A Maddening Myriad of Multiverses II: A Crash Course for Laymen into Multiverse Models
On multiversal models with 'Cosmic Cycles', Bounce, Simulations and Quantum Mechanics
In the previous part of ‘MMM’ we set the stage by talking, in extreme brevity, about what is the idea of ‘fine-tuning’ the universe, the dimensionless input parameters that exist in our current observable Universe, and the first 5 set of multiverse theories that ponder on what may exist beyond.
For the uninitiated (and the initiated as well), we spoke about the first five Multiverse Theories that are propounded to explain what might exist beyond our own ‘observable’ universe and these included the two versions of a ‘quilted multiverse’, an inflationary multiverse, a 9-dimensional string multiverse, and the brane multiverse. We look forward to the final six multiverses over the course of this (hopefully) brief article.
I. Can these Multiverse Theories even be validated?
The conduct of certain scientific experiments may validate key assumptions that we have posited for each of the multiverses, thereby bringing us closer to accepting the said multiverse as the correct theory. For instance, during exponential expansion (inflation), tiny quantum fluctuations in the gravitational field are stretched into long-wavelength gravitational waves1. It is predicted that these waves would polarize CMB photons in a pinwheel pattern. Thus, it can be said that confirming the polarization of CMB photons directly supports inflation. Also, more precise CMB data might validate the reasonableness of existing scalar field potential energy curves. Conversely, the legitimacy of the fundamental inflation theory is challenged by competing theories.
There are also several experiments attempting to discover extra dimensions and, if successful would so thoroughly shake the foundations of physics that it might make us willing to question basic, seemingly self-evident, elements of reality.
For instance, The Large Hadron Collider (LHC) experiments address extra dimensions in two ways. First, if energy is lost in LHC experiments, it may be escaping to other dimensions. And second, at short distances, gravity may have greater strength (not flow into other dimensions), allowing mini black holes to form more easily2. Since mini black holes should leave a unique signature when disintegrating, experimental data from LHC could validate their existence, inferring that at greater distances gravity does leak into other dimensions.
Additionally, in the case of a multiverse founded upon the idea that is ‘string theory’, a key aspect of string theory is supersymmetry that predicts a heavier partner particle for each existing particle in the Standard Model3. If the LHC identifies the matching new particles, then an important part of string theory would be validated. Since the ‘M-theory’ braneworld string vibration energies are considerably weaker than the conventional string energies4, it may be feasible for the LHC to detect new particles corresponding to expected vibration patterns. The cyclic process, relying on brane collisions, might produce gravitational waves5, but because there is no inflationary stretching, there should be no evidence of gravitational waves in CMB data. If detected, they would invalidate the predicted Cyclic multiverse. Also, were accelerated expansion of the universe to stop, the cyclic concept would be disproved because it predicts continuous expansion. In the bounce scenario, gravitational waves might have made an imprint on the cosmic microwave background radiation bringing information from a time near to, or even before, the big bang6. The larger point I am seeking to drive is that the achievement of any breakthroughs in the assumptions that we have listed down would bring us closer to the acceptance of one of the multiverse theories.
TLDR: A complete validation is not possible given where we stand - infact it may be far from achievable for now, but we can inch towards it with time; and have certain aspects or ‘building blocks’ of these multiverses validated.
II. Theory #6: A Cyclic Multiverse
The Cyclic multiverse is another prediction predicated on the ‘M-theory’. In a process that repeats over time, two colliding branes generate new universes (also referred to as the ‘Ekpyrotic Universe’)7. Although laws remain unchanged, the constants of nature may vary as the process repeats. Consequently, there is an explanation for fine tuning. Some theorists predict periodic collisions between braneworlds8 that produce Big Bang scenarios9. This proposed cycle, occurring over a trillion years, is: collision, expansion, cooling, dispersion and then another collision. Repeat. Repeat. Repeat. Although the abovementioned collisions occur in long cycles, infinite time generates an infinite number of universes that repeat in time.
The process excludes a dramatic “inflation” type of expansion.10 This theory not only addresses the fine-tuning issue but also the “infinite regress enigma” by answering the question, what does the last turtle holding up the universe stand on? Since the cycle is continuous, it needs no turtles. The Cyclic multiverse idea is founded upon the assumption regarding firstly, the process of branes physically colliding in trillion-year periods11 and, these brane collisions producing Big Bang-type events12 (inflation’s dramatic expansion not required).
III. Theory #7: The Big Bounce Multiverse
In the big bounce multiverse theory13, the expansion state is created from the rebound of a contracting universe (as an alternative to inflation). It is based on the concept of Loop Quantum Gravity (LQG). Loop quantum gravity suggests that the atomic structure of spacetime changes the nature of gravity at very high energy densities. Loop-based scenarios are founded on general principles of quantum theory and relativity theory and therefore avoid introducing new ad hoc assumptions (as with inflation).14
LQG predicts the existence of spacetime atoms each with a volume of 10(^-99) cubic cms. Particles and fields are described as spin networks. Spacetime is essentially conceived as a ‘spin foam’ with discrete time.
Sidebar 1: What are spin-networks in the LQG theory? LQG stems from the need to describe Einstein's theory of general relativity through the lens of quantum theory - that is, to describe the General Relativity theory quantum-mechanically. Put crudely, it is one of the ways of reconciling the theory of relativity with the theory of quantum physics / quantum mechanics - a conundrum that many describe as the most pressing challenge for any physicist to resolve. In order to take Einstein’s principle of background independence seriously one must generalize quantum theory. One such generalization which leads to a candidate for a quantum gravity theory is 'Loop Quantum Gravity (LQG)'. In LQG, at each instant of time, geometry is concentrated on one dimensional structures, called graphs, which can be arbitrarily complicated. A graph is simply a network of one dimensional, oriented lines which are linked together at their end points to form a kind of mesh. The arrows give each line a direction (in mathematics-speak, an orientation). Each line is labelled with a half-integral number. The mathematical background of this number is the same as that of spin numbers in particle physics, a type of number used to describe a basic property of elementary particles. Consequently, this number is called a spin label. The result is what is called a spin network, and it is taken to represent the quantum state of space at a certain point in time.
When a preexisting universe collapses under the attractive force of gravity, at one point the density grows so high that gravity switches to being ‘repulsive’ and the universe starts expanding again. Effectively, it ‘bounces’. In this scenario energy is recycled from the previous universe. The assumptions that are at the basis of LQG involve the existence of spacetime atoms, a repulsive-effect gravity15, and the ‘bounce from previous universe’.
IV. Theory #8: A Holographic Multiverse
The Holographic multiverse represents two physically equivalent universes, one existing on a two-dimensional boundary surface mirroring a second universe in three-dimensional space. Notably, the issue of fine tuning is not resolved in this version of the multiverse, and no plausible justifications are provided for how one is supposed to assume that it is bio-friendly. From black-hole theories, physicists conclude that the amount of information contained within a region of space is always less than the area of the surface that surrounds the region (expressed in square Planck units), a strange relationship, since volume increases with the cube of a sphere’s radius.16
Consequently, since all physical phenomena can be encoded on the surrounding surface, our three-dimensional reality could be a holographic projection from a two-dimensional surface – the holographic principle. Several scientists and physicists consider it to be an extremely interesting development, as Brian Greene puts it: “A particular nongravitational, point particle quantum field theory in four spacetime dimensions describes the same physics as strings, including gravity, moving through a particular swath of ten spacetime dimensions”.
Restated, under this version of the multiverse string and quantum field theories are equivalent approaches expressed in different languages. Others have proposed holographic models reinforcing the concept or incorporating other distinct elements into it. The Holographic multiverse assumptions include the creation of a ‘mathematical holographic theory’ (through an amalgamation string theory and quantum field theory), and that a two-dimensional surface projects a three-dimensional reality.
V. Theory #9: A Quantum Multiverse
In the Quantum multiverse, an apparent wave function collapse spawns a parallel universe in Hilbert space (an abstract space where the wave functions live). Rather than a wave collapsing abruptly when “observed,” it continues in a separate, previously identical universe. In this theory too, like the eighth multiverse concept, no insight into fine tuning is provided.
On the one hand, where quantum physics is governed by the ‘Copenhagen Interpretation’, the wave function collapses into one outcome – no other outcome occurs. On the other hand, where quantum physics is governed by the ‘Many Worlds interpretation’17, each quantum fork in the road splits the universe in Hilbert space18 (abstract infinitely dimensional space). The ideal way to visualize it is to think of it like a fork in the path.
Since quantum decisions are prevalent, the process naturally generates many parallel universes. The Quantum multiverse rests on assumptions including non-collapsing wave function (the ‘Many Worlds Interpretation’) and Hilbert theoretical space with possibly infinite dimensions.19
VI. Theory #10: A Simulated Multiverse
According to the hypothesis formulated under the ‘simulated multiverse’ argument, we live in a computer simulation. This option is significantly different from the preceding proposals, since a “computer program” may modify the basic laws of nature. Any number of simulations could exist so the simulated environment explains fine tuning.
The key underlying assumption is based on sentience, the ability to create conscious self-aware beings within a supercomputer. Whether this is possible is still uncertain and indecipherable. Science fiction writers have long envisioned integrating people into a simulated reality as in the Keanu Reeves starrer ‘The Matrix’. It is also founded on the belief that ‘consciousness’ in human beings is something that can be simulated through a computational process - a stance that transcends disciplinary distinctions and is heavily debated by scholars.20
Like all theories it has its share of detractors who do not think it is possible; they believe life is more than an array of physical atoms; life requires a non-physical component (spirit or soul) that is non-programmable. However, it may be possible for an inquisitive and advanced civilization possessing powerful computers to simulate virtual universes. If we exist in their simulation, hopefully, they do not lose interest and terminate the simulation any time soon. Our computers operate on computable functions and discrete numbers that would necessitate a digital simulation. If nature is continuous, a new, distinctive type of supercomputer must be employed, one capable of handling numbers with an infinite number of digits, such as, π or 2 (or possibly a new form of mathematics to express these numbers as speculated by mathematician Norman Wildberger). The Simulated multiverse assumes that there is an ability to simulate self-aware beings, an ability to compute consciousness, an advanced civilization with motivation to experiment, and discrete computations or unique supercomputers.
VII. Theory #11: The Ultimate Multiverse
The Ultimate multiverse is appropriately labeled since it assumes all mathematically possible universes exist. Thus, it encompasses all the above proposals plus others based on more fundamental and possibly different laws – in this context, different laws than we experience. All the characteristics can assume any value; and therefore, fine tuning can be explained. The key hypotheses predicting parallel universes are the External Reality Hypothesis (ERH) and the Mathematical Universe Hypothesis (MUH). As implied from their names, all reality is described mathematically; in fact, physical existence equals mathematical relationships and all defined mathematical relationships are real.
Since every imaginable universe may not have a mathematical definition, some would not exist. If there are relationships in nature based on continuous numerical values (infinite digits are needed to define continuous values) or endless computations, the MUH has a problem since non-computable relationships could never become reality. One solution is to allow only computable functions as Max Tegmark acknowledges in his seminal work on ‘Our Mathematical Universe’ a more indepth inquisition using his previous article ‘The Mathematical Universe’ as a foundation. Nonetheless, it is evident that ‘computable functions may be needed for Mathematical Universe Hypothesis to make sense’.
In the MUH interpretation, the passage of time is not fundamental – a time-dependent process is not required, the flow of time is merely an illusion. Among the related assumptions forming the foundation of the ‘Ultimate Multiverse’ is the belief that mathematic relationships define reality (MUH), that relationships exist outside space and time, and that time is not fundamental.
VIII. Where Do We Stand?
So we tried to trace the path from multiverse proposals to supporting theories to fundamental assumptions. I will be the first to admit that some assumptions, although inherent in leading scientific theories, appear quite strange when you think of it down to the last applicable principle. The Quilted Finite parallel universe, with a limited number of universes, establishes a safe baseline; but opinions on the other ten multiverse proposals are based on the validity of the assumptions.
I would also like to note that - as would appear to anyone reading the eleven theories we have set out - many of these assumptions are non-intuitive. This makes a challenge to their credibility inevitable at some point. Logically, this also makes one ponder whether intuition itself is an appropriate measure. Afterall, nature is weird, relativity and quantum mechanics both contradict intuition – so, in such a case should we expect the ultimate theory of physics, whatever it turns out to be, to feel weirder still? Like many aspects that I talk about, this is something that I cannot claim to have an answer to. I am not even close to having an answer and I can see myself arguing either ways given what we know as of now.
From this perspective, radical new ideas are anticipated. Since parallel universes reside forever outside our vision, future experiments and theoretical calculations can provide only circumstantial evidence for their existence. If experiments are not conclusive, then we are arguably in the realm of metaphysics where proposals by philosophers are as valid as theoretical scientific theories.
IX. So Which of the Theories Should We Believe?
I was here merely as someone who put all the theories and their foundations before you; the question of which of these to believe is as perplexing to me as it is to anyone. It can be difficult for some to grasp the mind-bending idea that there are many if not innumerable other universes beyond our own. There’s a vast amount of fun (and serious deliberation) to be had with the possibilities of multiple universes, and contemplating infinite possibilities soon gets silly. Several people with intellectual capacities far eclipsing ours have tried to reason for (or against) the ideas of multiverse models with their differing and overlapping hypotheses and terminologies to ensure q reality-blurring confusion for mere mortals like you and I. What have I done in this article? I have merely given you the fodder and (hopefully) the ability to bounce about between “parallel worlds”, “divergent timelines”, “quantum universes” and “n-dimensional branes”.
I don’t know which of these posited hypothesis is correct, but that was never my aim in the first place. The intent was to enable you to think for yourself. Think. Interpret. Argue. And Decide
Katrin Becker, String Theory and M Theory (Cambridge University Press)
Sir Roger Penrose, The Emperor’s New Mind