I'm writing a book about a civilization started by humans who sent a laser-powered sail ship with AIs and frozen embryos to the edge of the cosmological event horizon (the furthest we can travel before expansion of the universe disallows us to travel further). I want to be able to find possible real-life objects at a certain distance from earth that my ship would be able to travel to.

Things I'm assuming for the sake of my own sanity:

  1. There is a stable infinitely-working source of power driving the laser.
  2. This ship will be able to accelerate at a maximum of 100 Gs (~981 m/s2)
  3. This ship will be able to travel up to 30% of c, or the speed of light, (89937737.4 m/s)
  4. The ship leaves Earth in the "near future" from 2100-2200.

My questions:

  1. Would embryos be able to survive 100Gs for so long? If not, what is a reasonable acceleration?
  2. Using the acceleration answered above, if the ship constantly accelerates up to 30% of c, coasts for an x amount of time, and then constantly decelerates until they stop at their final destination, what is the furthest they would be able to travel in lightyears from earth?
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    $\begingroup$ I cannot see why the speed of the ship (with respect to Earth) would be limited to 0.3 c. Accelerating at 100 g, the ship will reach 0.3 c in about 28 hours, or a little more than one day. What do they do, stop the engines? (And the accelerating at 100 g won't save you all that much time; at a sedate 1 g acceleration the same speed would be reached in about 4 months; which is small compared to the travel time.) $\endgroup$
    – AlexP
    Oct 8, 2020 at 20:49
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    $\begingroup$ There's a physical phenomenon known as creep en.wikipedia.org/wiki/Creep_(deformation) where materials flow under stress despite being solid. In particular, "...ice will creep at temperatures below 0 °C (32 °F)." even at 1 G and 100 Gs is high-speed car crash levels of acceleration. Being at cryogenic temperatures will help but it's unclear how much. That suggests that a low human-tolerable acceleration might be preferable for preserving the viability of the embryos. $\endgroup$ Oct 8, 2020 at 23:07
  • $\begingroup$ Possibly relevant: Bacteria seem resistent to much higher accelerations (sciencedirect.com/science/article/abs/pii/S0012821X01003429) $\endgroup$
    – Daron
    Oct 9, 2020 at 2:00
  • $\begingroup$ How can one travel to an horizon, since the horizon is at a given distance from the observer? $\endgroup$
    – L.Dutch
    Oct 9, 2020 at 2:56
  • $\begingroup$ Thanks for joining us at Worldbuilding. Had you taken our tour, read our help center and looked through the reasons to close a question, you'd have discovered that asking more than one question is a reason to close a question. Also, note that #2 seems meaningless as the ship could travel as far as it wants, but it will never decelerate. You need an opposing laser to make that happen - and you don't have one. The only way to make such a journey would be to have an onboard means of slowing down. $\endgroup$
    – JBH
    Oct 9, 2020 at 3:57

2 Answers 2


Emily, below is a link (if it works ) to an article published by the European Space Agency's Advanced Concepts Team which seems to indicate it might be possible for living cells to survive accelerations up to the 100Gs in your question - if suspended in liquids. Given the small cross section of human eggs it is quite probable they would have an even better % survival rates than other living organisms

[Article dated 24 April 2007][1] [1]: https://www.esa.int/gsp/ACT/projects/liquid_ventilation/

I do question however whether a laser propulsion system is likely to get you anywhere near that level of acceleration - especially if we are talking about beamed propulsion as opposed to say laser initiated fusion.

While 'light sails' are being actively researched as a means of interstellar travel they generate only a tiny acceleration and as a result it would take many months for any such vessel to reach a significant % of the speed of light. And even then I think max velocity has been modeled to be around the 5% mark or something similar.

Also you have have difficulty finding materials that will resit that kind of acceleration you want for more than the briefest periods - unless you use lots of material - which means more mass - which means more energy for propulsion - which means longer times to reach a given speed. And then the whole cycle starts all over again. Rocket engineers hate physics.


This is an answer only to the second question.

By numerically integrating the geodesic equation I get that if you accelerated to 0.3c and coasted, you'd end up roughly 2.5 billion light years away in comoving distance = present-day metric distance. The distance is roughly linearly proportional to the starting speed up to around 0.5c. (Note that the farthest you can get at the speed of light is roughly 16 billion light years.)

You don't need to brake. You'll end up moving at about the same speed as the galaxies at your maximum distance. This is true in any expanding universe because of Hubble's law. The farther from your starting point, the larger the recessional speed of the galaxies; once you reach galaxies that are receding at around the same speed as you, you won't pass them. Even without the runaway expansion of ΛCDM, you can only get a finite comoving distance away from your starting point by moving ballistically in an expanding universe.

  • $\begingroup$ But doesn't "reference drag" would allow ship to reach much further? (If expansion is not accelerated?) $\endgroup$
    – ksbes
    Oct 9, 2020 at 12:24

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