On an utterly fascinating episode of Mythbusters, Adam and Jamie visited an observatory wherein a scientist pointed a laser at a Retroreflector on the Moon. Using sophisticated equipment, the scientist detected a signal coming back from the Moon, proving that, yes indeedy, somebody had been there and left the reflector there.
The laser pulse that the scientist sent to the Moon was some ungodly number (I think Jamie said it was in the quadrillions) of photons; they got about 3 photons back.
My question: how much light would they have had to send up there for Adam & Jamie to see the reflection? Would it even have been possible given current technology (and perhaps energy resources)?
Also, had someone been standing on the Moon when Adam & Jamie’s scientist friend sent up the laser pulse, would they have been able to see it (assuming they were in the right place)?
Well, theoretically, the photoreceptors in the retina respond to single photons, so there’s a chance that at the level used by the Mythbusters, a single molecule of pigment might catch one of the reflected photons and trigger a neurochemical signal to the optical nerve; however, there are neurological filters active that’d weed out such a signal, otherwise, our vision would be overcome by noise in low light conditions. So you’d need a number of signals in a certain time period to actually get a conscious reaction from it; how many, however, I couldn’t tell you. It’d be a few times the intensity the Mythbusters used, probably within one or two orders of magnitude; however, to actually notice this signal, you’d have to find a way to block out every other conceivable light source, which you probably won’t be able to do; by how much you would have to increase the intensity to get an actually visible signal back, I couldn’t tell you.
Problems you run into. First you’re going to need to make the return signal brighter than the moon itself. Otherwise the light will be obscured by the moon’s light.
Second is that the signal spreads out over distance. Even lasers aren’t perfectly columnated and spread. That’s why so very little amount of the original signal is returned. So you send a pulse 2 mm wide and it reaches the moon smeared across 5 square miles (my random guess*), then the reflected light from the retroreflector has an area of, say, 10 sq feet, which then is smeared across 3 counties when it reaches the ground. So I don’t think you would see a point source on the Moon, just an extra brightness, maybe with a green tinge (since they used a green laser).
*Maybe someone will come post with actual numbers.