Catching photons coming from the moon

Physicists from UCSD are using lasers to send light pulses in direction of an array of reflectors installed on our moon in 1969 by Neil Armstrong and Buzz Aldrin. The goals of these experiments are to check the validity of Einstein's theory of general relativity and to precisely measure the distance between the Earth and moon.

In "Shooting the moon," the San Diego Union-Tribune describes how and why physicists from UCSD are using lasers to send light pulses in direction of an array of reflectors installed on our moon in 1969 by Neil Armstrong and Buzz Aldrin. One of the goals of these experiments is to check the validity of Einstein's theory of general relativity. Another one is to measure the distance between the Earth and moon with a precision of one millimeter by catching photons after their round trip to the moon. But it is amazing to realize how difficult it is to capture photons after such a trip.

Here is how the San Diego Union-Tribune describes why these experiments are done.

Lunar ranging has revealed several insights into the interior and orbital mechanics of both the moon and Earth. One is that the moon is spiraling away from Earth at a rate of about 1.5 inches a year, due to ocean tides on Earth. Another is that the moon probably has a liquid core.
Lunar ranging has helped scientists better understand the rise and fall of tectonic plates -- which vary the Earth-moon measurements -- and the forces beneath the ground that cause those seismic movements. And then there's Einstein's theory of general relativity.

Please read the full article for more explanations about how this theory could be validated -- or not -- by these experiments. And let's focus on the new lunar ranging project led by Tom Murphy from UCSD. This research work is being conducted at the Apache Point Observatory in New Mexico and is called APOLLO (Apache Point Observatory Lunar Laser-ranging Operation). Here is a short description of the project.

APOLLO measures the round-trip travel time of laser pulses bounced off the lunar retroreflectors to a precision of a few picoseconds, corresponding to about one millimeter of precision in range to the moon. Using this information, we will be able to gauge the relative acceleration of the earth and moon toward the sun.

APOLLO started to send laser beams to the moon about a year ago. Below is a picture showing the first time that the laser was used in the APOLLO project to send light to the moon (Credit: Tom Murphy, UCSD). Here is what Murphy wrote: "On July 24, 2005, APOLLO shone its laser out of the telescope enclosure for the first time. We were able to spend several hours pumping light at the moon, coming to understand the system."

The APOLLO laser deployment

And below is "a picture from the August 2005 run by Gretchen van Doren, showing the laser beam making its way to the (over-exposed) moon. No, the moon is not exploding under the influence of our 2.3 Watt laser! The edge-brightening of the beam can be seen, as the telescope secondary mirror robs the beam of light in its center. Orion is seen at right." (Credit: Tom Murphy, UCSD)

The APOLLO laser beam going to the Moon

You can see larger versions of these images on the APOLLO Laser First Light page.

Now, you know why these physicists are doing these experiments. And, if you're like me, you probably think it's easy to send a light beam to the moon and to catch all the photons after their round trip. But i's not easy. It's extremely difficult. Let's go back to the San Diego Union-Tribune article for some details.

Every pulse of laser light that leaves the telescope contains about 300,000,000,000,000,000 (300 quadrillion) photons of light. Scientists are happy when just one of those returns to their detector.
Here's another way to look at it: One out of every 30 million photons sent in a laser pulse will hit the lunar reflectors. And only one in 30 million of the reflected photons will be recaptured by the telescope at Apache Point.
"If you hit the reflector, it's like you just won the lottery – it's a one in 30 million chance," Murphy said. "But imagine that you just won the lottery, and they tell you it's a one in 30 million chance the money will find it's way to your bank account."

The APOLLO web site gives additional details in its Basics of Lunar Ranging page which describes how the technique works.

Only about one part in 30 million of the light we send to the moon is lucky enough to actually strike the targeted reflector. But the reflector is composed of small corner cubes, and for reasons related to the uncertainty principle in quantum mechanics, the light returning from each of these small apertures is forced to have a divergence (called diffraction).
In the case of the Apollo reflectors, this divergence is in the neighborhood of 8 arcseconds. This means that the beam returning to the earth has a roughly 15 kilometer (10 mile) footprint when it returns to the earth. We scrape up as much of this as our telescope will allow, but a 3.5 meter aperture will only get about one in 30 million of the returning photons -- coincidentally the same odds of hitting the reflector in the first place.

And the researchers are catching photons at an unusual rate as writes the San Diego Union-Tribune.

During the best of times, Murphy and his colleagues have gotten back about 2,500 photons in a 10-minute period. That may not seem like a lot, but it takes three years for the McDonald Observatory in Texas -- the only other lunar ranging station in the country -- to gather that many return photons, Murphy said.

A last question remains: will these experiments lead to a revolution in physics, as the San Diego Union-Tribune article suggests? We'll know more in a few years.

Sources: Bruce Lieberman, San Diego Union-Tribune, July 13, 2006; and various web sites

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