Solar sailing DOESN"T break laws of physics'

In article , "Steve Harris" writes:

wrote in message
...
In article , "Steve
Harris" If there's no change in photon energy to first
order, then
obviously that's a breaking of Carnot's law to first
order,
since Carnot requires an decrease in photon temperature
(photon energy) for work to be extracted.

Two points:

1) Photon energy is *not* "photon temperature". Photon
(or any
particle) does not have a temperature). Temperature is an
ensemble
property.


Yes, yes, but the point is you can have a thermalized bath
of photons, and if you do, then it has to follow the same
thermodynamic laws as any heat source. to wit, if you turn
some of the photon energy of such a collection into free
energy (charging a battery on the rocket or pushing the
rocket faster), then you have to pay some entropy price for
destroying that much heat. In simple reflective sails that
means creating another bunch of (reflected) photons that are
effectively thermalized at a lower temperature (heat going
into a lower temp reservoir).

And, indeed, you will. If you take a, say, blackbody radiation
impinging on you sail, the reflected beam will have blackbody spectrum
corresponding to slightly lower temperature. Doppler shifted
blackbody spectrum is blackbody spectrum. No problem.

The confusion present arises from thinking that the reflected photons
are at same energy as the incoming ones. They're not.

Or else you have to heat up
some sail material which was previously at a lower temp than
the illuminating radiation, and dump your entropy that way.

If you have a monochromatic source of photons like laser or
a microwave beam, then (so far as I can tell) nothing
prevents you from converting such a beam *entirely* into
free energy (charging a battery on board the rocket) and
destroying the EM radiation completely, so that no photons
are left at all, and sail heating is minimized (perhaps
doesn't occur at all). All the energy could be extracted
from the beam and used to charge a battery or make chemical
fuel, or something.

How do you plan to use the energy to make chemical fuel? To use it to
create protons and electrons, then assemble these into atoms etc?:-)
I don't think so. You can much better carry the fuel with you, to
begin with. But that defeats the idea of the light sail which is to
get around the inherent reaction mass limitations.


For reasons that are obscure to me, there are additional
limitations for using the energy in photons that come in a
blackbody spectrum. They act like heat, and only some or
their energy is available for conversion into free energy at
the target, and thence into rocket kinetic energy.

The limits are the same as in any other process. Entropy cannot
decrease.

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

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).

Shows like Star Trek seem a little artificial sometimes because none of
that sort of thing creeps in when engineers talk to engineers. And also
because they spout off with all the technical words on the job, when
scientists in real life might ask for "the magic wrench", or suggest doing
"the hokey pokey" on a detector.

But I suppose if they try to get those sorts of language issues in there,
it would seem more artificial than otherwise.

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

(Gregory L. Hansen) wrote in message ...
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).

Hah. Similar to hacker humor: "For sufficiently large values of
zero".

Except ... gasp ... I can hear your line above not even being used
ironically.

Apropros the present problem: like interesting, man: colon: I suspect,
to first order, that when a wee tiny body recoils elastically off a
great massie body, and the great massie body is originally at rest,
that energy loss of the wee tiny body is, to first order in dp, zero.
My suspicions are also aroused to the possibility that once the great
massie body is put in your state of motion in your original state of
reference, that this is no longer true.

Which is, however, as Mati suggests, a complete red herring here;
though there are so many on the plate I think we bought them in tomato
sauce. It does sound like a nice little oolieish answer that the
second law of thermo is violated only to second order in dp, or some
such nonsense, though it is in fact nonsense and, whatsmore,
irrelevant nonsense. Is that an oxymoron?

Speaking of slang and argot and jargon* and all that, this thread
illustrates another habit of language. 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.

There was an amazing phenomena in NYC a few years back where sidewalk
scaffolding -- you know, that stuff erected under construction sites
to protect you from being hit on the head by the smaller debris --
suddenly became known as "sidewalk bridges". Nobody knows who started
this word virus, yet suddenly all discussion of sidewalk scaffolding,
themselves enjoying a temporary newsworthyness for equally obscure
reasons, had to refer to them by the new shiboleth, or reveal the
writer to be horribly, horribly technically inept and inadequate and
to have been sired out of wedlock in Spanish Harlem on a warm summer
night and grown up in the street!

If you know what I mean.

I believe the concept intended was the, now abundantly wrong,
assertion that solar sails "violated the second law of
thermodynamics". Repeat after me: "Solar sails violate the second law
of thermodynamics" -- except they don't.

Shows like Star Trek seem a little artificial sometimes because none of
that sort of thing creeps in when engineers talk to engineers.

I feel like an attempt was made once or twice, but maybe I err.

And also
because they spout off with all the technical words on the job, when
scientists in real life might ask for "the magic wrench", or suggest doing
"the hokey pokey" on a detector.

That's good!

Spock and Data may have been given the role of using technical
language in ordinary conversation; but you are right -- I never heard
the reverse phenomenon emulated, or even emulsified, though the
writers doubtless received substantial emollient to effect it.

But I suppose if they try to get those sorts of language issues in there,
it would seem more artificial than otherwise.

And most people, not even being around engineers and scientists ever,
just wouldn't get it.

*also shiboleths and sacred kine

In Steve Harris wrote:

[Geoffrey Landis had written]:
There is no energy cost to move the stationary sail to
first order.

If there's no change in photon energy to first order, then
obviously that's a breaking of Carnot's law to first order,
since Carnot requires an decrease in photon temperature
(photon energy) for work to be extracted.

For a stationary sail, there's no work extracted, so there is no
breaking of Carnot's law.

Let's phrase it slightly differently.
power (which is work per unit time) is force times velocity.

For a stationary sail, velocity is zero, so power is zero. No work is
extracted.

Carnot requires two thermal baths of different temperatures for
kinetic energy to be gained.

Right. And Carnot says nothing whatsoever about requirements for
momentum to be extracted.

But it's okay, because in any inertial frame where work is
being done on the sail, you see two populations of photons
(those coming and those leaving), and these two DO have two
different temperatures. That's it.

Correct, and since the reflected photon ensemble has a blackbody
spectrum (for a non-dichroic sail), that analysis works and gives the
right result. It was only the specific case of a stationary sail where
the analysis has to take into account the case of no change in photon
energy.

--
Geoffrey A. Landis
http://www.sff.net/people.landis

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

Speaking of slang and argot and jargon* and all that, this thread
illustrates another habit of language. 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.

But that does suggest a debating tactic that would probably have been old
news to Gorgias. 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?"
--
"Is that plutonium on your gums?"
"Shut up and kiss me!"
-- Marge and Homer Simpson

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?


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

wrote in message ...
In article , (Alex Terrell) writes:
The effect of solar radiation pressure has been observed. This seems
to be one of many experiments that appear to break a fundamental law
of Physics (Carnot's rule).

Nothing is broken here.

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


OK, none of us doubt solar sails work now?

But this is an interesting problem to consider. Firstly, we would not
expect to understand photons by analogy with classical particles, as
in some of the snooker ball arguments given above. Most importantly,
photon velocity is always the same when measured in any frame of
reference but this would not be the case with classical particles.

Thus any consideration of the motions involved must be related to a
frame of reference because special relativity leads us to expect
observers in motion with respect to each other to make different
measurements.

Surprisingly, we also need to be careful about frames of reference
when we make classical considerations. For example kinetic energy and
momentum are not unique properties of the sail and depend upon the
frame of reference of the measurement. Also, when two classical
particles collide the momentum transfer between them is unique but the
kinetic energy transfer depends upon the frame of reference. This is
because momentum transfer is equal to force times time but energy
transfer is force times distance. The time is unique (in classical
systems), but the distance covered during the collision depends upon
the observer's frame of reference.

The upshot of all this is that observers in different places will see
something different. An observer on the sail will see the sail as
stationary, with zero kinetic energy at all times. Light from the sun
will approach at velocity c and wavelength l and be reflected with the
same velocity and wavelength. She will see no energy transfer as
proposed by Gold.

An observer on the Sun would see photons reflected from the sail that
were doppler shifted because of the sail's velocity in his frame of
reference. He sees a different photon velocity relative to the sail
before and after reflection because the photon velocity must always be
c in his frame of reference. The energy lost or gained by the photons
would match the kinetic energy change of the sail, again in his frame
of reference.

Strictly speaking it may be invalid to apply special relativity to the
sail because it is accelerating, although with a large velocity and
very small acceleration this assumption may be OK. What is certain
here is that all observers will experience the same laws of physics in
their frame of reference and make the same measurement of the velocity
of light at all times. This appears to lead to the conclusions above.

wrote in message
...
In article , "Steve
Harris" For reasons that are obscure to me, there are
additional
limitations for using the energy in photons that come in
a
blackbody spectrum. They act like heat, and only some or
their energy is available for conversion into free energy
at
the target, and thence into rocket kinetic energy.

The limits are the same as in any other process. Entropy
cannot
decrease.


Yes, but I'm curious about the mechanism for why entropy
cannot decrease. Something about quantum states expanding in
phase-space, but can you put it into easier terms? Why (for
example) can you extract all the energy in a monochromatic
beam and use it to charge a battery (or make new atoms of
matter and antimatter if you must), but there are limits on
what fraction of a beam of blackbody radiation you can use.
Don't just give me an entropy argument (you were ragging on
me for "explaining" wing forces on winds by momentum
conservation, remember?).

What's going on *mechanistically* that you can in theory
completely convert monochromatic EMR to any other kind of
energy you like, but not the mix of wavelengths of EMR that
comes off a black body?

SBH

In article , Geoffrey A. Landis writes:
In Steve Harris wrote:

[Geoffrey Landis had written]:
There is no energy cost to move the stationary sail to
first order.

If there's no change in photon energy to first order, then
obviously that's a breaking of Carnot's law to first order,
since Carnot requires an decrease in photon temperature
(photon energy) for work to be extracted.

For a stationary sail, there's no work extracted, so there is no
breaking of Carnot's law.

Let's phrase it slightly differently.
power (which is work per unit time) is force times velocity.

For a stationary sail, velocity is zero, so power is zero. No work is
extracted.

Carnot requires two thermal baths of different temperatures for
kinetic energy to be gained.

Right. And Carnot says nothing whatsoever about requirements for
momentum to be extracted.

I'll piggy back here and add a comment because I came to realize what
is the source of some of the confusion present.

No, the second law (as Ed rightly pointed out, people here say
"Carnot" when what they really mean is the second law of
thermodynamics) does ***not*** say that you need two thermal baths of
different temperatures for kinetic energy to be gained.

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.

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

In article , "Steve Harris" writes:

wrote in message
...
In article , "Steve
Harris" For reasons that are obscure to me, there are
additional
limitations for using the energy in photons that come in
a
blackbody spectrum. They act like heat, and only some or
their energy is available for conversion into free energy
at
the target, and thence into rocket kinetic energy.

The limits are the same as in any other process. Entropy
cannot
decrease.


Yes, but I'm curious about the mechanism for why entropy
cannot decrease.

No "mechanism", just probabilities. And strictly speaking, it is not
"cannot decrease", just "is extremely unlikely to decrease".

Something about quantum states expanding in
phase-space, but can you put it into easier terms?

What you need here is a primer to statistical mechanics. That goes a
tad beyond a casual 1-2 paragraph note, some ot he concepts are quite
subtle. But no, you don't need to invoke QM, entropy was defined within
the classical framework to begin with (though it generalizes beautifully
to QM).

Entropy is (up to a multiplicative constant of no inherent
significance) the log of the number of microstates compatible with a
given macrostate. Now, given different macrostates realizable within
some given constraints (say, available energy), the probability of any
such macrostate is proportional to the number of microstates
compatible with it. So, higher entropy state is more probable.

Another take on this (for which you've to thank Shannon) is that
entropy of a system is proportional to the amount of missing
information about the system (i.e. of the information that would have
to be provided to characterize the system uniquely). Thus a fully
characterized system has zero entropy (no information missing).

Why (for
example) can you extract all the energy in a monochromatic
beam and use it to charge a battery (or make new atoms of
matter and antimatter if you must), but there are limits on
what fraction of a beam of blackbody radiation you can use.

For the same reason for which you can convert a perfect optical image
to a perfect digital recording of such, and vise versa, but you cannot
convert a blurred and grainy image into a perfect one. There is
missing information.

Don't just give me an entropy argument (you were ragging on
me for "explaining" wing forces on winds by momentum
conservation, remember?).

yep:-)

What's going on *mechanistically* that you can in theory
completely convert monochromatic EMR to any other kind of
energy you like, but not the mix of wavelengths of EMR that
comes off a black body?

It is not "machanistically". It is a matter of missing information.
You can shuffle an ordered deck, but it is highly improbable to get a
suffled deck back to an ordered one by continued shuffling.
For more than this, you'll have to consult textbooks.

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