Claims & Assumptions made in Eternity in Six Hours

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.


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 13 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.