Re: adapting to an open universe

The Equalizer (
Mon, 19 Jan 1998 17:13:26 GMT0BST

I believe that due to time being relative the effect of the space
expanding infront of the light beam is cancled out but the relative
change in time dialation.

E.G. The object sending out the light wave can not be traleving away
from us at a speed higher than the speed of light no matter of how
far away it is! Therefor the light will allways reach us.

> From: Nick Bostrom <>
> To:
> Date: Sun, 18 Jan 1998 17:05:21 +0000
> Subject: Re: adapting to an open universe
> Reply-to:

> Hal Finney <> wrote:
> > It may be that the universe expansion is worse than I thought.
> > Consider that the computer must, as time goes on, be composed more
> > and more of empty space, since the density of space is decreasing
> > (exponentially?). We must signal from one side to the other by sending
> > something that takes energy, whether gravitational radiation, photons,
> > or particles. Suppose we send a particle. The problem may be that if we
> > give the particle too small an initial speed, that it never gets there.
> > It loses ground due to the universe expanding out from under it, and at
> > some point it actually starts to go backwards.
> I had a similar thought the other day.
> I started out with the following idea. Suppose we construct probes
> that fly out in all directions, and when they reach a lump of matter,
> they use some of the energy it contains to shoot fragments back to
> origo (Earth). Some fragments have to fly in the opposite direction
> to conserve total momentum, but there should be a constant fraction
> of the energy in a lump of matter that we can send back to origo at a
> certain speed.
> As the colonization wave spreads, the frontier surface that they
> harvest matter from increases as the square of the radius of the
> colonization bubble. My idea was that even if the average density of
> the universe decreases over time as a result of expansion, this
> decrease might slow enough so that amount of matter that
> could be sent back to origo at any moment would diverge as t goes to
> infinity. This could allow an infinity of computations at origo.
> I did some back of the envelope calculations. I think the expansion
> rate of the universe (if it is open) will asymptotically approach a
> constant (could anybody confirm this?). So the density of the
> universe would decrease as t^3, for large values of t, and the volume
> from of the space where we can have harvested a constant fraction of
> its matter content grows as t^3 (if the probes have constant speed
> ans since the volume of a sphere is proportional to R^3). So far,
> things look bright.
> But then I thought of this problem. Suppose the probes travel with
> the speed v. Then there must be some distance d such that if the
> probes start at that distance, they will never return to origo. For
> if d is big enough, then by the time the probes have traveled 10% of
> the way, the remaining 90% will have grown larger than the original
> 100%, due to the expansion of space inbetween. So if that expansion
> rate settles down to a constant, then there must be a sphere centered
> around Earth such that nothing that is outside of that sphere can
> *ever* affect Earth, even if it travels with the speed of light.
> Hence the above proposal fails, unless somebody can point to some
> mistake in this reasoning.
> ________________________________________________
> Nick Bostrom

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