The Laser Elevator
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!”
February 15th, 2008 at 4:58 am
I lol’d, but was the point education or humor? I dunno.
February 15th, 2008 at 5:02 am
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.
February 15th, 2008 at 5:11 am
Awesomely funny :-)
February 15th, 2008 at 5:16 am
but they haven’t attacked us, yet!
February 15th, 2008 at 5:20 am
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.
February 15th, 2008 at 5:28 am
You mispelled “jigawatt.” Hope this helps.
February 15th, 2008 at 5:38 am
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…
February 15th, 2008 at 5:44 am
It’s about the amount of energy you need for time travelling. :)
Or about as much energy as an nuclear Powerplant produces.
February 15th, 2008 at 5:52 am
> 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.
February 15th, 2008 at 5:54 am
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.
February 15th, 2008 at 6:08 am
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?
February 15th, 2008 at 6:19 am
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.
February 15th, 2008 at 6:54 am
You should write/draw a physics textbook!!
February 15th, 2008 at 6:55 am
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.
February 15th, 2008 at 7:33 am
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.
February 15th, 2008 at 7:33 am
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)
February 15th, 2008 at 7:42 am
“(Remember, energy ? unlike momentum ? has no direction.)”
Actually, it has a well-defined direction–time.
February 15th, 2008 at 7:55 am
Actually, William, what you’re thinking of is entropy.
February 15th, 2008 at 7:56 am
>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.
February 15th, 2008 at 7:56 am
A lasing medium between the Sun and your sail, you say?
http://laserstars.org/history/mars.html
February 15th, 2008 at 8:12 am
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).
February 15th, 2008 at 8:28 am
Alicia got there before me, but you should intersperse comic strips and physics lessons. Please?
February 15th, 2008 at 8:33 am
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.
February 15th, 2008 at 8:33 am
And we’ll nickname the dyson sphere “The Death Star” then, and leave a handy hole for proton torpedoes to blow up the sun?
February 15th, 2008 at 8:37 am
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.
February 15th, 2008 at 8:38 am
get back to work. now.
February 15th, 2008 at 8:43 am
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?
February 15th, 2008 at 8:45 am
Brilliant.
Fuck mollecular biology, I should have chosen something with more lasers and squirrels and intergalatic wars and stuff.
February 15th, 2008 at 8:51 am
You misspelled “Centauri” (Cesium got it right in that email in)
(yes, one of the English majors reading the comic despite the warning…)
February 15th, 2008 at 8:52 am
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.
February 15th, 2008 at 8:57 am
Actually, it would take a fair 1.21 megawatts to lift the squirrel using this method.
February 15th, 2008 at 9:34 am
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!
February 15th, 2008 at 9:35 am
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!!!
February 15th, 2008 at 9:40 am
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?
February 15th, 2008 at 9:41 am
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.
February 15th, 2008 at 9:52 am
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.
February 15th, 2008 at 9:55 am
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!
February 15th, 2008 at 10:07 am
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…
February 15th, 2008 at 10:20 am
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.
February 15th, 2008 at 10:20 am
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.
February 15th, 2008 at 10:22 am
Geeks are children who never grew up.
I got married. :-(
February 15th, 2008 at 10:22 am
Dyson spheres…. Why must our sun be a death star?
February 15th, 2008 at 10:31 am
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.)
February 15th, 2008 at 10:53 am
> 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.
February 15th, 2008 at 11:00 am
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.
February 15th, 2008 at 11:06 am
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
February 15th, 2008 at 11:08 am
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.’
February 15th, 2008 at 11:11 am
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.
February 15th, 2008 at 11:15 am
Thats funny.
Yesterday i was learning for a physics test and
because most textbooks are really crappy, thought about writing one too.
February 15th, 2008 at 11:20 am
@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. :)
February 15th, 2008 at 11:23 am
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…
February 15th, 2008 at 11:24 am
“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.
February 15th, 2008 at 11:28 am
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.
February 15th, 2008 at 11:35 am
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.
February 15th, 2008 at 11:55 am
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. :x
February 15th, 2008 at 12:03 pm
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.
February 15th, 2008 at 12:15 pm
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.
February 15th, 2008 at 12:23 pm
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.
February 15th, 2008 at 12:28 pm
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.
February 15th, 2008 at 12:33 pm
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…
February 15th, 2008 at 12:50 pm
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.
February 15th, 2008 at 12:53 pm
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….
February 15th, 2008 at 12:59 pm
Now witness the power of this fully armed and operational … Dyson Sphere.
February 15th, 2008 at 1:06 pm
Dyson-Sphere-Planet-Killing-Laser Machine: bringer of interstellar war…or the coolest way of killing vampires ever?!
February 15th, 2008 at 1:09 pm
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.
February 15th, 2008 at 1:24 pm
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.
February 15th, 2008 at 1:30 pm
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.
February 15th, 2008 at 1:33 pm
The thing about solar sails is, how the hell do you STOP them? Engage the space brakes?
February 15th, 2008 at 1:36 pm
so how far could we lift the squirrel then?
February 15th, 2008 at 1:38 pm
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…
February 15th, 2008 at 1:38 pm
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…
February 15th, 2008 at 2:08 pm
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!
February 15th, 2008 at 2:11 pm
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 :)
February 15th, 2008 at 2:14 pm
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?
February 15th, 2008 at 2:22 pm
“Nicoll-Dyson Laser”
February 15th, 2008 at 2:22 pm
Why does each reflection of the photon result in +2P in the body of which it reflects?
February 15th, 2008 at 2:29 pm
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.
February 15th, 2008 at 2:36 pm
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.
February 15th, 2008 at 3:06 pm
squirrels , if pushed - will attack your nuts
February 15th, 2008 at 3:22 pm
What’s the point of a solar sail?
February 15th, 2008 at 3:27 pm
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.
February 15th, 2008 at 3:28 pm
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!
February 15th, 2008 at 3:46 pm
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!
February 15th, 2008 at 3:49 pm
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).
February 15th, 2008 at 4:06 pm
o.o
that is absolutely wonderful.
February 15th, 2008 at 4:07 pm
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
February 15th, 2008 at 4:28 pm
Far-fetched, and creative.
February 15th, 2008 at 4:41 pm
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)
February 15th, 2008 at 5:12 pm
“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…
February 15th, 2008 at 5:18 pm
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 :) )
February 15th, 2008 at 5:33 pm
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.
February 15th, 2008 at 5:50 pm
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.)
February 15th, 2008 at 5:55 pm
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.
February 15th, 2008 at 6:13 pm
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.
February 15th, 2008 at 6:34 pm
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.
February 15th, 2008 at 7:01 pm
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…
February 15th, 2008 at 7:05 pm
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!
February 15th, 2008 at 7:17 pm
What about a huge lens? Or have I missed something?
February 15th, 2008 at 7:56 pm
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?
February 15th, 2008 at 9:02 pm
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’
February 15th, 2008 at 9:14 pm
Didnt the moties in The Mote In God’s Eye build a giant planetary laser to ower a solar sail spave vessel? If only we had moties to enslave to actually build such a laser… :(
Also, what building the reflector on the dark side of the moon, since the moon is tidally locked anyways we dont have to worry about it rotating/revolving? away. Only worry would be that the moon would inch closer towards the earth and cause planet wide doom, like when in Bruce Almighty….can you say ginormagantuan tidal waves?
Besides long ranged laser exchanges sound boring, we gotta find a way to use mass-drivers and launch killer asteroids, like in Footfall.
I think ive referenced enough today..
February 15th, 2008 at 9:56 pm
I (N + 2)th the request for a xkcd-illustrated and written physics book with lasers and blowing things up.
I (N ^ N)th it!
I Ackerman(N, N)th it!
February 15th, 2008 at 10:13 pm
I love it!
February 15th, 2008 at 11:46 pm
OK, so we see the absurdity of depending on solar sails for providing the main propulsion of our spacecraft when we are far from the source of photos (a star). Remember that when you’re near a star, you get a lot more light, and therefore a lot more ‘kick’ out of the sail. Pretty convenient when you don’t want to fly into the sun. But like real sails, you can tack into the wind, around the sun, using not only the sun’s gravity but its’ photons and phlogistons., gaining velocity as you do. through some very careful and clever use of orbital calculations and precisely timed steering rocketry, you take a swing by Jupiter, using its gravity to swing back around toward the the sun. Wash, Rinse, Repeat. Like a Comet! but much faster, and without all that vapour trail. at tome point when you’re going fast enough, you break out of the sol-Jupiter trajectory, maybe being extra clever and stealing some gravitational energy along the way from Saturn, Uranus, or Neptune, and break out of the solar system in the vector you want. Of course, you still have to do the reverse at your destination, which means that you’ll have to have pretty good maps of the destination, in order to know how to use the destination’s massive bodies to slow down. And of course you bring out the ’star sail’ (because we’re not using it on Sol now) to help in doing so.
February 15th, 2008 at 11:51 pm
Interstellar communication, blah, blah, travel, blah, blah, war, yadda, yadda, yadda -
What happened to the *squirrel*?
February 16th, 2008 at 12:13 am
I am going to be tacky and reply to me own message. Sorry about the typos.
s/photos/photons/ probably others as well. But what I am really here to add is that we need to take advantage of that technology which our species has mastered so well: our computers. We haven’t managed to harness any large-scale energy technologies (fission and fusion explosions we can do, but they’re so messy without the proper containment), we we have to be clever little apes+ and do the best with what we’ve got. The computation is probably the easy part. We also need data..that is, much, much better data about planetary systems than we already have, so we’ll know about how to slow down at our destination - and that we’ve chosen a destination that is of some value (i.e. has a planet which can easily be terraformed, or a moon made entirely of cheese, etc.). To do this, we need to build a system of telescopes that are spread out throughout the solar system, which transmit very high resolution data to earth (probably via the moon), to a data center where the data is all correlated and we get some idea of what’s going on a hundred and fifty light years away.
We’ll also need some pretty heavy interstellar spacecraft. I imagine they’ll probably be some sort of hollowed-out asteroid, or perhaps built of lunar cement. But they need to have a large ablation shield on all sides, and storage for reaction mass (that matter we throw overboard really fast so we can change movement vector and speed), and room for all the cargo - humans, their supplies, and the tools we’ll need to terraform the destination, plus all the stuffed animals people couldn’t bear to leave behind on earth.
it’s probably easier to start closer to home, terraforming Venus and Mars, perhaps mining the asteroid belt and the gas giants. Some of the same technology mentioned above can be used to get some really good velocity going, but using the destination planets as the massive bodies instead of going another time around the sun. And of course We’ll have to use some sort of transport tech to put up these really accurate high-res observatories all throughout the solar system, so it will probably be same-old, but I am not sure what’d mass we might use to slow down at the destination above and below the elliptic plane (Pluto?) - we might have to use up a real lot of reaction mass and energy, making telescope placement very expensive.
Anyhow Randall, if any of your former buddies at NASA want to give me a job as mad scientist, just drop me some email. Either that, or we should go out for a few brews at the Thirty Ear and figure out which one of us is more of a mad scientist.
February 16th, 2008 at 12:28 am
scotb,
WHATTAHELL is jigawatt?!?
:-P
February 16th, 2008 at 2:00 am
This is probably a stupid question, but if we’re capturing all the sun’s light and focusing it into a giant death ray, what’s lighting - and heating - the Earth?
February 16th, 2008 at 3:02 am
A better alternative to a solar powered death ray would have to be a Death Star approach. You remove large portions of the moon’s inner mass and replace it with a particle accelerator capable of launching aircraft carriers. Hurl a few of them, and they die the death of the dinosaurs.
February 16th, 2008 at 3:14 am
El ascensor laser…
xkcd explica de forma amena el funcinamiento de las velas solares. ¿Cuanta energía es necesaria para elevar una ardilla con un laser? La respuesta en el interior…
February 16th, 2008 at 3:42 am
@Raize:
We’d live on the Dyson Sphere, guh! ;)
February 16th, 2008 at 4:28 am
[...] It’s a good thing I found xkcd - I think it was via language-log and, btw, and I have no idea how I got to reading language log. Anyway, thanks to a friend I learned about the xkcd-blog (or “blag”), and so my morning was slightly happier, after reading about a more efficient method for using light-sails, with this helpfull illustration (blogentry on blags.xkcd): [...]
February 16th, 2008 at 4:52 am
so more thoughts here on just this whole spaceship powering thing….now that they’ve discovered that Saturn’s moon Titan has more liquid hydrocarbons than earth, several times over, i am sure there’s some people at Exxon trying to figure out how to claim all that oil/gas and sell it on earth.
Just think about how bad that would be - not only would we be putting more carbon into the atmosphere than the planet had to begin with, but we’d also lessen the amount of elemental (O2) oxygen on the planet as it combines with these Titanic hydrocarbons to produce water and carbon dioxide.
A far better idea would be to bring a whole bunch of water up from Earth to Titan, and see if we can make it rain there. Then introduce some interesting life forms to make it get all exciting…some algaes and bacteria could have a lot of fun.
February 16th, 2008 at 5:18 am
This post was cool as, love the webcomic! Just commenting to let you know you have a fan base in Australia.
February 16th, 2008 at 5:37 am
Just how funny that I was talking to some friends about the need for this elevator, but how?! I´m running to tell them, gotta keep on building up our future world. Love your site!
February 16th, 2008 at 5:39 am
You should switch your comic to this text-and-pictures exposition. It’s better.
You could get 1.21 gigawatts with a bolt of lightning. But how would you know where and when one would strike?
February 16th, 2008 at 6:16 am
Would the lasing cavity be enough to let all the excess energy escape? If the walls of the Dyson sphere are reflecting all light, it might cook us all. Even if it doesn’t get to that extreme circumstance it would eliminate nighttime and the need for clothes.
Good plan, actually.
Where can I donate republic credits?
February 16th, 2008 at 6:18 am
The answer to that is quite complicated, but it all starts with a nasty bump on the head…
February 16th, 2008 at 8:25 am
Forgive my ignorance, but why not just paint a solar sail black and that way it can completely absorb the photon (and presumably it’s energy)?
I have to admit, though, that I’m more of a fan for more manly forms of space travel.
February 16th, 2008 at 8:26 am
Oops, my hyperlink didn’t work. Here’s the URL:
http://en.wikipedia.org/wiki/Nuclear_pulse_propulsion
February 16th, 2008 at 9:33 am
Yes, textbook! I didn’t care much for physics at my university (I’m a biochem major/math minor). Now, pchem, that rocks. But in general physics I never really got into it. I learn more from this than I did in class. (don’t tell my profs that though!) Our physics book was horrid. I couldn’t even do the homework by reading the chapters - I had to look online!
On another note, I have no clue what a Dyson sphere is *shy* sometimes although I am far from a liberal arts major these do go over my head. I guess my true nerd-ness comes out though in that i.must.go.research.it.NOW.
February 16th, 2008 at 9:53 am
Yes! Death ray! That is so freaking sweet!
February 16th, 2008 at 11:22 am
I mean, by the time we can build a Dyson sphere, we’ll probably have a gravity-wave drive or something.
Gravity wave drives are pretty easy to make actually.
You will need:
- Two reasonably massive bodies (like jupiter not Barry White* btw, you can never compress Barry White’s body enough to make a blackhole, trust me on this).
- A Mr. Fusion™ generator and some banana peels (for those extra zesty 1.21 Gigawatts**)
- A 1.21 gigawatt lightbulb
So take your two massive bodies, compress them until they form non-rotating black holes and use homeopathy and a pinch of tarragon to keep them near enough to each other that their gravitational fields can interact when you begin to spin them, but seperate enough so you still have two black holes. Meanwhile, you should use your ginormous lightbulb to keep the two blackholes from evaporating thanks to hawkin radiation and ideally should actually be causing your two non-rotating ‘holes to increase in size, due to the amount of energy going into them exceeding the amount being radiated outward.
Now use an eggwhisk and start rotating your blackholes so they are counter rotating in relation to each other (i.e. one is spinning clockwise and one is spinning counter-clockwise). Now if some of the current theories on stellar mechanics of blackholes is correct, the blackholes should start emitting gravity waves in a single direction (defined by the spin of the ‘holes) due to both frame dragging and the Generally Mischievous Leprechaun of Relativity, and those gravity waves should push the pair of blackholes in the opposite direction. This is where it’s very important your lightbulb should be pumping enough energy into the ‘holes that you’re making up for the loss in mass that occurs as result of the blackholes radiating both hawkin radiation AND the additional gravitational waves, which will robb the ‘holes of mass to stop 19th cnetury physicists looking silly.
Now build a space ship around the black holes et voila! A gravitational wave drive.
And a neat side effect is that you also have a space drive that happens work as a super duper powerful mass driver just by chucking things between the photosphere’s of the ‘holes.
On the flip side you should probably check that Centauri’s haven’t built their own dyson shell and turned it into a stellar engine of some kind, it’s no good shooting things if it takes 4 years to see that the entire star system swerve at the last second.
* I figured someone would make a “compress a fat person” joke if I didn’t. Damned to you who ever you would have been.
** For years I thought gigawatt was pronounced “gig-a-what” rather than “jig-a-what”, but now I see the error of my ways.
February 16th, 2008 at 12:17 pm
“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.”
Well, except that L1 is unstable, and the Earth rotates. You’d have to have the station orbiting around L1, and have the power stations constantly traveling around the equator while doing a dance to keep the beam aligned.
“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?”
Easy. Build the sphere outside Earth’s orbit. Then you’d only have the problem of light reflecting off the non-lasing parts of the sphere, which I’m sure could be managed.
February 16th, 2008 at 1:23 pm
Nice idea.
But, having spent much of the 80’s leading a major team on railguns, could we PLEASE remove any mention of that technology from all this earth / space drive stuff???.
The answer is no.
No, really no.
And linear motors are too inefficient at high speed, and yes, I did know Prof Laithwaite who invented them.
So sorry.
February 16th, 2008 at 1:31 pm
Oh, and a tank gun propellant charge produces 5 gigawatt, but only for a a few millisconds.
155 artillery? aboiut 35GW, for a little longer.
Just thought you would like to know.
Its not about peak power, its about the amount of time time the thing is turned on.
So I am going to stop now before I get carried away.
February 16th, 2008 at 2:10 pm
luckily, by the time we have a dyson sphere death ray, we’ll have colonized a few hundred other planets.
hmm, wouldn’t it be hard to aim though, i mean, you would have to be almost impossibly precise
i say we just figure out a way to travel faster than light (Shaw, Fujikawa, get on that guys, we’re waiting)
February 16th, 2008 at 2:30 pm
Someone’s already made a laser-powered craft, but I think the squirrel might get dizzy.
February 16th, 2008 at 3:37 pm
rail guns rail gunz rail gunz ha ha Paul. The 80s were a long time ago.
So if we can use laser cooling on atoms, can’t we just do the opposite and lase them towards the speed of light? But that’s probably boring, and BoseNovas are cool.
Someone else mentioned time travel. That’s my main area of research “right now”. So far I have been successful in traveling forward, as I lost the entire last monday. But that’s just an effect of too much klonopin. I don’t think any pharmeceuticals are going to help me go back in time (except maybe high doses of deliriants)
February 16th, 2008 at 6:46 pm
Hirop says:
>Actually, William, what you’re thinking of is entropy.
Wrong. Energy’s direction is in fact time (it’s relativity). That’s the relation the uncertainty principle, and some other awesome things in physics, work on:
Momentum—>Space (standard directions)
Angular Momentum—>Space (angular directions)
Energy—>Time
February 16th, 2008 at 10:34 pm
I don’t know if someone already corrected you “xkcd” on the 15th of February, but making the sail black would actually half the amount of kinetic energy being transfered to the sail. According to the conservation of momentum laws, if a photon comes in with a momentum of P and is absorbed, all of its energy is being transfered to the solar sail, hence it gaining exactly 1P of kinetic energy per photon. If, however, the sail is reflective, the photon still collides with the sail, but is then sent back at 3.0 x 10*8 m\s in the exact opposite direction. In order for conservation of momentum to be true, the sail will have to gain exactly 2P of momentum; 1P for the work done to bring the photon to a rest, and instantly another 1P to accelerate it backwards in the opposite direction to leave the net momentum constant. On top of that, this same photon can then return to the planet, be reflected by the same method, in which of course the resulting acceleration on the earth will be negligible, then return to the sail, further accelerating it.
I hope that made sense!
February 17th, 2008 at 1:33 am
What about magnetic sails?
http://en.wikipedia.org/wiki/Magnetic_sail
February 17th, 2008 at 1:51 am
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
Oh man, this is the most awesome idea I’ve heard all year long.
also Nthing the xkcd physics book, and make sure this one gets in!
February 17th, 2008 at 3:27 am
Why not just manipulate the large-scale structure of dark matter in the universe, using galaxies as missiles and causing the targets to be swallowed up in a couple billion years by mergers of supermassive black holes? (No, not the song.)
February 17th, 2008 at 4:25 am
You know, 1.21 Gigawatts of power a flux capacitor and a De Lorean DMC-12 going 88 mph will get you a time machine. A far better use than squirrel lifting.
February 17th, 2008 at 7:00 am
omg LOL
beuhheuheuhe
ALL SERIOUS AND THEN…
“DEATH RAY”
killed me
February 17th, 2008 at 10:45 am
So.. war… really good use for science…
February 17th, 2008 at 11:53 am
> I don?t know if someone already corrected you ?xkcd? on the 15th of February, but making the sail black would actually half the amount of kinetic energy being transfered to the sail.
> I hope that made sense!
It did not — you are repeating exactly what I said in a comment, and not correcting anything in the article. It sounds like you want to argue with someone who left another comment. And minor correction to you — it halves the momentum transferred to the sail — not the kinetic energy, necessarily.
> 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.
This concept is hard to get into words properly.. I understand that they are different units; I’m saying that compared to the sorts of projectiles we’re used to, the amount of momentum compared to the transferable energy (kinetic, for objects we’re used to) is very high — momentum of a photon is energy / c, and c is very large in everyday terms. I do understand what you’re saying, but there’s only so far I was willing to delve into mathematical pedantry in one blog post :)
> 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
This was discussed in some source — I can’t remember which — as a method to propel a sun, in effect turning the entire solar system into a spacecraft. You can move the sphere slightly back from the point where the energy is emitted and make its mass unbalanced such that the gravitational attraction counterbalances the energy of the exiting photons. Your acceleration is supremely slow (I think it would take you millennia to get up to noticeable speeds) but eventually you coul get the entire star gliding along, death ray flowing out behind it. That’s a story for another post, though :)
February 17th, 2008 at 12:58 pm
Okay, so assuming that we have a Dyson Sphere around our sun and a circular series of lasing cavities at one point on said Dyson Sphere
Isn’t that the Death Star?
And also, wouldn’t it take four years for our Death Ray to reach Alpha Centauri? That’s one slow-ass battle. We’re going to have to do better than that if we’re going to wage a good interstellar war.
February 17th, 2008 at 12:59 pm
Did anyone say laser?
http://www.physorg.com/news122298608.html
Worlds most powerful laser, to date
February 17th, 2008 at 2:49 pm
What about the relativistic effects and the resulting reduction in momentum with increasing velocity. I would do the math but if I had it would be where this comment is.
February 17th, 2008 at 3:15 pm
“This was discussed in some source — I can’t remember which — as a method to propel a sun, in effect turning the entire solar system into a spacecraft. You can move the sphere slightly back from the point where the energy is emitted and make its mass unbalanced such that the gravitational attraction counterbalances the energy of the exiting photons. Your acceleration is supremely slow (I think it would take you millennia to get up to noticeable speeds) but eventually you coul get the entire star gliding along, death ray flowing out behind it. That’s a story for another post, though :)”
“And also, wouldn’t it take four years for our Death Ray to reach Alpha Centauri? That’s one slow-ass battle. We’re going to have to do better than that if we’re going to wage a good interstellar war.”
Simple. Just use the above idea, but add a smaller lasing cavity on the other side. In a few thousand years we’ll be hurtling toward Alpha Centauri spouting an annihilating death ray of doom. Assuming we don’t develop FTL, the best we can do is shorten the distance. In fact, install lasing cavities symmetrically around the Sun. Then send off ships sailing on lased sunlight toward nearby stars, construct Dyson spheres, and voila, you have a couple dozen interstellar missiles, and it’s sayonara for the Centauri. Language mixing aside.
February 17th, 2008 at 3:19 pm
I do hope this technology can be used to offset the Raptor threat. However, one must consider a growing threat after the raptors… sharks with laser beams on their (frickin) heads!
February 17th, 2008 at 5:08 pm
I like! You should explain cool science stuff in this way more often!
February 17th, 2008 at 5:54 pm
What about the relativistic effects and the resulting reduction in momentum with increasing velocity.
Well what’d actually happen is that the wavelength of the light emitted would change (which sounds a bit besides the point but the speed of light in a vacuum is always constant, irregardless of how fast you’re moving or in what direction in relation to a beam of light you happen to be moving, only the wavelength changes), but that’d be true for a regular, non-bouncy, solarsail anyway, so the bouncy-solarsail would still be better.
On the flip side however, if we’re moving towards Centauri we might be able to swing the gun around (giant flywheel?) and use the blue shifting to increase the power and lethality of the Dyson Cannon while also deccelerating so we can hang around the centauri’s home sytem and write obnoxious messages in their puny alien public toilets afterwards.
And we’ll decelerate even harder if they foolishly try to thwart us with a giant reflective surface of some kind (or we’d accidentally fling them into interstellar space while their impromtu lightsail smashes into the side of their planet/star, but either way, those godless alien communists ain’t got a prayer).
Remember that when you’re near a star, you get a lot more light, and therefore a lot more ‘kick’ out of the sail.
though you should probably also remember that the nearer the sun you are the greater the pull from the star’s gravity - and as both the light emitted AND the gravity of the star drops off with distance according to the inverse square law you’ll get just as much thrust from the solarsail no matter how far from it you are - it’s only the doppler shifting due to relative motion that will change the entry/exit thrust
Which means that the solarsail’s deltaV is probably ultimately dependant on the frequency of light being emitted by the star which the sail can use for power, if you can make a material that reflects x-rays and you happen to live near or on a pulsar then interstellar travel is a piece of cake really. Also, you’d be able to make the most powerful Dyson Cannon possible.
February 17th, 2008 at 6:25 pm
[...] Laser elevators, squirrels, and death rays. I love xkcd, even the blog stuff. [...]
February 17th, 2008 at 6:30 pm
“you’ll get just as much thrust from the solarsail no matter how far from it you are”
Well, modulo steering, of course.
February 17th, 2008 at 7:13 pm
Error?
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 :’(
Sorry.
:)
February 17th, 2008 at 7:16 pm
So, in pondering the idea of the particle loosing some of its momentum with each reflection, I came across an interesting possible phenomenon. If the particle starts out with energy at an ultraviolet or higher level, will it eventually go through the spectrum as it transfers momentum? if it were 100% efficient, you wouldnt see it unless you got in the beams way (cooking you probably) but the idea of a rainbow of propulsive light is at least artisticly pleasing (in the same manner as a prism).
Or am I misusing a physics concept.
February 17th, 2008 at 8:18 pm
Surely the dyson sphere would be pushed into the sun by the reverse momentum of all of the light leaving the lasing cavity? What you would need is some structure to hold the sun in the center of the sphere. Of course, the entire sun and dyson sphere structure would then be pushed away. No, the only way you could keep things stable is to have lasing cavities on both sides of the sphere.
February 17th, 2008 at 9:12 pm
“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.
February 17th, 2008 at 9:42 pm
you guys make me sad.
February 17th, 2008 at 10:39 pm
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:
]
} -o [
}] -o[
}]o- [
}] -o[
Like a never-ending game of pong. With lasers.
February 18th, 2008 at 12:37 am
Chris (one of the first posts) misspelled “misspelled”.
And I think “Gigawatt” is correct.
February 18th, 2008 at 1:07 am
Dyson spheres might exist someday, but Wilson spheres are here today, and can be mass produced from renewable resources.
February 18th, 2008 at 1:20 am
Keep that squirrel under 88 man, I don’t think a squirrel could make an N-jump on it’s own.
February 18th, 2008 at 2:27 am
> 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.
February 18th, 2008 at 2:42 am
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?
February 18th, 2008 at 2:49 am
“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.
February 18th, 2008 at 3:43 am
>”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
February 18th, 2008 at 5:26 am
“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.
February 18th, 2008 at 5:33 am
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.
February 18th, 2008 at 7:52 am
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. ;)
February 18th, 2008 at 8:53 am
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……………..”
February 18th, 2008 at 9:22 am
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)
February 18th, 2008 at 9:29 am
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 .
February 18th, 2008 at 9:31 am
dammit there should be the word war between interstellar and shall. I blaming this late hour at least where I live
February 18th, 2008 at 9:47 am
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.
February 18th, 2008 at 10:06 am
>>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.
February 18th, 2008 at 10:10 am
<3
February 18th, 2008 at 10:39 am
[...] early today to see if I could get up to page 7 or 8 before too late in the afternoon) I came across this in xkcd’s newsblag. Which I thought was absolutely hillarious, and of course utterly [...]
February 18th, 2008 at 11:00 am
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.)
February 18th, 2008 at 11:10 am
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.
February 18th, 2008 at 11:50 am
joules not gigawatts (I have jigawatts on the brain)
February 18th, 2008 at 11:51 am
Any physics lecture that included the phrase “supplemented by a Dyson Sphere…” would be greatly improved, IMHO
February 18th, 2008 at 11:56 am
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.
February 18th, 2008 at 12:08 pm
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.
February 18th, 2008 at 1:30 pm
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.
February 18th, 2008 at 1:39 pm
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.
February 18th, 2008 at 2:43 pm
ahahha, deathstar.
February 18th, 2008 at 3:38 pm
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.
February 18th, 2008 at 3:49 pm
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.
February 18th, 2008 at 4:01 pm
@ 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.
February 18th, 2008 at 4:14 pm
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).
February 18th, 2008 at 4:21 pm
@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!
February 18th, 2008 at 4:44 pm
The drawings make this amazing. Munroe, you should really replace Wikipedia’s sterile and boring diagrams with your nice and friendly hand-made ones. :)
February 18th, 2008 at 4:51 pm
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?
February 18th, 2008 at 5:00 pm
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?)
February 18th, 2008 at 7:42 pm
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!
February 18th, 2008 at 10:12 pm
“Actually, William, what you’re thinking of is entropy.”
No, I’m thinking of special relativity, where energy is momentum in the time direction
February 18th, 2008 at 10:29 pm
Haha, this is FANTASTIC.
February 18th, 2008 at 11:02 pm
@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).
February 18th, 2008 at 11:15 pm
I don’t spend enough time thinking.
February 18th, 2008 at 11:43 pm
Preemptive strike?
February 19th, 2008 at 12:01 am
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.
February 19th, 2008 at 12:11 am
Squirrels and lasers don’t scare me…
SHARKS with lasers- now that’s trouble.
February 19th, 2008 at 2:06 am
PKM: “Also I’m pretty sure that wouldn’t work in space because I would guess it requires a medium to alter the refractive