Laser cooling
Frank Palmer
>>> 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
flpa...@ripco.com
Brian Davis
>> Get a hurkin' big laser, though...refrigeration is easy.
>
> A laser generates heat (outside the laser beam, which
> obviously generates heat) it doesn't absorb it.
Then again, a refridgerator also generates heat - it absorbs it from
one place, and radiates it (and more) to somewhere else.
> This involves elementary thermodynamics. Entropy increases.
> Concentrating heat in one spot and direction...
...is done all the time and...
> is a major decrease in entropy.
...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
jesusX
> ...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).
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.
Ian
>Frank Palmer wrote:
>
>>> Get a hurkin' big laser, though...refrigeration is easy.
>>
>> A laser generates heat (outside the laser beam, which
>> obviously generates heat) it doesn't absorb it.
>
> Then again, a refridgerator also generates heat - it absorbs it from
>one place, and radiates it (and more) to somewhere else.
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).
>> This involves elementary thermodynamics. Entropy increases.
>> Concentrating heat in one spot and direction...
>
> ...is done all the time and...
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.
>> is a major decrease in entropy.
>
> ...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).
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.
Aaron Bergman
>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).
>
>university used it to actually create a real live Bose-Einstein
>Condensate in the lab.
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/>
Aaron Bergman
>
> ...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
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.
jesusX
> 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.
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... =-]
Frank Palmer
: In article <378953...@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
flpa...@ripco.com
Nout Hoozemans
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?
Brian Davis
> 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).
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.
>>> This involves elementary thermodynamics. Entropy increases.
>>> Concentrating heat in one spot and direction...
>>
>> ...is done all the time and...
>
> 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.
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).
> If waste heat is leaving a spaceship, it _must_ be in a high entropy
> form.
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
c36...@sp2n21.missouri.edu
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
Nout Hoozemans
<c36...@sp2n21.missouri.edu> schreef in berichtnieuws
7mcuij$faq$2...@news.missouri.edu...
exactly my point. it seems to me a very difficult way to get rid of excess
heat.
Erik Max Francis
> 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.
... But laser cooling is certainly used to get in the microkelvins, as I
recall. It was used in generating the Bose-Einstein condensate, but it
wasn't the cooling method used when the condensate was created -- it was
used to cool it to the point where other methods could get it cold
enough.
--
Erik Max Francis / email m...@alcyone.com / whois mf303 / icq 16063900
Alcyone Systems / irc maxxon@efnet / finger m...@members.alcyone.com
San Jose, CA / languages En, Eo / web http://www.alcyone.com/max/
USA / icbm 37 20 07 N 121 53 38 W / &tSftDotIotE
\
/ Laws are silent in time of war.
/ Cicero
Brian Davis
> well, wouldnt it be much easier then to use superconductors and a
> supply of volatiles?
Umm, while we're at it - superconductors are not perfect thermal
conductors (regardless of how Niven might like to use them). Or at least
so I was taught in grduate school (appeals to authority bug the heck out
of me too, but I've been unable to find confirmation one way or the
other in my books here at home).
--
Brian Davis
Erik Max Francis
> Umm, while we're at it - superconductors are not perfect thermal
> conductors (regardless of how Niven might like to use them).
To be fair to Niven, he made it clear that he was talking about some
science fictional variant of traditional superconductors, which he
referred to as "heat superconductors."
> Or at least
> so I was taught in grduate school (appeals to authority bug the heck
> out
> of me too, but I've been unable to find confirmation one way or the
> other in my books here at home).
No, you're right. Superconductivity and Niven's "heat
superconductivity" are not the same thing.
Ian
>Ian wrote:
>
>> 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).
>
> 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.
It can't cool itself unless it is hotter than the external environment. In
which case it would naturally cool anyway. Remember, even generating power
with a battery still produces heat. To go back to the "Sundiver style"
issue, it can't cool itself by emitting low-entropy energy.
>>>> This involves elementary thermodynamics. Entropy increases.
>>>> Concentrating heat in one spot and direction...
>>>
>>> ...is done all the time and...
>>
>> 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.
>
> 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).
Remember that the laser isn't perfectly efficient. Physics prevents it
from being so. It's also VERY low-entropy. It's operation simply
generates more waste than it gets rid of. And by operation I mean the
firing of the laser itself, not generating the power to fire it.
I don't see the point of trying to externally power it, either. That's
like bringing energy into the system just so that you can beam part of it
out of the system and leave the rest in the system as waste heat.
>> If waste heat is leaving a spaceship, it _must_ be in a high entropy
>> form.
>
> Maybe, but you can't prove that by a simple application of
>therodynamic law.
You can combined with some other bits of simple physics, namely that the
stupendous volumes of energy necessary to get the entropy of released
energy really low are essentially impossible.
>Afterall, there are chemical reactions that lower
>their temperature, but increase the (global) entropy, a similar
>situation.
Huh? That's converting heat into entropy... which is not a "similar
situation" to getting rid of waste heat as low-entropy energy (you could
dump it into MASS, which changes things, but that wasn't the subject).
> Sorry, I'm just not getting it. As an engineering problem I see it,
>but not as a consequence of the laws of thermodynamics.
Thermodynamics include such consequences as the maximum efficiency of any
heat engine.
jesusX
> Superconductors wouldn't be much use, and
> heat (as opposed to electric) superconductors apparently don't
> exist.
That's because modern day superconductors have to be cooled to extremes
before the superconducting effect takes hold. Warm it up and it fizzles
out...
Frank Palmer
Ho> -- The original context was taking a laser INSIDE a spaceship and
Ho> using it to
Ho> -- shoot off all the waste heat of the spaceship.
Ho> well, wouldn't it be much easier then, to use superconductors and a
Ho> supply of volatiles?
Well, a supply of volatiles would be quite effective as long
as it would last. Superconductors wouldn't be much use, and
heat (as opposed to electric) superconductors apparently don't
exist.
Another model is to use rather normal refrigerator/heatpump
technology to put all the heat in a coiled tube outside the
ship and radiate that to the coolest part of the sky.
Ian
>Frank Palmer wrote:
>> Superconductors wouldn't be much use, and
>> heat (as opposed to electric) superconductors apparently don't
>> exist.
>
>That's because modern day superconductors have to be cooled to extremes
>before the superconducting effect takes hold. Warm it up and it fizzles
>out...
No, that's because "heat superconductivity" has absolutely nothing to do
with electrical superconductivity.
Nout Hoozemans
conducting heat?
wait, i've got it.
we put all the hot bits in the one space ship,
and let the people leisurely float behind it
in the cool ship.
no need for cooling devices!
Nout Hoozemans
Ian <iadm...@undergrad.math.uwaterloo.ca> schreef in berichtnieuws
37abd26a....@news.netcom.ca...
extremely low temperatures, let's just stuff the ship full of 'em.
there won't be a heat problem!