This is a list of claims and assumptions made in the FHI paper, Eternity in Six Hours. It is not exhaustive. I collected this list as part of my attempt to answer the questions:

Since my interest is writing this is on the feasibility of intergalactic colonization, I've neglected claims in the paper about the Fermi paradox.

Abstract

The Fermi paradox is the discrepancy between the strong likelihood of alien intelligent life emerging (under a wide variety of assumptions), and the absence of any visible evidence for such emergence. 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. This results in a considerable sharpening of the Fermi paradox. [emphasis added]

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 grenade blast (800kJ).
  • Redundancy required for a probe to make it to a galaxy is given by R = exp(dAρ ) where is d is distance to be travelled (in comoving coordinate), A is cross-section of the probe, and ρ is the density of dangerous particles.
    • Dangerous particle size given as a function of speed of the probe by equation in the paper.
    • From slower probes (80%c and 50%c) redundancy required is low, two probes are enough to ensure one survives.
    • If you have a 500T replicator, you have more cross-section but also better ability to shield.
    • Density of matter in space is much higher in interstellar space compared to intergalactic space. Might not be possible to launch universe-colonization directly from our sun.
  • Dyson spheres are very doable. Assumed to have 1/3 efficiencies over sun's output (3.8x10^26)
    • We could disassemble Mercury and turn it into a Dyson sphere.
  • Launch systems could achieve energy efficiency of 50%.
  • Apart from risks of collision, getting to the further galaxies is as easy as getting to the closest, the only difference is a longer wait between the acceleration and deceleration phases.
  • Travelling at 50c% there are 116 million galaxies reachable; at 80% there are 762 million galaxies reachable; at 99%c, you get 4.13 billion galaxies.
    • For reference, there are 100 to 400 billion stars in the Milky Way, and from a quick check it might be reasonable to assume 100 billion is the average galaxy.
      • The ability to colonize the universe as opposed to just the Milky Way is the difference between ~10^8 stars and ~10^16 or ~10^17 starts. A factor of 100 million.
  • On a cosmic scale, the cost, time and energy needed to commence a colonization of the entire reachable universe are entirely trivial for an advanced human-like civilization.
  • Energy costs could be cut by a factor of hundred or thousand by aiming for clusters or superclusters [of galaxies] and spreading out from there.

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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.

I would disagree on large colonization fleets being ruinously expensive, the best case scenario for large colonization fleets is if we have direct mass to energy conversion, launching say 2 probes from each star system that you spread from. Each probe would use half the mass energy of the star. Converting a quater of its mass to energy to get ~0.5c

You can colonize the universe even if you insist on never going to a new star system without bringing a star with you. (Some optimistic but not clearly false assumptions)

Accelerating a probe to ~0.5c isn't the biggest issue. It will be much harder to deaccelerate once you are at the target location.

I think the expansion of the universe means you don't have to deaccelerate.

For some destinations, but not for most of them (I'm pretty sure). At least Eternity in Six Hours spends a great detail of time discussing deceleration.

If you wouldn't fly faster then a given universe moves, I don't see how you can ever catch up to the universe.

It's a question of acceleration, not just speed.

The main assumption, imho, is that if we input very large capabilities, we will get very large achievements. That is, if we we assume possibility of superintelligence and nanotech, we will get a possibility of quick intergalactic travel.

However, arguing from infinities could be problematic, as it could produce contradicting results, as is well known in the philosophy: https://en.wikipedia.org/wiki/Omnipotence_paradox