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. I’ve only read posts down to late February.
    My initial thought was that this isn’t a normal dyson sphere. Its reflective, which as Aniviller pointed out means you would probably cook your sun into a red giant about half way though constructing the sphere. Also as Seb pointed out you would need a counter balance laser to handle the death ray’s kick back.

    Someone suggested using Pluto for target practice, or preparing for Alpha Centaurus’s Allies both cases illustrates a major engineering problem that needs to be addressed which is turning the thing.

    I propose the entire dyson sphere be made out of reflective cavity so that any part of the suffice can be polarized to act as the death ray’s eye. This won’t cost much more than a reflective sphere. It allows us to dissipate energy we can’t handle. The turning problem is solved. We can handle kick back.


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  3. I’m 64th’ing the requests for a textbook, and I even have a starter forum for it…

    The University of Waterloo holds an annual physics competition, the “Sir Issac Newton” contest, or SIN for short. They try to make the questions funny and employ stick diagrams to illustrate problems: (4th link). Note that your raptor test (#135) would probably fit right in!

    If you pair up with these guys, you would probably become an unstoppable force in the realm of physics contests. And the students that give up their social lives to study hard and do well in this contest in the hopes of winning a scholarship might actually stand a chance of developing a sense of humour in their later years. I’ve got a book of about 10 years of SIN contests that I can pass along if you’re interested.

    Also I ‘get; all of your crypto jokes. Keep ’em coming 🙂


  4. I’ll sign the petition for the XKDC Physics Comic/Textbook 🙂

    It would be really great. I would get it even I’m from Paraguay. 😀


  5. [Patrick says: Also as Seb pointed out you would need a counter balance laser to handle the death ray’s kick back.]

    Considering the weapon in question is a laser (or high energy particle beam/plasma beam), would there even BE a kickback? To my knowledge, whenever I’ve seen a laser used in experiments, there’s never been a kick back or recoil.

    Would the sheer scale of the device in question actually end up producing this kick? Or would there be a kick at all? I’d be more concerned with the lens or focusing device melting under the heat of its own projected blast. I mean, if it’s a real Dyson sphere, wouldn’t the lens (I use that word for whatever would focus the energy in the direction of a rival system) be incredibly large? Even a small lens would be immense. (personally, I’m picturing a lens the diameter of jupiter, or larger.)

    I’m not sure on the mathematics or physics of something so large… is there someone well versed enough in theoretical particle physics who could comment on it? Would there be a kick from such a laser/particle beam? Or no?


  6. According to Newton’s third law, the sphere around the sun would also begin accelerating at the same rate as the light it emits. The “normal” sun emits light in all directions, however if this new system of the sphere and the sun emits lazed light in just one direction, it will move in the opposite direction.


  7. [Isaac Newton says: According to Newton?s third law, the sphere around the sun would also begin accelerating at the same rate as the light it emits. The ?normal? sun emits light in all directions, however if this new system of the sphere and the sun emits lazed light in just one direction, it will move in the opposite direction. ]

    So if the lens of the ‘cannon’ was fixed solid into the hull of the Dyson sphere, you’d have two forces against it wouldn’t you? You’d have the internal pressure of the focused energy pressing outward against it, as it passes through the lens, thus pushing it away… and at the same time, the ‘kick’ effect on the outside of the lens thanks to the actual emitted blast. I can only assume the ratio wouldn’t be 1:1, otherwise the lens (and by proxy, the rest of the sphere, since it’s solidly built into it) would be stock still. So, if the ratio isn’t 1:1, then one of the forces is probably stronger… Probably the kick effect, actually now that I think about it. It’d be somewhat like a regular gun then, with pressure built up inside, then released. The pressure inside doesn’t push it, but once released it does.

    Huh… Would love to see this on freakin’ Discovery. I can just see it. “Can we build a Mega Laser Cannon? Recent theories suggest we can!”


  8. In the normal setup (i.e. Earth, mirror and the sail, marked with 9P and 8P), there is at least one problem. After starting a number of sails this way, Earth will gain momentum and may be kicked out of orbit, and that’s then how to meet our doom 🙂


  9. Quote cjk:
    “In the normal setup (i.e. Earth, mirror and the sail, marked with 9P and 8P), there is at least one problem. After starting a number of sails this way, Earth will gain momentum and may be kicked out of orbit, and that?s then how to meet our doom :-)”

    Nah we’ll meet our doom because some other intelligent race will beat us to making an interstellar deathray like the one proposed here (that I’ve affectionately come to call “The Eye of Ra”) and destroy us.


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  11. You are a genius! Your idea for the death ray completely explains dark matter! All the aliens have decided to nuke [nuke doesn’t begin to cover it] the heck out of each other. The sun being covered by reflective material explains why we can’t see them! And since nobody’s figured out how to move beyond the speed of light, but everybody’s afraid that the other one is going to figure it out first, there are billions of solar systems out there which aren’t being inhabited. [everyone’s wiped out]

    And if anybody’s been wondering why we haven’t received any transmissions from other solar systems, it’s because nobody really wants to shout “shoot at me”.

    Of course, we’re genius’s enough to do it.


  12. i think that magician sayed hosseini is beter than criss angel and david blaine becuase sharifi hosseini the use mind and real people to do magic .but blaine and criss is unlimite in country but sharifi is limied by iran. becuase iran not importat magic and sharifi is do magic for love people iran only


  13. The lightsabers in Star Wars are not be able to move things other than another lightsaber.

    A 1.21 gigawatt laser will burn through everything dependning on the wavelength. It wouldn’t get the time to lift something.


  14. not sure if anyone touched on this.
    sorry, randall. Assuming elastic collisions, momentum is conserved, true.
    But so is kinetic energy.
    You can’t gain energy. Conservation of energy and mass.
    you would accomplish the same thing by making the shield black, and use the radiation from heat.
    Though it would be totally cool if this worked.


  15. I think your sun-turned-death-ray idea is awesomestandingtacular, but the main problem I see is that directing all the sun’s energy into a lasing cavity via a dyson sphere would invariably necessitate the not-so-healthy abscence of that sun-shiny warmth we’ve all come to know and love here on Earth… maybe a dyson half sphere? Or what if you polarized the dyson sphere? Then it’d be like we were wearing sunglasses ALL THE TIME! Then I could finally live my dream of being the next Blues Brother! Did I just digress there? I think I did…

    To digress even further – WD Velociraptor vs F/A-22 Raptor vs Jurassic Park velociraptor… who would win?


  16. You might find that stuff to be interesting. Might be the best scheme for solar powered lasers.


  17. The laser reflection lifting concept wouldn’t work on Earth because of atmospheric absorption. Even the best wavelengths are about 90% transmitted, so 0.9*0.9*0.9 etc a thousand times = 1.75e-46, really no beam. In only 30 reflections (15 round trips) the beam is at less than 5% it’s original strength.


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  22. In the normal setup (i.e. Earth, mirror and the sail, marked with 9P and 8P), there is at least one problem. After starting a number of sails this way, Earth will gain momentum and may be kicked out of orbit, and that’s then how to meet our doom 🙂


  23. Well, if the output by the lasing cavities actually made the sun start moving in the opposite direction, the gravity of the sun would cause Earth to follow it around. So then we could go taking trips around the universe, and what not.


  24. Dear people;
    We wouldn’t get kicked out of orbit by this any more than blasting off with a rocket would kick us out of orbit. Stop being silly and look at the mass of the Earth compared to the tiny momentum being added.

    There would be kickback because light has mass; lasers concentrate and focus light by bouncing it between a couple mirrors; when the light exit one end, it’s not canceling its force on that end, meaning there’s a net force on the other end. Since we’re firing the laser outward, the net force is inward; we would have to cancel it with an equal laser/force the opposite way.

    Sorry about the late post, I only keep up with XKCD blag through stumbleupon.


  25. (I am a french canadian and i’m in arts so I’m putting great effort to actually explain my idea lol)

    What I would do is a ship with multiple ”photon containers” around it. A photon container would actually be a crate with a tiny hole but an incurved surface inside to make the photon bounce indefinitly(until it loses all his energy). If you are able to calculate the resulting angles from the bounce and making it so that the photon never goes out the crate it will result in a great amount of energy(especially if your ship has a lot of them ) I will post eventually a blue print on my website to explain it better.


  26. It was my understanding that a laser elevator relied on the gas under the elevator being heated by the laser. This then raises the pressure under the elevator, creating an upward force. The pressure is the driving force for raising the elevator, not the momentum of the photons. This only works in an atmosphere, of course.


  27. Diffraction and Doppler shifts aside, wouldn’t most of the photon’s energy be lost to heat from imperfect reflection? An “infinite hallway” with mirrors at both ends appears to get quite dark after only about 10 reflections. Is there some new material with significantly better reflection; perhaps at a specific frequency?


  28. dunno about the dyson sphere…anyway, since it seems a heck larger than earth, from where do we get the material to build it?????


  29. there isn’t enough material in the entire solar system to make the sphere the sun consist of like 98% of the mas of the solar system it would be eaiser to make a giant sheild that would create a giant death ray blocking shadow that would protect the earth


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