When ordinary people ask, where are the aliens? We usually just laugh it off and take it as a joke. However, when this sentence is uttered from the mouth of a scientific authority, it becomes a question that is terribly thoughtful. The sun is a relatively young star, which means that there are terrestrial planets that are much older than the earth. In theory, their civilization should be far more developed than ours. Earth is 4.54 billion years old. Assuming we compare with a planet P which is 10 billion years old. If P’s experience is similar to that of Earth, their civilization should be 5.46 billion years ahead of us. Their technology will be beyond our comparison. This gap is like the monkey seeing human civilization. So why haven’t we discovered alien civilization so far?
One explanation for the Fermi Paradox is that alien civilizations tend to self-destruct before making contact with other civilizations (such as Earth). e.g. climate change, nuclear war
Another explanation is that intelligent civilizations are rare, either because the conditions necessary for the evolution of intelligent life are scarce, such as humans needing oxygen to live.
Or the alien civilization has received the signal sent by the earth, but they may not reply to us for some reason.