He concedes that the Principle can be quite sound under controlled circumstances: If most balls in a box are red, you are more likely to randomly draw a red one. Where stars are otherwise alike, the Principle can be useful for making probability decisions in astronomy.
But the probability of life on other planets presents us with a very different situation: The Principle assumes, in the absence of any evidence, that Earth is typical in terms of its properties and life forms, not simply its position. And that is what he sees as an unwarranted assumption:News, “Astronomer: We can’t just assume countless Earths out there” at Mind Matters News (November 14, 2021)
A quick look at our solar system neighbors should dispel this notion. Mars is a frozen desert; if it had life in its early years, it didn’t offer enough stability to support it for very long. The same applies to Venus, now a hellish furnace. Farther away, there are many “Earth-like” exoplanets, but only in the sense that they have a similar mass and orbit a star at a distance that is within the habitable zone, where water, if present on the surface, is liquid. These preconditions for life are a far cry from life itself.Marcelo Gleiser, “The mediocrity of the mediocrity principle (for life in the universe)” at Big Think (October 6, 2021)
Realistically, he notes, life must exist on a planet for a long time before the ways it changes a planet’s atmosphere could be detected from many light years away. Intelligent life may take longer and be far more tenuous. Put another way, on our own planet, water bears can survive many catastrophes that humans cannot. But they don’t think or seek to communicate with anyone and likely never will.
Takehome: Marcelo Gleiser notes that the starting point of the Mediocrity Principle assumes countless Earths. That’s not a conclusion from evidence. It’s bad logic.
You may also wish to read: Physicist: Copernican Principle doesn’t make Earth insignificant. That, Marcelo Gleiser says, is a philosophical attitude, unrelated to the science. Theoretical physicist Gleiser notes that we’ve only begun to point huge telescopes at exoplanets. There are too many unknowns to be sure of our status.