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Catching Photons Coming from the Moon
Roland Piquepaille writes "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. I also have up a summary, which contains additional details and pictures, if you just want to learn how difficult it is to capture photons back from the moon."
"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."
I.e. 1 out of 30,000,000^2 photon's come back to be captured.
What's new here is how short a pulse they're sending. The light pulse is only about 0.1ns long (the article says "an inch"), which is actually quite good for a big pulsed laser. That's why they get so few photons back.
On the other hand, detecting single photons is no big deal; that's what photomultipliers are for.
Nope, photons are bosons and not fermions. If the Pauli Exclusion Principle applied to photons then we wouldn't have lasers.
There had already been a few robot landers. Three Rangers, which crashlanded; five Surveyers (1966-68) which successfully softlanded. The Apollo 12 astronauts visited the Surveyer 3 site.
Are you serious? All objects will fall to the earth at the same rate at the same distance.
This is pretty basic. It's one of the first observations of classical physics.
F = G * m(1) * m(2) / (r^2) = m(1) * a
(equate Newton's second law with Newton's theory of gravitation where a is acceleration, m1 is the body being accelerated, and m2 is the massive body m1 is being accelerated towards.)
If you cancel m1 on both sides you get G * m2 / (r^2) = a
This means that the gravity of a massive body is always going to accelerate an orbiting body at the same rate if that body remains at the same distance. So, two masses let go at the same height above the earth will fall to the earth at the same rate (9.81 m/s^2). They each have different *forces* responsible for that acceleration, but since m*a = F, that extra force for the more massive object is needed to accelerate it at the same rate.
Actually the said 'single photon' that comes back from the retroreflector arrives with millions of others coming from everywhere around (from our atmosphere to the neighboring moon land), and is totally unvisible within this "noise".
n ade/pages2/269.html shows a lunar shot... within an entirely french page, sorry)
The issue here consists in estimating the presence of photons *below noise level*, which you only can do by statistically studying series of shots. (or, in a simplified form: by averaging hundreds of shot results, you lower the noise and end in seeing a small peak around the time where you expected the photons to come back)
Incidentally these experiments have been and are done today routinely in many observatories worldwide; the originality here may be an increase of precision but the mehod is very classical. Here in France I have a neighbor observatory which organizes visits to this setup, for instance (the last photo of http://www.bdl.fr/fr/ephemerides/astronomie/Prome
Herve S.
Hubble can see items of 50 metres size in UV wavelength on the moon's surface. Seems it's resolving power is related to the wavelength of the "light" it is using, same as in photolithography used in producing nanometre scale details on semiconductors. http://www.newscientistspace.com/article.ns?id=dn7 880
1 mm at lightspeed is about 3.3 picoseconds. First, what photon detector has a rise time in that range? Second, atmospheric conditions will dynamically affect the measurement, I suspect with significantly more than a few picoseconds of noise. Tidal effects on both the Earth and the Moon will change the distance. Finally, what Time Interval Analyzer are you going to use? The SR620, one of the better units on the market, does 25 ps resolution, and accuracy is closer to 100 ps.
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