Multiverse ideas are common but often ill-formed since they rarely seriously interrogate the question of "which universes exist?"
Max Tegmark formalized a simple idea that any universe is "real" if it has a complete mathematical description
a consequence of this idea is every computer program is a universe
in other words humans create tiny self-contained universes that we can insert avatars into and move around in, extract sounds and sights from
and we treat this as utterly mundane
Science is inherently incapable of answering the question of first origins
science is an infinite descent of "but why?", a five year old responding to each answer with the same question
if we say the big bang created the universe we are scientifically obligated to ask what caused the big bang
religion solves this problem by simply defining a primal mover and naming it "God"
but really all this means is religion allows itself to respond to "but who created God?" with "shut up"
There are really two problems: "Why something and not nothing?" and "Why this and not something else?"
the first you can maybe just ignore because, demonstrably, something exists
the other question is rough, our universe is weirdly specific. there's only one universe so why this specific one
multiverses offer an answer to first origins that are compatible with a scientific worldview
the reason why things are this way and not some other way is that somewhere else, things are different
Scientists call this the "fine-tuning problem"
the Standard Model of physics has about 25 arbitrary numeric constants that are just hardcoded into the equations
but these numbers are not quite arbitrary enough, that is some of them if adjusted even slightly appear to produce a universe with no useful chemistry and as far as seems possible no life
they seem "finely tuned"
if the constants are random, how did we luck out to get the ones supporting self-aware structures such as ourselves?
So multiverses can provide a satisfying answer to the fine tuning problem
(similar to how the "rare earth" problem is resolved by the existence of a visible universe)
(the solar system is finely tuned to support life, but we can assume this happened by chance because there are many visible star systems and therefore many trials to get it right)
however it is important to remember this is a philosophical answer and not a scientific one
in fact this is not scientific thinking at all
Try defining things this way: science tells us what is happening and philosophy tells us how to feel about it
(this framing sounds like it is belittling philosophy, but it isn't)
(feelings are important)
(you can't do science without philosophy because you need philosophy of science)
(philosophy of science tells us which scientific theories to prefer over others, in other words, scientists ultimately pick the theories they feel good about)
The multiverse answer to origins is a philosophical trick for letting ourselves feel comfortable with an uncomfortably random universe
the philosophizer concludes an infinite number of universes feels more plausible than a single overly arbitrary one
but has no evidence any of these additional universes exist
and has no model for how the multiverse is structured, or if they do, they have no way to evaluate it against competing models
nor is the idea of the multiverse falsifiable
Despite this some physicists have started leveraging the philosophical idea of the multiverse as if it were a scientific one
mostly string theorists
the 11-dimensional M-Theory bulk or the ensemble of universes possible under the KKLT construction
they call this the "anthropic landscape" and propose logical reasoning about it should be considered a new field of science
they seem to be unusually good at securing book deals
This is how it works
consider the "landscape" of all possible universes
applying the "anthropic principle", discard (as irrelevant) all universes with no life
perform statistics on the average universe within the set that remains
this is the universe you are probably in
There are so many problems with this
the construction admits outlier universes exist, but self-aware structures in the outlier universes would find the anthropic argument exactly as plausible as lifeforms in typical universes do
the philosophizer assumes they know which ensemble of universes exist in the multiverse, and can predict which environments self-aware structures can endure, all baselessly
and again there is zero evidence for any of this
castles of air built atop castles of air
The "multiverse" of star systems that solves the rare earth problem is scientific
we can create a model of stellar evolution and evaluate it against competing models, say Giordano Bruno's
we can experimentally confirm or falsify the model using telescopes
whereas multiverse anthropics adopts an idea based entirely on how convenient it would be for academics if it were true
and then imagines it can be used to solve real-world questions
That the string theorists got so lost in their own thoughts so quickly should serve as a warning
if a multiverse is to be considered at all the concept needs to be used in as limited a way as possible with as few assumptions as possible
hence to me the attraction of the Tegmark multiverse, if any
the simplest possible hypothesis with the largest possible ensemble
literally every conceivable universe with systemic behavior included by definition
Best to keep a theory made of feelings within the realm of feelings
an intellectual shrug for the question of first origins
why do we exist
because everything exists
why not
