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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."
When you have a cool concepy like a solar sail, why let little things like laws of physics get the in the way?
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
Has Thomas Gold seen an anemometer?
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...
*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.
Didn't Voyager and Galileo take advantage of the solar wind to get way out there in a short time? How different a concept is using a "sail" to go if it's riding the same (or related) mechanism?
Be excellent to each other. And... PARTY ON, DUDES!
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.
"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.
NO ID: BEING FREE MEANS NOT HAVING TO PROVE IT
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.
...a radiometer.
The material must absorb the photon to convert it into kinetic energy.
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.
IANAPhysicist, but its recently been suggested here that quantum mechanics might allow us to extract energy in situations involving a single heat bath.
...vividly encapsulates that post-Watergate/pre-punk/coked-up moment when you could trust no one, least of all yourself.
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
Karma: NaN
"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
A solar sail is essentially a giant mirror that reflects photons of sunlight back in the direction they came from
Great, so in a few hundred years the granola crunchers will start complaining that we're causing Solar Heating.
Trolling is a art,
I hope Gold didn't OK that, because it's physical nonsense. What it is trying to imply is that if the photons are emitted with the same energy as they are absorbed, the solar sail can't gain any kinetic energy (because of conservation of energy--the photons kept all theirs, so the sail gets none). But what about the conservation of momentum? The photon was headed rightward in the first diagram and headed left in the last one. That's a next change in momentum--where did it go, if not the sail?
Never mind whether the rocket's moving or not. There is no such thing as a perfect mirror. Some of the photon's energy will be lost when they strike the mirror, so the photons will be red-shifted in that way. They will be re-radiated with less energy (and hence momentum) than they hit the mirror with, hence a net "push" on the mirror.
Stick Men
So dumb... and yet, so funny. :-D
(and you didn't even need to mention Natalie Portman, In Soviet Russia, Beowulf clusters, etc...)
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.
You are correct. This omission appears as an explicit assumption in Professor Gold's "Paper". "If this was a perfect mirror, the two temperatures will be the same". Cheers!
NO ID: BEING FREE MEANS NOT HAVING TO PROVE IT
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.
I agree, at least to some extent. In the paper Gold commented on the energy of the photons entering and leaving the sail, but that was all in the reference frame of the sail itself. Of course the sail isn't moving relative to itself.
Escher was the first MC and Giger invented the HR department.
The Planetary Society already has a solar sail project going and is preping to launch soon.
http://www.planetary.org/solarsail/
I wonder if they've tried mithril from Khazad-Dûm? It's as light as a feather and hard as dragon's scales.
796F75617265616E65726400
human-made solar sails were not only good enough for inter-system travel, but they were speedy as all hell too.
that count dooku guy must be using his evil midichlorians to break the laws of physics.
(as if super jumping, telekinesis, and finger lightning weren't outrageous enough. he really slaps physics in the face with those sails)
// "Can't clowns and pirates just -try- to get along?"
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?
Anybody knows what happened to Star of Tolerance? It was to be a demonstraton model for solar sailing, planned to be launched ahead of other experiments by various governmental agencies, the guy behind it was on the front page of Scientific American or New Scientist, a while ago... They launched a test vehicule, but something went wrong. Their webpage has been pulled.
Ooops, I'm talking rubbish (at least the latter part): forget the "and hence a net push" part.
Stick Men
Get me the patent on this!
.. basically pump all the heat to some single point where it can radiate out. You'd need to power it with an RTG or solar cells.
According to the article solar sails only work if the object is of lower temperature.
Ok so you have a cooling system to cool down the solar sail
Ok guys this is kinda OT, but I always wondered for a long time. Let's say you have a hollow sphere. THe inside of the sphere is lined with mirrors (basically, a mirror sphere turned inside out) Now what will happen if you flash a beam of light into that sphere?
Let's say that you were able to flash that light through a small hole and then replace that hole back with a mirror. Will the light keep flashing back and forth to infinity??
If you were viewing from the inside without affecting the path of the light, would it be shiny all over the place? What will happen?!!!!
Ok, after rereading your post several times and the article twice, I think you're right.
I just like how you threw in the word "Duh". It lends surprising weight to your rebuttal.
Solar Sailing?! Sweet! Let's strap on skiis and hit the cosmos!
"Those who would sacrifice liberty for security deserve neither!"
Carnot's principle and thermodynamics in general only applies where you are very close to thermodynamic equilibrium. Even the concept of temperature does not apply unless there is equilibrium.
To me, the obvious come-back to Gold is an example of a laser that emits photons of a single energy. Shoot the photons at a target, mirror or otherwise, and momentum is transferred to the target. Presto, a laser sail. The sun is not a laser, but it emits photons which are one-by-one indistinguishable.
The solar system is not in thermodynamic equilibrium. All these photons are basically shooting from the sun outward. Look at the Sun, you see 5000 K. Look outward, you see 3 K cosmic background. Don't you feel the push?
Physics is wonderful!
Fiat Lux.
When placed in the sun radiometers do spin the wrong way if radiation pressure was what caused it to spin. Instead the black sides of the paddles are pushed along by heated vapor inside the radiometer. The vapor near the black side of the paddle heats up faster and pushes against the paddle.
Now, if you put the radiometer in a microwave that's when it gets interesting. It glows like a light bulb and spins like crazy one way. When you stop the microwave it eventually slows down and starts spinning the other way. I don't remember which one is which, but I think it spun the correct way for radiation pressure when it was being microwaved. I wouldn't recommend trying this though. My radiometer eventually exploded leaving little shards of glass everywhere inside my microwave.
Now as for the guy refuting solar sails, he is a crackpot. Some red flags are:
1. Explaining what c stands for
2. Mentioning Einstein
3. No math to back it up
4. Using an argument to essentially say the conservation of momentum is wrong
Just about anyone can get something placed in arxiv but the crackpots rarely get published in the real journals.
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.
Tron: Solar Sailer - one of the FINEST Intellivision games ever produced. Just awesome!
"End of line"
Lisa, get in here. In this house we obey the laws of thermodynamics!
Comet tails.
Comet tails *always* point away from the sun.
Must be something pushing the tail particles away.
QED
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Sticking feathers up your butt does not make you a chicken - Tyler Durden
It's already been done, I believe: Mariner Mercury used the orientation of its solar panels for attitude control to extend the mission by reducing fuel usage.
Though, to be fair, at that distance I'm not sure whether most of the resultant force came from photons or the solar wind.
viewing photons == absorbing photons. stick an observer in the sphere (or poke a hole in it again - it makes no difference.) and they would see a flash of light as they absorbed all the photons.
I saw this documentary on Discovery about a russian torpedo that creates it's own air bubble around it so that it can use a different propulsion system to increase the speed of it underwater.
Given the fact that space is a vaccuum, and that we're talking about solar sails here, obviously anything remotly related to gas emition wouldn't work, since any gas propelled would immediately get sucked out into space.
My question is, wouldn't it be possible to contain the solar power, have the heat transferred into gas (again in a contained field), and use the gas emition as an exhaust instead, like gas propelled? I'm guessing you could still use the sail as a way of collecting energy to created this gas. Of course this is still not quite effective the more distant to the sun your ship travels.
In which case wouldn't solar sails only be good for a Sun-present quadrant? And this being relatively close to it in any event?
Just a few lingering thoughts.
QD
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.
They don't know even know if it will work theoritically but they are giving it ago. I love it. Whos paying though cuase if it goes wrong thats an expensive piece of scrap metal floating around up there.
-- Karma Karma Karma Karma, Karma Chameleon - Boy George
As far as I can tell, you could use our old friend gravity to return to earth on a budget. All you have to do is slow down your revolution around the sun and you'll start falling closer to earth's orbit.
There would be timing issues, but what interplanetary expedition wouldn't?
"Sometimes, I think Trent just needs a cup of hot chocolate and a blankie." -Tori Amos on Nine Inch Nails
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 |
Yes the sail is moving relative to itself- it's accelerating. There is no reference frame on the sail (atleast not an inertial one).
-WolfWithoutAClause
"Gravity is only a theory, not a fact!"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...)
It seems like almost all of you have neglected, in your consideration of perfect mirrors, that the velocity of a photon can NOT change. The speed of light is constant. Rather, the effects of surface diffraction (scattering) causes photons to be absorbed, while a fraction of them are reflected, but at the same speed they had going in. Suffice it to say, no one might have a sufficient understanding of the mechanisms involved to predict the outcome. Wait and see what happens when the thing gets in space.
QUIT FEEDING THE TROLLS!
This is not a closed cycle, the sun is actually an energy source. Therefore, obtaining energy from it and converting it to useful work is quite possible. Not only that but momentum is given by m*V where the V is a velocity (i.e. a vector and direction matters). If the photon was reflected in the opposite direction, it underwent a significant change of momentum (unless there is some caveat in the physics of photons that I'm not aware of - and that may be possible).
A goal is a dream with a deadline
I am no dolt at science but without /. I probably would ahve read the article and gone maybe. But after reading the comments the general consensus is that Gold is wrong. And this is through some pretty sound scientific logic. Its great how most people here are generally pretty science minded. I wish this was the rule for the rest of the world.
You know what I am getting at.
-- Karma Karma Karma Karma, Karma Chameleon - Boy George
Wrong! A blackbody would radiate equally on all sides, but a reflective body does not. A perfectly reflective body would radiate the energy back in the direction from which it came. But an imperfect reflective body will send some radiation in all directions, but more radiation will be reflected back toward the source than in the other directions. Hence, a net non-zero force does exist that pushes the imperfect reflective body away from the radiation source.
Gold Makes the following claim:
sss
Gold claims that conservation would "accelerate the body to an energy content of Mvc." Assuming the object of mass M is intially at rest and radiation of energy E is shined upon the object and absorebed, then the "Resulting Kinetic Energy from conservation of momentum" is E^2/(2 c^2 M ) which is much not the same as Mvc unless v = 2 c. (A rather high velocity.) Check my math here.
Conservation of momentum:
Momentum from Light = E/c
Momentum imparted to object = E/c
Resulting velocity = v = E/(cM) Resulting Kinetic Energy from conservation of momentum = RKEFCOM = 1/2 M v^2 = Mvc * (vUnder the link 'Can it really sail away?' there's a diagram saying that since, on a perfect mirror the photons will not lose energy, the mirror can't receive any energy. This is wrong:
The photons change direction when they are reflected; this means they have changed momentum, which corresponds to a force exerted on the mirror. The situation is equivalent to a little steel balls hitting a hard surface - if the 'reflection' is perfect, no energy is lost as heat. However, both photons and steel balls will lose energy by transferring it to the reflecting surface.
In the case of photons the force exerted on the surface is very small per photon as well as of very short duration, so the mirror isn't accelerated very much, and the energy transfer is very small, but it is not 0!
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.
A solar sail is essentially a giant mirror that reflects photons of sunlight back in the direction they came from.
Wrong! Even I know that solar sails are not driven bz reflectens of the photons, but get their momentum very normal from the solar particles.
These are the particles that also cause the aurora borealis. These have rest mass, normal momentum and anything normal.
Simple as that. So can somebody explain what the fuzz is about?
--
Karma 50, and all I got was this lousy T-Shirt.
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.
the universe gives us the finger
l is t=true
http://antwrp.gsfc.nasa.gov/apod/ap030630.html?
But I thought the solar sail worked by capturing the kinetic energy of ions, alpha particles, and other particles ejected by the sun, not the incoherent light. Then again, I doubt I know more about then someone who works for the "Center for radiophysics" :P
autopr0n is like, down and stuff.
Well, you could also just set a solar sail in motion and have it go off into space. The system you're talking about might just accelerate 'forever' with the limit of the kenetic energy being equal to the energy imparted by the bomb. In general a 'perpetual motion machine' runs in a cycle and has some drag. There are lots of cycles in space that run forever anyway, like the earth's orbit around the sun, polar stars, etc.
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this is really funny. Where are my modpoints when I need em?
Well, when you bounce light off a mirror when that mirror is moving relative to the direction of the light, then the frequency of the light changes as well. It's called redshifting (as you probably know). If a mirror is moving away from you, it's frequency is diminished. Less frequency means less energy, so each bounce produces less energy then the last.
Personaly, I think the thing would accelerate 'forever' but that acceleration would have a limit of zero as t->infinity, and the KE would have a limit of the energy of the bomb, assuming that all the energy was reflected by the mirrors. (of course, you do have gravity pulling the mirrors back together)
autopr0n is like, down and stuff.
They are vaccum tubes. No air.
autopr0n is like, down and stuff.
The crucial bit is, Carnot's argument holds for a heat engine, a device that executes a cycle and returns to its original state. The solar sail is not returning to its original state.
:P
:P
I think for human transportaion, this might cause some problems
(but yeah I agree, any system can continue to operate while it gets new usable energy (like one-directional kenetic energy of the photons).
This is begining to sound more and more like the "Rockets will never be able to go to the moon, since there is no air to react against" thing
autopr0n is like, down and stuff.
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.
They also have a good overview of solar sailing on the project page here.
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
Light reflected off a moving surface is redshifted, accelerating or not. As long as something is bounced back, there should be some energy returned. And yeah, the energy will be less
autopr0n is like, down and stuff.
The black side and the shiny silver side may be at the same temperature but their emissivity (tendency to radiate) is different. That's why dark colored radiators work better than light ones. Look at the radiator inside you car.
No, the sail is not moving relative to itself - the concept is nonsense. GR says reference frames don't have to be inertial.
Since the mirror will move away from the sun, the light will be lower in frequency on the way back, and thus the photons will have lower energy :)
autopr0n is like, down and stuff.
other than photons...these and the photons do exert a pressure that has been measured. I don't think the skeptics conclusion that "C's______ heat transfer equations are valid because this clearly is a heat engine" is a valid conclusion. Then his entire paper falls apart if that's the case.
Want to see every step I took to start my company? http://www.rowdylabs.com/blogs/pitchtothegods
Even if it works, it will spawn nothing but a bunch of millionaires named Biff or Muffy sitting on teak paneled solar yachts cluttering up space while going for a weekend jaunt to the moon.
Cue the Rodney Dangerfield quotes from Caddy Shack...
I agree this would deliver energy to the sail. Now, what makes us think it will all go into acceleration? This, to me, means that the sail will probably heat up.
I didn't see any reference to this article which nicely explains how Crooke's Radiometer works, with references. Note in particular the reference to an experiment showing that when the vacuum is perfect and friction is minimized, the vanes move away from the reflective side.
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?!
Thermodynamics doesn't talk about momentum. It talks about heat energy. This solar sail isn't using the tremendous amount of heat energy that falls upon it and is wasted. This solar sail uses the momentum from the photons (and other ions?). It uses Newton's older simpler laws. Most people don't try to get energy from the momentum of light as it is tiny compared to the energy in the light itself.
Ok so we all have seen Leik Myrabo on TV demonstrating that a laser can push a small sail in vacuum. Now, this should work equally well with a bigger sail in a bigger vacuum with a bigger light source, right?
Not a bad idea until you run out of gas.
Actually, since you are in space, you don't really even have to heat the gas. Just open your (relative to space) pressurized bottle of gas, and you have an instant rocket. Problem is, it's very expensive to haul up a bunch of atoms up through the atmosphere for the sole purpose of throwing them away in space. And help you should you run out, as you would no longer have any propulsion system.
I'm amazed that someone at a Center for Radiophysics and Space Research does not realize that E is a vector quantity. This is one of the most basic principles of radiation transport. The trasfer of momentum from particles and photons is fundamental and established in models of great predictive value. I'm afraid Mr. Gold has steped outside his field of expetise, which looks mostly geophysical.
Friends don't help friends install M$ junk.
1. If the sail translates any of the energy particles absorbed into heat, it will radiate more of it from the sun side than the dark side. It is recieving the engergy from the sun side and there should be a thermal gradient in the material, such that the sun side would be hotter, and radiate more engery back into space, and produce thrust, no matter how small. 2. Of the energy particles striking the sail that are not absorbed, energy will still be transfered while maintaining velocity. As many have said, there is a red shift - The particles may change from visible or ultraviolet to infared wavelengths, but they will still maintain velocity while giving up energy to the sail. These particles didn't give up kinetic energy (velocity was constant in the interaction), so my question is how does it get translated as so? Might the craft just sit there (or fall towards the gravity well) and heat up? Does this energy translate back into #1 above and produce thrust? would the definition of the perfect mirror mean that no heat is absorbed from a particle impact, only kinetic energy? 3.Ultamately this only considers the energy particles, and no the physical particles emmitted from the sun that could be turned to propulsion. These can lose velocity on impact and directly translate this to the sail as acceleration. Tom
Not this totally inappropriate set of arguments that
are a combination of empirical/statistical approximations and Newtonian bafflegab.
Photons ALWAYS travel at relativistic speeds.
The sail will eventually accellerate to near-relativistic speeds. This will further decrease the number of photons incident/second further than the inverse square law from distance from the light source.
The imperfect mirror will definitely shift wavelengths/direction of the photons.
The imperfect mirror will heat up because part of its imperfections. The necessary re-radiation of the heat (if directed in the same direction as the original light source) will transfer momentum OUT of the sail.
Could someone with a PHD in physics working with relativistic equations please explain this to us?!?
but if they aren't inertial, then there can be red shift, and the point is still correct...
"I'll have a Guinness, no wait, make that a Coors Light" -Grad student I work with, who shall remain anonymous...
First off, he's using Carnot's figures wrong; in the heat pump sence, shouldn't he use T1 as the sun's output and T2 as the temperature in the shadow of the sail?
:)
I mean, that's the temperature delta he should be using, not the difference between incoming and outgoing radiation (which, due to the fact that a perfect mirror isn't used, seeing as we can't make those, loses energy anyway...now who's not ignoring physics?).
Not only that, shouldn't he be using a different set of equations anyway? Why use those meant for a Carnot process when he's dealing with a different set of energy trandsformations?
's Like using the equations for an inelastic transformation when one should be using those for an elastic one...(well, not exactly, but kind off...sort off...well, not at all, but you know what I mean
-- Waht? Tehr's a preveiw buottn?
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.
Hot air will have lower density in a closed system, so while the molicules hit harder, there are less of them. The motion is caused by the way air moves around the edges.
autopr0n is like, down and stuff.
If the comets were hot enough not to be heated by the radiation, or were 100% reflective, the tail wouldn't exist. Even if that were true, which it's not, all we would need to do would be to make the surface of the sail not 100% reflective and cold.
autopr0n is like, down and stuff.
though intellectual honesty impels me to concede it also says
The Mongrel Dogs Who Teach
With all the arguments using various forms of physics, people have forgotten one basic fact: The solar sail cannot work until some sort of particle screen is created to protect them. The sails would have to be so large that without a particle screen, micro-meteors and space dust would tear the sails to shreds before you could get out of Mars orbit.
Light Speed Limits... Not just a good idea.. its the law. B. Lightyear
"I drank what?" - Socrates
Actualy, in a perfict vaccume they spin the other (correct) way, assuming that the non-black side is a mirror.
autopr0n is like, down and stuff.
That wouldn't be any diffrent then the heat generated by nuclear energy or burning oil or anything else. In fact, it would cause less heatup then burning oil and other carbon based fuels, which cause greenhouse gasses, trapping more light in addition to creating heat energy.
autopr0n is like, down and stuff.
Looks from the diagram like Gold's notion violates conservation of linear momentum. But I haven't yet read the article.
The sun gives off not only light, but also spits off extremely fast moving plasma, called the solar wind. The wind, though not very dense, typically moves as a speed of several hundred miles per second. That's around a million miles an hour. (You can check current contitions here). The idea is to use the momentum from the solar wind, which is mostly ionized hydrogen, not the light from the sun, to propel the spacecraft. (There are some very good images on the website.)
The most interesting approach was suggested by Robert Winglee of the University of Washington. He suggest using a giant magnetosphere (essentially a magnetic field stretched out by a plasma) as a sail. The magnetophere deflects the solar wind, transferring momentum into the spacecraft. There is also another advantage - the magnetosphere works as a shield to keep the wind from damaging the spacecraft itself.
The "sail" is made from an ionized gas trapped in a magnetic field. It's easy to let the sail out and take it in, and if the sail ever "breaks," you can just make a new one using more plasma.
So, despite what the paper has to say, solar sail research is alive and well. It's just that the most promising designs work a little differently than the author thought.
At that point, the incident surface of the mirror will be hotter than the other side, at which point both can be treated as black-body radiators, with a defined temperature and surface area for each surface. Then, both will emit radiation proportional to T^4. Radiation that is emitted toward the sun will decrease the efficiency of the mirror. As such, any flaws in the mirror directly translate into less than 100% efficiency.
I don't usually flame, but that article was absolutely worthless and looks like it was written by a guy who got out of thermo. And has he never seen one of those little devices that has a little spinner on a needle in an evacuated piece of glass? The spinner has a bunch of vanes on it, each painted black on one side and white on the other. Surprisingly, when exposed to sunlight, it spins. So the proof-of-concept is there for the solar wind mirror.
-Looking for a job as a materials chemist or multivariat
The article refutes the idea of solar sails designed to use momentum from photons to move a sail through space. The article seems to confuse thermodynamics and mechanics, ignoring the conservation of momentum to make its point. The article makes points out the fact that a perfect reflector would not drop the photons temperature, so it cannot be used as an engine (no temperature drop in a Carnot cycle).
Since a photon's 'temperature' is proportional to its frequency, I guess this is true. If there is no frequency change when the photon is reflected back in the opposite direction from a perfect reflector there is no 'temperature' change. But the direction of the photon is changed by reflection, and momentum must be conserved. an imperfect reflector would probably result in some 'temperature' change. How you could use this in a Carnot cycle I don't know.
The article quotes a Steven Soter: "Steven Soter, an astronomer at the Hayden Planetarium in New York, is open to Gold's idea. He says applying conservation of momentum to photons could be a mistake. 'Light is very different from matter, and one may wonder if the momentum rules are also different.'" Soter works at Hayden planetarium and was a collaborator with the late Carl Sagan. I don't know his background or credentials.
His statement shows a lack of understanding. The momentum of the photons should be p=hf/c. Where p is momentum, h is planck's constant, f is frequency, and c is the speed of light. If the photon is reflected perfectly the sail must pick up twice the original momentum in order to balance out. Momentum has both direction and magnitude. If you start out with one photon moving away fron the sun (call it one unit of momentum) and a stationary sail. After reflection you have one photon moving torwards the sun. The reflected photon has the same magnitude, but opposite direction, or -1 unit of momentum.
But conservation of momentum means that the total system should be the same before as after or one unit (positive direction) total. So to get a total of one positive unit the sail must have two positive units of momentum.
before collision the photon has one positive unit, and the stationary sail has zero units for a total of one positive unit. After collision the photon has one negative unit, the sail has two positive units for a total of one positive units. I think that the conservation of momentum has been tested for photons.
The article also states that the first flight of a solar sail will take place this fall, but the Russians have already launched.
The article also incorrectly explains why most crooke's radiometers move in the direction of the white side, and are propelled by the black side, here is a good link that explains why.
Whew! Both Steven Soter and Thomas Gold seem to have good reputations. I think Gold's arguments about a sail not being a carnot engine are accurate, and are being applied out of context. It does not matter if a solar sail is not a good Carnot engine, any more than it matters if a wind sail on a ship is a good Carnot engine. It does matter that a steamship have a good Carnot engine.
Mr. Soter's quote is disturbingly inaccurate.
Actualy 'true' radiometers have been built, but you have to use a pure vaccum, and cover the materal in a glass coating to prevent offgassing. It does work. see here
autopr0n is like, down and stuff.
via:
article
"Indeed, a small "kite" or solar sail (31 cm x 76 cm in area) was actually added to the Mariner series of spacecraft to balance the solar pressure on the solar cells. However, it wasn't until the Mariner 10 mission that solar sailing techniques were used for maneuvering by using the pressure of sunlight reflecting off of the solar panels for attitude control. By using the ballast solar sail for attitude control manuevering, the project was able to extend the planned life of the mission to get more data."
NASA's asymetrical capacitor thruster will work if this doesn't! :-)p ://www.vacuum-energy.com/nacm.htmll ogy.nasa.gov/scripts/nls_ax.dll/twDi spTOPSItem(111;TOP8-80;0;1)
http://jnaudin.free.fr/lifters/main.htm
htt
http://techno
The complete cycle in the case of a solar sail includes the manufacture of the "heat" ie the sun itself. When you mesure the efficency of the the entire cycle, you see that it does obey the law. QED.
Well.. maybe. Or Maybe not. But Definitely not sort of.
did somebody's reality cheque bounce? Long, long ago, when solar sails were proposed, they were intended to use the mass emitted from the sun to move them along and it was clarified that they were not intended to use light which they said would not work. In the past few years the discussion of solar sails has been revived but has been focused exclusively on using light to move them. So what has changed? Has some miracle occurred whereby radiation has become better than mass at pushing these big sail thingies around? Perhaps we've been totally wrong all these millenia about using sails to catch the wind at sea, as well?
Its just so simple nobody makes a fuss about it. This article is just like all the others claiming einstein is wrong or "the universe is 5731 years old".
You can levitate cones or spheres even in earths graviation well (if you use a light source bright enough).
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.
There are problems with solar sails. But they are engineering prolems, not concerning the scientific background.
HI O WISE PRINCE. WHT TOOK U SO DAM LONG?
If an object has energy then it has mass. Just plug the energy into E=mc^2 and solve for m
Thomas Gold has a long history of fringe stuff, but this is the first time I've seen so directly how he can get something so obviously wrong. Did he even run this by anybody knowledgable before posting it?
Aside from the steady-state cosmology somebody mentioned earlier, Gold was the one who predicted in the 1960's that the Moon was covered with a thick layer of dust that would doom the Apollo missions - he wasn't entirely wrong, but the dust layer (regolith) turned out to be compacted and very strong, and not exactly a problem. And since then Gold has been the big proponent of primordial hydrocarbons (or at least carbon) in the Earth's deep interior, and the interior of other planets and the Moon too. Most recently he's been pushing his Deep Hot Biosphere proposal.
I haven't looked closely at his other stuff, but if it's as poorly thought out as this one, no wonder almost nobody believes him!
Energy: time to change the picture.
QM and particle colliders have definitely proven the concept that momentum is vector dependent for even quasi-particles. That's why you can predict the trajectories of particles. But there is no evidence that this concept remains true when you have a large amount of particles interacting. Gold's argument seems to be that on macroscopic scales, the vector dependence gets swamped out by the intrinsic random nature of many quasi-particles interacting and that's the root cause of Carnot's laws.
It's great that the experiment is happening soon, since we'll get to see the results soon. I'm expected Gold to be proven correct. The many-body estimations of QM theory have a lot of assumptions and holes in them. Simply do to math limitations, the best we can do with QM these days is fully account for 3 particles.
on a planet with no biosphere?
nice
I think this is all /. needs to know to not listen to the damned new scientist.
The sad thing is that Carnot equations aren't even designed for this sort of thing. Has the guy who wrote this never HEARD of black-body radiators?
The efficiency will be:
Efficiency = (Incident - BBdark)/Incident,
where BBdark is black-body radiation radiated by the non-illuminated side. How is this hard?
-Looking for a job as a materials chemist or multivariat
You know what I'm tired of doing twice the work required, the purpose of a mission is to get there, getting back is just this moral "extra credit" if we make it a policy that no more return trips we could spend that much more in getting farther plus there is not as much of a setback when space ships blow up. People wont mourn the human lives lost just the fact that they didn't accomlish their task. Besides, there are a lot low income kids out there that truely want to become astronauts, lets give them a chance for goodness sakes!
It seems to me that Gold's main argument is that radiation pressure won't affect a perfect mirror. I contend that he forgot about relativity.
Gold says that a photon striking a perfect mirror will be reflected in the opposite direction with exactly the same energy. Since a photon's momentum is linked to its energy, it will also have the same momentum. Thus, no momentum could have been imparted to the mirror.
I would argue that this is true, but only from the photon-mirror reference frame. From the reference frame of the sun, the photon will have been red-shifted: it will have lost a little energy and thus some momentum, and the mirror will have been accelerated.
Consider this thought experiment. Construct a huge mirror in the shape of a half sphere (shiny side in). Put it around the sun. Then, all radiation emitted in the direction of the mirror will be reflected the other way. Thus, all of the radiation from the sun will be moving in roughly one direction. Gold contends that neither the mirror nor the sun would move. But, photons have momentum. The net momentum imparted to the sun from radiated photons is normally zero because the photons are radiated equally in all directions. With the mirror in place, the photons would not be radiated equally in all directions. The net momentum would not be zero, and something in the Sun-mirror system would have to accelerate. I'll leave it to you to decide what would move.
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
Assume we have inertias, m1 and m2. m1 starts with velocity v1 and mass two starts with no velocity. m1 ends with velocity v1' and m2 ends with velocity v2 (these will be 1-dimensional vector quantities for this discussion).
Inelastic Collisions: v1' = v2
Elastic Collisions: kinetic energy in = kinetic energy out
Scientist A: "So, if we slam the Earth into our spaceship, it we can get it going really fast, right?"Scientist B: "Nah, let's use Jupiter instead!"
"There are a dozen opinions on a matter until you know the truth. Then there is only one." - CS Lewis (paraprhase)
In his paper, Dr. Gold wrote:
"If this was a perfect mirror, the two temperatures (of incoming and reflected light) 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."
His argument fails because the premise is wrong. In fact, the reflected light will be doppler shifted because there is a small but finite time between absorption and emission of the photons. I'm not going to bother with the exact proof (though I'll describe the process if someone else wants to do the calculations), so I'll have to admit that my argument is not conclusive. I will state, though, that without further calculations, I see no reason to accept the premise he presents.
The calculation: Figure out how much power would go in to a perfectly absorptive body (intensity times area). Then, for the body to be in thermal equilibrium with the radiation, it will have to emit exactly that much power as blackbody radiation. Figure out the blackbody spectrum from the power output, and calculate the temperature of the body from that spectrum. Do the same for the reflected and doppler shifted (in fact, doubly doppler shifted: once to put our observations in the frame of the mirror, once again to return to the sun's rest frame, and don't forget that the doppler shift back is infinitesimally greater in effect than the original doppler shift to the mirror frame) radiation from the mirror. I suspect that all laws of physics will hold fine.
BlackGriffen
For one thing, since the photons are reflected you need twice the impulse. For another, why use something has heavy as a full gram? I think the illustration showed a sphere the size of a dust grain.
Is this really a thermodynamics issue?
What about this:
Casimir effect
-... ---
Here's why the article is wrong. Carnot equations are used to describe a machine that derives heat energy from a temperature gradient. This thing doesn't (strictly) work that way - it works by having a gradient of photon flux on either side. More photons strike one side than the other, imparting momentum when they bounce off. If more photons bounce off one side than the other, then we have a net force.
I mean, you can easily explain this through Newtonian mechanics and the De Broglie relationship. This is what suggests to me that the original author lacked a decent college physics class.
-Looking for a job as a materials chemist or multivariat
The solar sail is not a thermal engine. It does not attempt to absorb heat and transfer it to a heat sink. To the contrary, it reflects photons.
So Carnot's laws are not involved here, much less violated. Therefore, the whole article is bull.
"New Scientist" is reaching a new low of bad editorial control. How can such a huge mistake go unnoticed? I am very disappointed.
--
Mad science! Robots! Underwear! Cute girls! Full comic online! http://www.girlgeniusonline.com/
I can honestly say that the new scientist, in which this was published, is a joke. It is not a journal. It has garbage like this from people who have no idea what they're talking about.
Please, people of slashdot, STOP believing the junk you read in there.
-Looking for a job as a materials chemist or multivariat
You, like Open_The_Box, are failing to make the distinction between mass (m) and rest mass (m_0). In my book, the equation you mentioned is written with m_0 instead of m. m can be derived from E^2 = m^2 c^2, it is finite and non-zero for photons, and it is useful in some cases.
-- Repeat with me: "There is no right to profits".
The children's toys operate with another effect than the proposed one.
:-).
The children's toys, known as Crooke radiometers, contain a vacuum of 0.05 mBar. In this environment, the effect shows as follows:
Photons hit black side of paddle -> paddle gets warmer -> molecules near the paddle have a higher impulse -> push paddle away with higher impulse
Photons hit reflecting side of paddle -> paddle doesn't get much warmer -> low impulse -> paddle gets pushed less.
Interestingly, Maxwell proposed a different effect, using the difference in density of the gas near the colder/warmer side. This was shown to be wrong: due to the free path being extremely long (because of the low pressure), there is no interaction between the gas molecules, and therefore no pressure density.
So, Crooke's radiometer doesn't show the effect. What does, you ask?
In 1901 Piotr Lebedev was able to show (with the by then much better vacuum) that the effect does indeed exist. However, you need a much much better vacuum, and you have to glass-coat your paddles to prevent paddle molecules gasing out, "dirtying" your vacuum.
Then, of course, your paddles turn in the opposite direction, because they have twice the impulse on the reflecting side).
Not that I'd heard a lecture on exactly this topic this week or something
As pictured in the New Scientist graphic, there is indeed a problem. You could take two solar sail craft - face them in opposite directions with their sails precisely parallel - then fire a laser at one of them. The laser light would bounce endlessly back and forth between the two perfect mirrors - imparting a little push to each one as it did so. The two craft would accellerate away with no energy being consumed in the process.
So, we must conclude that either the craft won't move - or that the photons are somehow transformed in the process of being reflected such that they impart a smaller and smaller push with each reflection until their energy is 'used up'. Since no mirror is ever likely to be 'perfect', that seems a reasonable explanation.
If the photons ARE transformed to some lower energy state by an imperfect reflection - then there is energy to be gained by the sail and it CAN work.
I've got a crazy idea. If it is all about the thing getting hotter, why not just store it and then drop it to back to Earth? We could then make use of that energy in the old fashion way we always do: the steam engine.
http://www.spaceandtech.com/digest/flash2001/flash 2001-059.shtml
I also note that it is common to see people who have to control spacecraft (especially those not in earth orbit) say that you cannot neglect the effects of solar radiation pressure on the spacecraft's attitude and trajectory. For example NASA's Microwave Anisotropy Probe plans to use solar radiation pressure for backup attitude control (see page 8) and at the very least, cannot neglect this force on the spacecraft.
I would tend to conclude that this guy doesn't really know what he's talking about.
One way to look at it is conservation of energy. It takes energy to climb out of a gravitational well. Higher-frequency photons have more energy, lower-frequency photons have less. Climb up, you drop the frequency, you increase the wavelength, and it's a red shift.
This is how black holes work. Ever wonder why light can't escape from a black hole even though the speed of light is a constant? It's redshifted by an infinite amount.
If that toy was the same one I had, the glass wasn't evacuated--it was full of dry air. It worked not on radiation pressure, but on temperature. You could test this by running the thing under a hot-water tap.
This guy seems to know what he is talking about...
With the side benefit that you're keeping the charged particles away from the passengers and/or electronics. If the magnetic field is reflecting them, they're not giving the payload radiation poisoning.
One proposal is to deploy a superconducting wire in a ring around the spacecraft.
Gold's article says that 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.
This is either incredibly naive or incredibly disingenuous. Crooke's radiometer is well-understood: air heated by the black side of the paddles expands and pushes the paddles away from it. Operate the radiometer in a vacuum, and it doesn't move---proving only that dim light bouncing off tiny paddles cannot overcome significant friction. Reduce the friction, and the radiometer works just as expected (see the link).
The thing about unrefereed, unedited pubs is that these days it is not that hard to get anything even slightly clueful published in either a refereed conference or journal or a reasonably-edited popular publication: there are lots of venues. So when an extraordinary claim shows up on arxiv, you should probably figure it's nonsense unless experts tell you otherwise.
A little internet research here reveals that Gold is full of it. First of all, if his thermodynamic explanation of "light pressure" is correct then the Crookes radiometer should stop rotating after a few seconds (once the black side reaches thermal equilibrium and the veins slow down under the influences of friction). Secondly the obvious question is this: If Reynold's explanation is INcorrect(and Gold is right) then the Crookes radiometer should rotate the "wrong" way even in a perfect vacuum. But guess what: Pyotr Lebedev did the experiment in 1901, creating a sufficiently good vacuum, and the thing rotated the "correct" way (according to the photon model of light). In sum, without resorting to complicated explanations of what is going on (Reynolds etc...) the experimental evidence clearly indicates that Gold is wrong.
Viewing a mirror in terms of thermodynamics is clearly a misconception. What I wonder is: How did this guy get to be quoted in New Scientist? Or should I not be wondering these things?
m = E / c**2
Photons have mass. What you're thinking of is that they have a zero *rest mass*. Rest mass is the mass you'd measure if you were moving at the same speed as the object being measured. If photons had a nonzero rest mass, they couldn't travel at the speed of light.
Lets skip down to the "formal" argument: From a formal point of view, it is clear that one could not equate radiative momentum content with Newtonian momentum. Newtonian momentum is Mv, clearly a vector, while the momentum attributed to radiation is E/c, a scalar, since E is a scalar and c is a universal constant of nature. He is arguing that momentum carried by sunlight somehow has become a scalar. This is complete garbage. Momentum is a vector quantity, period. While it is true that the magnitude of the momentum vector is proportional to the scalar energy it is not equal to a scalar. If you (wrongly) grant him that momentum is not a vector then sure a sail won't work, what direction would it go in? This guy is a crackpot.
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.
He misses two points:
- The "cold" end of the heat engine is not the mirror, but the dark sky behind it.
- In the coordinate system of the center of mass, the "gas" of photons is "cooled" by the red-shift of the mirror as it moves away.
In heat-engine terms, the mirror is a piston, photons on the sun side are a gas, the lack of photons on the dark side is the lower (near-zero) ambient pressure behind the piston, and the red-shift is the cooling of the gas by expansion as it pushes the piston.
The mirror-piston accellerates as long as the pressure on one side is higher than on the other - which it is as long as it's below lightspeed, which it always is since it has rest mass. Terminal velocity is less than c becaue the "pressure drops" both from the blue-shift and from a loss of photon density due to the inverse-square law.
And replacing the mirror with a black object means the piston "leaks" the gas, reducing the force from it by a factor of two - exactly what you'd expect.
No problems with carnot. (You can even "heat the sun" a bit by pushing the mirror toward it, and create somewhat leaky "walls" to the "cylinder" by using a retro-reflector rather than a simple mirror.
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Yeah, honestly. PhysicsGenius saying something correct about science? I thought that was a sign of the apocalypse...
Momentum is mv only for massy particles. For photons it is E/c = hf/c.
Using the "rest mass definition of mass" the equation should be E == mc**2/sqrt(1-v**2/c**2), which is clearly the same for v == 0.
another star trek moment :) they showed something like this on deep space 9 if I recall. About the bajorans trying it out, and succeeding at it too.
ok, where to start?
It appears that the author of this article needs a little bit more physics before they should start claiming that things won't work... First, It should be noted that high profile places like Slashdot and the New Scientist *do* have an impact on where and how research money is spent. As such they have a certain responsibility to make an effort to verify the content (to a limited degree) of what they post.
As such, the article makes several assumptions which are patently false. The first is that the solar sail is a purely thermodynamic reaction which requires that it be governed by Carnot's law. The fact is that such a sail would in fact be affected (a very little) by an additional reaction which would add to the overall thrust provided. This additional reaction would be governed by Carnot's law, but would not contribute or detract in a significant way from the actual process used. The process which is being tapped for energy in the solar sail is the interaction of two bodies in motion ( the photon and the sail) this interaction, while requiring vast numbers of photons to have any noticeable affect, is *not* governed in any way by Carnot's equation. Saying that it does is akin to saying that an internal combustion engine will only work until it warms up! As we know from the daily commute, this is only true on Friday afternoons on the way home for the weekend.
Second, The author is assuming that the main interaction between the photons and the sail is simply absorption, or that if it is reflection then the sail itself only gains some heat from the interaction, and the photon looses some. The simple fact is that the sail will simply reflect the photons back in the direction that they came from, with a lower energy level (i.e. lower frequency) thus absorbing *kinetic* energy from the interaction. The absorption of photons is a parasitic affect that can be minimized by using the right materials for the sail. Of the remaining energy that is transferred to the sail, most will be kinetic energy, and some will be heat energy (and thus the material must be highly resistant to heat or it will be destroyed shortly after deployment).
The author has confused the idea that the sail will then radiate photons as it warms up. The difference is that the sail will radiate in all directions producing a null vector thrust, while the "solar wind" is unidirectional producing a vector thrust away from the sun. When coupled with gravity's vector thrust towards the sun, this can be used to produce thrust in almost any direction (much the way a sailing ship can sail almost directly at the wind by rearranging its sails)
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I agree with his article largely, but I have one minor quoible with it that I think is a point he overlooks.
Admittedly, I'm working on one year of college level Physics, and one year of high school level physics, so I'm by no means an authority.
He assumes that a black sail would reach an equilibrium temperature with the light source, and at that point the propulsion would cease. With a light weight material, this would be very quickly.
Does this happen with currently known black materials, or materials that are very close to black? No, how can we tell? They are still black. Thus, despite being exposed to extended light sources, they continue to maintain their blackness. This light energy is being converted in to heat energy. This heat energy is being dispersed through out the area radiantly.
In a black sail, light and heat are being applied to it from the sun. The law of thermodynamic conservation says that this energy cannot be destroyed, and so must be converted in some other fashion, or stored. The light->heat conversion is stored for as long as the sail is a lower temperature than the total thermal energy of light+heat from the sun. Once the sail and its inbound energy have reached equilibrium, this inbound energy can no longer be stored, and so must be deflected or converted. If the light energy is deflected, then the material ceases to be black and turns white (in fact, this is basically what happens when you turn a light switch on in a room, no?), but since we know from empirical evidence that black materials maintain their blackness, they must instead be converting this light energy in to some other sort of energy. If this energy is heat, and our material is already at heat capacity, it must be radiated outwardly (and backward). If 100% of this light energy that was absorbed (keeping in mind that we have no perfectly black surface) is converted to radiant heat, then no forward momentum would be achieved. Otherwise the rest of that energy MUST go somewhere, and that somewhere would almost certainly be forward thrust. It may not be a lot of thrust, but it should be some.
On another point:
The perfect mirror sail couldn't possibly work because 100% of the light energy would be reflected back. Because you can't alter the speed of light, in such a situation you cannot harvest any of its energy (it's a theoretical perfect mirror, 100% is being reflected, so 100% of it is keeping 100% of its original energy level, thus none is imparted to the sail; any of its energy that was absorbed due to it being a non perfect mirror would fall under the same category as our previous black sail discussion).
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Most of us geeks have seen the little science fair deals that spin when light is shone upon them. To describe: an assembly with spires coming out horizontally at 90-degrees is placed upon a pin to reduce friction. At the ends of each spire is a panel parallel to the spire but flat on the vertical. One side of each panel is black, the other is white. When light (or other EM waves) shines on the white side, it generates repulsion, because white reflects all visible light. When it shines on the black, nothing happens, because black absorbs all visible light.
The result? You can get this assembly to rotate on the pin quite quickly using just photons as the thrust. In an environment with no gravity and no atmospheric viscocity, I don't see why a solar sail wouldn't work. I've got one on my room at home. It works fine.
Buy one
Very interesting. So, is it possible (or even necessary) to take into account the fact that the sun, from whence the photons come, is moving too? Or does the sun's gravitational pull make it essentially stand still as far as the sail is concerned (i.e. the sail and everything else in the solar system are moving with the sun, within the galaxy)?
Your fantasies contain the seeds of important concepts.
On another point:
The perfect mirror sail couldn't possibly work because 100% of the light energy would be reflected back. Because you can't alter the speed of light, in such a situation you cannot harvest any of its energy (it's a theoretical perfect mirror, 100% is being reflected, so 100% of it is keeping 100% of its original energy level, thus none is imparted to the sail; any of its energy that was absorbed due to it being a non perfect mirror would fall under the same category as our previous black sail discussion).
This is not as obvious as you state. Some physicist can correct me of course, but here is the intuitive part you must leap too... light is both a wave and a particle, depending on what you are doing. As a wave contacting the mirror, you are correct, the 100% reflective mirror will just bounce all the light back in the other direction. But when you consider that light also acts upon objects as a particle, then as a particle a photon must have momentum. The mirror has nothing to do with reflecting particles, only waves. The mirror and its attached structure is then simply a large ball bearing being shot at by a bunch of small ball bearings from the sun at high energy.
However, the light must also maintain its wave characteristics with the mirror, so when the light reflects from the mirror, but imparts some momentum, then the drop in energy for the photon is registered as a drop in the frequency of the reflected light.
Light is cool.
The other way is to consider the sail as a Steady State, Steady Flow machine. Photons come in on one vector in the reference frame, bounce off the (moving) sail, and leave on another vector. For a sail thrusting forward in its orbit or moving away from the Sun, the photons will be red-shifted by some tiny amount; this loss in energy is equivalent to the delta-temperature between the inlet and outlet of the "machine". As with any Carnot-cycle engine running on a very small temperature difference, the efficiency is lousy.
The real problem is that the efficiency you measure depends on the reference frame, but you have this same issue when trying to compute the energy change from gravity-assist maneuvers so anyone doing astronautics should have some idea of what to use.
Scientists restrict study to entire physical universe; creationist
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
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A solar sail is a one-way trip only - away from the sun. If you want to get back you need to use some other form of propulsion.
Wrong.
to return to a lower oribit, think angular momentum:
1) use the So.Sa. to reduce your angular momentum (ang. vel.).
result: orbit becomes more excentric (have radial velocity). Perihelion decreases.
2) when droping towards sun, use So. Sa. to reduce your radial velocity, while increasing your angular momentum.
result: you will now be at a lower orbit with less excentricity. e.g., the Earth's orbit.
QED.
Working for necessity's mother.
... makes me realise that in amongst all this talk of photon-momentum and reductions in frequency of departing photons, that I have no idea what the actual mechanism is that's involved in taking a little packet of energy travelling at the speed of light in one direction, and turning it around so it flies back the way it came.
Presumably, when a photon passes in amongst the surface of the mirror, it interacts with the particles in some way that causes it to turn around. I don't exactly think the photons just hit the nucleus of some atom and ricochet off - I'm guessing some repulsive force coming from electron shells or nuclei must be involved somewhere, but since the photon has to always be travelling at the speed of light, we're not talking about the photon gradually slowing down as it approaches.
Surely its the detail of this interaction that actually transfers momentum and energy from photon to mirror... not some theoretical inelastic collision transferring momentum and energy. The effects we see as conservation of energy and momentum at the macro level must be the result of some kind of particle interaction at the subatomic scale...
You can find an article explaining the long history of Crooke's Radiometer explanations at:
? mo de=1&faqID=42
http://www.stillmoving.ca/physics/usenetFAQ.php
The true answer is more complicated than either radiation pressure or the "hot side bounces air molecules" explanations.
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I would do it myself if I had the points...
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Like most such arguments, this one will be resolved in a time-honoured fashion. Physicists will debate whether or not such a device is theoretically possible, and engineers will build one and see if it works.
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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.
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.
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I've sometimes wondered:
;)
{
suppose I had a machine that converted mass to energy. If a mass enters the machine with angular momentum (or high temperature, etc), that energy would have to go "somewhere" right?
}
if so then, in the case of Feynman's experiment, maybe the redshift on the up trip would balance the acceleration on the down trip?
It feels like the machines Feynman presupposes couldn't exist without piling entropy into the universe or 100% efficiency if not both.
TSWRRWTBOTNB!!
Let's see if we can beat this dead horse a little bit more. There's still some visible meat....
Gold is arguing that Solar Radition Pressure (SRP) does not exist. However SRP models are confirmed daily when estimating satellite orbits.
SRP is a small force, compared to, say, gravity. The acceleration due to SRP is on the order of 1 ten millionth of a meter per second squared. However over the course of an orbit, even a 3 hour orbit, the cumulative effect of this acceleration is on the order of several meters.
A familar example is the Global Positioning System. The effect of solar pressure moves a GPS satellite tens of meters during just one of its twelve hour orbits. If this effect were not taken into acount by the control segment of GPS, hundreds of thousands (perhaps millions) of GPS users would see the comparable errors (10's of meters) when performing stand-alone positioning.
It's not a heat engine, duh! IANAP, but I got halfway through the article and said to myself "This man doesn't know what he's talking about!" It's just as AlecC says, heat is disorganised energy, solar radiation is organised. Haha! I'm smarter than a Cornell Professor. ;-)
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I am a physics student at Cornell University and I personally attended the talk where Gold first unveiled this theory of his. I was also given a pre-publication copy of his paper and had the opportunity (along with several other students) to give him feedback on it.
Gold is a really interesting person, and he has come up with some wonderful ideas in his time (he is not a young person by any stretch of the imagination), but he is also the type of person who likes to constantly challenge our basic assumptions about physics. This is a wonderful and very necessary thing, however as a side effect he also tends to come up with a lot of crazy and quickly dismissed theories.
I'm afraid to say that this is one of those theories. I'm rather surprised it's appearing on Slashdot now as it was debunked some time ago. Scrath that, I'm not surprised at all. This is slashdot after all.
Anyway, I'm just saying that we probably should spend too much time worrying about this.
It will work using photons. The Casmir effect has experimentally been proven. FYI the casmir effect is attractive force between two mirrors that face each other due to the momentum of virtual photons.
Well.. maybe. Or Maybe not. But Definitely not sort of.
Main point: he goes into the frame of reference of the light sail, notes that light_energy_in == light_energy_out, therefore no energy is transferred to the sail, and therefore there must not be any force on the sail.
But Energy is the dot-product of force . distance (likewise, power is force . velocity). 'No energy is transferred' is a different statement than 'there is no force' in the frame of reference of the sail (distance moved = 0, velocity = 0)
Radiation pressure has been successfully used by spacecraft for attitutde control and station-keeping.
Crookes radiometer does not turn due to radiation pressure.
He also has troubles with the concept of thermal equilibrium and inelastic collisions.
This is not a prediction that comes from observing sailboats, rather it comes directly from relitivity.
Well.. maybe. Or Maybe not. But Definitely not sort of.
The reply from Ungrounded Lightning is correct. The bounced light is red shifted if the sail is moving. Thus energy is extracted from the light.
Also don't forget that much of the solar wind pressure is not from light, but from solar protons and other particles, going several hundred kM/S, depending on solar storm activity.
Amazing how much noise a not-well-thought-out criticism can generate.
I beleive you have it pretty much down so i shall ask you the thoughts i have in mind. I was just thinking, will this actualy generate enough speed/acceleration to fight against the gravity of surounding plants and the sun. Although im sure some crazy equations would be needed to solve such a thing, I would think not though. It doesnt seem like a photon (an miniscule mass) accelerating into a large mirror could cause it to move the object, unless already in motion, but if it does keep it going how do the creators plan on making sure it stays 'on path'(not straying off course toword a ever nearing planet. Once we it has gotten far out what exactly does it run from then. The whole pushing the light sail toword the sun completely confused me, but im assuming that is how it would return. I do not at all understand how that would work though. Light pressue? I would like to point out i think this would work technicaly but to what extent i must ask. Yes I am fairly confused.
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.
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Assume space is at 0 degrees.
More like 3 degrees, actually, (Big Bang leftovers) but I concur on the main points.
-- Alastair
And yet, we've never seen the Silver Surfer use a windsail on his surfboard. Go figure.
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Yeah, he probably doesn't know shit
Ok, I read the article a few times and now I understand why he thinks a perfect mirror wouldn't work.
I don't understand why a balck body would only accellerate for a brief time. I would understand that if it were the same temperature on both side it wouldn't move...
I wonder, would a peltier heat pump make a good engine?
The incoming gas (the air) is at the essentially the same temperature as that leaving the the paper and so, if this were a carnot heat engine, the paper could do no work.
This exposes the fallacy in his argument. The proposed solar sail is not a heat engine i.e. it is not extracting useful work from the heat energy of the photons. Were it to rely on lowering the photons energy to do work then it would be a heat energine. However instead it relies on the mechanical energy of the photons to do work. The fact that photons are radiated heat is irrelevant for this excercise, they could just as well be ping pong balls.
You could use the same argument to show that you couldn't play billiards, which is obviously nonsense
The fallacy in his argument is that Carnot's rule applies to thermal energy. Thermal energy is microscopic motion with an overall total zero momentum for any macroscopic chunk you look at. Carnot's rule tells us nothing about what you can do with macroscopic momentum: it doesn't tell us about billiard balls, for example. But the total momentum of the light hitting a solar sail does not have a total zero momentum. The solar sail manages to carve out a macroscopic chunk of all the radiation leaving the sun whose total momentum is non-zero. Since it has a non-zero total macroscopic momentum, considerations of heat engines don't come into play, and it can be used for propelling a solar sail just like you can propel one billiard ball by hitting it with another one.
And no worries about micrometeroid impacts shredding your gosamer sail like so much wet kleenex.
Such an impact with an M2P2 field would, of course, strip off some of your plasma, but that's what a reserve tank is for. Your 'sail' is self-healing.
Just because it works, doesn't mean it isn't broken.
Another clue: confusing a light sail with a radiometer (which is, in fact, a heat engine and works because it's not run in a complete vacuum). The guy knows nothing.
Incidentally, when I was back in school, I got to see a film of an experimental verification of light sails. A small mirror was suspended on a hair in a fairly hard vaccum. Then a bright light was flashed off and on until the mirror started swinging. The effect was so small, and the time of the experiment was so large, that any thermal equilibrium would have long been achieved. The only problem is that the effect was so incredibly small, that I wonder if it's practical actually to make one. The need to have high enough strength and low enough mass to get anything useful out of it seems some pretty severe engineering requirements.
The Eurostar 2000+ platform (used for commercial geostationary satellites (and 3 axis stabilised)) uses solar sailing to provide torque for pointing.....
(The solar sailing is acheived by rotating the solar panels at a slight offset, and the use of an asymetric tab on the panels). The exact details of this are now a little hazy, since it was some years since I was trained on the platform (and I am not going to search out the manuals now (midnight in Europe)).
A torque in this case is nothing more than an unbalanced force.
there's no point in constructing a spacecraft that can't travel well beyond the speed of light. physics as we know it is entirely irrelevant. the power needed for a real intergalactic spacecraft- one that that can travel to distant stars that we cannot even see from earth with the most powerful telescope in less than a fraction of a second, will require technology that transcends our current understanding of the physical world and energy relationships. this entire discussion is retarded- if you were planning a trip to the crab nebula would you build a sailboat? even traveling at the speed of light you'd be a pile of dust before you passed pluto. these solar dream ships are great for cheezy ST scripts and cheap sci-fi novels.. but GMAB the idea is unpractical even on a purely theoretical level.
Radiation pressure do exist. It has nothing to do with the quality of the mirror but a perfect mirror is preferable. But a piece of scratched metal would recieve radiation pressure.
What happens is this:
A photon traveling towards the mirror. The photons frequency is n. The energy of the photon is related to the frequency through plancks constant.
E = hn
Einsteins theory of special relativity relates energy to mass though the equation E=mc^2.
The mass equivalent of the photons energy is thus:
m=hn/c
The impulse of any particle is defined as the mass times the velocity. Here it is important to remember that the velocity is a vector pointing in the direction of travel. Impulse is therefore also a vector pointing in the same direction as the velocity. The equation becomes:
p=mc=hn/c
What creates radiation pressure is the fact that the impulse of a closed system is always preserved. That is the impulse of the photon added to the impulse of the solar sail MUST remain constant at all times. Why? Because Sir Isaac Newton says so, thats why!
The photon now hits the solar sail and is reflected. Since the impulse of the photon now points in the opposide direction of before it hit the sail, it now has an impulse that is opposide of before.
The photon has had a total change in impulse of an amount 2*hn/c, and since the total amount of impulse must be preserved we are forced to conclude that the sail has recieved an equal amount of impulse in the opposide direction.
Every photon that hits the sail delivers impulse to it. For a big sail this adds up to a very significant amount, enough to gain speeds comparable to the speed of light (like 1/20th or more). You can help it gain speed by shining powerfull lasers on it, when the light from the sun dims as the sail moves away.
Solar sails work. They have been tested experimentally already by shining powerfull lights on thin mylar in a vacuum chamber. I just saw this experiment the other day on Discovery Channel in fact, and it was easy to see the miniature sail blowing in the photon wind.
Carnot's law does not say that the temperature must go down...it says that the entropy of the universe must go up. For thermal systems, there is a temperature driven maximum efficiency. For non thermal systems, there is no such limitation, hence fuel cells, which use gibbs free energy to calculate efficiency versus carnot efficiency, have very high efficiency rates. Photons reflected from a solar cell (due to compton scattering) are at a lower energy ... Energy of photons e=hv (where h is planks constant and v is frequency) and E=mc2 (where E is energy, c is speed of light and m is mass, photons have relativistic mass = hv/c2 since momentum p is = mass x speed , thus hv=pc..h and c are constants, so if p goes down (loss of momentum, and momentum is always conserved) v (frequency) must go down, and energy was hv, thus energy went down. Since energy was transfered, the future usefulness of the reflectant photons goes down, and thus entropy of the universe goes up. The photons cannot possibly transfer more momentum than the difference between current momentum (which was reduced by the solar cell) and the momentum due to brownian motion of the interstellar hydrogen (about 3K)with which they must be discharged. The crookes radiometer is consistent, since the mirrored surface encounters much less compton scattering it reflects most photons at a higher energy (thus on that side less momentum is transfered) than the black side which absorbs the high energy photons (visible light) and emmits lower energy photons at near infrared (it will heat up more). Thus the black side is absorbing more energy, and hence transfering more momentum, so the net moment arm pushes forward on the black side.
Whenever the offence inspires less horror than the punishment, the rigour of penal law is obliged to give way...
<sarcasm>Yeah, he probably doesn't know shit</sarcasm>
The proof is in the pudding. This is basic physics that even an undergraduate who's on the ball should be able to figure out. If he really is such an accomplished physicist, then he's also an idiot. Let's go through a few examples:
With a few exceptions, reflection is the absorption and re-emission of photons (TIR being one. Also, there are fancy materials I read about on slashdot that use microscopic bubbles that preclude light of certain wavelengths, allowing perfect reflection of certain wavelengths). Gold argues that a blackbody that absorbs a photon receives a push of momentum.
So, what, re-emitting that photon causes an opposite momentum kick that cancels the first momentum kick? A far-fetched claim, to be sure, but the only one that preserves any semblance of a mathematical foundation. Or does it... It means that if you played backwards a movie of the absorption/re-emission, you would see the photon being absorbed, and the solar sail accelerating toward the sun, then the photon being re-emitted, and the solar sail being accelerated away from the sun. Which is of course the exact opposite of what happens when the movie is played forwards, and hence a violation of the principle that at the quantum level, events have no preferred direction in time. Well, I suppose quantum mechanics could be wrong...
Next, let's address his argument about equilibrium. Quoth:
Would it be better to place a black sheet there instead of a mirror-faced one? Unlike the mirror, this could absorb energy and the momentum associated with that. But it would do this only from the moment of its exposure until it reached thermal equilibrium with the available radiation. Then energy absorption would cease, and with that the delivery of momentum to the sheet would also cease. For any lightweight sheet, this time would be only seconds.
Hmm, as I recall, the solar sail was to be extremely thin, right? So the blackbody radiation that it would emit would be in both directions, imparting no net acceleration or deceleration. Yet energy would continue to flow from the direction of the sun. Even if we accepted Gold's bogus claim that only absorbed (non-reflected) light imparts momentum, clearly there is still energy pouring in from only one direction, even at "thermal equilibrium". Another way to look at it is that the side that is exposed to space is not in thermal equilibrium, and continues to emit blackbody radiation into space. Sorry Gold, no twinkie.
As to his point that the "temperature" of the reflected light doesn't drop, that's crap. If he's going to assign a temperature to the incoming light, then he can't compare the "temperature" of the refelcted light in an accelerating frame.
Quoth:
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.
These are two completely different reference frames. When a single photon is reflected off the solar sail, the solar sail is ever-so-slightly accelerated. That means the reference frame he chose for the reflected light is not the same as the reference frame for the incoming light. The photon is ever-so-slightly red-shifted in the frame of reference that the solar sail was in before the photon struck it; hence, there actually is a drop in its "temperature".
Here's my question. In the article, Gold says that yes, if the reflected photon has lower energy then the sail will move, but if it has the same energy then it won't move.
You seem to be saying that reflected photons will always have a lower energy? Does this mean that when I look at myself in the mirror, I am actually seeing the colours with a slightly longer wavelength than they are? And if I set up 2 opposing mirrors, can I shine a blue light and see green , red etc. too as the light reflects back and forth and gradually loses energy?
Finally, how does temperature relate to momentum, exactly? I was under the impression that temperature meant internal kinetic energy, which photons can't have (but obviously they do, otherwise the COBE wouldn't have anything to measure). So how do you incorporate temperature when you write the equation for a photon's total energy?
Red shift is relative to the observer, and is an indication of how fast an observer is moving relative to the place that the radiation was, well, radiated.
Let's say you have a sun that we arbitrarily assume is fixed, and a smaller radiation source that is moving directly away from the sun. The radiation that this source emits is not red shifted with respect to it in any direction, as long as it doesn't accelerate.
Let's say, now, that it isn't a radiation source, it is a detector. The radiation FROM THE SUN is red-shifted, because the detector is moving with respect to the sun. That is what a red shift is.
Now it's a mirror. Someone in the same frame of reference as the sun looks at the light that has bounced off the mirror. It appears red-shifted to them, because the mirror that is the 'source' of it is moving away. In fact, it looks exactly as red-shifted as the light looked to the detector, since it is moving the same speed with respect to the sun as the sun is with respect to it.
Or does it look exactly the same because it bounced off the mirror, and is therefore faithfully reflected?
Or does it look TWICE as red-shifted, because the red-shifted light was reflected off the mirror, and now it's red-shifted even MORE because the mirror is receding?
Or is it no longer red-shifted at all, because we're back in the same frame of reference as the light originated?
The point is, just saying 'the light is redshifted' is utterly meaningless without a frame of reference.
-fred
Sign #11 of Slashdot overdose: You see the phrase 'moderate Republican' and you wonder if that would be a +1 or a -1.
I doubt it. When sailing boats go to windward they do so largely because the sail is operating as an airfoil due to the bournoulli effect, and the boat effectively gets "sucked" to windward by the low pressure region on the leeward side of the sail -- exactly the same way an aircraft flies. The keel prevents side slip and keeps the boat oriented the right way.
Only when sailing boats go dead down wind does the sail act as a drag device or parachute. For most boats, this is the least efficient point of sailing. It's also worth mentioning that boats going into the wind create their own "apparent" breeze, thereby creating even more lift from the wing (sail).
Because the solar "sail" is more like a parachute than a "wing", it's not going to generate lift on its leeward side, and therefore can't go to windward the way yachts can.
Even old square riggers are not a good analogy because, although they don't have airfoil cross-sections on their sails, they could sail across the wind by means of vector resolution between the force of the sail and the force of their keels and hulls on the water. The spacecraft has no "ether" for it's keel to push sideways against.
"Let's do a gedankenexperiment (thought experiment)."
I thought it was German for "can't get a research grant."
Yes, but M2P2 has a maximum speed of about 180,000 miles per hour. It is expected that solar sales can reach up to 25% the speed of light, which puts it at about 180,000 miles per second.
I'm wrong and so are you.
Not to say that his arguments are correct (I don't know)
They're not, as even basic college level physics would dictate, as I pointed out.
-Looking for a job as a materials chemist or multivariat
It seems that the author of the paper ignored completely the effect of the constant cascade of charged particled that stream out of the sun on a second to second basis. The effect of these particles is vastly greater than the photonic pressure that seems to be the basis for his argument against the usefullness of the sail.
In addition, the author's use of examples ignores the effect as well. A Crookes radiometer rotating backward is doing so inside a protected environment completely sheltered from the effects of a stream of ionized gas. Although we consider this space "vacuum", it is actually wuite full of high velocity (and therefore high inerta) particles. So, the solar sail is not a heat engine at all. In some ways, it operates similarly to the way a sailing ship works, with the exception that a sailship is actually driven by the aerodynamic force of the wind moving AROUND the sails, rather than impacting the sails. Will the decrease in vacuum pressure on the shadow side of the solar sail be enough to effect the acceleration of the craft? I dunno, only experimentation will tell for sure. (just theorizing here, but the mechanics of solar sails and sailing vessels may be more closely related that we know right now.)
Inside the earth's magnetosphere, the solar sail may have some difficulty gaining momentum. However, once outside, in the full solar wind, the efficiency should increase.
== That terrible green-green grass, and violent blooms of flower dresses, and afternoons that make me sleepy.==
Gravity.
"Verbing weirds language." -- Calvin
Thanks for all the replies. I've learned of a newer way of looking at the event horizon.
On that same note, Maxwell's Demon doesn't violate any laws, because it can somehow "see" the the gas molecules and thus is not limited by thermodynamics. Though I have read claims that the entropy difference is balanced by information entropy somehow, I'm not very familiar with that concept.
BlackGriffen
Well, in the first year we had to do a couple of other subjects but from the second year on it was purely physics, plus some maths of course. In fact I liked it this way, in the first year I did things like chemistry and materials which turned out useful later on.
Funnily enough, physics one of the few natural sciences @cam where you could specialize so early on. In the second year you had to do three subjects in total, but you could take 'double physics' and maths.
Escher was the first MC and Giger invented the HR department.
Solar wind is generally accepted to be composed not only of photons, but also masses of particles.
Many of the particles are high-energy/high velocity ionised particles (c.f. the earths magnetic field and resultant aurora).
Now I assume that Gold has not managed to "disprove" the use of standard particle force transmission in collisions (if he has he should inform all thos misguided sailors).
This article is some of the poorest science I have seen in a long time.
One definition of the term "solar wind", a quick digression into the material properties of the sail, and any scientist/engineer/sentient being should have been able to make a qualatative judgement that it will provide a measure of motive force.
That is not to say that the gravitational attraction at a given point will be outweighed by solar wind propulsion, but that and other much more interseting issues have not even been considered in this issue.
Quinkin.
Insert Signature Here
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."
Um, from my reading of the paper, the same 'logic' should prove that solar cells can't work once they've achieved thermal equilibrium with their environment.
I have a few gadgets around the house that prove otherwise.
I think his mistake is in assuming that everything is a 'heat engine'. There are non heat-engine processes going on here which he's not accounting for.
Quidquid Latine dictum sit, altum videtur (anything said in Latin sounds important)
This reminds me of a lightsail story in the sixth grade reader book "Galaxies" (you know, the one that started out with Tigers, Lions, and Dinosaurs in first grade, than had things like Rainbows and I forge
OMG
Google!
Bye!
OMFG, Google has failed...on locating a heavily-used reader series from the '70's.
Anyhoo, the story was about peeps setting off in a lightsail ship from Earth. It was huge news. They were being talked to on worldwide TV by radio, but in short order that wasn't fast enuf. So they used laser, but that wasn't fast enuf to keep up with them! They were off!
All I can remember is thinking how stupid that was, they could never accelerate that fast!
And what's up with that other story, about the Mexican kid in the southwestern US who wants a haircut but can't get one because the racist who runs the barber shop tells him he has a "greasy head"? All I can remember thinking is why doesn't the dope go wash his hair? It's kinda rude to want the barber to get his tools all greasy because you're such a slob.
"Has [being a kidnapped teenage girl, raped repeatedly for months] changed you?" - Katie Couric to Elizabeth Smart
In High school i read an excelent Science fact article in the Analogue that talked about magnetic solar sails... the concept is basically you unfurl a big coil of superconducting wire around the ship and charge it with a large power source and create a giant electro magnet...the field then intereacts with the Solar Wind. The charged ions and particles impact the field and ipart momentum. THe best was the author described how one could enter and leave a planet's gravity well with a strong enough magnetic field. very interesting stuff
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
This is MY galaxy...go find your OWN!
As other posters ably explained, you must be running toward (or away from) the mirror to see any color shift.
As I recall, Echo I, the balloon satellite launched in 1960, had its orbit significantly affected by light pressure.
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.
It is if you do round trips. B-)
By the way: The original article's argument could also be used to claim that hot gas can't push a piston which is covered with a REALLY good (i.e. perfect) thermal insulation, OR if the piston was at the same temperature as the gas, because the molecules would rebound with the same energy they had on approach. (Gas pressure? What's that?) In fact, an insulating coating on a piston improves the efficiency of a heat engine because it reduces the amount of heat that transfers between the gas and the piston without doing any work.
Interestingly, a perfect mirror in a light sail is EXACTLY equivalent to perfect insulation on the piston of a gas-based heat engine, while an imperfect mirror is similarly EXACTLY equivalent to a piston with a less-than-perfectly insulating surface.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
will this actualy generate enough speed/acceleration to fight against the gravity of surounding plants and the sun.
No - but neither will a rocket.
Instead of fighting it directly you ORBIT the sun, and use the sail at an angle (or a rocket) to pump up the orbit, until you're far enough from the sun that the attraction is negligible.
As for the planets - if you're not going to or from them (in which case you orbit THEM and use the sail to pump the orbits up or down) you can plot a course to use their orbital motion and gravitational attraction to give you an extra yank on your way - transferring some of their orbital momentum to the ship. (It will take a LOT of ships to steal enough momentum from, say, Jupiter to make a detectable difference in its orbit.)
Although im sure some crazy equations would be needed to solve such a thing, I would think not though.
You can figure the thrust from the momentum of the light very easily. From there it's the same math as a rocket - but simplified, since you're not constantly changing your ship's mass as you burn away the fuel.
It doesnt seem like a photon (an miniscule mass) accelerating into a large mirror could cause it to move the object, unless already in motion,
But the sun gives off a LOT of photons. B-)
Again it's just like a more typical heat engine using a gas as the working fluid (such as the one in your car): One gas molecule doesn't do very much to a half-pound piston hooked up to a piston rod, crankshaft, and VERY heavy flywheel. But there are a LOT of gas molecules zipping around in a cylinder full of burning fuel-air mixture.
but if it does keep it going how do the creators plan on making sure it stays 'on path'(not straying off course toword a ever nearing planet.
Compute the path just like any other spacecraft. Then measure the deviation from the desired path partway along the trip and tweak it for any errors. (Just as with a rocket ship, this is the famous "mid-course correction". If your destination is nearby you do it about halfway along your trip, because you're most efficient when you use half the trip to let the error grow before you measure it, then use the other half for the correction to take effect.
Once we it has gotten far out what exactly does it run from then.
By the time your so far away that there isn't enough light from the sun to be worth collecting, you're REALLY moving. And there's nothing else very large around for lightyears. Then you coast until you're near your destination.
The whole pushing the light sail toword the sun completely confused me, but im assuming that is how it would return.
Yep. That's how you stop - or actually manouver into a parking orbit.
But I really talked about it to show the equivalence to the flip side of a carnot-cycle heat engine - when the piston is compressing the fluid and transforming mechanical energy to heat energy in the workin fluid.
I do not at all understand how that would work though. Light pressue?
Yep.
Flying away from the sun the light pushes you, speeding you up and getting red-shifted (cooled and expanded) in the process.
Flying toward the sun the light still pushes you, but it is slowing you down and getting blue-shifted (heated and compressed) in the process.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
Buy Steampunk Clothing Online!
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.
"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."
Methinks that you've let the cat out of the bag. Now the question is, was it (the cat) previously owned by Schrödinger ?
"Oh drat these computers, they're so naughty and so complex, I could pinch them." --Marvin the Martian
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
look, I'll say this once and you'll never read it but what the hell.
Gold *knows* all this, he knows how to do that calculation. he just thinks it's wrong!
Just as Einstein *knew* what Galilean transformations were and how to use them, he thought them wrong.
(trouble is of course, Einstein was right, and Gold is probably wrong here).
how Gold explains the fact that radiation pressure is easily observable (very easy with a big laser) I have no idea. but he isn't a total cretin.
How do you explain Doppler redshift in terms of the photon model?
Is the photon red-shifted at the source, or when you see it? Presumably when you see it, because when it is emitted it doesn't yet 'know' how far it's going to travel before it hits something, or how fast that something is going to be moving. (A galaxy that appears redshifted to us would not appear redshifted to eg. a stationary (wrt it.) observer close to it, even though it might be the same photons that get seen).
But when you see the photon, it's a photon, and it has no memory of how fast the object it was going was actually moving, so how does it know how much to get redshifted?
How about the physics of reflection?
Correct me if I am wrong, and I admit I didn't read all the posts, but:
When a photon hits the mirror, it knocks an electron or two off to a higher orbit. Then, unable to sustain such an orbit, they fall back releasing the energy which creates an identical photon, but with an opposite vector. Same polarization, same amplitude (in a "perfect mirror" only, of course), but different direction.
I can't say one way or another, but when tons of photons are hitting a large "perfect mirror" surface at the same time, all the time, they do create a constant change in the mirror (hmm, how about a degrading mirror that records what it reflects?), that's why it reflects.
Now, photons act as a wave. What if it were a perfect laser hitting a perfect mirror? The chances are that when (imagine two sine waves coinciding in a fiber-optic cable) the incoming photons hit the photons that are reflected, their amplitude increases (or decreases). Thus, before being dispersed, there is bound to be some amplification going on right next to the mirror.
Does it serve anything? I dunno, I am not a physicist, it's just common knowledge, and that was but a rant.
QED, Gold is wrong.
You have clarified everything perfectly, and cleared up the questions I had. Thanks. I hope I will one day be able to answer such questions. Im only 16 right now so I have a while to expand my knowledge and lots of time to ask questions. As long as there are people you you willing to answer I shall be a genius in no time! Thanks a lot.
As another junior physicist, I was warned by my thesis advisor not to even consider going to Cornell. I've since met a few "scientists" from that club, and haven't yet met one with a brain or basic understanding of their field. With people like Gold and Kalos in charge, it's no wonder only the brainless brown-nosers have careers there.
But I'm sure the ignorant upper classes will continue to send their precious morons there for education and degrees and then continue to brag about them.
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.
So, let's extend this logic:
Let's say that you are in a fixed position with respect to the sun, and you reach out and drop this solar sail thingie. Now we are all in the same frame of reference. There is no red shift to the photons with respect to the observer. THEREFORE there can be no acceleration. Right? Even though the photons are hitting the wings and everything, since there is no red shift, the photons aren't losing energy and therefore there can be no gain of energy by the sail.
(Now, you *could* say that the red shift is instead the result of the ACCELERATION of the object BY the light. But that's equally silly, and five minutes' thought could tell you why.)
AND, to carry this further, the FASTER the solar sail is moving away from the sun, the MORE energy it gets from the photons. Power your solar sail with a laser beam of very high energy and coherance, and you could, if this were true, approach the speed of light. After all, you gain mass in the rest frame of reference, but at the same time you're gaining more speed from each photon. If I had to guess, I would suspect a congruence here, though I haven't actually written anything out.
This is more than a little silly. Look elsewhere for your acceleration, but don't look at red shift, unless you care to explain this. Red shift is a result of relative velocities, and does not depend in any way upon acceleration.
-fred
Sign #11 of Slashdot overdose: You see the phrase 'moderate Republican' and you wonder if that would be a +1 or a -1.
Well, I don't think you can use these forms of the equasions, since photons have no mass. They do have kenetic energy, IIRC.
Momentum at relativistic speeds is different than classical momentum (mv).
Photons, mesons, all those stupid "carrier" particles. They don't make any sense! They come out of nowhere whenever you need to transfer energy, then they disappear again. That's silly. Can't we forget all this stuff and just go back to regarding energy as an abstraction?
So I finally figured out that some physicists want a new def of "mass" that would give photons a rest mast even though they never rest. I guess I can live with that. But it sounds like one of the quantum definitions ("spin" comes to mind) that starts out in the macroscopic world and retains its old label in the quantum world, even though it's refined until it's a completely different concept. But I could be wrong!
If the scheduled launch succeeds, we'll find out soon enough if a solor sail can work.
But, here's what I want to know: How do you stop the thing? Or even brake? Get to the half-way point, pull the sail down, and coast to a stop? Carry some retrorockets? Throw out a space anchor?
-- Slashdot: When Public Access TV Says "No"