Poof goes the solar sail ?

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drag

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Greetings !

Please read this short story about solar sails' physics:
http://www.spacedaily.com/news/rocketscience-03zg.html

Can the experts here provide their opinions, please !

I personally thought that was all simple and basic stuff
long since solved by physics. I mean, a photon has
energy and hence momentum which it transfers to the
sail and then it is reflected - reemmited thus again the
momentum exchange occurs only that this time the photon
and the sail push against each other (actualy kin'na smells
fishy in terms of energy if I put it this way).

But the light pressure experiment with the reflective and
absorbing surfaces clearly draws a different picture. I've
heard of it before but no details were given about the
surface (or, in fact, I think it was said that a reflecting
surface would move, which is clearly a book's or my memory's
mistake ?).

So, what actualy happens here ?!

The only possible conclusion here as I see it is that
the reflected - reemmited light "drags" the reflective
material back, but how can that be ?!

Is that really so complex that nobody thought about it before ?!

Does this actually doom the whole idea of solar sails - they'll
just evaporate if they're absorbing or produce no thrust
if they are reflecting ?!

HAS ANYBODY EVER DONE AN EXPERIMENT, EXCEPT THE ABOVE MENTIONED,
BEFORE AN ENTIRE SOLAR SAIL SPACE MISSION WAS READY FOR LAUNCH ?!

Thanks ! :smile:

Live long and prosper.
 
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I don't believe Mr. Gold

Anyone wanna place any bets on this one?

Photons will reflect off the whole mirror surface and transfer momentum and energy to the sail. And a reflecting sail will work twice as well as an absorbing one.
 

drag

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Re: I don't believe Mr. Gold

Originally posted by Tyger
Photons will reflect off the whole mirror surface and transfer momentum and energy to the sail. And a reflecting sail will work twice as well as an absorbing one.
So the experiment brought as an example in the story is a lie ?
(SpaceDaily ussualy doesn't post false info.)
 
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Re: Re: I don't believe Mr. Gold

Originally posted by drag
So the experiment brought as an example in the story is a lie ?
(SpaceDaily ussualy doesn't post false info.)
The Crooke's radiometer has been around a long time, and versions have been tested with a high vacuum and bright light on the reflecting surface. They are the relevant experimants.
 

Hurkyl

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I wonder if he considered the doppler effect.
 

HallsofIvy

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I don't know this "Thomas Gold" of Cornell but it makes you wonder what the physics department at Cornell thinks of this.

Yes, of course, a photon (or anything else) reflecting perfectly from a "sail" maintains the same energy. However, momentum, unlike energy, is a vector quantity. If the photon has momentum m intially and reflects off the sail in the opposite direction, it imparts momentum 2m to the sail. I believe that's the way it was explained to me when I was in highschool (more years ago than I care to remember) and don't see anything in the article that contradicts that.
 

Ace-of-Spades

I don't think conservation of momentum can be applied to massless particles such as photons. While it is true that they do have momentum associated with them, it is a different type of momentum, originating from the photons frequency.

I guess Quantum mechanics must be able to explain this.

I assume that when a photon strikes a reflective surface, it is absorbed and excites the atom absorbing it. That atom will return to its rest state, releasing a new photon having the same energy.

Since conservation of energy will hold, even at the quantum level, no energy can be imparted to the mirror, and thus it can't be accelerated.

(Maybe I'm wrong, but that seems to be the only way to make sense of the problem) [?]
 

jcsd

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Originally posted by Ace-of-Spades
I don't think conservation of momentum can be applied to massless particles such as photons. While it is true that they do have momentum associated with them, it is a different type of momentum, originating from the photons frequency.

I guess Quantum mechanics must be able to explain this.

I assume that when a photon strikes a reflective surface, it is absorbed and excites the atom absorbing it. That atom will return to its rest state, releasing a new photon having the same energy.

Since conservation of energy will hold, even at the quantum level, no energy can be imparted to the mirror, and thus it can't be accelerated.

(Maybe I'm wrong, but that seems to be the only way to make sense of the problem) [?]
No your wrong, a photons momentum is given by p= h/wavelength. Therfore as Planck's constant is always constant an inelastic collision results in a change of wavelength.
 
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Ace-of-Spades

I didn't deny that photons have momentum.
I only said that I don't think the law
of conservation of momentum,
(which I know is supposed to be universal)
would apply in this case.

Once again I said I *think*
So I might be wrong!
 

jcsd

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What actually happens is that from the reflectors point of view the photons momentum stays the same but from a 'stationery' point of view the phtons are redshifted and thus lose momentum.
 

Ace-of-Spades

YES!!!
Ive solved the problem for once and for all...
Here it goes...

Momentum is always conserved yes? - NO!
Momentum is always conserved in a collision? - YES! :smile:

When a photon strikes a reflective surface, or any
other surface for that matter (could be a brick wall)
It does not undergo your ordinary everyday classical
collision. Instead it is absorbed and anihilated by an
atom of the surface it strikes. This excites the atom
which will then jump back down to a more stable
state releasing a DIFFERENT photon.

Since this is not a collision, Momentum need not be
conserved, and thus no energy needs to be transfered.

So... Therefore the Mirror sail thing will NOT work!
 

jcsd

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No, momentum is ALWAYS conserved in some form or other, when the photon is absorbed (tho' in the case of a perfect reflector though it's specifically NOT absorbed but reflected) the electron aquires the photon's momentum. Of course in this case were talking about free electrons and collison not absorbtion.

Rememberit is only the iew of one scientist that the solar sail won't work and from what I know I'm not convinced he is correct.
 

Ace-of-Spades

I don't know, I think you're wrong!
Photons are not classical particles, and
so will not behave like pool balls which conserve
momentum when they collide.

Photons are quantum particles, which behave
quite differently. Photons do not collide with
things, which bounce them back.

When you look at a red sheet of paper, what is
hapenning to the photons hitting the paper??

QM says...
- The photons hitting the paper are absorbed,
they will excite atoms within the paper, the
atoms in the paper will have energy levels which
correspond to a red frequency.

The atoms in the paper will then drop back down
to their ground state emitting a NEW photon
having a red frequency.
The same applies for all matter, including
reflective surfaces.
 

jcsd

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In QM mometum is still conserved.

Compton scattering is the QM process of the scattering of phtons by free electrons, there is no absorbtion in this. The idea of a perfect refelctor is that it doesn't absorb any of the light as opposed to it's polar opposite the perfect absorber.

The electrons in a reflective surface are free electrons so they cannot absorb the photons, they scatter them.
 

Ace-of-Spades

Oh yeah... he he he
My bad (I forgot about Compton Scattering)
I guess youre right!:smile:

........

So, would your ordinary household
mirror not be perfect reflector?
 

jcsd

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Actually the sail being a perfect refelector would, I believe use Thompson scattering but still no absorption.
 

Ace-of-Spades

Whats the difference between Thompson
and Compton scattering?
Ive never heard of Thompson!
 

jcsd

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Thompson scattering is classical and elastic, Compton scattering is non-classical (i.e. QM) and inelastic.
 

Ace-of-Spades

Then why on earth would you
call this a classical collision?
 
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Ace, if the photon

is reflected it will be from the whole surface of the mirror and impart momentum to the whole mirror. If it's absorbed it will only be absorbed by a little part of the mirror and it will heat that part up.
 

drag

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Originally posted by HallsofIvy
Yes, of course, a photon (or anything else) reflecting perfectly from a "sail" maintains the same energy. However, momentum, unlike energy, is a vector quantity. If the photon has momentum m intially and reflects off the sail in the opposite direction, it imparts momentum 2m to the sail. I believe that's the way it was explained to me when I was in highschool (more years ago than I care to remember) and don't see anything in the article that contradicts that.
Precisely my thoughts, but the fact that an absorbing surface
will receive momentum while a reflective one will not in
the experiment mentioned does appear to contradict this.
What's up ?

Live long and prosper.
 

jcsd

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It's a simple elastic collision:



(p= 1m) @--> £ (at rest) total mometum = 1m


(p= -1m) <--@ £--> (p= 2m) total mometum = 1m
 

jcsd

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btw the above diagram is not meant to be an ilustration of how the solar sail works (for a start the diagram contravenes the conservation of energy) it's just to show how the conservation of momemtum works.
 

drag

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O.K. But again, what about the experiment when the
absorbing surface is pushed and the reflective surface
is not (that is not supposed to happen then) ?

Thanks ! :smile:

Live long and prosper.
 

jcsd

Science Advisor
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There must be an exchange of momentum when something is reflected.
 

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