[QUOTE=chalsall;539126]I'm /loving/ this thread![/QUOTE]Have you read/listened to "The Three Body Problem"? If you haven't you might like it. The sequel "The Dark Forest" deals with some of this in an interesting way. I recommend that if you don't let too long lapse between them. It is better to the the first in mind while absorbing the second.

[QUOTE=Uncwilly;539134]If you haven't you might like it.[/QUOTE]

Thanks for the recommendation. Added to my "to procure" list. :smile:

[QUOTE=kriesel;539133]Either by mechanically rotating antennas, or more recently with phased arrays, radar is a swept beam, which to any relatively fixed point looks like a repeating brief pulse of higher peak power than the omnidirectional equal power. Radar also tends to be a flat beam, pretty tangential to the planet's surface. No point in checking for airplanes underground or in outer space.[/QUOTE]Yup, but the radar's horizon sweeps out a wide solid angle as it rotates.

The phrase "all directions" is somewhat hyperbolic because most high power radars have been located in the northern hemisphere. Nonetheless, a large fraction of the celestial sphere has been covered by their signals.

[QUOTE=xilman;539103]Aiming is not really a problem.

Exercise: compute the width of a 10[SUP]-7[/SUP] radian beam at a distance of 100 parsecs. I've already told you that 1 parsec is 2×105 astronomical units[/QUOTE]
Even with a shotgun one must lead the moving target. A parsec is defined as 3.258 light years. So the center of the transmitting antenna signal strength lobe should be aimed at a point equivalent to up to 326. years of target lead from actual target position at the time the photons depart, and allowing for "windage and elevation" lensing either atmospheric or gravitational or any spatial gradients in the weak interstellar medium that contribute any refraction. (One can hope that gravitational and WIM lensing change slowly relative to the 655.6 year photon round trip, if they are large enough to matter, while atmospheric lensing may be correctable in real time by use of guide stars.)

10-7 radians x 100 parsecs x 3.09 10[SUP]13[/SUP]km/parsec =~3.09 x 10[SUP]8[/SUP]km. Earth's orbit about Sol has mean diameter ~3x10[SUP]8[/SUP]km.
Sol is considered to be orbiting in the galaxy at a tangential velocity of 230 km/sec = 7.26 x 10[SUP]12[/SUP]km/year or 2.36x10[SUP]12[/SUP]km/325.8 solar years (~0.00077 c). That lead distance is ~7653. times the signal cone width at the far end.

The lead to beam us a light is twice that relative to apparent position, since our observed location is 326 years old at that distance. If they image us or Sol today, they see where it was 326 years ago, make a relative angular velocity measurement, and need to aim their photons to where we'll be in 326 years in the future. And they won't be able to get away with rounding like I've used here. 1/7653 is the beam width.

[URL]https://en.wikipedia.org/wiki/Galactic_year[/URL]

Adjust all the preceding for the effect of the source's movement, which I've omitted. If the other point is at the same galactic radius and near us in angle and velocity, riding along with us, lead considerations go away, and the signal is blue or red shifted slightly during the trip, depending on who's in front, and shifted back at the receiver antenna.

This galaxy is around 120,000 light years in diameter, so this technique reaches only a very tiny fraction of the volume.

[QUOTE=kriesel;539178]10-7 radians x 100 parsecs x 3.09 10[SUP]13[/SUP]km/parsec =~3.09 x 10[SUP]8[/SUP]km. Earth's orbit about Sol has mean diameter ~3x10[SUP]8[/SUP]km.[/QUOTE]Jolly good!

You have the required result: that to a first approximation you need only aim at the star where it will be when the photons arrive. To attain a much better approximation, use direct imaging and Doppler information to know where the planet will be with respect to the star to an accuracy of better than a million kilometres.

We already know the space-motion of stars pretty well. Radial velocity measurements well under 1km/s are routine. GAIA gives us their instantaneous projected positions to 2x 10[SUP]-5[/SUP] arcseconds or better, which is 10[SUP]-10[/SUP] radians, three orders of magnitude better than required. The accuracy of proper motions is dependent on the length of time for which positional measurements are made; the nominal GAIA mission is five years. Extrapolation over a thousand years will inevitably magnify errors markedly, but this can be reduced by using the bulk data to compute the local gravitational field for us and the target star, thereby taking curvature of their paths into account. Repeating GAIA a few decades from now will reduce the error in the predicted position to a negligible value.

All of the forgoing computations in this thread show us that interstellar communication to a distance of at least a kiloparsec is well within currently available technology and levels of expenditure. As retina has pointed out, the only significant hindrance is political will and patience.

[QUOTE=xilman;539182]All of the forgoing computations in this thread show us that interstellar communication to a distance of at least a kiloparsec is well within currently available technology and levels of expenditure. As retina has pointed out, the only significant hindrance is political will and patience.[/QUOTE]

Well, maybe one other.
[quote]2X2L calling CQ . . . 2X2L calling CQ . . . 2X2L calling CQ . . . New York. Isn't there anyone on the air? Isn't there anyone on the air? Isn't there anyone . . . 2X2L --[/quote]

[QUOTE=Uncwilly;539134]Have you read/listened to "The Three Body Problem"? If you haven't you might like it. The sequel "The Dark Forest" deals with some of this in an interesting way. I recommend that if you don't let too long lapse between them. It is better to the the first in mind while absorbing the second.[/QUOTE]

Regarding "The Three Body Problem", the following quote was lifted from this link: [url]https://www.amazon.com/Three-Body-Problem-Remembrance-Earths-Past-ebook/dp/B00IQO403K[/url]

"I'm about 25% into the book. I doubt I'll finish it. I have a Ph.D. in Physics, and my graduate program had a strong Particle Physics department (though I specialized elsewhere). The author shows a startling lack of understanding of what motivates Physicists and how they think, to a point where the book is ruined for me."

I've read some of the recommended books in this forum such as "Dragon's Egg" and others and while I can suspend belief reading speculative science I find wrong science fiction "almost" unreadable.

Although not scifi, this kind of a book is a good read after seeing the "Hollywood" movie: [url]https://en.wikipedia.org/wiki/The_Science_of_Interstellar[/url]. Other worthwhile books of this genre (in my opinion) were the novels published by reputable scientists in response to both Velikovsky's and von Daniken's speculations. Recently I came across a TV series hosted by an investigative journalist in which one episode dealt with UFO's and an instance in Canada...Mission Declassified. The other episodes are also worthwhile but don't concern this thread.

Regarding the "three body problem" there exist stable solutions which are interesting in their own right. To close, I live close to one of the best DX'ers in the world (who is a retired secondary school geography teacher) and some of the Morse contacts he makes are truly remarkable. However, I cannot fathom how an advanced civilization could use EM radiation as THE required form of interstellar communication..even if their internet has 5-6-7.. G.

[QUOTE=jwaltos;539314]However, I cannot fathom how an advanced civilization could use EM radiation as THE required form of interstellar communication..even if their internet has 5-6-7.. G.[/QUOTE]

It propagates well? Slow, but well. :smile:

And what do you propose instead? Spooky entanglement?

[QUOTE=chalsall;539319]It propagates well? Slow, but well. :smile:

And what do you propose instead? Spooky entanglement?[/QUOTE]

I will propose that you look at van Raamsdonk's papers and watch a YouTube lecture, specifically where he develops an equivalence between the structure of space-time and entanglement where entanglement has already been shown to be a possible mode of communication. One thing I appreciate is when someone publishes a paper that develops the equivalence between two different (usually mathematical) formal systems of expression. Expressions like the Dirac Equation are like gems that need to be studied under multiple and possibly simultaneous perspectives. Developing a mathematical proof of such results and understanding said proof should advance the formal foundation AND the interpretation relative to known and unknown physics. Prediction in such a case is not a random result from a random process.

[QUOTE=jwaltos;539321]...where entanglement has already been shown to be a possible mode of communication.[/QUOTE]

Sure. But that involves "touch" (the passing of at least one atom) before faster than c bit-exchange is even theoretically possible.

EM is going to be the initial OSI layer 1 for such comms, most likely.

Again, slow as heck. But there now.

Here is a paper that touches on aspects of what I alluded to: Emergent Gravity and the Dark Universe, Erik Verlinde.
Here is a book that explores a certain form of biological communication: Bridging the Gap between Life and Physics, Ron Cottam, Willy Ranson (p.153. "A generalized form of relativity applies to any communication....Communication between glial cells....")

Communication intrinsically involves sentience, regardless of its form, purely biological, hybridized or otherwise. In my opinion, there are many things being developed in isolation which which will die on the vine unless associated in more imaginative ways than exist presently. Serendipity happens through the coincidental interactions of some of these isolated projects and only when someone recognizes the significance of what they are looking at.