Cris Moore <moore@santafe.edu> wrote:

Sorry, I should have said that there were about 1000 different sets, with ratios measured to about 5 digits of accuracy each...

That might work, but I'm not sure how much the frequency of each tuning fork might drift due to physical changes over millions of years. My idea was that the tuning forks would have some simple constant difference or constant ratio of frequencies, and that the message would be in ternary, with each trit being conveyed by the polarization of that frequency's tuning fork. (Sound in solids have three possible polarizations: Horizontal, vertical, and longitudinal. The first two are analogous to EM waves, the last one to sound waves in gases and liquids.) I'm reminded that Touch Tone frequencies were chosen so that none of them would be close to the sums or differences of frequencies (or of integer multiples of frequencies) of the others, to prevent aliasing due to nonlinerarities in telephone circuits. I don't know whether that's an issue in sound propagation in rocks, but if it is a similar approach could be used. What's the math behind such frequencies?

your point about the gravitational redshift is well taken.

All sorts of subtle effects become important when discussing very high precision measurements. (I remember when I first learned about pi, and the fact that it was then known to thousands of places of precision, I was envisioning mathematicians very carefully drawing as nearly perfect a circle as possible, and measuring it very carefully with a very good ruler, with the aid of a powerful microscope.) Warren D Smith <warren.wds@gmail.com> wrote:

I kind of liked your idea for searching for ET by "looking down." The particular details re tuning forks were stupid, but...

Why? Is there something I'm overlooking?

I think it is an interesting idea in the abstract that the aliens would want to "leave a message" so that they would be heard at any time in the future, not just one time like the stupid idea of sending a radio message once. Also, I point out that this idea was already in Arthur Clarke's "2001" sci-fi.

Yes, and in his earlier (1951) short story "The Sentinel."

His aliens left a monolith made of magic indestructible material on the moon, etc.

It was neither magic nor indestructible. It contained a superconductor with a strong current in it. This resulted in a strong magnetic field, to make it findable, and gave it power to transmit a radio signal when it was exposed to sunlight and warmed by its discoverers.

Seems to me, the Moon is a lot better place to leave a message than Earth. It'd last longer.

That depends. Clarke's message was intended to last only a few million years. In his context, that made sense. (The builders of the monolith had recently uplifted our ancestors into sapience, and reasonably expected that we would soon develop science, technology, and space travel.) But in general, time is deep, and there's no reason to expect that we and aliens are neighbors in time. Over a longer time span, if on the surface of the Moon or shallowly buried, the message would be likely to be destroyed by a meteor impact. It could be deeply buried, but that would make it harder to find. Also, it's difficult to bury things deeply. On Earth, shallow burials often spontaneously become deep burials, due to Earth's geological activity, and aliens could rely on that. And things on Earth will always be more findable by us than things elsewhere, since we will probably always study and explore the Earth more than any other planet or moon.

Perhaps a satellite in Earth orbit would be even longer lasting, and anyhow very easy to spot.

My understanding is that no Earth orbits last more than about ten million years, due to air drag in low orbits and perturbations from the Moon in high orbits.

As far as I understand they claim to be tracking everything in orbit larger than X, where X is what? The size of a golf ball? And nothing was there?

Right. And that was strong evidence that there aren't any long-term orbits, since otherwise natural debris would have accumulated there, as it has around Jupiter and Saturn.

Certainly a golf ball sized satellite would be adequate to carry an enormous amount of info from aliens, terabytes easily. How long would it last?

That depends on where it is. 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. The downside is that they're as close to absolutely unfindable as you can get. Suppose our galaxy contains about a trillion randomly distributed Voyager-sized probes. Think about how any technology, no matter how advanced, could go about finding any of them, ever. I worked this problem a few years ago, and came up with absurd answers, such as converting most of the mass of the galaxy into an immense fleet of starships each equipped with such a powerful radar that not only would we have noticed if any was operating anywhere in the whole universe in our past light cone, but Marconi would have noticed too. Any science fiction novel which depicts a chance meeting in interstellar space, I will throw across the room in disgust. That will never happen, for the same reason that you'll never suffocate from all the air in the room being on the opposite site of the room from you. Physically it could happen, but it's too absurdly unlikely to think about.

Supposedly the layer of dust on the moon is only about 2 cm thick, suggesting that if you are willing to sacrifice an outer layer a few cm thick your satellite has decent odds of lasting a few Gyears?

The Moon has significant gravity. When meteors blast debris from the Moon, almost all of it falls back to the Moon. Not so with reasonable- sized artificial satellites. Anyhow, below the few centimeters of dust isn't a pristine surface, but smashed up debris of all sizes, getting gradually larger as you dig deeper. The Moon is impacted by meteors of all sizes, and is saturation-cratered. (That's another reason why spheres with tuning forks would be more findable on Earth. To reach any large body of undisturbed bedrock on the Moon, you'd have to dig quite deep.)

And dust particles in space obviously do last for Gyears in the sense of not evaporating,

Again, it depends on where they are. The sun's light pressure exceeds the sun's gravity for dust particles, similarly with other stars, so whatever dust doesn't land on a planet is quickly pushed away from solar systems into interstellar space.

So... I doubt ET left any messages for us.

Again I ask if anyone has done a search like Project Durin.