Solar Sailing and Physics 651
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."
Unfortunately (Score:5, Interesting)
Photons vs Gas... (Score:3, Insightful)
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 (o
Photons vs Gas... Orders of magnitude? (Score:5, Informative)
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...
Re:Photons vs Gas... Orders of magnitude? (Score:3, Interesting)
No, sorry, gparent is correct (Score:3, Insightful)
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 lambd
Re:Photons vs Gas... Orders of magnitude? (Score:5, Informative)
And from quantum mechanics, p = h / lambda (Plank's constant divided by wavelength, the result being a vector in the direction of propogation), tells you what the momentum is for a given photon wavelength.
If photons had mass (even 'relativistic' mass), they could not travel at the speed of light. And the symmetries would be different, none of the equations would work out properly (the polarization formulas, for example, would fail completely).
The rest mass MUST be zero, if photons travel at the speed of light, otherwise the relativistic mass (gamma * m) would be infinite (since gamma approaches infinity as velocity -> c).
The reason why gravity is affected by photons (and vice versa) is because the coupling is between the curvature and the mass-energy tensor, and the mass-energy tensor (as the name implies) contains more than just the mass.
So, while photons have some of the properties of massive objects, they also have some significant differences.
Re:Photons vs Gas... Orders of magnitude? (Score:3, Interesting)
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 tha
Re:Photons vs Gas... (Score:3, Informative)
Re:Photons vs Gas... (Score:3, Informative)
I'd highly recommend Helliwell's Relativity, though I think it's only sold through the Claremont colleges bookstore, so I doubt that's very helpfull. Beyond that, I Feynman's lectur [amazon.com]
Re:Unfortunately (Score:5, Funny)
OUCH...my doctor usually just uses an index finger!
Re:Unfortunately (Score:3, Funny)
Solar Intensity (Score:3, Insightful)
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 powere
irrelevant (Score:3, Insightful)
Well, IANAP (Score:3, Interesting)
Re:Well, IANAP (Score:2)
Re:Well, IANAP (Score:3, Informative)
Re:Well, IANAP (Score:3, Interesting)
Well in this particular example, using the sun's energy to create a perpetual motion machine, really doesn't work out. IANAP, but, If I recall correctly, a perpetual motion machine is based on the idea that once the machine is started, it would be able to operate as desired indefinately, since the energy that is used to work the machine would be supplied by the working of the machine. Invariably the su
Re:Well, IANAP (Score:4, Informative)
Re:Well, IANAP (Score:5, Informative)
It doeds, because the important photons are those which are reflected. Hypothesize that the mirrore is perfect: 100% of all photons are reflected, none absorbed, and the mirror does not heat up at all. The paper would assert that the mirror therefore feels no force at all. And yet, on one side it is being battered by photons, whcih come in with a momentum vector one way and leave with the vector reversed. ISTM that this purely theoretical mirror must feel some force. The question is whether a real mirror can come close enough to the theoretical to get perceptible thrust. The thermal mechanism described is in fact a leakage mode of the system - a defect to be minimoised.
The fallacy is that a sail is a heat engine. It is not. Heat is disorganised energy - atoms flying around in random directions. Heat engines convert disorganised energy - the random motion of atoms - into organised energy. One example of such an engine is the Stirling engine, which converts hot gas into rotary motion - and obeys Carnot's law. Solar radiation is organised energy - and the sail converts one form of organised energy into another; it is a form of gearbox, if you will, and can approach 100% efficiency. The analog is to the sail, not the Stirling engine. A sailing boat can convert wind into movement as long as the wind lasts. (The fact that the wind is created by a heat engine is irrelevant - the wind is organised and they thermodynamic price has been paid before it reaches the sail.)
I actually am a physicist and Thomas Gold is wrong (Score:5, Informative)
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.
Re:I actually am a physicist and Thomas Gold is wr (Score:3, Informative)
Re:Well, IANAP (Score:3, Interesting)
> of all photons are reflected, none absorbed, and the mirror does not heat up at all. The paper would assert that
> the mirror therefore feels no force at all. And yet, on one side it is being battered by photons, whcih come in with a
> momentum vector one way and leave with the vector reversed.
This is a beautifully classical way of looking at it, but it is taking a classical idea
Conservation of momentum (Score:3, Funny)
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
Re:Well, IANAP (Score:3, Informative)
Re:Well, IANAP (Score:4, Interesting)
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.
Seems like we already have a proof-of-concept (Score:2, Interesting)
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...
Re:Seems like we already have a proof-of-concept (Score:5, Informative)
Re:Seems like we already have a proof-of-concept (Score:3, Informative)
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
Wrong, mod it up (Score:3, Informative)
For a complete explanation of Crooke's radiometer, see:
Re:Seems like we already have a proof-of-concept (Score:2)
Actally no - different mechanisim there (Score:5, Informative)
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?
Woohoo solar sailing (Score:3, Funny)
Sun, here I come!
The article is wrong (Score:5, Informative)
Re:The article is wrong (Score:2, Interesting)
perfect mirror (Score:3, Interesting)
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, signify
Re:The article is wrong (Score:2, Interesting)
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
Re:The article is wrong (Score:3, Informative)
Oh, it's worse than that... (Score:5, Insightful)
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.
Re:So, how do we get back? (Score:3, Insightful)
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.
think angular momentum (Score:5, Interesting)
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.
Re:The article is wrong (Score:3, Informative)
The article is bogus.
Arg. Too bad I don't have my mod points. I don't see why on Earth that was modded as a troll. Yeah, other particles are emitted from the sun. It's called the solar wind, of course. The massive particles in the solar wind would provide a good proportion of the thrust to a solar sail.
There are some propulsion systems that rely entirely on the solar wind, such as the M2P2 [space.com], which is an interesting conce
Solar wind and Voyager (Score:2)
Re:Solar wind and Voyager (Score:5, Informative)
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 [spacedaily.com] couple of years ago but it suffered a launch failure).
Re:Solar wind and Voyager (Score:3, Informative)
Mariner 10 [solarviews.com] 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?)
I don't think so (Score:5, Insightful)
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.
No, its an adiabatic expansion (Score:5, Informative)
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).
Re:No, its an adiabatic expansion (Score:3, Insightful)
Thermodynamics would only come up if someone claimed to be sailing on microwave photons from the 3-degre
Re:I don't think so (Score:5, Informative)
In the way of background, note that Gold is the same Gold of the Bondi-Holye-Gold steady state cosmological model, proposed in the 1940s and 1950s as an attempt to "fix problems" with the big bang model, and has long held non-conventional views on light. Gold and others invoked "tired light" -- photons which became redder from their point of emission, even though doing so contradicted momentum-energy conservation. It's a archetypical example of a theory trashing a fundamental principle in order to exaplain last week's cosmological observations. We should always be wary of our assumptions, but all too often, cosmological theorists will attempt to make a splash by abandoning them in favor of explaining very tenuous and often incorrect observations.
Gold has always been an outsider in the astrophysics community, but has done some very good work over the years; including some seminal work on pulsars. He was Peter Goldreich's (major figure in theoretical astrophysics, for those not familiar) Ph.D. advisor.
Those interested in the history of the steady-state model, including attempts to resurrect it, and the many errors it commits, can check out this page [ucla.edu].
Re:I don't think so (Score:3, Insightful)
It's perfectly fine for a physics outsider not to bother understanding, say, standard thermod
Obligatory Simpson's Quote (Score:2, Funny)
If it does work... (Score:3, Insightful)
Re:If it does work... (Score:3, Insightful)
I think it's even easier in the case of solar sails: all you need to do is angle your mirror 45 degrees to the sun (reflecting the solar wind toward your direction of travel) and gravity will do the work for you. You'll slow down into an elliptical transfer orbit that will take you closer to the sun.
Re:If it does work... (Score:3, Informative)
Actually, a solar sail makes use of solar radiation pressure (i.e. pressure exerted on a surface due to momentum transfer from reflected photons). The "solar wind" is a stream of energetic particles (not light) emitted from the sun. It would produces several orders of magnitude less force on a solar sail than would solar radiation pressure.
That said, you are correct that it is possible to move in a sunward direction by orienting the sail in a way t
Re:If it does work... (Score:4, Informative)
Nope. They could maneuver in a way similar to that sailing ships use to go upwind. By angling the sails correctly and using the sun's gravity field, a solar sail vessel can fly "upwind" toward the sun. See this NASA reference [nasa.gov] for a basic primer.
Tacking sail boats (Score:5, Insightful)
The other comments about gravity doing the moving-towards-the-sun bit sound right, though.
Ignores Red Shift (Score:5, Funny)
"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.
ALMOST tested (Score:3, Interesting)
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.
the links to previous Slashdot stories... (Score:3, Informative)
http://science.slashdot.org/article.pl?sid=01/07/ 2 0/1246254 [slashdot.org]
http://science.slashdot.org/article.pl?sid=01/07/2 2/0321239 [slashdot.org]
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 [slashdot.org]
It mentions a test
Photon Pressure (Score:5, Interesting)
Re:Photon Pressure (Score:3, Informative)
You can check this: just ask where aT^4/3 (radiation pressure) is equal to the product of density*N_A*k*T/mu (gas pressure), with mu the mean molecular weight, Na and k atomic constants, and T the temperature. You'll get
density = 1.5 x 10^-23 T^3 g cm^-3
meaning that radiation pressure dominates gas pressure only for very high temperatures and low densities. T
Re:Photon Pressure (Score:3, Informative)
nitpicking point in the article (Score:4, Interesting)
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.
Re:nitpicking point in the article (Score:5, Interesting)
Flaw: Light is red-shifted climbing out of the gravity well. So when it reaches d2 there is not so much energy as when it left D1, so a smaller amount of mass will be produced. When it falls back down, the mass difference is equivalent to the kinetic energy gaind from falling by the equivalence relation E=mc^2.
Re:nitpicking point in the article (Score:4, Informative)
Can you expand on this? I've never heard of this, and I can't think of anything in my 40+ years of layman's reading on physics that could be expressed this way.
Here's a few links. Google for "gravitational redshift" and you'll get lots more.
Link [wolfram.com].
Link [std.com].
To summarize, the gravitational redshift (or blueshift, for light falling into a gravitational potential well) is a real effect. It was demonstrated by Pound and Rebka at Harvard University in 1960. They used essentially monochromatic gamma ray photon sources at the top and bottom of an elevator shaft, and measured the shift in frequency for photons traversing the shaft each direction. Kudos to Einstein--General Relativity gets another check mark.
Re:nitpicking point in the article (Score:4, Informative)
Light does indeed do both. As it drops deeper into a gravitational potential, it blueshifts; as it emerges, it redshifts. Its path will also appear to be bent by the gravitational field. (I could introduce you to a number of physicists who will adamantly insist that the light follows a straight path along curved space, but that's getting awfully picky.)
Light cannot emerge from a blackhole because any photons emitted inside the event horizon get redshifted down to nothing on their journey out. They may indeed follow a curved path while they do so, but this is not essential to the process. Some physicists speculate that this bending has an interesting consequence right near the event horizon. In principle, photons on the right trajectories could orbit the black hole--kind of a neat thought experiment. Of course, any speculation about what goes on inside a black hole is merely that, since actual observations are rather difficult to come by.
Re:nitpicking point in the article (Score:5, Interesting)
Can you expand on this? I've never heard of this, and I can't think of anything in my 40+ years of layman's reading on physics that could be expressed this way.
It's a well-known effect in General Relativity (well, to General Relativists!) and it is called the gravitational redshift effect. In fact, GPS software has to take in effect the gravitational time dilation of radio photons 'falling' from the satellites to the receivers [virginia.edu], amongst some other relativistic corrections, in order to get a triangulation down to a few meters.
Sorry if I've borked up the details, haven't had coffee yet!
Dr. Fish
Re:nitpicking point in the article (Score:3, Interesting)
Re:nitpicking point in the article (Score:3, Interesting)
According to General Relativity, transporting the energy from d1 to d2 in the first place will _decrease_ the energy by the same amount, so you can't create energy in this closed process.
This indeed happens to photons -- Pound and Rebka measured the effect, known as a gravitational redshift.
Re:nitpicking point in the article (Score:3, Interesting)
Re:nitpicking point in the article (Score:5, Funny)
the second law of thermodynamics is only a statistical law
I'm not a physicist, but I thought the first two laws of thermodynamics were:
1. You do not talk about thermodynamics.
2. You do not talk about thermodynamics.
Maybe I'm getting confused with something else.
Why use a mirror? (Score:3, Interesting)
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.
Re:Why use a mirror? (Score:3, Informative)
Carnot's Law (Score:2)
Other particles are available apart from photons (Score:5, Interesting)
Ahhh... but Gold has forgotten the Doppler effect (Score:5, Informative)
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.
Physics (Score:5, Interesting)
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.
Re:Physics (Score:3, Informative)
Has essentially been done in 1901. See P.N. Lebedev, Ann Phys. (Leipzig) 6:433 (1901).
Re:They have done it 1000ths of times. (Score:3, Informative)
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".
Already being done (Score:3, Informative)
http://www.planetary.org/solarsail/
Excellent Solar Sailing Book (Score:5, Informative)
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 [amazon.com] (yeah, yeah, it's amazon, thbbbppp) and here [planetary.org] 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...)
Actual Physical Expermental Proof (Score:5, Informative)
Comet tails *always* point away from the sun.
Must be something pushing the tail particles away.
QED
Re:Actual Physical Expermental Proof (Score:3, Insightful)
Absolute rubbish (Score:5, Insightful)
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!
Why bother with giant mirrored sheets? (Score:5, Interesting)
Not photons doing the pushing (Score:3, Interesting)
IAAP and he's almost certainly wrong (Score:3, Informative)
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...)
The light *does* cool down so Gold's wrong (Score:5, Informative)
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
Misconceptions (Score:3, Interesting)
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...
Gold's a senile crank (Score:3, Interesting)
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.
Tommy Gold. (Score:3, Informative)
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."
No more absorption (Score:3, Informative)
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!
Re:Laws? Who needs them? (Score:5, Informative)
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 [nasa.gov]: 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.
Re:Laws? Who needs them? (Score:5, Informative)
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.
Duelling authorities (Score:3, Informative)
though intellectual honesty impels me to concede it also says
No, it IS a heat engine. But he's still wrong. (Score:4, Insightful)
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.
Re:No, it IS a heat engine. But he's still wrong. (Score:3, Insightful)
Re:Laws? Who needs them? (Score:3, Interesting)
That turns out not to be the case. If you model the reflection process closely, you'll see it's equivalent to absorption and reemission, so the photon imparts twice its momentum.
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. I
Re:Laws? Who needs them? (Score:4, Insightful)
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.
Re:Laws? Who needs them? (Score:3, Informative)
Re:Check My Math and Gold's Math (Score:3, Interesting)
(Oops, accentally hit submit. Here is the correctly formatted version.)
Gold Makes the following claim: