2024-07-2613:16
Status:Complete
Tags: Astrophysics
The observable universe is 90,000,000,000 light-years in diameter, with 100,000,000,000 galaxies, each with 100,000,000,000 to 1,000,000,000,000 stars each with trillions of habitable planets which would imply that there should be several lifeforms.
Even if there are aliens, we would likely not know about it because they are outside our local group: it would take billions of years to reach other galaxies even with the fastest spaceships. So in the Milky Way alone there are 400,000,000,000 stars. Just in the Milky Way, there are an estimated 1,000,000 habitable planets. Earth is only approximately 4 billion years old, and some of these other planets would have had 6-7 billion more years to develop past Earth.
On the Kardashev scale there should be at least some Type I or Type II civilization, observable from Earth given it would only take a few million years to colonize the Milky Way. So the question arises: Where are all the aliens?
There are three answers to this question but one of the most popular is the Great Filter:
The Great Filter
A great filter is a set of circumstances that makes life possible or impossible to exist. There are two sub scenarios under this theory: We have passed the great filter and are lucky and special, or the filter is yet to come (ooh scary!).
Already Past
We grossly overestimated the difficulty of life to form. The conditions may be unique and circumstantial, the early universe may have been very hostile, etc.
Yet to Come
There are other civilizations, but they have to overcome several filters to allow for space travel: nuclear war, then climate change, then the graveyard of life (super ominous) e.g. super awesome tech exists it just destroys planets in its wake⌠or some Type III civilization exists and gets rid of all the threats of Type II civilizations.
Source(s)
https://www.youtube.com/watch?v=1fQkVqno-uI https://www.youtube.com/watch?v=sNhhvQGsMEc