Solar sails suck.
In a 2002 paper, Laser Elevator: Momentum Transfer Using an Optical Resonator (available at your local school/library, possibly electronically — J. of Spacecraft and Rockets 2002), Thomas R. Meyer et. al. talk about a neat way to get a lot more speed out of light reflection than with a regular solar sail. The basic physics are pretty simple, and it’s a fun subject to think about.
When a photon hits a solar sail, it gives the sail momentum. If the photon has momentum P and bounces off a stationary sail, it looks like this:
Think of where the energy is in this system. Before it hits, the photon has energy E. After it bounces, the photon still has roughly energy E. But the sail’s moving, so where did it get its kinetic energy? (Remember, energy — unlike momentum — has no direction.)
The answer lies in the word “roughly”. The photon loses a tiny fraction of its energy to Doppler shifting when it’s reflected, but only a tiny fraction. It is this tiny fraction that goes into pushing the sail. This is a phenomenally small amount of energy — far less than a percent of what the photon has. That is, not much of the photon’s energy is being used for motion here.
This is why solar sails are so slow. It’s not that light doesn’t have that much energy, it’s that it has so little momentum. If you set a squirrel on a solar sail and shone a laser on the underside, do you know how much power would be required to lift the squirrel? About 1.21 gigawatts.
This is awful. If we were lifting the squirrel with a motor, railgun, or electric catapult, with 1.21 gigawatts we could send it screaming upward at ridiculous speeds.
This is where Meyer and friends come in. They’ve point out a novel way to extract momentum from the photon: bounce it back and forth between the sail and a large mirror (on a planet or moon, perhaps).
With each bounce, the photon loses a little more energy and adds another 2P to the sail’s momentum. The photon can keep this up for thousands of bounces — in their paper, Meyer et. al. found that with reasonable assumptions about available materials and a lot of precision, you could extract 1,000 times the momentum from a photon before diffraction and Dopper shifts killed you. This means you only need 1/1,000th the energy to levitate the squirrel — a mere megawatt.
This isn’t too practical for interstellar travel. It requires something to push off from, and probably couldn’t get you up to the necessary speeds. It may, they suggest, be useful for getting stuff to Pluto and back, since (somewhat like a space elevator) it lets you generate the power any old way you want (a ground nuclear station, solar, etc). But more importantly, it’s kind of neat — it helped me realize some things about photon momentum that I hadn’t quite gotten before. It’s like Feynman says, physics is like sex — it may give practical results, but that’s not why we do it.
Now we’ll let things get sillier. I spent a while trying to brainstorm how to use this with a solar sail (that is, using the sun). I imagined mirrors catching the sun’s light and letting it resonate with a sail.
But you really need lasers for this — regular light spreads out too fast. Maybe a set of lasing cavities orbiting the sun …
Supplemented by a Dyson sphere …
And since by this point we’ll probably have found aliens …
Why settle for interstellar communication when you can have interstellar war? And we could modulate the beam to carry a message — in this case, “FUCK YOU GUYS!”
xkcd 
“The mirror reflecting the sun will, after reflecting the sun-light, be in the wrong angle to keep the light bouncing between it and the solar sail. It will just keep bouncing it back to the sun :’(”
Well, see the third picture. You can’t have it bounce indefinitely, but you can keep it up (assuming arbitrarily precise placement) for a while, by reducing the angle until the sun would be eclipsed by the mirror.
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you guys make me sad.
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Forgive me if anyone else has already postulated this, but could you not fire the proton, slide a reflective cover over the firing mechanism, and let the photon bounce between the two objects to be pushed apart until it dies?
Time of events:
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} -o [
}] -o[
}]o- [
}] -o[
Like a never-ending game of pong. With lasers.
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Chris (one of the first posts) misspelled “misspelled”.
And I think “Gigawatt” is correct.
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Dyson spheres might exist someday, but Wilson spheres are here today, and can be mass produced from renewable resources.
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Keep that squirrel under 88 man, I don’t think a squirrel could make an N-jump on it’s own.
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> Alpha Centauri is an over-ambitious target, but this would certainly work within our own system. Goodbye, invasion fleet! Also, fuck Pluto. We could use it for target practice. Full planet or toasted space cinder, that?s my position on its status.
Weak.
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To XKCD:
I am sorry, as you said, it was not actually you I was correcting. I was skimming through posts and noticed the comment that you were corrected, and when I typed “awesome” into the text finder in Firefox, it found the quote your correction, not the original post.
Yours was actually correcting the same thing that I was, only yours was actually right!
Where I said “1P of kinetic energy, I actually meant 1P of momentum; my mistake!
Sorry again about correcting your correct corrections =).
P.S:
I’m curious whether a ultraviolet/very small wavelength photon of light would actually cause more momentum to be transfered than a a long wavelength/infrared photon?
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“Forgive me if anyone else has already postulated this, but could you not fire the proton, slide a reflective cover over the firing mechanism, and let the photon bounce between the two objects to be pushed apart until it dies?”
You could, but that would pretty much cut the energy available in half – you fire a beam, reflect it when it comes back, rinse and repeat. I doubt there would be a great increase in efficiency, although I haven’t done the math, and you’d need the same precise alignment both ways.
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>”but eventually you coul get the entire star gliding along, death ray flowing out behind it. That’s a story for another post, though :)”
^winning idea
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“Please, please,” pleads a historian, “write a textbook, or at least something along the lines of The Cartoon Guide to Physicis.”
It’s an enchanting break from war, diplomacy, and short, brutish lives, not to mention I want to understand what my boyfriend is talking about without asking *him* to explain it!
P.S. I have a great photo of an African red spider for you.
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Once again I’m reminded why visiting XKCD is more important than working on fixing my computer. That means a lot, since for the last 20-24 hours all my computer would tell me is “Error loading OS”. It finally started on it’s own (Either I tricked it or it also is an XKCD fan) and naturally I had to check for new comics.
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textbook_requests++;
The physics one would be cool, but I think there could be a range:
Basic Geometry with Raptors…
Black Hat Guy’s Guide to Computer Security and Hacking…
Heck, even the world’s first stick-man version of the Karma Sutra. 😉
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so let me get this strait.
you’ve turned the sun, into a DEATH STAR.
i can imagine the pitch to the military. “right well first we need to build a dyson sphere around the sun and……………..”
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Building a dyson sphere is a neat idea, but what about a nuclear fusion powered 300 TW laser, that laser exists rigth now at the Mitchigas University (http://www.physorg.com/news122298608.html), and the nuclear fusion power plant may be here in next 5 years (source:http://www.news.com/8301-11128_3-9866626-54.html)
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to paraphrase einstien. I know not what weapons the first interstellar shall be fought with but surely the second shall be fought with squirrel based weaponry.
Also I look forward to the publication of Black Hat guys guide to personal relations .
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dammit there should be the word war between interstellar and shall. I blaming this late hour at least where I live
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Actually, i think aplha centauri could cook our asses pretty much anytime they wanted to, as they probably don’t follow earth laws of physics in their degenerate hellhole of a planet. by now they should have their own brand of physics, postulated by their local einstein , newton and dr.who in which planets and like are actually floating butterscotch candy, stars are floating butterscotch cnady, and we are also butterscotch candy. and since it is common fact that alpha centaurians hate butterscotch candy, theyre probbaly setting up a mass butterscotch candy detonation device, that will throughly annihilate everything in teh known universe..
although the incoming rays will only be powerful enough to destroy a piece of butterscotch candy.
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>>What about the relativistic effects and the resulting reduction in momentum with increasing velocity.
As you start solar sailing towards the those “freedom hating” Alpha Centurions (Hundreds of them I say) the incident light would be redshifted increasingly with velocity. At let’s say 0.1c the energy of the incident light would have be 0.9 its relative stationary energy. Additionally in terms of decelleration without a prepositioned parallel mirror at our destination the the decelleration would be 1.1/1000 that of our outward accelleration accounting for an equilvalent blue shifting.
Unless the solar sail is actually a kinetic energy weapon.
Our 0.5kg would now weight squirrel would have an equivalent energy 22 tera Joules. Now lets say this was’nt a squirrel but instead a cannister of a 1:1 deutrium/hydrogren 1.404*10^33 eV could be fusion boosted by a factor of 5.03 to 112 tera joules < a lot.
Possibly better that a directed laser beam which could be reflected back using advanced centurion optics… assuming they knew in advance where we where aiming.
Wow that was so far off the point.
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❤
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There’s something that I’ve never understood about solar sails: How is there any momentum in the photon? I thought that photons had a mass of zero, which means that the momentum in the system will be zero. Result: no proplusion.
Will someone please explain why I’m wrong? (I know I must be, but I’m not sure why.)
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Did you really do the math and come to 1.21 gigawatts, or did you just watch Back to the Future recently and pull that out?
As for those naysayers that claim that Alpha Centauri hasn’t attacked us yet, you must realize that they will only respond to force. Trying to communicate with them will yield no results. They only understand your foot on their chest.
And if that wasn’t obvious enough for you, go read the prophecy of Niven & Pournelle.
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joules not gigawatts (I have jigawatts on the brain)
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Any physics lecture that included the phrase “supplemented by a Dyson Sphere…” would be greatly improved, IMHO
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Something I’ve always wondered about: How do you start construction on a Dyson sphere? You can’t start at one end; you’d fall into the sun. You’d have to start with a ring around the sun, which poses its own problems a la ‘Ringworld.”
In any case everyone asking for a an interesting and fun book about science should pick up ‘A Short History of Nearly Everything’ by Bill Bryson.
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Ignore my previous statement
In response to Joshua’s question
photons have momentum De Broglie (the genius) got a Nobel prize for it which he sold for cakes (unsubstantiated). Where by the photons momentum (p) plancks constant (extremely small) divided by its wavelength (lambda, also prety small) . Creating an effective exchange in momentum without a large increase in entropy is what make the solar sail generally high impractical.
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couple points:
Seb Sez:
“One problem with the dyson sphere lasing cavity, is that the momentum imballance from the lasing cavity means that there will be a net thrust on the dyson sphere in the opposite direction from the laser?”
Unless I’m mistaken, the thrust would actually be applied to the sun? The dyson sphere is equally reflective all the way around unless it’s being fired, in which case there is only a slight imbalance caused by the lasing cavities (which are essentially semi-transparent). The cavities themselves essentially just reflect the light over and over again before allowing it to pass thru in the initial direction, so the net thrust applied during this process should approach zero.
Fatibel Sez:
“I?m not a scientist so forgive my terminology? but wouldn?t it be possible to hoist a one-way mirror (very thin and lightweight version) parallel to the solar sail. This way the photons get through, bounce off the sail and are reflected back by the mirror, ad infinitum, maximizing the force exerted on the sail?”
Joel makes a good point, but there’s kinda a way to make this work. If you were to put a diffraction grating lens a couple milimeters away from the reflective surface, and put a small mirror at the focal point (accounting for the fact that light passes thru the lens twice), it would reflect back to hit the full surface each time before being re-focused back down to the small mirror. If the mirror had 1% the surface area of the sail, you’d let 99% of the energy thru while providing a close-range reflective surface. Essentially a one-way mirror.
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And I’m gonna be classless and reply to my own comment… I just realized the other flaw with this plan. I guess you’d have to swap my mirror out with some sort of adjustable reflective lens that would NOT be attached to the ship, so that it could be allowed to accelerate away from the sail. The best way to use this would probably be to deploy it right about the time that your initial reflective source starts to diffuse too much, allowing you to use a new, closer one for a time.
Other idea, maybe just drop the diffraction grating off at the point where the laser starts to diffuse, and give it a VERY long focal length, maybe a few hundred thousand AUs. It would get pushed a lil bit, but would remain essentially stationary and increase the feasible range at which your laser-elevator still provides thrust.
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ahahha, deathstar.
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First Dyson Root Cyclone technology and now the Dyson Sphere Death Ray? What can’t Dyson do? But as interesting as a death ray that’s effective at over 40 trillion km distance is, I’d like to know how they’re going to apply this “sphere” in terms of vacuum cleaner technology – vacuuming is my life.
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I just researched something awesome…
1 second of Dyson’s Sphere focused sunlight is the equivalent energy of (no exaggeration) 56 billion trillion kg of raw nitrogen (N2) reacting with a like amount of raw oxygen (O2) (taken at accepted 180 kilojoules/mol and a molar weight of 28 g). And even better, Newton’s Law of cooling say that we won’t all die if we lost a full second of sunlight.
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@ allabouttrust February 15th, 2008 at 5:16 am :
So what? Did Iraq attack the US? What about Panama? Vietnam? Korea? Or Germany now that I think about it?
It’s called a preemptive strike.
“If you have a hammer, everything looks like a nail” or something similar.
Whose quote was that by the way? I’m sure one of our intelectual readers knows.
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One of our “intelectual [sic] readers” might know that a massive death ray is the only way to solve our problems with the Alpha Centaurs (aside from perhaps gelding them, but that would just be rude).
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@Nick Johnson: Yep someone has done experiments lifting something light using light. (pardon the pun) I saw a TV programme on Discovery a couple of years ago where some guy used laserpulses to shoot something akin to a spinning flying saucer up a guide-rope. When I saw the same man (again on Discovery Channel) a year or so later, he did the same thing minus the guide-rope. So there!
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The drawings make this amazing. Munroe, you should really replace Wikipedia’s sterile and boring diagrams with your nice and friendly hand-made ones. 🙂
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In Charles Stross’s Iron Sunrise there is the idea of a kinetic energy weapon as an interstellar deterrent. Essentially if a planet is threatened, the weapons, which are stationed in the Oort cloud with crew in hibernation, accelerate toward the target. The crew eject in a small lifeboat, leaving the massive body to slam into the destination at relativistic speeds.
“Something I’ve always wondered about: How do you start construction on a Dyson sphere? You can’t start at one end; you’d fall into the sun. You’d have to start with a ring around the sun, which poses its own problems a la ‘Ringworld.””
Why not? Just start construction in orbit. Even better, launch satellites into an orbiting shell, and add material between them.
“Once again I’m reminded why visiting XKCD is more important than working on fixing my computer. That means a lot, since for the last 20-24 hours all my computer would tell me is “Error loading OS”. It finally started on it’s own (Either I tricked it or it also is an XKCD fan) and naturally I had to check for new comics.”
Someone write a sappy love note to your boot sector?
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I read some time ago a SF book with a spaceship that was propelled (at least initially) using exactly the method described here (I think it was a book by Stanislaw Lem: can anyone remeber which?)
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Oh my God! This is exactly how to create a Death Star! I mean, the space station part is straight forward, but using a Dyson sphere to generate the rediculous amounts of power necessary for destroying a planet?!? GENIUS!
I mean, it would still take a hella long time, but once people make machines that make machines that make machines that make things super quickly, we’ll be out of the woods, and into the interstellar war!
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“Actually, William, what you’re thinking of is entropy.”
No, I’m thinking of special relativity, where energy is momentum in the time direction
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Haha, this is FANTASTIC.
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@manta1976
That seems to be ‘Fiasco’, the last novel by Lem. I vaguely remember that the laser was placed on one of the Jupiter’s moons (effectively dismantled in the process).
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I don’t spend enough time thinking.
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Preemptive strike?
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Dude… you do not want to do that. If you blow up Alpha Centuri then sometime in the distant future (or the past… I don’t really know) Ford Prefect won’t be born. And then Arthur Dent won’t survive the destruction of the earth and nobody will have it recreated. Then we’re all F^kd.
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Squirrels and lasers don’t scare me…
SHARKS with lasers- now that’s trouble.
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PKM: “Also I’m pretty sure that wouldn’t work in space because I would guess it requires a medium to alter the refractive index of, which the Vacuum Of Space doesn’t count as.”
Space, however seemingly empty it is, is not completely empty. There _IS_ medium there, just very few and far between. Someone correct me if I’m wrong.
Also, why was I the first to bring this up?
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@Sagran: Ford’s from a small planet in the vicinity of Betelgeuse, not Alpha Centauri.
Curious, expanding on Fatibel Sez’s earlier idea about self-initiated and self-sustaining reflection, couldn’t one emit the photons and use a small mirror to send the photon into the loop of reflection?
Being 2 in the morning it’s a little hard to describe exactly what I mean, but I picture it similar to this, only a LOT more precise…stupid ascii…
/
/_ _ _ _
/| | |
| | |
| | |
Emitter -> – – – /| |
| |
|_ _ _ _|/
/
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Okay…so it didn’t turn out like it’s supposed to…sorry…
Hopefully someone gets my drift.
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Theres a similar idea out there, Ion Propulsion. It doesn’t use a sail but it emits ions at extremely high velocities, we are talking .99C, and uses the momentum produced to accelerate the craft. The acceleration it’s self is slow but, over the vast areas of space could potentially propel a craft at quite large speeds.
Link to the all powerful wiki:
http://en.wikipedia.org/wiki/Ion_engines
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