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. All this Lunacy with Sails, and mirrors is just rubbish.
    Such technology is just too frail and hypothetical to be applied to space travel.

    But What i’ve got here , is something exactly the opposite. Somthing that can take you right up to the edge of Death and back. Its the new Lamborghini Murcielago Spyder Superleggerra GT28000 Coupe Roadster,
    with a VVV64 engine capable of producing 28000 BHp. And Just look at it , its beautiful, even when its blasting down the Jupiter-Io flux tube, hoverpads screeching. It does 0 to 6c in just under 5 seconds and 10c in half that time…
    Half!!
    i wouldnt be able to sel ‘Solar Sail”by that time.
    sure the handlings pretty stiff and its does, a lightyear per gigagallon, but itll get you where ever the hell your going faster than James May gets lost. even if you are a squirrel….

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  2. Rubbish!
    By the time anyone even starts to set off to where ever they want to go in that travesty of an Audi, theyll be stuck at a galactic filling station filling their tanks up, changing their tyres, cooling their engines, or calling the cops to report the theft of the thing when you went to get a spot of a drink.

    What you actually need for such a journey, any journey, in fact land ,water ,space or hyper is of course this Alfa Romeo 156c Galacticostrino-briggerish-er-ino . its light , seats four and doesnt look as though its a moving portrait of a bunch of Alterian Rhombic Crystals.
    Beneath the Vaccum-hood it has a 16-liter Old BMW X6 engine, thatll trudle about for lightyears all ..year, & not raise so much as squeak the whole journey.
    In the murky edges of space, all you need is a Reliable car, and this one is as reliable as the universe itself

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  3. So, James has Brought a can of tomato soup and Jeremy’s brought a Chainsaw without any controls on it.
    I meanwhile , i’ve done this sensibly and got a Porsche 911 Turbo GT7 rs Turbo Carrera S GT Turbo, the new one. It is the epitome of intellignet design. see those flaps? they control the lateral angle of the ride automatically, bringing fresh star-bred horse power to which ever hoverpad needs, whenever and wherever from the Triple turbocharged -40 Liter Flat 20 Engine. Yes it may be a diesel but under these circumstances its Perfect! The Sequential gearbox, the adjsutable suspension, the CrystalScan Sat-Nav, its beautiful!
    Its even got VAccum-conditioning! In a sports car!
    On top of all this is the cost , a mere 30 Altarian Dollars.
    Honestly this’ll be the only one back here next week.

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  4. unfortunately for the Top Gear presenters, the rest of the episode featured The Stig running circles around them in a Solar Sail.

    And on that bombshell, see you next blag!!!

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  5. Suppose we attack pluto… just suppose, and we find out
    that plutonium was not named after pluto, but pluto named after the vast reserves of plutonium on it. needless to say , Pluto, will make for a beautiful first target for the Death Star. Much more wicked that Alderaan.
    We could show the Rebel-Centaurians of the might they face here with minimal amount of energy.

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  6. Dantooine! They’re on Dantooine!

    “Theres a similar idea out there, Ion Propulsion. It doesn’t use a sail but it emits ions at extremely high velocities, we are talking .99C, and uses the momentum produced to accelerate the craft. The acceleration it’s self is slow but, over the vast areas of space could potentially propel a craft at quite large speeds.”
    Yes, but there you need to carry vast supplies of reaction mass in order to throw it into space. Ion engines have been around for years and are on several space probes, but they don’t match the cheap, limitless energy of the sun, plus you don’t have to drag all that mass around.

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  7. If part of the photon’s energy is transferred to the solar sail, is it possible that each reflection has slightly less energy than the last?

    In other words, if the photon had 100% of its kinetic energy before the first bounce, and .000000001% was transferred, then it had .999999999% left, and it would then take .000000001% of the .999999999%, and so on.

    It’s practically a moot point, since we only expect it to be reflected 1000 times or so, but if there was a way to continue the reflection up to a few million times, that would be something to figure into the calculations.

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  8. The problem of the Dyson sphere bouncing around (with reference to it’s position around its star) is easily solved. Use solar sail material of adjustable reflectivity (or opacity, perhaps?) on the inside of the sphere. By modulating the opacity/reflectivity, the dyson sphere can re-orient itself before anything disastrous (to the sphere, at least) results from its positional changes. The sphere itself is another sail, just a hollow one.

    Built this way, the sphere might even let through most of the light unless lasing were needed. We’d have a “dark moment” as the sphere formed a reflective shell and all the photon energy was released through the lasing aperture, then things would go back to normal. If we were really slick, we’d work out a material which could serve as a lasing, reflective, transmissive or opaque medium at our lesiure, so there’s no qualms about repositioning or focusing – we just need to send the right signals to the right panels of the sphere and pow – instant lightshow (or solar rave).

    The force of evil within me should stop now, or otherwise we’ll have a solar-sized disco ball on our hands.

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  9. Does anyone know if the Einstein field equations have been solved for the energy-momentum in a column of intense light between two parallel mirrors? What happens when you keep pumping energy in? Assume the mirrors are stellar in mass. It would be neat if the resulting spacetime could be exploited to decrease the travel time.

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  10. It’s all very interesting. You were talking about how diffusion would screw the whole thing up, but what if lasing cavities were placed at some optimal distance to prevent this? You could make a kind of space highway.

    Or death ray depending on which movie we are in. 🙂

    I could get rid of my squirrels. 🙂

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  11. Space highways would be interesting. Of course, you’d have to wait until your origin and destination are aligned with one of the lasing cavities orbiting around the sun (unless they’re in a Dyson sphere, where I suppose you could just rotate the sphere around/Luno’s idea). And there’s no getting off to go to the bathroom.

    “If we were really slick, we’d work out a material which could serve as a lasing, reflective, transmissive or opaque medium at our lesiure”
    Why do I foresee a giant Coca-Cola logo formed out of sunlight?

    “It’s practically a moot point, since we only expect it to be reflected 1000 times or so, but if there was a way to continue the reflection up to a few million times, that would be something to figure into the calculations.”
    Yeah, there’s no way they could be positioned precisely enough with any size of mirror we could feasibly build right now. It probably would converge down to zero, though, instead of suddenly vanishing at some point.

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  12. I thought they proved a while back that you don’t actually get momentum from light…

    My understanding that the whole theory was based on the vacuumed glass balls with spinning paddles. Which, incidentally, it turns out they’re not perfect vacuums, which is why they spin.

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  13. Cesium, you’re dead-on about the “space highway” idea. It would take tremendous initial effort to build the spheres in the first place, but once you’ve built them, you’d be able to develop a pulsed laser energy beam matrix between neighboring stars that interstellar ships could use as an energy source.

    If, like me, you aren’t a laser guru, google “300TW laser” for more info on pulsed beams that are on the cutting edge (no pun intended) in both Michigan and China. It appears that femtosecond (haha, Firefox doesn’t spell-check that as valid!) pulsed lasers are the hot shiznit. With pulses so small, it should be possible to concentrate beam power for a very-brief moment, then resume daylight as usual without anyone noticing.

    I have to admit, earlier in reading the comments here, I was first thinking about an “ordinary” Dyson sphere around Sol, and wondering if the teaming masses would object, to wit: “Hey, we were using that! You can’t turn off our sun!”

    To integrate our ideas, let’s take my “smart material” and stretch it on the scaffolding between the geostationary satellites (thanks, Cesium), forming a giant buckyball (thanks, Mr. Fuller) around each star within our navigational matrix (thanks, Wachowski brothers – no, waitasec).

    Now we not only have a source of energy for interstellar transit, but we also have a damn fine way to defend our solar systems if anybody starts itchin’ for trouble. [Yes, we use femtosecond pulses for transit. Wanna see what a full second of solar output does to your ship, mister nasty alien?]

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  14. Cool, but I can see this going like the giant mirror in space designed to reflect the sun (and cool Earth) used in Futurama. A meteorite hit it, rotating the mirror and sending a ray of deadly amplified sun across Earth.

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  15. Maybe nobody read this or It’s have been said before but, Why put a planet in the system? It’ll be possible to put something in the “fixed” mirror to use that 9P? Maybe a mirror-propeller? Is this the beginning of the First Solar Pedal Boat?

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  16. Off topic I know – Could you possibly hold and store energy (such as 1.21 Gigawatts) if you had two mirrors continually reflecting this light or laser or whatever back and forth? Imperfections in the mirrors and all practical flaws barred from consideration.

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  17. I would assume so, but it’s the practical flaws that get you. The mirrors would have to be perfectly aligned and 100% reflective, and there’d have to be perfect vacuum between them, which is completely infeasible.

    “It appears that femtosecond (haha, Firefox doesn’t spell-check that as valid!) pulsed lasers are the hot shiznit. With pulses so small, it should be possible to concentrate beam power for a very-brief moment, then resume daylight as usual without anyone noticing.”
    But those are mostly for precise applications (and to conserve power, but the Sun isn’t in danger of running out soon). I don’t foresee that being much of a concern for an interstellar weapon, and once you get pretty far from the sun every femtosecond of light matters.

    “Cool, but I can see this going like the giant mirror in space designed to reflect the sun (and cool Earth) used in Futurama. A meteorite hit it, rotating the mirror and sending a ray of deadly amplified sun across Earth.”
    Except for the astronomical (pun intended) mass of any such mirror made of currently known materials. Momentum would prevent it from rotating that quickly. Although NEOs (thanks again, Wachowski bros) would present a danger of smashing the mirror and earning seven years of bad luck (but is that an asteroid year or an Earth year?).

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  18. 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.

    This effect in fact exists. A rotating asteroid will be spun up (or spun down, depending) by photons absorbed from the Sun, being reradiated as heat (infrared). This changes not only the spin of the asteroid but its orbit as well. If it weren’t late at night I might even remember the name of the effect…

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  19. So coincidently a friend and I were discussing a similar idea today. We were handling the question of what to do if we needed to evacuate the solar system in say the next million years and wanted to send more than a few generation ships. The real problem here is where does the momentum come from (since you need to push off of something). The easy solution is to use Jupiter as fuel, but this might not be too efficient. The better solution we came up with is build a giant parabolic reflector behind the sun and use it to push a solar sail (actually you can absorb the energy and use it as a power source too). Our rough calculations say that you accelerate at about 10^(-10) m/s^2 meaning that you travel your first light year in about 100,000 years. The only problem is that you still need to drag Jupiter in order to decelerate once you get there…

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  20. There’s an idea I’ve read (sorry, can’t remember where) that lets you decelerate your solar sail when it gets where it’s going.

    The idea is to bring along another mirror (or use part of your primary mirror). When you’re wanting to decelerate, you turn your sail around, throw the deceleration mirror behind you and let it reflect the laser beam from home back at your sail.

    The deceleration mirror accelerates away, but who cares? Your sail slows down.

    Perhaps you could use this idea for increasing thrust. Instead of leaving the reflecting mirror in the solar system, bring several along with you. Drop a new one when the last one is too far away to reliably focus.

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  21. Given that the Alpha Centaurians have about a 2 billion year head-start on us, technologically speaking, there’s a good chance they are already aware of the potential death-star uses of the various spare stars they have laying about otherwise useless. We are relatively resource-contrained for stars compared to Alpha Centauri, and we’d have to go a long ways to get additional stars to use for sending intergalactic fuck-you messages. As such, I really think it’s in our collective best interest NOT to attack Alpha Centauri. Frankly, they’d probably kick our ass. We should probably stick to bullying smaller things, at least until our technology is better able to support our bravado. Like squirrels.

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  22. “There is no life Beyond your own planet”

    — Disembodied Radio Voice On Unlooked For Frequency

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  23. @Allabouttrust: Preemptive strike!

    @Doug Weathers: The problem with this is that it takes a long time for a solar sail to produce any appreciable acceleration, and you’d likely be moving at a pretty decent clip by the time you want to slow down, so dropping a mirror behind you (in your direction of movement once you’ve turned around) would most likely only lead to a collision with the mirror.

    @Luno: Great. Now I’m going to be reading about that when I should be studying for a philosophy exam. =P

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  24. Why couldnt we rather use the Dyson Sphere to make earth habitable or something other useful? We could cool down Venus a little, so we could live there too 😀

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  25. Hi Euler,

    You said:

    “The problem with this is that it takes a long time for a solar sail to produce any appreciable acceleration, and you’d likely be moving at a pretty decent clip by the time you want to slow down, so dropping a mirror behind you (in your direction of movement once you’ve turned around) would most likely only lead to a collision with the mirror.”

    Nope! At the moment you detach the deceleration mirror, it’s moving at the same speed as the rest of the starship. No danger of collision. In fact, since you’ve probably gave it an impulse to get it away from you, it’s heading away from your ship (towards your destination).

    The laser beam is no longer accelerating you – you’ve tacked out of it so the beam can hit the deceleration mirror.

    Now the beam is hitting the deceleration mirror. It’s accelerating away from you, and reflecting the beam back at your sail. In reaction, your ship is decelerating.

    This means that your ship, and the deceleration mirror, are moving *apart* and not together.

    It’s true that having two huge sails close together will be ticklish. It might make sense to wait until the deceleration mirror is a few radii away before unfurling it.

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  26. what about mounting the laser on the ship? sort of like pointing a giant fan at a sail??

    that way you don’t need … laser stations and diffraction shouldn’t be a problem, but the onboard nuclear plant might be…

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  27. “what about mounting the laser on the ship? sort of like pointing a giant fan at a sail??”
    No net force on the system. Nothing moves. Sort of like sitting at one end of a cart and pushing on the other end — nothing happens.
    Actually, on second thought, the reflected photons would have momentum backwards. So you’d have some thrust. But it would be less than the thrust you get just by pointing the laser backwards and firing off into space; think about it. And there you can’t use two mirrors, because subsequent reflections pretty much cancel each other out. It might be worth it to mount a laser on the ship in the middle of interstellar space, but I think the mass requirements of a laser and power plant would exceed the benefits of the extra thrust.

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  28. Apropos/Cesium, the energy has to go somewhere, so the cart stretches. 😉

    Seriously, that’s a fine example of a closed-system problem. It’s like sticking a propeller and a generator on the back of your car, and then saying “I get free electricity from the generator!” No, you don’t. Just check your mileage, chum.

    > But those are mostly for precise applications (and to conserve power, but the Sun isn?t in danger of running out soon). I don?t foresee that being much of a concern for an interstellar weapon, and once you get pretty far from the sun every femtosecond of light matters.

    The idea for propulsion is to modulate the timing of the solar-laser to small (in the time-domain) pulses, so we don’t fry our target ship. As has been said here, the sun (or any other star) isn’t short on power. If I mirrorize all but one panel of my Dyson sphere and turn the remaining panel into a lasing cavity, even the MOST reflective surface that we target will get really hot, quickly. Without super-short pulses, I don’t think the sailors would survive for long. It’s kinda challenging to shed heat when there’s no atmosphere (except the one INSIDE the ship).

    The idea for the weapon is to modulate the timing of the solar-laser to either a stream or a high pulse rate, so we fry our target ship. Presumably, we’d figure out a way to overwhelm their cooling systems (guessing that they might also have figured out a highly-reflective surface). To be crass, we’d simply bombard them with energy until they pop like a bottle of coke and mentos.

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  29. 1.21 Gigawatts.

    Precisely what is needed to travel Back to the Future.

    Lasers are the key to time travel.

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  30. You could get Mr. Fusion to lift the squirrel. It’ll be available from Black and Decker in about 8 years.

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  31. I’m all for an xkcd textbook too, but even with so many refractions, why lift the squirrel? So it could pee on the new textbook?

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  32. ahh, cheez…

    since when do physicists need a reason to do things?

    “Let’s spend billions of dollars building a mammoth machine so we can smash particles together at uber-wicked speeds!”

    How could one possibly be skeptical of that?? When the novelty of it is so strong!

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  33. Why not just take a laser and mount it on a ship with a mirror on the back. then fire your laser ate a second mirror previously released into space and placed very close to your ship. you could release new mirrors when ever the energy loss became to big.
    Laser—-()
    Laser—-(–)
    Laser—-(—-)
    Laser—-(——)
    etc.

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  34. What if you just built the dyson sphere around the squirrel?

    (do I have a potential career in middle management or what?)

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  35. Wow, I’m late on commenting, but a thought occurred, and I had to bring it up, since I personally lack education in all forms:

    Could bouncing the photon around like in Meyer’s paper increase output on a solar panel? Or is this already done? Because it seems to me that this would be an easy way to increase the efficiency of solar power, something that is much needed, with dismal efficiencies of 14% being the norm, for commercially available models.

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  36. “The idea for propulsion is to modulate the timing of the solar-laser to small (in the time-domain) pulses, so we don’t fry our target ship. As has been said here, the sun (or any other star) isn’t short on power. If I mirrorize all but one panel of my Dyson sphere and turn the remaining panel into a lasing cavity, even the MOST reflective surface that we target will get really hot, quickly. Without super-short pulses, I don’t think the sailors would survive for long. It’s kinda challenging to shed heat when there’s no atmosphere (except the one INSIDE the ship).”
    Right, but if you’re a couple lightyears away, I don’t see much danger in frying the ship.

    “What if you just built the dyson sphere around the squirrel?”
    Then it wouldn’t be a Dyson sphere. Unless the squirrel were in space and sustaining nuclear fusion, in which case it would probably be dead, and something weird going on.

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  37. Seriously, constructing a dyson sphere is completely impractical. Not to mention that light of that intensity would fade and shift way to much to be a practical death ray. even to alpha centauri it would take way to long to be a practical weapon. You would have to gurantee that peace wouldnt break out within the initial use of the weapon and the time it takes to hit. also the death ray would have to be in continual use unless you decide to use it it power something somehow. I wouls suggest mining to be a much better aplication for the “Death Ray” melt the surface of a planet so materials are easier to exract, not to mention global warming could easily be dealt a “fuck you” Not to mention solar flares would DESTROY the sphere. The high energy particles that are emitted by the sun would fuck with anything you contstruct around it. and a magnetic field large enough to negate the particles would utilize a TON of energy considering that you would need to ensure that its intensity constant and very very high. deflecting millions of particles isnt exactly energy poor work. beyond all that you could i suppose use this technique to construct stellar transport that is relatively cheap and then use surface to station transports.

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  38. That’s great, bouncing the light back and forth and all but there are two potential problems.
    1> The planet’s going to move.
    2> What happens when you’ve slowed down all the photons and changed the speed of light?

    Duh. Oil crisis all over again but with light. LEAVE ME LIGHT ALONE!

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