Suppose for a moment the Earth had no atmosphere and I took a laser pointer (like the ones you use to play with your cats) and pointed it at the moon. If there were another person standing on the moon, right where I was pointing at, would they be able to see the dot?
Almost certainly not.
The small, low power laser pointers we have aren’t going to hold a tight beam for very far. You can even start seeing how diffuse the ‘dot’ gets across a large room.
Even for the high power ones NASA and other scientific organizations use to hit the moon, it’s unlikely you’d see it (especially the infrared lasers which aren’t visible anyway).
Here’s an article about it. They have to deal with atmosphere, but even without atmosphere, the beam wouldn’t stay very tight across that distance.
Laser beams aren’t perfectly parallel so there’s still some spread over long distances. The massively powerful laser they use to hit the reflectors left on the moon still only return enough photons to be barely detectable by sensitive instruments.
Relevant XKCD What-if: Laser Pointer
According to Wikipedia, when the laser is reflected back to earth, “The reflected light is too weak to see with the human eye,” But there’s no mention of whether it might be visible to someone on the moon.
I’d also presume that “the ones you use to play with your cats” are a LOT weaker than NASA’s.
Typical consumer-grade laser pointers have a divergence of about 0.001 radians, or 0.0573 degrees. At a range of 240 miles, this means your beam will be spread over a circle with a diameter of about 240 miles. Pretty sure the resulting intensity will be below the threshold of detection for the human visual system.
Pretty sure that can’t be right. That means over 50’ it would diverge 50’, which is definitely not the case.
If I did the calculation right, .001 radians over 240 miles is about .5 miles of spread.
If the moon is approx. 400000 km away and the beam divergence is 1 milliradian then you are talking about a spot with 200 km radius.
The eye may be able to see a single photon under some circumstances, but in any case there would not be a huge visible “dot” with a 5 mW laser pointer. In fact the xkcd comic discusses this
My bad. I meant to write:
At a range of 240,000 miles, this means your beam will be spread over a circle with a diameter of about 240 miles.
240,000 miles is how far away the moon is.
That makes a lot more sense. I actually did the calculation in my link wrong, I should have halved the angle, so it would be more like .25 miles at 240 miles.
One night, when I was tired of driving the dog crazy with my laser pointer, I aimed it at the wall of a school building near my house. Google Maps put the distance at 528 feet. It was very diffuse and very dim. The moon would be approximately 2.4 million times as far.
I asked a similar (but, more reasonable) question awhile ago:
I’m reminded of a science fiction book where a military starship arrives in the far outskirts of a hopefully-friendly system, and immediately fires its primary laser weaponry at the system’s habitable planet. Because at those ranges, laser weapons are the only way to communicate.
And the reverse: Larry Niven’s “The Warriors”: a hostile and extremely warlike alien race encounters an early Human sublight exploration ship. The aliens have a long-range telepath, and the telepath says that the humans don’t even know what weapons are. (Humanity has spent three centuries being bred and conditioned into pacifism; interpersonal violence is treated as mental illness and large-scale violence is literally unthinkable.)
The human ship is indeed unarmed, but one “borderline unstable” (not completely pacifistic) human turns the ship’s comm laser on the aliens when they start to attack the human ship.
Nitpick: That was a propulsion laser, not a comm laser.
There is no such thing as an unarmed starship.
I recall that David Brin reversed that concept as a plot point in Sundiver.
After a manned (and aliened) probe descends into the photosphere of the Sun, it is sabotaged by one of the aliens (because reasons). With the main drive offline and the heatshields failing, the pilot further reduces the heatshield and uses the communications laser (which doubles as a refrigeration system) as a propulsion mechanism to lift the probe out of the Sun, at the expense of a frozen crew who were recovered and revived.
And at 1000’, a diameter of 1 foot. This answers the narrow question, and, I think, the broad question I was trying to ask in another thread: if you point your tiny little laser pointer, that you use as a laser pointer in your classroom, at an aircraft, no, they can’t see it. It’s people with powerful laser ‘pointers’ that cause the problem.
??
lasers get hot, right? Which is why they need a cooling system of their own…
Hyper-efficient alien tech, probably.
The idea is that, in order to work at all, a refrigerator must have a part that’s hotter than its ambient environment, in order to dump heat into that environment. And thus, a refrigerator that’s designed to keep a spacecraft at survivable temperatures even while that spacecraft is inside of a star must have a heat-dump that’s very hot indeed: So hot that it’s easier to make the heat-dump a laser. Ludicrously advanced tech, of course, but the fundamental physics checks out.
Though, it’s a plot point that that specific piece of hyper-advanced tech is purely human, not alien: The aliens all have the attitude that if there were any reason to want to dive into a star, then someone else would have done it already, and since nobody ever had, that was proof that it was useless. Which becomes relevant in the last book of the series.
Yeah, this is wrong.