what if: millions of laser pointers aimed at the moon....

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Steve K

Steve K

Flashlight Enthusiast
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hi gang,

the folks at XKCD took a shot at answering the question "If every person on Earth aimed a laser pointer at the Moon at the same time, would it change color?". The analysis starts out with 5mW laser pointers, and bumps up the power level over and over, until it reaches ludicrous levels! Very interesting reading. 🙂

http://what-if.xkcd.com/13/
 
It is amazing to think of how much power is needed to even know if your laser is hititng the moon. I think the MythBuster guys did one story where they got an observatory to aim the observatory's laser at some retro-reflectors on the moon that one of the Apollo missions left behind, and then detect the very tiny signal that was reflected back. There's a YouTube of this part of the episode.... I think they said that the laser could produce a pulse with a peak power of a giga-watt or so.
http://www.youtube.com/watch?v=VmVxSFnjYCA
Very cool stuff!
 
..."If every person on Earth aimed a laser pointer at the Moon at the same time, would it change color?"...
Click to expand...
It most definitely would not change color. With enough power perhaps it could be made to appear another color but then when you turned them off you'd find it was still that same old moony color that we're used to.

To really change the color of the moon with lasers you'd have to have enough power to actually burn the surface somewhat.

Er...literally. 😀
 
Yeah, I remember that episode, they used a whole lot of power, and were able to count a measly few photons of 532nm returning from the corner cube reflectors
 
You can get an idea of the amount of light energy required to change the apparently color of the moon. Elsewhere I am well known for my 'thumbnail calculations', so try this one.

The solar Flux on the ground at the equator on earth is about 750w/m^2. The moon is essentialy the same distance form the sun as the earth, so the solar flux on the moon would be about the same as it is on the earth (actually somewhat higher because there is no atmosphere on the moon).

The diameter of the moon is about 3500 Km, So the area of the apparent disc to illuminate would be 1750km x 1750km x pi kilometers^2. Roughly 10 million square kilometers or 10^14 square meters, making the solar incoming energy on the moon about 750 x 10^13 watts. There are roughly 10^10 inhabitants or earth, so to just reach the same order of magnitude as the sun shinning on the moon, you would need each earthling to point a laser with an energy of about 75,000 watts at the moon, to match the solar flux, 750,000 watts per earthling. That of course ignores the problems involved in simply providing sufficient energy to power all of those lasers, let along building enough multi-megawatt lasers. This of course ignores amospheric losses, which would only make things worse.

When I was a University Student, I once had a homework problem in thermodynamics called the 'meaning of never'... This one falls within the parameters of never. You cannot generate anywhere near enough light energy to even change the apparent color of the moon while illuminating it with millions of lasers, even if each one has output in the megawatt class.
 
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mattheww50 said:
You can get an idea of the amount of light energy required to change the apparently color of the moon. Elsewhere I am well known for my 'thumbnail calculations', so try this one.

The solar Flux on the ground at the equator on earth is about 750w/m^2. The moon is essentialy the same distance form the sun as the earth, so the solar flux on the moon would be about the same as it is on the earth (actually somewhat higher because there is no atmosphere on the moon).

The diameter of the moon is about 3500 Km, So the area of the apparent disc to illuminate would be 1750km x 1750km x pi kilometers^2. Roughly 10 million square kilometers or 10^14 square meters, making the solar incoming energy on the moon about 750 x 10^13 watts. There are roughly 10^10 inhabitants or earth, so to just reach the same order of magnitude as the sun shinning on the moon, you would need each earthling to point a laser with an energy of about 75,000 watts at the moon, to match the solar flux, 750,000 watts per earthling......
Click to expand...

and not to mention that half of those people are on the wrong side of the earth! I suppose you could just send their lasers over to someone on the other side of the earth, though...

has anyone run the numbers on what it would take to light up a spot on the new moon sufficiently for someone on earth to see it with the nekkid eye? Should be quite a bit easier than competing with the sun's flux, but still quite a challenge.
 
One of the first things that scientists did with the newly invented laser was to point it at the moon, which was already being done with radar. Hitting the moon with laser light was so important, they took a heavy retroreflector up for the first landing! If someone was on the moon, they'd be able to see the light from a few watts of green if properly collimated with a telescope. These days, that laser could easily be battery operated and attached to a really cheap telescope.
 
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