Thus is the excellence of God magnified and the greatness of his kingdom made manifest; He is glorified not in one, but in countless suns; not in a single earth, a single world, but in a thousand thousand, I say in an infinity of worlds.
Giordano Bruno, On the Infinite Universe and Worlds (1584)
How like science fiction our lives are, she thinks. The alternate universe in which, innocently, ignorantly, we continue to exist as we'd been, unaware that, in another universe, we’d have ceased to be.
Joyce Carol Oates, DIS MEM BER and Other Stories
of Mystery and Suspense
One of the greatest mysteries is why our universe seems fine-tuned for life (see Idea 2). We can appreciate this better by examining the best tool we currently have to describe the universe: The Standard Model. Despite the word “model” in its name, the Standard Model is a comprehensive theory that identifies the basic particles and specifies how they interact. It has been extremely successful in practice, generating values that fit the observed data to an extraordinary degree. Yet the model has several constants, such as the speed of light, the masses of elementary particles, and the so-called cosmological constant, whose values no theory has been able to explain so far. In all, the Standard Model has twenty-odd, unfixed parameters.[1]
As it happens, nature is surprisingly finicky and only a narrow range of settings for these constants is suitable for the evolution of life. For example, if the constants that govern nuclear physics changed even slightly then nucleosynthesis and the creation of the heavy elements in the stars and supernovae might become impossible (see Idea 1, “The particles that make ‘us’”). We know that much of the carbon and oxygen needed for life is produced by the fusion of helium atoms in stars called red giants. But a change of only 0.5% in the strength of the so-called strong force that governs nuclear structure would be enough to prevent those reactions from occurring. The result would be a dearth of raw materials needed for life.
Similarly, a number known as the “fine structure constant” characterizes the strength of electromagnetic forces. If it were a little larger, stars could not burn, and if it were only a little smaller, molecules would never form. It also seems that the laws of nature were fine-tuned just enough to keep the cosmological constant (a number that measures the amount of cosmic repulsion caused by the energy in empty space) from being a deadly danger to the formation of life.
Evidently, we are here today to ask questions about the universe because the values of the constants of nature are compatible with life. This is the basis of what is known as the anthropic principle. In its weakest form, the anthropic principle simply states that the fact that intelligent life exists in the universe should be taken as an experimental fact that helps us understand the constants of the universe. As the late physicist and Nobel Prize winner Steven Weinberg explained it, “the world is the way it is, at least in part, because otherwise there would be no one to ask why it is the way it is.”[2] Stated this way the weak version is hard to argue with as it becomes a tautology.
It is the strong version of the anthropic principle that troubles most scientists. This version establishes that God or another powerful agent has chosen the physical constants of the universe to make life possible. Today, practically all reconcilers of science and religion invoke the anthropic principle: That the values of physical constants appear to be fine-tuned to produce a universe hospitable to the rise of conscious, worshipful life. “Clearly, theism can provide a coherent response to the anthropic question,” writes the physicist-theologian John Polkinghorne. “Those who believe in God do not regard the universe as being just ‘any old world,’ but they understand it to be a creation whose Creator may be expected to have endowed it with just those finely tuned laws and circumstances that will enable it to have the fruitful history that would be a fulfillment of the divine will.”[3]
Not surprisingly, cosmologists revile the anthropic principle as a piece of vacuous sophistry. In a heated debate with Polkinghorne, Weinberg referred to the strong anthropic version as “little more than mystical mumbo jumbo.”[4]
Extending reality: string theory and the multiverse
The strong anthropic principle may lose its mysticism if physicists are able to develop a “theory of everything” that explains why the constants of nature such as the speed of light and the mass of the electron have the values that they have. That is, physicists would like to find out whether the laws of physics are unique or “whether God had any choice,” as Einstein once famously put it. Some physicists are hopeful that string theory, a theory that views elementary particles as small strings vibrating in several dimensions, will do the trick.
According to string theory all the different particles and forces in the universe are composed of wriggling strands of energy whose properties depend solely on the mode of their vibration. String theory describes how these strings propagate through space and interact with each other. For mathematical consistency, these strings vibrate in 10-dimensional spacetime. And for consistency with our familiar everyday experience of the universe, with three spatial dimensions and the dimension of time, the additional six dimensions are “compactified” to be undetectable. The physicists who work with string theory envision our universe as an eerie place with at least ten spatial dimensions, six of them hidden from us.[5]
The great advantage of string theory is its promise of unifying the two pillars of modern science: quantum mechanics and Einstein’s theory of gravity. Einstein’s equations break down at enormously small distances and large energies found at the origin of the universe. At distances of the order of 10-33 centimeters, quantum effects take over from Einstein’s theory of gravitation. Quantum theory, however, is at this point unable to incorporate gravitational effects, which are needed to explain the evolution of the universe. Making these two theories compatible with each other remains one of the main challenges of the modern era. In string theory, one of the many vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus, string theory is a theory of quantum gravity.
Despite initial successes, many physicists are now doubtful that string theory fits the bill. In addition, scientists are now finding that string theory allows for not one, but a huge number of solutions to its equations; about 10500 or perhaps even more, where each solution provides the kinds of equations physicists need to describe reality. In string theory, a “solution” implies a vacuum of spacetime that is governed by Einstein’s theory of gravity coupled to a quantum field theory. Each solution describes a unique universe, with its own set of particles, fundamental forces and other such defining properties.[6]
This embarrassment of riches, however, opens another way to demystify the strong anthropic principle: Our universe may be just one of many, some of which may be barren while others, like ours, harbor life. Indeed, the theories allow for the creation of not just one universe but many, possibly infinitely many. As we saw in our discussion of Idea 1, scientists believe that the universe “inflated” a few moments after the Big Bang. One natural extension of inflation, according to cosmologists, is that the realm we call the observable universe could be only a small patch of a vast bubble or “pocket” in a much vaster group bred endlessly in a chain of big bangs. “Once you’ve discovered it’s easy to make a universe out of an ounce of vacuum,” writes Craig Hogan, a cosmologist at the University of Washington “why not make a bunch of them?”[7]
There is no reason to expect that these universes will be identical. Even within our own bubble, tiny random non-uniformities in the primordial raw material would cause the cosmos to look different from place to place. What is more, cosmologists say, the laws of physics themselves, as experienced by creatures like us, confined to four dimensions and the energy scales of ordinary life, could evolve differently in different bubble universes. The theory that our universe is just one of myriad expanding bubbles, each with different laws, goes by the name of eternal inflation.[8]
If there are indeed other universes compatible with the standard model, but with different constants than ours, then the strong anthropic principle simply becomes a prescription for deciding which universes can support life. Effectively, the fundamental constants of nature could look different in different bubbles. The truly fundamental equations of the Standard Model may be the same everywhere in all universes but may not completely determine the values of all constants in each one. As a result, different universes may have different values for the speed of light, the mass of the electron, and the so-called cosmological constant. The set of these hypothetical universes (including our own universe) is known as the multiverse. The multiverse theory posits that these universes comprise everything that exists and can exist: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them.[9]
If the multiverse theory is right, it will clarify many issues considered too complicated and metaphysical. It would not only explain why our universe appears fine-tuned for life, but it can potentially explain why the universe may be intelligible to us, and why the past is different from the future. In the multiverse every alternative is realized, which means that there must be at least one universe in which scientists are able to discover how to extend life indefinitely. Thus, it could be that in one universe an exact replica of me, and you, are able to live for thousands of years. In fact, the multiverse may be the only chance for humans to escape extinction (see Idea 1, the accelerating expansion of the universe). The multiverse has then many deep and promising ideas and it has expanded our “cosmic perspective” beyond anyone’s imagination. [10]
Parallel universes: real or imaginary?
But is the multiverse for real? After all, we may never be able to directly observe or detect another universe. Not surprisingly, many people both inside and outside the scientific community consider the multiverse metaphysics, not physics. For example, in a 2003 New York Times opinion piece, the physicist Paul Davies argues that multiverse theories are non-scientific:
For a start, how is the existence of the other universes to be tested? To be sure, all cosmologists accept that there are some regions of the universe that lie beyond the reach of our telescopes, but somewhere on the slippery slope between that and the idea that there is an infinite number of universes, credibility reaches a limit. As one slips down that slope, more and more must be accepted on faith, and less and less is open to scientific verification. Extreme multiverse explanations are therefore reminiscent of theological discussions. Indeed, invoking an infinity of unseen universes to explain the unusual features of the one we do see is just as ad hoc as invoking an unseen Creator. The multiverse theory may be dressed up in scientific language, but in essence, it requires the same leap of faith.[11]
Box 3. Four Levels of Parallel Universes In an article entitled “Parallel Universes”1 the Princeton physicist Max Tegmark discusses four distinct levels of parallel universes: Level I This level multiverse assumes that the universe is infinite so that everything that can happen will happen, so that if we could look out far enough, we would eventually discover an exact replica of ourselves. According to this model, you have a twin in a galaxy about meters from here (1 followed by 1029 zeros). This distance is so humongous that it is infinite for all practical purposes. Yet, remarks Tegmark, it does not make our alter ego or doppelganger any less real. The estimate is derived from elementary probability and does not even assume speculative modern physics; merely that space is infinite in size and almost uniformly filled with matter, as observations currently indicate. In infinite space, even the most unlikely events must take place somewhere. Level II This multiverse is the eternal inflation scenario (see main text), in which “pocket” universes inflate continuously, each with different fundamental constants. As I mentioned, string theory allows for many solutions to its equations. The Stanford physicist Leonard Susskind calls the enormous range of possible solutions the populated Landscape. The idea behind this Landscape is that mechanisms that rely on well-tested physical principles gave rise to a huge number of pocket universes, with each and every valley in the Landscape being represented. The Landscape has perhaps 10500 or even 101000 valleys, each one of which corresponds to a set of laws of physics that may operate in vast bubbles of space. Thus, in each bubble an observer will see a specific four-dimensional universe with its own characteristic laws of physics. Our own visible universe would be one relatively small region within one such bubble.[12] Level III This multiverse is based on the many-worlds interpretation of quantum mechanics, first proposed in 1957 by Princeton graduate student Hugh Everett. If the fundamental equations of physics are what mathematicians call unitary, as they so far appear to be, then the universe keeps branching into parallel universes: whenever a quantum event appears to have a random outcome, all outcomes in fact occur, one in each branch. Although more controversial than Level I and Level II this level adds no new types of universes. “The passionate debate about Everett’s parallel universes that has raged on for decades,” writes Tegmark, “is reminiscent of the famous Shapley-Curtis debate of the 1920s about whether there were really a multitude of galaxies (parallel universes by the standards of the time) or just one, a storm in a teacup now that research has moved on to other galaxy clusters and superclusters. In hindsight, both the Shapley-Curtis and Everett controversies seem positively quaint, rejecting our instinctive reluctance to expand our horizons.” Level IV This multiverse is the most extreme version, as it proposes that all mathematically self-consistent world descriptions enjoy real existence. This Platonic view proposes that our physical world really is a mathematical structure. It rests on two separate assumptions. First, that the physical world is a mathematical structure. Second, that mathematical existence and physical existence are the same, so that all mathematical structures exist “out there” in a physical sense. In this view, we could also consider mathematics as a “disciplinarian” forcing reality to obey laws rather than an approximation we use to talk about reality. If this interpretation is correct, then in principle an infinitely intelligent mathematician could derive all properties of all parallel universes (including the subjective perceptions of self-aware beings in them). 1. Tegmark, M. (2004). Parallel Universes, In Science and Ultimate Reality: Quantum Theory, Cosmology and Complexity, by Barrow, J. D., Davies, P. C. W. & Harper, C. L. (eds.) Cambridge University Press.
Not all physicists agree. One of them is the Princeton physicist Max Tegmark, who posits four levels of the multiverse (see Box 3). According to Tegmark, the key question is not whether the multiverse exists but rather how many levels it has, I, II, III, or IV. The multiverse theory, he argues, fulfills the basic criteria of an empirical science: it makes predictions, and it can be falsified (that is it can be proved wrong by scientific evidence, see Idea 12, forthcoming). Recent evidence, for example, points to a finite universe that is “only” 70 billion light-years in diameter. If this finding is confirmed by further observations, it will dispose of the Level I multiverse, the idea that every person on Earth has an infinite number of alien doubles leading parallel lives.
In the coming decades, dramatically improved cosmological measurements of the micro-wave background radiation and the large-scale matter distribution will test Level I by further constraining the curvature and topology of space and will test level II by providing stringent tests of inflation. The Large Hadron Collider (LHC), which became operational in 2010, provides a powerful tool to test various predictions in high-energy physics, including those that will directly or indirectly support or disconfirm the existence of the Level II multiverse.
Physicists hope that the LHC, or more powerful accelerators, will help answer many of the most fundamental questions in physics, questions concerning the basic laws governing the interactions and forces among the elementary objects, the deep structure of space and time, especially regarding the intersection of quantum mechanics and general relativity, where current theories and knowledge are unclear or break down altogether. These tests may clear up the mystery of dark matter ( see Idea 1) and shed light on the existence of extra dimensions, as predicted by various models inspired by string theory.
The refinement in the methods to measure the constants of nature will also clarify the extent to which they are fine-tuned, thereby weakening or strengthening the case for Level II. If the current effort to build quantum computers succeeds, it will provide further evidence for Level III, since they would, in essence, be exploiting the parallelism of the Level III multiverse for parallel computation. Conversely, experimental evidence of “unitarity” violation would rule out Level III. Finally, success or failure in the grand challenge of modern physics, unifying general relativity and quantum theory, will shed light on Level IV. “Either we will eventually find a mathematical structure matching our universe, or we will bump up against a limit to the unreasonable effectiveness of mathematics and have to abandon Level IV.”[13]
Indirect proof in science
Despite these arguments, many physicists remain unconvinced. Like Paul Davies, other physicists argue that most multiverse theories lack empirical testability and are unfalsifiable; they are outside the methodology of scientific investigation to confirm or disprove. According to most multiverse theories, other universes are in a different spacetime framework, so in principle they cannot be observed. For example, this is what the physicist Carlo Rovelli of the Center for Theoretical Physics in Luminy, France, says about the multiverse theory:
It is easy to write theories. It is hard to write theories that survive the proof of reality. Few survive. By means of this filter, we have been able to develop modern science, a technological society, to cure illness, to feed billions. All this works thanks to a simple idea: Do not trust your fancies. Keep only the ideas that can be tested. If we stop doing so, we go back to the style of thinking of the Middle Ages.[14]
Nonetheless, the idea that our universe may not be the only one out there keeps recurring, and it may not go away anytime soon. One important aspect of the multiverse is that it comes about as a byproduct of other theoretical ideas, especially cosmic inflation, and string theory. Thus, people should take the idea of multiverse seriously if there is further empirical support for these theories; science after all depends on being able to observe something, but not necessarily everything, predicted by a theory (see Idea 12, forthcoming).
There are two remarkable examples of this. One is the discovery of the positron. In 1928, the English physicist Paul Dirac published a paper in which he introduced an equation intended to describe the essential nature of electrons in a manner consistent with both the theory of relativity and quantum mechanics. The equation miraculously accounted for the previously unexplained fact that electrons had been observed to spin about their own axes as well as for other subtleties in the behavior of the electrons. Also emerging from Dirac’s equation were solutions that corresponded to electrons that had positive charge (or rather, negative energy.) His equation implied the possibility of an electron spontaneously jumping between positive and negative energy states.[15] However, no such transition had yet been observed experimentally.
Dirac himself was puzzled by the equally valid negative-energy solution that the mathematical model allowed. In a subsequent paper, published in 1931, Dirac predicted the existence of an as-yet unobserved particle that he called an “anti-electron” that would have the same mass as an electron and that would mutually annihilate upon contact with an electron (this was the first mention of antimatter). Understandably, the physics community reacted skeptically to Dirac’s ideas. Then, in 1932, Carl Anderson at Caltech discovered a positively charged particle in cosmic rays that, except for the sign of its charge, behaved exactly like the electron. Anderson called this particle positron. He wasn’t looking for it, but it was Dirac’s antiparticle.
The second even more remarkable example is the case of virtual particles. In physics, a virtual particle is a particle that exists for a limited time and space. As it happens, the laws of quantum mechanics allow for the possibility of matter-antimatter pairs to spontaneously appear from nothing for a bit before annihilating each other again. The so-called vacuum, it seems, is teeming with virtual particles. Indeed, this is how our universe probably got started (see Idea 1).
This means that at a given time a hydrogen atom is composed not only of a proton and an electron, as stated in chemistry books, but also of a virtual electron and a virtual positron. The thing is no one has been able to observe such particles as they exist only for a fleeting instant. However, while they are not directly observable, they can be detected by indirect means.
Dirac’s equation can predict quite accurately the spectrum of hydrogen (the specific colors of light emitted by hydrogen when heated up), one of the most precisely measured in physics. The equation was able to closely reproduce the general structure of the observations, but there was a small deviation from the measurements. For many years, physicists tried to resolve the discrepancy to no avail. Finally, it was realized that the equation precisely gives the correct answer only if the effect of virtual particles is included. When such particles are considered, the result obtained with Dirac’s equation matches observations to better than 1 part in a billion. “Virtual particles therefore exist,” as the physicist Laurence Krauss writes.[16]
To conclude: Expand your notion of reality
I have reviewed here some of the key theories that aim to explain where the complexity of the universe comes from, without invoking a supernatural entity. It is hard to say if a scientist will ever be able to “discover” other universes experimentally. Nonetheless, they may be as “real” as are positrons and virtual particles. The question is not if we will ever be able to see other universes (or other dimensions); it is whether we will ever be able to test the theories that predict they exist. These theories may be our only window to a reality “out there” that we are unable to observe because of our limited senses.
The controversies generated by string theory and the multiverse have made both physicists and laymen rethink their views on “what exists out there” and at least consider the possibility of universes other than ours. They have also prompted scientists to reconsider supposedly well-established terms such as “evidence” and “proof” in the context of the scientific method. We will have an opportunity to revisit these critical issues when we discuss Idea 12 (forthcoming).
[1] “The dawn of physics beyond the Standard Model,” by Gordon Kane, Scientific American, June 2003 [2]Quoted in “A New View of Our Universe: Only One of Many,” by Dennis Overbye, The New York Times, October 29, 2002. [3]Polkinghrone, J. (2004). The inbuilt potentiality of creation, In Dembski W. A. & Ruse, M. (Eds.) Debating Design: From Darwin to DNA, Cambridge University Press, p.251. [4]“Crossing Flaming Swords Over God and Physics,” by Carey Goldberg,” The New York Times, Apr 20, 1999. [5]One of the best expositions on string theory can be found in Greene, B. (2003). The elegant universe: Superstrings, hidden dimensions, and the quest for the ultimate theory, W. W. Norton & Co. [6]Wolt, P. (2007). Not even wrong: The failure of string theory and the search for unity in physical law, Basic Books. [7]C. J. Hogan (2000). Why the universe is just so, Reviews of Modern Physics, 72, 4, p. 1149. [8]“Found: A Quadrillion Ways for String Theory to Make Our Universe,” by Anil Ananthaswamy, Scientific American, 28 Mar 2019. [9]Greene, B. (2011). The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos, Knopf. [10] Linde, A. (2004), Inflation, quantum cosmology and the Anthropic Principle, In Science and Ultimate Reality: Quantum Theory, Cosmology and Complexity, by Barrow, J. D., Davies, P. C. W. & Harper, C. L. (eds.) Cambridge University Press. [11]Davies, P. (12 April 2003). "A Brief History of the Multiverse". The New York Times. [12] Susskind, L. (2006). The cosmic landscape: String theory and the illusion of intelligent design, Little, Brown & Co. [13] Tegmark, M. (2004). Parallel Universes, In Science and Ultimate Reality: Quantum Theory, Cosmology and Complexity, by Barrow, J. D., Davies, P. C. W. & Harper, C. L. (eds.) Cambridge University Press, p.486. [14]Scoles, S. (19 April 2016). “Can physicists ever prove the multiverse is real?”, Think Big, Smithsonian magazine special report. [15]Dirac, P. A. M. (1928). The quantum theory of the electron, Proc. R. Soc. Lond. A: 117, 610-624. [16]Krauss, L. M. (2012). A universe from nothing: Why there is something rather than nothing. Free Press, p.69.
Comments