Catching Photons Coming from the Moon 146
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."
nice (Score:1)
Re:nice (Score:2)
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)
If you look close... (Score:4, Funny)
Re:If you look close... (Score:2)
Re:If you look close... (Score:2)
Good. (Score:2)
Re:Good. (Score:2)
Re:Good. (Score:2)
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)
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.
Re:Good. (Score:2)
Re:Good. (Score:2)
"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
Re:Good. (Score:2)
Snippet describing how difficult it is ... (Score:5, Informative)
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.
Re:Snippet describing how difficult it is ... (Score:2)
"Why?"
"Because you are The One".
Re:Snippet describing how difficult it is ... (Score:2, Offtopic)
Re:Snippet describing how difficult it is ... (Score:3, Informative)
(c = f * lambda, E = h * f)
60 Watts = 60 Joules/second = 1.5*10^20 photons per second from a 60 Watt lightbulb.
30000000^2 = 9*10^14
1.5*10^20/9*10^14 = 170000 photons returned per second from a 60 Watt beam, at least according to the back of this envelope.
I just thought I'd put your number into some context. A 60 Watt searchlight pointing at the moon will get a lot of photons back - of course, you can't tell which are
Re:Snippet describing how difficult it is ... (Score:2)
You do have the advantage of knowing (a) roughly when it's coming, and (b) what color it is. That certainly helps.
Re:Snippet describing how difficult it is ... (Score:2)
Seems a bit frivolous... (Score:1)
Re:Seems a bit frivolous... (Score:4, Insightful)
Re:Seems a bit frivolous... (Score:2)
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.
Re:Seems a bit frivolous... (Score:2)
Didn't know there was a lesbian sex technique called the "Trib".
I'd question whether 1 mm is even possible... (Score:3, Informative)
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
Re:I'd question whether 1 mm is even possible... (Score:2)
Re:I'd question whether 1 mm is even possible... (Score:2)
Who knew that a 2 credit course at the 100 level in a community college would be so useful?
Re:I'd question whether 1 mm is even possible... (Score:2)
This is right - for a single count. However, if your equipment is stable (i.e. it has no unpredicta
Statistical averaging... (Score:2)
title? (Score:3, Funny)
Re:title? (Score:2)
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.
Re:title? (Score:2)
Re:title? (Score:2)
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
The "Moon": A Ridiculous Liberal Myth (Score:3, Funny)
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.
Re:The "Moon": A Ridiculous Liberal Myth (Score:1)
Re:The "Moon": A Ridiculous Liberal Myth (Score:1)
Re:The "Moon": A Ridiculous Liberal Myth (Score:1)
Re: The "Moon": A Ridiculous Liberal Myth (Score:2)
Re:The "Moon": A Ridiculous Liberal Myth (Score:2)
Re:The "Moon": A Ridiculous Liberal Myth (Score:2)
I think that some people (such as the poor soul who wrote this) DESERVE to be dragged into the street and shot.....several times..... for the good of humanity
One who would DESERVE that would be someone who would be gullible enough to think that the guy in question was being serious, in other words, the subtle person that you are ;-)
Re:The "Moon": A Ridiculous Liberal Myth (Score:2)
Also. consider the spam factor. If just 800,000 people take 5 minutes to read it, he's already cost society one lifetime in agregate. And actually much worse if most of those people took time from their regular wakefull time rather than from their sleep.
question (Score:1)
How did Neil Armstrong and Buzz Aldrin install reflectors on the moon from a soundstage in Burbank?
Re:question (Score:5, Funny)
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.
Re:question (Score:2)
At least... (Score:1)
Keep experiments running (Score:4, Interesting)
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)
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.
It's not like detecting a single photon, alone! (Score:4, Informative)
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
Re:It's not like detecting a single photon, alone! (Score:2)
Seeing the surface (Score:2, Insightful)
Re:Seeing the surface (Score:4, Informative)
Resolution of Hubble (Score:4, Informative)
Re:Resolution of Hubble (Score:1)
Wouldn't silence the idiots (Score:1)
How would they work out the distance? (Score:2)
Re:How would they work out the distance? (Score:1)
Other earth-moon projects might rely on knowing the distance, but then they just have to calculate it.
Re:How would they work out the distance? (Score:1)
Problems with this article (Score:1, Funny)
(As to whether hotter objects fall faster than colder objects, I don't know yet. That's something else I've been wondering.)
Re:Problems with this article (Score:5, Informative)
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.
Re:Problems with this article (Score:1)
Re:Problems with this article (Score:2)
Re:Problems with this article (Score:2)
Re:Problems with this article (Score:2)
Re:Problems with this article (Score:2)
Re:Problems with this article (Score:2)
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.
Re:Problems with this article (Score:2)
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.
Re:Problems with this article (Score:2)
Sigh.
Re:Problems with this article? or with assumptions (Score:2)
Re:Problems with this article (Score:1)
Re:Problems with this article (Score:3, Funny)
Force of Gravity = 6.67 * 10- mass1
Re:Problems with this article (Score:2)
Re:Problems with this article (Score:2)
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
Re:Problems with this article (Score:2)
Equivocations like these obscure the fact of the matter, which (as he said in his very first post) is that "[m]ore massive objects will reach their destination quicker than the less massive objects." This is true by Newton's equations regardless of differences in scale.
Re:Problems with this article (Score:2)
Re:Problems with this article (Score:1)
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)
Re:Wouldn't... (Score:1)
Not yours! (Score:2, Funny)
Hey! Just cause you Yanks got there first doesn't make it yours, m'kay?
Obligatory movie qoute (Score:2, Funny)
Quite far from difficult, can be done by almanyone (Score:4, Interesting)
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.
Re:Quite far from difficult, can be done by almany (Score:2)
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.
A step in the right direction (Score:1)
1.5" a year (Score:2)
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 (Score:2)
Re:Nothing to see... (Score:1)
You are aware that the moon's orbit is slowly going further out and eventually it will be flung out and lost? I'd wager the distance is minutly further out now than in the 60's.
Re:Nothing to see... (Score:2, Funny)
Re:Nothing to see... (Score:2)
The Earth doesn't have enough angular momentum to do that. Eventually earth and moon will be tide locked to each other and recession will stop.
Re:Nothing to see... (Score:1)
it's people like you that make us depend on alien engineers.
Re:Nothing to see... (Score:1)
Because it's the first time the journalism majors at the San Diego Union-Tribune have heard about it, compounded by the fact that they had reason to bury some of the real news back in the C section where they hope noone will notice it.
And for what it's worth I've just come back from an hour or so of catching photons from the moon without any sort of instrumentation at all. Piece of cake.
KFG
Re:Nothing to see... (Score:2)
Bluish green, if I recall correctly. On a slightly misty night I drove by one facility in Greenbelt, Maryland, and saw a freakin' laser beam shooting off the dome. Scare the hell out of me.
Why this is front page news is beyond me.
It may have to do with the submitter.
NASA laser ranging history (Score:2, Interesting)
In 1980 I visited the MOBLAS-5 Yarragadee [nasa.gov] station in the Western Australia outback, which was
Re:Mirrors? On the moon! (Score:2)
Re:Mirrors? On the moon! (Score:4, Interesting)
The Hatter idea is that no living thing can escape the atmosphere and survive (due to radiation [wikipedia.org] 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 [wikipedia.org] 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 [wikipedia.org].
The credible conspiracy theory: Send the humans into orbit, camp in orbit while the robot [nasa.gov] fetches the samples, reunite humans with the 'bot, then land as heroes.
Re:Mirrors? On the moon! (Score:1)
Re:Mirrors? On the moon! (Score:3, Funny)
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 th
Re:Mirrors? On the moon! (Score:2)
We have the ALSJ [nasa.gov] 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.
Re:Mirrors? On the moon! (Score:1)
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.
Re:Pretty cool... (Score:3, Informative)
Re:I must register a strong objection (Score:1)
Re:Boring.... (Score:2)
Re:Observe, our penninsula of masculinity (Score:2)
"Yeah, put 'em on the glass!"
Spacecataz