Thread: "The Great Silence"

Originally Posted by Syme

And they wouldn't be able to detect ours unless they are building radio telescopes with 500-mile-wide dishes (which isn't strictly not out of the question, we must admit).

First, you wouldn't need a single 500 mile wide dish. You can construct arrays of much smaller radio telescopes that combined have effectively enormous collecting areas. In fact, LOFAR is going to have an effective diameter of more than 500 miles: http://en.wikipedia.org/wiki/Lofar


Also, if what you're saying is the case, then using a continuous laser won't buy you much either.

The advantage of using a continuous beam from a laser is to transmit a signal is that its energy subtends a relatively small solid angle compared to a dipole emitter, whose radiation is emitted in all directions.

However, this is at most three or four orders of magnitude increase in intensity, I believe...

EDIT: Also, if you poke around on google a bit, you will find plenty of serious discussions about the advantages of using lasers for interstellar communication.
http://adsabs.harvard.edu/full/1992lbsa.conf..637S
http://www.coseti.org/ross_02.htm
http://oreilly.com/catalog/alien/chapter/ch07.html

In fact apparently there is a whole parallel SETI effort, called OSETI, focused on looking for optical communications from other star systems instead of radio signals. Again, the lasers considered are usually actually visible-light lasers, so the problems of making a raser that operates on long wavelengths is probably irrelevant.

Well, first of all, the reason we use radio waves for long distance communication is because there's a window of wavelengths to which our atmosphere is transparent.

So I read one of your links:

http://oreilly.com/catalog/alien/chapter/ch07.html

and now I get why lasers could be useful in interstellar signalling. Even more important than focusing the beam energy into a small solid angle is that lasers can release their energy in very intense pulses. Consider that the "habitable zone" of most stars is considered to be within a radius of 2 million miles and a radius of 1 billion miles. This paper claims that we can make such an intense laser pulse over that gigantic annular region that the pulse can seem appear brighter than nearby stars when observed by an alien whose planet lies within this annulus. That seems really hard for me to believe, but that's what they're claiming....

The second link you provided:

http://articles.adsabs.harvard.edu/c...IF&classic=YES

is talking about making entire planets into enormous, extremely high intesity lasers using the CO2 gas in the atmospheres of either Mars or Venus as the gain medium for microwave lasing. A satellite orbiting the planet would be synchronized with the beam so that the beam bounces off of it at just the right moments to effectively form a resonant cavity for it. Its orbit and mirrors would have to be extremely tightly controlled. Very interesting, but purely speculative.

The third link looks like it's more about what frequency of carrier wave would be best to use and pulse width/period between pulses. I think it's more or less indirectly advocating a pulsed laser scheme to send interstellar messages.

Originally Posted by sycld

First, you wouldn't need a single 500 mile wide dish. You can construct arrays of much smaller radio telescopes that combined have effectively enormous collecting areas. In fact, LOFAR is going to have an effective diameter of more than 500 miles: http://en.wikipedia.org/wiki/Lofar

Yes I know, you'll note I have already mentioned multi-aperture (multi-dish array) radio telescopes earlier in this thread. I was being broad when I spoke of aliens constructing 500-mile-wide dishes. Obviously they would actually use distributed arrays like we do.

I don't really want this thread to become bogged down into the technical merits of laser vs. radio for interstellar communications. Both methods are possible (whether you believe it or not). What I'm interested in talking about in this thread is the "Great Silence" problem and the question of why, if life is common throughout the universe (which many people think it should be), we don't see any evidence of alien civilizations.

Originally Posted by Atmoscheer

And when there are an infinite number of thousand-planet groups, the limit on the equations both approach infinity.

I'm not quite sure what you're getting at. There aren't an infinite number of thousand-planet groups. Large, but not infinite. Again, using human history thus far as a model, intelligent life in general will be vastly more common than intelligent life capable of engaging in interstellar communications. For every alien civilization out there that's shooting radio waves into space and listening to radio waves from space, there are ~1,000 that aren't. How does that not put the lie to your claim that, if we were to find a planet that has life, the odds of finding communication-capable life there should be "pretty much the same" as finding intelligent life in general there?

Originally Posted by Syme

Both methods are possible (whether you believe it or not).

You obviously didn't even bother to read my post. I never said optical communication isn't possible or feasible, so don't say I did.

Again, using human history thus far as a model, intelligent life in general will be vastly more common than intelligent life capable of engaging in interstellar communications

That seems to be a wild claim to make. As a species, humans are only 200,000 years old. Agriculture is only 15,000 years old, and what we consider to be the history of civilization is only aroun 7 thousand years old. We have only had a conception of "interstellar communication" at all for a little over 100 years. As a species we are young, and as a highly organized, "civilized" species, we are even younger, so there is very little reason to believe that most civilizations would be as young as we are.

Also, we have no idea if humans developed civilization faster or slower than other intelligent species would, nor do we know how common it would be for intelligent species to develop civilization. Maybe the probability for an "intelligent" (or rather "sapient") species to produce civilization given enough time is unity, or maybe it is far less than that.

And we really don't know how long a civilization survives on average after it has any ability to achieve interstellar communication. Drake and his contemporaries estimated 10,000 years (rather arbitrarily), and so we, by comparison, are a barely nascent "interstellar" civilization, an arguably we haven't even entered this phase since we don't really have as of yet a firmly established protocol for interstellar communication.