de...@premier1.net (Devin L. Ganger) writes:

 >>> I sorta doubt that dumping excessive heat will be a big problem once
 >>> you are a few AU out from the nearest star....
 >> You might doubt it, but you'd be wrong.

 De> Well, if you're trying to do it via convection, etc., yes, there would
 De> be a big problem.
 De> Get a hurkin' big laser, though...refrigeration is easy.

Um, no.

A laser generates heat (outside the laser beam, which
obviously generates heat) it doesn't absorb it.

This involves elementary thermodynamics.  Entropy increases.
Concentrating heat in one spot and direction is a major
decrease in entropy.

___ Blue Wave/QWK v2.12

--
Frank Palmer
flpal...@ripco.com

   Then again, a refridgerator also generates heat - it absorbs it from
one place, and radiates it (and more) to somewhere else.

   ...is done all the time and...

   ...only in a local (not global) sense. I failt to see how this simple
argument invalidates using a laser as a refridgeration device (is it
possible? I'm not sure - but there seems to be some holes in your
argument presented above).

--
Brian Davis

Not only is it possible, but it's been used. Some gents as a US
university used it to actually create a real live Bose-Einstein
Condensate in the lab. Really nifty stuff... A good link for details is
here: http://www.nist.gov/public_affairs/gallery/bosein.htm

--
jesus X [jesusx{at}who.net] [jesusx{at}depechemode.com]

Everything not Strictly Forbidden is now Mandatory.

Think Cosmically, Act Insignificantly.

A refrigerator is a device which removes heat from one location, at the
cost of generating additional waste heat at various locations.  The system
as a whole grows hotter (the system includes not only the refrigerator but
the power plant generating electricity for it).

Only for _part_ of a system.  You can't charge a laser beam with waste heat
and use that to cool the system, it's in violation of thermodynamics.  You
can't take waste heat, which is high entropy, and send it off in a laser
beam, which is low entropy, without generating even more waste heat.

You can't use a laser as a refrigeration device in the senses referred to
earlier (a place to put waste heat, eg. in Brin's Sundiver books).  His
argument is quite right in that respect.  If waste heat is leaving a
spaceship, it _must_ be in a high entropy form.

I'm fairly sure that lasers can't get you down to BEC temperatures.
Atleast they couldn't back when BECs were first done -- I suppose
they might have done it now, but I doubt it -- there's too much
thermal motion involved. You need to use different techniques to
make BECs.

Aaron
--
Aaron Bergman
<http://www.princeton.edu/~abergman/>

I'm not sure what the original context of this was, but laser
cooling is certainly possible and won the Nobel prize in physics
a couple of years back. It's really quite an idea -- you tune the
laser just beyond a resonance of the material you're trying to
cool. Thus, only the stuff that's travelling towards the laser
and sees it Doppler shifted get hit by the laser. By arranging
the lasers in various geometries, one can do all sorts of cool
stuff with this.

Aaron
--
Aaron Bergman
<http://www.princeton.edu/~abergman/>

No, you're right they can't. But IIRC, the latest attempt got down to
about 4 degrees K, they used some funky devices after that... But I was
just trying to prove the point that they DO exist... =-]

--
jesus X [jesusx{at}who.net] [jesusx{at}depechemode.com]

Everything not Strictly Forbidden is now Mandatory.

Think Cosmically, Act Insignificantly.

: In article <378953A8.4...@pdnt.com>, Brian Davis wrote:
: >
: >   ...only in a local (not global) sense. I failt to see how this simple
: >argument invalidates using a laser as a refridgeration device (is it
: >possible? I'm not sure - but there seems to be some holes in your
: >argument presented above).

: I'm not sure what the original context of this was, but laser

The original context was taking a laser INSIDE a spaceship and using it to
shoot off all the waste heat of the spaceship.

: cooling is certainly possible and won the Nobel prize in physics
: a couple of years back. It's really quite an idea -- you tune the
: laser just beyond a resonance of the material you're trying to
: cool. Thus, only the stuff that's travelling towards the laser
: and sees it Doppler shifted get hit by the laser. By arranging
: the lasers in various geometries, one can do all sorts of cool

[I bet Bergman claims that this pun was unintentional.]

: stuff with this.

Sure you can.  And I'm going to save this explanation.  But it doesn't
deal with the original claim.

--
Frank Palmer
flpal...@ripco.com

-- The original context was taking a laser INSIDE a spaceship and using it
to
-- shoot off all the waste heat of the spaceship.

well, wouldnt it be much easier then to use superconductors and a supply of
volatiles?

   And the huge heat sink of the enviroment outside (ie- a materially
closed system of powerplant + refridgerator can still dump heat outside
- think of a refridgerator powered by a battery for instance). The point
is that the powerplant + refridgerator can indeed cool itself, according
to the laws of thermodynmaics.

   Here's my conceptual problem: imagine a black box (how most of my
thermo was taught to me, and obviously about how good a grasp I have on
it now ;-). You say I can't take waste heat and pump it into a low
entropy form (like a laser) *without generating even more waste heat*.
In other words globally entropy must increase. I agree.
   But you can arrange (or at least imagine) a system that does this in
a local sense (refridgeration). The catch is that you need an outside
source to power it (ie- waste heat cannot be the only think powering
it), and obviously since net entropy increases, if I'm lowering the
internal entropy of the system (temperature) I must be increasing the
external entropy (since this is the "temperature" of the laser (low, in
a thermodynamic sense), I need to dump a lot of photons per second (a
*lot*!) - quite an engineering feat, but not a violation of
thermodynamics).
   See my problem? It's quite easy to see how to cool something, and a
laser represents one way to pump entropy around (a terribly inefficient
one? Certainly - but one that can work against a huge temperature
gradient).

   Maybe, but you can't prove that by a simple application of
therodynamic law. Afterall, there are chemical reactions that lower
their temperature, but increase the (global) entropy, a similar
situation.
   Sorry, I'm just not getting it. As an engineering problem I see it,
but not as a consequence of the laws of thermodynamics.

--
Brian Davis

This could work, sorta - as long as the source of energy powering
the laser was hotter than the local sun.  But then, you wouldn't
need the laser ;-)

Ike

<c369...@sp2n21.missouri.edu> schreef in berichtnieuws
7mcuij$fa...@news.missouri.edu...

exactly my point. it seems to me a very difficult way to get rid of excess
heat.