Consider this. SETI pins it's hopes on a narrow bandwidth where the universe is relatively tranparent. The resoning being that if someone wanted to send a signal, this is the best frequency to use.

Well, there are several problems associated with this logic.

They become apparrent if we ask ourselves whether someone living on, say, alpha centauri could detect that there is intelligent life on the earth using the same techniques.

• The first problem is that we don't broadcast in that narrow band of frequencies that SETI is scanning. In fact, I believe this part of the spectrum is deliberately set aside for the purposes of astronomical research.
• Where we do broadcast, it's in a mess of domestic signals that are jumbled together at various frequencies and bandwidths. For example; think about all the radio and television signals you can pick up at your home. Your friend in a neigbouring state can get a completely different set of signals. Imagine this multiplied accross the entire globe - that's a lot of signals, isn't it? That many signals recieved all at the same time by an alien civilisation would look like noise.
• We actually did send 2 signals out on the particular frequency that SETI monitors, back in, oh I thik it was 1974, from the Aricebo Radio Telescope. So anybody listening would have to be listening to US at the EXACT time that the signal from us went past them. Pretty unlikely.
• The signals we did send in 1974 were in a narrow, 'searchlight' beam. That means that they would also have to have been in the beam to recieve anything, again, pretty unlikely, and from memory, one was sent to Sirius, and the other to the Plieades.

So we haven't exactly been very communicative ourselves, have we?

In fact, we couldn't even detect if there was intelligent life on Alpha Centauri, let alone more distant star systems.

Maybe that's a smart move, given the competitive nature of natural selection and humanity's track record, there's a fair arguament to say that a more technologically advanced civilisation (read "dominant civilisation") would exploit us, as richer, more advanced nations have done to poorer nations throughout history.

Imgine that. We could finally make contact with alien life, only to find the Earth made into a gigantic sweatshop making expensive running shoes for our alien 'saviours'.

-Frisbee

Or cut into strips and marinated...

Yes. The other part of this question seems to confuse people, Why do we think that sending and recieving radio signals of any frequancy would happen any where else in the universe? ... its nonsence.
You may be familuar with Douglas Adams 'Hitch hickers guide to the galaxy' In this trilogy he sagests that finding a cup of tea any where else in the universe would be against the probabilaty factor so it could only happen if there was a common link ie; distant descendance, out cast or other related factors. To take this a little deeper.. Firstly you would need a society that wanted to drink water poluted with the dried leaves of a plant, hot and with milk. where else are we going to find lactating cows?... tea trees, people that would dare to drink this stuff, and an enviroment that could suport all this stuff.?..... :blink: So what I'm trying to say is that a society could develope into a nuclear capabilaty, and posibly evan space travel without ever transmiting a raidio signal. would it not be posible to develope light comunication, laser or optic cableing. Using light or infared or whatever. or minnute electrical impolses, any way thats an idea I would like to see some descusion on.

It's very difficult to communicate in other regions unless your technology evolves via an understanding of electromagnetism and the radio region of its spectrum. Its survival value gives it one up on the alternatives.


Why SETI probes the 21 cm hydrogen region is not clear to me. The universe is mostly hydrogen and must generate a lot of noise in this portion of the spectrum. As we go interstellar, would we communicate at this frequency?

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OMG It looks like you've agreed with me.... My logic must be flawed.

There really is no fundamental difference between radio and laser communication, just which frequency you happen to like using. Certain frequencies would be transparent to an oxygen atmosphere. In space, if you wanted to transmit your TV broadcasts in orange visible light, nothing would stop you. And yes, we don't make too many powerful coherent siginals ourselves for ET to pick up on.

Our best bet is to look for other earthlike planets, not hunt around for a jackpot civilization that happens to be broadcasting ridiculously strong radio siginals on a bizzare frequency that we have found no reason to use ourselves.

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You folks have touched on just a few of the many considerations of transmitting and receiving radio signals. Fresbie mentioned that our domestic radios often use some of the same frequencies in many different places all over Earth, and asked how could an advanced civiliation may decipher so many signals.

Seperating the different signals isn't too difficult with good multichannel technology. However, those frequencies couldn't even be considered because of their extreamly short range. That is exactly why here on Earth, different radio signals can be transmitted from many different locations.

Why, many of our domestic radio, and television signals would be hard pressed to be received even on the Moon which is quarter of a million miles away.
Travel the half billion miles to Saturn, with the most powerful receiver which can possibley be built, and you still won't pick up our domestic radio traffic.
At absolute best, you might pick up a very weak and static filled television satelite transmission, if the satelite was misdirected and turned directly toward you.
Let alone any thought of receiving our domestic radio and television signals at Alpha Centauri, which is 25 trillion miles away.

Not when we have Voyager and Pioneer probes which are barely twelve hours out from Earth at the speed of light, and can no longer receive instructions from Earth using our most powerful radios with the biggest transmitting antennas on the planet.

Transmitting to Sirius, which is something like 50 or so trillion miles or 8 light years distant is impossible in radio. Oh you could send a signal, but it would be too distorted to be received or understood after eight years in space.

Laser communication would be more practical, but still take as much time.

We would be best off to stop transmitting, and even stop listening in radio band, and use our radio astronomy to help find the exo-planets. Then decide what if anything is there to communicate with. Which is not very likely.

I agree with you. wish i had said it all myself...

We don't broadcast in visible light because:

a) we see visible light, so it would be one of the most annoying things if we did (technically we do broadcast in visible light, you know that light in your house? well it would be transmitting light at 50 or 60 Hz)

b) the shorter the frequency we go the more energy to requires to transmit it. The frequency of visible light is so high that to transmit in that spectrum would be so coslty companies wouldn't do it, and we'd go through the fossil fuels like there's no tomorrow. Radio waves are much cheaper to transmit.

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If we want to draw attention to our selves or make contact we need to make a signal that could be seen or other wise be detected by the targets of our quest. This idea has little or no merit at all, and I am being very silly. but here we go... Could we send a craft out to Alpha Centari. I sagest it would take 10 to 15 years maybe. Once there this robotic vesil would need to change the stars output to a polsating or whatever would get noticed from the region around this part of our galaxy.. Its just that any spiecies that can distroy or change a stars output is going to get the attention of any technoledgy orentated alians around here.. maybe, or not... Next question is can we do this or not? Not. I would think.

No, there is a definite difference between raio and laser; it is collimation; focussing.

If all the energy of a signal is focussed on a particular distant system then it is much more difficult to pick up the transmission accidentally at a system which s not the target.
Focussing is more efficient the shorter the wavelength.

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I say not. Furthermore, I don't know if I want to get aquainted with a civilization with technology sufficient to destroy a sun, or change the output of a star.

Imagine what such a people could do TO us. Never mind what they could do FOR us, because why would they want to be so benovolent to a sociaty as violent as the human race?

I can almost hear them now. "You will be assimulated. Resistance is Futile."