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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
*picks up fire-proof boat*
Sun, here I come!
Of course it runs NetBSD. BTC: 1NT7QvbetmANwaMzhpVL6
The article is wrong in the sense that it treats the photons from Sun to be in the form of heat - they are not, because their velocities are not randomised - there is a net momentum radially away from the sun. Carnot's cycle assumed the thermal energy to be in the thermodynamic form, i.e. say internal KE of gas, etc... You could simply adapt this guy's argument to a bullet hitting a plank of wood and show that wood gaining motion would break carnot's law - this is not the case.
Here's the crux of his argument: But what will be the performance of the mirror as a heat engine? If the mirror receives heat energy from the Sun and converts some of this into free energy, namely the kinetic energy of its motion, it falls into the strict definition of a heat engine, and Carnot's rule defining the maximum efficiency for this energy conversion must apply. 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. It would be that fraction of the heat flow that could maximally appear as kinetic energy gained by the mass of the mirror. If this was a perfect mirror, the two temperatures will be the same, and it follows that the mirror cannot act as a heat engine at all: no free energy can be obtained from the light. The proposed solar sail cannot be accelerated by sunlight.
Carnot only applies to closed systems. In textbook examples of heat engines, the engine, the heat source and the heat sink are all included in the analysis. Gold has included the engine (the sail) and heat source (the sun), but he's neglected the heat sink (the almost-perfect blackbody of intergalactic space). It isn't the temperature difference between absorption and emittance that matters, it's the temperature difference between source and sink, and that difference is huge here.
Yes, but for a different reason... The black side heats up more than the shiny side and it recoils molecules more... - the photon effect works the other way round but is negligible in presence of air...
"We obey the laws of thermodynamics in this house young lady!" -Homer Simpson
Won't it only be useful for travel away from the sun? So, it might be used in say space probes, but nothing like a Mars mission or at least only one way in a Mars mission.
'He was a dreamer, a thinker, a speculative philosopher... or, as his wife would have it, an idiot.' - Douglas Adams
The Solar Sail and the Mirror says:
"If this was a perfect mirror, the two temperatures will be the same"
This ignores the frequency shift due to the moving mirror. Proof falls down. Thermodynamics and conservation of momentum both still hold.
Physics correct! News at 10!
Yawn.
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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.
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?
Gold's theory ignores one important aspect- the doppler effect.
Let's do a gedankenexperiment (thought experiment).
Hypothesize that you have such a solar sail and it's already in orbit and starting to pull away from earth (say, because an ullage rocket has initiated a short accelleration)
The incoming solar photons - IN THE FRAME OF REFERENCE OF THE SAIL - enter and leave at constant wavelength. But the sail is moving with respect to the rest of the solar system (the ullage rocket kick-started this motion)... so the wavelength measured in the frame of reference of the sail mirror is not correct.
In the external (non-accellerated) frame of reference of the solar system, the photons hit the mirror at some particular wavelength, but exit at a longer wavelengh (because the mirror is moving).
The count of photons is the same- but their energy is lower.
So, where did the energy go? Draw the Feynmann diagram: there's only one place it could have gone- and that's into the sail itself. Therefore the sail accellerates further.
A similar gedankenexperiment will show that a sail moving _toward_ the sun pumps energy into the photons, and so decellerates.
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
Actually, the number of misconceptions and errors in this "article" boggle the mind... For example,
Except, of course, that that expression is for the magnitude of the momentum. Duh. The momentum carried by the photons emitted by the Sun lies in the direction those photons take; for any given photon, the momentum is radially away from the Sun. For all of them together, the momentum is zero because they all cancel -- but that happens only when you integrate over the entire sphere. For the tiny portion hitting a sail, there would be net momentum.
The Mongrel Dogs Who Teach
Didn't Voyager and Galileo take advantage of the solar wind to get way out there in a short time?
No (human) spacecraft to date has used the solar wind for propulsion - the solar sail is the only realistic mechanism for doing so, and that's never actually been tried (there was to have been a test of the Cosmos 1 couple of years ago but it suffered a launch failure).
Nae bother
Also, its not a perfect vacume, and look real close,,,no reflectors..black side and white side..and look which way it spins. What happens is the light heats the black side and the few molecules floating around in there hit the hot side and bounce off faster, having grabbed heat, imparting motion, or something to that general effect.
"The sun provides negligable energy out past the orbit of Mars"
I doubt this approach uses light as a form of energy. The idea here is to think of the light photons as 'mass' rather than 'energy'. Since E=mc^2, it follows:
m=E/c^2. since c=velocity of light (10 power 10) and E could be 10 power -24, the mass of a photon could be infinitesmally small, and negligible.
My chief concern here would be, if a satellite can be propelled by reflecting photons, then the 'deflection' caused by a single hydrogen atom (of which there could be lots in space, besides dust and gases) could cause deviations, millions of times greater in magnitude compared to the desired motion.
Simply stated - unworkable, but then, try telling that to 'scientists', specially those reading Slashdot!
If you keep throwing chairs, one day you'll break windows....
The solar sail and the mirror
Thomas Gold (Center for Radiophysics and Space Research, Cornell University)
The radiation pressure exerted by incoherent light on diverse surfaces is examined. The thermodynamic rule, first given by Carnot in 1824, describes the limitation to the amount of free energy that can be obtained from a source of thermal energy, and he gave the compelling reason for this rule, that if more free energy than he had prescribed could ever be extracted, then a heat pump could use that free energy and re-create all the heat energy that had been consumed. A perpetual motion machine could then be constructed. Now, 179 years later, it is proposed to fly a spacecraft that is expected to gain velocity from the radiation pressure the sunlight is expected to exert on solar sails, panels of thin plastic sheets, mirror surfaced on the side facing the sun. However a detailed examination of this proposal shows it to be in direct conflict with Carnot's rule, and no such pressure can be expected. Either Carnot's accepted rule is in error, or the solar sail proposal will not work at all.
Carnot, a French engineer had described in 1824 a basic law of thermodynamics: heat energy can be converted into "free" energy, such as mechanical energy of motion, but only in an engine which must have certain properties. Heat must enter it at a temperature which we will call T1, and it must then be degraded in the engine to a lower temperature, T2. A certain fraction of this flow of heat energy can then be converted into free energy. The maximum fraction that can be so converted is given by (T1 - T2)/T1. He had shown that the cycle is reversible, so that a heat pump can be constructed that would use free energy to deliver heat; moreover that it would be able to reverse the heat flow from T1 to T2 precisely, if given the maximum free energy obtainable from the heat engine. Thus he showed that a perpetual motion machine could be constructed if either the heat engine or the heat pump could achieve a higher efficiency than that which he had stipulated. Any device that can obtain free energy from a supply of heat, by whatever means, is thus covered by Carnot's rule.
179 years have gone by during which all the heat engines we now employ for every aspect of our civilization have been designed, and all their designers have recognized Carnot's rule as the ultimate aim of their designs. The physicists of that long period have all agreed with that rule. Lord Kelvin based his deduction of the absolute zero of temperature on Carnot's considerations. The absence of perpetual motion machines seems to show that no one has succeeded in overcoming the limitations prescribed by Carnot.
Yet now, we have a proposal on the table that runs counter to the rule of Carnot. It is proposed that the radiation pressure on a mirror from a hot body, the Sun, could be used to supply propulsion energy and momentum to a spacecraft, and thus facilitate interplanetary travel of vehicles, without the need for any other means of propulsion. What a desirable solution this would be! The Sun would pour out its energy whatever we do to it, and the momentum associated with that, calculated by Maxwell and confirmed later by Einstein, would be E/c, where E is the amount of energy emitted in a given interval of time, and c is the velocity of light. If a perfect mirror is used to receive the sunlight and its momentum, the re-emission of that light would gain the same momentum once more, and thus the force exerted on a perfect mirror would be doubled. The best mirrors are not completely perfect, but this would cause only a small loss of efficiency. It is proposed to use thin plastic sheet with aluminized mirror surfaces for these "solar sails". The speeds were calculated for a certain speeds of interplanetary travel to be obtained. A fund of several million dollars was assembled for the first space experiment of the new technology is proposed to be launched within a few months of writing this.
But what will be the performance of the
When you can bandy about cool names like "the Carnot cycle", why let actual facts get in the way?
This guy is wrong. Period. The solar sail would not be a heat engine -- it's not an engine of any kind -- so Carnot's analysis does not apply. Radiation pressure does exist and can be used to blow objects away from the Sun. Don't believe me? Too bad, because we have photographic evidence: The tails of comets always point away from the Sun (during the approach and departure of the comet), in part due to the radiation pressure on the dust that makes up the tail.
The Mongrel Dogs Who Teach
http://science.slashdot.org/article.pl?sid=01/07/2 0/1246254 2 2/0321239
http://science.slashdot.org/article.pl?sid=01/07/
There was another Slashdot article about solar sails (from 2002-04-29) here:
http://science.slashdot.org/article.pl?sid=02/04/2 9/1246221
It mentions a test to be launched "sometime after September". I don't know if that test ever launched.
The trouble with that analogy is that you can only tack a sail boat because it has a centreboard to stop it from going sideways, which is difficult to arrange for in space. Try it without sometime.
The other comments about gravity doing the moving-towards-the-sun bit sound right, though.
The Planetary Society already has a solar sail project going and is preping to launch soon.
http://www.planetary.org/solarsail/
There is a very introductory book about solar sailing by Louis Friedman, Executive Directory of the Planetary Society: it's Starsailing: Solar Sails and Interstellar Travel (yeah, yeah, it's amazon, thbbbppp) and here are some selected chapters. It rocks because it walks you through the equations and such. It's really NOT hard to understand. I found it in my high school when I was a kid and really fell in love with the whole concept. It really rocks.
In the book it points out that the concept was tested with the Mercury probe that NASA sent way back (Mariner-10) in that they used the solar panels to get a spin from light pressure (iirc, it's in the book and I haven't read it in 10 years...)
Do you know why the road less traveled by is littered with the bones of the unwary?
The black vanes on the spinning apparatus absorb light more efficently than the white ones.
This absorption of energy causes the black side of the vane to be hotter (by a very small amount, i'm sure) than the white side.
The heat radiating off of the dark side of the vanes works much in the same manner as a jet engine (without the need for a compression chamber). Note that the reason it's in a glass bulb is to impose a partial vacuum is to reduce air friction which would keep the vanes from moving. Also the use of a needle point piviot further reduces the friction.
So, in a word no. The solar sail intends to gather energy by photons bouncing off of a sheet, while your example is really just a simple heat engine.
You can verify this independantly with a little obseration and thought. After all wouldn't the white side of the vanes be providing the thrust if the energy was harnessed from potons bouncing off of it?
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.
Mariner 10 did in 1974, although not as a primary means of propulsion. (I assume that by "solar wind" you mean radiation pressure, not actual solar wind?)
There's no conflicts here with comet tails. Comet tails form because the solar wind (which no-one doubts exists) causes a *temperature* change in the frozen particles, just like the black side of the blade in a radiometer. If the comets were hot enough not to be heated by the radiation, or were 100% reflective, the tail wouldn't exist.
As for the tail pointing from the comet, that's true for one of the tails. There's at least three for each comet:
1: The radial tail, which points away from the sun.
2: The debris tail, pointing in the wake of the comet.
3: The plasma/ion tail, which is caused by heavier particles sent out by the sun.
(4): Some even claim a fourth tail, pointing towards the sun, which is interesting, because the release of that matter would be a newtonian force pushing the comet further away.
And no, there is no
5: Profit!
Anyhow, there's been no tests showing that a solar sail *could* work, unless it can bleed off material and retain a cold temperature like a comet does.
Comet tails.
Comet tails *always* point away from the sun.
Must be something pushing the tail particles away.
QED
www.christopherlewis.com
You are correct that the Carnot efficiency cannot be calculated for this case, as it is not returning to it's original position. However, in the postulated case of a perfect mirror, there is no heat sink, as the photons will be reflected back towards the sun, and not radiated from the back of the sail. If the photons are absorbed by the sail, then some are radiated to the heat sink, and the expansion is no longer adiabatic (which is consistent with the lower momentum transfer). Note that if the mirror ~was~ returned to its original position, and the sun was insulated against losses in directions other than towards the sail, you would have an adiabatic expansion and then compression of the photons, leaving you with no net change in energy.
Gold complains in his paper of physicists not treating photons thermodynamically. That's funny - I seem to remember working out a problem using an adiabatic expansion of a photon "gas" in my undergraduate days. I don't remember the equation of state off the top of my head, but I guarantee you can find it in Callen or any other thourough thermo book.
Yes, IAAP. I also think that slashdotters should note that this was published in the Arxiv, which is NOT a peer reviewed publication (although I must say that the Arxiv rocks!). The Arxiv is sort of like an open source scientific journal, or a BBS for scientific papers. I highly recommend wandering around in it for a few hours, but remember to take everything in there with a grain of salt.
Darn. I was going to try NOT posting to slashdot today. Oh, well. Feed the addiction (sticks needle labeled "/." in arm).
Um, sorry, no. Photons have no mass. You need the full form of the equation: E^2=(pc)^2+(mc^2)^2 which for a massless photon (m=0) would become E=pc. p in this case is momentum, which photons do have.
When the photon strikes the sail it imparts momentum to it. LOTS of photons=lots of momentum and near the Sun there are a LOT of photons. Bear in mind the sail will need to be HUGE. Also momentum has direction (vector not scalar).
Oh, and the effect of the odd hydrogen atom should be very small in comparison to the LOTS of photons constantly (alright, discretely if you're delving into duality) striking the surface of the sail. It will be slowed down slightly by atoms floating through space but if the sail is large enough and light enough then theory says it should be able to break free of the Sun's gravity (of course that's what the original argument is about). Do you really think one or two little hydrogen atoms will be much of a problem?
No numbers were harmed during the course of this diatribe, but a few ego's were bruised...
If you can't think of something nice to say then don't say anything at all. No, REALLY.
The article is cluttered with flaws and unfortunate misinterpretations of laws of physics. These flaws do not turn better if they are part of a complex theoretical explanation.
1.) Electromagnetic radiation has momentum, otherwise, there would be no electromagnetic forces. Period.
2.) Light is not heat - it is a directed stream of photons and a solar sail is by no means a heat engine being limited by the Carnot principle.
3.) Energy conversion holds due to doppler effect - reflected photons get their frequency shifted to red.
4.) Sailing boats have been working perfectly for thousands of years, using a similar principle with air carrying the momentum. And the air was not absorbed by the sails!
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.
Well, I am a physicist (PhD MIT '97) and unless I'm missing something *really* subtle, Gold's argument is just plain wrong. As earlier posters pointed out, this is not a system in equilibrium, nor is it closed. Conservation of momentum (which applies perfectly well to photons with the relations
energy=speed of light * momentum or e=c*p or p=e/c
and
energy=planck's constant*frequency=h*nu=h*c/lambda
p=e/c=h*c/lambda/c=h/lambda
When I reflect a photon from an object the total momentum of the system = photon + object must remain constant (if we're scattering elastically)...
Inelastic scattering effects (doppler, etc.) don't change the basic story here. SOME momentum will get transferred to the mirror with each photon bounce...
Also, there's no reason I can't reflect the photons from the solar sail at a slight angle to the sun (just like tacking into the wind) at the cost of losing some of the momentum kick from the photons, the photons now travel off into empty space (as opposed to the sun) making the system clearly open... It will continue to be accelerated so long as photons hit it (albeit with decreasing numbers of photons/sec as it gets farther out...)
Yes, but for a different reason
The author of the original article seems to be unaware of this:
Crookes' radiometer has invariably rotated in the opposite sense to the expected one. The black side of the paddles invariably recedes from the light, and many explanations have been offered, but not including that which would seem the most obvious: the absence of radiation pressure on the bright side
Sure, the temperatur change is necessary for the comet's body to evaporate partially, so that the tail can form. But that doesn't have anything to do with the direction in which the tail points.
The illegal we do immediately. The unconstitutional takes a little longer.
--Henry Kissinger
Glacial? Hardly.
Solar sails should be able to pull 1/1000th of a G. That's Pluto and back in under six months.
In the mid-late 1800's, traders got rich running clipper ships to Asia and back using the same time frame.
Heinlein has an essay on it in his Expanded Universe collection. I seem unable to find a web version of it, though.
I'm not crazy,I'm actively irresponsible.
Under 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!
I see two problems with the reasoning.
1) if the mirror is moving (fast) away from the sun, the reflected light will have lower frequencies (doppler effect) than the incomming light, so that the reflected light is of `lower temperature'.
2) When defining the two Carnot's temperatures, T1 and T2, I think T1 is rightly set to the sun's temperature; but T2 should be that of the `outer universe', the 3K background radiation. Then one sees that the max efficiency of the aparatus is 6000K/3K, a rather high efficiency indeed. The 3K would be the push back on the mirror on the other side.
Yes, the mirror may have a black back side, but that doesn't matter. After some time, the mirror would become 3K (equilibrium, if the black side is turned to the background radiation), and it will radiate 3K ratiation.
(Like it would do if the back side of the mirror would be a mirror).
Summary: on the sun-side of the mirror, there is a push of 6000K radiation (sun temperature), on the back-side is a push of 3K radiation. There you have your efficiency.
Well, I don't think you can use these forms of the equasions, since photons have no mass. They do have kenetic energy, IIRC.
autopr0n is like, down and stuff.
(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?
Q: If the solar sail works by using the sun's photons to push you away, then how do you get home again? A: Come back at night.
Gold says that as the sail is a perfect mirror the light is reflected at the same temperature and so Carnot's law applies. But of course, if the light were reflected at the same temperature it wouldn't be losing any energy so a working solar sail would violate conservation of energy and the laws of thermodynamics would be the least of our problems.
Gold's problem is that he forgets both conservation of momentum and conservation of energy. What happens is that as the sail is kicked forward (gaining both energy and momentum) the photon is reflected with slightly less energy than it arrived with (ie is cooler) and conservation of momentum, conservation of energy and the laws of thermodynamics still apply.
My back of an envelope calculations tell the fractional energy loss is E/(M*c*c) where E is the photon energy and M the sail mass.
You will notice that Gold's article doesn't appear in a refereed journal: this is because most referees have heard of conservation of energy.
Dr John Rowe
School of Physics
Exeter
UK
The same way sailing ships move against the wind... A zig-zag course, called tacking. Umm, tacking only works because there are two media; the water and the wind. A aerodynamic shape (sail) uses the wind to create a thrust vector in the direction the wind is blowing. A hydrodynamic shape (the hull) uses the water to create a thrust vector perpendicular to the keel pointing in the general direction the wind is coming from. Add these together and you end up with a vector that points parallel to the keel, either forward or backward. In free space you only have one medium, the solar wind. This is like a balloon rather than a ship, and the solar sail (like a balloon) will go the direction the wind is blowing no matter how it is aligned to it. It is possible to get a perpendicular (to the wind) vector component based on what direction you deflect the solar wind, but without a second medium to interact with you cannot get a negative parallel component.
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...
I am disrespectful to dirt! Can you see that I am serious?!
The sun provides negligable energy out past the orbit of Mars.
Not negligible, but solar intensity does fall off as 1/r^3. In Mars orbit, the solar radiation on a surface normal to the incipient light is about 60% of that in Earth orbit. This represents the fact that the photon are spreading out in a sphere from their source--though there are just as many as there were on the surface of the Sun, there's now a whole lot more space in between them.
I'm most familiar with this in the context of solar powered spacecraft. To operate a solar S/C near Mars, you need massive unwieldy solar arrays that are expensive to launch. The only other viable power source for space, currently, is thermoelectric conversion from the heat generated by nuclear decay (not a live reaction), and is only 6-7% efficient.
We still need someting like Prometheus in order get around and about in places where the sun doesn't shine brightly.
Agreed. The Nuclear Space Initiative is the only way we will ever get something sizeable to the outer planets--and back. Spaceflight requires too much power to be generated any other way, at least with the science we can do now.
-Carolyn
Like Daddy always said: if you can't dazzle 'em with brilliance, baffle 'em with bullshit.
Is that like saying that sailing on an ocean into the wind is impossible? Hate to tell you, but it can be done. The only problem with doing this on a solar sail is figuring out exactally how one 'tacks' into the light.
Of course, this also discounts that one can use other gravitational bodies in order to change trajectory, and if you're going to Jupiter and comming back in one nonstop trip, you can always fold/destroy the sail and coast on the initial momentum from the trip out.
though intellectual honesty impels me to concede it also says
The Mongrel Dogs Who Teach
Fortunately it is also irrelevant. When dealing with photons, the kinetic energy equation is E=cp as parent stated. And no, you don't substitute mass into p - De Broglie's relationship makes this E=hc/(lambda), where h is Planck's constant and lambda is the wavelength of the light.
-Looking for a job as a materials chemist or multivariat
Actually, with the gravity of the sun, the angle of the sail and the direction of motion relative to the orbit the vessel is in, it is perfectly possible to tack. If you direct the sail so that your thrust is oppossing your orbital velocity or adding to it, you can alter the radius of your orbit.
>The author is just an idiot. Its like he picked of a tv show "physics for dummys", remembered a few complicated words and wrote some superduper article.
Ph.D. from Cambridge, faculty at Cornell. The best explanation I can think of is today's fortune on the Slashdot home page, "An expert is someone who avoids the small errors while sweeping toward the grand fallacy".
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.
I know you claim to know what you're talking about but you might want to go back and look at those plans again because voyager had absolutly no ion engines on it what so ever, it used hydrazine thrusters. And it also uses gyroscopes to change the directions it points. And at least for voyager 2 its generator is still moderately powerful, about 300 watts extrapolating from 330 watts in 1997. And while many of the instruments are turned off they still won't all be turned off totally until 2020. When it really won't be able to power its instruments.
A Bugg
You are incorrect - the tubes have only a partial vacuum. In a hard vacuum, the motion stops. Only with a very delicate instrument can you get a radiometer to function in a hard vacuum, because radiation pressure is a much smaller effect than the complicated gas phenomenon really behind it, and in fact you'll notice that a Crooke's Radiometer spins the wrong way - it spins away from black, where a radiation pressure effect would spin away from white.
. php?mo de=1&faqID=42
For a complete explanation of Crooke's radiometer, see:
http://www.stillmoving.ca/physics/usenetFAQ
My amazing wife - Artist, Author, Philosopher - Laurie M
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.
It is obvious that if he is at the Cornell Center for Radiophysics and Space Research, then he is not an actual physicist there. I actually suspect some ulterior motive behind this.
There is no thermodynamic problem with radiation pressure and solar sails, Carnot, Maxwell and Einstein are correct, and Thomas Gold is wrong.
The photons that leave from the Sun are at a Maxwellian distribution at the temperature of the Solar photosphere, many thousands of degrees.
The photons which are re-emitted by the heat of the sail are at the temperature of the sail, which is obviously less than the temperature of the Sun.
There is a temperature gradient, hence work is done.
Gold is also obviously uneducated in physics because of his assertions about how momentum is 'E/c' and how 'E' is a scalar but momentum is a vector, blah blah blah.
The correct way to do an electromagnetic problem like this in purely classical E&M (which is all that is necessary to solve this problem) is to use the stress energy tensor, which is certainly non-zero for propagating electromagnetic waves.
You then use this to find the forces on material bodies, e.g. dielectrics or conductors. You have to think about the stress-energy tensor on both sides of the material body and add up forces.
(interesting note, you can also get angular momentum out of this too, appropriately polarized electromagnetic waves can exert a torque).
I learned this in my sophomore year physics class.
Actually, this sounds like a good problem set problem. "Hmmm....now that you ask about it--------yes, it WILL be on the final exam! muahahha."
Given a plane wave source of incoming radiation with flux F_0, with a certain amount of it R1 reflected and the remainder absorbed, what would be the net acceleration in the direction of the radiation of the solar sail with emissivity E once it has reached thermal equilibrium, accounting for all radiation forces. Assume space is at 0 degrees.
What you would do is to put in on the incoming side, the stress energy tensor for the incoming radiation, the outgoing radiation, plus, on both sides, radiation emitted from a black body.
You first get the energy balance right, and then solve for the acceleration.
If you want to be tricky, you would go into the accelerating frame of the sail, and then appropriately red-shift down the incoming frequency of the radiation.
Here is my suspected ulterior motive. This thermodynamic fallacy is exactly the same one that creationists use to deny the obvious explanation for why plants live and complex life evolved.
Namely that the Sun has been providing input energy at 10,000 degrees and the Earth has been re-emitting energy at 300 degrees K and the temperature difference over 4 billion years can do a fair amount of work.
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!
-- ;-)
Kuro5hin.org: where the good times never end.
The solar sail is a heat engine. But he's still wrong.
The basic claim is that the photon doesn't lose energy to a perfect mirror. But that's wrong. It neglects both the ACCELLERATION of the mirror due to the impact of the photon, and the red/blue shift of the photon when reflected from a mirror in motion relative to the observer.
It's easy to understand the lightsail/sun/photons system as a heat engine: The lightsail is the piston and the photons are the working fluid.
Just as with a piston, if the lightsail were held still (and the mirror were perfect or imperfect but at solar temperature) the photons would rebound without loss of energy. But the high photon-gas "pressure" on one side of the "piston" versus the near-vacuum (dark sky) on the other side means there is a force on the mirror. If not held it will accellerate.
Just as with a piston, no work is done on it until it starts to move - and the faster it moves the more work is done on it. But the faster it moves the more the light is red-shifted, i.e. the "gas is cooled", so the more rapidly work is done. Exactly what you expect in a piston engine.
You could also push the light sail toward the sun (as when decellerating at the far end of the trip). In this case the photons would be blue-shifted and the work from pushing the sail against light pressure would thus go into "heating" the photons - and the sun, if the sail was pointed properly so the "photon gas" hit the far end of the "cylinder" rather than escaping.
His analysis assumed the sail was unmoving and unaccellerating, which is just plain confused.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
On a more technical level, the incoming photons do not obey Maxwell-Boltzmann statistics (not even vaguely approximately), so it is not semantically valid to make thermodynamic statements about them.
As to energy conservation, photon reflection is physically an absorption followed by a reemission. Since the mirror is accelerated by the process, an observer in the rest frame sees a doppler redshift of the reflected photons, and thus energy balance is maintained.
Finally, even if you wanted to sprinkle goat blood on the photon spectrum and call it a thermodynamic quantity, the redshifting preserves the blackbody spectrum (one of physic's remarkable results) while making it "colder", and thus the "temperature" decreases appropriately.
Show me the Planckian radiators, Gold! And then we can talk thermodynamics.
-- ;-)
Kuro5hin.org: where the good times never end.
OK, the bit about the photon gas being the working fluid is clever. But Gold's point is, solar sails would violate the Carnot condition. Carnot's analysis applies only to closed, cyclic engines. No one is proposing this as a closed, cyclic engine.
The Mongrel Dogs Who Teach
Yeah, he probably doesn't know shit
Professor Gold is not only the progenitor of the primordial mantle methane, errrr, hypothesis, he is also the progenitor of the idea that the Lunar landings would not have been feasible because the surface dust was not solid enough to support the weight of a lander.
This idea is rumored to have prompted the entire "Ranger" series of spacecraft (at umpteen million dollars) primarily to demonstrate that the Lunar surface was solid.
Oh, and the supporting statements by Steven Soter in the New Scientist article regarding the thermodynamics of photonic momentum-transfer? Three guesses who was Steven Soter's Ph.D. adviser...
Just repeat after me: "Tenure is *good* for science. Tenure is *good* for science. Tenure is *good* for science."
The Planetary Society as mentioned in the article has their own solar sail which is going to be launched in a few months.
They are the ones actually doing something about this, NASA and the ESA have yet to get off their butts and actually do any real tests.
You can join them and give support to these and other space projects. Much better than complaining about NASA.
This guys argument is based on the fact that after a certain time, the light is no longer absorbed because the mirror is at it's equilibrium temperature. This proves that he does not grasp simple physics. The equilibrium means that the material emits as much light as it absorbs. The reason the sail works is that the light is emitted equally in the forward and backward direction, but is only absorbed from the back. This means there is a net momentum transfer.
Now, one obvious remark would be that there is no energy transfer, because as many photons are emitted as there are photons absorbed. This means that the energy of the mirror cannot change and therefore we have a contradiction (mirror gains more momentum, but not more energy). However, this all works out if you take into account the Doppler shifts of the emitted radiation with respect to the absorbed radiation.
This stuff is all understood, I cannot understand where this guy gets his ideas. His arguments can equally well be applied to laser cooling and we know that that works!
-- Please put this in your sig if you think
>causes a *temperature* change in the frozen particles
k no w.pdf ...as I could in 30 seconds.
/ 51 090979.pdf ... contains a detailed study of the radiation pressure effects on a satellite going to the moon -- even though the effect is very small, it is required in this case to accurately calculate the orbit.
p re ssure.html
That does not provide propulsion, only the ability to leave the nucleus. The _dust_tail_ of a comet is still generally accepted to be due to radiation pressure.
> Anyhow, there's been no tests showing that a solar sail *could* work
Type "laser levitation" into Google. You'll get just under 10,000 hits, the first few pages of which are almost all examples of real-world experiments. People are doing this as I write this.
Your statement above is ill-informed. Then again, apparently Gold is too, because he makes direct claims that this everyday practical system does not exist, even though he could just as easily find this:
http://astp.msfc.nasa.gov/ast/presentations/3g_
More to the point, if you type in "radiation pressure" you'll get an equally useful list of examples of space-related examples. The very first hit:
http://www.terrapub.co.jp/journals/EPS/pdf/5109
Or you could read this short little blurb, which is quite useful:
http://yarchive.net/space/spacecraft/radiation_
Solar sails are not heat engines. Period. Neither are electric motors, balls rolling down hills, springs, or lots of other things. All of these "other things" have effeciencies much higher than the ideal heat engine.
Consider: what is the temperature of the electricity after it comes out of a motor? Does that explain the power you're getting from it?
Heat engines are only one way of many to extract energy, one our culture has become dependant on because we can dig up fuel cheaply. It is not a particularily good one however, and our widespread use of heat engines is likely to change in the next 20 years or so.
For instance, take an existing car that gets, say 30mpg. Now take out the normal engine, and replace it with an electric motor, fuel cell, and a reformer that extracts the hydrogen from the gas. This will improve milage to about 40mpg, even though reformers are rather ineffecient. This is because the energy in the fuel can be still extracted with higher effeciency as electricity than as heat, notably in the motor which is close to 90%.
Gold's argument is just plain dumb. I can't imgine how anyone could seriously suggest it. I wonder if even he believes it. In fact, a radiometer DOES spin towards the black side in a high vacume, a fact he should be familiar with.
A friend of mine came up with some additional bits of the theory of conservation of momentum a while back.
See, we were wondering what happens when, say, a car hits a snow bank. Car slows to a stop. Snow bank is, broadly speaking, immobile.
So he decided that it must be that the momentum was now stored up in the snow bank, as *potental* *momentum*. Unfortunately, before we could figure out how to harness this (or indeed where it went once the snow had melted), the pub closed.
-fred
Sign #11 of Slashdot overdose: You see the phrase 'moderate Republican' and you wonder if that would be a +1 or a -1.