Telescopes and Signals Question

[jshuford]

O.k., I'm not sure how to phrase this, but I'm going to try. If I'm confusing in my question, just ask for a clarification. I want to know how detecting the source (or direction) of an interstellar signal works? Specifically, a theoretical signal sent from an alien civilization at us.

For instance, if a signal is coming from another planet to the Earth, it would have to be hitting the whole planet, right? It's not like the signal is aimed right at our telescope. I mean, one way they identify possible intelligent interstellar communications is to cross-check telescopes hundreds of miles away to look for Doppler shift in the signal caused by the Earth's rotation. So we (the Earth) would be getting a signal on several telescopes simultaneously, so the signal has to be hundreds of miles wide right? Probably hitting the entire Earth (it would have to, otherwise it'd be almost impossible for us to find in the first place). So if the signal is hitting the entire earth, how can we tell what direction it's coming from?

For instance, I know that SETI points its telescopes at sun-like stars, but if the signal they're detecting is hitting the whole earth, why do they need to point at all? And why don't all the signals just get mixed up?

I may be thinking about this all wrong, but I hope someone can help. :) Thanks in advance!

 

[berkeman]

O.k., I'm not sure how to phrase this, but I'm going to try. If I'm confusing in my question, just ask for a clarification. I want to know how detecting the source (or direction) of an interstellar signal works? Specifically, a theoretical signal sent from an alien civilization at us.

For instance, if a signal is coming from another planet to the Earth, it would have to be hitting the whole planet, right? It's not like the signal is aimed right at our telescope. I mean, one way they identify possible intelligent interstellar communications is to cross-check telescopes hundreds of miles away to look for Doppler shift in the signal caused by the Earth's rotation. So we (the Earth) would be getting a signal on several telescopes simultaneously, so the signal has to be hundreds of miles wide right? Probably hitting the entire Earth (it would have to, otherwise it'd be almost impossible for us to find in the first place). So if the signal is hitting the entire earth, how can we tell what direction it's coming from?

For instance, I know that SETI points its telescopes at sun-like stars, but if the signal they're detecting is hitting the whole earth, why do they need to point at all? And why don't all the signals just get mixed up?

I may be thinking about this all wrong, but I hope someone can help. :) Thanks in advance!

The parabolic dish antennas used in Radio Telescopes are very directional. They only have gain in a very narrow angle, so they detect only sources that they are pointing right at.

 

[DaveC426913]

You are thinking of the signal as if it were a flashlight beam with a particular beam width. It's not, it's just like a lightbulb or starlight, shining in all directions.

For instance, if a signal is coming from another planet to the Earth, it would have to be hitting the whole planet, right? It's not like the signal is aimed right at our telescope.

Correct. Signals coming from some distant source are, for all intents and purposes, emanating spherically i..e in all directions, like a lightbulb or the starlight from Sirius. If there were a telescope the same distance from Sirius but 5 light years away from us, it too could see Sirius.

I mean, one way they identify possible intelligent interstellar communications is to cross-check telescopes hundreds of miles away to look for Doppler shift in the signal caused by the Earth's rotation.

No. The two scopes will detect where in the sky the signal is coming from. The same way we could locate Sirius using two telescopes to triangulate its position.

So we (the Earth) would be getting a signal on several telescopes simultaneously, so the signal has to be hundreds of miles wide right? Probably hitting the entire Earth (it would have to, otherwise it'd be almost impossible for us to find in the first place). So if the signal is hitting the entire earth, how can we tell what direction it's coming from?

The signal is effectively infinitely wide, like starlgiht from Sirius.

We can tell the direction it's coming from the same way we can tell what direction Sirius' starlight is coming from: we point our eyes up at the sky and we see light coming from that point in space, right there.

For instance, I know that SETI points its telescopes at sun-like stars, but if the signal they're detecting is hitting the whole earth, why do they need to point at all? And why don't all the signals just get mixed up?

The dishes are pointed in order to cover a very narrow area of sky and amplify it. This is done to pick up extremely weak signals (which they will be, over interstellar distances).

If the signals were strong enough that we didn't need to amplify them with a parabolic reflector, then you are correct, it could (and would) be picked up any any receiver - such as a ham radio - without need for poiting the antenna. However, that would tell us nothing about where the signal is coming from - and where the signal is coming from is at least as significant as what the signal contains.