The late famous astronomer, Carl Sagan posed a very important question.

“What do civilizations look like that have lived for millions of years? We've only had radio telescopes and space shuttles for a few decades; Our technological civilization has seen only a few centuries. That being the case, what about some multi-million year old technological civilizations? Do they see us, that is, this human race, as monkeys?”

A blind question that remains unanswered to this day is whether we are alone in this vast universe. Although there is no evidence of life anywhere other than Earth, most scientists believe that there must be other intelligent beings like us in this vast universe.

Even if we cannot observe such extraterrestrial civilizations at the moment, we can certainly, using the laws of science and physics that we know, predict the things that might exist in a civilization. Using the laws of quantum physics, general relativity, and thermodynamics that we now know, we can infer how large and technologically advanced civilizations may have been before our own.

"Is there life anywhere else than on Earth?" Although the question asked is purely speculative, we are now discovering many planets outside our solar system. Sometimes, evidence of extraterrestrial life can also be found very quickly. Although there are many planets that are wind giants like Jupiter, we have also discovered planets similar to Earth. When we look at the night sky in the future, it will not look the same as our ancestors did. In the future we may have an encyclopedia of the composition of life on the planets orbiting those star points visible in the sky.

Well, while we cannot predict the exact features of advanced civilizations on such planets, we can outline their basic features. No matter how technologically advanced a civilization is, it cannot violate the basic laws of physics. As we now know the laws of physics from atoms to galaxies, we can certainly explore how these advanced civilizations could exist within a scientific framework.

We can classify a civilization based on its power requirement based on the following factors:

Based on the principles of thermodynamics: No matter how advanced civilizations are, they cannot overcome thermodynamics, that too the second law of thermodynamics.
Theoretical Laws of Static Matter: Common atomic matter is found in the universe in three forms: planets, stars and star clusters. So the power of a civilization may be found in a planet, or a star or constellation.
Planetary Dimensional Development Theories: The power needs of an advanced civilization must be greater/faster than massive biocatastrophes (meteor collisions, supernovas, etc.) on the civilization's host planet. If not, the civilization may end up in total extinction.