The fundamental allegation is that after Aldridge's Freep log-in was blocked for egregiously violating their acceptable use policy, he used multiple e-mail accounts to get more than 50 log-ins under various names, and continued posting in violation of the AUP. Much as I dislike them, if there is any truth to that, they are in the right and Aldridge is wrong. It's a private web site. They have to right to kick you out if they want to. You don't have the right to sneak back in disguise.
One problem here is that we have a URL to the freepers' allegations, but nothing about Aldridge's response. Now and then I've noticed a conservative to twist the truth, or even to (gasp!) lie. (For instance, Gore never said he "invented" the internet.) Obviously Aldridge didn't convince the judge, but maybe the judge is a conservative Republican too...
Sounds like the company made three of the biggest mistakes it could possible make:
1) They didn't back up the source.
2) They left the source entirely in the control of one person. Say he skis head-first into a tree, then where are you?
3) They knew he thought he was being screwed over, and they still didn't back up the source!
To start with, if they were financially provident, they wouldn't have been buying lottery tickets in the first place. You get better odds from a Mafia bookie than from a gov't run lottery (at least in the USA) -- not that bookies are a wise investment strategy either...
A faraday cage the size of a PC built on gov't contract might cost $5K by itself. But you've also got to shield and filter all cables that run through it. A power cord can pick up emissions and carry them outside -- so you've got to put a low-pass filter where it goes through the box. VGA cables are notorious emitters (the signal frequency is high enough that a few foot of wire makes a decent antenna). So unless you faraday cage the whole room, you're going to have to use multiple layers of cable shield.
A room-size faraday cage is the simplest solution, but there's more to it than chicken wire. The chicken wire (or aluminum or copper foil) has to be electrically connected all along the edges -- and construction workers don't know how! Doors need conductive gaskets. Power lines need filters. I think it would cost $20K to build as a commercial project. As a government project, with union work rules and inspectors tripping over each other... The armed services do have electronics shops inside faraday cages so top-secret electronics can be worked on, but don't ask me what they spent on them. The one I was in had a bank vault door that must have cost $10K all by itself. (This opened inside the building. But then there was a fire exit door to the outside that was just an ordinary metal door with added gasketing -- and when the air conditioning wasn't working hard enough, the techs would open it to let some air in... 8-)
I know more about Tempest than you do -- it's not a spy program, but the armed forces program to make their computers proof against this kind of spying by heavily shielding everything. It also adds $5K or more to the cost (or more like $20K under a gov't contract). The strongest signal would probably be from the video cable, but the keyboard and disk drive cable also emit. Even signals on the motherboard give some emissions. Descrambling the bits (particularly from parallel busses) is quite a challenge, but from the attention given to controlling even motherboard emissions in Tempest systems, apparently the experts don't think it's impossible. Modern FCC-compliant enclosures really do cut down on emissions quite a lot. My guess would be that the easiest thing to tap would be video, LAN, keyboard, mouse, and other external serial interfaces would be considerably harder but possible, and internal parallel interfaces would take technology beyond anything I know of. That is, a really good spy organization might be able to record you typing in your password, then have to break in to use it. If you run with the covers off, you are theoretically at risk of internal SCSI, IDE, or motherboard signals being intercepted, but I have no idea how anyone could intercept 8 to 64 bits parallel and make sense out of it.br>
It isn't a risk I would worry about at present, unless you've really made enemies in high places -- and they'd probably go for a more direct attack anyhow. But technology keeps advancing. To really be safe, you'd want your keys to stay inside of a shielded calculator- or credit-card-sized device, preferably one that had to be unlocked by biometrics (fingerprints, e.g.), read on its own scanner. This device would have to do encryption itself, so as to never send out a private key. However, you'd have to type a message on another machine and send it to the security device in the clear, so there's a little vulnerability there...
All this is highly theoretical -- most users leave their systems open to much cruder methods, like read the password off the post-it note, recover secret documents from the trash, ask someone to let you in so you can do some maintenance...
And all that (ROM locked in safe, sneakernet, etc.) is in vain if, when you load the password into your non-networked PC, someone across the street is picking up the emissions from the NNPC and reading the data flowing through it.
One point you are missing: to use an encryption key, at some point it has to travel around the computer in the clear -- and apparently organizations like the CIA can pick that up from an antenna across the street, if for some reason they put their best people and equipment on it. Of course, it's much easier to just snatch you and wire up your genitalia until you give them the keys, but sometimes that method has unacceptable repercussions...
Anyway, the equipment to intercept computer emissions is subject to Moore's law too, so someday soon it will be within the price range of industrial spies, and the next year PI's will be using it to dig up dirt for divorce cases. We can ban it. That will work, sure, just like banning guns...
I suspect the reset button did exactly what it was supposed to. It reset the computer. This is NOT going to fix a bad hydraulics line. In fact, it makes the computer forget what was just going on, so it's probably going to make a bad decision as soon as it starts taking control. It is not impossible to store enough status information so the computer could have picked up just where it left off before -- but that isn't such a great idea when the pilot punches the button because the computer was screwing up.
I did mix up the dates a bit. It's at least 20 years since the first Blackhawk protos flew, and over 15 since they started using them -- however it wasn't flying right until 1988 or so, and I know that because I was peripherally involved in a critical avionics fix. The tail-plane was electronically controlled, the cable to that controller wasn't adequately shielded and filtered, and radio signals in the military voice band could be picked up and jam the tail-plane controller to full nose-down. In the typical Blackhawk mission, it would be at 100 foot or so above ground, and so it was almost impossible for the pilot to pull it out -- and difficult afterwards to tell whether something malfunctioned or the pilot just misestimated wind and trees... There were a number of crashes before the Army would admit there was a problem. I think a system I was working on in 1987 forced them to fix it -- it included a transmitter powerful enough to would drive the chopper full nosedown every time, and because it was usually fired up at 5,000 feet there was time to turn off the transmitter and recover control, so the pilots got back alive to report. And there is nothing angrier than a pilot whose aircraft just tried to kill him...
IIRC, the original design required the builder to thread hundreds of rods through the whole multi-storied catwalk, from bottom to top. I'm not an expert on these things, but it would be difficult to put together a scale model on a tabletop, and to build it while hanging in midair? So I'm not surprised the builder changed to short rods -- but it meant that instead of the load being carried through the intermediate box beams by the long rods, it transferred from top rods to bottom rods through the box beam. Furthermore, the box beams were formed by welding two U-channels together -- so the rods were secured by nuts bearing on the welds. The structure had only a fraction of the design strength, unfortunately it was enough to hold until a crowd went out on the catwalk...
As I said, I'm not a structural engineer, but it sounds like the engineer on that design had no experience with actual building operations. And the builder should have gone back to the engineer with the proposed changes to re-evaluate the strength... I am an EE at an electronics manufacturing plant. A large part of my work is dealing with board designs that are unbuildable and untestable. Obviously the engineering schools still don't teach the practical aspects of making boards. We get the bad designs changed, but we get them changed by going back to the designer, not by unilaterally changing them -- whether or not the board is going into a safety-critical application...
By the accounts I've seen, there was a light which lit up saying "reset". So the pilot hit the reset button. And that caused erratic prop and engine operation. (You try to write a program so it boots up and takes control of an on-going process without glitches.) And since even a recruit headed for cook school should be able to figure out that when doing that made things worse, you stop doing that, if the reports were accurate, the pilot had to be panicky.
Of course, 75% of what you see in the news is wrong, so maybe the pilot was indeed trying to use the reset button to keep the computer off-line. In that case, double-shame on the system designers for not providing a manual override on each control!
Having once worked on a project where, if I told you what it was I'd have to kill you, I find the notion of open-sourced military software very funny... But aircraft control software could easily be open-sourced, even for military aircraft.
In other engineering disciplines, it's usually the case that the designer and the builders are distinctly different people, and it's easier to say whether the problem was a design issue. I remember one case where a bridge falling down was the fault of both -- the designer designed something that couldn't be built exactly that way, and the builders made changes without consulting the designer or getting new strength calculations. (This was a multi-story foot bridge inside a fancy hotel; if I remember right, it was in the early 80's.)
Actually, a decent embedded system (RTOS or not) will re-boot itself when it needs to. That is, you got a "watchdog timer" which the main program loop keeps resetting; if it doesn't get reset, it resets the computer. But apparently they gave the pilot a pushbutton just in case the watchdog timer crapped out too. But he pushed it when the real trouble was a chafed-through hydraulic line. And when it didn't help, he pushed it nine more times...
As I see it (and I'm not claiming expertise), the problems were, in descending order of importance:
1. The designers failed to route and fasten hydraulic lines so they wouldn't chafe.
2. The Marine Corps and aircraft mfg both ignored warnings about chafing lines.
3. The system for some reason told the pilot he needed to reset the computers although the computer had nothing to do with the hydraulics failure.
4. The software changed propellor settings erratically while booting up.
5. They didn't give the pilot an effective manual override for any controls the computer might be messing up.
6. The pilot panicked a little. Understandable, but pilots and Marines are supposed to be panic-proof...
Correct. One thing is that sailboats use two "wings", the sail above and the keel below, in fluids with different velocities, and so the vector sum of the "lifts" can be partly into the wind. Lightsails cannot create thrust towards the source of the light, and in practice they'll always have significant thrust away. But orbital dynamics comes to the rescue: angle the sail to reduce your orbital velocity and you will drop, even though part of the thrust is still outwards.
It receives MOMENTUM from the photons bouncing off it. Momentum change = mass * change in velocity (and it's a vector). If the mirror is at right angles, you get twice the momentum of each reflected photon, because you get both the incoming momentum and the reaction to the outgoing momentum. The photon gets mass from e=mc^2, or m = e/c^2. c^2 being very big, the mass of even a whole lot of photons isn't much, hence thrust is rather low. But it's free (aside from the cost of the sail, lifting it to space, and repairing meteor holes....)
The spacecraft receives ENERGY (mv^2) only to the extent that it changes velocity in response to that momentum transfer. It's going to be a small fraction of the energy in the light.
As for how you angle the thrust: someone used a tennis ball analogy, that's probably the easiest way for most non-physicists to see it. Or if you want a mathematical treatment: let the mirror be angled at 45 degree to the incoming light. Incoming photons are reflected at 90 degrees to incident. The momentum transfer to the mirror is the vector sum of the momentum of the incoming light (x at 0 degrees) and the outgoing light (x at 90 degrees). The resultant is sqrt(2)* x at 45 degrees. But notice that you've reduced both the amount of light you intercept by angling the mirror and the impulse you get from each photon intercepted (2x at 90) -- so a 45 degree angle cuts the thrust in half. (This assumes 100% reflectivity -- with a real mirror, your results are somewhat worse, since some photons are absorbed and so only contribute outwards thrust. And outwards thrust won't help you spiral outwards -- you aren't going to get enough thrust from sunlight to just fly straight out. The animation in How Things Work is wrong.)
At arbitrary angles (still assuming 100% reflection): The thrust will be right down the axis of the mirror. With the mirror and thrust axis at angle T to the sun (that is, 0 degrees is dead-on and 90 is turned edge to the light), the thrust is (cos(T))^2 times the maximum thrust. 60 degrees cuts the thrust to 1/4 of maximum, but it directs most of it along the orbit where it will actually be useful. And at high angles, you get better reflection. Possibly it would be best to plan on never setting the sail closer than 60 degrees, so you can make the spars weaker and lighter.
It _is_ like blowing into your own sails. But you would get a little thrust by shining the laser backwards. The thrust in newtons is power in watts divided by the speed of light (3E8 m/s), you need a heck of a good source of power, like a big fusion generator or tons of antimatter. And it doesn't have to be visible light -- microwaves, gamma rays, or infrared would do just as well as long as it's directional. Note that all practical energy sources emit more power as waste heat than as usable energy -- so maybe the best light-rocket would simply radiate heat out one end.
However, if we do get fusion powered craft, I expect they would put the usable energy into accelerating the fusion products. You get more thrust from a given power by accelerating helium than by shining a light. For interstellar flight, most of your craft would have to be fusion fuel, so you might as well use the byproducts... Light-propulsion makes sense only as a way to use some power supply you don't have to take along. (Except maybe in a warcraft where the big laser could do double-duty?)
Because you'd need something like 99.9% of the weight of the vehicle to be fuel. That's with chemical rockets. With fusion, you could do considerably better, maybe only 90% fuel. And that fuel should be something like hydrogen or deuterium -- pretty hard to keep it from leaking away in a ten year flight...
Re:Negative Refractive Index stuff and Solar Sails
on
How Solar Sails Work
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· Score: 2
I found a reference to that here.
The negative refractive lens can (in theory) be created with a 40 nanometre thick silver film. But to work, it must be at a "distance of 20 nanometres from the light source, since the system uses its near field."
The sails are reflective (mirror-like), so by angling them you can get sideways thrust -- a combination of the incoming momentum of the sunlight and the reaction from the momentum of the reflected light. So angling the sails at 45 degrees gives you thrust at 45 degrees to the sun, but it cuts the thrust by half. (sin(45) squared -- once for the reduction in light intercepted by angling the sail, once for the off-axis thrust.)
You can't tack like a ship tacking into the wind (where the vector sum of forces on the sails, keel, and rudder gives a net force partially into the wind) because you don't have any equivalent to the keel. But you can use an angled sail to reduce the orbital velocity, so the ship drops sunward. And the best way to go outwards is to use sideways thrust to increase the orbital velocity.
The real problem with solar sails is the very tiny force per square meter that is possible. It should be enough for minor orbital adjustments. If you plan way ahead, it might be possible to use light sails to slowly spiral in or out (like weeks or months to the moon, years or decades to Mars). Or you find a way to augment the thrust -- like building a giant battery of lasers on the moon to provide much more intense light. In a couple of Larry Niven stories, they used lasers to launch an interstellar lightsailer, but to brake at the destination, it had to almost dive into the sun...
I couldn't find anything directly about the microwave refractor, but a search for "negative index of refraction" turned up three short pieces on the basic physics: 1 2 3.
Note that these involve putting conductors into the near field of the emitter -- that means within a fraction of a wavelength of the light source or microwave antenna. But to me, that just makes the metal an added piece of the antenna, and one well known directional radio antenna uses an array of metal rods in the path of the emitted waves...
I don't know if the rings and fiberglass arrangement is just a variation of this.
Or for a more relevant comparison: Hollywood's most successful movies get more than $100M just in US ticket sales. If that $300M is gross revenue, then 3 Hollywood successes beat the whole porn industry. It is possible, though, that porn studios average more profits than Hollywood -- they don't pay the leads $20M each, or spend $50M on special effects.
Re:"Wings" in politics
on
Republic.Com
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· Score: 2
Meant to say "the right was on the defensive for 200 years." Darn. Proofread, then submit.
Slashdot Mirror
The fundamental allegation is that after Aldridge's Freep log-in was blocked for egregiously violating their acceptable use policy, he used multiple e-mail accounts to get more than 50 log-ins under various names, and continued posting in violation of the AUP. Much as I dislike them, if there is any truth to that, they are in the right and Aldridge is wrong. It's a private web site. They have to right to kick you out if they want to. You don't have the right to sneak back in disguise.
One problem here is that we have a URL to the freepers' allegations, but nothing about Aldridge's response. Now and then I've noticed a conservative to twist the truth, or even to (gasp!) lie. (For instance, Gore never said he "invented" the internet.) Obviously Aldridge didn't convince the judge, but maybe the judge is a conservative Republican too...
Sounds like the company made three of the biggest mistakes it could possible make:
1) They didn't back up the source.
2) They left the source entirely in the control of one person. Say he skis head-first into a tree, then where are you?
3) They knew he thought he was being screwed over, and they still didn't back up the source!
To start with, if they were financially provident, they wouldn't have been buying lottery tickets in the first place. You get better odds from a Mafia bookie than from a gov't run lottery (at least in the USA) -- not that bookies are a wise investment strategy either...
A faraday cage the size of a PC built on gov't contract might cost $5K by itself. But you've also got to shield and filter all cables that run through it. A power cord can pick up emissions and carry them outside -- so you've got to put a low-pass filter where it goes through the box. VGA cables are notorious emitters (the signal frequency is high enough that a few foot of wire makes a decent antenna). So unless you faraday cage the whole room, you're going to have to use multiple layers of cable shield.
A room-size faraday cage is the simplest solution, but there's more to it than chicken wire. The chicken wire (or aluminum or copper foil) has to be electrically connected all along the edges -- and construction workers don't know how! Doors need conductive gaskets. Power lines need filters. I think it would cost $20K to build as a commercial project. As a government project, with union work rules and inspectors tripping over each other... The armed services do have electronics shops inside faraday cages so top-secret electronics can be worked on, but don't ask me what they spent on them. The one I was in had a bank vault door that must have cost $10K all by itself. (This opened inside the building. But then there was a fire exit door to the outside that was just an ordinary metal door with added gasketing -- and when the air conditioning wasn't working hard enough, the techs would open it to let some air in... 8-)
I know more about Tempest than you do -- it's not a spy program, but the armed forces program to make their computers proof against this kind of spying by heavily shielding everything. It also adds $5K or more to the cost (or more like $20K under a gov't contract). The strongest signal would probably be from the video cable, but the keyboard and disk drive cable also emit. Even signals on the motherboard give some emissions. Descrambling the bits (particularly from parallel busses) is quite a challenge, but from the attention given to controlling even motherboard emissions in Tempest systems, apparently the experts don't think it's impossible. Modern FCC-compliant enclosures really do cut down on emissions quite a lot. My guess would be that the easiest thing to tap would be video, LAN, keyboard, mouse, and other external serial interfaces would be considerably harder but possible, and internal parallel interfaces would take technology beyond anything I know of. That is, a really good spy organization might be able to record you typing in your password, then have to break in to use it. If you run with the covers off, you are theoretically at risk of internal SCSI, IDE, or motherboard signals being intercepted, but I have no idea how anyone could intercept 8 to 64 bits parallel and make sense out of it.br>
It isn't a risk I would worry about at present, unless you've really made enemies in high places -- and they'd probably go for a more direct attack anyhow. But technology keeps advancing. To really be safe, you'd want your keys to stay inside of a shielded calculator- or credit-card-sized device, preferably one that had to be unlocked by biometrics (fingerprints, e.g.), read on its own scanner. This device would have to do encryption itself, so as to never send out a private key. However, you'd have to type a message on another machine and send it to the security device in the clear, so there's a little vulnerability there...
All this is highly theoretical -- most users leave their systems open to much cruder methods, like read the password off the post-it note, recover secret documents from the trash, ask someone to let you in so you can do some maintenance...
And all that (ROM locked in safe, sneakernet, etc.) is in vain if, when you load the password into your non-networked PC, someone across the street is picking up the emissions from the NNPC and reading the data flowing through it.
One point you are missing: to use an encryption key, at some point it has to travel around the computer in the clear -- and apparently organizations like the CIA can pick that up from an antenna across the street, if for some reason they put their best people and equipment on it. Of course, it's much easier to just snatch you and wire up your genitalia until you give them the keys, but sometimes that method has unacceptable repercussions...
Anyway, the equipment to intercept computer emissions is subject to Moore's law too, so someday soon it will be within the price range of industrial spies, and the next year PI's will be using it to dig up dirt for divorce cases. We can ban it. That will work, sure, just like banning guns...
I suspect the reset button did exactly what it was supposed to. It reset the computer. This is NOT going to fix a bad hydraulics line. In fact, it makes the computer forget what was just going on, so it's probably going to make a bad decision as soon as it starts taking control. It is not impossible to store enough status information so the computer could have picked up just where it left off before -- but that isn't such a great idea when the pilot punches the button because the computer was screwing up.
I did mix up the dates a bit. It's at least 20 years since the first Blackhawk protos flew, and over 15 since they started using them -- however it wasn't flying right until 1988 or so, and I know that because I was peripherally involved in a critical avionics fix. The tail-plane was electronically controlled, the cable to that controller wasn't adequately shielded and filtered, and radio signals in the military voice band could be picked up and jam the tail-plane controller to full nose-down. In the typical Blackhawk mission, it would be at 100 foot or so above ground, and so it was almost impossible for the pilot to pull it out -- and difficult afterwards to tell whether something malfunctioned or the pilot just misestimated wind and trees... There were a number of crashes before the Army would admit there was a problem. I think a system I was working on in 1987 forced them to fix it -- it included a transmitter powerful enough to would drive the chopper full nosedown every time, and because it was usually fired up at 5,000 feet there was time to turn off the transmitter and recover control, so the pilots got back alive to report. And there is nothing angrier than a pilot whose aircraft just tried to kill him...
IIRC, the original design required the builder to thread hundreds of rods through the whole multi-storied catwalk, from bottom to top. I'm not an expert on these things, but it would be difficult to put together a scale model on a tabletop, and to build it while hanging in midair? So I'm not surprised the builder changed to short rods -- but it meant that instead of the load being carried through the intermediate box beams by the long rods, it transferred from top rods to bottom rods through the box beam. Furthermore, the box beams were formed by welding two U-channels together -- so the rods were secured by nuts bearing on the welds. The structure had only a fraction of the design strength, unfortunately it was enough to hold until a crowd went out on the catwalk...
As I said, I'm not a structural engineer, but it sounds like the engineer on that design had no experience with actual building operations. And the builder should have gone back to the engineer with the proposed changes to re-evaluate the strength... I am an EE at an electronics manufacturing plant. A large part of my work is dealing with board designs that are unbuildable and untestable. Obviously the engineering schools still don't teach the practical aspects of making boards. We get the bad designs changed, but we get them changed by going back to the designer, not by unilaterally changing them -- whether or not the board is going into a safety-critical application...
By the accounts I've seen, there was a light which lit up saying "reset". So the pilot hit the reset button. And that caused erratic prop and engine operation. (You try to write a program so it boots up and takes control of an on-going process without glitches.) And since even a recruit headed for cook school should be able to figure out that when doing that made things worse, you stop doing that, if the reports were accurate, the pilot had to be panicky.
Of course, 75% of what you see in the news is wrong, so maybe the pilot was indeed trying to use the reset button to keep the computer off-line. In that case, double-shame on the system designers for not providing a manual override on each control!
Maybe so, but since they haven't got any new helicopters since the Blackhawk (30 years old?), they do really need the Osprey now.
Having once worked on a project where, if I told you what it was I'd have to kill you, I find the notion of open-sourced military software very funny... But aircraft control software could easily be open-sourced, even for military aircraft.
In other engineering disciplines, it's usually the case that the designer and the builders are distinctly different people, and it's easier to say whether the problem was a design issue. I remember one case where a bridge falling down was the fault of both -- the designer designed something that couldn't be built exactly that way, and the builders made changes without consulting the designer or getting new strength calculations. (This was a multi-story foot bridge inside a fancy hotel; if I remember right, it was in the early 80's.)
Actually, a decent embedded system (RTOS or not) will re-boot itself when it needs to. That is, you got a "watchdog timer" which the main program loop keeps resetting; if it doesn't get reset, it resets the computer. But apparently they gave the pilot a pushbutton just in case the watchdog timer crapped out too. But he pushed it when the real trouble was a chafed-through hydraulic line. And when it didn't help, he pushed it nine more times...
As I see it (and I'm not claiming expertise), the problems were, in descending order of importance:
1. The designers failed to route and fasten hydraulic lines so they wouldn't chafe.
2. The Marine Corps and aircraft mfg both ignored warnings about chafing lines.
3. The system for some reason told the pilot he needed to reset the computers although the computer had nothing to do with the hydraulics failure.
4. The software changed propellor settings erratically while booting up.
5. They didn't give the pilot an effective manual override for any controls the computer might be messing up.
6. The pilot panicked a little. Understandable, but pilots and Marines are supposed to be panic-proof...
Correct. One thing is that sailboats use two "wings", the sail above and the keel below, in fluids with different velocities, and so the vector sum of the "lifts" can be partly into the wind. Lightsails cannot create thrust towards the source of the light, and in practice they'll always have significant thrust away. But orbital dynamics comes to the rescue: angle the sail to reduce your orbital velocity and you will drop, even though part of the thrust is still outwards.
It receives MOMENTUM from the photons bouncing off it. Momentum change = mass * change in velocity (and it's a vector). If the mirror is at right angles, you get twice the momentum of each reflected photon, because you get both the incoming momentum and the reaction to the outgoing momentum. The photon gets mass from e=mc^2, or m = e/c^2. c^2 being very big, the mass of even a whole lot of photons isn't much, hence thrust is rather low. But it's free (aside from the cost of the sail, lifting it to space, and repairing meteor holes....)
The spacecraft receives ENERGY (mv^2) only to the extent that it changes velocity in response to that momentum transfer. It's going to be a small fraction of the energy in the light.
As for how you angle the thrust: someone used a tennis ball analogy, that's probably the easiest way for most non-physicists to see it. Or if you want a mathematical treatment: let the mirror be angled at 45 degree to the incoming light. Incoming photons are reflected at 90 degrees to incident. The momentum transfer to the mirror is the vector sum of the momentum of the incoming light (x at 0 degrees) and the outgoing light (x at 90 degrees). The resultant is sqrt(2)* x at 45 degrees. But notice that you've reduced both the amount of light you intercept by angling the mirror and the impulse you get from each photon intercepted (2x at 90) -- so a 45 degree angle cuts the thrust in half. (This assumes 100% reflectivity -- with a real mirror, your results are somewhat worse, since some photons are absorbed and so only contribute outwards thrust. And outwards thrust won't help you spiral outwards -- you aren't going to get enough thrust from sunlight to just fly straight out. The animation in How Things Work is wrong.)
At arbitrary angles (still assuming 100% reflection): The thrust will be right down the axis of the mirror. With the mirror and thrust axis at angle T to the sun (that is, 0 degrees is dead-on and 90 is turned edge to the light), the thrust is (cos(T))^2 times the maximum thrust. 60 degrees cuts the thrust to 1/4 of maximum, but it directs most of it along the orbit where it will actually be useful. And at high angles, you get better reflection. Possibly it would be best to plan on never setting the sail closer than 60 degrees, so you can make the spars weaker and lighter.
It _is_ like blowing into your own sails. But you would get a little thrust by shining the laser backwards. The thrust in newtons is power in watts divided by the speed of light (3E8 m/s), you need a heck of a good source of power, like a big fusion generator or tons of antimatter. And it doesn't have to be visible light -- microwaves, gamma rays, or infrared would do just as well as long as it's directional. Note that all practical energy sources emit more power as waste heat than as usable energy -- so maybe the best light-rocket would simply radiate heat out one end.
However, if we do get fusion powered craft, I expect they would put the usable energy into accelerating the fusion products. You get more thrust from a given power by accelerating helium than by shining a light. For interstellar flight, most of your craft would have to be fusion fuel, so you might as well use the byproducts... Light-propulsion makes sense only as a way to use some power supply you don't have to take along. (Except maybe in a warcraft where the big laser could do double-duty?)
Because you'd need something like 99.9% of the weight of the vehicle to be fuel. That's with chemical rockets. With fusion, you could do considerably better, maybe only 90% fuel. And that fuel should be something like hydrogen or deuterium -- pretty hard to keep it from leaking away in a ten year flight...
I found a reference to that here. The negative refractive lens can (in theory) be created with a 40 nanometre thick silver film. But to work, it must be at a "distance of 20 nanometres from the light source, since the system uses its near field."
The sails are reflective (mirror-like), so by angling them you can get sideways thrust -- a combination of the incoming momentum of the sunlight and the reaction from the momentum of the reflected light. So angling the sails at 45 degrees gives you thrust at 45 degrees to the sun, but it cuts the thrust by half. (sin(45) squared -- once for the reduction in light intercepted by angling the sail, once for the off-axis thrust.)
You can't tack like a ship tacking into the wind (where the vector sum of forces on the sails, keel, and rudder gives a net force partially into the wind) because you don't have any equivalent to the keel. But you can use an angled sail to reduce the orbital velocity, so the ship drops sunward. And the best way to go outwards is to use sideways thrust to increase the orbital velocity.
The real problem with solar sails is the very tiny force per square meter that is possible. It should be enough for minor orbital adjustments. If you plan way ahead, it might be possible to use light sails to slowly spiral in or out (like weeks or months to the moon, years or decades to Mars). Or you find a way to augment the thrust -- like building a giant battery of lasers on the moon to provide much more intense light. In a couple of Larry Niven stories, they used lasers to launch an interstellar lightsailer, but to brake at the destination, it had to almost dive into the sun...
Sorry, here's #3.
I couldn't find anything directly about the microwave refractor, but a search for "negative index of refraction" turned up three short pieces on the basic physics: 1 2 3.
Note that these involve putting conductors into the near field of the emitter -- that means within a fraction of a wavelength of the light source or microwave antenna. But to me, that just makes the metal an added piece of the antenna, and one well known directional radio antenna uses an array of metal rods in the path of the emitted waves...
I don't know if the rings and fiberglass arrangement is just a variation of this.
Or for a more relevant comparison: Hollywood's most successful movies get more than $100M just in US ticket sales. If that $300M is gross revenue, then 3 Hollywood successes beat the whole porn industry. It is possible, though, that porn studios average more profits than Hollywood -- they don't pay the leads $20M each, or spend $50M on special effects.
Meant to say "the right was on the defensive for 200 years." Darn. Proofread, then submit.