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Catching Photons Coming from the Moon 146

Posted by Zonk
from the need-better-than-a-jar dept.
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."
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Catching Photons Coming from the Moon

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  • we finaly have the tech to prove the theory(or at least try)
    • There have been experimental verifications of General Relativity for quite some time now. IIRC, Einstein himself noted how his theory accounted for a slight deviation in Mercury's orbit that Newtonian mechanics could not. And, if you don't consider astronomy quite "experimental" enough, there have been experiments with clocks and other such things. The first Google hit got me this page [utoronto.ca], which looks like an understandable enough summary.

    • Re:nice (Score:4, Insightful)

      by Umbral Blot (737704) on Saturday July 15, 2006 @12:18AM (#15723581) Homepage
      Oh we had the technology to verify the theory long ago (the atom bomb was one such verification of E=mc^2, the slower decay of fast moving particles is a verification of time dilation, the bending of light arround the sun observable during an eclipse is a verification of the curvature of space time, and the explanation of Mercury's orbit is a verfication of E=mc^2 in the other direction), this is simply an additional check.
  • by DumbSwede (521261) <slashdotbin@hotmail.com> on Friday July 14, 2006 @11:41PM (#15723478) Homepage Journal
    You can just make out the begin of what looks like the word "chair"
  • Well good, at the least now the conspiracy nuts will now have to admit that aliens from Area 51 put up that pesky reflection array on the moon. But damnit, men did not walk on the moon.
    • A robot probe could easily have carried a reflector. People wouldn't be required.
      • It would have had to have some pretty impressive computer controlled landing software for 1969!?!
        A lot of people worked on the moon shots, so many that something did go to the moon in July of 1969. I believe that people went because they could handle all the problems easily (e.g. landing) that computers of the era could not easily do.
        People also tend to think that finding one case where a general rule fails invalidates the entire rule. The entire moon shot could have been faked at great expense at the t
        • Re:Good. (Score:5, Informative)

          by 1u3hr (530656) on Saturday July 15, 2006 @02:22AM (#15723850)
          It would have had to have some pretty impressive computer controlled landing software for 1969!?!

          There had already been a few robot landers. Three Rangers, which crashlanded; five Surveyers [nasa.gov] (1966-68) which successfully softlanded. The Apollo 12 astronauts visited the Surveyer 3 [nasa.gov] site.

        • I don't personally doubt that the Apollo missions happened more or less as NASA claims. But the presence of a reflector is not the reason I believe it. I believe it for other, more compelling reasons. I pointed out that there are other ways for a reflector to be there because I don't want to see my position supported by such a weak argument.

          "It would have had to have some pretty impressive computer controlled landing software for 1969!?!"

          Indeed. And it's even more impressive that a Soviet computer did it fi
    • No, no, see, this whole thing is a conspiracy, too. There is no reflector array on the moon, the scientists are lying to us. All the footage of "laser pulses" is faked in a Fox News studio.
  • by Hulkster (722642) on Friday July 14, 2006 @11:50PM (#15723502) Homepage
    "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.
  • Is it just me or does it seem that goal #2 is a bit excessive. Knowing the distance between the Earth and the moon to the point of a millimeter.. Exactly what does that gain us in, say, accuracy of experiements, etc.?
    • by calidoscope (312571) on Saturday July 15, 2006 @12:02AM (#15723539)
      Well if you read TFA from the Union-Trib, the whole point was getting enough accuracy to see if the orbit of the moon followed the predictions of General Relativity exactly. A deviation from those predictions would mean that General Relativity needs amending. The beauty of this experiment is that it is relatively inexpensive - the reflector is already on the moon.
      • Many of the experiments which measured up in cms, ad many other data proved that for those ranges the theory is correct.
        But as was the case of Newton, wherein his theories of gravity was proved right for a huge range of velocities/distances.

        Only more experiments, in smaller ranges, would even put a question mark on the Einsteins theory of gravity.
        If it holds up, well and good.
        If not, time for change (either the experiment or the theory)

        So, this experiment and millions of others are very useful indeed.
    • Considering all the variables plus measurement accuracy.

      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 [thinksrs.com], one of the better units on the market, does 25 ps resolution, and accura

      • With one measurement, you're totally correct. But using statistical techniques from multiple measurements you can cancel out all the random errors that occur. You can do the same thing using a GPS receiver and get centimeter accuracy from the at best 3 meter accuracy of GPS.
        • I love you, data and error analysis. With you, I've taken an uncertainty that, with one measurement would be in the 20+ cm range, and reduced said uncertainty down to ±2cm, all using statistical techniques. Now, if only you could have dinner ready by the time I got home, all would be well in the world.

          Who knew that a 2 credit course at the 100 level in a community college would be so useful?
      • 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... 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.

        This is right - for a single count. However, if your equipment is stable (i.e. it has no unpredicta
  • title? (Score:3, Funny)

    by binarybum (468664) on Friday July 14, 2006 @11:53PM (#15723512) Homepage
    must say that title is a bit vague. I was just outside last night getting bombarded with photons from the moon. I'm betting technology circa 1888 is capable of capturing photons coming from the mooon.
    • must say that title is a bit vague.

      Yeah, surely you couldn't be bothered to read the short summary to figure out more specific info than the title can convey.

      Really, give me a break. To make matters worse, you didn't even bother to suggest an alternative title. You're just making pedantic complaints.
      • Yeah, well at least he read the whole line. I just read the word catching, and can't seem to find any references to baseball. What madness it this?
      • Yeah, surely you couldn't be bothered to read the short summary to figure out more specific info than the title can convey.

        from the summary: "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" sorry, I'm still at a loss - if their finding it difficult, perhaps they're just not sending enough photons - I can assure you it is no difficult task to capture photons back from the moon.

        you d
  • by Anonymous Coward on Friday July 14, 2006 @11:57PM (#15723523)
    It amazes me that so many allegedly "educated" people have fallen so quickly and so hard for a fraudulent fabrication of such laughable proportions. The very idea that a gigantic ball of rock happens to orbit our planet, showing itself in neat, four-week cycles -- with the same side facing us all the time -- is ludicrous. Furthermore, it is an insult to common sense and a damnable affront to intellectual honesty and integrity. That people actually believe it is evidence that the liberals have wrested the last vestiges of control of our public school system from decent, God-fearing Americans (as if any further evidence was needed! Daddy's Roommate? God Almighty!)

    Documentaries such as Enemy of the State have accurately portrayed the elaborate, byzantine network of surveillance satellites that the liberals have sent into space to spy on law-abiding Americans. Equipped with technology developed by Handgun Control, Inc., these satellites have the ability to detect firearms from hundreds of kilometers up. That's right, neighbors .. the next time you're out in the backyard exercising your Second Amendment rights, the liberals will see it! These satellites are sensitive enough to tell the difference between a Colt .45 and a .38 Special! And when they detect you with a firearm, their computers cross-reference the address to figure out your name, and then an enormous database housed at Berkeley is updated with information about you.

    Of course, this all works fine during the day, but what about at night? Even the liberals can't control the rotation of the Earth to prevent nightfall from setting in (only Joshua was able to ask for that particular favor!) That's where the "moon" comes in. Powered by nuclear reactors, the "moon" is nothing more than an enormous balloon, emitting trillions of candlepower of gun-revealing light. Piloted by key members of the liberal community, the "moon" is strategically moved across the country, pointing out those who dare to make use of their God-given rights at night!

    Yes, I know this probably sounds paranoid and preposterous, but consider this. Despite what the revisionist historians tell you, there is no mention of the "moon" anywhere in literature or historical documents -- anywhere -- before 1950. That is when it was initially launched. When President Josef Kennedy, at the State of the Union address, proclaimed "We choose to go to the moon", he may as well have said "We choose to go to the weather balloon." The subsequent faking of a "moon" landing on national TV was the first step in a long history of the erosion of our constitutional rights by leftists in this country. No longer can we hide from our government when the sun goes down.
    • I want the three minutes of my life it took to read that back.
    • You, sir, are trying to spin the facts to spread blasphemy and heresy. It is well documented throughout numerous sections of the bible that God Himself created the Moon, and that all things Moon-Related are God-Related. How can you state that light "just randomly emits" from the Moon? That the nuclear reactors which power this "just randomly work"? Clearly there is an intelligence far beyond us at work here. Only God could create something so massive with such powerful light-reflecting abilities, perfe
    • OK, this is a troll. However, I've never seen a reference to the daytime moon in any pre-1950 literature, ever. The moon is frequently visible during the day. And yet every piece of literature from before 1950 talks about the moon solely as a nighttime phenomenon. Look through art galleries and you won't find a single painting of the moon during the day. So it seems pretty clear that the daytime moon is in fact a new phenomenon.
  • 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.

    How did Neil Armstrong and Buzz Aldrin install reflectors on the moon from a soundstage in Burbank?
    • Re:question (Score:5, Funny)

      by Anonymous Coward on Saturday July 15, 2006 @12:15AM (#15723572)
      How did Neil Armstrong and Buzz Aldrin install reflectors on the moon from a soundstage in Burbank?

      They didn't. The Burbank soundstage looked fake, so they had to build one on the Moon.

      The Burbank Landing is a hoax. We never went to Burbank. Going to Burbank requires resources and capabilities far exceeding those we possess or will be able to possess in the foreseeable future.
  • ...this should theoritcally put to rest the conspiracy theories having to do with the Apollo mission and a Western US military base of which the government doesn't acknowledge.
  • by MichaelSmith (789609) on Saturday July 15, 2006 @12:11AM (#15723559) Homepage Journal

    The LR^3 retroreflector featured here was part of the ALSEP station on several of the apollo missions. In the years since these missions the ALSEP stations have been shut down. The reflectors are passive devices and don't have an off switch, which is why they are still working.

    In particular the seismonitors which were a part of each system could still be operating today, and delivering new scientific results.

    I think this article is an example of why experiments should not be shut down before they really stop working.

  • Narrow output pulse (Score:3, Informative)

    by Animats (122034) on Saturday July 15, 2006 @12:32AM (#15723617) Homepage

    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.

    • by Herve5 (879674) on Saturday July 15, 2006 @02:45AM (#15723884)
      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".
      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/Promen ade/pages2/269.html [www.bdl.fr] shows a lunar shot... within an entirely french page, sorry)
  • Seeing the surface (Score:2, Insightful)

    by FractusMan (711004)
    This is slightly off topic, but related to sending light and receiving it. From the Earth's surface, just how good of a resolution can we get of the lunar surface? I mean, can we put the 'We never landed on the Moon' theories to rest simply by pointing a good telescope up there and looking for footprints/lunar rover tracks?
  • Considering neither the Earth or the Moon have a perfectly flat surface, millimetre-precision reading will only be useful if they know to the millimetre how far away the mirrors are from the centre of the Moon.
    • If they know the height of the refelctor on the moon and the height of their laser, then you will be able to calculate the distance between two other points - I guess. But I think the most importain part is that we know the distance over time and then it dosen't matter where you messure from.

      Other earth-moon projects might rely on knowing the distance, but then they just have to calculate it.
    • You misunderstand. The 'millimeter' rating is only that: a rating. It states that they can measure, 'to the millimeter', the distance between the reflector and the laser. Of course, there will be other measurements relative to this figure which will be important - the shift between subsequent measurements, for example.
  • Gravity accelerates all objects at the same rate? What does this mean? My understanding is that two objects of difference masses will fall at different rates. More massive objects will reach their destination quicker than the less massive objects. This has been mathematically proven. I even had a physics teacher help me write out the hypothetical trials.

    (As to whether hotter objects fall faster than colder objects, I don't know yet. That's something else I've been wondering.)
    • by monoqlith (610041) on Saturday July 15, 2006 @02:24AM (#15723854)
      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.

      • Don't bite the trolls.
      • Actually, he's completely right [slashdot.org], for any usual meaning of the word "fall." Check out that discussion.
        • not really. the gravitational acceleration that the hammer(or any other small object) imposes on the earth is so small as to be completely negligible(it is 1 For all meaningful purposes the hammer and the feather will drop at the same acceleration in a vacuum. So, for any usual meaning of the world "fall", (i.e. the one that you and I use to talk about objects day to day), two objects with the same mass will fall to the Earth at the same acceleration. For big objects you have to do a force balance for both
          • Then again, the original poster said nothing about hammers, or any other small objects. There's just no disputing that "more massive objects will reach their destination quicker than the less massive objects."
            • Sorry, the comment you linked to used the example of a hammer. And no, actually, you can dispute this "fact" in principle if we're talking about the rate at which the objects fall. It involves the idea of an accelerating frame of reference. See, in typical usage we refer to the "falling" object as the less massive one. It falls towards the center of gravity of the more massive object. If you fixed the more massive object(which is how we tend to think of these systems), two objects, regardless of mass, will
              • The word "rate" is the confusing part. Like I said though, more massive objects will reach their destination quicker than less massive objects, even if on a very small scale.

                I don't know how relative time fits into all of this. I don't know if the formulas I'm told are adjusted for relative time, or if the formulas assume a universal time constant. I believe time slows down near massive objects.
          • Let us say we're going to do an experiment on the Moon, which has no atmosphere.

            We're going to do two seperate trials so the objects don't affect each other at once.

            Both objects have same volume.

            Trial one, we're going to drop a 1 kilogram bowling ball 1 mile away from the Moon's surface.

            Trial two, we're going to drop a bowling ball that has the same mass as Jupiter.

            Trial two, both will hit each other in less time than trial one would. By the word trial, I mean seperate experiments/whatever.
    • Care to enlighten us with your great discovery? I would have to say your physics teacher was actually a lit major who was masquerading. You've been duped.
      • Below are some of the formulas. I haven't been taught how to derive the bottom one. If you do a couple trials, objects A and C in trial one, and objects B and C in trial two, with A being more massive than B, with center of gravities being dropped from the same distance, the more massive one will reach it's destination "sooner" than the less massive one. This is math. It's provable. I need to go to sleep now, so maybe later I'll be posting the complete example with trials.

        Force of Gravity = 6.67 * 10- mass1
        • To get acceleration from a force, use Force = mass * acceleration. Equating the two cancels out the mass of the falling object. The acceleration of the object depends only on the mass of the other object. In other words, massive objects fall with greater force because a greater force is required to accelerate them. If it somehow seems strange that only the mass of one of the objects affects acceleration, realize that it works both ways. The earth's acceleration towards a falling object does not depend on th
          • (Hopefully I didn't make any typos while copying this from a piece of paper.)

            Object A is 2kg
            Object B is 1kg
            Object C is 1kg

            Trial 1: The center of gravity of Object A is 10m from the center of gravity of Object C.
            Trial 2: The center of gravity of Object B is 10m from the center of gravity of Object C.

            Force of Gravity = 6.67 * 10- mass1 * mass2 / distance

            Trial 1: Fg = 6.67 * 10- * 2kg * 1kg / (10m) = 1.334*10^-12 Newtons
            Trial 2: Fg = 6.67 * 10- * 1kg * 1kg / (10m) = 6.67*10^-13 Newtons

            distance = ½ * accel
    • If you aren't making a joke, then your physics teacher should be fired.

      As has already been posted, due to the equivalence of inertial mass and gravitational mass, the object mass in the equation you have posted cancels. Also, if you really did do an experiment then I would question your methods. How did you account for air resistance. A 1992 article in Physical Review Letters details an experiment performed in an ultra-high vacuum where the universality of the free fall acceleration of objects in a gr
  • Wouldn't... (Score:1, Interesting)

    by Anonymous Coward
    ... the reflector be covered in dust now? And what has 30+ years of solar wind done to the reflector?
    • Actually the moon is a pretty static place. No air means no wind which means all the 'dust' stays where it is. Armstrongs original footprints should still be there (unless the lander wiped them out on take off.
  • Not yours! (Score:2, Funny)

    by badzilla (50355)
    ...installed on our moon in 1969...

    Hey! Just cause you Yanks got there first doesn't make it yours, m'kay?

  • Fire the "L.A.S.E.R."!
  • by Ancient_Hacker (751168) on Saturday July 15, 2006 @06:41AM (#15724198)
    It's not all that hard to bounce photons off the Moon. The US Army Signal Corps did it in 1947, using very mediocre WWII radar sets. Radio amateurs have been doing it since around 1960, with limited equipment, skills, and very limited transmitter power.

    What's difficult is doing it with nanosecond resolution. That requires very wide bandwith antennas and receivers, which also let in a lot of wide band background noise.

  • I think we should keep a few hundred lasers trained on the moon at all times... just in case. (cue background music)
  • I have noticed reports about this phenomenon before, that the moon's distance from earth is increasing. I assume it's because assymmetric tidal forces are dragging the moon forward.

    Assuming that's the case, I did a quick calculation of how large the forward dragging force on the moon would have to be. Assuming I did it right, the force is about 1.31e11 Newtons (roughly 2.94e10 pounds). That compares to the gravitational force between the earth and moon of 1.98e22N, 11 orders of magnitude bigger.

  • "The moon belongs to America, and eagerly awaits the arrival of our astro-men."

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