The Multiverse gets its name from the idea that there may not just be one Universe, the one in which we live, but many different Universes. These Universes would be completely hidden from us, we would not know of their existence – they would be as real as this Universe, but only our abstract mathematical and theoretical physics models would give us any clue as to their existence. So the first question that comes to mind is does the multiverse really exist?

Unfortunately we won’t get a definitive answer to that question in this article, but some compelling information will be presented suggesting that the Multiverse may really exist. There will also be a discussion as to why there may not be a Multiverse, so as to make this discussion as balanced as possible.

## Beginning Of The Multiverse Concept

The first solid concept of the Multiverse can probably be traced back to 1952 when Erwin Schrodinger, a Nobel prize winning physicist who formulated the famous Schrodinger matter-wave equation which is one of the foundations of Quantum Mechanics, gave a lecture in Dublin, Ireland describing the Multiverse concept(although he didn’t use the word multiverse).

Warning his audience at the beginning of the lecture that what he was about to say might seem lunatic, he went on to elaborate about how his equations, notably the matter-wave equation, described not just alternate possibilities, but alternate histories that actually did exist simultaneously with the one history that we know.

A little bit later on, in 1957, the American physicist Hugh Everett published a paper on his ‘many worlds’ interpretation of Quantum Theory. There are two main interpretations of Quantum Theory(although there are some less well known ones) – the Copenhagen interpretation and the many-worlds interpretation, from Hugh Everett.

The Copenhagen interpretation of Quantum Theory is probably the most commonly taught in physics classes, and very simply put, describes the collapse of the waveform of various super-positional probabilistic states when it interacts with consciousness. The Schrodinger matter wave equation describes many different super-positional states going forward in time, only one of which collapses into reality, according to the Copenhagen interpretation. By the way, just for the sake of completeness, there is a complex conjugate part of the Schrodinger matter-wave equation which describes super-positional probabilistic states going backwards in time, although this aspect of the equation is usually ignored.

In contrast Hugh Everett’s many-worlds interpretation of Quantum Theory states that there is no collapse of the waveform at all – the different super-positional states described in the Copenhagen interpretation do not exist at all in their probabilistic form. What this means is that all possible solutions of Schrodinger’s matter-wave equation actually exist as reality somewhere, although we can only experience one at a time.

The many-worlds interpretation gets more complicated than this, with the idea that our consciousness can cause a ‘split’ of the Universe we are in when we are making an observation of something – an alternate Universe is created with the alternative outcome of the event being observed. An infinite, or almost infinite, number of alternate Universes are eventually created, with an infinite or nearly infinite number of possibilities. For example, there could be a Universe where Germany won World War 2, another where JFK wasn’t assassinated and served two terms, still another where the Apollo 11 mission blew up on its launchpad, yet another where dinosaurs still exist and are roaming around, and on and on – you get the idea.

This is the basic idea of the Multiverse, that there are a huge number of Universes, perhaps even an infinite number, that actually do exist although we can never see them. The many-worlds interpretation of Quantum Mechanics is one scenario of how the Multiverse might exist, although there are other possibilities as well which we will explore in detail in the sections below.

## Multiverse Hypothesis

The Multiverse hypothesis basically is idea that the Universe in which we live is only one of many, perhaps even an infinite number of Universes, all existing simultaneously, though they are unseen. This concept has many different names – multiverses, parallel universes, alternate universes, parallel worlds, many worlds, quantum universes, quantum realities, parallel realities, alternate realities, alternate dimensions, dimensional planes, and so on. It may well be that the Multiverse hypothesis will never be completely provable; although our advanced abstract mathematics and physics theories may strongly suggest that it is the case.

The concept of multiple Universes have been hypothesized in a number of different area; astronomy, physics, religion, philosophy, cosmology, and science fiction. Physicists are divided on whether some type of Multiverse exists with some thinking that multiple Universes are at least a strong possibility, while others dismiss the idea.

Some physicists believe that the concept of multiple universes don’t belong in the scientific discussion at all. First of all, the concept is currently unprovable by the modern scientific method, and second if the Multiverse does in fact exist it means that the physical laws in each Universe may be different than in the others making it impossible to have any kind of standardization of physical laws and concepts across the entire Multiverse. Physical laws could be so different in most of the Universes that life itself would not even be possible. That brings to question does something really exist if it can’t be observed, but that discussion is beyond the scope of this article.

It would seem that for the foreseeable future the Multiverse hypothesis will have to be relegated to only speculation, however compelling the possibility might be as we develop more advanced mathematical and theoretical physics concepts suggesting its actual existence.

## Proponents & Skeptics Of The Multiverse

What follows is a list of some of the more prominent scientists on either side of the Multiverse debate – first the proponents and then the skeptics.

### Proponents

Some of the more prominent scientists in favor of the Multiverse hypothesis are as follows: Sean Carroll, David Deutsch, Hugh Everett, Brian Greene, Alan Guth, Stephan Hawking, Michio Kaku, Andre Linde, Laura Mersini-Houghton, Yasunori Nomura, Raj Pathria, Leonard Susskind, Max Tegmark, Neil deGrasse Tyson, and Alexander Vilenkin.

### Skeptics

Prominent scientists who are skeptical of the Multiverse hypothesis include: Jim Baggott, Paul Davies, George Ellis, Adam Frank, Marcelo Gleiser, David Gross, Anna Ijjas, Abraham Loeb, Viatcheslav Mukhanov, Roger Penrose, Carlo Rovelli, Joe Silk, David Spergel, Paul Steinhardt, Michael S. Turner, and Neil Turok.

## Evidence For The Multiverse

The search for evidence in favor of the Multiverse hypothesis has not yielded any compelling results so far, although this should probably be expected since these other Universes, if they are real, exist completely outside of our physical laws. This makes the search for evidence extremely difficult at best, if not impossible.

Although there are a few scientists claiming to have found evidence that our Universe has collided with other parallel Universes in the distant past, by examining the cosmic microwave background and the radiant heat from the Big Bang among other things, an examination of this data by other scientists has found no significant statistical evidence of parallel Universe collisions.

## Evidence Against The Multiverse

In much the same way that it is almost impossible to prove the existence of the Multiverse, it is also virtually impossible to disprove it’s existence. Rather than scientific evidence against the Multiverse it is really more of a philosophical argument the existence of multiple Universes.

Our modern scientific method of examining various hypothesises, such as that of the Multiverse, is contingent upon all of the physical laws of OUR Universe, and as such are really useless in either proving or disproving the Multiverse hypothesis. It would seem likely that the physical laws in other Universes outside our own are different, maybe a LOT different – trying to make any comparisons or extrapolations based on our Universe would only be an exercise in futility and prove completely fruitless.

Whether the Multiverse exists or not will have to remain in the realm of speculation dependent upon what our most advanced cosmological models and theoretical physics models might suggest. This type of speculation, whether in the scientific community or the realm of science fiction, will have to suffice for the foreseeable future, and maybe even forever.

## Possible Multiverses

In this section we will be discussing the various types of Multiverses that could exist based on our current understanding of cosmology and physics. We will be drawing heavily on the work of physicists Max Tegmark and Brian Greene since they have some especially interesting things to say about this topic.

### Max Tegmarks Four Levels Of The Multiverse

Max Tegmark is a Swedish-American physicist and cosmologist and also a professor at the Massachusetts Institute of Technology. His classifications of the Universe are arranged in four levels, with each subsequent level built and expanded upon the previous one.

#### Level 1 – Extension Of This Universe

This level predicts that as a result of a hypothetical eternal inflationary expansion of our Universe, which is an extension and variation of the cosmic inflationary model, there will exist a virtually infinite number of Hubble volumes which realize all of the initial conditions. A Hubble volume is the spherical region of the observable Universe(estimated at about 10 nonillion – 10 million trillion trillion – cubic light-years) surrounding an observer, beyond which everything is receding greater than the speed of light, due to the expansion of space, probably caused by dark matter – everything outside of this sphere is unseen and forever hidden from the observer.

With an infinite Universe there will be an infinite number of Hubble volumes which, since they all started with the same initial conditions, will all have the same physical laws and constants, although their distribution of matter and configuration is likely to be different.

Since the number of Hubble volumes is infinite in this infinite Universe, there will be a finite number of Hubble volumes with a similar configuration to the one in which we live. So Level 1 is basically one infinite Universe with an infinite number of Hubble volumes – much like bubbles in a liquid.

#### Level 2 – Universes With Different Physical Constants

Level 2 builds on Level 1 – in this level an eternally expanding and infinite Universe some regions of space will stop expanding at the same rate and distinct bubbles will form – analogous to gas bubbles forming in a loaf of rising bread. These bubbles are themselves Multiverses in a beginning stage of development. In these bubbles there may be different spontaneous symmetry breaking, resulting in different physical laws and constants.

#### Level 3 – Many Worlds Interpretation Of Quantum Theory

Hugh Everett, an American physicist, was the first to come up with the many-worlds interpretation of Quantum Mechanics. Very simply put, the basic idea is that any time an observation is made there is a split in the Universe which results in a different parallel Universe, at least different with respect to the possible outcomes of the observation that was made. This results in a virtually infinite number of possible worlds, or Universes, which contain every possible result of every observation which has ever been made or that ever will be made.

Like Levels 1 and 2 there is an infinite number of Universes, called Hubble volumes in the first 2 Levels. Furthermore, in the many-worlds model, the assumption will be made in Level 3 that the physical laws and constants will be the same much like they are in the Level 1 model. The only difference between Level 1 and Level 3 is where the parallel selves of the observer exist. In Level 1 they live elsewhere in three-dimensional space, and in Level 3 they live on a completely different quantum branch in what is called Hilbert space(a mathematical concept named after the German mathematician David Hilbert who was one of the most influential mathematicians of the late 19th and early 20th centuries), which has an infinite number of dimensions.

In a similar way all Level 2 Hubble volumes can result in worlds coming from the spontaneous symmetry breaking in a Level 3 many-worlds model, which would imply that the worlds in Levels 1, 2, and 3 are really the same thing, except for the quantum branching effect of the observer in Level 3 which results in a virtually infinite number of parallel selves. This hypothesis is equating the Multiverse to Quantum Many Worlds.

#### Level 4 – The Ultimate Ensemble

Level 4 is Max Tegmarks own hypothesis which considers all Universes to be equally real but which have different mathematical descriptions. As Max Tegmark himself writes: Abstract mathematics is so general that any Theory of Everything which is definable in purely formal terms(independent of vague human terminology) is also a mathematical structure.

What Max Tegmark seems to mean from this statement is that any many-worlds, or parallel Universes, theory which can be conceived can be completely described at Level 4 bringing closure to the hierarchy of the types of Multiverses, so there can be no higher levels.

### Brian Greenes Nine Types Of Multiverses

Brian Greene is an American theoretical physicist, mathematician, and string theorist and currently is a professor at Columbia University. What follows is a description of the nine types of Multiverses that he believes could possibly exist.

#### Type 1 – Quilted

With this model every possible event will occur an infinite number of times in an infinite amount of space. The limitations of the speed of light would prevent us from seeing these other identical areas. This quilted model of the Multiverse will only work in an infinite Universe.

#### Type 2 – Inflationary

In the inflationary model of the Multiverse it is composed of various pockets in which inflation fields collapse, forming new Universes.

#### Type 3 – Brane

The Brane model of the Multiverse requires an extra-dimensional theory such as string theory to work. This Brane theory of the Multiverse states that our Universe and many other Universes, perhaps an infinite number, exist on a membrane, or brane, for each Universe which is floating in some higher dimension in a sort of hyperspace.

Floating in this higher dimensional hyperspace these branes will sometimes interact with each other through gravity or some other unknown force, perhaps every few trillion years or so, which result in a violent collision, which we would know as a Big Bang. These violent interactions would keep occurring, which would result in multiple, or cyclic, Big Bangs.

#### Type 4 – Cyclic

The Cyclic type of Multiverse is somewhat of a variation on the Brane model. In the Cyclic model there are many Universes, maybe an infinite number, which are floating on their own membranes, or branes, in a sort of extra-dimensional hyperspace.

This model differs in that these Universes can bounce back through time where they can be pulled together again creating another Big Bang. The contents of the old Universe are thus destroyed and everything starts all over again – this process keeps repeating itself, maybe forever through eternity.

#### Type 5 – Landscape

This type of model of the Multiverse relies on the concept of Calabi-Yau spaces in string theory. Calabi-Yau space is also known as a Calabi-Yau manifold in algebraic geometry and has special properties that are applicable to theoretical string theory in physics.

Here when quantum fluctuations occur they drop the shapes to lower energy levels which result in pockets with different physical laws and properties than the surrounding space. This is a multiverse containing as many as an infinite number of Universes.

#### Type 6 – Quantum

This is basically the many-worlds interpretation of Quantum Mechanics where new Universes are created when an observation occurs. Thus an infinite number of Universes are created in the Multiverse.

#### Type 7 – Holographic

The holographic model of the Universe is simply based on the hypothesis that the surface area of a region of space can completely encode the contents of the volume of that space.

#### Type 8 – Simulation

This type of model, as the name implies, hypothesizes that our entire Universe is only a simulation on some complex computer system which is likely far beyond our understanding. Not only that, but this complex computer system is running simulations of many Universes(a finite number) with different physical laws and properties in each individual program, or Universe, giving rise to what we would call the Multiverse.

This seemingly bizarre hypothesis of the Multiverse seems to be gaining more traction, in large part because of our own rapid technological advances in computers and virtual reality. If we can advance this far in only decades, so the argument goes, how far could virtual reality technology advance in a thousand, hundred thousand, or even million years?

Given this argument there are more and more who believe that we may be living in a sort of computer simulated virtual world – what would lie outside of this simulation is beyond the scope of our imagination.

#### Type 9 – Ultimate

Simply put, the Ultimate model of the Multiverse would contain every possible Universe which is mathematically possible with every possible variation of the laws of physics.

### M-Theory Of The Multiverse

This is a somewhat different kind of Multiverse using an extra-dimensional theory called M-theory, and also string theory. This hypothesis of the Multiverse requires 10 or 11 dimensions in the space-time continuum where the extra 6 or 7 dimensions could either be compacted on a very very small scale, invisible in our every day activities, or simply localized on a dynamical type of D-3 brane – this is short in string theory for Dirichlet membrane, a class of extended objects upon which open strings can end with Dirichlet boundary conditions. Then there is the possibility that there are many other branes which could support multiple Universes, the Multiverse.

### Black-Hole Cosmology Of The Multiverse

Black-Hole cosmology describes a cosmological model of the Multiverse where our entire observable Universe exists inside a black hole, which itself exists in a much larger Universe containing many such black holes, each with a Universe inside.

## Anthropic Principle Of The Multiverse

The anthropic principle of the Multiverse is a type of reasoning used to explain the existence of our Universe and the fact that we exist in it. It has been used as an argument in physics since the mid 1970s. It goes something like this: If there are a gigantic, maybe even infinite number, of Universes – the Multiverse – each one might have different physical laws and constants, and out of all these there are bound to be at least a few where these physical laws and constants are such that a Universe like ours, with all the structures necessary for life to evolve and emerge such as planets, stars, galaxies, matter with a wide range of elements, and so on, would exist.

We can then apply the weak anthropic principle to this to conclude that intelligent and conscious beings such as ourselves can only exist to observe such a Universe if the physical laws and constants are finely tuned enough to allow us to exist in it.

In other words, although the probability of this happening in any one Universe would be exceedingly small, perhaps almost infinitesimal, in an infinite(or almost infinite) number of Universes, it would be likely that there would be at least one, and probably more, where this type of Universe supporting an intelligent conscious form of life would exist.

With this line of reasoning a Creator who intelligently designed a Universe with the conditions necessary for intelligent life would not be needed, although such a possibility would not necessarily be ruled out either.

## Does The Multiverse Exist?

We will end with the question – does the Multiverse really exist? And the answer is, of course, we don’t know. Our most advanced theoretical models of physics and the cosmos suggest that it may exist, but we simply don’t have the means to test such a hypothesis and come up with a definitive conclusion.

Perhaps our technology and theoretical concepts of physics and the Universe will evolve to such an advanced state in the distant future, that such a proof will be possible, but I think it is doubtful. After all, we would be trying to prove a concept that is completely outside of our reality as we know it, where our known physical laws would probably not apply. But then again, we are such curious creatures, if there ever is to be a way to prove or disprove the existence of the Multiverse, then we might find it.