If seti is looking for EM signals that are a byproduct of a civilization's global or planetary communications system wouldn't the strength of those signals be limited by the power necessary to broadcast them to the intended receiver? I know that the signal would keep traveling far beyond the intended target but at what point (in light years) do those signals get too weak for us to detect? What i mean is, with our current sensitivity, how far away is too far away for us to detect a signal that was intended only to reach a receiver that was within a civilization's own planetary system? Has anyone made these calculations yet? Is this a valid question or am i missing something fundamental? I'm assuming that a broadcaster wouldn't expend more energy than was necessary for their signal to reach the intended receiver. I'm also assuming that the EM signals are for global/interplanetary communication because a civilization that has interstellar travel would need some kind of FTL communication (who wants to wait 20 years per exchange when you're 10 light years from home) which we have no way to search for. Maybe my assumptions are wrong though.

Electromagnetic radiation follows the inverse square law. Therefore, it would require a tremendous amount of energy to transmit something across the galaxy. It is quite a long shot that a signal will be detected, but nothing can be detected if we don't even try.

Oh i agree that we must try. I'm just wondering at what distance a signal intended to only reach within a planetary system becomes too weak for us to detect. There's also the possibility that a civilization might use way more power than necessary because they're trying to announce themselves, which is another reason we should keep searching.

Oh i agree that we must try. I'm just wondering at what distance a signal intended to only reach within a planetary system becomes too weak for us to detect. There's also the possibility that a civilization might use way more power than necessary because they're trying to announce themselves, which is another reason we should keep searching.

Ooh hello :) Welcome to the boards Andrew! Very interesting question! Probably veering a little off to the side here, but you've given me the courage to ask the following question (which may well be one the most stupid ever asked on the SETI boards :)) which is... is it true that "in space... no one can hear you scream". If it is, why is it that we are able to pick up sound waves emanating from beyond our planet? Or does the phrase simply point out the inadequacy of our ears in a vacuum? (See! Told you it was a stupid question! :))

Hi also to Convergence! Don't think we've chatted before :)

Hi Anniet, good question but you need a bit of information. It IS true that you can not hear a scream in space as there is no medium to transmit the signal - no air or solid material. Sound is what is called a longitudinal waveform using a meduim (say air) to pass a series of compressions and rarifications (sp) of the molecules in the medium. No air no sound.

A beginning explanation that will need data added by others:

The signals that are being looked for are electromagnetic and they do not require a medium. No air no problem. At a high enough frequency it would arrive in the form of light, a transverse waveform. In this waveform you are actually seeing energy packets that are traveling at right angles to the wave direction - note the word energy.

So we are not looking for sound waves.

Regards
Robert

edit for grammar

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Hi Anniet, good question but you need a bit of information. It IS true that you can not hear a scream in space as there is no medium to transmit the signal - no air or solid material. Sound is what is called a longitudinal waveform using a meduim (say air) to pass a series of compressions and rarifications (sp) of the molecules in the medium. No air no sound.

A beginning explanation that will need data added by others:

The signals that are being looked for are electromagnetic and they do not require a medium. No air no problem. At a high enough frequency it would arrive in the form of light, a transverse waveform. In this waveform you are actually seeing energy packets that are traveling at right angles to the wave direction - note the word energy.

So we are not looking for sound waves.

Regards
Robert

edit for grammar

Hi Robert! :) That definitely rings a bell in the, er... medium between my ears. :) I know I'd grasped the concept once before... but couldn't remember the significant bits... so thank you! You have gone a long way to providing them :)

Anytime Anniet, my high school physics to the rescue.

As a side note you can not hear: phasers, explosions, spaceships flying .....

LOL

Be Well
Robert

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I'm just wondering at what distance a signal intended to only reach within a planetary system becomes too weak for us to detect. There's also the possibility that a civilization might use way more power than necessary because they're trying to announce themselves, which is another reason we should keep searching.

Hi Andrew - I found this on a blog :) (I know everyone... all hail the blog not :))

Radio was invented just before 1900 and radio waves move at the speed of light, so the farthest from Earth that the earliest radio signals could have travelled by now is about 110 - 115 light years. However, many scientists believe that after about 4 light years, radio waves become so weak as to be indistinguishable from the background radiation.

Obviously that relates to our radio signals - which were not intended to announce anything other than ourselves to ourselves. Someone wanting to announce themselves to others would probably try making far less general racket and much more concentrated "noise" but I can't find any blogger who's addressed the issue as to what strength it would need to be to be distinguishable for more than about 4 light years.

I will keep looking though :) *gone blogging - back later*

@ anniet, thanks for the welcome and also thank you for looking that up! that's exactly the kind of answer i was looking for. So really, unless a civilization was intentionally trying to announce themselves we probably wouldn't pick up their signal if it was farther than proxima centauri. that's kinda depressing :( well, lets hope that there ARE some brave intelligences out there doing exactly that.

@ anniet, thanks for the welcome and also thank you for looking that up! that's exactly the kind of answer i was looking for. So really, unless a civilization was intentionally trying to announce themselves we probably wouldn't pick up their signal if it was farther than proxima centauri. that's kinda depressing :( well, lets hope that there ARE some brave intelligences out there doing exactly that.

+ 1 ... they could be underestimating just how brave they'd need to be once we turn up on the end of their signal :)

Another slightly depressing thing is that so much time would have elapsed, there would be no knowing for sure if they were still around. Unless of course they came up with a way of folding/compressing time or exceeding the speed of light.

[disclaimer]do not be disturbed by the latter sentence - it is coming from a wishful brain - not a scientific one[/disclaimer] :)

I'm not so worried about the time issue. It would be enough for me just to know that a civilization other than ours existed at ANY point in time, even if they were long extinct by the time their signal reached us. Actually, i should say it would be enough for me to have evidence that they exist/existed. I don't know that their are other intelligent beings out there but i do believe that there are, it would be nice to have some evidence to validate my belief.

I'm not so worried about the time issue. It would be enough for me just to know that a civilization other than ours existed at ANY point in time, even if they were long extinct by the time their signal reached us. Actually, i should say it would be enough for me to have evidence that they exist/existed. I don't know that their are other intelligent beings out there but i do believe that there are, it would be nice to have some evidence to validate my belief.

Welcome Andrew to the SETI Forums!

Maybe one day we will get proof, of intelligent beings.

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I'm not so worried about the time issue. It would be enough for me just to know that a civilization other than ours existed at ANY point in time, even if they were long extinct by the time their signal reached us. Actually, i should say it would be enough for me to have evidence that they exist/existed. I don't know that their are other intelligent beings out there but i do believe that there are, it would be nice to have some evidence to validate my belief.

Agree 100% :)

Welcome Andrew to the SETI Forums!

+1

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Hi everyone! thanks for the warm welcome :) Sorry my reply is lagging but its finals week here. Stayed up all night studying for chem final :( not that i dislike chem i just dislike that i have to study so hard to do well, i wish it was as easy as calc. Okay, bbl. Wish me luck, i need it :/

Good luck Andrew:)

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Good luck Andrew:)

+2

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Good luck Andrew:)

+2

+3 :)

Good luck Andrew:)

+2

+3 :)

+4

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What i mean is, with our current sensitivity, how far away is too far away for us to detect a signal that was intended only to reach a receiver that was within a civilization's own planetary system? Has anyone made these calculations yet?

This is a recurring topic in the forum. It depends on the receiving antenna and the transmitting power. See these papers:

For Astropulse-like transmissions: http://www.centauri-dreams.org/?p=29589

For narrow band transmissions: http://www.centauri-dreams.org/?p=3986