probably a minor and tedious point, but it bugs me when journalists do this:
> (3) The universal force of gravity is very stable. Newton's gravitational constant G has changed less than 1 part in 100-billion since the laser experiments began.
technically worded correctly, but this language implies that we know it IS changing, by some small but non-zero amount.
actually, we haven't detected any change at all - so there's every chance it's NOT changing - but if it is, it's changed less than this small amount which our instruments won't measure more accurately than.
> No variation of the gravitational constant is discernible,
(dG/dt) / G = (0.0±1.1)x10–12 /yr
Can you explain how that reconcile with this recent news?[0] "Their tests yielded a new G value of 6.67545 × 10−11 m3⁄kg s2, which is higher than the current accepted value by about 240 parts per million"
Of course, that particular experiment could just have had some corrupting factors, but how do you know what experiments support a constant value and which ones don't without approaching tautology?
Short version is you're talking about a value, they're talking about a first derivative of that value.
I can take two COTS cheap canned 64 MHz oscillator modules like the ones on your computer motherboard, and the mfgr claims they'll output something within 100 ppm of 64 MHz. Once the temp stabilizes they'll individually be WAY more stable than 100 ppm comparing right now to an hour ago, but whatever it is they oscillate at, it'll be within 100 ppm of each other.
If you graph the output of NTP, perhaps via Munin or any number of other sysadmin tools, you'll see most of those devices are very stable from hour to hour, at a basically fixed offset from the correct frequency.
A couple years ago there was a ham radio construction project in QEX etc along the lines of using a crude computer grade COTS osc, feeding it thru the equiv of a hundred-something times multiplier, and using it as a local oscillator. I built a couple and they're ridiculously stable, although all over the map. I don't have an 1152.000000 MHz oscillator although from memory they were all within 10 or so ppm of each other (better than the rated 100ppm)
Edited to add, if you're running Debian Linux and NTP, take a look at /var/lib/ntp/ntp.drift. The generic COTS dell 6-U server one of my images runs on, ticks very stable and precisely -40.069 ppm low compared to reality and it never varies. On the other hand, my "desktop" running basically the same software but on generic dell desktop hardware is at present running +9.615 ppm fast at this instant although it varies a bit more (probably due to temp swings).
> Improvements in our knowledge of the absolute value of the Newtonian gravitational constant, G, have come very slowly over the years. Most other constants of nature are known (and some even predictable) to parts per billion, or parts per million at worst. However, G stands mysteriously alone, its history being that of a quantity which is extremely difficult to measure and which remains virtually isolated from the theoretical structure of the rest of physics. Several attempts aimed at changing this situation are now underway, but the most recent experimental results have once again produced conflicting values of G and, in spite of some progress and much interest, there remains to date no universally accepted way of predicting its absolute value.
The Wired article confirms that it's hard to measure, with some examples of the difficulties.
However, G* M is not hard to measure accurately. We believe the Earth gains about 100,000 kg per year from meteors, and in any case, the Moon and the Earth have changed by much less than one part in a million over the last few decades. So any change in G* M should be due to a change in G, even if we can't measure G directly.
This is of course also why we don't know the mass of the Earth better than a few hundred parts per million.
It's precisely because almost all gravitational dynamics are controlled by G*M (for some M) that G is so poorly-known. We're forced to do terrestrial experiments, and they're extremely difficult.
G is very seductive. It seems obvious that we should be able to measure a fundamental constant of nature to better than 14 ppm (or, if you take the scatter of current measurements, 100s of ppm), but it's really really hard.
> No variation of the gravitational constant is discernible, (dG/dt) / G = (0.0±1.1)x10–12 /yr
If we're obsessing over details, I'm wondering if 0.0 is indeed the center of the confidence interval, or if this is just a way to say they have bounded |G'/G|. The cited paper ("Lunar laser tests of gravitational physics") just states the result without giving much detail.
In general, because G' is a signed quantity, I would expect any experiment to be quoting double-sided errorbars. If G increases, one thing happens, if G decreases, the opposite happens, so the difference is discernable.
> (1) The moon is spiraling away from Earth at a rate of 3.8 cm per year. Why? Earth's ocean tides are responsible.
What a terrible explanation. I've done a physics course or two so I understand the physical process of how ocean tides are responsible (I think). But for your average person that's just silly.
TL; DR - The earth's rotation drags the tidal bulge faster than the moon's orbit slowing the earth and giving more energy to the moon which moves it into a higher orbit.
... I've known about the widening orbit of the moon, but I only just realized that if the moon was rotating in a different direction, it would eventually spiral down to the earth.
Imagine living on such a planet, with a large gravity capture moon, that provides a definite sell by date to living on the surface.
In such a world all the extra energy would have to be captured by tides.
Right now a portion of the energy is used to "fling" the moon farther away, and a portion is used by tides.
In the other scenario not only is all the energy available to tides, but you also have the extra added energy of the moon "falling" toward the earth.
I'm not sure tides could capture such energy without smoothing the borders of the oceans, which would cause less friction, and then consequently less momentum transfer to the moon.
Wouldn't the moon leaving the Earth's orbit give us a sell-by date too? I thought the removal of a large object like the moon would change our orbit as well, causing things to be dramatically different on Earth.
Is that wrong? What would happen to the Earth when the moon breaks away?
That's a pretty big if, actually. It may be possible for the earth to give up all its angular momentum without giving the moon enough energy to escape. But then we have eternal day and night on opposite sides of earth, so that's probably not a big improvement.
Actually the angular momentum transfer will stop when the moons orbit is the same length as the earth's rotation. So earth would still have day and night but would only have 10-12 per year.
We already have a sell-by date. Long before the sun goes red giant, in fact in about a billion years from now, solar output will increase to the point that liquid water (and therefore life) will be impossible on the surface of the earth.
Was thinking about that too. It is sad that the moon is leaving us, but the opposite would be a lot worse. Could we survive such an event? Perhaps by chilling on Mars for a few thousand? years.
Few thousand would be lowballing it rather lot. An impact of that magnitude would melt the entire surface, boil the oceans, and strip off the atmosphere. It would take hundreds of millions of years for earth to recover, if it ever would.
Also, Mars is not that hospitable to life. The best option I see would be putting as much people and industry in orbit as possible, and try to live from asteroids.
I've always found it rather interesting how the combined mass of Mars and Venus is just about the same as Earth's. Can't we move Venus out to a higher orbit where it would impact with Mars and produce an Earth 2?
It wouldn't be an impact. For the moon to reduce its orbit to the point that it impacted the earth it would be moving rather slow as orbital objects go.
It would be more of a gradual merging as chunks of rock and ocean moved between the two bodies.
Yes, I was thinking a -3.8cm per year rate, and did contemplate the "squishing" that the moon Io has to deal with. Just pulled the timeframe from my rear end, probably would take that long just to get the process started. :/
And of course for the curious, no the moon was not 'skimming the surface of the earth' a million years ago as would be implied by a 'continual' increase of 3.8 cm/yr in the orbit.
The earth is not homogenous, and so the gravitational force experienced by the moon is not constant. This results in a transfer of angular momentum between the two, speeding up the moon slightly (and slowing the earth even more slightly as the transfer is proportional to mass)
Even over a billion years that would only change the distance by 38,000 km which means it wasn't 'skimming the surface of the earth' by any imagination. Unless there is a 260,000 Km inclusive definition of skimming. Even at 4.6 billion years (assuming a linearity for that 3.8 cm / year that likely isn't there) that would still only account for just over half the earth-moon distance.
So I don't think the skimming was implied in any way!
You are correct of course. Now I feel sad that I didn't do the math correctly in my head (in my world apparently kilometers are like 100M or something :-()
I wouldn't worry about it, the mental image of the moon skimming the surface of the earth was worth it :) But at the required orbital speeds the tidal effects would be another matter. Talk about a rip-tide :)
> TL; DR - The earth's rotation drags the tidal bulge faster than the moon's orbit slowing the earth and giving more energy to the moon which moves it into a higher orbit.
Is there no way that this is a misinterpretation of the facts? Couldn't the answer be that the Earth is losing mass through its atmosphere (into space), and due to having less mass, the moon has less gravitational pull with the Earth, and the orbit becomes larger?
A good question. However if the Earth was becoming lighter then that would affect the gravitational interaction with lots of things like satellites, passing asteroids and maybe our orbit around the Sun. I'm quite sure the change with all the satellites would be easily noticed.
Thanks for the link, I appreciated the short but informative explanation.
Today I learned the moon is spinning away from the Earth while the Earth's rotation is slowing down, and by the time this dance movement ends, a day on the Earth will last a month!
Yes but last I checked when the tide went out it doesn't go out to the moon and push it a little bit, so blaming the tide is nonsensical at best. If you want to make a one line quip about it. It would be more accurate to say it is because the Earth spins faster than the moon revolves around the Earth. After all once the Earth stops spinning faster than the moon revolves there will still be a tide but the moon will stop moving away from the Earth.
Is it possible for anyone to use that reflector to calculate the earth-moon distance? What kind of laser is used for this? Do we know the precise location of the reflector?
There is a Mythbusters episode about moon landing hoax conspiracy theories where the hosts visited an observatory to test the lunar retro reflectors.
The laser they used was pretty big (don't know/remember the numbers) but the more impressive part was the detector. Despite the huge laser, just a handful of photons make it there and back and twice through the atmosphere.
Overall it didn't look like something you do on your back yard with off the shelf equipment, but it might be doable by a motivated hackerspace crew with a lot of time and some money on their hands.
I was looking at the moon the other night and though, "Holy fuck the sun is powerful." The light is reflected off not that reflective grey dust/rock and is fairly freaking bright.
best issue a 'notice to airmen' if you go this route, pointing multiple-tens-of-watt lasers skyward is going to get you into a lot of trouble if you're not careful about it.
Yes, better report to the authorities before you go waving a huge laser at the moon. In my home country, owning a laser this big requires a permit in the first place. You would definitely need to report it to the local aviation authority, who would re-route flights and notify aviators.
A few weeks ago there was a music festival with some big laser decorations and they actually changed the approach path to the largest airport in the country for the weekend.
The lasers used in the lunar ranging experiments are pulsed, in much the same way radar is pulsed, so even though the laser's peak power is very high, the average power could be less than a watt and accomplish the desired objective.
The other thing about this experiment is that it relies on autocorrelation to avoid a need for very much power. Autocorrelation depends on knowing the departure times of the outgoing laser energy and accumulating many samples at the expected return time, which is predictably and reliably related to the transmission time. This integration technique produces a clear return signal that would be lost in noise if only one sample were collected.
In principle, if someone was patient and had good enough equipment, one could detect the reflection of a very small outgoing power level by collecting hundreds or thousands of samples. Autocorrelation is an amazing technique.
It's actually not all that powerful (probably still strong enough to be subject to all kinds of regulations though). The most cost and effort seems to go into the optics (read: telescops) for targeting and detection.
> Do we know the precise location of the reflector?
how can people still deny that we went to the moon when that thing has been sitting there since? Id think that those people who think that there was no moon landing would have done a fair amount of research about it, and would have learned of the existence of this man made thing there.
The typical moon-conspiracy-theorist doesn't claim that nothing man-made has reached the moon, only that humans haven't been there. Unmanned probes or laser reflectors don't present a problem to their world view.
For that matter, from Earth examples, its not terribly hard to find places with volcanic obsidian laying around vs nearly pitch black non-reflective areas.
Doesn't help that people were not randomly bouncing laser beams off the moon before the landings.
"so there exists a peculiar shiny field of obsidian rocks on the surface in that particular area, so what?"
If it were a religious claim that (fill in the blank) put a gold tablet up there, skepticism would be seen as healthy.
Personally I think the ham radio AMSAT guys should drop an active transponder on the moon, so EME antennas don't have to be so elaborate. Of course it would only work half the time and temperature cycles would be brutal on the poor thing, but still...
> "so there exists a peculiar shiny field of obsidian rocks on the surface in that particular area, so what?"
It's not sufficient for the surface to be shiny. The retroreflector array needs a really specific configuration of mirrors to reflect incoming light from any direction back to it's source. That is rather unlikely to form naturally. Without that, it would only reflect the laser back to earth for a very small portion of moon's orbit, as it doesn't always point exactly to the same direction.
Assuming solar not RTG for power. And the battery bank to make it thru the weeks long lunar night would be enormous. If AMSAT (or partners) did a RTG power source it would certainly be a first, probably not going to happen along with a lunar AMSAT also being a first. Unless they hitch-hiked on a bigger mission.
There is some weirdness with certain mountain peaks near the poles being in continuous sunlight. Great spot for a manned colony. Of course that is a lot different in scale from just dropping an unmanned AMSAT transponder.
It would be much easier to lunar orbit the device, although less cool than being on the surface. A reasonably low orbit could keep transmitting even in the shade.
I don't think humans have walked on the moon. I wouldn't bet my life on it, and I don't have a complete narrative as to the why/how of it, but I'd put the odds at over 50%.
It's something of a technological anomaly. A few years back the Apollo program had been in complete shambles, burning the Apollo 1 astronauts to death. 30-40 years later, we are still crash landing probes into the surface of planets and blowing up space shuttles. But for a few years starting in 1969 we routinely soft landed a rocket on an unmapped alien surface, and then launched it again and docked with the command module orbiting at 3000 mph? It just doesn't seem to "fit".
I've seen enough historical examples to know that you CAN dupe most people most of the time.
Most people seem to be convinced by the "social proof". I really believe if it were the other way around, no one had heard of the moon landings, and you were going around telling people "no really, we landed on the moon, drove a car around on it and played golf!" ... that you would be completely ridiculed.
Anyway, not here to start a huge debate, just providing an answer to a question.
> But for a few years starting in 1969 we routinely soft landed a rocket on an unmapped alien surface
Er, no, we didn't. Before we started landing people on the moon, we sent a series of unmanned mapping missions to the moon so that we wouldn't be trying to land on an unmapped alien surface. [1]
The problem with moon hoax theories is that, in the modern day, we're accustomed to extremely realistic CGI. What we don't realize is that in 1969, it would have been harder to FAKE the moon landings than to actually land on the moon.
> completely secret for decades. Virtually no secrets survive that long.
How do you know that? If a secret did last for decades, you wouldn't know about it.
I can actually give a couple counterexamples: two big government secrets that lasted 3 decades:
(1) A US bomber accidentally dropped a hydrogen bomb out of an airplane into the dessert near Albuquerque, New Mexico, triggering a conventional but non-nuclear detonation. This happened in 1957 but was kept secret until 1986 -- a span of 29 years. "It was only in 1986 when an Albuquerque newspaper published an account based on military documents recovered through the Freedom of Information Act." (ref: http://www.hkhinc.com/newmexico/albuquerque/doomsday/ )
(2) The British were regularly reading encrypted German messages by around 1940. The codebreaking of the German Enigma machine was one of the greatest secrets of World War 2, and the British shared the knowledge with the Americans. This secret was revealed in 1974--after 34 years--because of two books by key intelligence figures. (ref: http://en.wikipedia.org/wiki/Ultra#Post-war_disclosures )
In both cases, at least dozens of people--but more likely hundreds of people--would have been privy to the secrets.
I'm certainly not endorsing conspiracy theories about fake moon landings. I'm commenting only about the persistent meme that big secrets are quickly exposed: it's not necessarily true.
>> completely secret for decades. Virtually no secrets survive that long.
> How do you know that? If a secret did last for decades, you wouldn't know about it.
No, I meant secrets that quickly came out (that weren't secret for long), versus things that were only revealed after a long time. It's a reasonable yardstick for the degree that things can be kept secret "for decades", my claim.
* What women want. Freud famously asked. but no one knows, certainly not women. This may belong in a different category, since it's not clear that anyone knows the secret.
> A US bomber accidentally dropped a hydrogen bomb out of an airplane into the dessert near Albuquerque, New Mexico, triggering a conventional but non-nuclear detonation. This happened in 1957 but was kept secret until 1986
So that's in the never-revealed secrets column? Just checking.
> This secret [Enigma] was revealed in 1974--after 34 years--because of two books by key intelligence figures.
Actually, it was because of the British Official Secrets Act, that required silence on sensitive matters until the government granted permission, and that was obeyed by all concerned. The books were't simply published, after due consideration they were vetted by the authorities in advance of publication. That example belongs in the well-kept secrets act, because everyone involved obeyed the rules until given permission to do otherwise. If it had been a 50 year silence requirement, I suspect that it would still have been obeyed.
But I think you'll agree that rare exceptions don't disprove rules.
> I'm commenting only about the persistent meme that big secrets are quickly exposed: it's not necessarily true.
Not "quickly exposed", that's not a position I took. Thirty or forty years is sufficient validation for my original claim.
There is a question I've always wanted to ask someone who's convinced that the moon landings were faked.
Where the line is between fake and real? Was it the entire Apollo program or just the six landings? Obviously there has been some space travel by humans at the time. In your view how far did we get before faking the rest? Did we just stop with Gemini and say 'screw it the rest is too hard'?
I can acknowledge that it seems superficially anomalous to have people flying to the moon using slide rules, but it's not like NASA just pointed a finger and decided to go. There were a number of steps along the way where each step makes sense compared to the next one. If Apollo 8-10 were real, then 11 seems entirely plausible.
In what way does this demolish the various conspiracy theories? When confronted with evidence of man-made items on the Moon, the tin-foilers just say they were put there by robots.
It always puzzles me that most people think that only Armstrong and Aldrin went to the Moon, although 12 men have walked on its surface.
May be it's me but I couldn't help thinking that the title of this article seems to help spread that idea: only Neil & Buzz went to the Moon, and it happened only once.
TFA does go on to reference "reflectors in the Sea of Tranquility (Apollo 11), at Fra Mauro (Apollo 14) and Hadley Rille (Apollo 15), and, sometimes, in the Sea of Serenity. There's a set of mirrors there onboard the parked Soviet Lunokhud 2 moon rover..."
That would sort of imply that it happened more than once. I do wonder, though, why the Sea of Serenity doesn't get a mission name tied to it.
"I do wonder, though, why the Sea of Serenity doesn't get a mission name tied to it."
1) Too depressing that 17 was probably the last time a human being will ever visit the moon, so lets not talk about it.
2) General lack of selenological (is that even a word?) knowledge. Its like saying I've never visited Ireland, although I have been to Dublin and Galway and a couple places in between, or some similar static type language failure like that. The armchair lawyers like to point out that the mission planners didn't use that name on the map for the landing, but the scientists in charge of the reflector did, or something along those lines.
> (3) The universal force of gravity is very stable. Newton's gravitational constant G has changed less than 1 part in 100-billion since the laser experiments began.
technically worded correctly, but this language implies that we know it IS changing, by some small but non-zero amount.
actually, we haven't detected any change at all - so there's every chance it's NOT changing - but if it is, it's changed less than this small amount which our instruments won't measure more accurately than.
> No variation of the gravitational constant is discernible, (dG/dt) / G = (0.0±1.1)x10–12 /yr
http://ilrs.gsfc.nasa.gov/docs/williams_lw13.pdf