Would extraterrestrial civilizations know that there is life on Earth?
If there are intelligent lifeforms out there, what are the chances of them being aware of life here on Earth? (No, I am not scared of extraterrestrials invading Earth for resources :-). I'm just curious.)
We really don't have any way of knowing this for sure because we have no idea of the number of intelligent civilizations out there, or where they are. However, we can say something about the conditions that would be required for them to know that we exist.
In order to know that there is life on Earth, an extraterrestrial civilization would probably at least need to be technologically advanced enough and have good enough astronomical instruments to "resolve" Earth from the Sun -- that is, to take an image of our Solar System in which Earth appears as its own separate point of light, not completely washed out by the Sun's light. This is very hard to do -- we certainly can't resolve Earth-like planets around faraway stars yet! (However, certain space missions that are just now in their planning stages have this as their goal; for example, the Terrestrial Planet Finder.)
If they were able to resolve the Earth and take a spectrum of its light (i.e. look at how much light of each color they were receiving), they would be able to tell that Earth was hospitable to life, by determining its temperature and the presence of large amounts of water on the surface and oxygen in the atmosphere. They might even be able to figure out that there is life on Earth in the form of vegetation -- chlorophyll, the chemical in plants responsible for photosynthesis, has a relatively distinct spectral signature known as the "red edge."
As for the question of whether extraterrestrial civilizations might be aware of intelligent life on Earth, most of them probably would not. Our civilization has only been sending detectable signals of our existence into space for a very short time. The television broadcast during the 1936 Olympics in Berlin, Germany is often regarded as the first time that humans produced a radio signal that was strong enough to be easily detectable from other stars. (If you've ever seen the movie Contact or read the book, you probably already know this because it's an important part of the plot.)
This means that we've only been sending out detectable signals for the last 70 years or so. Since the signals we send out travel at the speed of light, they would only have been able to reach stars within 70 light-years of us, and only extraterrestrial civilizations located within this distance would have been able to receive evidence of our technology. This is a very small region of space -- it covers maybe a few thousand stars out of the 100 billion stars in the Milky Way galaxy! Therefore, if there are other intelligent civilizations spread throughout our Galaxy, the vast majority of citizens who live in them would not know of our civilization's existence (barring the possibility that they had some way of surpassing the speed of light barrier, of course).
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