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BACK to the timeline: Perspectives on faster-than-light and time travel ("warp drive") technologies circa 2400 AD-2450 AD...
The parallel universes of a Level One multiverse
As of early 2003 it appeared that our reality consisted of a counter-intuitive 'multi-verse' of sorts, with scientists attempting to get a grasp on how many levels it might possess, and how (or if) those levels might be bridged in some fashion-- even if only in theory.
In early 2003 space appears to possibly be infinite in its depth and distance, even if mankind can only 'see' with his instruments out to as many lightyears as has been possible for light to travel in our direction since our local Big Bang. Infinite space looks to lie beyond the visible horizon.
Within the infinite distances of the infinite space over the horizon, parallel versions of ourselves look likely to live and toil on parallel versions of Earth. There may be only the tiniest of differences in chronologies or other characteristics of these parallel worlds to our own. Or there may be enormous differences. And everything in-between. Every possible different version of your own existence and life likely is transpiring somewhere among those worlds.
"In infinite space, even the most unlikely events must take place somewhere" -- Max Tegmark
This level of the multi-verse (level one) enjoys the greatest theoretical certainty of existence circa 2003.
The laws of physics on these parallel worlds are identical to our own. Only the circumstances present in their local Big Bangs were different from ours, thereby initiating the infinite number of parallel tracks and timelines.
It appears extremely unlikely we could ever travel to these parallel worlds to meet our other selves, or even communicate with them, based on the theoretical limits of technology as we currently understand them, and the unimaginable distances involved. The closest Hubble volume most near to possessing a being virtually identical to ourselves may be some 10 to the 1,028 power meters away. If you wanted not just an inhabitant identical to yourself, but the entire Hubble volume nearest identical to your own (we're talking maybe some 14 billion lightyears worth of galaxies here), that would be some 10 to the 10,118 power meters away.
On the other hand, the more and wildly different a given parallel universe is to our own, the closer it apparently could be. Which means on an individual level it'd be far easier for us to get to a parallel universe where Earth and humanity doesn't exist at all, and aquatic silicon lifeforms are dominant in the 'other' Milky Way galaxy, than to any sort of reality which would seem more like home.
But alas, even the nearest bizarre parallel universe might still be far too distant for even an effectively immortal being to reach via conventional travel-- before our local universe itself dies.
The infinity of wildly diverging multiple sets of parallel universes of a Level Two multiverse
In this theoretical multiverse, there's an infinite number of enormous bubbles, with each bubble containing an entire level one multiverse, incorporating the infinity of parallel universes/timelines described before. However, in level two, each bubble can have its own physical starting conditions, constants, and particles, independent of the others (though physical laws may remain the same throughout). So travel between bubbles could be pretty hazardous without something akin to a level two protective suit or other means of life/reality support. But of course the distances separating such bubbles would far surpass even those between parallel universes within each bubble: so only God-like beings need even apply for such risky travel. Indeed, in a level two multiverse, even the distances inbetween the parallel realities within a single bubble would be expanding so fast that travel between such realities themselves would be vastly more difficult than described before for a simpler level one multiverse.
If we live in a multiverse of a level two nature, the odds of our ever meeting our parallel selves would seem vanishingly small, even if and when we attained God-like technological powers by which to attempt it.
The probability stream of a Level Three multiverse
Here is a superverse much resembling a vast river of possibilities in what is otherwise a void, with at least one and perhaps many major 'currents' vying for dominance and direction at its core. This great stream of chance is most vital nearer the center, becoming in many spots more dilute and diffuse and inconsistent as one approaches the outer edges. This metaphysical flowing sea harbors strong, perhaps bizarre and unexplainable preferences for certain kinds of events over others near its core, but allows almost any sort of thing to happen nearer the periphery.
Here a different kind of parallel universes/timelines is generated, based on slight differences in decisions and actions made by each realm's inhabitants. A single particular universe at the Big Bang over time multiplies into an infinite number, branching like a tree to incorporate every different possible outcome. This superverse may have began with an infinite number of such starting universes. Or not-- as the probabilities involved from beginning to end can all be prejudiced (the causes of such bias may remain unresolved for a long long time to come).
In this superverse the parallel inhabitants perceive the birth of whole new universes merely as a random element in their own choices or environment. Only a truly outside observer can perceive the non-randomness at the center of the superverse stream, as well as all the parallel (and at times opposing) courses which develop within it. Perhaps the only way to gain some measure of external observer value independent of the stream would involve some manipulation of level four multiverse mathematics.
In a level three multiverse this branching of realities is somewhat similar to that of a level one. A major difference is that the parallel universes of level one are located a vast distance away in something much like the normal spacetime with which late 20th century humanity is already familiar. But in level three these parallel universes exist within an infinity of dimensions of Hilbert space. The branching of realities and timelines in Hilbert space is quantum-based-- at least superficially.
In a level three multiverse, it looks like the perception of time may be one key way for folks like us trapped within it to best exploit our circumstances. For it may be that in one way or another, all possibilities across the entire multiverse may always exist simultaneously in both space and time-- with our conscious measure and perspective of the moment being the main thing separating us from other universes and timelines.
Some possibilities for how individual exploitation of such a level three multiverse's opportunities might be perceived by observers in the immediate vicinity may be found here.
In some ways the probability stream may resemble a vast probabilistic biosphere, with event wells acting something in the medium as gravity wells do in spacetime, only in the stream arbitrating the likelihood of events subject to conscious perception by living entities, from the scale of microbes up through sentients, both organic and not. This phenomenon may serve to prejudice certain timelines towards extropic activity and the creation of dissipative structures and away from the entropic, in a sort of counter-balance to other cosmic forces. Something like how dark energy may exert an effect opposite to that of gravity.
It may also be that such event wells may themselves be sentient, or appear that way to certain of those capable of detecting them. The consciousness of such event wells (if and when it exists) may be of the collective sort, perhaps even a superset of all the lesser consciousnesses whose existence are dependent upon its whims. Such an entity may not be subject to the same spacetime constraints as we: its group consciousness may simultaneously include all living minds which existed in the past, exist now, and will exist later within a given region of spacetime over millions or billions of years.
The level three multiverse may be the least popular concept among scientists circa 2003. Perhaps partly because it implies that "anything goes" in some respects.
The mathematical magics of a Level Four multiverse
In a level four multiverse, the full possibilities of mathematics may reign supreme, breaking many of the typical connections among physical laws and constants found in the previous levels, to spawn universes which may bear no resemblance at all to any real world our most imaginative scientists and story tellers could come up with.
Unfortunately, there are vast voids and potentialities in mathematics which could make such a multiverse intimidating even for gods to traverse.
At this level perhaps all the realities of the lower levels could be explored, at least in some limited ways. However, it appears that such exploration is strongly constrained by such things as Godel's Incompleteness theorem and others. The flip side of restrictions is that quite a bit of mathematics may actually reach up from level four and into still higher multiverse realities (if such truly exist)-- though the usefulness or practicality of such realms may be negligible for living beings of parameters such as 21st century humanity and even the most advanced future progeny of such which can be imagined. And the dangers of tapping such realms for such as we may be infinite.
It may be that if the level three multiverse truly exists, then someone who fully mastered its concepts could conquer space and time, and perhaps even go on to use level four principles to explore whole new realms beyond anything ever imagined before.
All the above is based upon my own interpretation and understanding of the article(s) cited below, as well as other references included on this page and elsewhere.
-- Parallel Universes: Not just a staple of science fiction, other universes are a direct implication of cosmological observations By Max Tegmark; April 14, 2003; Scientific American
-- A Brief History of the Multiverse By PAUL DAVIES (science.slashdot.org citing the New York Times, April 12, 2003)
-- Slashdot: A Skeptical Look At The Multiverse posted by timothy on Sat Apr 12, 2003; science.slashdot.org
-- Information in the Holographic Universe Theoretical results about black holes suggest that the universe could be like a gigantic hologram By Jacob D. Bekenstein; August 2003 issue; sciam.com; July 14, 2003
"...an infinite number of worlds could exist with 10^1,000 different types of physical laws operating among them."
"We finally learned that the inflationary universe is not just a free lunch: It is an eternal feast where all possible dishes are served."
-- Universe offers 'eternal feast,' cosmologist says; eurekalert.org; 18-Feb-2007; Contact: Dawn Levy email@example.com 650-725-1944 Stanford University
"...there may be clones of ourselves with separate lives."
-- Michio Kaku: Mr Parallel Universe By John Crace; February 22, 2005; education.guardian.co.uk
"There are good reasons to believe that the universe is infinite. If so, it contains an infinite number of regions of the same size as our observable region (which is 80 billion light years across)."
"...we have an infinite number of regions like ours and only a finite number of histories that can play out in them. It follows that every possible history will occur in an infinite number of regions."
"...countless identical civilizations are scattered in the infinite expanse of the cosmos."
-- ALEXANDER VILENKIN
-- THE WORLD QUESTION CENTER 2005; edge.org
"...certain special states of a system are promoted above others by a quantum form of natural selection..."
"If it wasn't for quantum darwinism...the world would be very unpredictable..."
"...the macroscopic...world...emerges from the quantum world...through...decoherence..."
"Decoherence selects out of the quantum...states that are stable, that can withstand the scrutiny..."
"The environment is modified so that it contains an imprint of the pointer state..."
"...each individual's observation is based on only a tiny part of the environmental imprint..."
"They survive monitoring by the environment to leave 'descendants' that inherit their properties..."
-- Natural selection acts on the quantum world [I believe the quotes, title and URL to be originals; author and date info acquired second-hand, due to original report being made inaccessible to the public-at-large; the quotes may be listed out of original order, since I couldn't check the original] by Philip Ball; 04/12/23