Solar sailing DOESN"T break laws of physics'

wrote in message
...
In article ,


Consider an ensamble of particles (and it doesn't matter
in the least
whether they're photons or anything else). The energy of
the ensamble
can be represented as a sum of two parts:

1) The kinetic energy of the CM motion (i.e. the energy
associated
with the total momentum of the ensamble).

2) The "internal" energy, i.e. the energy of the ensamble
in its CM
frame.

Now, the first part is freely transferable to anything the
ensamble
interacts with, subject only to conservation of energy and
momentum
laws. The second law is not involved at all. It is only
when you
want to extract some of the second part (the internal
energy) that the
second law comes into play.



Well, here's the source of come of my confusion. You tell me
that I can't extract the energy of a beam of photons that is
coming from a pinhole in a blackbody and has been focused by
a parabolic reflector (I'll call this a heat beam), because
some of the information is missing WRT a beam of the same
power that has been generated in the same fashion, which is
non-coherent but monochromatic. But if all the photons are
moving in the same direction in the "heat beam," why *can't*
we convert all of their energy to free energy?

It seems to me that a very important thing about loss of
free energy in heat energy is the loss of *directionality*
of the kinetic energy of the particles, and I'm thinking
this must also translate in some analogous fashion to heat
beams as well, when you dispense with material particles
entirely. If you focus heat EM into a beam, you've partly
directionalized it. And yet nature doesn't allow you to take
a confined gas of some temperature and pass it though a
pinhole and bounce if off a reflector and thereby somehow
gain more than the thermodynamically available amount of
energy out of it. Er--- or does it? You're letting the gas
expand into a larger volume as you "focus" it, and you pay
the entropy cost that way. Perhaps if you're allowed to use
volume liberally in this fashion you can extract all the
heat energy in a hot gas that you like (get as close as you
like to all of it out as free energy). Yes? And the same
for a thermalized photon gas?

SBH

In article , "Steve Harris" writes:

wrote in message
...
In article ,


Consider an ensamble of particles (and it doesn't matter
in the least
whether they're photons or anything else). The energy of
the ensamble
can be represented as a sum of two parts:

1) The kinetic energy of the CM motion (i.e. the energy
associated
with the total momentum of the ensamble).

2) The "internal" energy, i.e. the energy of the ensamble
in its CM
frame.

Now, the first part is freely transferable to anything the
ensamble
interacts with, subject only to conservation of energy and
momentum
laws. The second law is not involved at all. It is only
when you
want to extract some of the second part (the internal
energy) that the
second law comes into play.



Well, here's the source of come of my confusion. You tell me
that I can't extract the energy of a beam of photons that is
coming from a pinhole in a blackbody and has been focused by
a parabolic reflector (I'll call this a heat beam), because
some of the information is missing WRT a beam of the same
power that has been generated in the same fashion, which is
non-coherent but monochromatic. But if all the photons are
moving in the same direction in the "heat beam," why *can't*
we convert all of their energy to free energy?

Oh, you can absorb them, converting *all* their energy to heat. No
problem. It is only when you try to convert all of it into
macroscopic motion that you run into themodynamic limits. And, by the
way, same is true for your "monochromatic but non-coherent" beam
(though, strictly speaking, if it is non coherent it cannot be quite
monochromatic, either). You're still missing phase information and
you cannot fully convert the beam into useful power.

It seems to me that a very important thing about loss of
free energy in heat energy is the loss of *directionality*
of the kinetic energy of the particles, and I'm thinking
this must also translate in some analogous fashion to heat
beams as well, when you dispense with material particles
entirely. If you focus heat EM into a beam, you've partly
directionalized it. And yet nature doesn't allow you to take
a confined gas of some temperature and pass it though a
pinhole and bounce if off a reflector and thereby somehow
gain more than the thermodynamically available amount of
energy out of it. Er--- or does it? You're letting the gas
expand into a larger volume as you "focus" it, and you pay
the entropy cost that way. Perhaps if you're allowed to use
volume liberally in this fashion you can extract all the
heat energy in a hot gas that you like (get as close as you
like to all of it out as free energy). Yes?

You just described the principle behind a rocket nozzle. That's
exactly what it does.

And the same for a thermalized photon gas?

Can't think of an exact analogy, off hand, but in principle I see no
reason why not. Well, come to think of it, I can think about an
analogy, though it is a bit artificial.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

(Gregory L. Hansen) wrote in message ...
In article ,
Edward Green wrote:
(Gregory L. Hansen) wrote in message

While I am familiar with the
second law of thermo, and Carnot cycles, and the approximate relation
of these concepts, I have never made the aquaintance of "Carnot's
Rule", or whatever the precise form being brutish about here.

Now maybe my thermo education is incredibly inadequate or non-standard
or shows my heritage as a failed p-chemist rather than a failed
physicist, but it seems to me just _possible_ that other threadies had
no greater familiarity with this rule than I did -- though probably
understanding the second law perfectly adequately. Yet here we are,
present company (inconsistently) excluded, all brandying about
"Carnot's rule" as if it is something we have known and loved from
childhood, and never spoke about violating the second law of thermo in
any other way that "violating Carnot's rule".

I don't think it's so.

The habit of language illustrated is that if somebody uses a
catch-phrase which we are not _quite_ familiar with, but feel we ought
to be, we are apt to immediately adopt it to show that we too are
totally hep, cool and jargonationally hep with it, dadeo.

Interesting. I assumed Carnot's Rule was something like that the
efficiency of a heat engine was the Carnot efficiency or less, but I don't
think it was ever really defined. So I Googled "Carnot's Rule", and the
only hits on it link to Gold and solar sails. And no hits on a Web of
Science search. Or in the McGraw-Hill Dictionary of Scientific and
Technical Terms.

Beautiful! _I_ assumed it was at least a known, if obscure, concept,
which we were picking up as jargon for "the second law", whereas you
went the extra mile and found out that if it's obscure, it's _really_
obscure! That's too good.

Anyway, we both know why at gatherings it's only the most senior
emeritus emertus who will dare ask questions like "Er, excuse me: what
is 'Carnot's Rule'?". It's not even necessarily that they are braver
than everybody else: it's just that, given sufficient experience, they
_know_ if they haven't heard of something that probably nobody else in
the room has either -- so they can ask the question without fear of
looking foolish.

But that does suggest a debating tactic that would probably have been old
news to Gorgias.

Who's Gorgias? (He asked with a certain embarassment :-).

Invent almost correct terms that your opponent is
embarassed to question. "But the Green's polynomials don't converge...",
and watch the discussion turn to the important of convergence rather than
"What the hell is a Green's polynomial?"

Thanks, Gregory! You've really made my day. LOL, etc.

(Gregory L. Hansen) wrote in message ...
In article ,
Gregory L. Hansen wrote:
In article ,
Edward Green wrote:

But that does suggest a debating tactic that would probably have been old
news to Gorgias. Invent almost correct terms that your opponent is

And did I mention making casual historical references that your opponent
feels shamed not to know?

You *******. ;-)

In article ,
Edward Green wrote:
(Gregory L. Hansen) wrote in message
...
In article ,
Gregory L. Hansen wrote:
In article ,
Edward Green wrote:

But that does suggest a debating tactic that would probably have been old
news to Gorgias. Invent almost correct terms that your opponent is

And did I mention making casual historical references that your opponent
feels shamed not to know?

You *******. ;-)


Heh! Gorgias was an ancient Greek sophist that peddled debating skills
for money. His thing was that it didn't matter whether you're right or
wrong, he'd teach you how to win the argument. I'd remembered there was a
name associated with that, but it took a bit of Googling before I found
the reference to casually drop. Couldn't have happened in real-time
conversation.


--
"Is that plutonium on your gums?"
"Shut up and kiss me!"
-- Marge and Homer Simpson

Laurel Amberdine wrote in message ...
On Tue, 8 Jul 2003 00:58:23 +0000 (UTC), Gregory L. Hansen wrote:
In article ,
Laurel Amberdine wrote:
On Mon, 7 Jul 2003 18:45:12 +0000 (UTC), Gregory L. Hansen
wrote:

In this particular discussion, I don't think "first order" really means
anything, it's just something that people are saying.

I don't know if the phrase is being used technically now or not, but it
does seem to have nearly become slang.

It is actually slang, in the right groups. Hang around with physicists
during lunch time and eventually someone will say something like "To first
order, it rained during the entire vacation", or "A three sigma apple is
still a good apple" (when comparing to e.g. peaches).

Uh huh. And you guys wonder why no one else sits with you at lunch.

I am, of course, only kidding. I think it's cute, and would happily hang around
and listen, but there aren't any physicists around where I am.

Besides, he didn't specify 3 sigma which way.

Apropros of nothing, I am totally targetted by the new Chrysler ad
campaign. I totally wanted one _before_ they mentioned casually it
was clocked at 150 mph on the autobahn -- just based on its looks. Of
course, don't try this at home guys. Right. Now of course, my
Maseratti does 185, they took away my license and now I can't drive,
and etc. But at $35,000 "fully equipped", this is a fast car for
pocket change... well, within reach of Joe Average. AND the added
zinger "be one of the first 9000 to call to get one in 2003. And, it
looks cool. I must be in the target audience, and they hit about 5
sigma on my buying buttons: ACT NOW! GOES REALLY FAST! LOOKS REALLY
COOL! GREAT VALUE!! (Yeah, I'm enough of a Jewish grandmother to think
"great value" adds to a product's cache ;-).

Sigh ... now, just where did my Joe Average disposable income go ...

On Fri, 4 Jul 2003 11:13:08 -0700, "Steve Harris"
wrote:


"Geoffrey A. Landis" wrote in
message
om...

This is exactly what Gold argues.

Not quite. You understand this first point? If the sail
is
stationary, then a reflection of a photon results in zero
change in
energy (to first order), and thus the sail can gain
momentum at no
energy cost?

Come on. You're like the guy who says: when I heat up an
object, its weight doesn't change (to first order).
Therefore the equivalence of mass and energy is violated.
Duh.

There is no energy cost to move the stationary sail *to
first order.* Carnot's law is broken to exactly the degree
that you simplify the problem with approximation. But don't
confuse your approximation with violation of physical law.

What's Carnot's Law?

I looked in both my thermodynamics books, Halliday & Resnick, and
Feynman, but couldn't find anything about it. I found a bunch about
the Carnot cycle, naturally, but no Law. I know Carnot is long dead,
so he can't have invented it in the last thirty years, either.

This is annoying, because I can't look up anything about airplanes or
aeronautics because all those books are packed away, which irritates
me, and what I can look up, about physics, I can't find.

Mary

--
Mary Shafer Retired aerospace research engineer

"Turn to kill, not to engage." LCDR Willie Driscoll, USN

Mary Shafer wrote:

On Fri, 4 Jul 2003 11:13:08 -0700, "Steve Harris"
wrote:


"Geoffrey A. Landis" wrote in
message
om...

This is exactly what Gold argues.

Not quite. You understand this first point? If the sail
is
stationary, then a reflection of a photon results in zero
change in
energy (to first order), and thus the sail can gain
momentum at no
energy cost?

Come on. You're like the guy who says: when I heat up an
object, its weight doesn't change (to first order).
Therefore the equivalence of mass and energy is violated.
Duh.

There is no energy cost to move the stationary sail *to
first order.* Carnot's law is broken to exactly the degree
that you simplify the problem with approximation. But don't
confuse your approximation with violation of physical law.

What's Carnot's Law?

I looked in both my thermodynamics books, Halliday & Resnick, and
Feynman, but couldn't find anything about it. I found a bunch about
the Carnot cycle, naturally, but no Law. I know Carnot is long dead,
so he can't have invented it in the last thirty years, either.

This is annoying, because I can't look up anything about airplanes or
aeronautics because all those books are packed away, which irritates
me, and what I can look up, about physics, I can't find.

Mary


Carnot's Law == The 2nd law of thermodynamics

a short mention he
http://www.benwiens.com/energyFAQ.html#energyFAQ.25

Mark

Steve Harris wrote:
But if all the photons are moving in the same direction in the "heat
beam," why *can't* we convert all of their energy to free energy?

You can, but only if you have an infinite heat sink at absolute zero,
and are willing to take literally forever. (Radiating heat at low
temperatures is slow. Radiating heat at absolute zero is infinitely
slow.)

If you have monchromatic light, you can have each photon strike an
easily ionized target and cause one electron to be emitted for each
photon. Since the photons all have the same energy, then so will the
electrons. The electrons can be made to go "uphill" by an amount
equal to their voltage, to charge a capacitor.

But if the photons all have different energies, then so will the
electrons. Each electron will have a different voltage (momentum).
You will have replaced your "hot" photons with "hot" electrons.

If you try to use those hot electrons to charge your capacitor, some
will have too much energy. The excess energy will be wasted heating
the capacitor plate, which will lose that heat either by radiating
electrons or photons. Other electros will have too little energy.
They'll fall back to the emitter, and heat *it* without doing anything
useful whatsoever.

A similar argument can be made for any other kind of photon collector.

It seems to me that a very important thing about loss of free energy
in heat energy is the loss of *directionality* of the kinetic energy
of the particles, ...

If the thermalized photons are coming from all directions equally then
the whole environment is equally hot, and you can get no useful work
whatsoever. Well, maybe a little, but only until your absorber heats
up to the same temperature as the background. After that, each part
of it will be just as likely to emit photons as to absorb them, so
your engine is just as likely to run backwards as forwards, regardless
of the details of its construction. Everything will be at maximum
entropy for that temperature.

(Well, actually, you could have thermalized photons are come from all
directions equally, but *not* at the equilibrium temperature of the
environment. A cloudy day on earth is an example. Solar panels still
work pretty well even if the light is not at all directional.)

Perhaps if you're allowed to use volume liberally in this fashion
you can extract all the heat energy in a hot gas that you like
(get as close as you like to all of it out as free energy). Yes?

Yes. The coldest things in nature -- colder than the 3 K background
-- are espanding gas clouds from supernovas, ironically enough.

But this cooling sure takes a lot of space and time.

And the same for a thermalized photon gas?

Yes, given sufficient space and time. You may want to wait for the
cosmic background to cool off some first.

Exercise for the student: What's the optimal radius to build a Dyson
sphere around the sun to capture as much energy as possible? Would it
be useful to build several concentric spheres, each using the waste
heat from the next one inwards?
--
Keith F. Lynch - - http://keithlynch.net/
I always welcome replies to my e-mail, postings, and web pages, but
unsolicited bulk e-mail (spam) is not acceptable. Please do not send me
HTML, "rich text," or attachments, as all such email is discarded unread.

On Mon, 7 Jul 2003 19:24:20 GMT, in a place far, far away,
(Henry Spencer) made the phosphor on my monitor
glow in such a way as to indicate that:

In article ,
Laurel Amberdine wrote:
Ignorance time: people are saying "to first (second, zeroth) order" etc,
quite often lately. What does it mean, anyway?

While those terms have precise meanings, they are often applied somewhat
more loosely and generally. Speaking loosely and generally...

"To first order" means "considering only the biggest and most obvious
effects, neglecting details which don't change things very much".

"To second order" means "including the most significant of the details,
but neglecting really small ones which have rather smaller effects".

And so forth.

And by analogy, "to zeroth order" means "making drastic simplifications to
get an answer which will be somewhere in the right ballpark".

This is a useful discussion in the context of space policy as well.
Most discussion in Congress is about second and third order items,
when in fact the problem is at the level of first and zeroth order, or
as Tom Rogers says, "page 1."

--
simberg.interglobal.org * 310 372-7963 (CA) 307 739-1296 (Jackson Hole)
interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org

"Extraordinary launch vehicles require extraordinary markets..."
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