The Laser Elevator

Posted byRandall 426 Comments on 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!”

426 replies on “The Laser Elevator”

  1. 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..

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  2. 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!

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  4. 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.

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  6. 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.

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  8. 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?

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  9. 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.

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  12. Pingback: Lifting Squirrels by Laser

  13. 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.

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  14. This post was cool as, love the webcomic! Just commenting to let you know you have a fan base in Australia.

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  15. 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!

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  16. 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?

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  17. 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?

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  18. The answer to that is quite complicated, but it all starts with a nasty bump on the head…

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  19. 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.

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  21. 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.

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  23. 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.

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  24. “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.

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  25. 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.

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  26. 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.

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  27. 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)

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  29. 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)

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  30. 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

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  31. 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 ms 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!

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  33. 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!

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  34. 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.)

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  35. 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.

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  38. > 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 🙂

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  39. 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.

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  41. 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.

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  42. “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.

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  43. 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!

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  45. 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.

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  47. “you’ll get just as much thrust from the solarsail no matter how far from it you are”
    Well, modulo steering, of course.

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  48. 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.
    🙂

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  49. 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.

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  50. 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.

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