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The multiverse (or meta-universe, metaverse) is the hypothetical set of multiple possible universes (including our universe) that together comprise everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them. The term was coined in 1895 by the American philosopher and psychologist William James. The different universes within the multiverse are sometimes called parallel universes.
Recent developments in cosmology and particle physics, such as the string landscape picture, have led to the remarkable realization that our universe - rather than being unique - could be just one of many universes. The multiverse proposal helps to explain the origin of the universe and some of its observational features.
Astronomers like Martin Rees have speculated that the apparent fine-tuning in the universe exists because at least one of the infinitely many universes will have the particular constants and conditions that made life possible. He admits, however, that the scientific case for a multitude of universes lies on the speculative fringe of cosmology; the idea is built on guesses not laws or evidence.
The multiverse theory is an old one, and often goes under the name "the anthropic principle." What is new is that physicists now have some halfway reasonable conjectures about how all these other universes might come to be. John Wheeler, together with Hugh Everett, suggested an early possibility, in which the alternative worlds are different branches of the wave function in a quantum superposition. (In later years Wheeler moved on from this interpretation of quantum mechanics.) Another idea is that, what we normally call "the universe," is but a small component in a much larger system, in which other spatial regions have different laws and properties. Perhaps the most extreme version of a multiverse has been suggested by Max Tegmark, a theoretical physicist at the University of Pennsylvania. Tegmark proposes that all mathematically self-consistent world descriptions enjoy real existence. There is thus a sliding scale of extravagance, ranging from multiplying worlds with the mathematical laws fixed, to multiplying laws within an overall mathematical scheme, to multiplying the mathematical possibilities too.
Level I: Beyond our cosmological horizon
A generic prediction of cosmic inflation is an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.
An infinite universe should contain an infinite number of Hubble volumes. All will have the same physical laws and physical constants. However, almost all will be different from our Hubble volume regarding configurations such as how matter is distributed in the volume. But since there are an infinite number of such volumes, then some of these will be very similar or even identical to our own. Thus, far beyond our cosmological horizon, there will eventually be a Hubble volume identical to our own. Tegmark estimates that such an identical volume should be about 10(10115) (larger than a googolplex) meters away
Level II: Universes with different physical constants
"Bubble universes", every disk is a bubble universe (Universe 1 to Universe 6 are different bubbles, they have physical constants that are different from our universe), our universe is just one of the bubbles.In the chaotic inflation theory, a variant of the cosmic inflation theory, the multiverse as a whole is stretching and will continue doing so forever, but some regions of space stop stretching and form distinct bubbles, like gas pockets in a loaf of rising bread. There exists an infinite number of such bubbles which are embryonic level I universes of infinite size. Different bubbles may experience different spontaneous symmetry breaking resulting in different properties such as different physical constants.
This level also includes John Archibald Wheeler's oscillatory universe theory and Lee Smolin's fecund universes theory.
Level III: Many-worlds interpretation of quantum mechanics
Hugh Everett's many-worlds interpretation (MWI) is one of several mainstream interpretations of quantum mechanics. In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations each with a different probability. According to the MWI, each of these possible observations correspond to a different universe. Suppose a die is thrown that contains 6 sides and that the result correspond to a quantum mechanics observable. All 6 possible ways the die can fall correspond to 6 different universes. (More correctly, in MWI there is only a single universe but after the "split" into "many worlds" these cannot in general interact.)
Tegmark argues that a level III multiverse does not contain more possibilities in the Hubble volume than a level I-II multiverse. In effect, all the different "worlds" created by "splits" in a level III multiverse with the same physical constants can be found in some Hubble volume in a level I multiverse. Tegmark writes that "The only difference between Level I and Level III is where your doppelgängers reside. In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional Hilbert space." Similarly, all level II bubble universes with different physical constants can in effect be found as "worlds" created by "splits" at the moment of spontaneous symmetry breaking in a level III multiverse
Level IV: Ultimate Ensemble
The Ultimate Ensemble hypothesis of Tegmark himself. This level considers equally real all universes that can be defined by mathematical structures. This also includes those having physical laws different from our observable universe. Tegmark writes that "abstract mathematics is so general that any TOE that is definable in purely formal terms (independent of vague human terminology) is also a mathematical structure. For instance, a TOE involving a set of different types of entities (denoted by words, say) and relations between them (denoted by additional words) is nothing but what mathematicians call a set-theoretical model, and one can generally find a formal system that it is a model of." He argues this "it implies that any conceivable parallel universe theory can be described at Level IV" and "it subsumes all other ensembles, therefore brings closure to the hierarchy of multiverses, and there cannot be say a Level V."
A multiverse of a somewhat different kind has been envisaged within the multi-dimensional extension of string theory known as M-theory. In M-theory our universe and others are created by collisions between p-branes in a space with 11 and 26 dimensions (the number of dimensions depends on the chirality of the observer); each universe takes the form of a D-brane. Objects in each universe are essentially confined to the D-brane of their universe, but may be able to interact with other universes via gravity, a force which is not restricted to D-branes. This is unlike the universes in the "quantum multiverse", but both concepts can operate at the same time.
Is there anybody out there? In Alejandro Jenkins' case, the question refers not to whether life exists elsewhere in the universe, but whether it exists in other universes outside of our own.
While that might be a mind-blowing concept for the layperson to ponder, it's all in a day's work for Jenkins, a postdoctoral associate in theoretical high-energy physics at The Florida State University. In fact, his deep thoughts on the hypothetical "multiverse" — think of it as a mega-universe full of numerous smaller universes, including our own — are now receiving worldwide attention, thanks to a cover article he co-wrote for the January 2010 issue of Scientific American magazine.
In "Looking for Life in the Multiverse," Jenkins and co-writer Gilad Perez, a theorist at the Weizmann Institute of Science in Israel, discuss a provocative hypothesis known as the anthropic principle, which states that the existence of intelligent life (capable of studying physical processes) imposes constraints on the possible form of the laws of physics.
Strong anthropic principle (SAP) (Barrow and Tipler): "The Universe must have those properties which allow life to develop within it at some stage in its history."
This looks very similar to Carter's SAP, but unlike the case with Carter's SAP, the "must" is an imperative, as shown by the following three possible elaborations of the SAP, each proposed by Barrow and Tipler:
"There exists one possible Universe 'designed' with the goal of generating and sustaining 'observers.'"
This can be seen as simply the classic design argument restated in the garb of contemporary cosmology. It implies that the purpose of the universe is to give rise to intelligent life, with the laws of nature and their fundamental physical constants set to ensure that life as we know it will emerge and evolve.
"Observers are necessary to bring the Universe into being."
Barrow and Tipler believe that this is a valid conclusion from quantum mechanics, as John Archibald Wheeler has suggested, especially via his participatory universe and Participatory Anthropic Principle (PAP). However, the argument leads to a chicken-and-egg problem, for those observers must exist in some universe of their own in order to act (unless the observer(s) is self-existent, which returns to the prior possibility).
"An ensemble of other different universes is necessary for the existence of our Universe."
By contrast, Carter merely says that an ensemble of universes is necessary for the SAP to count as an explanation.
What theorists like Dr. Perez and I do is tweak the calculations of the fundamental forces in order to predict the resulting effects on possible, alternative universes," Jenkins said. "Some of these results are easy to predict; for example, if there was no electromagnetic force, there would be no atoms and no chemical bonds. And without gravity, matter wouldn't coalesce into planets, stars and galaxies.
"What is surprising about our results is that we found conditions that, while very different from those of our own universe, nevertheless might allow — again, at least hypothetically — for the existence of life. (What that life would look like is another story entirely.) This actually brings into question the usefulness of the anthropic principle when applied to particle physics, and might force us to think more carefully about what the multiverse would actually contain."
Google Video Link
The Simulation Hypothesis or Simulation Argument proposes that reality is in fact a simulation of which those affected by the simulants are generally unaware. The hypothesis does not have global (there exist reality that is not simulated) scope since, if true, the laws of physics in our known universe require that there is a reality that is not a simulation as there must be a place housing the machinery on which the simulation is being run. The hypothesis itself relies entirely on the development of analog of Cartesian Dieu Trompeur futuristic hypothetical simulated reality, currently regarded as a fictional technology. This technology has been a central plot device of many science-fiction films, most notably Star Trek, The Truman Show, Dark City, The Thirteenth Floor, The Matrix and Total Recall, as well as stories such as I don't know, Timmy, being God is a big responsibility and A Very Special Shutdown Notice. The Simulation Hypothesis has become the subject of serious academic debate within the field of transhumanism, via the work of Nick Bostrom and others
/Simulated reality is the proposition that reality could be simulated—perhaps by computer simulation—to a degree indistinguishable from "true" reality. It could contain conscious minds which may or may not be fully aware that they are living inside a simulation. In its strongest form, the "simulation hypothesis" claims it is entirely possible and even probable that we are living in a simulated reality.
Now a New Zealand scientist is saying that physicists should seriously explore the idea. Brian Whitworth at Massey University says that it is perfectly reasonable to conjecture that "the world is an information simulation running on a three-dimensional space-time screen". Deciding whether or not this is true is a matter for science to resolve.
Assuming Whitworth is serious, what should we make of this idea? He readily admits that this is a weird idea but points out that it is no more strange than many widely held views in physics such as the many worlds interpretation of quantum mechanics, the big bang and Boltzmann brains.
[edit on 17-1-2010 by constantwonder]
So how would we be able to tell if our universe was a simulation? Whitworth says that if reality was to do something that information processing cannot, then it cannot be virtual. But he falls short of suggesting what this might be.
(As an aside, there are plenty of mathematical algorithms that are incomputable. They are the products of a physical human mind, so if they count as something that information processing could not come up with, Whitworth's idea is already dead in the water.)
Whitworth goes on to suggest various ways in which phenomenon associated with quantum mechanics and relativity can be explained in terms of VR.