Brin's sundiver ships

[David Higgen]

Just been rereading Brin's Sundiver. In which his sun-navigating ships
refrigerate themselves by pumping out the heat energy via a laser beam.

Now, I hesitate to take issue with Mr Brin, who does have a real technical
training: but isn't this an egregious and impossible violation of
thermodynamics? Converting low grade (heat) energy into high grade
(coherent light) energy is surely a thermodynamically inefficient
process, producing more heat as a waste product.

Any real physicists out there care to comment?

Dave Higgen (da...@xtenk.asd.sgi.com)

[Bruce James Bell]

da...@xtenk.asd.sgi.com (David Higgen) writes:

> Dave Higgen (da...@xtenk.asd.sgi.com)

I am a physicist wannabe (undergrad). Will that do?

I thought that the thermodynamic thing was a problem, myself. if you have an
internal temperature which is lower (in this case, much lower) than the ambient,
it is impossible to pump heat out *unless* you have a heat source that is at
a higher temperature than the ambient, or some direct source of power. Laser
beams are even worse, since they require thermodynamic conditions that are
*more* severe than infinite temperature (a population inversion, which is
sometimes expressed as a *negative* temperature!). And, of course, most
lasers produce great amounts of waste heat, and even the most efficient ones
(diode lasers, I think) are fairly inefficient.

Of course, the Uplift Universe might have superior energy sources, that are at
a greater effective temperature than the outer regions of the sun. Or, if you
aren't actually *inside* the sun, you can take advantage of the ultimate in
solar power, and dispose of your waste heat skywards. However, with present
technology, it is thermodynamically impossible to make such a refrigeration
laser system which works inside the sun.

Bruce J. Bell br...@tybalt.caltech.edu

[William Richard Russell]

In article <p96...@sgi.sgi.com> da...@xtenk.asd.sgi.com (David Higgen)
writes:

Well, I don't know if I'm quite a real physicist, but here goes...

There are two critical concepts to think about when you consider what
the Sundiver ships are doing. The first is the ``heat engine'', where
incoming heat (i.e., from the sun) is used to create something else,
like motion (i.e. a steam engine), electricity (i.e. a thermocouple), or
coherent light (i.e. Brin's refrigerator laser).

If you did this experiment in any normal environment, you would take in
heat from one end, turn as much into {motion|electricity|laser light} as
possible, and then eject the waste heat into the cooler environment. In
that case, the (maximum) efficiency of the system would be the Carnot
coefficient of heat engine efficiency,

eta = (Th - Tl)/Th,

where Th is the temperature of the reaction (e.g. the boiler temperature
for a steam engine), and Tl is the temperature of the environment into
which the waste heat is ejected. Eta, then, is the amount of work you
get per unit input of heat. For example, if a heat engine has efficiency
0.8, then you get 8 joules of work for every 10 joules of heat you put
in.

You'll note immediately that eta is maximized when Th is much bigger
than Tl. That's why engines (steam or gasoline) always perform more
efficiently at hotter internal temperatures (Th) vs. cold enviromental
temperature (Tl).

The second critical concept is that of the refrigertor, which works
opposite to the heat engine because it takes in work (in the form of
mechanical work usually) and moves heat from an area of low temperature
to an area of high temperature.

The maximum efficiency for a refrigerator is

gamma = Tl/(Th-Tl),

where Tl is the temperature inside the refrigerator and Th is
temperature outside. Gamma, then, is the amount of heat moved into the
environment for each unit of work invested in moving it. When Th is very
high (or Tl becomes quite low), it takes a LOT of work to move heat from
the working area (i.e. the inside of the refrigerator) to the
environment.

Brin's Sundivers don't really make sense as heat engines or
refrigerators. In the heat engine case, the temp of the enviroment (Tl)
is the same as the temp of the working substance (i.e. the heat outside
the Sundiver), so the efficiency is zero.

As a refrigerator, the Sundiver could work by investing a tremendous
amount of mechanical energy into moving heat from the inside to the
outside. But since the temperature of the environment (now Th) is so
high, the efficiency of the refrigerator would be very low, and that
still fails to explain what the refrigerator laser is doing.

So, in other words, Sundivers don't seem to hold up under thermodynamics
as they are now understood. Much of the Sundivers' mechanisms were the
product of Galatic technology, however, and therefore may violate this
understanding.

Now perhaps you COULD drive the refrigeration process with some other
energy resource (i.e. batteries or somesuch which which are NOT heat
engines), and then use the waste heat from the refrigeration process
(i.e. all of the work which isn't used to move heat [according to the
coefficient of refrigerator performace] becomes waste heat) to drive the
heat engine for the refrigerator laser. But then there would be no place
to eject the waste heat from the laser, and the system falls apart
(again). Besides, Brin mentions nothing of this in the book (Sigh...).

By the way, when you say "low grade (heat) energy" I think you actually
mean low entropy energy, and your "high-grade (coherent light)" energy
is actually high entropy energy. So the Sundiver is ejecting entropy in
some way; although without knowing the real details, it's hard to say
what effect this has on refrigeration.

>
> Dave Higgen (da...@xtenk.asd.sgi.com)

Rick Russell
Atmospheric Scientist in Training
--
| Rick Russell | ri...@owlnet.rice.edu |
| WRUSS00@RICEVM1 (Bitnet) | RICE::RUSSELL (SPAN) |
| All opinions expressed are my own. |
| "Don't mess around with the Demolition Man!" |

[sudo...@gmail.com]

> By the way, when you say "low grade (heat) energy" I think you actually
> mean low entropy energy, and your "high-grade (coherent light)" energy
> is actually high entropy energy. So the Sundiver is ejecting entropy in
> some way; although without knowing the real details, it's hard to say
> what effect this has on refrigeration.

Isn't that backwards? Doesn't energy move from low entropy to high entropy? Such that the surface of the Sun is at a lower state of entropy than the surface of the Earth? I was under the impression that the Big Bang was the lowest point of entropy in history and entropy has been increasing ever since.

[Bill Snyder]

Yup. Penrose, in _The Emperor's New Mind_, has a nice explanation
of how the Sun and space act on the Earth as a sort of "entropy
bilge pump." We get energy on the day side as lower-entropy
visible light, and radiate it back out on the night side as
higher-entropy infrared.


--
Bill Snyder [This space unintentionally left blank]

[rickr...@gmail.com]

So I know you responded to me 21 years after the original post, and this response is 6 years after your reply, but yes your correction is accurate.

The Sundiver is absorbing higher entropy energy in the form of white light, and ejecting lower entropy energy in the form of its blue refrigerator beam (if I remember the details well enough 27 years later).

Consequently the refrigerator laser is an entropically unfavorable device, and should result in an inability to continue to direct the energy into the refrigerator laser, at least through such pedestrian means as chemical reactions. Perhaps some sink of entropy has been discovered at the nuclear level? Well, maybe Brin just didn't think it through.


rickr...@gmail.com , formerly WRU...@ricevm1.rice.edu