Does anyone take seriously the Many Worlds interpretation of Quantum Mechanics anymore?
It is a very serious possibility. The real problem with this concept this idea is not that it can occur (since quantum computers rely critically on it) but how to integrate the concept into our real world. It will one day be better understood but we are speaking about the superposition of states which represent alternative events that may happen in our World. In the end a single state will be the result. The other states may or may not represent events as we see them but they may have a reality despite our inability to record them. You must ask the question can you believe any event that you cannot confirm or record? Do you actually believe in the possibility of alternative realities?
Events in our Universe represent a continuum of "something" that is interconnected. Please do not think of the world as being made of little separate billiard balls... this is actually quite wrong. Everything are "electromagnetic waves". We choose different outcomes for our lives by a simple binary tree process. At each fork in the road a choice is made (an event), the outcome is then simple but practically "irreversible". It is not actually a conscious choice but this "infinitesimal" process of a single event has an outcome. Does the Universe make copies of us all the way out to "infinity" to support all the possible choices we are capable of or is there some other way in which to interpret this phenomenon? I like to think of it in a different way. We usually treat particles as "points" and the way the particle "evolves" in time is its world-line... a series of points tracing the particles path through time. As T increases, the places each atom and even each sub-atomic particle has occupied sweeps out a path throughout "history". Now each person is a "collection" of such particles... an ensemble. Around this person are other ensembles of other peoples and particles. It is impossible for our mathematics to track this complex function and it is customary to reduce this process to a much simpler system that we can actually "handle" of a single point or maybe a collection of a small number of points if we are feeling "up to it" computationally.
One day our science will make it possible to do a much better job of this abstraction we call the Universe but for now we have reduced it to almost a collection of the most simplest of functions possible. Something like the reductions a child would make when they draw their parents on paper ... simple stick people with only the most superficial features visible... a line for the mouth, another line for an arm etc... this is the nature of our present Physics. Now consider this world line for just one single sub-atomic particle and consider where it really was... it is not possible to actually trace its path since there are no records and its position was a lot less "distinct" than you are willing to admit. It is a collection of dynamically variable quasi-stationary electromagnetic waves filling the space that it is existing in... the wave functions... a collection of "Fourier Components" locally collected in the general vicinity of the original particle. They are effectively "space filling".
Every particle has such an ensemble of waves and each of these are also "space filling"... in one way they are able to "superimpose" and "spread" everywhere in the vicinity of a certain "individual" particle, perhaps up to many kilometers from where you are seeing the "center of this action". Now try to visualize this "volume" of events limited by the speed of light and by the strength of the interactions rapidly dying with distance (as little perfect waves) till maybe a few kilometers away the single event no longer is appearing to have any further influence. So what is happening here may be just one single event. Where is it?... when is it?... Most importantly ... what is it?... it is very fuzzy. The event in reality is actually confined to a volume of several cubic kilometers and it is happening to a single sub-atomic particle.
An "action" by a person will be the sum total of an uncountable number of these tiny sub-nuclear events... a vast bundle of intertwining world-lines reaching everywhere this event could potentially touch as interacting "wavelets" for all those kilometers. You will say this is not the case since we only witness "one" event and one outcome and it is confined to a very small space and to a very short window in time. This is true from a single point of truth as an "observer" interacting and "disturbing" this collection of wavelets since the observers wavelets are also interacting with the "scene" as well. In this respect the observer is part of the scene and the evolving events. The interaction is far beyond our present science to compute and to evaluate but this is no impediment to the most powerful natural supercomputer that exists... it is so parallel, to almost an infinite degree, and is able to calculate this out with "natural" algorithms because the algorithm is built basically into the very base nature of the "space" this is happening in.... a kind of "resonant chamber" that is ideally conditioned to resolve these wavelets into "phenomena". At each tiny point in our many dimensioned space, its nature is a system of "components" based on energy alone and working with geometry and the natural elasticity of the "medium"... "computes" a function that localizes the tiny sub-atomic particle wavelets dynamically
as if being focused by a lens. In fact a lens is the most powerful mental tool to conceptualize this "engine". To me this is the most important principle of our Universe since it "supports" everything that is happening.
The wave is "everywhere" but the particle is found to be just where you expect it to be. The particle's expectation value is "highest" where the particle's field is now becoming "evanescent". A tiny moment later this same particle is found somewhere close bye, maybe rotated through a small angle and now interacting with some other sub-atomic particle with a gluon or something. This shift in energy density affects the entire "scenario" out to a number of kilometers. It is just an interplay of waves and nodes and caustics. This "mechanism" is the
only mechanism that provides us all the Laws and Physics we are able to understand... there is "nothing" else, but what more do we need? It is still way beyond the human mind to be able to produce a model that will operationally simulate this "Universe".
It is most probable (according to Feynman-Wheeler Theory of Advanced and Retarded Potentials) that half of these waves are focused from the past and the rest are from the future so that the entire ensemble make up a whole. Just as you need the whole of a hologram to produce the sharp pictures they can make. Even though a small bit of a hologram has all the "picture"... the detail is missing... so too is this "snapshot" of the local World also holographic... and to create the sharp definition we call reality... we need all the detail out to several kilometers to "resolve" everything that is occurring inside that tiny volume where this single "tiny" event is proceeding. The spatial volume itself that it is embedded in is the "machine" that captures that "dynamic" element and instantly processes it. Space may be empty... but it is not without a very important use... it is an "optical processor" for the waves. Just do not think this is only an "optical" processor since the "optics" is all there really is and it is responsible for all the electromagnetic forces including gravity and the "illusion of hardness" which is only electromagnetic force as well.
So just what are these "Many World" possibilities? What is seen by an observer and what events are occurring are not etched in stone and the
phase of all these interacting phenomena... uncountable to our present technology, and for "some time to come"... actually are occurring "right now" simultaneously in that space as interacting waves. We
"resolve" one aspect of them as if this was a polarizing filter at a certain angle. We make an "observation". That polarizing filter may have been rotated through a tiny angle... an angle so small that you could not resolve it on a dial... this is more than enough to change the entire phenomena way out to those kilometers distant. A different event will have occurred. The sub-atomic particle will move "imperceptibly" into a different direction and the knock on effect is a different sequence of "causes and effects" resulting in an altered history. The Universe cannot remember all this happening (there is nowhere to store it) so it "forgets" instantly what has happened and is already "computing" the next event. So in a way for this one tiny particle all histories have in one way "occurred", it remains for the quantum theorist to try and calculate the probability of each of these possible worlds, what are all the possible states for this system (a single quantum particle) when the polarizing filter is rotated through the full 2pi radians and what appears to be a "continuum" of states is resolved into one.
But suddenly a traveler from the future "pops" into the scene and the quantum theorist is aghast because he was not able to calculate this eventuality... You may ask why not?... It is not a surprise to me. It is something the quantum theorist could not have known, no matter how smart he thought he was to predict the future this possibility was always unforeseen. Time travel "opens the box" to events beyond prediction since this adds another ensemble of world-lines to the already fuzzy mess that is beyond the limits of prediction. Time travel is possible for single events... and by extrapolation... much bigger collections of events, this is noted at the particle level. All you can say here is you can never know when things may happen that cannot be predicted beforehand to happen. Will the Universe prohibit this phenomena?... I doubt it... since the Universe has no memory it just responds to the new events like the waves in a child's bathtub respond to the child splashing about... the waves cannot anticipate when the "rubber ducky" is thrown in to change the overall "equation". It will respond in a predictable way. It was always a possibility in the bathtub since it was the intention to throw the rubber ducky in all along. Our Physics is without that "mind" aspect attached to it, it deals with mostly dumb random motion and simple linear dynamics. It does not understand the dimension that the act of a predetermined mind may cause... especially if it comes from the future.
A Time Traveler can select between the many world interpretations possible and potentially choose an outcome, even an unexpected outcome. This ruins the predictions of quantum theory proving it has "limitations". The traveler and his ensemble of wave-functions will be seamlessly integrated into the "scene" because the phenomenon is "bosonic interaction". His materialization should occur beyond the earth since the fermions do not obey Bose-Einstein Statistics... it is best to "arrive" where there are no other particles to interfere with the traveler's system. From that point on we only have events occurring and these have a limited "range" so the arrival of a traveler can modify the history of the evolving system and thus change the course of a history. The time traveler is a useful concept to resolve some ideas regarding the Many World Hypothesis.
Cheers