Yay math! For the record, "giant rocket thruster burning for millions of years" seemed like a pretty stupid solution to a pretty predictable problem. But I dug the geeky nerdy giant number-ness of it. Thus, your answer is awesome. And while I'm right there with ya on the whole "death ray" aspect of it, and am not going to quibble about the orders of magnitude of doom projected, I do think there are some errors on your math. So let's presume that 1368 W/m^2 falls on the earth. That's an "at noon" number for the sun directly overhead. However, when it's noon in New York it's sunset in London and sunrise in Alaska - total energy does not fall on the surface of the earth uniformly, but rather proportionally. Our actual number, then, is not 68e6/(AreaOfEarth/2) but 68e6/piRadiusOfEarth^2) = 1.75e14 W. Whack 12 digits off that and you've got 175 terawatts hitting the earth at any given time. You get .13 terawatts. I get 175 terawatts. Actually, that is* four orders of magnitude. Do me a solid and double-check my math, wouldja? I gotta get back to work.
Oh dear... My brilliant post was actually pretty darn stupid. I had managed to grab a completely erroneous figure for the surface area of the earth - I didn't check my units, and used square miles instead of square meters. You're absolutely right about light falling on the surface of the earth proportionally, and thus you're absolutely right about that being the correct way to math this all out. First off, let's correct my own math, accounting for the new, proper usage of units. Keep in mind that all of this is wrong, due to light falling proportionally. Wattage supplied to the earth by the sun: 1368 * ((5.101 * 10^14) / 2) watts = 348,908.4 terawatts 99.9% efficiency spillover: 68,000,000 / ((5.101 * 10^14)/2) * 0.001 = 266 watts. Time to boil cube of water: N/A 99.999999% spillover: N/A Ok! Now let's do the math while taking into consideration Klein's point about the proportionality of light falling on earth: Wattage supplied to the earth by the sun: 1368 * (3.14159265 * 6378100^2) watts = 174,831.071 terawatts 99.9% efficiency spillover: 68000000 / (3.14159265 * 6378100^2)*0.001 = 532 watts Wow, what a difference! 532 extra watts, while over 1/3rd as bright as the noonday sun, definitely won't blind you instantly, and definitely wouldn't boil you or any water nearby! Your question about what it would look like is entirely valid, and I can't give you an answer better than "bright." It would probably still kill us all, eventually - Assuming it runs day and night, it'd add the equivalent of 1064 extra watts of daylight to every square meter on earth. My knowledge of climate suggests to me that nearly doubling the amount of heat supplied to the earth would be "bad" - Basically defeating the whole purpose of installing the giant light death beam in the first place. Another fun thought experiment would be to imagine what would happen if you pointed this "engine" at another, earth sized planet. 68000000 / (3.14159265 * 6378100^2)*0.999 = 531,547 watts per square meter. Assuming half of it is infrared, that's enough energy to boil everything on the surface of the planet in less than a half hour, even assuming an average starting temperature of 0C/32F. (Again using some mathematics from my game) While I suppose it makes an acceptable engine, it makes a truly terrifying weapon. Anyway, I guess this all just goes to show you - Don't instantly believe the confident guy, particularly when he starts spouting math! Check his claims out and see if they hold water. Cheers!
A couple points:
1) Just want to make perfectly clear you know I'm not Robert Zubrin. because while that would be dope it just ain't so. 2) Your 532W number is presuming 6 nines of efficiency. Reality begs to differ. I think we're still pretty squarely in "holy shit death ray" territory, particularly considering some process or other needs to turn mass into photons and it's going to be terrifying. We're going from (Niagara Falls is 2400 metric tons per second) To ...at which point I don't even care where it's pointed - the engine of doom necessary to drive the conversion scares the shit out of me. I'm almost certain I've seen this movie...Your question about what it would look like is entirely valid, and I can't give you an answer better than "bright."
about 3,780 metric tons per second, equivalent to a modest river 120 meters wide by 30 meters deep, following along at a leisurely 1 meter per second
an exhaust velocity of 60,000 meters per second
Another fun thought experiment would be to imagine what would happen if you pointed this "engine" at another, earth sized planet.