Solar sails - impossible?

Robert Schneider wrote:

I think that he is probably right. If reflected photons still have the same
momentum as incident photons, then a perfect mirror won't be able to gain
any momentum.

Since momentum is a *vector* quantity, they clearly do not have the same
momentum before and after reflection.

Paul

"Robert Schneider" writes:

"Stanislaw Sidor" wrote in message
...
What do you think about the Thomas Gold's article:

arXivhysics/0306050 v1 5 Jun 2003
"The solar sail and the mirror"
Thomas Gold (Center for Radiophysics and Space Research, Cornell
University)

http://www.arxiv.org/html/physics/0306050

Is he right or not?

I think that he is probably right. If reflected photons still have the
same momentum as incident photons, then a perfect mirror won't be able to
gain any momentum.

No, he and you are both very, very wrong. Momentum is a =VECTOR= quantity:
It has both a MAGNITUDE and a DIRECTION. When a photon reflects off a solar
sail, its =DIRECTION= will change, even if its magnitude does not; hence,
the momentum of the photon =WILL= be changed by the reflection, and the
sail must recoil with an equal and opposite change in momentum in order to
conserve momentum.


However, I don't think that his ideas dismiss solar sailing entirely.
A sail that absorbs energy and radiates it preferentially in specific
direction might work.

Please think =VERY= carefully about what difference there is between
"reflection of a photon in a specific direction" and "absorbing energy
and radiating it preferentially in a specific direction." If you do,
you will find that the primary difference is that the later is less efficient...


-- Gordon D. Pusch

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

(Gordon D. Pusch) wrote:

Gold fails to recognize that light
reflected by a sail moving away from a light source will be doppler-shifted
to lower frequencies

Well.... yes... but....

There's no such thing as an absolute system of rest that determines
whether something is moving or stationary.

The sail doesn't care whether it's being hit by a 5000 Angstrom
wavelength photon or by a photon that would have been 5001 Angstroms
but is red-shifted to 5000 Angstroms because of relative motion
between the source of the photon and the sail.

If you can have a force (which produces acceleration) on a "moving"
mirror it must also produce a force on a "stationary" mirror.

You must be able to obtain (i.e. calculate) the same force in any
intertial system as you can directly measure the acceleration of the
sail with sufficienly sensitive accelerometers attached to the sail.

I quickly did this calculation for a system where a mirror is
initially at zero velocity. Apologies for any slips in this - it must
be available elsewhere as well.

Start with a photon of frequency "f1" and a sail of mass m.
After the reflection the photon has a frequency f2 (approximately =
f1) and
the sail has a recoil velocity v.

For the photon momentum p = hf/c, energy E = hf
For the sail momentum = mv, energy = 1/2mv^2

The photon experiences a change in momentum of
delta p = (f1 + f2)h/c

The momentum change of the sail is
mv

From conservation of momentum:
v = (f1 + f2)h/(cm) 1)

The energy change of the photon is:
h(f1 - f2)

The energy change of the sail is:
1/2 mv^2

From conservation of energy:
h(f1 - f2) = 1/2mv^2

Substitute for v from 1) above -
h(f1 - f2) = 1/2 m((f1 + f2)h/(cm))^2

rearrange:
f1 - f2 = (m/2)h(f1 + f2)^2 h / (c^2 m^2)

make the approximation f1 ~ f2 and rearrange a bit:
delta f = f1 - f2 = 2hf/(c^2 m)
i.e.
(delta f)/ f = 2hf/(c^2 m) 2)

This frequency shift (in the frame in which the sail is initially
at rest) is just the same as the Doppler shift induced by the small
change
in velocity of the sail.

i.e. equation 1) above said that
v = 2hf/(cm)

The Doppler shift is:
(delta f) c / f = v

substituting for v and rearranging again gives:
(delta f) / f = 2hf(c^2 m)
as in equation 2)

Newsuser "Gordon D. Pusch" wrote: ...

He is about as wrong as it is possible to be, about both solar sails and
Crookes Radiometers.

Who has reviewed Golds' article? Why such a rubbish has been published in
NewScientist?

(STS)

Here, Gordon D. Pusch wrote:
"Robert Schneider" writes:

"Stanislaw Sidor" wrote in message
...
What do you think about the Thomas Gold's article:

arXivhysics/0306050 v1 5 Jun 2003
"The solar sail and the mirror"
Thomas Gold (Center for Radiophysics and Space Research, Cornell
University)

http://www.arxiv.org/html/physics/0306050

Is he right or not?

I think that he is probably right. If reflected photons still have the
same momentum as incident photons, then a perfect mirror won't be able to
gain any momentum.

No, he and you are both very, very wrong. Momentum is a =VECTOR= quantity:
It has both a MAGNITUDE and a DIRECTION.

Gold wasn't making a conservation-of-momentum argument. He was making
a conservation-of-energy argument.

He was wrong, but you haven't addressed his mistake. The energy of a
photon is not a vector quantity.

--Z

"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the borogoves..."
*
* Make your vote count. Get your vote counted.

"Stanislaw Sidor" writes:

Newsuser "Gordon D. Pusch" wrote: ...

He is about as wrong as it is possible to be, about both solar sails and
Crookes Radiometers.

Who has reviewed Golds' article? Why such a rubbish has been published in
NewScientist?

The New Scientist is =NOT= a peer-reviewed scientific journal --- it is a
media-owned "popular science" tabloid. It is written and edited by JOURNALISTS,
not scientists --- and journalists, even "science journalists," frequenctly
do not know squat about science.

Likewise for how Gold's paper got into arXiv: arXiv is =NOT= peer-reviewed;
ANYONE can deposit a paper into it, and some of what gets dumped into it is
utter crap written by giant raving looney crackpots.


-- Gordon D. Pusch

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

In article , Stanislaw Sidor
wrote:

Newsuser "Gordon D. Pusch" wrote: ...

He is about as wrong as it is possible to be, about both solar sails and
Crookes Radiometers.

Who has reviewed Golds' article? Why such a rubbish has been published in
NewScientist?

No one refereed Gold's paper, it was published on the e-print server,
which has no gatekeeper.
http://www.arxiv.org/html/physics/0306050

NewScientist is a newsmagazine, rather than a scientific journal, and
is a bit credulous.

--
David M. Palmer (formerly @clark.net, @ematic.com)

My previous posting had a couple of typos:

wrote:
The sail doesn't care whether it's being hit by a 5000 Angstrom
wavelength photon or by a photon that would have been 5001 Angstroms
but is red-shifted to 5000 Angstroms because of relative motion

blue-shifted

substituting for v and rearranging again gives:
(delta f) / f = 2hf(c^2 m)


(delta f) / f = 2hf / (c^2 m)

In news:jjustwwondering typed:
(Henry Spencer) wrote in message
...
In article ,
Stanislaw Sidor wrote:
What do you think about the Thomas Gold's article...

It has already been torn to shreds very thoroughly on some of the
other
sci.space.* newsgroups. Gold simply doesn't know what he's talking
about;
he's overlooked or misunderstood Doppler shift. Light pressure has
been demonstrated many times in the lab, and must be allowed for in
deep-space
spacecraft navigation and comsat attitude control. Its existence is
beyond question.

A comet tail is a solar sail...

Actually a wake as the tail has no effect on the comet head.
--
Mike
__________________________________________________ ______
"Colorado Ski Country, USA" Come often, Ski hard,
Spend *lots* of money, Then leave as quickly as you can.

"Mike Speegle" wrote in message ...
In news:jjustwwondering typed:
(Henry Spencer) wrote in message
...
In article ,
Stanislaw Sidor wrote:
What do you think about the Thomas Gold's article...

It has already been torn to shreds very thoroughly on some of the
other
sci.space.* newsgroups. Gold simply doesn't know what he's talking
about;
he's overlooked or misunderstood Doppler shift. Light pressure has
been demonstrated many times in the lab, and must be allowed for in
deep-space
spacecraft navigation and comsat attitude control. Its existence is
beyond question.

A comet tail is a solar sail...

Actually a wake as the tail has no effect on the comet head.

Nothing like a wake: the tail does not spread in the direction
opposite to the comet's movement - but in the direction opposite
to the sun - i.e., the direction in which it is pushed by
solar radiation.

The comet head is not significantly affected, but that
is beside the point: comparing the tail to a sail
does not entail comparing the head to a spacecraft.