We have now discussed all the multiverse models considered the most elaborate among theoretical physicists studying such questions. It is quite difficult to wrap your head around some of those models, but I hope that I provided the ideas behind each of them in a fairly comprehensible way. Today, in the last article of this series, I shall try to make some conclusions that are taken partly from Brian Greene’s book “The Hidden Reality” which I referenced in every article. Once again, I encourage all those readers who are interested in these questions to put this book onto their wish-lists, since all of the questions we’ve been considering here are explained by Brian Greene in all the necessary detail and a very clear way. Today we shall be mainly focusing on the question as to whether or not the existence of the Multiverse might someday be proven or, otherwise, disproven. But first, let me briefly remind you what these models are based on.

- The quilted multiverse – this model works only in an infinite universe. If space spreads out infinitely far, every possible event will occur an infinite number of times, but the finiteness of the speed of light will prevent people, or any other living organisms, from being aware of those identical areas.
- The inflationary multiverse – eternal inflation leads to a vast number of ‘bubble-universes’ one of which happened to become our home.
- The brane multiverse – this model follows from String theory (or more precisely, from M-theory) and states that our universe is located on a gigantic 3-brane, which itself exists inside a hyperspace – a space of higher dimensionality – that contains other branes which are separate universes. All the force carrier particles are bound to their branes except for gravitons.
- The cyclic multiverse – a pair of branes, each being a universe, goes through a cycle of colliding, causing Big Bangs, bouncing back and passing through time until they are pulled back and eventually collide again, destroying any complex structures and creating them anew.
- The landscape multiverse – based on String theory combined with Inflationary cosmology. Through the process of quantum tunneling a new ‘bubble-universe’ might be born inside another one. However, this new universe will have another shape of extra dimensions such that the two will vary in physical constants and, consequently, in physical laws.
- The quantum multiverse – an event with a number of possible outcomes that takes place in our universe ‘splits ‘ it such that all of these outcomes are realized each in its own universe.
- The holographic multiverse – derived from the holographic principle and states that any event taking place in our universe is generated on its 2-dimensional boundary.
- The simulated multiverse – exists on complex computer systems that are capable of simulating entire universes.
- The ultimate multiverse – states that every mathematically possible universe is physically real and exists within an all-embracing conglomerate of parallel universes.

What’s particularly interesting about the multiverse idea is that it could become one of the most important discoveries in all of physics. We have known for quite a long time that when exploring the realm of physics we are not to rely on our experience when we research various phenomena which are beyond the scope of ordinary events. After Newton discovered his laws, they have been so simple and elegant and have been describing completely diverse phenomena, such as the motion of planets and moons and the motion of falling apples, that it seemed that all of physics can be described in a similar manner. However, when other physicists continued to explore physical notions they entered the areas which are far beyond Newton’s laws applicability. The main physical theories of 1900s and 2000s, such as Special and General Relativity, Quantum Mechanics and String theory, work with concepts that have no similarities to our everyday experience whatsoever. This all means that when developing a new theory you do not know what place it will eventually take. And this is exactly what happens with the multiverse idea; it will either dramatically shift our understanding of reality or be thrown out as invalid.

Our familiarity with the physical laws that govern our Universe pass on from generation to generation constantly correcting and widening our views. Before Copernicus we thought that we, human beings, are in the center of the Universe which always revolves around our planet. Copernicus became the first person to develop a model which shows that this is not the case. Since then this model has evolved dramatically and is now known as the Copernican principle. Firstly, it showed that our neighboring planets don’t move about our planet, but instead, orbit the Sun. Then we figured out that the Sun itself is not located at the center of our galaxy. After that we realized that our galaxy is not at the center of all galaxies. Finally, we became confident that even the particles that our bodies – as well as planets, stars or galaxies – consist of represent less than 5 percent of the total mass. The existence of the Multiverse would push this concept even further saying that even our universe – the word that always meant ‘all there is’, *everything* – is itself a tiny spec within a vast multitude of other universes. But the main objective of science, and particularly of physics, is not in the further advancement of the Copernican principle. Physicists do not work on widening Copernican views, but instead, they do what they have always been doing: based on obtained data and performed experiments they construct new models which would be capable of explaining the data and make some predictions. It is remarkable that following this logic, researchers encounter various multiverse models all the time. It is far more plausible to encounter multiverse scenarios rather than elude them.

Although the idea of the Multiverse matches the Copernican principle really well, it differs from the former remarkably. When we receded from being the center of the Universe and realized that neither Solar system nor the Milky Way galaxy is, we could easily test and confirm these hypotheses. Many physicists believe, however, that the multiverse idea could never be tested and should be considered philosophy rather than physics. After all, how could this idea be tested if other parts of the Multiverse exist *somewhere* being not within our reach completely? This may sound reasonable, but actually, multiverse concept is much more elegant and might eventually lead to some testable consequences.

Let me mention a few such possibilities. Firstly, even though universes existing within a multiverse may differ from one another significantly, they might have some common properties since they all rise from one theory. If we are not able to find these properties in our universe, our trust in this multiverse model will waver. The confirmation of such properties, on the other hand, will raise our confidence in the model. Secondly, if common properties are not found, some correlations between them from one universe to another may lead to other predictions. For example, one model might say that a new form of particles must necessarily exist according to that model. The impossibility of finding these particles will rule out the corresponding model, whereas their discovery will certainly raise our confidence in it. Then, even if such correlations are not derived, we can apply another form of analysis. We can use the anthropic principle to figure out how common universes such as ours would be among those wherein complex forms of life could arise. If it proves to be fairly common, our trust in this particular model will increase. But if our form of universe largely differs from those which are common in the model under consideration, this model will surely lose its credibility.

We do not have such testable predictions yet, but the Multiverse idea has taken its place fairly recently, so new theoretical research and mathematical models could eventually shed light on testable and, therefore, falsifiable predictions.

This is an overall picture but what about those multiverse models that we were considering in this series? If future observations provide convincing evidence for the Universe being finite, the quilted multiverse hypothesis will be rejected. If future experiments and observations place reasonable doubts on inflationary cosmology, then the inflationary multiverse model will be much less satisfactory. On the other hand, if the inflation model gets conclusive evidence to support it, and if eternal inflation becomes an inevitable feature of this model, then it will be almost impossible to elude the existence of the Multiverse. If String theory fails to be confirmed, then various multiverse models based on it will also have to be discarded. But if the experiments carried out on particle accelerators confirm that gravitons escape our universe, it will give much credence to the brane multiverse model. The idea of quantum multiverse is based on the mathematical concept of linear functions. If it happens to be found that quantum mechanical equations should be modified and there is no place for linear functions in the modified ones, then the many-worlds interpretation will go off the rails. If the holographic principle is to be confirmed, this won’t persuade us that the Universe is indeed a hologram, but we will certainly need to pay attention to this hypothesis. If physicalism turns out to be the correct theory describing the workings of the brain, then computer simulation of a universe with reasonable inhabitants will be impossible. However, if we eventually manage to create such a universe with our computers, then the artificial nature of our own universe will be almost inevitable. Finally, if such computationally simulated universes are based on Gödel-complete mathematical structures, it will be an indication towards the ultimate multiverse hypothesis.

These all are possible implications which require a lot of experimental and mathematical work, and we don’t know how much time will pass before we can make any rigorous conclusions. But interestingly, we already have at least one testable prediction. It is based on the inflationary multiverse model and takes a very strong support from the landscape multiverse one. These models state that each particular universe is some sort of a bubble neighboring other bubbles within the Multiverse. In this case these bubbles can collide. And if our universe has gotten hit by another one, this collision should have produced a subtle fingerprint in the cosmic microwave background (CMB), that we might, one day, be able to detect. If it happens, this will be a direct confirmation of the existing of the Multiverse!

With that said, I want to mention the last (but not least!) important thing that will happen if we somehow manage to confirm the existence of the Multiverse. Sometimes science gives an eye to details. It explains why the sky is blue, why desk is solid, why two objects with different masses fall down with the same velocity under the force of gravity, and so on. Sometimes science takes a broader view and discovers that we live in a galaxy with hundreds of billions stars, that this gigantic structure is itself one of hundreds of billions galaxies, that the entire Universe is homogeneous etc.

But sometimes science does something else: it shifts our views on science itself. When physicians work on some problems they must take into account three things. The first one is the initial conditions of your system (the positions and velocities of the objects within the system, their angular momentum, and so on). The second is fundamental physical constants (such as the universal gravitational constant, Planck’s constant, fine-structure constant and others). The third is the mathematical equations describing the behavior of a system in question (e.g. Newton’s equations, Einstein’s field equations, Schrödinger’s equation etc.) Possessing information about initial conditions and applying mathematical equations to the system under consideration you can determine the result of your experiment (a certain outcome of a classical system and corresponding probabilities for a quantum one). This method has a very wide applicability which is not in question at all. However, it raises a very deep question as to how could we explain initial conditions themselves – i.e. the initial data at the very beginning of the Universe. Why did these conditions happen to be those that led to the Universe we see today? How could we also explain the values of fundamental constants? And finally, could we explain *why* certain mathematical rules describe the physical behavior of various objects?

Various multiverse models tell us that there are *no* answers to any of these questions whatsoever. If there are many universes instead of just one, then initial conditions may vary from one universe to another, which would lead to their different structures. Therefore, there is no fundamental explanation to the particular initial conditions which our universe had, and our question is simply misguided. Instead, a right question to ask would be: is there a universe where the configurations of particles would correspond to those that we see in our universe? If we are able to show that the number of such universes is high in a multiverse model under consideration, then our universe will certainly find itself in this model, and the question as to *why* the conditions in our universe are what we’ve got need not be answered.

Similarly, the question about the values of fundamental physical constants is also wrong-headed as it is raised by the logic concerning a single universe. In the inflationary multiverse model various properties of the inflaton field lead to various masses of particles and diversified interactions. Likewise, in the multiverse models based on String theory fundamental constants vary with the different shapes of extra-dimensions. In the case of the Multiverse we should change our question to: does the corresponding model allow a universe with fundamental constant such as ours to exist? And how often would you encounter such a universe, at least among those where these values provide a possibility for our form of life to take hold?

And even the question about mathematical structures, providing consistent physical laws, might be turned on its head. In the simulated and ultimate multiverse models these mathematical structures vary from one universe to another. In the first one, mathematical and physical laws are dependent upon the choice of a simulator, while in the second – *every* mathematically consistent structure is physically real.

All these three questions seem absolutely mysterious when a single universe is concerned, but in the multiverse approach there is no need in giving answers to them since there cannot be any answer at all.

I thank all the readers who have been following this series and see you all next time.

Thanks for a great series Aleksei, it has been a huge feat and absolutley fascinating.

I disagree that multiverse theories cannot be considered simply because they can’t be tested. Just because we cannot test something now, doesn’t mean that we will never be able to test it so therefor multiverse theories are definitely physics.

Nice work Aleksei!

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thanks a lot Peter! I really appreciate your thoughts and comments concerning these models!

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