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Solar Sail Will Work, says Planetary Society
degauss writes "In response to Cornell Physicist Thomas Gold's paper declaring the theroy behind solar sails flawed (previously mentioned in this Slashdot article), Louis Freedman, executive director of the Planetary Society (the organization behind the COSMOS project), has written a brief rebuttal to the claims in Dr. Gold's paper regarding the feasibility of solar sails for use as a method of transportation in space. He does not go in to detail with equations and such, but does give an overview of the reasons he believes Gold's hypothesis is incorrect."
Dear degauss:
"theory".
Thank you.
The Spelling Nazi.
Solar sails blow!
because if they can, would it not be a good idea to attatch solar sails to all NEA's? the force would push them out of earths path into the asteroid belt.
I am the Alpha and the Omega-3
What about the KOSMOS Project?
Putting the romance back into necromancer.
Just got to keep your eyes open for those Grid Bugs!!!!
In your case they don't. :)
This reminds me of some of the most common last words:
"Check this out! - It's gonna work!"
There is a simple, yet somewhat expencive, way to see who's right.
Built a satelite / spaceprobe with a whopping huge and light (mylar maybe?) sail. Launch into space (as the sail will be then main experiment on this one, it can be relatively light and might piggyback anotehr launch). Deploy sail. Wait and see what happens. THEN one can sit down ans find out if current theories are on the mark.
Everything in the world is controlled by a small, evil group to which, unfortunately, no one you know belongs.
Grid bugs and 32mb FX graphics in the eMac huh?
will the lameness filter catch all my caps cause they are like yealling?
Too bad the "Correct Spelling Filter" didn't catch you.
They are talking about the seas on the moon, of course.
The acceleration to get to 100 mph is what ?
100 years ?
therefore...
YOU FAIL IT!!!
YOU FAILed IT with such a lack of flair and style that I am forced to conclude that, despite your claims, you must be straight and white. go screw some fat, pimply, goth chick YOU geek loser, and leave us beautiful Nubian queers out of your crapflooding.
So, rather than going through all the trouble of avoiding air molecules, so that differential thrust due to heating does not propel the spacecraft, why not use the differential thrust to your advantage? After all propulsions is what they want. If differential heating of air molecules is capable of producing motion, so be it. Why not use it instead of trying to avoid it.
An Indian-American Hindu committed to non-violent thought/speech/action alarmed by the global explosion of radical Islam
You would look stupid putting a solar sail on each member of the nea.
Though no idea should be dismissed a priori in science, this alone calls into question Dr. Gold's ability. Unless they're using "steady state" in a manner unconnected to its traditional usage, Dr. Gold is on the side of a theory that has pretty much fallen by the wayside. Excluding the increasingly, um, eccentric Fred Hoyle, there are no real leaders among the handful of proponents of steady state.
The Mongrel Dogs Who Teach
Ever heard of Solar Wind?
Why couldn't the contributor just reflect over the rebuttal article before posting?
It was obvious from the original story that the guy was wrong. He made fundamental errors that you could spot after a freshman level course in physics 101. The rebuttal doesn't really address the specific flaws in the original paper- it has the character of "ahh this is nonsense" which it is. But the original paper has some obvious conceptual errors:
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.
The two temperatures are NOT the same. They are slightly different. The mirror is not infinitely massive, so in the mirror's own reference frame the photons reflecting from the mirror have a lower energy / longer wavelength after their elastic collision with it- the mirror receives a small bit of momentum from each photon in the collision. And in the sun's frame, the mirror is receding and the reflected photons are doppler shifted. He can't assume that the incident and reflected energy are the same and run off making derivations from that. They are extremely close, but the difference between them is not zero like he assumes.
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.
Does he even realize the sun is a point source? The sun shines on one side of the sail, not both sides! One side is exposed to radiation with a temperature of 300K. The other side sees only 3K radiation. The sail temperature will rise to some intermediate temperature between 3 and 300K and reach thermal equilibrium with all available radiation. So what? This means nothing for momentum transfer! Once it reaches thermal equilibrium, the sail is receiving X watts of radiation coming from one direction, and radiating X watts thermally in all directions! While the wattages are the same for both, the radiated energy has no overall momentum, while the incoming energy has a very definite momentum. The point isn't to heat the sail, it's to move it. He seems to be confusing the sail's kinetic energy of motion with its internal thermal energy.
The rebuttal is very sparing. I think it would probably have been more vicious if its author didn't "know Prof. Gold well" and didn't have any reservations about embarrassing him.
An Indian-American Hindu committed to non-violent thought/speech/action alarmed by the global explosion of radical Islam
Oh the need for the Score: -2, Troll... so even the Uncut threshold would skip over it...
I have learned to use the word 'impossible' with the greatest caution.
To make laws that man cannot, and will not obey, serves to bring all law into contempt.
--E.C. Stanton
As insulting, flamebait, and offtopic this is, it's still funny! You have to give him credit for the anagram use. Very creative.
I'm still trying to figure out WHAT that ascii art even IS ... o.O
And he's got the biggest feature of the crank, a martyr complex:
(That's from a journal article he wrote.) Now from a purely scientific point of view, one is inclined to accept that Gold was the victim of medical close mindedness. The notion of "active hearing" does make a lot of sense, and medicos are notoriously rigid with respect to scientific issues. But other physical scientists have managed to bridge this gap: Norbert Weiner comes to mind. In fact, the very theories that Gold was trying to apply to hearing were originated by just that kind of cross-discipline collaboration.I have to suspect that Gold likes to play the contrarian just to avoid dealling with his on collaboration issues.
A Rebuttal by Geoffery Landis.
Do you know why the road less traveled by is littered with the bones of the unwary?
Hide their protractors!
(Or is it their compasses? [compassi?] {I mean the pointy ones with the stabbing..and he eyes....glavin!})
James Gold, Older brother of Thomas, says "Will not!"
Secretary of Planetary Society claims "Liar, Liar, pants on fire"
Louis Freedman picks nose while he thinks, says wife
FLASH! LOUIS FREEDMAN PICKS NOSE!
The Case of Exploding Journalists [Dave Barry]
I Do not! claims Louis Freedman on Letterman show"
I'll update this list when I see more information. This looks really interesting -- it's hitting all the major media!.
Correct Horse Battery Staple: 72 bits of entropy. Enter "Correct H" into google. When it generates the phrase, that's
F=dP/dt. What more do you need.
who cares about some method of propulsion when it comes to space travel. not until we discover some type of yet to be disovered natural occuring phenomena will be able to travel with any meaning in this universe of ours.
what is the big deal of traveling thirty years to find yet another pile of dull looking rocks and funky barren planets. and find out wow our signals can travel that far.
> A number of colleagues have contacted me since the web posting
> (on a rather obscure British web site of "e-print physics archives",
> http://uk.arXiv.org.) The British publication, "New Scientist,"
> ran an article...
uh, excuse me, but arxiv.org is most certainly not a rather obscure web-site. In particle physics (and other fields), it has become the defacto standard place to search for articles (along with SPIRES at SLAC).
Maybe he is referring to the fact that articles posted there are not necessarily peer-reviewed (but many peer-reviewed articles have a copy there as well). If you want to tell another physicist where to find an article online, the easiest way is to provide its arxiv.org key (e.g. hep-ph/somenumber).
apart from that, Louis Friedman is right though - Gold appears to be a bit of a nutcase.
1. Observation and description of a phenomenon or group of phenomena. - In this case, all sorts of different light experiments
2. Formulation of an hypothesis to explain the phenomena. In physics, the hypothesis often takes the form of a causal mechanism or a mathematical relation. - In this case, Maxwell's theory, and then the Quantum Mechanical changes it suffered
3. Use of the hypothesis to predict the existence of other phenomena, or to predict quantitatively the results of new observations. - In this case, there are 2 predictions in consideration: Solar Sails work because of [read the article for info], Solar Sails don't work because of [read the older article for info].
4. Performance of experimental tests of the predictions by several independent experimenters and properly performed experiments. - In this case, build it and try it.
You are suppose to argue beforehand,that is step 3. That is the way you really understand your theory and its implications. The arguing beforehand is a very important step to clarify what someone is really saying with his/her theory. After observing a phenomena it is very easy to come up with hundreds of explainaitions, but the only good ones are the ones that predict new stuff, and clarifying the theory allows us to make more precise experiments that really show light into the important issues. Arguing the different theories is what makes them more specific that just betting for the outcome of an experiment.
"There is no teacher but the enemy."-Mazer Rackham
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
Sweet!
My career as a Solar Pirate is looking more promising everyday
Yarrr!
On Wall Street they say "buy low, sell high" On the pad we say, "buy high, sell high" Isn't that somehow better?
You'd better make sure you match velocities correctly, or the result would be like trying to stop a fastball pitch by holding up a kleenex.
Don't blame Durga. I voted for Centauri.
"Your years of toil,"
Said Ryle to Hoyle,
"Are wasted years, believe me.
The steady state
Is out of date
Unless my eyes deceive me,
My telescope
Has dashed your hope;
Your tenets are refuted
Let me be terse
Our Universe
Grows daily more diluted!"
Said Hoyle, "You quote
Lemaître, I note,
And Gamow. Well forget them!
That errant gang
And their Big Bang-
Why aid them and abet them?
You see, my friend,
It has no end
And there was no beginning,
As Bondi, Gold
And I will hold
Until our hair is thinning!"
Jurisprudence Fetishist Gets Off On A Technicality --theonion.com
In this country we follow the laws of THERMODYNAMICS. This type of hacking is ILLEGAL. If you want to hack Solar Sails YOU MUST PURCHASE SAID ENERGY. We are going to see that this website is taken down immediately. We will log IP addresses of anyone who visits this site and we WILL find you and prosecute you to the maximum extent permissible under the LAW.
The rebuttal is pretty interesting, it rests on a fairly simple principle:
When the photons are travelling towards the sail an observer at the light source will see a red shift (doppler effect at work here).
When the photons are travelling away from the sail an observer at the light source will see a blue shift.
Because the observer hasn't changed position the shift can be attributed to a change in energy, which must have gone into the sail (as the only thing the photons encountered, assuming a perfect vaccume) meaning that the increased KE of the sail breaks no laws of physics.
Beep beep.
You are making a personal attack, not attacking his ideas.
:
Here are two cases
(a) Another BIG proponent of the Steady State universe is Fred Hoyle.(While we are at it, let's throw in that Hoyle also supported life from space rocks theory). Is he a quack? No. He has good arguments. In fact, Fred Hoyle is sadly forgotten for his VERY seminal work on figuring out how the Sun nuclear engine works. Sadly the Nobel committee only awarded Willie Fowler the Nobel though Hoyle arguably did as much to solve the problem : an scandalous injustice that many astrophysicists now rued.
(b) You can also attack Friedman's comment about the "obscure british "preprint physics archives".
A number of colleagues have contacted me since the web posting (on a rather obscure British web site of "e-print physics archives", http://uk.arXiv.org.)
The arXiv is the main distribution of physics and maths papers nowadays. Everybody in the field reads the archives. In fact the uk.arXiv.org is just one of the many mirrors of the main site arXiv.org hosted, ironically by Cornell.
So Friedmann did a "personal attack" on Gold (implying that Gold only published his findings in some no-name website). THat's not a good thing. A good scientist names his source without judgement : the reader can decide for himself whetehre it is good or not by its contents.
Mode (3) smart-aleck mode. Press * to return to main menu.
He is off his rocker.
Applying the laws of thermodynamics that govern a heat engine to a sail is just not applicable. You might as well apply Kirchoffs current law to a water balloon.
This is a straight forward conservation of momentum problem. Give a sail of size A, with reflectivity R and a photon flux F(photons/sec) impinging on the sail with average frequency f you will have have momentum imparted to the sail of M=2*(F*R*h*f)+(1-r)*h*f. h is plancks constant. QED and very straightforward.
Well I was dissapointed by pons and fleischman, Golds theory about the inorganic, I now see how and why you can have otherwise respectable people make completely foolish statements
They'll just have to remember not to ignite the rockets when they get to the phlogiston.
I told you... simoniker means I'M ON ERIK'S... Eriks what? We can only imagine.
I suggest you go to tubgirl.com then...
-uso.
1) look at the border. discern the word/phrase there.
2) do a google image search for that word phrase with the filtering off.
3) feel eyes melt out of your skull in horror.
4) ????
5) profit.
my pet machine
Don't forget the whalers...
On Wall Street they say "buy low, sell high" On the pad we say, "buy high, sell high" Isn't that somehow better?
a presigious US astronomer, wrote a paper in 1902 in which he concluded:
p 16 7y1977-78.pdf
"Flight by machines heavier than air," Simon Newcomb declared one day in 1902, "is unpractical and insignificant, if not utterly impossible."
His arguments were quite reasonable on the surface - Imagine a bird as a model. If you increase the size of the bird, the mass increases proportionally to the third power of its wingspan. But the surface area of the wings only increases proportionally to the square of its wingspan. Thus something much larger than a bird would never be able to fly, and all attempts to build heavier than air flying machines capable of carrying a human would prove futile.
Fortunately, the Wright brothers never read his paper, or at least never took him seriously.
About 40 years later it was argued by learned men that manned supersonic flight would never be achievable.
http://www.garfield.library.upenn.edu/essays/v3
Marconi wasn't formally educated, and he was laughed at for spending vast sums of money to send a radio signal across the Atlantic ocean. Any fool knows that radio waves couldn't penetrate the earth, and was limited to line of sight communication! Yet despite all logic, the damned fool contraption eventually worked. It was only later that they discovered the ionosphere could reflect certain frequencies back to earth.
Even great men of science make mistakes sometimes.
My rights don't need management.
noted. thank you for your contribution
in more speed then any conventional propelled rocket that you send in space.
The main practical difficulty I see is stopping. You can't slap propelled rockets on the ship to do the job; if you did, I would want to know you didn't propell you ship with that to begin with.
Maybe some fancy gravity trickery... deaccelerate as a star's gravity starts to whip you around. Other than that, I don't see how you could do it. I still don't see a use for these other than minute corrections in satellite trajectories. They're not directional, and the methods by which, say, sailboats can sail against the wind won't work here. Only way to slow yourself down is to stop sailing and let gravity do it's thing... a big problem when you're somewhere in space in which gravity isn't acting against you. Moreover, we wouldn't get very far away, because the force provided by sunlight diminishes exponentially as you move further away. And going towards another star wouldn't help, because you can't sail against the "wind" in this case (ship sails can because of how the wind will curve and press on the sail in a different direction than what it was originally travelling, which won't happen with light).
We're getting to the point where it will just take too long to go where we want to go, and eventually it's going to make us ask if we really can go there. I mean, hundreds of years later, who's going to care that a probe, unable to communicate with us, is careening somewhere past Neptune? As for people, don't hold your breath on this transporting us; it just takes too long. I don't know about you, but going to another planet wouldn't be worth most of my life, if not the whole thing and part of my children's. And I don't even want to hear this whole "Once we figure out how to go faster than light" garbage. You've all been taking Star Trek too seriously. Granted, people didn't believe we could fly, or the earth was round, blah blah blah. As we progress we do realize that things were wrong, but some things become more compellingly right as well. The speed of light, the fact that we can't exceed it and its correlation with time are what defines our reality. Sending something faster than light, AND slowing it back down without the obvious logical and physical laws getting in the way is impossible. Sending something as complicated and sensetive as an organism is absurd. Sending an organism as intricate as a person should be grounds for insanity.
Sorry, that just bothers me when geeks worldwide sit back after watching something on the Sci-Fi channel and think "Man, once we learn to warp... that'll be good times my friend", to which his friend replies "Affirmative" and starts taking readings on his platic light-up box... er, tricorder.
It's always the turtle that wins...
Unless there's a dog running after you. I got 5 bucks on the hare.
Now that I think of it, it really looks like it shouldn't work.
Assume there are two ideal parallel mirrors. If you send a ray exactly perpendicular to the mirrors, it would bounce between them indefinitely, depositing an infinite amount of energy!
And if you think of red shift, then I can construct a scenario in which the ray will bounce on an infinite number of (initially static) mirrors-just send it a bit diagonally.
But if everyone on earth sucked 2.67 * 10^12 dicks per second, we could produce enough energy to get the job done. It's so crazy it JUST MIGHT WORK.
I'm not sure what, but I'm pretty "profit" needs to go in there somewhere.
- None can love freedom heartily, but good men; the rest love not freedom, but license. -- John Milton
He speaks of the "Temperature of the Photons" is same same, so no thrust because of 2nd law of thermodynamics.
If I'm right with my thermodynamics knowledge, the temperature of the photons would be their *unordered* move (that is, relative to their mass centre). And this is nearly zero.
Not to mention the frequent use of the term "rebuttal"...
Sci-fi fans (or naval adventure fans, or people who like good books) who dig this whole solar sail business are highly encouraged to check out Michael Flynn's recent book The Wreck of the River of Stars. Hardcore space adventure aboard the last of the great solar sail ships, put out of business by fusion technology. It's a romantic yet compelling look at space-travelin' technology. Flynn's a talented writer, the type who can name his chef Eaton Grubb and get away with it. Good summer read.
Heh heh. Off-topic. Bring it on.
Karma: T-rexcellent.
2. Formulation of an hypothesis to explain the phenomena.
3. Use of the hypothesis to predict the existence of other phenomena.
4. Apply for a scientific grant and have some graduate students to get a lil' profit
5. Perform experimental test.
6. Apply for a patent. You will get the patent, is too easy these days.
7. Sue someone and profit even more
"There is no teacher but the enemy."-Mazer Rackham
And in the sun's frame, the mirror is receding and the reflected photons are doppler shifted
./ article about solar sails, to explain why light striking a sail loses energy and increases the motion of the sail. My quesion is, isn't the redshift of the sail relative to the sun so small that any photons strinking the mirror would lose only a tiny fraction of their energies? This might mean that a sail would work in principle, but that the accelerations would be so small as to be impractical. (Yes, any acceleration is better than no acceleration, but most astronauts would like to get home in their lifetimes...)
This point was raised in the previous
Can any non-armchair physicists shed some red-shifted light on this?
Thanks,
Dexter
The Sun may be at 6000K, but it only occupies part of the sky on that side of the mirror - the rest of the sky is at 3K.
So now we have a nice little calculation which is left as an exercise for the reader. If 6000K Sun plus 4 steradians of 3K sky = 300K (the classic black-body-in-orbit case), then the equilibrium temperature of the Sun-facing side *only* will be quite a bit higher (because there's just as much Sun, but only 2 steradians of sky).
"This ship is unsinkable", says designer of the Titanic.
I should have thought that the momentum change in the photons (in the +x direction before collision, the -x direction afterwards) would have clued Gold in that he was wrong. The momentum must balance. Gold made an error in assuming the reflected energy was exactly the same as the transmitted energy which is not a valid approximation if you are trying to tell what motion the mirror might have. In fact the reflected energy is slightly less due to Doppler shift of wavelength. Dumb mistake and one I would have thought a theorist would catch. Perhaps it was not one of his better days. We all after all DO make dumb mistakes at times. Prof. Gold was unlucky enough to do so in a very public way.
Actually the redshift is misleading and completely irrelevant anyway. A sail will work fine with zero redshift.
The point is that the interaction between the sail and the photons is characteristic of an elastic collision, exchanging momentum, with an associated increase in entropy. It's a trivial and easily understood interaction. There's just no hole here that you can poke a thermodynamic argument into. If some guy wants to convince us that this is a "heat engine" and then derive the fact that it can't work because of general thermodynamic principles, the onus is on him to explain where the thing breaks down in a specific, non-abstract way. So far, it seems he hasn't convinced everybody that the sun+sail system even fits the definition of a heat engine in the first place.
Did anyone else catch this line in the article?
"He has forgotten more physics than I ever knew..."
At first it seems like a compliment, but on another level, it seems to carry the implication that he's getting a bit senile in his old age...just a thought
Masterful editing by the slashdot crew yet again.
How difficult would it really be to pipe all the story submissions through a spell checker before posting them?
If space isn't an ocean, what is?
"For years, I struggled with reality... but I'm happy to say I finally won out over it." -- Elwood P. Dowd
6. Apply for a patent. You will get the patent, is too easy these days.
/. crowd. Yes, when it comes to SOFTWARE patents the USPTO is f'n up feircely.
What bothers me here is that are trying to drum up karma by catering to the USPTO sucks, and lawsuits suck
Solar sail technology is legitimately patentable even though it is gritting on the nerves of those who have not made a considerable investment in the development of anything ever.
Readers and contributers to this site seriously need to learn that it takes capital investment to drive an economy. I can not understand how some people complain of an economic slump, specifically in the IT sector, and in the same breath make outrageous claims like software and information should be free.
Yes, SCO sucks. I know. They do, and I'm sick of reading out them. But remember, there are people there who are going to lose their jobs.
If you invested millions into a technology nobody else has wouldn't you like some guarantee that you have the ability to return the investment?
The List of Grievances with Slashdot.
Tommy Gold and others quoted in the article about solar sails really should consult some real spacecraft engineers. For us, solar radiation pressure is an everyday reality. Solar radiation pressure is a major perturbing force on GPS satellite orbits, for example.
AMSAT, a group of radio hams that builds its own satellites, has for decades used radiation pressure to impart slow spins to its satellites with "blade turnstile" antennas. Paint one side of each blade black and the other white, and the spacecraft slowly spins like a Crooke radiometer -- but in the opposite direction, away from the white surface.
A Crooke radiometer is a very different beast. The glass bulb is not evactuated, so thermal heating on the black side of the vanes heats and expands air, pushing the vane away from the black surface. This force overwhelms the much smaller photon pressure, but in the vacuum of space only the radiation pressure exists.
Gold's thermodynamic argument is silly and wrong. A solar sail is not a heat engine, so the second law doesn't apply. The first law (energy conservation) does apply in a very simple way: the photons reflecting off the sail are red-shifted by the sail's motion, removing energy from the photons and imparting it to the sail by accelerating it.
No. In addition to conservation in momentum, in an elastic collision we have conservation of energy. If the photons were returned with the same energy (that is color, wavelength, frequency, etc.) the kinetic energy of the mirror could not change. A mirror moving away from the sun increases its speed and hence its energy, while a mirror moving towards the sun is slowed down so the energy diminishes. In the first case the light is redshifted (getting lower energy), while in the second case it is blueshifted (getting higher energy).
No. In addition to conservation in momentum, in an elastic collision we have conservation of energy. If the photons were returned with the same energy (that is color, wavelength, frequency, etc.) the kinetic energy of the mirror could not change.
Yes, but the point is that they're not the same energy, because the sail has a finite mass. They will have a slightly redder color for having scattered off it. So the kinetic energy of the mirror can change and there is no problem.
A mirror moving away from the sun increases its speed and hence its energy, while a mirror moving towards the sun is slowed down so the energy diminishes. In the first case the light is redshifted (getting lower energy), while in the second case it is blueshifted (getting higher energy).
These are not connected statements. A force on the mirror exists even in absence of the type of redshift you are describing. As seen from its own reference frame, the mirror will experience a force from radiation pressure. This will happen whether the solar system frame observes a redshift or not.
There will be a miniscule "redshift" observed in the mirror frame, from elastic recoil, and this is what powers the sail. The additional redshift you're talking about is seen when you move from the mirror frame to the solar system frame. That isn't the same redshift attributable to the force on the mirror. If it were, then the motion would be affected by your choice of coordinate system. So a mirror starting at rest with respect to the sun will still work fine.
There are 3 sources of momentum transfer as I see it.
1) Light impinging on the sail and reflecting back towards the sun. The photons have momentum due to relativity.
2) Photons being absorbed by the sail (increasing its temperature and transferring momentum to the sail)
2) Photons being radiated in all directions by the sail (radiant heat).
Conservation of momentum shows that the sail has to accellerate, but he's right that it will start out increasing rapidly, and as it heats up it will slow down its accelleration (because it starts to radiate infra-red on the side away from the sun)
I think this is the sense in which it is a heat engine.
The conservation of energy and the fact that the sail accellerates away from the sun does imply red shifts of the reflected photons (ie. reduction in their energy). This doesn't seem to bother me at all. It seems to be what bothers Gold.
((lambda (x) (x x)) (lambda (x) (x x))) http://www.endpointcomputing.com a scientific approach to custom computing.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
Newcomb's essay had some grounding in observation. He started with the reality that flying animals top out around fifty pounds, and plausibly reasoned that their design quality would be tough to match.
Newcomb did't foresee lightweight power plants with output in the tens or hundreds of horsepower.
In other words, at least Newcomb got the physics right.
>The main practical difficulty I see is stopping. You can't slap propelled rockets on the ship to do the job; if you did, I would want to know you didn't propell you ship with that to begin with.
1. Angle the sail to oppose your orbital motion. In the extreme case, stop relative to the sun and fall inward.
2. Carry the rockets. At least you won't be incurring the cost of rockets to push the braking rockets out to where they're needed.
3. Aerobrake, as one of the sibling replies suggested.
4. Send a disposable mirror in front of you to concentrate and reflect light backwards onto your sail. This is the technique proposed for decelerating a laser-propelled sail.
Notice that Spock and Captain Kirk get around using some sort of engine that glows blue out the ass-end.
Not sails, mind you. How exciting would Star Trek be if Scotty hoisted sails? Would we all then be doing that when the light turns Green, rather than "stepping on the gas" and pretending we are part of the Star Trek world?
Don't buy that, ok, then try this:
What if when we flushed the toilet, we had to hoist a sail (gee, no sails available, how about hoisting some TP?) rather than hear an assuring Whoosh! as the toilet flushes, and for a brief moment, we pretend we are Star Trekers.
Not buying that either, huh? Well, darn it, I just don't believe in sails as a viable way to get anywhere quickly.
If this is an elastic collision, and the sail gains energy, then the photon must lose some energy. Where do you propose this energy comes from?
The photon's initial energy.
Clearly, the photon's velocity can't decrease. Hence the redshift.
That's the redshift from the elastic collision, not the redshift from the mirror's motion relative to the solar system. They are two different redshifts.
Not true. I'm sure you would not say this had the argument been based on conservation of momemtum, or conservation of mass/energy.
Usually it's obvious where momentum is going (energy is trickier to track down). And the cases where it isn't obvious sometimes turn out to be interesting phenomena, like neutrinos.
Thermodynamics, in comparision, is so abstract and so prone to misinterpretation that it becomes a reasonable question to ask for a direct mechanism. Often people make implicit assumptions, incorporate subtle errors, or use imprecise definitions in forming their logic. (Like in this case, the definition of a "heat engine".) Anyone who's ever heard the one about how "the Second Law of Thermodynamics disproves evolution" knows what I mean. And it's not as if the exact mechanism wouldn't be interesting here. Exactly where and how do the thermodynamics affect the situation? How do they manifest themselves?
Didn't we have a big reflective object in high orbit already? Do we not have orbital data from them that tells us if there is a solar wind pressure?
a ry /Echo/DI55.htm
http://www.centennialofflight.gov/essay/Diction
Improving on the idea of sending and receiving
signals from the moon, in 1960, NASA launched a
balloon satellite that would reflect communications
signals. Echo I was a balloon made of
aluminum-coated Mylar that was launched by a
rocket into space. When it reached orbit 1,000
miles (1,609 kilometers) above the Earth, it
inflated from inside a 26.5-inch (67.3-centimeter)
magnesium sphere to 100 feet (30.48 meters) in
diameter. Circling the globe every two hours, it
shone more brightly than the North Star in the
evening. The balloon captured the imagination of
people who had watched the first man-made object
in space, the Russian satellite Sputnik 1, orbit the Earth in 1957.
If Slashdot were chemistry it would look like this:Cadaverine
... would be to ask him to explain how comet tails form. Doh! I have to admit not even thinking of that when I read Gold's article.
They will argue more the more expensive it is as to if it is worth trying.
How big does it have to be?
Can you do it with something less then a kilo at least for proof of concept?
So the rebuttal says the article was flawed because it was written assuming a perfect mirror, and not assuming a perfect vacuum. While the part about not assuming a perfect mirror is very true and valid, the part about space being a perfect vacuum seems a little suspect. I mean, it might be close, but you have to consider the size that a solar sail would be. Especially if operating in a cloud of interstellar dust, etc. friction would be noticable.
====
Crudely Drawn Games
it will, i has too, don't you remember in grade 6 when you used the light bulb looking thing with the horizontal windmill in it ????? Light would cause the "windmill" to turn. This is the same idea. It will work. My grade 6 teacher can't be wrong.
For some reason I'm not surprised you have an AOL account. Dim for a reason. I'd bet your a republican too.
Clever dick, keep it up. You're an impressive fool.
Should be VladTheFag.
" Cornell Physicist Thomas Gold's paper declaring the theroy behind solar sails flawed'
Yes, but did he take into account bugs in the Matrix?
Granted it doesn't go into all that much detail, but it does discuss the possibility of solar sails and various other techniques being explored to reach alpha centauri. Yeah, yeah, you'll actually have to (*gasp*) read something that is printed on paper, sorry.
Solar sails operate on Newton's third law. IE, they impart force based on reflecting a particle (or lots of them), known as photons, and in that reflection, which is not directly reflective, take sin(angle of reflection)*mv in forward motion, while decelerating the photon. Assuming that Quantum ElectroDynamics is correct, and a photon is merely an electron at limit velocity, the kinetic energy is the mass conversion of the deceleration. IE, the equivalent Schrodinger value of the electron produced at the reflection by the deceleration of the photon, which would be approximated by sin(angle of incidence)c. Beyond that, the force generated by the solar sail would be the integral of the flux density of light-the relative velocity of the sail to the incident light over the incident angle of the radiation on the sail. Hardly an easy problem, but also not beyond undergraduate calculus.
Carnot was unaware of the wave theory of matter or quantum physics, as, apparently, is the author of this article. Adiabatic cycles work in classical, IE low velocity, non Boyle limit environments, but , like most classical physics, fall apart at the quantum level.
For further reading, I advise the Feynman lectures on Physics, or if you're feeling adventurous, you could always finish his doctoral Thesis: Wheeler & Feynman, Princeton, 1943 (the subject is the concept of unification under the principle of Least Effort), as yet, unsolved.
Since Fermat's last theorem has been proved, maybe we can NOW move on to unification?
Let me sum up quickly.
1) Photons bouncing off a mirror will impart momentum to the mirror. That's Newton's laws right there, as fundamentally a part of the fabric of the nature of reality as we know it as damned near anything else we have ever studied. If Gold were right and a Solar Sail wouldn't work, then it would mean Newton's 3rd law would be violated, and that's super bad mojo, mojo of a scale that Gold was unable to back up with sufficient evidence and argumentation in his penny-ante paper.
2) The laws of thermodynamics are not violated by the operation of Solar Sails. In any given inertial reference frame the reflected photons will be "red shifted" and have a slightly lower energy. This is how energy is conserved (since the movement of the sail represents work, and thus energy). This is also how the 2nd law of thermodynamics (non-decreasing entropy) is followed, since the redshifted photons are higher in entropy (for slightly complicated reasons) and balance the work done.
3) Light pressure is not theoretical, it has been detected, measured, and, indeed, used many times in many circumstances. Its properties have corresponded very closely (to about as many 9s as you'd like) to what has been theorized.
In short, Gold is full of crap and the New Scientist ought to be ashamed at printing his stupidity.
I'd heard a little of this argument from elsewhere, but seeing the source is pretty embarrassing. The Sun is rightly recognized as a heat source, but what isn't understood is that the other heat sink is the background of the universe which is somewhere around 5 degrees K plus any warmer objects (like the Moon or planets) that might get in the way.
"In response to Cornell Physicist Thomas Gold's paper declaring the theroy behind solar sails flawed (previously mentioned in this...."
I am usually not to much of a nit picker because my own typing sucks....but not taking the time to fix something as obvious as this? Come on you guys atleast read what you type.
OH THE SHAME I fell off the wagon and use sigs again!
If you read the article, you should have read the part that said solar pressure has already been established and mesured. It's not a theory, it's a fact. There's no need to do an expirement to disprove gold's paper.
autopr0n is like, down and stuff.
If this is an elastic collision, and the sail gains energy, then the photon must lose some energy. Where do you propose this energy comes from? Clearly, the photon's velocity can't decrease. Hence the redshift
Well, clearly the photon's speed can't change, but velocity is a vector, not a scalor. Sheesh, you learn that in the first week of physics. When the direction of the photon changes, so does it's velocity.
autopr0n is like, down and stuff.
If there were a correct spelling filter 99% of posts would be blocked.
I browse Slashdot at +3, Funny
fuck you homo, theres nothing wrong with circumcision
damn you are a waggler
waggly cocks
The best you can do on that point actually is that the center of our galaxy forms the center of the universe. Back to you, Bob.
-- thinkyhead software and media
Comment removed based on user account deletion
Thomas Gold: "Uh uh!"
Planetary Society: "Uh huh!"
And here's the New Scientist article about why the sail might fail...
In other news, warp drive has been invented...
So we think it's impossible now, hell it might very well be impossible but hey everythings worth a try once.
If you take a look, I said "the photon's velocity can't decrease". If you don't take that to mean "the photon's speed can't decrease" then I'd like to know what you think it means.
I think you'll live longer if you relax a bit and don't assume dumbest possible interpretation of things you read on Slashdot.Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
The rebuttal's reference to quantum effects are irrelevant, since the laws of thermodynamics are not violated by quantum physics (they seem to contain their own protection against that, which is why it is impossible to build a device implementing a "Maxwell's Daemon").
An interesting angle on this is to try to devise a perpetual motion engine using this effect. One could imagine using a second, fixed (or attached to something much more massive than the sail), mirror that would reflect the light back towards the sail, thereby reusing the same photons over and over. Of course, the energy of the photons would decrease over each iteration and the pressure of the reused light would decrease pretty quickly. There might be geometric reasons why this couldn't work, as well.
Freedman's rebuttal says that Gold is wrong to argue from a thermodynamic point of view, and that he ignores quantum mechanics. However, all the laws of physics must be consistent with each other. This consistency is what makes these intellectual arguments so interesting. For example, by insisting that electrodynamics was consistent with mechanics, Einstein developed special relativity.
Gold's arguments are simply wrong, but this incorrect rebuttal is not really that good. When debating with crackpots, it's important to be meticulous in your arguments, because they will seize upon any small error and attempt to make that the focus of the debate, not their own large, glaring errors.
It's not wasting time, I'm educating myself.
there is no spoon.
...that the concept can significantly move a spacecraft: TRY IT!
Table-ized A.I.
For a detailed explanation see the Dome News.
The article is a popular treatment without equations, but anyone who has completed a course in Modern Physics should be able to write down the equations.
How would the fact that you were relying on directional light from the sun affect the directions in which you wanted to sail? In other words, thinking like a boat sailor, would you be limited to certain angles from the sun? Or would you simply be able to use other mirrors to redirect the light to the angle you wanted in order to get to the destination of choice (although it sounds as if this would not be possible, since if the solar sails provided force the reflecting mirrors probably would also)?
I forget the title...there was a Larry Niven story about a race of alien traders who land and give us various technologies. In return, they want us to build a giant laser on the moon to power their solar-sail craft to the next star system. Our government doesn't want to do it...but then someone realizes that if we DON'T build the laser, the aliens' Plan B is to cause the sun to go nova, which would also produce the necessary thrust.
I believe it was "The Oldest Profession"?
- Sun emits "yellow" photons.
- Observer on runaway sail, due to Doppler, see "red" photons.
- Sail reflects perfectly "red" photons back to the sun.
Where you got an idea, that changing wavelength accomplished by loss of energy?
You can absorb as much enegy as you wish, but without matter it useless. E=m*c*c should give you an idea of how much reactive mass you can squeeze out of this.
I wonder if a solar sail would exhibit a radiometer effect.
If the sun-facing side of the sail were black so as to make that side heat up preferentially, wouldn't it get a bit of a kick from free atoms that gain kinetic energy after hitting it?
IIRC light energy flux from a point source decreases as the cube of the distance, not the square. To see why, imagine a unit sphere centered around a point source, and a two-unit sphere centered around the same source. The two-unit sphere has 8 times the surface area of the unit sphere, and the same total flux.
d^(-3) isn't exponential, but it's pretty quick. I'm sure the parties involved in the dispute have already taken this dramatic fall-off into account, but think a moment. A system that worked very well in our region of the solar system could be blown off course by the gravity of the tiniest speck of dust out around Neptune's orbit, if its only hedge against inertia was the sail. This is why it would be wise to bring rockets along even if you couldn't bring enough fuel to use them as propulsion.
I am programmed for etiquette, not destruction!
A great teacher knows how to challenge his students at just the right level. Hans Bethe, Gold's Cornell colleague, was famous for that.
I suspect Gold's paper is a challenge to his freshman physics students to find the flaws in his logic. It's very clever in that perspective! Or maybe it's an April fool's joke. BTW, a Google search on "Crookes Radiometer" yields fascinating results.