that power available >> power needed. So as part of assessing that, I identified all the components which contributed to each and examined how realistic/sensitive to assumptions they are.

To begin with, here are the factors from the paper which impact the energy required for colonizing the galaxy.

  • The mass of each payload / self-replicating probe to be sent.
    • Mass of the payload is a linear factor in the energy required.
  • The travel speed.
... (read more)
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4Ruby
Launch System Feasibility Efficiency To save on the mass which has to be launched (which requires squaring the rocket equation), the authors of Eternity in Six Hours favor an external fixed launch system which accelerates the replicating probe including both probe and fuel for its later deceleration. In the paper, the authors briefly list coilguns, quenchguns, laser propulsion, and particle beam propulsion as potential means of accelerating the probes. They state even though the theoretical energy efficiency of these systems could approach 100%, since one never obtains the theoretical efficiency, they assume 50% efficiency. The question here: 1. Is it possible to build a launch system which launches (possibly quite large) probes to significant fractions of the speed of light? 2. What efficiency is realistically achievable? When contacted for comment, one of the authors, Anders Sandberg, stated: Looking back at our paper, I think the weakest points are (1) we handwave the accelerator a bit too much (I now think laser launching is the way to go) . . . My shallow impression is that the proposed launch systems might only represent large engineering challenges more than difficult physics/designs breakthroughs. Coilguns have been constructed and laser propulsion has been demonstrated in the lab. What remains is a question of scale and efficiency. However, even a difference between 5% efficiency and 50% efficiency is only a single order of magnitude. Not a large difference in the overall picture here.
4Ruby
Number of Probes to be Sent The mass of replicator, specific impulse of the fuel, and travel speed determine the energy required to launch a single probe. The number of probes to be sent is determined by the number of destinations and the redundancy factor in number of probes sent to ensure that one probe arrives at each destination. Due to collisions with interstellar dust or other failures, we can imagine that not every self-replicating probe will arrive at its destinations. The number of destinations is limited by one’s travel speed since increasing large regions of space are moving beyond our reach due to expansion of the universe. The faster one travels, the more distant stars one is able to reach before they get too far away. The authors of Eternity in Six Hours (pg. 21) ``calculated that: * Travelling at 50% c, one could reach 1.16 x 10^8 galaxies * Travelling at 80%c, one could reach 7.62 x 10^8 galaxies * Travelling at 90%c, one could reach 4.13 x 10^9 galaxies For reference, an average galaxy might have 10^8 stars. The authors calculated travelling at 99%c, a redundancy factor of 40 is required, i.e. 40 probes for every destination. For 80%c and 50%x, the redundancy is less than 2. I did not prioritize looking into this calculation and am trusting the result that at lower speeds, the redundancy factor required is not very high.
4Ruby
Specific Impulse and the Fuel Component of the Probe’s Mass Importantly, part of what we need accelerate is the fuel required to decelerate the probe once it arrives at its destination. The greater the fuel we send along with a probe, the greater the initial launch energy required. We want fuel which is highly efficient by its mass, i.e, it has high specific impulse. The amount of fuel mass required to decelerate a probe is extremely sensitive to the specific impulse (Isp) of the fuel used. Oliver Habryka noticed that Eternity in Six Hours may be making unreasonable assumptions about achievable specific impulses attainable. Further investigation revealed that which specific impulses are attainable may determine whether or not space colonization is affordable at all. To me, it is a major sensitivity in the paper. Transformed to isolate initial mass, the relativistic rocket equation gives: m0: initial mass m1: final mass c: speed of light Isp: specific impulse Δv: change in velocity This formula for the initial mass is linear in final mass and exponential in specific impulse. For a fixed mass of 1kg, the initial fuel mass required to accelerate to different fractions of the speed of light are shown by: Isp can be measured in m/s; the x-axis gives Isp as a fraction of the speed of light. The dotted lines correspond to 4% of c (the Isp for fission given by the paper) and half that value, 2% of c. On page 11, Armstrong and Sandberg provide the following values of specific impulse (measured as a fraction of c). Of these, we have only actually attained nuclear fission, however not in efficient rocket form. As Habryka pointed out, the paper makes the assumption that almost all of the energy released by nuclear fission is converted into kinetic energy, however that this may be unrealistic. Habryka identified a nuclear rocket concept which may be capable of achieving an Isp of 3%-5% the speed of light. Fission fragment rockets, while not yet built, us
4Ruby
Energy Required to Accelerate Mass To travel between the stars it is necessary to be able to accelerate an interstellar probe to some very fast speed, let it travel through space (without friction, it will keep going due to energy), and then decelerate it once it arrives at the target destination. Space is insanely large and to get most places you really want to be traveling at a significant fraction of the speed of light; however, this requires enormous amounts of energy since by special relativity, the faster you are travelling, the more energy is required to accelerate further. This enforces the limit that you cannot go faster than the speed of light since this would require infinite energy.) The relativistic kinetic energy of a rigid body is given by: This formula is linear in mass (m) and considerably superlinear in velocity (v) approaching infinity as velocity approaches the speed of light (c). The y-axis is incremented in units of 100 million gigajoules (=10^17 joules). Even accelerating a 1kg mass to 10% the speed of light requires 4.5*10^14 joules. 50% of c requires 1.4*10^16 joules. For comparison, world energy consumption in 2013 was estimated by the International Energy Association to be 5.67x10^20 joules. In other words, accelerating a single 5 tonne probe to 10% c would require ~1% of Earth’s entire energy consumption. Accelerating to 80% could required 100% of 2013’s energy consumption. Now consider that to colonize the universe, we need to send upwards of 100 million (10^8) probes. Since we need to both accelerate and decelerate a probe, this energy is required twice over. Doubling the mass of the probe doubles the energy required, but doubling the target speed multiplies the energy required many times over even at very small fractions of the speed of light.
Answer by Richard Korzekwa 200

In the order that they appear in the paper, these are a few of the parts that seemed iffy to me. Some of them may be easily shown to be either definitely iffy, or definitely not-so-iffy, with a little more research:

As for nuclear fusion, the standard fusion reaction is 3H +2H→4He +n+ 17.59 MeV. In MeV, the masses of deuterium and tritium are 1876 and 2809, giving an η of 17.59/(1876 + 2809) = 0.00375. We will take this η to be the correct value,because though no fusion reactor is likely to be perfectly efficient, there is also the possibility of getti

... (read more)
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5habryka
Further comments on this rocket design: https://forum.nasaspaceflight.com/index.php?PHPSESSID=3omp04d25qbe0qj5l5n1qfl0hp&topic=47693.20
4Answer by Ruby
Extracting my response from this post. Claims and Assumptions (not exhaustive) * Self-replicating probes for colonizations could be launched to a fraction of lightspeed using fixed launch systems such as coilguns or quenchguns as (opposed to rockets). * Only six hours of the sun's energy (3.8x10^26W) are required to commence the colonization of the entire universe. * A future human civilization could easily aspire to this amount of energy. * Since the procedure is conjunction of designs and yet each of the requirements have multiple pathways to implementation, the whole construction is robust. * Humans have generally been quite successful at copying or co-oping nature. We can assume that anything done in the natural world can be done under human control, e.g. self-replicators and AI. * Any task which can be performed can be automated. * It would be ruinously costly to send over a large colonization fleet, and is much more efficient to send over a small payload which builds what is required in situ, i.e. von Neumann probes. * Data storage will not be much an issue. * Example: can fit all the world's data and upload of everyone in Britain in gram of crystal. * 500 tons is a reasonable upper bound for the size of a self-replicating probe. * A replicator with mass of 30 grams would not be unreasonable. * Antimatter annihilation, nuclear fusion, and nuclear fission are all possible rocket types to be used for deceleration. * Processes like magnetic sail, gravitational assist, and "Bussard ramjet" are conceivable and possible, but to be conservative are not relied on. * Nuclear fission reactors could be made 90% efficient. Current reactor designs could reach efficiencies of over 50% of the theoretical maximum. * Any fall-off in fission efficiency results in a dramatic decrease in deceleration potential. * They ignore deceleration caused by the expansion of the universe. * Assume probe is of sturdy enough construction to survive a gre
6Answer by Ruby
An argument sometimes given for colonizing space is as a measure against existential risk. Human settlements beyond Earth might offer some measure of redundancy and backup in the event of catastrophe on Earth. Whether one thinks this is a argument for space colonization will depend on what one thinks the likely catastrophes on Earth might be and how well space colonization overcomes them. Notably, many would consider space colonization (certainly "nearby" settlement) to offer at best limited protection from unsafe AI. I do expect the nature of this argument to shift depending on the timescale. On very short timescales in which humanity is at most aspiring to colonize Mars, then constructing refuges on Earth might be a better investment. On longer timescales (the timescales over which we might aspire to colonize interstellar and intergalactic colonization), we might imagine human civilization has matured past any significant existential risk. If not, there could certainly be "safety" in sending some of our civilization out at speeds which cause those refuges to be safely out of reach due to expansion of the universe.
4Answer by Ruby
The kinds of numbers thrown around in the astronomical waste argument are sometimes accused of being a Pascal's Mugging. Even if one has doubts about whether to work on existential risk reduction, it could be argued that because the Far Future has such overwhelming and immense value that the expected value of working on existential risk outweighs all other opportunities, e.g. near-term altruistic projects like global poverty, global health, and animal welfare. Having sharper estimates of the potential of the Far Future, bounded by how much of the universe we can actually reach, could help us relate to astronomical waste arguments with far more principle than "aahhh, these are such big numbers!!" They're big numbers, but not all numbers are equally big.
5Answer by Ruby
The assumption that we can colonize the stars is core to the Astronomical Waste Argument made in favor of working on existential risk reduction. If this assumption is weakened, so is the case for prioritizing work existential risk reduction. Most things are impossible. Perhaps our belief that we could possible colonize the stars is based only our ignorance. If we actually tried to colonize the stars (or simply tried to actually look into the possibility), we would find that we shouldn't take it for granted at all that space colonization is a realistic possibility. Summary of the Astronomical Waste Argument Nick Bockstrom's 2003 paper, Astronomical Waste: The Opportunity Cost of Delayed Technological Development: Bostrom arrives at different estimates of the potential number of human minds depending on whether we are satisfied with running "human" minds on computers or wish to stick with biological instantiation. Using digital instantiation: Using biological instantiation: Bostrom clarifies that not only utilitarians should care about this immense potential value which might be reached: Clearly, the extent to which we can actually colonize star systems beyond our own affects how strong an argument there is from astronomical waste (or as I would rather call it, our astronomical potential). If we can in fact be confident that we can colonize the entire reachable universe, that might be 10^17 stars instead of the 10^13 in just the Virgo Supercluster. An even stronger argument than Bostrom states. On the other hand, if we can't even colonize beyond our star system, we're just at 10^0 stars. Then there'd be no astronomical argument at all.
4Ruby
Addendum: In an Open Philanthropy Project blog post, The Moral Value of the Far Future, Holden Karnofsky mentions Nick Bostrom's Astronomical Waste argument to say that he does not consider it robust enough to play an overwhelming role in his belief systems and actions. Admittedly, Karnofsky proceeds to say that even if he fully accepted the reasoning, he isn't sure what implications it would have. Nonetheless, I suspect that were we to have a non-speculative, robust case about what is possible that this well might push our behavior in particular directions. For instance, perhaps we find that Bostrom's 10^38 humans lost per century of delay is extremely speculative, yet 10^20 is eminently attainable. I believe that if did have a robust case for the latter, this would shift the prioritization of some and likely bolster the altruistic motivation of those who right now are primarily sustained by the speculative plausibility of Bostrom's extreme case. Perhaps more importantly, if we are unable to even establish a firm lower bound much above what the Earth alone could sustain long-term, then those who have made Astronomical Waste arguments part of their belief systems and actions have reason to pause and reconsider how they should update given that the potential of space colonization might be much weaker than previously hoped.
4Answer by Ruby
How fast you need to go unsurprisingly depends on quickly you need to get there. I've estimated that 100kly is larger than the distance to most places within the Milky Way. * Travelling at 99%c, you can cover that in ~100,000 years. * Travelling at 50%c, you can cover that in 200,000 years. * Travelling at 10%c, you can cover that distance in 1,000,000 years. * Travelling at 1%c, you can cover that distance in 10,000,000 years. Recall that there are at least tens of millions of stars in the Milky Way. There are probably many stars within 50kly or even 25kly of Earth. Nonetheless, these distances mean that even at extremely fast speeds it would still take tens of thousands of years to millions of years. This may or may not be a problem. The universe will probably last for at least another few billion years, compared to which a million years is not much at all. The question is whether your expedition can survive that long between stars. (It might make a big difference whether you are sending only digital machines or humans too.) What are the distances? Taking stats from its Wikipedia entry, the Milky Way has a diameter of 150-200 kly (kilolightyears), however: However, I will assume that the upper bound given of 200kly captures most of the 100-400 billion stars. Our sun is 26.4 ± 1.0 kly from the Galactic Center (see image). It might be difficult to travel through the center of the galaxy, but let's assume that the distance you travel to get anywhere in Milk Way from our sun is no more than traveling to the Galactic Center (~25ky) plus the upper bound of the radius (~100kly), so approximately 125kly. That's the distance to the outer edge so actually the vast majority of destinations should be less than that. One could do some fancier trigonometry to get exact numbers and nice averages, but this gives us the order of magnitude: ~100kly to travel almost anywhere in the Milky Way. That is probably still well above average since the density of stars is much hi
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