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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."
we finaly have the tech to prove the theory(or at least try)
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You can just make out the begin of what looks like the word "chair"
Letter To Iran
At my university (Wesleyan) we have a duplicate of the reflector array (one of the physics professors was involved with the Apollo program) in the basement. Its says on the plaque that already know the distance to a few cm and it was enough to provide relativistic evidence. My guess is that this experiment is just a higher wavelength laser (think blue vs. red) and appropriate detector. I know that there are completing modifications to relativity to increase accuracy and that this experiment might help on that front, but such high precision measurements are commonplace. Why this is front page news is beyond me.
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.
Shh.
"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.
I guess the array must have left behind by the hollywood crew who faked the moonlandings.... :D
If I RTFA correctly, Snell's Law is limited by quantum uncertainty as the normal approaches 90 degrees. Never realized that before, but it makes sense, because the Exclusion Principle would be violated momentarily at exactly 90 degrees. (But: is the Exclusion Principle even supposed to apply to photons?)
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.?
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Just shooting the moon with a laser is boring.
Nuke The Moon
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.
ôó
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!)
.. 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.
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
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.
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?
Push Button, Receive Bacon
...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.
Moderators please...mod this guy Funny!
I almost wet myself laughing while I was reading this.
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.
http://michaelsmith.id.au
Don't tell Buzz, he's really committed to the lie.
"I use a Mac because I'm just better than you are."
I know you are trolling, but the real Moon-hoax tinfoilers never claimed that 'something' didn't land on the Moon, just that no humans from Apollo 11/12 landed on the Moon.
The Hatter idea is that no living thing can escape the atmosphere and survive (due to radiation or whatever reason the Hatters claim). And no, the Space Station and the sattelites are technically inside the atmosphere, well below the Van Allen belt.
The reflector delivery and the soil sample return could be done by a robotic probe, which in fact is what the Russians did with their Luna 16 mission about a year after the alleged Apollo 11.
Since the Russkies got the first sattelite (Sputnik), the first man in orbit, the first suit walk, the first docking, etc., the thinking was that we could sound-stage their glorious defeat, end the darn space-race, then go spend the money on something more profitable.
The credible conspiracy theory: Send the humans into orbit, camp in orbit while the robot fetches the samples, reunite humans with the 'bot, then land as heroes.
Obama likes poor people so much, he wants to make more of them.
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.
to the use of the phrase "our moon" in the article. While I realize that most of the Slashdot "readership" (given the relevance of posts to the articles, I hesitate to use the word) is USian, and that you fellows believe you own the moon by right of discovery, those of us in Canada beg to differ.
The moon does not belong to the States by discovery. It was discovered by the first hairy ape that looked up at night. We always knew where it was, you guys just got there first.
As intellectual leaders of a sort, or at least as the nearest thing down there, I would think that the slashdot editors would feel some bit of "white man's burden" to set a good example and make sure such phrases don't creep into the stories.
The funny thing being that with 1960s technology it would have been harder and more expensive to create thousands of hours of difficult fx than to land on the moon. This was before CGI.
http://science.slashdot.org/science/06/07/13/16542 00.shtml Apollo 11 TV Tapes Go Missing
Let me ask, have YOU seen any of these "thousands of hours" of high-res tapes that you refer to. Have you seen a single original frame? The fact is that Williams and Kranz (top brass in charge of the archives and missions at NASA) conceed that the original data is misplaced, believed wiped.
All we have now is re-filmed qvga-res shit: tv-grabs, literally.
But don't despair, for NASA, like the OJ, just might finally find the reel killer.
What's funny is that one would need post-doctoral training to even understand just the kind of info one could extract from high-quality TV scans; I do not expect you to understand.
The bottom like is that whether the TV feeds came from the Moon or from a set we will not know until the original tapes can be examined.
Obama likes poor people so much, he wants to make more of them.
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.
We have the ALSJ and I don't believe there was a writer in the world (let alone in the US) in the 1960's and '70s who could have written it from scratch.
http://michaelsmith.id.au
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.)
What are you trying to say here, exactly?
Are you claiming the aliens beamed ALSJ down on stardate 3134.0? Or the NASA con-men scripted it?
I just don't care either way. I was merely pointing out to GGP that the reasonable tin-foil hatters never claimed Apollo 11/12 was fake, just that the humans on the Moon part was fake.
Obama likes poor people so much, he wants to make more of them.
... the reflector be covered in dust now? And what has 30+ years of solar wind done to the reflector?
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.
They'd claim the telescope was controlled, or the results misreported.
To err is human. To forgive is good system design.
I need more power!
-m10
Hey! Just cause you Yanks got there first doesn't make it yours, m'kay?
"Don't belong. Never join. Think for yourself. Peace." V.Stone, Microsoft Corporation
http://www.lunarregistry.com/
(now if something, this type of shit needs to be stopped)
-m10
Fire the "L.A.S.E.R."!
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
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.
as they never landed on the moon, they can't have build up any reflectors, so, there can not be a reflected photon to catch.
::o)
cogito ergo sum.
sorry, HAD to write this. Resistance is futile.
I think we should keep a few hundred lasers trained on the moon at all times... just in case. (cue background music)
Heh, every once in awhile the ferengi come around to raise and lower the reflectors on us for a big mind fuck.
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.
"National Security is the chief cause of national insecurity." - Celine's First Law
I inspected the Auroral tracking station in Canberra Australia, in 1993. They even fired the laser up for us and explained that they bounced the laser off reflectors left on the moon.
The laser pulsed 10 times a second and I vaguely recall them saying it measured the distance to within a few centimetres.
They had been running this for quite some years before I saw it I am quite sure.
It's not all that hard to bounce photons off the Moon.
Hell, the sun's been doing it for a few billion years using a working fusion reactor. Seems to me that beats out US physicists and engineers by a long shot.
how does this affect our understanding of spatial dynamics? I only understand physics insofar as it affects 3D graphics (specifically, light and particle dynamics), so you'll have to forgive my relative ignorance. I just find myself wondering if it's going to be a minor amendment to the current theories, or a complete re-write. If it's the second option, I'll just sit and wait for the maths patches for my various 3D software tools, shall I? Also, on a totally puerile note: you can tell that they're physicists, and not web designers, can't you? Their page http://physics.ucsd.edu/~tmurphy/apollo/apollo.htm l looks like it was designed around the same era the rocket they're named from was launched...
http://xkcd.com/313/
Well, she's the main operator. She also works for the Apollo program now, and I occasionally sit on the catwalk watching for airplanes that wander into the airspace while the laser is "shining."
When you sympathize with stupidity, you start thinking like an idiot.
Sure, and that fusion reactor should be switched off ASAP! It constantly sends radioactivity to earth, and moreover we already know that some time in the future it will blow up and destroy all life on earth!
The Tao of math: The numbers you can count are not the real numbers.
Don't forget that Newtons law used here is a theory derived from empirical evidence. Ojects accelerate towards their common center of mass. For objects are small relative to the earth, e.g. a golf ball or a battleship, it's not significant and the classic newtonian formula is accurate. When the object are comparable in size the total mass, and therefore the attractive force, increases. If an object the size of say, the moon dropped to the earth from a given distance, at the end of a given time it would be traveling faster (as observed from earth) than a golf ball would under the same circumstances.
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.
Equine Mammals Are Considerably Smaller
Please Err, your buttocks are so small and square, it barely reads as a moon.
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Yes, but the parent poster seems to be under the impression that even objects that don't appreciably change the center of mass of their system vary in free fall acceleration. The classical universal theory of gravitation is very accurate even for systems where the center of mass is different from the center of mass of one of the bodies. It's just not *as* accurate as Einstein's General Relativity, which includes (and predicts) systems where the gravitational potential is close in proportion to c^2(black holes, and other incredibly massive systems)
I wonder if they are using GPS techniques to encode the laser pulse with a pseudo-random sequence. By synchronizing the return pulse train with the predicted sequence, they can significally increase the signal-to-noise ratio (a million-fold). Once the signal is locked-in, it should be fairly easy (but not trivial) to further refine the tracking of both the transmitter and receiver.
In 1980 I visited the MOBLAS-5 Yarragadee station in the Western Australia outback, which was custom-built for this purpose. MOBLAS meant MOBile LASer, and as you can see from the picture it is built in a trailer. But the equipment does not move. The site has clear night skies and no geological or human interference. And results from one location over time are valuable. It was beautiful and eerie to see the green laser beam. Even though the air was clear you could see it when standing around the pad. It seemed to last longer than the tiny fraction of a second it pulsed. At the time the operators would get a visual feedback from their instruments on who big the return signal was. And they would tweak the telescope tracking as required to get good returns. Tapes of the results were then sent back to NASA. I assume this is all automated now. Those guys looked extremely bored.
BTW, notice how big the empty tarmac is around the trailer. I was told NASA wanted a 100 by 100 foot pad and they mistakenly got a 100 by 100 metre pad, making it about 10 times bigger than they needed!
"The moon belongs to America, and eagerly awaits the arrival of our astro-men."
sig: sauer
can only eliminate truly random noise. It will not remove unpredictible, but non-random errors.
"National Security is the chief cause of national insecurity." - Celine's First Law
He's talking about extremely large masses (eg. a bowling ball with the mass of Jupiter) in which case he's actually right.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....