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This question has been taken from Randall Munroe's book What If?: Serious Scientific Answers to Absurd Hypothetical Questions.
Randall Munroe left NASA in 2005 to start up his hugely popular site XKCD 'a webcomic of romance, sarcasm, math and language' which offers a witty take on the world of science and geeks. It now has 600,000 to a million page hits daily. Every now and then, Munroe would get emails asking him to arbitrate a science debate. 'My friend and I were arguing about what would happen if a bullet got struck by lightning, and we agreed that you should resolve it . . . ' He liked these questions so much that he started up What If.
Firstly, we should know that not everybody can see the moon at the same time. Either we can gather everyone at one spot and decide a time the moon is visible to all. Since, 75% population lives between 0 degrees to 120 degrees latitude, the best place for everyone to gather is somewhere around the Arabian Sea.
Let's take the quarter moon. It would be easier for us to differentiate between darker and lighter sides.
And a good one is good enough to hit the surface of the moon. The atmosphere may distort the beam a little, and may absorb some of it. But, most of the light would make it.
Let's assume everybody has a steady aim towards the moon and the light is spread evenly across the surface. At 1230 hours GMT, everybody would aim at the moon at the same time.
Still, it makes sense. Moon needs a considerable amount of light to light itself, a little over a kilowatt of energy per square meter. Since the Moon's cross-sectional area is around 10^13 square meters, it's bathed in about 10^16 watts of sunlight-ten petawatts, or two megawatts per person. 5 MW is way too less, we can try some alternatives.
Suppose, we spend the $2 trillion to buy one-watt green lasers for everyone and use it.
The green lasers we're using lights out about 150 lumens of lights; which is more than flashlights. This lights up the surface of the Moon with about half a lux of illumination.
if we compare, the full moon bathes up the Earth with about one lux of illumination.
With the advancement of technologies, we are going to use Nightsuns. They are the searchlights used by cops and Coast Guard helicopters. The beam is several degrees wide. Let's see what's the result is.
It's hard to see, but we're making progress! The nightsun is providing 20 lux of illumination.
Let's try IMAX projector array. It is a 30,000-watt pair of water-cooled lamps with a combined output of over a million lumens.
Still barely visible.
At the top of the Luxor Hotel in Las Vegas is the most powerful spotlight on Earth. Let's try that and give one of them to everyone.
Oh, I see. Let's try to add a lens array to each so the entire beam is focused on the Moon.
Yes, it's visible now. Looks like we have accomplished our mission. But...
Let's use the megawatt lasers developed by the Department of Defense. The Boeing YAL-1 was a megawatt-class chemical oxygen iodine laser mounted in a 747. Let's try and give this to everyone.
Looks impressive, ain't it? We used five petawatts of power, which is double the world's average electricity consumption.
Now, let's try a megawatt laser on every square meter of the surface of Asia. This would use up use up Earth's oil reserves in approximately two minutes.
The moon will shine as brightly as the sun.
The confinement beam at the National Ignition Facility, a fusion research laboratory is the most powerful laser on Earth. It's an ultraviolet laser with an output of 500 terawatts. However, it only lights up for a few nanoseconds which use the energy equivalent to a quarter-cup of gasoline.
If we give that to everyone and aim at the moon, it would turn the atmosphere into plasma, instantly igniting the Earth's surface and killing us all.
What would happen to the moon?
The laser would radiate a considerable amount of pressure on the moon. After a few years, the pressure would be enough to push it free out of the Earth's orbit.