Re: poly: Bekenstein bound and expanding universe

From: Hal Finney <>
Date: Mon Jan 19 1998 - 10:24:52 PST

Mitchell Porter, <>, writes:
> Suppose you're a technological civilization in an open universe.
> You expand into space, but eventually run into others doing the
> same thing. You have control over a finite volume of space,
> but can't go beyond its boundaries, and can't obtain new energy
> from beyond them.
> Nonetheless, since space is expanding (on a supragalactic scale,
> at least), shouldn't the Bekenstein bound on your territory
> also be increasing? Without limit?

Wei Dai offered a similar speculation on the Extropians list.

Tipler says that as the amount of space grows without limit, the speed
with which distant parts of space recede increases as well, hence you
need more and more energy to send a signal from one side of space to
the other. Eventually you need more energy than is available, he says.

I don't fully understand this, but one example might be trying to send
a signal by sending a particle of matter from one end of your space
to the other. You need to send the particle with a velocity greater
than the expansion velocity, then while it is travelling the expansion
velocity increases, so you need to send it with an even higher velocity
initially just so it eventually reaches the other side. As time goes
on the necessary velocity increases without limit.

This is not a fully general demonstration, of course; I'm not sure how it
applies to photons or gravitational waves. Presumably the energy needed
there increases as well with increasing recession velocity, but I don't
know if it would have a threshold like a material particle would.

Received on Mon Jan 19 18:45:25 1998

This archive was generated by hypermail 2.1.8 : Tue Mar 07 2006 - 14:45:29 PST