What’s the application? Measuring the distance from the Earth to the Moon by bouncing a laser off one of the retro-reflector arrays left there by the Apollo missions.
What problem(s) is it the solution to? 1) “How does the distance from the Earth to the Moon vary over time due to things like tidal drag?” 2) “Does the strength of gravity change over time?” 3) “What can we do with a laser to really cheese off people who think the Moon landings were fake?”
How does it work? This concept is simplicity itself. You simply point a laser at the Moon, fire off a short pulse of light, and wait for it to come back, about two and a half seconds later. If you know when you fired the pulse out, and when it came back, that time tells you the distance the light traveled, because we know that the speed of light in vacuum is a constant 299,792,458 m/s.
This has been going on for pretty much as long as there have been lasers. The first range measurements in the early 1960’s bounced light directly off the Moon’s surface. The Apollo missions dropped off retroreflector arrays that greatly boosted the signal, and those have been in use for the last 40 years. The latest experiments are pushing the precision down to around a millimeter– that’s 0.001m out of just about 400,000,000m, which is pretty astonishing.
This is an impressive achievement for both scientific and technical reasons.
On a scientific level, these measurements have allowed scientists to deduce the presence of a liquid core on the Moon, to verify that the Moon is drifting slowly away from the Earth (about 3.8 cm/year, due to a sort of friction caused by the tides. This slows the rotation of the Earth a tiny bit each year; the Moon creeps outward slightly, compensating for the change in angular momentum), and to place stringent limits on any possible variation in the strength of gravity or deviation from the inverse-square law.
On a technical level, this is really impressive because the number of photons they’re working with is so small. They use short, intense pulses of laser light on the way out, but by the time they go to the Moon and back, these pulses are spread out over a huge area. The Apollo experiment is the best to date, and they detect three photons per pulse on average, on their best day (well, their best day as of a couple of years ago…). That’s requires some serious technical wizardry.
Why are lasers essential? While any sort of light would do, in principle, this is fiendishly difficult with a laser, and would be essentially impossible with any other source of light. A laser produces as intense and narrow a beam of light as you’ll ever see, but even a laser beam expands over a path as long as the distance from the Earth to the Moon. As they explain in their web introduction, the beam use by Apollo starts out around 3m across, and is about 2km across by the time it reaches the Moon, and is about 15km across by the time it gets back to Earth. And that’s the best you’re ever going to get– any other realistic source of light would be worse.
Why is it cool? Dude, they measure the distance from the Earth to the Moon to within a millimeter! You’ve got to be impressed with that.
And the scientific payoff is impressive. The lunar ranging measurements are some of the best constraints we have on possible deviations from General Relativity. They tell us about the composition of the Moon, and the behavior of the Earth. It’s amazing how much information you can extract from such a simple signal.
Why isn’t it cool enough? At the end of the day, it’s just using the laser like a really big flashlight. Also, the Moon landings were, like, totally faked. I read it on the Internet, it must be true.
(This post is the third of twelve highlighting amazing laser applications, in honor of the 50th anniversary of the first laser. These posts serve as a lead-up to an audience poll asking what the coolest laser application is, so if you like lasers and radio buttons, watch this space over the next week or so.)