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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.
My rights don't need management.
IANAP, but (And please correct my ignorance if need be)... Light is different than actual matter, so maybe the same laws of thermodynamics do not apply? If this is the case, could a perpetual motion machine be made harnessing the power of reflecting light?
Excuse me, I don't mean to impose, but I am the ocean
Don't those little kid's toys, with the white and black vanes in them (shaped like a lightbulb) spin when you put them in sunlight?
I suppose we could try one of these with a mirror in place of the black vanes...
I think this is really interesting technology, and hope to see a SUCCESSFUL test of it soon. I've been fascinated with the idea ever since reading about it in a short story by Arthur C. Clarke many years ago.
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.
yeah, but in the case of a bullet hitting a plank of wood and the wood gaining motion, the bullet loses KE and transfers it to the plank. Gold is claiming that the photon is bounced off the sail with the same energy with which it hit it - ie no transfer. IANAP but the part I don't understand is the part of the diagram saying "The sail is a perfect mirror". Surely such a thing does not really exist? Perfect?
evil math within Nature's Cubic Creation!
Seems like a black body surface treatment would be better.
Now that I think about it I remember those little evacuated glass bulbs with the a small turnstile with small paddles - one paddle is black and the other is white. When placed in the sun they turn. That should be enough to prove the concept.
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.
Finally! A year of moderation! Ready for 2019?
true, and a good point.
but, you didn't read his article carefully enough. look closer at his thought-experiment:
"We can determine the incoming temperature of the radiation by measuring the temperature an absorbing (black) body would reach when exposed to the radiation being sent to the mirror, and the temperature a black body would reach exposed to the outgoing radiation from the mirror, both measurements carried out in common motion with the mirror. Carnot's rule would then give the maximum efficiency as that fraction of the heat flow trough the mirror, given by the difference of the two temperatures, divided by the input temperature".
you see, they don't have to be a 'form of heat', but they do have a temperature. and this either changes or it doesn't, if it doesn't then you can do no work, if it does, then you need to drain the excess heat away (laws of thermodynamics)continually to keep the engine running.
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.
The dangers of excessive individualism are nothing compared to the oppressiveness of excessive collectivism
Ok guys this is kinda OT, but I always wondered for a long time. Let's say you have a hollow sphere. THe inside of the sphere is lined with mirrors (basically, a mirror sphere turned inside out) Now what will happen if you flash a beam of light into that sphere?
Let's say that you were able to flash that light through a small hole and then replace that hole back with a mirror. Will the light keep flashing back and forth to infinity??
If you were viewing from the inside without affecting the path of the light, would it be shiny all over the place? What will happen?!!!!
IMHO, the perfection of the mirror may be irrelevant, or at least nearly so.
The 'desired imperfection' of the mirror is that it's moving away from the light source, and that it can be accelerated. If photons were to bounce off of a perfect mirror, coming back with the same intensity and color, ie: total energy, then there would be no net energy to have moved that mirror.
I haven't thought this completely through, but it would seem 'obvious' that the frequency of the reflected light should be lower, signifying that energy has been transferred to the sail. (The intensity would be lowered because the sail is an imperfect mirror.) But in this light, I'm not sure what the story would be reflecting light off of a moving, but non-accelerating mirror. OT1H, it would seem that the velocity of the mirror would drop the frequency of the reflected photons. OTOH, since the photons didn't accelerate the mirror, and their frequency dropped, where did the energy go? OTGH, since we fixed the velocity of the mirror, perhaps that energy was dissipated as heat against the mechanism we used to do that.
Which finally leaves me wondering about the backside of the sail. I guess this works because the light from the Sun is non-random on one side, and starlight is random on all sides. Makes one wonder, a: how far from a star is this usable, b: can 'shaping' the sail somehow get net momentum out of starlight, or farther from a star?
The living have better things to do than to continue hating the dead.
I thought the comet's tail was the result of the solar wind, not radiation pressure
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.
blog |
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.
Gold Makes the following claim:
sss
Gold claims that conservation would "accelerate the body to an energy content of Mvc." Assuming the object of mass M is intially at rest and radiation of energy E is shined upon the object and absorebed, 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:
Momentum from Light = E/c
Momentum imparted to object = E/c
Resulting velocity = v = E/(cM) Resulting Kinetic Energy from conservation of momentum = RKEFCOM = 1/2 M v^2 = Mvc * (vUnder the link 'Can it really sail away?' there's a diagram saying that since, on a perfect mirror the photons will not lose energy, the mirror can't receive any energy. This is wrong:
The photons change direction when they are reflected; this means they have changed momentum, which corresponds to a force exerted on the mirror. The situation is equivalent to a little steel balls hitting a hard surface - if the 'reflection' is perfect, no energy is lost as heat. However, both photons and steel balls will lose energy by transferring it to the reflecting surface.
In the case of photons the force exerted on the surface is very small per photon as well as of very short duration, so the mirror isn't accelerated very much, and the energy transfer is very small, but it is not 0!
No, Voyager and other craft to this date took advantage of gravitational alignment of planets, but used no solar sails to capture solar "wind".
I am not sure about Galileo's design, but as my dad worked on Voyager, I still have a bunch of it's design specs around.
Voyager uses and ion drive to manuver, which is powered by radioisotope thermoelectric generators, which in turn are fueled by our friend plutonium 238. The ion drives are nothing like the "star trek" version, rather they are pointed wires which provide propulsion by throwing electrons off of its end.
Interestingly enough, the main problems with Voyager today tend to be that the radioisotope has degraded till less energy is available, the wear and tear on the couplings reduce the amount of electricity available to various components, and the extremely low temperatures require a lot of heat from the powerplant to bring them up to operating temperature.
As a result, nearly all of the Voyager subsystems have been shutdown, excepting those critical for operation.
Masses are always quoted as rest masses, otherwise it's not a constant but a variable which depends on your point of view. Nevertheless, the photon's mass is ALWAYS zero for the same reason that's velocity is always c. k*0 = 0 in most mathematical frameworks.
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.
If you can't think of something nice to say then don't say anything at all. No, REALLY.
Good thing the mirror doesn't convert heat energy into kinetic energy, or we'd be in trouble!
The mirror converts the momentum of electromagnetic particles into it's own momentum. A Carnot style heat engine is one that derives it's energy from the movement of heat from one portion of a system to another. Steam turbines are an excellent example of a carnot heat engine. A solar sail does not work on that principle at all.
I suppose next he'll publish a paper claiming that these http://www.lonezone.com/2000/catalog/lz888.html will never work either...
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I assume this is the same Thomas Gold, Cornell astrophysicist, who is best known for his Deep, Hot Biosphere theory, which says that oil and natural gas do not come from decomposing organic matter, but rather are inorganic products of the deep earth itself. Gold has predicted for decades that our oil and gas discoveries have but scratched the surface, and that there are incredibly more massive reserves waiting to be discovered below.
Unfortunately for Gold, no convincing evidence for his theory has ever been found, and he is widely considered a crank. Now that he has been retired for several years, we have to consider him a senile crank.
Certainly the current paper does nothing to change that opinion. Among the other obvious physics mistakes which have been pointed out, let's look a little closer at his final example, a light beam incident on a dark body. Gold purports to show that the body's velocity calculated based on momentum transfer is inconsistent with the velocity based on energy transfer. But this is an elementary mistake! Any calculation based on equating these two results for an inelastic collision (as when the incident particles are absorbed by the body) will show the same disagreement.
What Gold neglects to consider is that some of the energy is absorbed as heat. You can't calculate the body's velocity based on the assumption that all the energy becomes kinetic. It is the momentum-conservation formula which correctly tells us what the final speed of the body will be.
It's always sad to see a once-great mind descend into senility. I'm not sure whether it's even sadder when the mind was once a crank.
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.
But if you look even closer you have to account for the ever-increasing velocity of the mirrors. In a mirror's frame of reference, the photons become ever more redshifted, and so impart ever-decreasing momentum. In the rest frame, there's a Doppler shift in the absorption and reemission that progressively steals energy from the photons as seen by the rest frame, so they become progressively redder and lower-energy.
The converse experiment is to accelerate two mirrors towards each other, which progressively Doppler blueshifts the photons. This experiment has actually been done and turned red light into blue light!
-- ;-)
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