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Solar Sailing and Physics
Roland Piquepaille writes "In this article, the New Scientist writes that the next generation of spacecraft might be propelled with the help of the sun. "Both NASA and the European Space Agency are developing solar sails and, although never tested, the concept is quite simple. A solar sail is essentially a giant mirror that reflects photons of sunlight back in the direction they came from." But Thomas Gold from Cornell University in New York says the proponents of solar sailing have forgotten about thermodynamics, the branch of physics governing heat transfer." And this is where it's becoming interesting. Gold's paper, "The solar sail and the mirror," states that "either Carnot's accepted rule is in error, or the solar sail proposal will not work at all." So, as this illustration from New Scientist shows, the real question is: "Can it really sail away?" We'll know it in September when the first tests are done. In the mean time, read this summary for more details and read the original stories for far more information."
This works well for exploring the inner planets, or if you just want to do a flyby of the outer ones. The sun provides negligable energy out past the orbit of Mars. We still need someting like Prometheus in order get around and about in places where the sun doesn't shine brightly.
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Whe I was studying Astrophysics many years ago, we learned that photon pressure is what "keeps stars up" i.e. the pressure exerted by the photons produced in the star exerted on the matter comprising the star are what prevents it from collapsing under its own gravity. My mind is rusty, but we derived the equations and solved them for certain masses of stars. We also looked at solar sails using similar maths. I suspect that solar sailing is possible, since the physics is similar to what's going on inside a star...
Stick Men
From the article: 'The absence of perpetual motion machines seems to show that no one has succeeded in overcoming the limitations prescribed by Carnot'.
Although it is true that no perpetual motion machines have ever been built the second law of thermodynamics is only a statistical law and so can be broken in very special circumstances. Richard Feynmann once proposed a perpetual motion machine that should work in theory (on a small scale governed by the heisenburg uncertainty principle) even though we do not have the technology needed to make it. It works as follows:
you will need:
a device to turn mass into energy (d1) and a device to turn energy into mass (d2).
Place d1 at a point on the earths surface and d2 at a height above it. Use d1 to turn some mass into photons and shine these photons at d2 where they are turned back into mass. Let the mass fall down to d1 and harvest the kinetic energy released. Repeat ad infinitum.
Now as stated this would only work under a small distance were d1 and d2 were placed very close together so hardly any useful energy could be gotten out of it, but it does show that the 2nd law is not as undeniable as is often thought.
All that glitters has a high refractive index.
Even if it turns out that particles without rest mass, such as photons, cannot be used for solar sails, there is still a solar wind made of particles which do have a rest mass. Solar sails could still work. One interesting idea is a "virtual sail" made of a permanent magnet. In theory it should gain momentum when the electrically charged particles are deflected by the magnetic field.
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You would think that physicists should have solved simple problem like this by now. After all, how difficult can this be to prove in a fairly simple experiment on earth? If physicists are struggling with truly hard things like the quantum chromodynamic interactions inside a proton should this not be easy?
What about building a small sail, parking it in a vacuum tube and firing a somewhat powerful laser at it? If there is movement, it works. If not, then, well, no.
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M2P2 is a much more viable alternative -- no massive sheets to drag around, low power consumption, and a clever way of doing things. More on it here and here and here and here and here.
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My understanding was that solar sailing was not in fact using photons to push the craft, but in fact using the solar wind, which consists mainly of hydrogen streaming from the sun. Photons have no mass, therefore cannnot transfer momentum.
(Oops, accentally hit submit. Here is the correctly formatted version.)
Gold Makes the following claim:
Gold claims that conservation of momentum would "accelerate the body to an energy content of Mvc." Assuming the object of mass M is initially at rest and radiation of energy E is shined upon the object and absorbed, then the "Resulting Kinetic Energy from conservation of momentum" is E^2/(2 c^2 M ) which is much not the same as Mvc unless v = 2 c. (A rather high velocity.) Check my math here.
Conservation of momentum:This is clearly not Mvc as Gold claims unless v = 2 c. Maybe I am misunderstanding what he means by v. Also, if all the incoming energy were converted to kinetic energy, then we get a "resulting kinetic energy from complete conversion" of RKEFCC = E. Notice that
So, unless the incoming energy is less than twice the total energy obtained from converting the object to pure energy,
Is there anything wrong with this argument?
No they don't have mass. "Some say"? Einsteins theory of general relativity quite clearly shows the curvature of spacetime due to the presence of mass. I'm not really in the mood to have a cosmological argument right now but I happen to work in the field of gravitation and know a little something about this.
The properties you describe (momentum and being affected by gravity) are properties demonstrated by particles. Just because it's affected by gravity doesn't mean it's got mass. And one difference is that massive objects cannot travel at the speed of light. Also it's not gravity that affects the light directly - you don't see photons slowing down as they travel away from a massive object - it's the effect of a massive object on the curvature of spacetime which causes the distance traveled by the light on a given axis to change (depends on co-ordinate system chosen but hey, too much detail).
Black holes are black because the photons cannot escape from the gravitational potential well. But the differences are very important, very simple and quite profound. I'd suggest you have a look through some books on general relativity or even some basic wave/particle duality undergraduate physics notes. They might help your argument some.
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Ok, I'm sorry to seem old or non-progressive, but you CANNOT create a perpetual motion machine.
True - but the paper has used a broken argument. The statement
If a heat engine could exceed the Carnot efficiency
Then you could produce a perpetual motion machine
but we know you cannot create a perperual motion machine
Therefore no heat engine can exceed the Carnot efficiency.
is true.
The statement
The light sail is a heat engin and therefore cannot exceed the Carnot efficiency"
is false, becasue a light sail is not a heat engin. A heat engine works on dis-ordered energy and must pay "Maxwells Price" - the wages of Maxwells Demon - to order it. A light sail works on ordered energy, for which maxwells Price has already been paid by the sun dumping energy into outer space.
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A solar sail is a one-way trip only - away from the sun. If you want to get back you need to use some other form of propulsion.
Wrong.
to return to a lower oribit, think angular momentum:
1) use the So.Sa. to reduce your angular momentum (ang. vel.).
result: orbit becomes more excentric (have radial velocity). Perihelion decreases.
2) when droping towards sun, use So. Sa. to reduce your radial velocity, while increasing your angular momentum.
result: you will now be at a lower orbit with less excentricity. e.g., the Earth's orbit.
QED.
Working for necessity's mother.