# Telescopes and Signals Question

• jshuford
In summary, detecting the source or direction of an interstellar signal involves using directional parabolic dish antennas to pick up signals coming from a specific point in space. Signals from a distant source are effectively emanating in all directions, like a lightbulb or starlight, and can be located by pointing telescopes at the sky and triangulating the source's position. The purpose of pointing the telescopes is to amplify weak signals and determine the source's location, which is just as important as the content of the signal.
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?

jshuford said:
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?

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.

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.

jshuford said:
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.

jshuford said:
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.

jshuford said:
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.

jshuford said:
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.

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## 1. What is a telescope?

A telescope is an optical instrument used to magnify and observe distant objects in space.

## 2. How does a telescope work?

A telescope works by collecting and focusing light from distant objects using mirrors or lenses. The light is then magnified and viewed through eyepieces or captured by cameras.

## 3. What are the different types of telescopes?

There are three main types of telescopes: refracting telescopes, reflecting telescopes, and catadioptric telescopes. Refracting telescopes use lenses, reflecting telescopes use mirrors, and catadioptric telescopes use a combination of both.

## 4. What is the difference between a radio telescope and an optical telescope?

A radio telescope collects and analyzes radio waves emitted by objects in space, while an optical telescope collects and analyzes visible light. Radio telescopes are able to detect objects that are not visible to the human eye.

## 5. Can telescopes receive signals from other planets?

Yes, telescopes can receive and analyze signals from other planets. These signals can provide valuable information about the composition and characteristics of those planets.

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