Motie "Light Sail"

On Wed, 09 Jul 2003 13:47:16 GMT, Des Kavanagh
<"deskavanagh-AT("@)earthlink-DOT(.)net> wrote:

 >A recent article in New Scientist suggests that Light Sails
 >cannot be used to propel spacecraft. The argument is made that
 >such a system would contravene the First Law of Thermodynamics
 >and would constitute a perpetual motion machine of the first kind.
 >http://www.newscientist.com/news/news.jsp?id=ns99993895
 >A more detailed explanation from the original proponent of the theory:
 >http://www.arxiv.org/html/physics/0306050

I only checked out the New Scientist article, and it seems strange
to me. I thought that the mechanisms of solar sails were fairly well
documented, and the thermodynamic issues fairly straightforward.

There are hassles with them, of course. For one thing, if you're
talking solar wind, that is mostly particles, not photons. Particle
hits sail and sticks, kinetic energy transfer is obvious, method works
until the velocity of the sail exceeds the particles (not going to
happen within a solar system).

Photons have similar issues. Reflection is an energy transfer
process. Some energy is lost in near perfect reflection, but the
vehicle itself is unchanged. Allowing most thermal radition to
release in one direction rather than the other also helps account for
any losses due to inefficiency.

Since the "fuel" is free, it doesn't matter that the solar sail is
an inefficient energy transfer mechanism. The Sun will put out solar
wind for a long time.

 >NASA are currently developing Light Sail propelled vehicles.

Are they going to do a test one in space soon?


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 >> Stay informed about: Motie "Light Sail" 

Erik Max Francis wrote:
 >
 > Des Kavanagh wrote:
 >
  > > A recent article in New Scientist suggests that Light Sails
  > > cannot be used to propel spacecraft. The argument is made that
  > > such a system would contravene the First Law of Thermodynamics
  > > and would constitute a perpetual motion machine of the first kind.
<font color=green>  > > <a style='text-decoration: underline;' href="http://www.newscientist.com/news/news.jsp?id=ns99993895</font" target="_blank">http://www.newscientist.com/news/news.jsp?id=ns99993895</font</a>>
  > > A more detailed explanation from the original proponent of the theory:
<font color=green>  > > <a style='text-decoration: underline;' href="http://www.arxiv.org/html/physics/0306050</font" target="_blank">http://www.arxiv.org/html/physics/0306050</font</a>>
 >
 > It's a pretty bizarre stance, since light sails operate by the momentum
 > contained in photons, not their use in heat engines. I'm surprised New
 > Scientist even printed it.
 >
I deliberately referred to the 1st Law rather than Carnot's
earlier and limited expression of it. 1st Law is basically
conservation of Mass/Energy.

In the Light Sail, a photon strikes a mirror, at optimum efficiency
at normal incidence (not that that matters, but for simplicity)
and is reflected. Photon does not change frequency, no energy loss.

How can the Sail gain kinetic energy?<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

Des Kavanagh wrote:

 > In the Light Sail, a photon strikes a mirror, at optimum efficiency
 > at normal incidence (not that that matters, but for simplicity)
 > and is reflected. Photon does not change frequency, no energy loss.
 >
 > How can the Sail gain kinetic energy?

Throw an elastic ball at an object. The ball comes off the object with
the same speed it hit it with, and the object recoils. Where did that
kinetic energy come from?

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<< It's a pretty bizarre stance, since light sails operate by the momentum
contained in photons, not their use in heat engines. I'm surprised New
Scientist even printed it. >><BR><BR>


Intuition lies, but mine tells me that such a solar sail would never even get
the first puff of solar wind within them. I may side with the New Scientist
until there is a working prototype.
 >> Stay informed about: Motie "Light Sail" 

GrapeApe wrote:

 > Intuition lies, but mine tells me that such a solar sail would never
 > even get
 > the first puff of solar wind within them. I may side with the New
 > Scientist
 > until there is a working prototype.

Solar sails operate from photon pressure, not the solar wind. (Ones
that operate off the charged particles in the solar wind are usually
called magnetic sails, not solar sails.) The sails themselves are kept
oriented and aligned by struts or by spin, not by photon pressure.

You're using your intuition from normal sails to a situation where it
simply doesn't apply.

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 >> Stay informed about: Motie "Light Sail" 

"Erik Max Francis" <max.RemoveThis@alcyone.com> wrote:
 > GrapeApe wrote:
 >
  > > Intuition lies, but mine tells me that such a solar sail would never
  > > even get
  > > the first puff of solar wind within them. I may side with the New
  > > Scientist
  > > until there is a working prototype.
 >
 > Solar sails operate from photon pressure, not the solar wind. (Ones
 > that operate off the charged particles in the solar wind are usually
 > called magnetic sails, not solar sails.) The sails themselves are kept
 > oriented and aligned by struts or by spin, not by photon pressure.
 >
 > You're using your intuition from normal sails to a situation where it
 > simply doesn't apply.

This issue has been pounded into a kajillion pieces, each so
thin they would make decent light sail material, in the
sci.space.* newsgroups.

Quick summary: Mr. Gold's article in the New Scientist if
full of shit. I say that not solely out of derision but
because the article is so grossly in error that it is far
beyond the pale of legitimate scientific debate. Really,
it's not as if photon pressure is some theoretical
concept, it's been detected, measured, and used before
many times. And the physics details involved are so
rudimentary that it's hard to imagine any sizeable number
of legitimate experts in physics or related subjects
failing to understand them.


One quibble about the Motie sail though, I believe it did use
the solar wind to deccelerate, but that's rather a different
scenario.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

Erik Max Francis wrote:
 > GrapeApe wrote:
 >
 >
  >>Intuition lies, but mine tells me that such a solar sail would never
  >>even get
  >>the first puff of solar wind within them. I may side with the New
  >>Scientist
  >>until there is a working prototype.

The New Scientist article used the spinning of a Crookes Radiometer as
evidence of Gold's claim. The devices down here aren't really in a
vacuum. The thin air on the dark side expands from heat and pushes the
radiometer. I'm told if the device were opened to the vacuum of outer
space light pressure would dominate and the wheel would spin the other way.

Henry Spencer has cited the example of Radarsat 1, which has a
sun-synchronous orbit. Continuous sunlight pushes it's solar arrays and
radar antenna towards the earth. 2/3 of its station keeping fuel needs
to be used to counteract this push, an effect its designers hadn't
expected. I believe acceleration due to light pressure is already well
supported with empirical evidence.


Hop
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 >> Stay informed about: Motie "Light Sail" 

Des Kavanagh wrote:

   >>> A recent article in New Scientist suggests that Light Sails
   >>> cannot be used to propel spacecraft.

Yes, and I'm with Erik - I still don't understand why the New
Scientist printed this. Reading Gold's original stuff (the 2nd of your
links?), he makes some very very basic errors. As Erik mentions, the
Crookes radiometer is well understood, even at an undergraduate level.
Gold's out in left field.

 > In the Light Sail, a photon strikes a mirror, at optimum efficiency
 > at normal incidence (not that that matters, but for simplicity)
 > and is reflected. Photon does not change frequency, no energy loss.

Photon does not change frequency? Think Doppler shift. Forward (as
well as many othes) got this a long time ago in "Flight of the
Dragonfly".

 > How can the Sail gain kinetic energy?

Because the photon looses kinetic energy.

Heck, the biggest argument against Gold's position is to try the
experiment. The early Mariner spacecraft use solar "sails" for
steering, and they required taking both the solar wind and light
pressure into acccount.

--
Brian Davis<!-- ~MESSAGE_AFTER~ -->
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Erik Max Francis wrote:

 > Throw an elastic ball at an object. The ball comes off the object with
 > the same speed it hit it with, and the object recoils. Where did that
 > kinetic energy come from?

Actually, it *can't* come off with the same speed it impacted with
(not if the impacted object gains any momentum). Conservation of
energy & momentum implies it bounces off slightly slower (lower speed
for a ball, lower momentum and thus frequency for a photon). Slightly
misleading analogy unless you're careful.

--
Brian Davis<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

Erik Max Francis wrote:
 >
 > Des Kavanagh wrote:
 >
  > > In the Light Sail, a photon strikes a mirror, at optimum efficiency
  > > at normal incidence (not that that matters, but for simplicity)
  > > and is reflected. Photon does not change frequency, no energy loss.
  > >
  > > How can the Sail gain kinetic energy?
 >
 > Throw an elastic ball at an object. The ball comes off the object with
 > the same speed it hit it with, and the object recoils. Where did that
 > kinetic energy come from?
 >
Consider two identical perfectly elastic billiard balls, one stationary
and the other one moving toward it on centerline. the moving one does
not
recoil after the collision, it stops dead and the stationary one moves
away
from it at the same velocity that the "cue" ball had had. Kinetic energy
and
momentum are conserved.

What I don't understand about the Light Sail (I'm NOT saying it can't
work)
is that the photons are reflected with no change in frequency, hence no
loss
of total energy, and yet impart energy to the sail. Where does it come
from?<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

Thanks, didn't think of the Doppler shift.
Restored my faith


Brian Davis wrote:
 >
 > Des Kavanagh wrote:
 >
   > >>> A recent article in New Scientist suggests that Light Sails
   > >>> cannot be used to propel spacecraft.
 >
 > Yes, and I'm with Erik - I still don't understand why the New
 > Scientist printed this. Reading Gold's original stuff (the 2nd of your
 > links?), he makes some very very basic errors. As Erik mentions, the
 > Crookes radiometer is well understood, even at an undergraduate level.
 > Gold's out in left field.
 >
  > > In the Light Sail, a photon strikes a mirror, at optimum efficiency
  > > at normal incidence (not that that matters, but for simplicity)
  > > and is reflected. Photon does not change frequency, no energy loss.
 >
 > Photon does not change frequency? Think Doppler shift. Forward (as
 > well as many othes) got this a long time ago in "Flight of the
 > Dragonfly".
 >
  > > How can the Sail gain kinetic energy?
 >
 > Because the photon looses kinetic energy.
 >
 > Heck, the biggest argument against Gold's position is to try the
 > experiment. The early Mariner spacecraft use solar "sails" for
 > steering, and they required taking both the solar wind and light
 > pressure into acccount.
 >
 > --
 > Brian Davis<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

Des Kavanagh <"deskavanagh-AT("@)earthlink-dot(.)net> wrote:
 > What I don't understand about the Light Sail (I'm NOT saying it can't
 > work) is that the photons are reflected with no change in frequency, hence no
 > loss of total energy, and yet impart energy to the sail. Where does it come
 > from?

Not ALL of the photons are reflected _unchanged_.
Some of them are absorbed (giving up all of their energy), others are
send out again at a lower energy (frequency). This latter is called
Raman refraction (and I did my experimental stage of my physics study
on it, ok, it didn't have anyting to do with light-sails).
Both kinds transfer energy TO the sail.

PS: the energy that isn't used for kinetic change is sent out again
later, mostly as infra-red (low energy) photons.
--
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 >> Stay informed about: Motie "Light Sail" 

Des Kavanagh wrote:

 > Erik Max Francis wrote:
  > >
  > > Des Kavanagh wrote:
  > >
   > > > In the Light Sail, a photon strikes a mirror, at optimum efficiency
   > > > at normal incidence (not that that matters, but for simplicity)
   > > > and is reflected. Photon does not change frequency, no energy loss.
   > > >
   > > > How can the Sail gain kinetic energy?
  > >
  > > Throw an elastic ball at an object. The ball comes off the object with
  > > the same speed it hit it with, and the object recoils. Where did that
  > > kinetic energy come from?
  > >
 > Consider two identical perfectly elastic billiard balls, one stationary
 > and the other one moving toward it on centerline. the moving one does
 > not recoil after the collision, it stops dead and the stationary one
 > moves away from it at the same velocity that the "cue" ball had had.
 > Kinetic energy and momentum are conserved.
 >
 > What I don't understand about the Light Sail (I'm NOT saying it can't
 > work) is that the photons are reflected with no change in frequency,
 > hence no loss of total energy, and yet impart energy to the sail. Where
 > does it come from?

Look at the collision in the frame of reference in which the center
of mass-energy is at rest. The photon comes in from the left at c,
the sail comes from the right at velocity -v; v <<<< c. They collide.
The photon goes back to the left at -c and the sail goes off at v.
Kinetic energy and momentum conserved -- check.

Now you can do the Lorentz transform into any other reference frame,
e.g. the one in which the sail's initial speed v' = 0. The sail's
increased kinetic energy and momentum must be balanced by the photon's
loss. You can calculate the increase in the photon's wavelength.

--
Bill Woods

Here's a thought: When the World Bank is finished with Chad,
it should come to California, whose public finances these days
resemble those of a Third World corruption pit more than those
of a modern, presumably enlightened, industrial society.

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