Motie "Light Sail"

What about the dust and gas particles in a comets tail? They are
certainly undergoing trajectory modification as the comets path brings
it near the sun.
Bill
 >> Stay informed about: Motie "Light Sail" 

If a solar sail has no merit, how is it that a comet's tail is pushed away from
he sun?
Hey, Mr. Taliban, tally me munitions.
Here they come, and they gonna go boom
 >> Stay informed about: Motie "Light Sail" 

Barlow2 wrote:

 > If a solar sail has no merit, how is it that a comet's tail is pushed
 > away from
 > he sun?

Actually, comets have two tails, and neither are from photon pressure.
One is from the solar wind, and the second is ions redirected due to the
Sun's magnetic field. (Actually, there is a third tail, a neutral
sodium tail which was discovered with Hale-Bopp, but it is not generated
from photon pressure, either.)

--
Erik Max Francis && max DeleteThis @alcyone.com && <a style='text-decoration: underline;' href="http://www.alcyone.com/max/" target="_blank">http://www.alcyone.com/max/</a>
__ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE
/ \ There is nothing wrong with sobriety in moderation.
\__/ John Ciardi<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

John David Galt wrote:

 > The photon's energy of motion is a vector quantity.

Energy is a scalar. In relativity there's a combined
entity called '4-momentum', but that's not energy either.

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

In article <%I2Pa.92748$Io.8003424@newsread2.prod.itd.earthlink.net>,
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.

Wrong premise.

When a photon strikes a moving mirror, the photon changes frequency due
to redshift/blueshift--there is energy gain or loss depending on
whether the mirror is moving toward or away from the light source,
which is balanced by the energy loss or gain of the light sail as it is
slowed down or sped up.

--
David M. Palmer dmpalmer DeleteThis @email.com (formerly @clark.net, @ematic.com)<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

On Wed, 09 Jul 2003 22:25:06 -0700, Hop David
<hopspage.DeleteThis@tabletoptelephone.com> wrote:

 >
 >
 > 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
<font color=purple> > <a style='text-decoration: underline;' href="http://clowder.net/hop/index.html</font" target="_blank">http://clowder.net/hop/index.html</font</a>>
 >
 >

If you pump them down far enough they do spin the other way. & bizarrely
IIRC Jeffrey Lewins had both on his study window sill.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

In article <3F10ECA0.922C5FEF.TakeThisOut@diogenes.sacramento.ca.us>,
John David Galt <jdg.TakeThisOut@diogenes.sacramento.ca.us> 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?
 >
 >The photon's energy of motion is a vector quantity. Reflection imparts
 >velocity to the sail equal to the difference between the photon's old
 >and new kinetic-energy vectors. The fact that they are the same size
 >does not mean that the (vector) difference between them is zero.

In the limit of an infinitely massive lightsail, the photon would be reflected
back at the same frequency, and the same "kinetic energy", as before, but
with the momentum reversed. The lightsail would absorb the moemntum
change but not (because its mass is infinite) acquired any kinetic energy.
A real lightsail would be *nearly* infinite in mass compared to a single
photon, so a "real" lightsail would reflect photons back at a frequency
*immeasurably* lower than the original, with the *almost exactly* the
same momentum but in a different direction. The amount of KE the sail
picks up from a photon is so small and so difficult to calculate that
it is much, much easier to figure the momentum transfer (2 times h-bar
times k, with an angle-of-reflection factor thrown in and compensating
for doppler shift as the sail picks up speed) and calculate the kinetic
energy transfer from *that*.

This isn't just a photon thing. In any elastic collision between something
very massive and something tiny, almost all the kinetic energy stays with
the light object.

--
pciszek at TheWorld dot com | "Evolution is a theory that accounts
| for variety, not superiority."
| -- Joan Pontius<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

"David M. Palmer" <dmpalmer.RemoveThis@email.com> wrote in news:130720031109476604%
dmpalmer.RemoveThis@email.com:

 > In article <%I2Pa.92748$Io.8003424@newsread2.prod.itd.earthlink.net>,
 > 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.
 >
 > Wrong premise.
 >
 > When a photon strikes a moving mirror, the photon changes frequency due
 > to redshift/blueshift--there is energy gain or loss depending on
 > whether the mirror is moving toward or away from the light source,
 > which is balanced by the energy loss or gain of the light sail as it is
 > slowed down or sped up.
 >

What if the sail is (currently) at rest WRT the light source (or in a
perfect circular orbit)?

--
Ross Presser -- rpresser AT imtek DOT com
"... VB is essentially the modern equivalent of vulgar Latin in 13th
Centurary Europe. Understand it, and you can travel to places you never
heard of and still understand some people." -- Alex K. Angelopoulos<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

"Ross Presser" <rpresser DeleteThis @NOSPAM.imtek.com.invalid> wrote in message
news:Xns93B89EBADC2DCpt101594@129.250.170.93...

 > What if the sail is (currently) at rest WRT the light source (or in a
 > perfect circular orbit)?

If the light sail is stationary, light pressure exerts a force on the light
sail - but the force does no work. work=force*distance
(power=force*velocity), and the sail isn't moving, so the applied force is
doing no work and generates no power.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

[This was originally a response to a post in abl-n. Although
cross-posted, for some reason it didn't show up here in rasfs.]

Mike Williams wrote:

 > Wasn't it pervect who wrote:
  > >
  > >"Ross Presser" <rpresser.DeleteThis@NOSPAM.imtek.com.invalid> wrote in message
  > >news:Xns93B89EBADC2DCpt101594@129.250.170.93...
  > >
   > >> What if the sail is (currently) at rest WRT the light source (or in a
   > >> perfect circular orbit)?
  > >
  > >If the light sail is stationary, light pressure exerts a force on the light
  > >sail - but the force does no work. work=force*distance
  > >(power=force*velocity), and the sail isn't moving, so the applied force is
  > >doing no work and generates no power.
 >
 > Doesn't that end up with you getting into something like a "Zeno's
 > paradox" situation? If the force does no work, then it doesn't start the
 > sail moving, so light pressure can never move a stationary sail.
 >
 > To go even further, if we start with a sail moving at a constant
 > velocity, we should be able to perform the energy calculation in an
 > inertial frame that's moving with the sail and get the same result. So
 > light pressure would never accelerate or decelerate a moving sail.

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.

<a style='text-decoration: underline;' href="http://www.sacbee.com/content/politics/columns/walters/story/6944770p-7894047c.html" target="_blank">http://www.sacbee.com/content/politics/columns/walters/story/6944770p-...4047c.h</a><!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

Mike Williams wrote:

 > I don't think so. Can you apply calculus to a single photon?
 >
 > If the energy of the photon doesn't change (see earlier in the thread,
 > not quoted here) then the energy of the sail doesn't change, and it
 > doesn't move.

The fact that the momentum and kinetic energy of the sail are different
in different frames (which in every other frame would be nonzero and
thus the sail would move) is a good example that something's wrong with
your thinking.

If in all other frames the sail accelerates but the precise one you've
chosen to look at it doesn't, and thus you conclude that it can't
accelerate at all, that's a good indication that you're looking at
things too literally.

--
Erik Max Francis && max.DeleteThis@alcyone.com && <a style='text-decoration: underline;' href="http://www.alcyone.com/max/" target="_blank">http://www.alcyone.com/max/</a>
__ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE
/ \ You win the victory when you yield to friends.
\__/ Sophocles<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

Wasn't it pervect who wrote:
 >
 >"Ross Presser" <rpresser.RemoveThis@NOSPAM.imtek.com.invalid> wrote in message
 >news:Xns93B89EBADC2DCpt101594@129.250.170.93...
 >
  >> What if the sail is (currently) at rest WRT the light source (or in a
  >> perfect circular orbit)?
 >
 >If the light sail is stationary, light pressure exerts a force on the light
 >sail - but the force does no work. work=force*distance
 >(power=force*velocity), and the sail isn't moving, so the applied force is
 >doing no work and generates no power.

Doesn't that end up with you getting into something like a "Zeno's
paradox" situation? If the force does no work, then it doesn't start the
sail moving, so light pressure can never move a stationary sail.

To go even further, if we start with a sail moving at a constant
velocity, we should be able to perform the energy calculation in an
inertial frame that's moving with the sail and get the same result. So
light pressure would never accelerate or decelerate a moving sail.

--
Mike Williams
Gentleman of Leisure<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Motie "Light Sail" 

In article <pRZWKAAKV3E$Ewwe@econym.demon.co.uk>,
Mike Williams <mike.DeleteThis@econym.demon.co.uk> wrote:

> Doesn't that end up with you getting into something like a "Zeno's
> paradox" situation? If the force does no work, then it doesn't start the
> sail moving, so light pressure can never move a stationary sail.

Like Zeno's Paradox, this "problem" is easily resolved through the
appropriate application of calculus.
 >> Stay informed about: Motie "Light Sail"