New Scientist Article on Solar Sails

(Gordon D. Pusch) wrote in message ...
(JimM) writes (in response to "Geoffrey A. Landis"
):

The energy transfer to a solar sail can be accounted for from the
Doppler shift of reflected photons; even when the reflectivity is 100%,
a photon looses energy when reflecting from a moving sail. This effect
exactly corresponds to the energy increase of the sail. No sophisticated
physics is needed to analyze this effect, it is a problem suitable
for a homework assignment for a college undergraduate.

I had not thought of it exaxtly that way when I saw the item in New
Scientist.

But is seemed to me that the assumption of a 100% effective mirror
could not be correct, so that some light striking the mirror must turn
to heat. Thus the 2nd Law of Thermodynalics is preserved (I see the
2nd law as being the key to proper understanding of this) and so the
light sail functions.

Nope, sorry --- it's a simple matter of Newtoniam mechnaics and
conservation of momentum: If the photon reflects off the sail,
it's momentum changes, and the sail must therefore experience an equal
and opposite change of momentum in recoil from the photon's reflection.

The 2nd Law and thermodynamics plays no part in the simple Newtonian recoil
of the sail, and indeed absorbtion would actually _decrease_ the sail's
performance, since if the photon is absorbed, it only transfers =HALF=
as much momentum to the sail as if it is reflected back in the opposite
direction from whence it came --- and when the heat energy is re-radiated
as infrared photons, they are emitted in a RANDOM direction, resulting in
no _AVERAGE_ change in the sail's momentum --- so absorbing a photon is a
_LOSS_, not a gain.

The 2nd Law of Thermodynamics is in many ways the most fundamental law
of physics--indeed, of all science. No physical process can be
properly described without reference to it. If you do not know this
you are not much of a physist and should consider some other line of
work.

The 2nd Law says, among other things, that no machine (more generally,
no time-wise phyisical process) can ever be 100% efficient. A solar
sail is a machine. Ergo a solar sail (or indeed any mirror) cannot be
100% efficient.

The sail is "recoiled" * from sunlight, but the process is not 100%
efficient, and the light energy lost takes the form of heat. If
efficiency were too low, the sail would melt or evaporate.

In the light of above your description of what happens is nonsense. Go
back to Physics 101.

JimM

* As you quaintly put it. Where I come from I'd certainly talk in
terms of Newton's First Law of Motion -- "action and reaction are
equal and opposite" -- but that would only translate to a 100%
effective process in the case of entirely inelastic materials, which
is a theoretical construct that can never exist in the real world,
like absolute zero for example.

(JimM) writes:

Ian Stirling wrote in message ...

2nd law isn't really very important.
Reflection can easily be above 95%, so that'd be over 97.5% of the
thrust without absorbtion.

The 2nd Law is always important. It's more important that Relativity
and Quantum Mechanics.

The 2nd Law of Thermo is important, but it is =NOT= as important
as Newton's Three Laws.

Newton's Three Laws tell you DETERMINISTICALLY what a system sufficiently
simple to be govern by the =WILL= do --- and reflection of light off a
plane mirror definitely falls into that category! One needs only apply
Conservation of Momentum: The simple fact that every action has an equal
and opposite reaction.

By contrast, the Three Laws of Thermodynamics are =NOT= predictive,
but rather are mere CONSTRAINTS. They tell you what a system that is
so hopelessly complex that one has no hope in hell of ever fully analyzing
or controlling =CAN'T= do, when the best one can hope to do is merely draw
a "black box" boundary around it and balance the mean fluxes in and out,
which no reall clue of what those fluxes do while they are "inside the box."
While the constraints of Thermo are important and useful, there is _NO WAY_
that they are =MORE= important than Netwon's Three Laws of Motion.
Newton's Laws are the Mother of =PHYSICS= --- the Queen of All the Sciences.
Thermodynamics is mere Engineering --- an occupation for second-class grunts. :-)


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

[edit]



Thus the 2nd Law of Thermodynalics is preserved (I see the



2nd law as being the key to proper understanding of this) and so the



light sail functions.







Nope, sorry --- it's a simple matter of Newtoniam mechnaics and



conservation of momentum: If the photon reflects off the sail,



it's momentum changes, and the sail must therefore experience an equal



and opposite change of momentum in recoil from the photon's reflection.







The 2nd Law and thermodynamics plays no part in the simple Newtonian recoil



of the sail, and indeed absorbtion would actually _decrease_ the sail's



performance, since if the photon is absorbed, it only transfers =HALF=



as much momentum to the sail as if it is reflected back in the opposite



direction from whence it came --- and when the heat energy is re-radiated



as infrared photons, they are emitted in a RANDOM direction, resulting in



no _AVERAGE_ change in the sail's momentum --- so absorbing a photon is a



_LOSS_, not a gain.







The 2nd Law of Thermodynamics is in many ways the most fundamental law



of physics--indeed, of all science. No physical process can be



properly described without reference to it. If you do not know this



you are not much of a physist and should consider some other line of



work.



[edit]





I would recommend that perhaps you hold of a few "introduction to
physics" texts books and read up on the subject of physics, which you
obviously haven't done yet. The 2nd law of thermo..(2LTD) can be
applied to this problem if you really want, but its not very
informative.



But the subject of the solar sail has been (well still is) discussed
in other sci.space.* NG. No need to add my bit.



But if you really want to extend your understanding of the 2LTD, i
recommend the following:
http://www.phy.auckland.ac.nz/Staff/smt/453320FC.html



Greg

The 2nd law of thermo..(2LTD) can be
applied to this problem if you really want, but its not very
informative.

Of course it is. It is central. Newton's Laws of Motion are a given
here. The REAL (but unstated as such) question is, how efficient is a
solar sail as a machine? IOW, "Will a solar sail work, and if so, how
well?"

Glory be, we are right back at the 2nd Law!

I don't nead a lesson in the basics of Thermodynamics thank you very
much, but perhaps you do on why it is so central to all time-wise
processes and hence why it is the key to the question we are really
considering here.

For what it's worth, I'm sure it will work but will not be as
effective as some people believe--particularly those who mistakenly
suppose that the 2nd Law does not apply in this case and hence a
mirror can be 100% efficient.

JimM

By contrast, the Three Laws of Thermodynamics are =NOT= predictive,
but rather are mere CONSTRAINTS. They tell you what a system that is
so hopelessly complex that one has no hope in hell of ever fully analyzing
or controlling =CAN'T= do, when the best one can hope to do is merely draw
a "black box" boundary around it and balance the mean fluxes in and out,
which no reall clue of what those fluxes do while they are "inside the box."

Yes, thermodynamics is statistical in nature. If you think that means
it is not predictive, you clearly don't really undertand the first
thing about modern physics.

While the constraints of Thermo are important and useful, there is _NO WAY_
that they are =MORE= important than Netwon's Three Laws of Motion.
Newton's Laws are the Mother of =PHYSICS= --- the Queen of All the Sciences.

Nope. Strictly, Newton's Laws are actually incorrect but remain
convenient for approximate answers. Relativity and Quantum Machanics
(QM) deliver much more precise answers, and QM, for your information,
tells us that the entire universe is a 'statistical' place, as was
'predicted' (not really, but the trail was laid) by thermodynamics in
the 19th Century. It was the 2nd Law that lead to the realisation that
time had to be seen as a dimension, and it is the 2nd Law that makes
possible our undertanding of the transition from the very small
(qantum) world where time appear directionless to the macro world we
live in where it does have direction. Some theoroticians even believe
that our sense of the direction of time is in itself a thermodynamic
process, and temporal direction ('time's arrow') was determined at the
Big Bang itself.

But of course you can't be expected to comprehend this, as you are
seeming still stuck in the 17th Century...

Newton's Laws are not the 'Mother of =PHYSICS=' any more, but are very
handy for mere Engineering --- an occupation for second-class grunts
(as you so eloquently put it yourself.)

What happens when light strikes a mirror is actually a QM process.
We've given up on these tiny little marbles of Newtonian physics some
time back, you know.

JimM

(JimM) wrote in message om...

snip

The 2nd Law of Thermodynamics is in many ways the most fundamental law
of physics--indeed, of all science. No physical process can be
properly described without reference to it. If you do not know this
you are not much of a physist and should consider some other line of
work.

The 2nd Law says, among other things, that no machine (more generally,
no time-wise phyisical process) can ever be 100% efficient. A solar
sail is a machine. Ergo a solar sail (or indeed any mirror) cannot be
100% efficient.

The sail is "recoiled" * from sunlight, but the process is not 100%
efficient, and the light energy lost takes the form of heat. If
efficiency were too low, the sail would melt or evaporate.

In the light of above your description of what happens is nonsense. Go
back to Physics 101.

snip

Ok, here's a couple of problems I see. First, how can you describe all
physical observations in terms of the Second Law of Thermodynamics?
Ie, how does entropy manifest? Where do we get four dimensional space
from and other things that we observe in the real world?

Second, reviewing your comments, you seem to be implying that if one
were to construct a perfect reflecting surface, then it would not work
like an almost perfectly reflecting surface because entropy was
conserved. No momentum would be transfered to a perfect reflecting
surface while almost all of the momentum would be transfered to a
almost perfectly reflecting surface. But wouldn't it make vastly more
sense to treat the "perfect reflecting surface" as what it is. A model
which is the limit of constructable objects that do obey the Second
Law?


Karl Hallowell

JimM wrote:

The REAL (but unstated as such) question is, how efficient is a
solar sail as a machine?

Note that that is not the main thrust of Gold's abstract - he
maintains a number of rather ludicrous things, like mis-understanding
how a radiometer works, and "proving" that a reflective surface is
*less* "efficient" and providing thrust than an absorbing one.

Glory be, we are right back at the 2nd Law!

You may be, but I suspect we should define what "efficiency" we're
discussing. If you wish to analyse a solar sail as a heat engine, you
are welcome to. Doing a better job than Gold shouldn't be an issue
(for one thing, Gold doesn't seem to understand what the hot reservoir
and cold reservoir are in the problem - it's *not* the sail material,
and a 3K heat sink can make things rather nicely efficient, in a
Carnot sense).

particularly those who mistakenly suppose that the 2nd Law does
not apply in this case and hence a mirror can be 100% efficient.

I've yet to see anyone say that the 2nd law doesn't apply - mearly
that the 2nd law is not close to the easiest way to evaluate this mans
claims. And if you don't understand the difference betweeen a 100%
"efficient" (reflective) mirror, and a 100% "efficient" heat engine,
than you really need to carefully define your terms.

--
Brian Davis