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Lasers? To the Moon?

I was talking with friends today about an earlier blog entry I wrote on green lasers, and one of them speculated that at some point someone would use a laser to illuminate the moon. With a laser powerful enough, we could imagine a logo being painted on the surface of, say, the new moon, which would be amazing (the first time) and then annoying (thereafter).

An artist named James Downey thought of having millions of people simultaneously point their lasers at the moon, but that idea has been debunked:

It seems there are not enough people in North America to make Downey's idea work.

"As I suspected, the number required is not millions of people, but more than millions of millions of millions of people," said Donald Umstadter, a laser expert at the Center for Ultrafast Optical Science at the University of Michigan, Ann Arbor.

Okay, so, since I don't have the physics background to estimate the numbers myself, and since more detail wasn't provided, let's make some assumptions:

  • Millions of millions of millions equals quintillions.
  • The typical handheld laser produces 5 milliwatts.
  • 1 quintillion times 5 milliwatts equals 5 * 10^15 watts, or 5 petawatts.
Now, as it turns out, a 5 petawatt laser does exist: the aptly-named Petawatt, at Lawrence Livermore National Laboratory in California. This is deceiving, however, because the Petawatt fires pulses that last less than half a picosecond. To operate the laser continuously would, in theory, take 1,200 times the entire power generating capacity of the US -- and that was at 1.25 petawatts. Presumably 5 petawatts would consume 4,800 times the US power generation capacity.

So could someone paint logos on the moon with a laser beam? It's theoretically possible, and the laser exists to do it, but there's not enough power to make it practical.

A better idea -- okay, a less crazy idea -- would be to position a mirror in space capable of reflecting the sun's rays into a point on the moon that would be visible from Earth. This would obviate the need for power generation and would dispense with the problem of the laser's power attenuating as it rises through the atmosphere. The question is, how big would that mirror have to be?

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