[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]

What I had in mind is what is known as, if I have the terminology correct, "proper motion."

[QUOTE=Dr Sardonicus;539107]What I had in mind is what is known as, if I have the terminology correct, "proper motion."[/QUOTE]Our posts crossed. Mea culpa. I hit save before addressing your point.

Further on the background radiation. In reality, the receiver would almost certainly be using a coronagraph to shade the stellar radiation, thereby removing the almost background completely. It is how we image exoplanets right now.

Does anyone have the feeling that I may have been thinking deeply about interstellar communication for a few decades now?

I have also been wondering about another aiming problem. I draw a distinction between [i]prearranged[/i] communication, and [i]establishing contact[/i] with a civilization whose location you don't know; and, in fact, which you don't even know exists.

With prearranged communications (as with a spacecraft launched from earth) you have precise knowledge of the frequencies being transmitted, where to aim the transmitters and receivers, and how to extract information from the transmissions.

If you're searching for ET, or trying to tell all and sundry, "I'm here!" you don't know any of this.

Highly directional transmitters and receivers are of limited utility if you don't know where to point them.

[QUOTE=xilman;539111]Does anyone have the feeling that I may have been thinking deeply about interstellar communication for a few decades now?[/QUOTE]

So have I..for 30 years+. In the 80's while in the military, I did some magical things using simple Morse transmissions by understanding what I was doing during the time I was doing it in the part in the world where I was doing it within. EM theory, Meteorology, Geography...etc were brought to bear on the theoretical and applied resolution of a point to point contact. Presently, van Raamsdonk has some interesting ideas where integer factorization may have an important part to play.

Have you a clear set of theoretical parameters that must be fulfilled for your possible envisioned communications to exist? I've worked out some details regarding two "intelligent" connected objects within different "light cones (which is anachronistic)" requiring some new theoretical results regarding those things called "fields" that spawn mass/energy(what is it exactly?) and space-time. Can you predict anything within your thinking that can be tested and that is new?

In its rawest form (in my opinion), quantification requires a structure that is discrete. Whatever is considered "discrete" must be better understood..the "all or nothing" or "mathematics of superposition" work astonishingly well but a great deal is missing it seems to me. Symbolic reasoning works great but how great is it? Godel..P vs NP..the marriage between symbolic formalisms that look different but mean the same thing are indicators. I believe a mind like "Euler's" would work some magic in this day and age.

I just got back from a month in the Bahamas so I had some time to contemplate .. like Jimmy Buffett watching the tides..[url]https://www.youtube.com/watch?v=HhNt04Dg1HE[/url]

To close, Arthur C.Clarke made a couple of great observations regarding communication being a basis for civilization and being educated beyond your intelligence. Jack Vance wrote more about social dynamics than simple technological change (Languages of Pao) - Babel-17 is worth a look too.

[QUOTE=Dr Sardonicus;539119]I have also been wondering about another aiming problem. I draw a distinction between [i]prearranged[/i] communication, and [i]establishing contact[/i] with a civilization whose location you don't know; and, in fact, which you don't even know exists.

With prearranged communications (as with a spacecraft launched from earth) you have precise knowledge of the frequencies being transmitted, where to aim the transmitters and receivers, and how to extract information from the transmissions.

If you're searching for ET, or trying to tell all and sundry, "I'm here!" you don't know any of this.

Highly directional transmitters and receivers are of limited utility if you don't know where to point them.[/QUOTE]The first stage in communication may well be the one-bit message "I exist" which does not require highly directional signals but does assume that any potential recipient is monitoring a wide range of frequencies.. Our military radars have already sent that signal out to 20 parsecs or so in all directions. Given that, you can very easily determine from whence the signal came. Examination of the time variation of intensity will tell you whether the transmitter is on a planet (signal strength drops to zero when the planet is in the way) or in orbit. Doppler shifts then give you the orbital velocities and, if appropriate, the planetary rotation period (which can also be cross-checked with intensity information). With that information, stellar astrometry and stellar models (i.e. the mass of the star) tell you the orbital parameters.

A fair assumption is that anyone transmitting at a particular frequency is quite likely to be listening at a similar frequency. Further, given that information is being provided at one frequency, it would seem to be a good idea to look in more detail for other transmissions coming from the same source at different frequencies and to transmit likewise.

Extracting more information than the above is an interesting exercise in pattern recognition. It seems a reasonable assumption that if you believe that someone may be listening, you will take steps to make the patterns easily recognizable.

[QUOTE=xilman;539125]Doppler shifts then give you the orbital velocities...[/QUOTE]

I'm /loving/ this thread!

Probably stupid question: Don't you need to know the carrier frequency of the transmission(s) in order to determine the red-shift? Or can that be determined observationally?

[QUOTE=chalsall;539126]I'm /loving/ this thread!

Probably stupid question: Don't you need to know the carrier frequency of the transmission(s) in order to determine the red-shift? Or can that be determined observationally?[/QUOTE]The latter.

It doesn't actually matter what the carrier frequency may be as long as you can detect (at least some of) the transmission.

Once you detect it, you set your detectors with a wide enough bandwidth to pick up all the signal. The initially observed frequency, together with plausible orbital and rotational velocities (a few tens of km/s at the most), will tell you how wide you need to look. As more Doppler information comes in that and the stellar radial velocity (measurable to under 1 m/s with current technology --- stellar spectral lines [b]are[/b] standard frequencies) will tell you the carrier frequency.

[QUOTE=xilman;539125]The first stage in communication may well be the one-bit message "I exist" which does not require highly directional signals but does assume that any potential recipient is monitoring a wide range of frequencies.. Our military radars have already sent that signal out to 20 parsecs or so in all directions.[/QUOTE]
"In all directions?" That gives me a [i]great[/i] lead-in to the (dis)advantage of (non)directionality. I'll assume we have a beam spreading out uniformly within (one nappe of) a right circular cone, and that a line on the cone passing through the vertex forms an angle [tex]\theta[/tex] with the axis of symmetry. If I did my sums correctly, the beam subtends a solid angle of

[tex]2\pi(1\;-\;\cos\theta)\text{ steradians.}[/tex]

A truly omnidirectional transmission would have

[tex]\theta\;=\;\pi[/tex]

and would subtend a solid angle of [tex]4\pi[/tex] steradians. If [tex]\theta[/tex] is very small, one has the excellent approximation for the solid angle,

[tex]2\pi(1\;-\;\cos\theta)\;\approx\;\pi\theta^{2}\text{ steradians.}[/tex]

Thus, compared to your highly directional beam with

[tex]\theta\;=\;10^{-7}\text{,}[/tex]

a truly omnidirectional beam is attenuated by a factor of very nearly 4 x 10[sup]14[/sup]. That might cut down the range of detectability a bit.

I imagine the military radar signals galloping through the ether have [i]some[/i] directionality, but I'm pretty sure there will still be a huge attenuation factor in comparison to a highly directional beam with [tex]\theta\;=\;10^{-7}[/tex].

[QUOTE=Dr Sardonicus;539130]I imagine the military radar signals galloping through the ether have [I]some[/I] directionality, but I'm pretty sure there will still be a huge attenuation factor in comparison to a highly directional beam with [tex]\theta\;=\;10^{-7}[/tex].[/QUOTE]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.