Are we meant to be here? Anthropic Coincidences and the Multiverse
(The image above is of Barred spiral galaxy NGC 1300 and was taken by the Hubble Space Telescope.)
Are we meant to be here? Or is mankind just a fluke in a meaningless universe?
According to Scripture and Christian tradition we are meant to be here and the human race has an important place in the divine plan. The Letter to Diognetus, a Christian work of the early second century, said, “God loved the race of men. It was for their sakes that He made the world.” 1 The first creation account in Genesis culminates in the creation of mankind. And St. Paul told the Ephesians they were chosen by God and destined to be His sons and daughters “before the foundation of the world.”
On the other hand, many atheists tell us that science has shown the human race to be just an accidental by-product of blind material forces in a universe that has no point or purpose. A classic statement of this view was given by the physics Nobel laureate Steven Weinberg, who died just a few months ago. In his 1977 best-seller, The First Three Minutes: A Modern View of the Origin of the Universe, he famously wrote,
“It is almost irresistible for humans to believe that we have some special relation to the universe, that human life is not just a farcical outcome of a chain of accidents . . . but that we were somehow built-in from the beginning … . It is very hard for us to realize that [the entire earth] is just a tiny part of an overwhelmingly hostile universe … . The more the universe seems comprehensible, the more it also seems pointless.” 2
Weinberg’s view is understandable. The universe is indeed inconceivably vast compared to the earth and its inhabitants. The human race is indeed the outcome of a long chain of events that could have gone otherwise. The forces of nature, while not literally “hostile” to us, are certainly blind, unfeeling, and often harsh. The universe goes about its business without any concern for us. Tsunamis are unaware of the villages they drown. As the atheist Richard Dawkins often reminded his readers with evident and perverse relish, nature’s relation to us is one of “pitiless indifference.” 3
Isn’t it foolish then, as Weinberg suggested, to imagine that “we were somehow built-in from the beginning” and meant to be here? Hasn’t the whole thrust of modern science, from Copernicus and Darwin down to the discoveries of the last century, been away from such naïve anthropocentrism? Hasn’t the story of science been about the “successive dethronement of man,” as the Harvard biologist and popular science writer Stephen Jay Gould used to say? 4
Well, maybe not. It is beginning to look very much like we may have been “built-in from the beginning” after all.
In the last fifty years, particle physicists have begun to notice many features of the laws of physics and the structure of the universe that appear to be just what is needed for the existence of living things such as ourselves to be possible. These features are often called “anthropic coincidences.” 5 Until about thirty years ago, it was almost taboo to mention, let alone discuss, such things in the physics research literature.6 The idea that there might really be anthropic coincidences — or, if there were, that they had any significance — was generally dismissed out of hand as “unscientific.” Gradually, however, attitudes changed, in part because a number of leading physicists (not a few of them avowed atheists, including Steven Weinberg himself) began to take anthropic coincidences seriously as calling for explanation. It is now fairly widely accepted among particle physicists that some of these coincidences are real and may be telling us something important about the world.
For example, Stephen Hawking and Leonard Mlodinow, in their 2010 popular book, The Grand Design, wrote
“What can we make of these coincidences? … Our universe and its laws appear to have a design that … is tailor-made to support us and, if we are to exist, leaves little room for alteration. That is not easily explained, and raises the natural question of why it is that way.” 7
Edward Witten, widely regarded as the most brilliant theoretical physicist of his generation, said in an interview,
“The laws of nature are very delicate. … [The fact] that galaxies, stars and planets roughly like ours could have formed, and that living things roughly like us could have formed depends on many details of the laws of physics as we currently know them being just the way they are and not being slightly different. [I think] we’ll never resolve the sense of wonder about that.” 8
Before going into the question of the possible significance of anthropic coincidences, it may be helpful to give some examples of them.9
A famous example concerns the “strong force,” which binds neutrons and protons together to make atomic nuclei. If this force had been weaker by about 20%, it would have been unable to hold together a certain nucleus called “deuterium” (the nucleus of hydrogen-2). This would have been disastrous, as deuterium was a crucial stepping stone in the processes by which the nuclei of all the elements of the Periodic Table heavier than hydrogen were produced in the early universe and in stars. Without those elements, life based on chemistry would not have been possible. On the other hand, if the strong force had been stronger by a few percent, it would have been strong enough to bind two protons together into a nucleus called the “diproton” (or helium-2). That would have radically altered the ways that stars burn their nuclear fuel. It is unclear whether the kinds of stars that would have existed would have had the right properties to allow life to evolve on their planets. So the strength of the strong force seems to be in a “sweet spot” for life, neither too strong nor too weak.
Another famous example, noticed in the 1950s by the astrophysicist Fred Hoyle, has to do with the so-called “three-alpha process” by which nearly all of the carbon in the universe was (and is) made in the interiors of stars as they burn helium. It involves three helium-4 nuclei (aka “alpha particles”) happening to collide at almost exactly the same instant and fusing to form a carbon-12 nucleus. This otherwise very unlikely process is greatly facilitated by the fact that the nucleus of carbon-12 just happens to have an “excited state” at precisely the right energy. If it were not for this coincidence, there would be almost no carbon in the universe, preventing carbon-based life — and therefore probably any life — from arising.
An even more dramatic example of “anthropic fine-tuning” concerns a certain basic parameter called “v” in our present fundamental theory of physics — “v” is short for the rather forbidding technical term “vacuum expectation value of the Higgs field.” The parameter v is very important, because it is like a knob that controls the masses of many basic particles (such as electrons and quarks), the strength of the “weak force,” and (indirectly) many other features of the physical world as well. It turns out that the numerical value of v has to lie within an extremely narrow range of values for life to be possible.10 If the value of v were much larger than it is, all neutrons would have decayed, making hydrogen the only element. If v had been larger still, then protons would have decayed also, leaving as the only possible kind of atomic nucleus an exotic particle called the Δ++ (“delta-plus-plus”), whose atoms would be like helium and thus chemically inert. On the other hand, had v been much smaller than it is, then certain processes that are exceedingly rare in our universe, called “sphaleron processes,” would have erased all the matter from which atomic nuclei of any kind could be made, meaning that there would be no atoms at all.11 So v is in a very sweet spot for life.
Not all “anthropic coincidences” have to do with certain quantities being “fine-tuned” to have just the right values (like v and the strength of the strong force). Certain gross qualitative features of the universe and its laws are also important for making life possible.
An especially important example is that the universe is governed by the principles of quantum mechanics. Quantum mechanics tells us that at very small distances the physical world is grainy, rather than smooth. The very fact that matter is made up of particles, such as protons and electrons, out of which complex structures can be built — including living things — is a consequence of this quantum graininess. Moreover, the structures built out of those particles are only stable because of quantum mechanics. It is the Heisenberg Uncertainty Principle that prevents electrons from getting pulled inside atomic nuclei by their electrical attraction, causing all matter to collapse catastrophically.
Furthermore, even if electrons could somehow be kept orbiting atomic nuclei in a non-quantum-mechanical universe, rather than being pulled inside them, they would have an infinite number of ways of doing so, meaning that no two atoms would be alike, and no atom would behave consistently over time. Matter would be constantly and very rapidly morphing in unpredictable ways, without stable properties.
The ability of the universe to have life also depends greatly upon the kinds of matter particles that exist and their properties. It is especially important that there are particles with the properties of electrons, neutrons, protons, pions, and photons. Each plays crucial roles. The same is true of each of the known fundamental forces, namely gravity, electromagnetism, the weak force, the strong force, and the Higgs force.
For example, electromagnetism holds atoms, molecules and all ordinary matter together and also gives rise to the existence of light. The strong force, as already mentioned, holds atomic nuclei together. Without it, nuclei would fly apart due to the electrical repulsion of the protons in them. The strong force also provides the nuclear fusion energy that powers stars, which in turn is needed to warm planets and generate and sustain life. Without gravity, matter would not condense into stars or planets, nor planets retain their atmospheres. Moreover, the gravitational expansion of space and the gravitational clumping of matter was key to generating the reserves of all usable energy in the universe. The weak force makes possible certain nuclear reactions that generate the energy of stars like the sun. The Higgs force is needed to give mass to particles such as electrons and quarks.
It is not only the existence of these forces, but their specific characteristics, their delicate and complex interplay, and the fact that their strengths are in just the right relation to each other that makes life possible. For example, it is important for the existence of a rich variety of stable elements that the mass of the proton and the mass of the neutron be not too different from each other and that the neutron be slightly heavier than the proton. Otherwise, all the protons would decay into neutrons or vice versa. And, in fact, neutrons weigh 939.6 while protons weigh 938.3 (in a certain standard unit of mass). This near equality is due to the properties of the strong force, while the fact that the neutron is slightly heavier is due to the way the Higgs force affects quarks.
The characteristics of space and time are also critical. We take for granted that there are three space dimensions and one time dimension. But there is no law of logic that says that the world had to be made this way. Particle physicists study hypothetical universes with different numbers of dimensions. Indeed, in superstring theory, which many experts think may be the correct unified theory of all the forces of nature, there are nine space dimensions, six of which are curled up into little surfaces of subatomic size. It seems to be crucial for life that there are exactly three dimensions of space that extend for large distances, as in our universe. If there were more than three, the force of gravity between two objects would depend on their distance from each other in a different way than the famous “inverse square law” of Newton. Gravity would be comparatively stronger at short distances and weaker at long distances than Newton’s law says. As a result, planets would either escape the pull of stars, or would plunge into them, but could not stably orbit them.12 And the electrical force would behave the same way, making atoms unstable. If there were fewer than three extended space dimensions, there would be other problems. Complex neural circuitry, as is needed in brains, would not be possible. If one tries to draw a complicated circuit diagram on a two–dimensional surface, one finds that the wires have to cross through each other many times, leading to “short–circuits.” In three or more dimensions, however, wires (or neurons and axons) can go around each other without touching.
This a small sampling of the many anthropic coincidences that physicists have discussed. What is one to make of them? One rather obvious possibility is that a Mind framed the laws of this universe and did so with the intention that it should bring forth living things, including beings such as ourselves. That certainly accords well with the Christian view. There is, however, another way to explain at least some of these coincidences, which does not invoke God, called the “multiverse” hypothesis.
The multiverse idea is very speculative and comes in a variety of versions. It does not necessarily posit the existence of “many universes,” as is widely supposed.13 The versions thought about by physicists all assume that the multiverse is a single universe governed by a single set of fundamental laws of physics. (In some versions, the universe can split and recombine, but nevertheless forms a single interacting system.) What makes a universe a multiverse is that the fundamental laws of physics have a flexibility that allows certain physical quantities and features that were traditionally thought to be the same everywhere throughout the universe to vary from one place to another within it. For example, the strength of the strong force, the types of particles that exist and their properties, the value of “v”, and so on, might be different in different regions or “domains” of the universe. If there were a sufficiently large number of domains, there could be a high probability that in some of them all the physical quantities and qualitative features would be just right to make life possible.
Of course, to account for all the anthropic coincidences in this way, the multiverse would have to be such that all the quantities and features that have to be “just right” for life would vary from place to place. And not only that, they would have to vary over a sufficiently wide range to include the “just right” values. In other words, the multiverse would have to be extremely rich in the possibilities that were realized in its various domains.
To many people, including many physicists, the multiverse idea sounds far-fetched, and even kooky. Some religious people therefore imagine that it is just a desperate idea cooked up by atheists to avoid the obvious conclusion that the universe was designed with us in mind. But these dismissive judgments are not quite fair.
In the first place, many and probably most atheist scientists dislike the multiverse idea. The main reason is that it seems impossible to test it by any observation or experiment. The quantities and features that are postulated to be different in other domains of the universe are known not to vary within the part of the universe that we can see (the so-called “observable universe”). In other words, the other domains posited by the multiverse idea must lie outside our cosmic “horizon.” For many atheists, unobservable domains of the universe are just as hard to accept as an unobservable God. It is true, however, that some atheists (e.g. Steven Weinberg and Stephen Hawking) have realized that the multiverse is the only hope of explaining the anthropic coincidences in a naturalistic way and have therefore been willing to swallow whatever epistemological scruples they might have had.
Second, however weird the multiverse idea may sound to most people, it is really not so strange from the perspective of our present theories of physics. There are reasons, regarded by most cosmologists as compelling, to believe that the universe is exponentially larger than the part we can see.14 The real question, then, is not whether there are vast regions of our universe that are unobservable to us, but how dissimilar to ours those regions are likely to be. We know of ways that different parts of a universe can appear quite different from each other, even to the point of having different kinds of particles and forces, despite their all obeying the same physical laws at the deepest level. This could happen if the matter in different parts of the universe were in different “phases,” analogous to the way H2O can be in different phases (water and ice) in different places. The present best candidate for the ultimate theory of physics — superstring theory — is believed to have an exponentially large number of possible phases, which differ dramatically from each other in the kinds of particles and forces they have, and therefore may imply that the universe is a multiverse. This is one reason that the hostility to the multiverse idea among fundamental physicists has weakened in the last twenty years.
Given that there might be a naturalistic explanation of some of the anthropic coincidences, does that mean that they don’t in any way point to life being “built-in from the beginning”? Are we back to the idea that we are simply accidents in a vast and pointless cosmos? I don’t think so, for the following reason. In order for the universe to have a multiverse structure, the fundamental laws that govern it must have enough flexibility to allow many important physical quantities and qualitative features of the universe to vary from place to place. That is a highly remarkable characteristic for the fundamental laws to have. There is no a priori reason to expect a universe to have laws that would make it a multiverse, let alone a multiverse with the enormous richness of possibilities that would make life possible.
The take-away lesson of the anthropic coincidences, then, is that if a universe is to be life-bearing, its laws must be very special in one way or another. They may be special in having many important quantitative and qualitative features be everywhere “just right” to allow life, or they may be special in allowing all these features to vary from place to place in the universe. Therefore, we should not at all take it for granted that the universe has the right properties to make life possible. It did not have to be that way. Rather, it should be a source of great wonder, as Edward Witten, whom I quoted earlier, rightly said.
Having said all this, we remain with a question very troubling to many people: Why is the universe so large? How can we claim to be important in a universe that dwarfs us in its scales of space and time? There is at least a paradox here.
One answer is very traditional one. The universe was not made only for our benefit. As Psalm 19 declares, “the heavens proclaim the glory of God.” If it is the glory of God that they proclaim, then there is no particular reason why they should have to be made to human scale. In fact, an important thinker of the fifteenth century Cardinal Nicolas of Cusa argued that a universe of infinite extent would more aptly reflect the infinite splendor of God.
That is a good answer, but there is another. It turns out that the very age and vastness of the universe themselves have “anthropic” significance. Life emerged in our universe in a way that required great stretches of time. Most of the elements needed for life were made deep inside stars, which had to explode and disperse those elements into space to make them available to be formed into planets and living things. That alone required billions of years. Biological evolution required billions of years more. Thus, the briefness of human life spans, and even of human history, compared with the age of the universe may simply be a matter of physical necessity, given the developmental way that God seems to prefer to work. It takes longer for a tree to grow to maturity and be able to bear fruit than the fruit of the tree lasts. It took much longer for the universe to grow to maturity and be able to generate life than a human life lasts.
Physics also suggests why the universe has to be so large.15 The laws of gravity discovered by Einstein relate the size of the universe directly to its age. The fact that the universe is many billions of light-years across is a direct consequence of the fact that it has lasted for several billion years. Perhaps we would be less daunted by a cozy little universe that never got larger than the size of, say, North America or Asia. But such a universe would have lasted only a few milliseconds. Even a universe that never exceeded the size of the solar system would have lasted for only a few hours. A universe constructed in such a way as to evolve life had to extend vastly in space as well as in time. So that the frightening expanses that are so often said to be a sign of human insignificance are actually preconditions for our existence. And so, like so many other features of our strange universe, they point to man, as they also proclaim the glory of God.
1.. The Letter to Diognetus, 10, in Early Christian Writings: The Apostolic Fathers ed. Betty Radice, trans. Maxwell Staniforth (New York: Penguin, 1968) p.180.
2.. Steven Weinberg, The First Three Minutes: A Modern View of the Origin of the Universe (Glasgow: William Collins, 1977) p. 148.
3.. Richard Dawkins, “Science and God: A Warming Trend?” Science 277 (1997) 890.
4. Stephen Jay Gould, Full House: The Spread of Excellence from Plato to Darwin (New York: Harmony Books, 1996), 17.
5.. The word ‘anthropic’ comes from the Greek ‘anthropos’, meaning ‘human being.’ So an anthropic coincidence is a feature of the universe allowing the existence of human beings. The term “anthropic principle” is often used. This is highly unfortunate, since the anthropic coincidences are facts to be explained, not principles. Moreover, it is confusingly ambiguous, since several quite different “principles” have been proposed to explain the coincidences, including the “weak anthropic principle” and the “strong anthropic principle.”
6.. Steven Weinberg wrote a paper in 1989 discussing the so-called cosmological constant problem as an anthropic coincidence, which helped to break the taboo. Even so, in 1998 some colleagues and I had difficulty getting a paper published in Physical Review that pointed out an important anthropic coincidence. (See Reference 10 below.) The journal editor who handled our paper took the very unusual step of vetoing its publication despite positive referee reports, saying it was “not science.” This editor reversed himself, however, when Physical Review Letters (a much more prestigious journal) accepted a condensed version of the paper based on the strong positive recommendation of one of its referees: Steven Weinberg! Since then, the paper that was at first rejected as “not science” has become highly cited in the research literature, including by many top theorists, which illustrates the changing view of the subject among physicists.
7.. Steven Hawking and Leonard Mlodinow, The Grand Design (New York: Bantam Books, 2010), p. 162.
8.. Wim Kayzer, Of Beauty and Consolation, Episode 9, https://www.youtube.com/watch?v=RfwsvSjXkJU
9.. All the examples given here are discussed in much greater detail in Stephen M. Barr, Modern Physics and Ancient Faith (Notre Dame: Univ. of Notre Dame Press, 2003), pp. 118-137. There are many good books on anthropic coincidences: John Barrow and Frank Tipler, The Anthropic Cosmological Principle, rev. ed. (New York: Oxford University Press, 1988); Paul Davies, The Goldilocks Enigma: Why is the Universe Just Right for Life? (New York: Houghton Mifflin, 2008); Sir Martin Rees, Just Six Numbers: The Deep Forces that Shape the Universe (New York: Basic Books, 2000); Geraint Lewis and Luke Barnes, A Fortunate Universe: Life in a Finely Tuned Cosmos (Cambridge: Cambridge University Press, 2016).
10.. V. Agrawal, S.M. Barr, J.F. Donoghue, and D. Seckel, Physical Review D57 (1998) 5480; ibid. Physical Review Letters 80 (1998), 80.
11.. Private communication from Nima Arkani-Hamed. Sphaleron processes are non-perturbative electroweak processes that violate baryon and lepton number. They are exponentially suppressed at temperatures small compared to v. If v had been very small compared to its observed value, however, sphaleron processes would have wiped out all the baryons in the universe shortly after the Big Bang, leaving no atomic nuclei and therefore no atoms.
12.. The argument for this is actually quite simple. When the orbit of a planet is decomposed into radial and angular variables, the “effective potential” that appears in the “equation of motion” for the radial variable r (the planet’s distance from the star) has two pieces: (a) the Newtonian gravitational potential energy which is negative and proportional to r -(d-2) (where d is the number of dimensions of space), and (b) a “centrifugal barrier” potential, which is positive and proportional to r -2. When d = 3, the sum of these has a stable minimum. But when d > 3, it has no minimum. This can be easily seen by sketching the potential.
13.. The multiverse idea should also not be confused with the “Many Worlds Interpretation” of quantum mechanics. The two ideas are quite distinct, though there are scenarios that combine them.
14.. It is believed by most cosmologists that very soon after the Big Bang the universe underwent an enormous expansion called “cosmic inflation.” This seems to be the only way to resolve certain cosmological puzzles, especially the so-called “flatness problem” and “horizon problem.” If such inflation occurred, the universe is likely to be much larger than the part we can see.
15.. Stephen M. Barr, Modern Physics and Ancient Faith (Notre Dame: Univ. of Notre Dame Press, 2003), ch. 18.