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 nice thing is that the dyson sphere (which doesn’t necessarily need to be built in full) solve the space travel problem within it. you can just tie anything you want onto it and pull…
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“if it takes 4 year for the laser to reach Alpha Centauri for obvious reasons it would take the same time for them to get any information on our plan of blowing them with our wicked laser and 4 other years for any thing they do in response to our laser to reach us, that gives us about 8 years to get the laser ready before any retaliation from those guys.”
Not if you have hyperdrive. Obviously the laser can’t jump to hyperspace.
“Interesting, Star Wars literature says that the Death Star is a Dyson Sphere around a nuclear fusion core. It also says that the opening is a “lasing cavity”. PSYCHE!!!”
Hm. As I recall the core is about half the diameter of the whole thing. So it’s a bit thicker than a Dyson sphere.
“Why does the reflecting mirror have to be mounted on a planet? Why not just leave it freely in space? Sure, it will accelerate away from the sail — it picks up 9p momentum, after all — but so what? We can pick it up later.”
Well, that would take energy, if you’re sending off spacecraft regularly. I mean, you don’t want it falling into the sun or crashing into the Moon while launching something.
“Marty, I’m sorry. But the only power source capable of generating 1.21 gigawatts of electricity is a bolt of lightning. Unfortunately, you never know when or where it’s ever gonna strike.”
I’m sure by the time we can actually construct something like this we’ll have mastered weather control.
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Awesome post.
I Nth the textbook request. It wouldn’t even have to be a textbook at a specific level, just a general physics book similar to all those science-fact books Isaac Asimov wrote, but with more sketches. Man, I learned a lot more from Relativity of Wrong and Beginnings then I did from Chem 1.
Interesting science non-fiction for the non-doctorate is an underrepresented genre. There’s Asimov, the Bad Astronomer, and not much else. We need more straight-up science.
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Well, you can always move up to X-ray lasers if you want to go farther…
but you did not take the idea far enough: Alpha centauri will no doubt have their own mirror, and our little inter-solar squabble with slowly push our two systems apart. At last an explanation for the expanding universe: intersolar antisocial tendencies. Background radiation is not from the big bang, it is from ancient flame-wars.
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I (N+1)th the textbook.
Just take the Feynman Lectures, screw half the math, keep the interesting stuff he throws in but doesn’t extrapolate on, and have at least three chapters ending up with Death Stars.
The fact that your comics are printed in just about every engineering/science student newspaper I’ve seen so far, I’m sure would be mouth-watering for any publisher.
And if you don’t I’ll hold my breath. 😡
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so if you lift the squirrel at an acceleration of 88 mph, it travels back in time and plays guitar at its parents’ prom?
I think we need to discuss the political ramifications of such a device.
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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…
Without a beefy rocket on the outside of the dyson sphere, I suspect us poor old terrans will be splatted when the dysonsphere crashes into the sun (there being no effective graviational field inside a hollow shell, thereofre nothing keeping the whole thing centred on the sun) before the alpha-centaurians get zapped. Then again, when you are building a dyson sphere, a big rocket on the outside would probably be advisable anyway for the same reason.
Alternatively, you need another lasing cavity on the other side firing at the same time.
We could go one up and make a free electron laser out of a series of paired neutron stars for the magnetic fields and a black hole acretion disks particle jet :), for those occasions when you absolutely have to kill every last person in the neighbouring galaxy.
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Please remove references to squirrels as we feel this could lead some of your readers to experiment needlessly on these poor defenseless creatures.
If anyone finds a homeless squirrel, please bring it to a PETA shelter, where it wil be housed for a week, if nobody claims it we will do the right thing and end its life of tree climbing.
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Heh… after reading this, the only thing I could think of was letting the squirrel rise up a couple hundred feed then blocking the beam. Ahhhhhhh!!!!
Maybe it’s the cold medicine.
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True that the dyson sphere solar lasing devices, and subsequent variations of such, are useful the op has a limited viewpoint regarding effective application.
The main action the lased solar output has in our current case-study is the excitation and pulsed modulation of Uranus to affect transposition of the various vectors and qualities of the organisation of Near-Galactic Space ( a somewhat large toroid from just beyond the Oort Cloud centered into Sol ) and the transporting quanta of said NGS as mutual balancing for the Far Galactic-Disc the NGS we’re in has been entering into at a very strange oblique. In order for the cohesion of NGS to be maintained while entering in to the main phasing and energetic phenomenon that is the Milky Way from the angle we’ve been travelling in orbiting through the milky way about 4 times so far from the Sagitarius Dwarf Galaxy that orginially spawned NGS will require Uranus to very creatively release a lot of gas.
😉
CA
ps. I so wish I could shout ‘YHBT!’ Really, think for a moment how difficult the above is going to be. The paperwork’s been checked out and everything… the funding’s already been secured and traffic is being routed as we speak. Good pay, long hours, hard work.. mainly it’s a bonus for my resume. She’s been getting sparse and kinda dated…
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Thank you for explaining this subject using relevant examples. We look forward to levitating at the expense of extreme amounts of energy. Be assured that we will find some way to work this into our inevitable conquest of the planet and will not forget your contribution thereto.
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Seb,
Valid points and all easily recitified. Lasing such an intnse amount of energy over an large area of space has in the past generated potential to increase the flow of dark energy and it’s interaction with normal matter has a few phases to utilize.. the one we’ll get the better use of here is the manner of simulating gravitational density when twisted around other flow of dark energy timed properly. This is where part of the infrastructure of our meta-DS is importnat to fine tune. Placing a ring of increased density (keep from increasing it to much… pulling apart Sol is non-conducive to our current efforts) in a plane tangental to the vector that the meta-laser is pointing will create a situation where the mass of sol pulls the MDS along.. Though again this assumes that main thrust on Sol will be generated be the direct effect of the interaction of the push of photnic elemental energy away from sol… There are at least two to three spiraling supportive inter-changes before the main amount of energy utilisation creates such major shifts as is needed.
Re: the killing the whole galxy is the worst case scenario that NGS/Milky Way interactioon could form if left to ‘chance’ aka fate… NGS could be like a proverbial lit cigarette thrown out of a car on a mountain side road that drifts down a canyon wall into a firecracker factory… Boom!! Flipside: NGS might be a large scale lymphocyte coming back into a Thymus to relay better means of self-defense and healing… I’m working towards the latter, btw.
Cheers to your cupboard,
Cosmic-Anon
PS . Captcha gonna getcha with her freaky tangentia: “surf tainted”….
Woot and RandomAnon Delivers….
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Now witness the power of this fully armed and operational … Dyson Sphere.
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Dyson-Sphere-Planet-Killing-Laser Machine: bringer of interstellar war…or the coolest way of killing vampires ever?!
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That’s no star, it’s a space station! Unfortunately, moving it would probably present the problems Seb mentioned, barring the invention of stellar-sized propulsion devices.
However, I don’t see much of a problem in normal operation. Although there would be no net gravitational field inside the Dyson sphere due to the sphere, the sun’s own (significant) gravity should act to keep the sphere symmetrically centered on the Sun, as long as the laser isn’t fired long enough continuously to propel the Dyson sphere into the sun and destroy part of it.
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Correct me if I’m wrong (which I probably am), but wouldn’t a Dyson Sphere not work? The light would just reflect back into the sun. We’d need some sort of Dyson Paraboloid with the sun at the focus and the lasing cavities at the directrix.
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Maybe collaborate with the author of “The Cartoon Guide to…” series of books. I think there’s already one on physics, but you could pitch to him that it’s the 200-series level of books, less focus on art, more focus on using physics to destroy things.
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The thing about solar sails is, how the hell do you STOP them? Engage the space brakes?
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so how far could we lift the squirrel then?
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Nope, you weren’t the only one who caught the Back to the Future reference. It would be funny if it were accurate, though, peta- though they may be. I seriously can’t think of anything funnier to lift. Maybe an albino space squirrel…
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OMG you are my hero! I recently discovered your webcomic, and now I check it every day on my lj..
What makes it better is that you are a Physicist and so am I, well I am trying to be one…
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Eh, a few corrections here.
1. To say that a photon loses only a small portion of its energy because its momentum is so much smaller than its energy would be false. Just look at natural units where c=1, and you’ll notice that E=pc=p for a photon. Rather this is because of conservation of energy, and because the object the photon hits is so massive (and therefore has so much energy), that the momentum it gains from the photon barely increases its velocity (which is inversely proportional to mass), so the change in energy is very small, so the photon in turn loses very little energy.
2. I love the idea for the dyson sphere, but unfortunately i think it would be very short lasting, as eventually the dyson sphere would start moving to conserve momentum from the photons leaving it, and would collide with the sun (of course, who can say how long that would take, maybe you could wipe out a few civilizations and create your own short lived sith empire in the process, nothing says iron fist quite like a death star). But, just slap on some stabilizers on the opposite side of your death beam to even everything out. If you’ve got the energy to build a dyson sphere, you’ve got the energy to move a dyson sphere to increase the longevity of your giant ‘laser’.
>>>>but they haven’t attacked us, yet!
Screw them! we built the giant laser first!
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http://www.lightcrafttechnologies.com/
This is what I thought you meant by “laser elevator” 🙂
I agree with the guy who says you should write a physics textbook 🙂
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I brought something like this up on sci.physics a few years ago 🙂 Only instead of a mirror, can you use a blackhole to slingshot the light back at the sail?
http://groups.google.com/group/sci.physics/browse_thread/thread/24d4925112f87575/718168ef5ce77b5f?
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“Nicoll-Dyson Laser”
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Why does each reflection of the photon result in +2P in the body of which it reflects?
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How about modifying the idea for energy generation. Put the lasing medium at L1 (earth-sun), and focus it on Earth-based power stations. This should massively amplify the amount of kw/m2.
Though unlike microwave-transmitting solar sats, you would have a SimCity-style disaster if the beam’s aim was off. And you’d cook birds and have an airplane navigation hazard. But you’d easily have enough power to levitate a squirrel.
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I hope somebody approaches you to diagram a physics text book… If I had to calculate the power of a deathray, rather than with one bloody something in some obscure condition…
I might actually be a physics major not a cosc.
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squirrels , if pushed – will attack your nuts
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What’s the point of a solar sail?
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I know part of this was for fun, but what about a magnetic sling of some kind?
The only problem would be mounting them, finding the magnets, and overcoming the physical stresses on a ship.
If we could construct a ship that was magnetic then we could pass the ship through a series of ring-shaped tracks that hold high-power magnets. As the ship passes through the ring the magnetic field of the magnets being fired down the track would carry the ship with it, propelling it forward at the momentum of the magnet. Then it passes through a series of magnetic rings, each with a higher velocity (that is the magnet along the track) and each imparting higher speeds onto the ship.
Like I said, the difficulty falls back to maintaining the sling’s positions in space, creating the magnets, and constructing a ship that could fly within these magnetic fields without the rings themselves pulling them apart from the outside. Electromagnets might help, and high-speed docking capability so the ship simply sticks to the magnet which deactivates at the end of the rail.
I guess that is a rail gun isn’t it? Never mind.
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I dont pretend to know a lot about physics, so be gentle with your “no”. Does lifting the squirrel takes 1.21 gigawatts in earth’s gravity?
If so, wouldn’t you be able to take that energy into space where things weigh nothing (or a lot closer to nothing), and have much more spectacular effects?
Im not sure why I’m asking, because I bet you and other physics doctors already thought of it.
I love your comic! Thanks for all the work!
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miik, I think you mean – Squirrels , if pushed – will attack. You’re nuts!
bryan, 1p for the hit and 1p for being pushed off of. I would have to check in my “xkcd friendly guide to physics” just to make sur… oh wait – that doesn’t exist – Yet!
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jldugger said: The thing about solar sails is, how the hell do you STOP them? Engage the space brakes?
Easy. Halfway there you use a small engine to rotate the solar sail and then you use the star you are approaching as a break. It was done in “The Mote in God’s Eye” by Larry Niven and Jerry Pournelle.
The real problem I have with solar sails is how do you steer them? There’s a fair amount of dust in the universe and hitting it would destroy all your momentum (not to mention the sail itself).
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o.o
that is absolutely wonderful.
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Hi,
My school newspaper is publishing a column on the best webcomics on the Internet, and we were all hoping that we might publish one of yours as an example. If you don’t object to this, could you reply back with your consent? The newspaper would be sure to credit you and link back to this site and whatnot.
Just so you know, the newspaper in question is the Cavalcade, and we’re located in the outskirts of DC. We’ve got a pretty small staff, but we put out a quality publication every month, and would be thrilled if you’d consider letting us publish one of your comics.
Thank you,
Katie
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Far-fetched, and creative.
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1.21 gigawatts? 1.21 gigawatts? Great Scott!
and i can’t really believe i was the first to say it. seriously.
or is it passe and i just goofed?
(the youngsters amongst you may find this reference useful http://www.imdb.com/title/tt0088763/quotes)
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“So it’d be a good chunk of the United State’s energy output at any given point to…”
How big a chunk is 0.1%? US nameplate generation capacity is 1,075 GW (per the EIA). Power stations are rated in megaWatts, yes, but baseload plants tend to run in the 500-800 MW range.
The real advantage to getting the required power down to 1.21 MW is that you could use a single wind turbine to power it. As long as the wind is blowing, that is…
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I love the diagrams. (Stick people, and “photon has momentum. that way.”)
In response to seanb… Back to the future is on my list of movies to see. (I guess i’m a youngster 🙂 )
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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?
Space brakes? The way you slow down a sailboat is drop, trim, shorten or otherwise alter the amount of sail catching wind. That or toss out an anchor. Turning the boat into the wind might work but it’s much easier to pull down the sails. (you can also see where you’re going 🙂 ).
Sign me up for a textbook.
Dyson spheres … giant lasers… houses full of popcorn … Man, I miss Villians Supply dot com.
And BTW, if you go back in time you’ll always know when, where and just how hard the lightning is going to hit.
Just two cents from a tech writer plotting from inside the Air Force.
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Uh, I think you meant Doppler, not Dopper.
You guys will like this paper:
dx.doi.org/10.1007/3-540-56343-1_301
Larry Niven ambitions, on a Kim Stanley Robinson budget.
(If you can’t access it, the gist is that some planet and comet atmospheres show population inversion; all you’d have to do is build a cavity like the ones shown, since it’s already pumped by the sun, though some fresnel lenses to boost the pumping might help.)
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fatibel:
A one-way mirror of the sort you describe could also be used to build a perpetual motion machine. You could just place it between two thermal reservoirs, and have IR flow one way, from the cold side to the hot side, until the temperature difference is enough to drive a heat engine.
Unfortunately, the one-way mirrors we currently have work by being more-brightly-lit on one side than on the other. They are just as reflective in both directions.
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Chris:
It isn’t spelled “jigawatt.” It is pronounced that way, though. For any other value, the word is pronounced with a hard g. For the specific value of exactly 1.210 Gigawatts, it changes pronunciation.
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Robert L. Forward played with this stuff in “Flight of the Dragonfly”. He used a statite fresnel lens instead of orbiting laser cavities (which couldn’t remain on target because they have to stay pointing at the aliens, not at a given point on the solar surface). He had the centre of the solar sail decouple from the fat rim, which focussed the light back on the centre portion for braking.
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hey guys,
seems that lots of problems about levitating a squirell are linked to the production of the gigawatt… I found something interesting on wikipedia: try the compression flux generator:
http://en.wikipedia.org/wiki/Explosively_pumped_flux_compression_generator
just an intelligent way to vary the surface crossed by a magnetic field… and it provides a terawatt…
the squirell would fly for only some nanoseconds, though…
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oh, and I want a textbook too!
This is really a brilliant idea! personnally, xkcd tought me a lot, through looking up in wikipedia concepts that I didn’t know!
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What about a huge lens? Or have I missed something?
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I’m sure somebody already mentioned this, but if you turn the sun into a death ray, won’t it be hard for us to have “the light of day”, or for that matter, some wonderful summer days sitting around in the 75 degree (Fahrenheit) temperatures?
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Tristan > Destroying Alpha Centaurii is much more ‘constructive’ than summer dawns… sorry but geeks are less than romantic if lasers beam are involved, plus I think that the beautiful nightview of the resulting explosion will be a good message to future generations… ‘make (intergalactic) war, not (unprotected) love, son’
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