Re: the ultimate refrigerator

michael k teehan (
Fri, 19 Dec 1997 19:40:52 -0500

> From: Wei Dai <>
> To:
> Subject: Re: the ultimate refrigerator
> Date: Friday, December 19, 1997 7:45 PM
> On Fri, Dec 19, 1997 at 11:57:33AM -0800, Robin Hanson wrote:
> > In making my contribution to reversible computing
> > ( I learned enough say with
> > confidence that there is no particular advantage to erasing bits at
> > temperatures. If it were otherwise you could make a perpetual motion
> > erase bits (= replace unknown bits with known bits) at low temps and
> > reverse the operation (replace known bits with unknown bits) at high
> > By the "costs less energy" intution this cycle would create available
> Isn't that exactly what a heat engine does, when you think of it in
> information theoretic terms? Of course it is not a perpetual motion
> machine since when it erases bits, it raises the temperature of the low
> temp heat sink, and when it randomizes bits, it lowers the temperature of
> the high temp heat source, so eventually the temperature difference
> disappears. (Of course I'm excluding black holes here, since
> when you "heat up" a black hole by dumping energy into it, it actually
> gets cooler.)
> I don't see how you can say there is no advantage to erasing bits at
> temperatures, since your free energy is limitied, and at lower
> temperatures the same amount of free energy allows you to erase more
> The trick is to obtain a low temperature without spending energy on
> cooling, and that's what a black hole allows you to do. Perhaps what
> you're saying is that it wouldn't help to cool your computer to 0.1K if
> the best heat sink you have access to has a temperature of 3K. I
> agree with this.
> I think we're probably not really disagreeing, just emphasizing different
> aspects of the same idea.