Drake's Equation [0], the Fermi Paradox [1], and the Great Filter [2].
Drake's Equation basically tries to guess the number of civilizations in our galaxy by multiplying the rate of star formation, the fraction of stars with planets, the average number of planets that could support life, the fraction that actually DO develop intelligent life, the fraction that develop a method of communication that sends signals into space (such as radio), and the length of time those civilizations exist. Obviously, a lot of unknowns here, but if you take some guesses, you can create estimates.
The Fermi Paradox is the contradiction that the galaxy is so vast that it's incredibly improbable that we're the only intelligent civilization in existence, so how come we haven't found any evidence of other civilizations? Considering how relatively young the Earth and our Sun are, some other intelligent life should have expanded into a multi-star species by now, and we should have seen evidence for it.
The Great Filter is basically an answer to the Fermi Paradox. Basically, there are several steps to go from "a star system that might support life" to "interstellar colonization". Life likely needs to start from reproductive molecules, eventually evolving to complex multi-cellular life, to tool-using intelligence, to technology advancement, and eventually towards space exploration and an explosion of space colonies. The question is which step is hardest, least likely to be achieved? Which step is the Great Filter that has led to the apparent lack of evidence of other civilizations? If the filter is, say, the development of complex multicellular life, then it's very possible that Earth really is the only planet with intelligent civilization. If the filter is the final step, then we haven't reached it, and our outlook is bleak, as it would indicate that other intelligent civilizations have gone extinct despite their intelligence.
Obviously, these are very very abridged descriptions. Each of them has entire books dedicated to them.
Drake's Equation [0], the Fermi Paradox [1], and the Great Filter [2].
Drake's Equation basically tries to guess the number of civilizations in our galaxy by multiplying the rate of star formation, the fraction of stars with planets, the average number of planets that could support life, the fraction that actually DO develop intelligent life, the fraction that develop a method of communication that sends signals into space (such as radio), and the length of time those civilizations exist. Obviously, a lot of unknowns here, but if you take some guesses, you can create estimates.
The Fermi Paradox is the contradiction that the galaxy is so vast that it's incredibly improbable that we're the only intelligent civilization in existence, so how come we haven't found any evidence of other civilizations? Considering how relatively young the Earth and our Sun are, some other intelligent life should have expanded into a multi-star species by now, and we should have seen evidence for it.
The Great Filter is basically an answer to the Fermi Paradox. Basically, there are several steps to go from "a star system that might support life" to "interstellar colonization". Life likely needs to start from reproductive molecules, eventually evolving to complex multi-cellular life, to tool-using intelligence, to technology advancement, and eventually towards space exploration and an explosion of space colonies. The question is which step is hardest, least likely to be achieved? Which step is the Great Filter that has led to the apparent lack of evidence of other civilizations? If the filter is, say, the development of complex multicellular life, then it's very possible that Earth really is the only planet with intelligent civilization. If the filter is the final step, then we haven't reached it, and our outlook is bleak, as it would indicate that other intelligent civilizations have gone extinct despite their intelligence.
Obviously, these are very very abridged descriptions. Each of them has entire books dedicated to them.
[0] https://en.wikipedia.org/wiki/Drake_equation
[1] https://en.wikipedia.org/wiki/Fermi_paradox
[2] https://en.wikipedia.org/wiki/Great_Filter