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. Easy, build the sphere outside Earth’s orbit. Sure, it would take more material, but once we’ve built one lasing cavity and fried Alpha Centauri, we can take their resources at will.


  2. Hey! Nice interblagsite you got here. Love the comic, the alt texts, and these posts. Its intelligent and fun. Funtelligent?
    Anyway, I was reminded of this blog post when I read this comic:
    Hmmh, all these webcomics are interwoven in ways incomperhensable to us, beings with limited intelligence.
    Keep up the great work, and take it easy. 😉


  3. i don’t spend many time thinking. by the way, i report a red news about a controvercial site, it enable rich women to have more chance of finding handsome and charming soul mates.


  4. the problem with using angled mirrors to trap the photon is that you’re talking about moving one of the surfaces away from the other, so the photon would get bounced out as the focal point changes – possibly you could use separate mirrors joined by a hinge, so that some pistons could alter the angle as the platform was projected….


  5. I’m not seeing why the low efficiency of a solar sail is a problem. It’s not like there’s any shortage of energy, and the low force isn’t much of a problem, either.

    A vehicle that carries its own power source usually has to worry about at least one of power and total energy. A vehicle that gets energy from the sun doesn’t need to worry about total energy, and despite the low efficiency, power is unlikely to be a problem.

    The low force is an issue if you’re on a planet and need to overcome gravity, but once you’re in orbit you just *some* acceleration, even if it’s only 0.0001g. You’re not time limited: you can just carry on applying that acceleration for as long as you need.


  6. Well, because at 0.0001g, it would take as long to get around the solar system as to get to the nearest stars at the speed of light. Then you might as well launch an unpowered craft from orbit and coast there.


  7. This is the best thing I’ve ever read. On the internet, at least.

    Dammit, now I want to get a physics degree! All this anti-liberal-arts peer pressure is getting to me!


  8. Crap! I missed out on so much when I skipped science in Alpha C. … However, I think a 3D interactive tutorial, (rather than a XYZD?), book will do. My hat is out to you humor filled terrestrial science gods.


  9. I couldn’t possibly read all the comments until the latest so I’ll wager against a fact that this suggestion has been posted: I read somewhere that Solar Panel turn about 20-25% of the light energy to electrical one so instead of trying to reflect light more efficiently try to absorb it (as much as possible without taking an overheating risk) and shoot the photons the opposite direction of designated movement direction. in other words, this has the potential of pushing a body at 1/4P which is better than rebounding of nearby planets with the multi-reflection idea.

    just a thought anyway…


  10. Y’know, it looks like Pong to me. Why have interstellar war if you could play the most epic game of Pong ever conceived?


  11. “Y’know, it looks like Pong to me. Why have interstellar war if you could play the most epic game of Pong ever conceived?”
    Pretty hard to move the paddles.


  12. @Arancaytar — they must’ve somehow heard about our plans and set this up in case — obviously they can go faster than light and change the past….


  13. Very interesting, I read this a few days ago and since then i keep on thinking up new applications for this and ideas to extend it. i had heard of this idea before, but the way you re-explained it really inspired me. Thanks, please post more of these whenever a concept captures your imagination, i’m sure it’ll capture ours too.


  14. @the baeinstitute thing: Wait. They’re actually focusing the laser onto the “sail”? That must be hard to adjust. Also, you can’t really keep the laser in orbit while aiming it at Mars. And “photonic” is really redundant with “laser”.


  15. Maybe we could set up a business partnership?
    Also, feel free to plug your lasers in to my mains feed to my shed, as Ive wired it into the neighbor’s supply anyways.

    On a technical note, providing the squirell with folded-paper wings may provide a means to reduce the effects of thru-mirror heating (animal flaps wings, possibly providing course corrections in the first few seconds of launch). As the squirell travels further up through the atmosphere, the reduction in air pressure will negate this effect somewhat, but at least it will be cooler.
    If this proves difficult, consider a water cooling arrangement, but this may require the squirrel to have a polished bottom carefully mated to the sail with a good quality thermal tranfer compound. (I can supply such animals to special order)
    I recommend consideration of diverting the bowel output of the aformentioned creature as any pressure betwixt the sail and theanimal may cause an uncommanded separation of the sail and animal, leading to a total loss of the platform and possible damage to the squirell. In the microseconds following seperation I predict a sudden increase in bowel output that may hasten the demise; this might be mitigated by some form of diet restriction in the pre-launch phase. I propose therefore to supply squirells in an unfed state.


  16. It occurs to me that a Type-II-civilization death ray would be sending the message “FUCK YOU GUYS!” … no matter how it’s modulated.


  17. Laser elevator..probably not…laser shocks for a car? maybe…since you’ve got a lot more bounces in smaller spaces. but i r not physics person. What you guys think?


  18. The rutherford lab in oxford UK has a Petawatt laser, you could probably lift quite a few squirrels with that one.


  19. “Laser elevator..probably not…laser shocks for a car? maybe…since you’ve got a lot more bounces in smaller spaces. but i r not physics person. What you guys think?”
    You’d need very reflective surfaces and shock-resistant lasers, otherwise you risk destroying the laser or burning a hole through your car.


  20. Disclaimer: I have no idea what I am talking about most of the time.

    Thus I pose to the experts..

    “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.”

    “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?”

    Is there a way to use this self-guiding technique to slightly alter the direction of the light? Could you use an external source? e.g. I think black holes bend light don’t they? Some other source of gravity put near the photon for just long enough to alter it’s trajectory so it is perpendicular to both mirrors?

    Step one: Perpetual motion
    Step two: ???
    Step three: Profit!

    Enjoy your pondering.


  21. Also, if you didn’t want to have to get the timing perfect on the large mass influencing light’s path, you could keep a constant laser, move the mass close, remove it, then shut the laser. Some part of that stream was caught in between.

    See above disclaimer if I am totally off.


  22. I would prefer if the laser would be modulated showing the physics equations that are killing them at that very moment.


  23. I failed to mention on my other post that I spent the last 3 hours reading every comment made. I am currently mind numbed at this train of thought and plan on coming back and posting more useful and slightly more realistic thoughts.

    p.s. Using magnetics you would have the capability of moving and aiming the sun itself, therefore negating any random pushes, or loss of energy in the Centauri cannon. *You shoot in one direction, and use full sphere magnetics to push the sun back in to alignment.*


  24. You’ve managed to find a proper application for a Dyson sphere!

    But, theoretically, wouldn’t the freely reflected energy (provided it was a small enough/ reflective enough sphere to create the aforementioned death ray) have an adverse reaction with the fusion of the sun. Put simply, wouldn’t you, uh, you know, ‘blow up’ the sun?

    … not that thats a bad thing.


  25. I only made it about a third of the way through the preceding 339 comments, so this might have been mentioned before. However, in response to petschge*:

    This is a very good point. As a materials chemistry student, working in occasional collaboration with NASA Langley, making highly reflective, space-durable thin films, that squirrel is fucked. We make great films, and about the best I’ve ever seen was 90-92%, relative to an aluminum mirror whose absolute reflectivity was 92%. 0.92*0.92 = 0.846…

    Some light will be diffusely reflected (as opposed to reflected straight back [specularly], which is what we measure). This is good news for the squirrel. But some must be absorbed, presumably by the metal layer that’s doing the reflecting, as electron microscopy indicates the breaks in the metal surface are smaller than the wavelength of light.

    Since we can make the films highly electrically conductive, which implies thermal conductivity, we could just mount some major heat sinks, right? Well, even then I expect we can’t get to your 99.999% power dissipation requirement, and the squirrel will probably get toasted.

    I think the solution is to make a robotic polyimide squirrel. It would then be at least as thermally stable as the elevator itself, and also, be a robotic squirrel.

    *(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))


  26. Above, a woman named Alicia says you should write/draw a Physics textbook. I agree, wholeheartedly! I did okayish in Physics, but if there had been more squirrel-levitation and fewer hopelessly dull primary-colored line drawings of various planes, solids, and simple machines, I might not have spent most of class doodlng in the margins of my notebooks.


  27. P.S. Your Captcha is very entertaining (at least, to me). I just cycled through about five sets and each of them is a good character name (e.g., “Doris Wall”) or serious issue (“Nader Quarantine”)

    VVVVV”Station Mr”VVVVVVV


  28. A decent material for large-aperture solar-pumed lasers is Nd/Cr co-doped YAG ceramic.

    I make a similar material…it has the potential to be fabricated much larger than single crystals can be, and to be more transparent. The problem is getting a large enough volume, with good enough vacuum and high enough heat; these problems cease to exist in orbit.

    It might also be worthwhile to use reflectors to concentrate the light, even if you don’t get around to building a full-on Dyson sphere.


  29. As to Mr. Munroe drawing a physics book…some free-content options exist, with fairly light illustration. Wikibooks comes to mind…



    Sine the late 50’s (prior even to the invention of the laser) alarmist military strategists have had to exhort the need to protect the ‘high, nutty ground’ of the upper atmosphere. This region of security instability extends back to our primordial ancestors, who combated (and lost) their battle for the forest canopy with that most vile and evil of axis-oriented species… the squirrel.

    it took an amusing accident involving five glasses of vodka and a small pitcher of unsymmetrical dimethyl hydrazine to cause a dog to ‘accidentally’ be loaded onto Sputnik 2. While the US spent tens of billions on simulations of particle beam weapons for enticing funds from Congress for further simulation funding, the Soviet engineers were able to develop a laser capable of lifting a squirrel 50,000 angstroms with only a 99.7% ablation rate.

    Even as we speak, the Chinese are hard at work, violating the unratified Strategic Squirrel Use of Outer Space treaty, developing their own means of launching rodents that could chew through the wiring and insulation of the International Space Station! THIS MUST NOT BE ALLOWED!

    The Civilized, Free and Psychopathic World Order must stand united in our relentless struggle to Regain The Treetops, and return the balance of power to the lab rats – where it has been from the bold days when Isaac Newton cracked the infamous Squirrel Code, and gave hairless monkeys the stars!


  31. There is an error in the calculation for the sun-sail: The transfered momentum is not two times the momentum of the photon. As you said it looses some energy – so it has a lower freuquency – and so the absolute value for the momentum is in fact lower for the reflected photon.


  32. Better Yet!

    Add a gyroscope to the ship and simply bounce the same laser back and forth between two mirrors


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