Final answer:
According to Fermi's Paradox, we could reasonably expect our galaxy to be bustling with intelligent life due to the significant amount of time available for civilizations to develop interstellar travel, relative to the age of the galaxy. However, the paradox lies in the lack of evidence for such civilizations. Several hypotheses, including rarity of intelligent life or technological advancement, non-detection of advanced life-signals, non-interference policies by extraterrestrials, or self-destruction of civilizations, have been proposed to explain this absence.
Step-by-step explanation:
According to Fermi's Paradox, we might reasonably expect our galaxy to be full of intelligent life because d) The time it takes to cross the galaxy at a small fraction of light speed is much lower than the age of the Galaxy. Physicist Enrico Fermi, noted for the formulation of this paradox, pointed out that given the vast number of stars in the Milky Way, many of which could have planets capable of supporting life, and the age of the galaxy allowing plenty of time for civilizations to rise and possibly develop interstellar travel, we should have already seen evidence of extraterrestrial life if it were common. However, there is yet no credible evidence of advanced civilizations, which leads to the question: why are we not aware of them?
Fermi's Paradox suggests several possibilities, including but not limited to the rarity of intelligent life or technological civilizations, the potential self-destruction of advanced societies, the possibility that such civilizations avoid contact with less developed ones like ours, or that we might not be advanced enough to detect their presence. The Drake Equation is often used to estimate the number of intelligent species, factoring in the probability of life developing to the point of establishing communication and the duration of such civilizations. Philosophers of science reference the Copernican principle to argue there's nothing special about Earth's position in the universe, leading to the expectation that life probably exists elsewhere in the cosmos.