"In classical physics, [Deutch] says, there is no such thing as 'if'; the future is determined absolutely by the past."And this is still true in a multiverse model, as long as the individual universes remain independent of one another.
Adding humans into the model: as long as the humans within an individual universe remain able to only perceive the single universe they are in, this determinism also remains true. Under the presupposition of humans experiencing only the universe they inhabit, information transfer and flow remains within the closed system of the individual universe. Things happen (information flow/transfer is initiated), other things happen as a result (information is received), and still other things happen from that (information is retransmitted). Humans have generally instantiated this in the form of "linear time" and "cause and effect".
Keep in mind that these "events" or "things happening" are also instantiations, not the "real thing" -- the events and things happening are given form by humans who perceive them and then give them names, define them, and get enough other humans to agree with the instantiation so that the social weight of the group instantiation makes the humans believe it to be real. -- Thus, for example, things that are seen in hallucinations are not "real" because they are a perception that only one human shares, while the color blue is "real" because when you point to a color and call it blue, most everyone will agree with you and even those who don't agree are disagreeing on the details of the definition, not the fact that a definition of "blue" exists. The world as a flat object used to be real until enough humans shared a different perception of the Earth.
Continuing with the article I cited in my previous post: "In the multiverse, however, there are alternatives; the quantum possibilities really happen"
We can see this scientifically (scroll to the bottom to the section "Interfering with the multiverse)
You can see the shadow of other universes using little more than a light source and two metal plates. This is the famous double-slit experiment, the touchstone of quantum weirdness.But we can also see things in everyday life that should make us stop and consider how we should understand them: as a previously unrecognized part of the individual universe we live in, or as a glimpse of a different universe. A third option might be that it's something imaginary, but that is an option that shouldn't be chosen lightly -- it's easy to use that as a catch-all category for anything that we haven't been able to understand enough to believe in -- but we need to also realize that the fact that something was imagined by someone might be enough to make it a real part of the multiverse.
Particles from the atomic realm such as photons, electrons or atoms are fired at the first plate, which has two vertical slits in it. The particles that go through hit the second plate on the far side.
Imagine the places that are hit show up black and that the places that are not hit show up white. After the experiment has been running for a while, and many particles have passed through the slits, the plate will be covered in vertical stripes alternating black and white. That is an interference pattern.
To make it, particles that passed through one slit have to interfere with particles that passed through the other slit. The pattern simply does not form if you shut one slit.
The strange thing is that the interference pattern forms even if particles come one at a time, with long periods in between. So what is affecting these single particles?
According to the many worlds interpretation, each particle interferes with another particle going through the other slit. What other particle? "Another particle in a neighbouring universe," says David Deutsch. He believes this is a case where two universes split apart briefly, within the experiment, then come back together again. "In my opinion, the argument for the many worlds was won with the double-slit experiment. It reveals interference between neighbouring universes, the root of all quantum phenomena."
And continuing from my previous post again: "'By making good choices, doing the right thing, we thicken the stack of universes in which versions of us live reasonable lives,' [Deutsch] says. 'When you succeed, all the copies of you who made the same decision succeed too. What you do for the better increases the portion of the multiverse where good things happen.'"
I'm not sure I agree with this view of the interaction between universes, apart from the idea that spreading good is beneficial and the good you spread in your universe can seep into other universes when the universes brush up against one another.
My view at the moment is that awareness of the multiverse does other things for us here in our individual universe. For one thing, it's a useful explanation for those moments of deja vu and synchronicity that are otherwise inexplicable. It also can help us keep things in perspective: there really is only today to live in because everything else is just an echo of today. And whatever we think of our current universe, there are countless others that are both better and worse.
Also consider the question of mental illness or other memory or reality disorders... We should think of treatment as nothing more than methods for helping the person deal with the universe he physically shares with the rest of us, rather than an attempt to cure the person of whatever we think he has.
The concept of many universes also applies to spirituality and religion -- in what we call mystical, pagan, or shamanistic practices, the goal is to reach into the other universes while keeping a tie with the current one. The break from those, notably with the Judeo-Christian-Muslim practices, the goal is to close off the other universes and maintain control and clarity of perception within the current universe.