Re: adapting to an open universe

Hal Finney (
Sat, 17 Jan 1998 08:31:02 -0800

Wei Dai, <>, writes:
> On Fri, Jan 16, 1998 at 10:02:17AM -0800, Hal Finney wrote:
> > [Tipler] may be saying that at some point there won't be enough energy in
> > the whole universe to create a single photon of enough energy to signal
> > from one side of the computer to the other. Apparently there is only a
> > finite amount of energy in the (accessible?) universe. But I'm not sure
> > that there is a threshold energy which would be necessary to create the
> > signalling photon; perhaps photons can have arbitrarily small energies,
> > as long as they are hotter than the background.
> If your computer is reversible, you shouldn't have to use up any energy
> for computation, so this situation shouldn't come up.

I think you need to use some energy temporarily, although you should be
able to recover it.

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. This means that there
is a certain minimum energy necessary for this kind of signalling, and
that minimum energy will increase as the structure grows larger.

I'm not 100% sure about this but Tipler describes a more moderate effect
like this in a closed universe, and it should be worse in an open one.
Also it's not clear how this generalizes to the other forms of signalling,
but maybe something similar happens.