Re: the ultimate refrigerator

michael k teehan (
Fri, 19 Dec 1997 19:40:18 -0500

> From: Robin Hanson <hanson@econ.Berkeley.EDU>
> To:
> Subject: the ultimate refrigerator
> Date: Friday, December 19, 1997 2:57 PM
> Wei Dai wrote:
> >Advanced civilizations probably have extensive cooling needs. ...
> >However, even at the cosmic background temperature of T=3K, erasing a
> >still costs a minimum of k*T*ln 2 = 2.87e-23 J. What is needed is a way
> >efficiently cool a system down to near absolute zero. I think the only
> >to do it is with black holes. ...
> I replied:
> >I think this analysis is confused. Erasing a bit costs one bit of
> >regardless of what temperature you do it at. ... Negentropy is the real

> >resource ...
> Wei then asked:
> >Would you please explain?
> John Clark also responded:
> >I don't know what you mean by the "cost" of entropy. Entropy is free,
> >is not, because energy is conserved, entropy is not. I don't want to
> the
> >growth of entropy, it's possible that entropy will keep on increasing
> forever
> >and I certainly hope it does because the only alternative is the heat
> >of the universe. Free energy is related to entropy but if the universe
> >open then it's energy any intelligence must be very stingy with if it
> >to survive for long.
> >...
> >With reversible computing, ... Landauer, Bennett and Merkle have shown
> >the amount of energy needed to make a calculation can be made
> >small by slowing down the calculation a little.
> Wei posed the problem of cooling to get rid of excess local heat, by
> contact with distant sources of negentropy (= max sys entropy - actual
> entropy)
> such as the cosmic background. For this problem, the conservation of
> constraint is much less important than the constraint that total entropy
> cannot
> decrease. In making my contribution to reversible computing
> ( I learned enough say with great

> confidence that there is no particular advantage to erasing bits at lower
> temperatures. If it were otherwise you could make a perpetual motion
> erase bits (= replace unknown bits with known bits) at low temps and then
> reverse the operation (replace known bits with unknown bits) at high
> By the "costs less energy" intution this cycle would create available
> Robin Hanson
> RWJF Health Policy Scholar, Sch. of Public Health 510-643-1884
> 140 Warren Hall, UC Berkeley, CA 94720-7360 FAX: 510-643-8614