Laser Pointer
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Laser Pointer
If every person on Earth aimed a laser pointer at the Moon at the same time, would it change color?
—Peter Lipowicz
Not if we use regular laser pointers.
The first thing to consider is that not everyone can see the Moon at<br>once. We could gather everyone in one spot, but we learned our lesson<br>about that a few weeks ago. Instead, let’s<br>just pick a time when the Moon is visible to as many people as possible.<br>Since about 75% of the world’s population lives between 0°E and<br>120°E,<br>we should try this while the Moon is somewhere over the Arabian Sea.
We can try to illuminate either a new moon or a full moon. The new moon<br>is darker, making it easier to see our lasers. But the new moon is a<br>trickier target, because it’s mostly visible during the day—washing out<br>the effect.
Brightness aside, an ideal time would probably be 2:00 PM EST on<br>December 27th, 2012, when a full moon will be high in the sky above<br>Mumbai and Islamabad. At that point, the Moon will be visible to<br>approximately five billion people—most of Asia, Europe, and Africa—about<br>as many as can ever see it at one time.
But let’s pick a quarter moon instead, so we can see the effect on the<br>dark side. We’ll avoid the December 21st quarter moon to avoid<br>encouraging any Mayan nonsense, and pick the one on January 4th, 2013,<br>half an hour after midnight (GMT). It’ll be day in East Asia but night<br>in Africa and Europe.
Here’s our target:
The typical red laser pointer is about 5 milliwatts, and a good one has<br>a tight enough beam to actually hit the Moon—though it’d be spread out<br>over a large fraction of the surface when it got there. The atmosphere<br>would distort the beam a bit, and absorb some of it, but most of the<br>light would make it.
Let’s assume everyone has steady enough aim to hit the Moon, but no more<br>than that, and the light is spread evenly across the surface.
At half an hour after midnight (GMT), everyone aims and presses the<br>button.
This is what happens:
Well, that’s disappointing.
It makes sense, though. Sunlight bathes the Moon in a bit over a<br>kilowatt of energy per square meter. Since the Moon’s cross-sectional<br>area is around 10^13 square meters, it’s bathed in about 10^16<br>watts of sunlight—ten petawatts, or two megawatts per person—far<br>outshining their five milliwatt laser pointer. There are varying<br>efficiencies in each part of this system, but none of it changes that<br>basic equation.
5 milliwatts is wimpy. We can do better.
A 1-watt laser is an extremely dangerous thing. It’s not just powerful<br>enough to blind you—it’s capable of burning skin and setting things on<br>fire. Obviously, they’re not legal for consumer purchase in the US.
Just kidding! You can pick one up for<br>$300.
So suppose we spend the $2 trillion to buy one-watt green lasers for<br>everyone. (Memo to presidential candidates: this policy would win my<br>vote.) In addition to being more powerful, green laser light is nearer<br>to the middle of the visible spectrum, so the eye is more sensitive to<br>it and it seems brighter.
Here’s the effect:
Dang.
The laser pointers we’re using put out about 150 lumens of light (more<br>than most flashlights) in a beam 5 arc-minutes wide. This lights up the<br>surface of the Moon with about half a<br>lux of illumination—compared to<br>about 130,000 lux from the sun. (Even if we aimed them all perfectly, it<br>would only manage half a dozen lux over about 10% of the Moon’s face.)
By comparison, the full moon lights up the Earth’s surface with about<br>one lux of illumination—which means that not only would our lasers be<br>too weak to see from Earth, but if you were standing on the Moon, the<br>laser light on the landscape would be fainter than Moonlight is to us on<br>Earth.
With advances in lithium batteries and LED technology over the last ten<br>years, the high-performance flashlight market has exploded. But it’s<br>clear that flashlights aren’t gonna cut it. So let’s skip past all of<br>that and give everyone a Nightsun.
You may not recognize the name, but chances are you’ve seen one in<br>operation: It’s the searchlight mounted on police and Coast Guard<br>helicopters. With an output on the order of 50,000 lumens, it’s capable<br>of turning a patch ground from night to day.
The beam is several degrees wide, we’ll want some focusing lenses to get<br>it down to the half-degree needed to hit the Moon.
Here’s the effect:
It’s hard to see, but we’re making progress! The beam is providing 20<br>lux of illumination, outshining the ambient light on the night half by a<br>factor of two! However, it’s quite hard to see, and it certainly hasn’t<br>affected the light half.
Let’s swap out each Nightsun for an IMAX projector array—a 30,000-watt<br>pair of water-cooled lamps with a combined output of over over a million<br>lumens.
Still barely visible.
At the top of the Luxor Hotel in Las Vegas is the most powerful<br>spotlight on Earth. Let’s give one of them to everyone.
Oh, and let’s add a...