Warren D Smith <warren.wds@gmail.com> wrote:

--well, storing over 50 bits in a tuning fork frequency is absurd, for one thing...

I never suggested storing 50 bits in a tuning fork frequency. Indeed, I pointed out how absurd that was. My idea was one large tuning fork to get the sphere noticed, and lots of small ones to convey a short message. I don't know enough about sound propagation in rock, or about the relevant signal-to-noise ratios, to know how high a frequency could be used. I do know that if the wavelength isn't much larger than the size of the mineral grains, the sound will be scattered, like light in a fog. But if the S/N ratio is good enough, a fair amount of scattering and absorption can be tolerated, since there's no limit to how much a receiver can amplify a signal.

--I am unconvinced. It is not mathematically possible for any lone body to be "captured" into a high earth orbit, so any would have to have been there since formation.

It's just as possible for a body to be captured as for it to be ejected. It can happen from interactions with the Moon, from collisions near Earth, or from outgassing (e.g. comet remnants). Once it's captured, tidal drag can circularize the orbit, and can either raise or lower a circular orbit. It's believed that Mars' two small moons are captured asteroid or comet remnants, as are the smaller moons around the outer planets.

Also if KFL is correct, then I ask him: where are the tiny satellites of Venus?

Good question. The answer probably involves either the Sun's high tidal effect (2.6 times that on Earth) or Venus' very slow rotation or both. Also, stuff in solar orbit with any given aphelion will have a much higher speed relative to the inner planets when in the vicinity of those planets, making capture more difficult.

"Keith F. Lynch" <kfl@KeithLynch.net> wrote:

...

The Voyager records are expected to remain playable for at least 10 billion years, which is a respectable amount of time even by my standards.

--but you were claiming my 2 cm thick dust estimate was way too small, which seems like it would contradict this?

Interstellar space, which the Voyager probes are entering, is much, much emptier than our solar system. Anyhow, I'm not sure what you think the 2 centimeters of lunar dust is a measure of, other than itself.

--the moon may have formed for a few km of rocks of all sizes in top, already there at formation, not due to meteors. And then, after that short era of formation and initial blasting ended, then the rest of time (several Gyr) only sufficed to cause 2cm of dust to form on top of all that, in most places.

No, the bombardment is still ongoing. It has slowed since the early days of the solar system, but it's far from over. The Apollo seismometers recorded plenty of natural impacts.

If so, indicating decent chance of multi-Gyr survival in space (if willing to sacrifice a few cm thick outer layer) might be feasible.

In solar orbit? It's possible. There are reasonably stable solar orbits, but they're mostly high in debris. However, debris impacts won't just erode the surface; they are likely to either shatter the message probe or knock it into an unstable orbit, i.e. an orbit in which its eccentricity will be pumped up by Jupiter until it hits a planet, plunges into the sun, or is ejected from the solar system. Even if it survives in good condition, how would anyone ever find it and recognize it as artificial? The solar system is very large and contains lots of debris. In interstellar space, as I said, pristine survival is highly likely but anyone ever finding the message probe is all but impossible.

It's an interesting idea, but clearly neither one of us has thought it through enough.

Yes. I need to know more geology, more about sound propagation in rock, and more about how materials behave at high temperatures and pressures for long periods.

(Also, are aliens really going to work that hard to leave a Gyr message for us? Why should they want to?)

Because it's a lot cheaper and easier than pointing a huge radio dish at us and transmitting for a billion years? But people spend time and effort listening for such messages.

If "Durin" aliens were going to make meter-size titanium balls and land them on Earth, it would seem easier for them to do other stuff instead that'd work better.

What do you think would work better? I was envisioning spheres about an order of magnitude larger than that. Pressure vessels scale linearly, i.e. if a one-meter sphere requires a ten-centimeter-thick wall, then under the same pressure a ten-meter sphere would require a one-meter-thick wall. But that assumes uniform pressure in all directions, which may be far from the case in rock. I don't know. I wonder if anyone does. Bill Gosper <billgosper@gmail.com> wrote:

Stromatolite junk DNA?

How do you prevent random mutations from accumulating over the ages?