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 
I lol’d, but was the point education or humor? I dunno.
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The movie Real Genius sets unrealistic standards about lasers, even today. *sigh*
My life won’t be complete until we can use a laser to pop a houseful of popcorn, from space.
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Awesomely funny 🙂
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but they haven’t attacked us, yet!
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There is another way to make this even more awesome, make the sail black so that it absobes the photon, then it gets all the momentum of the photon. It will also get very hot, releasing black body radiation, that with a little ingenuity and some insulation, can be used to propel the sail even faster.
Unfortunately, there is a caveat that kind of kills this method for interstellar travel and that is diffraction. Even lasers diffract (duh). A laser sent out from a 1m aperture on earth will have a spot size of 5km when it reaches the moon. Any further than that and lasers will be quite useless for energy transport.
A dyson sphere would be cool though.
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You mispelled “jigawatt.” Hope this helps.
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So just how much power is a Gigawatt? I’m fairly certain Power Stations are rated in Megawatts, right? So it’d be a good chunk of the United State’s energy output at any given point to… lift a squirrel a few feet into the air. Wow. Marvelous. Stunning. I can see the Nobel Prize now…
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It’s about the amount of energy you need for time travelling. 🙂
Or about as much energy as an nuclear Powerplant produces.
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> There is another way to make this even more awesome, make the sail black so that it absobes the photon, then it gets all the momentum of the photon.
Common misunderstanding. If you make the sail black, it ends up with only half the momentum that it would if it reflected it — the rest just turns to heat.
> It will also get very hot, releasing black body radiation, that with a little ingenuity and some insulation, can be used to propel the sail even faster.
This, on the other hand, is more practical — I actually read a paper talking about using microwaves and principles like this one to propel something.
> Unfortunately, there is a caveat that kind of kills this method for interstellar travel and that is diffraction. Even lasers diffract
Yeah, which is why I said this was no good for interstellar travel. Most of the paper is spent determining how much of a boost you can get before diffraction makes the laser useless.
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I think that lifting a squirrel is a perfectly good reason to dedicate a full power plant to it.
This post is perfect, even if it didn’t had the xkcd-style drawings (maybe classhole should be operating the death ray). You should write things like this more often.
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Actually FYI when you have a laser pulse at the penta-watt level you don’t get diffraction. It is so intense it alters the refractive index of the medium it is traveling through, causing the laser beam to self focus (Well technically its called “self-guiding”) kinda cool. Amyone know where I can pick up a pentawatt laser cheap? Ebay?
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You could also use such a setup to steer potentially earth-hazardous asteroids into a greater orbit by planting a bunch of mirrors on its surface (up to a certain size of asteroid granted, and within a longer time frame than other methods.) I don’t know if it would be any more cost effective than using a solar collector though, safer probably.
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You should write/draw a physics textbook!!
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ipsi: Google’s shiny renewable energy site RE<C tells us that a gigawatt is about one San Francisco worth of power consumption. San Francisco < squirrel on laser elevator?
Darcythomas: are you sure you don’t mean a petawatt?
http://en.wikipedia.org/wiki/Peta-
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.
As for Meyers’ “novel” way of extracting more energy from a photon- this is how I was taught electrostatic repulsion is mediated, but that was only A-level physics which is full of LIES so it’s probably not that simple. I love how a physics paper inescapably logically leads to us building a death ray out of the sun.
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In “Ringworld” Larry Niven came up with solar-scale weapon system that used the sun as a giant laser. In that case, a solar flare was induced by means of a superconducting grid built into the ringworld itself, then (by some unspecified means I think) the flare lased when it got to a a certain size.
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The squirrel is going to have a bit of a thermal problem. Even if the mirror is 99.999% reflecting you cook the squirrel with 12 kilowatts. (which is roughly 10 times a microwave oven)
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“(Remember, energy ? unlike momentum ? has no direction.)”
Actually, it has a well-defined direction–time.
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Actually, William, what you’re thinking of is entropy.
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>Common misunderstanding. If you make the sail black, it ends up with only half the momentum that it would if it reflected it — the rest just turns to heat.
my mistake, although I’m not the one messing up the units. =).
Got to stop thinking I can do this type of thinking that early in the morning.
Wait, is that recursion… aaaahhhrg.
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A lasing medium between the Sun and your sail, you say?
http://laserstars.org/history/mars.html
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STUPID HUMANS: WE HAVE MADE A SIMILAR STELLAR-GUN MILLIONS ON YEARS AGO POINTED TO YOU. NOW WE ARE KEEPING YOUR PLANET UNDER CONTROL AND SURFING YOUR INTERNET SEARCHING FOR SMART IDEAS (and nasty human porn).
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Alicia got there before me, but you should intersperse comic strips and physics lessons. Please?
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There’s a little problem you didn’t see 🙂 Dyson sphere would cook the sun. Good news is that resulting supernova would probably kill aliens on alpha centauri.
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And we’ll nickname the dyson sphere “The Death Star” then, and leave a handy hole for proton torpedoes to blow up the sun?
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Dyson sphere – cool idea. It would look like the Death Star, with a hole in the side that kills stars. Of course, it would take 4 years to reach Alpha Centauri. By then I’m sure the Centauri would be able to jump here and destroy us all.
In Clarke and Baxter’s novel “Firstborn” they talk about using a space elevator to fling stuff away from Earth, centrifugally.
Now, the problem with your concept that I can see is that the laser, or whatever light, has to be directed precisely at the sail so it reflects a good number of times. This is hard, unless you make the sail and mirror absurdly huge (maybe not half the size of Earth or Sol, but…) I mean, by the time we can build a Dyson sphere, we’ll probably have a gravity-wave drive or something.
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get back to work. now.
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There is enough 10MW lasers I think, has anyone tried the non-fried squirrel elevator? In case she jumps down cross two “non-” part out. What kind of dielectric mirror would be needed for 1-5 second squirell levitation? Or would be plain silver satisfactory?
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Brilliant.
Fuck mollecular biology, I should have chosen something with more lasers and squirrels and intergalatic wars and stuff.
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You misspelled “Centauri” (Cesium got it right in that email in)
(yes, one of the English majors reading the comic despite the warning…)
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Cesium,
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.
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Actually, it would take a fair 1.21 megawatts to lift the squirrel using this method.
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I second Alicia. A physics textbook with xkcd’s drawings and explanations could be awesome. The Black Hat Guy can make some neat appearances there 🙂
The only question is – what target audience should the book address? Undergrads? Liberal-art majors (god forbid!) ?
Anyway, that’ll be definitely a book I am going to buy. In fact – place me in the preorder list now!
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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!!!
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Admit it, you chose a squirrel because that was about the right weight to consume that much energy.
If this method is so much more efficient, has anyone tried elevating something light and reflective using this method yet?
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The Dyson Death-ray is simple enough that you could commoditize it – create a death-ray-building machine, make 1000 copies of it, and launch them at 1000 stars near somebody you don’t like.
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So I guess that was the purpose for building the DeLorean all along, to lift a squirrel (Come on, don’t tell me I was the only one who thought of Back to the Future with that 1.21 Jigawatts line.)
Other than that, I have absolutely no idea what this, the previous, or many of the blogs are talking about.
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LSK wrote:
> Actually, it would take a fair 1.21 megawatts to lift the squirrel using this method.
Then we need to think bigger. It’s time to launch the Thousand Squirrel Army into space!
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I’m also in favour of Alicia’s suggestion – I want nothing but the best for my kids (I like to think ahead), and your style puts foot to backside!
Please put me in for pre-order too 🙂
Oh, right – my intended comment. Instead of moving asteroids out of our path (which involves getting over there, doing the “Armageddon” thing, and getting safely back), we could set up the mirroring system to move ourselves out of the way. A little further back, maybe that’ll give us a bit more time to deal with global warming as well…
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Your lasing cavity is an unstable optical resonator(a light beam bouncing within the cavity will tend to leave it very quickly). You’d be better off with a stable one (think two concave mirrors enclosing the active medium, so subsequent reflections make the light go through the active medium again and again).
And even then, laser beams spread(gaussian beams are popular here on earth. They spread). By the time it reaches the nearest star, it might be too weak for their DSN antennae to decipher our message. 😦 Yet another dream quashed by physics.
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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.
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Geeks are children who never grew up.
I got married. 😦
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Dyson spheres…. Why must our sun be a death star?
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An XKCD physics textbook would be the best textbook ever.
And Itay, the audience for the book would be the same as the audience for the webcomic, only slightly less educated. (before reading the book, that is.)
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> And even then, laser beams spread(gaussian beams are popular here on earth. They spread).
Of course they do. But keep in mind how much power we’re talking about. It could spread out many orders of magnitude and still outshine their local sun, gradually cooking the planet.
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.
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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.
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If you weren’t straight and I weren’t already married* I’d totally propose to you, dude. Any man who can talk that way about lasers already owns my heart anyway.
*stupid laws
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i think your theory puts you in the mad scientist domain, but obviously a scientist wouldn’t be this imaginative or make such sweeping assumptions without referring to 15 published articles per paragraph. i therefore have no choice but to confer onto you the title of ‘mad nerd.’
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I’m all for an XKCD physics textbook! I have always been a firm believer that science isn’t required to be hard to understand – it just has to be well-presented. Look at the work of Carl Sagan, and even Feynman’s lectures.
As for the audience, few assumptions are required if a complete whistle-stop tour of physics is to be completed. About the only requirement is that the audience is expected to be able to grok various geek culture references – mainly popular sci-fi and films, with some of the more commonly known computer culture stuff. I’d also think that the audience would want a bibliography so that they can easily read more into any given area (I love books which have those! – the absence was my main complaint about some of hawking’s work).
Why stop at physics? The natural world is a wondrous thing indeed. I loved Sagan’s approach in Cosmos, touring the history of science and covering evolution.
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Thats funny.
Yesterday i was learning for a physics test and
because most textbooks are really crappy, thought about writing one too.
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@Chris: “You mispelled ?jigawatt.? Hope this helps.”
Not true, actually – it used to be quite commonly pronounced with a soft g (as recently as the 80s even, when the film was made), but it’s always been spelt “giga”.
I massively lost an argument over this once, and have been correcting others for it ever since to compensate. 🙂
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