A common argument by atheists is that belief without evidence is irrational and unjustified, and that those arguing for the existence of God have the burden of proof.  Bertrand Russell famously argued that if one claims that there is a teapot orbiting the sun, the burden of proving the existence of the teapot is on the person who asserts the existence of the teapot, not the denier.  Christopher Hitchens has similarly argued that “What can be asserted without evidence can also be dismissed without evidence.”  Hitchens has advanced this principle even further, arguing that “exceptional claims demand exceptional evidence.”  (god is not Great, pp. 143, 150)  Sam Harris has argued that nearly every evil in human history “can be attributed to an insufficient taste for evidence” and that “We must find our way to a time when faith, without evidence, disgraces anyone who would claim it.”   (The End of Faith, pp. 25, 48)

A demand for evidence is surely a legitimate requirement for most ordinary claims.  But it would be a mistake to turn this rule into a rigid and universal requirement, because many of the issues and problems we encounter in our lives are not always rich with evidence.  Some issues have a wealth of evidence, some issues have a small amount of indirect or circumstantial evidence, some issues have evidence compatible with a variety of radically different conclusions, and some issues have virtually no evidence.  What’s worse is that there appears to be an inverse relationship between the size and importance of the issue one is addressing and the amount of evidence that is available.  The bigger the question one has, the less evidence there is to address it.  The questions of how to obtain a secure and steady supply of food, water, and shelter, how to extend the human lifespan and increase the economic standard of living, all have scientific-technological answers backed by abundant evidence.  Other issues, such as the origins of the universe, the nature of the elementary particles, and the evolution of life, also have large amounts of evidence, albeit with significant gaps in certain details.  But some of the most important questions we face have such a scarcity of evidence that a variety of conflicting beliefs seems inevitable.  Why does the universe exist?  Is there intelligent life on other planets, and if so, how many planets have such life?  Where did the physical laws of the universe come from?  What should we do with our lives?  Will the human race survive the next 1000 years?  Are our efforts to be good people and follow moral codes all in vain?

In cases of scarce evidence, to demand that sufficient evidence exist before forming a belief is to put the cart before the horse.  If one looks at the origins and growth of knowledge in human civilization, belief begins with imagination — only later are beliefs tested and challenged.  Without imagination, there are no hypotheses to test.  In fact, one would not know what evidence to gather if one did not begin with a belief.  Knowledge would never advance.  As the philosopher George Santayana argued in his book Reason and Religion,

A good mythology cannot be produced without much culture and intelligence. Stupidity is not poetical. . . . The Hebrews, denying themselves a rich mythology, remained without science and plastic art; the Chinese, who seem to have attained legality and domestic arts and a tutored sentiment without passing through such imaginative tempests as have harassed us, remain at the same time without a serious science or philosophy. The Greeks, on the contrary, precisely the people with the richest and most irresponsible myths, first conceived the cosmos scientifically, and first wrote rational history and philosophy. So true it is that vitality in any mental function is favourable to vitality in the whole mind. Illusions incident to mythology are not dangerous in the end, because illusion finds in experience a natural though painful cure. . . .  A developed mythology shows that man has taken a deep and active interest both in the world and in himself, and has tried to link the two, and interpret the one by the other. Myth is therefore a natural prologue to philosophy, since the love of ideas is the root of both.

Modern critics of traditional religion are right to argue that we need to revise, reinterpret, or abandon myths when they conflict with new evidence.  As astronomy advanced, it was necessary to abandon the geocentric model of the universe.   As the evidence for evolution accumulated, it was no longer plausible to believe that the universe was created in the extremely short span of six days.  There is a difference between a belief formed in the face of a scarcity of evidence and a belief that goes against an abundance of evidence.  The former is permitted, and is even necessary to advance knowledge; the latter takes knowledge backward.

Today we have reached the point at which science is attempting to answer some very large questions, and science is running up against the limits of what is possible with observation, experimentation, and verification.  Increasingly, the scientific imagination is developing theories that are plausible, but have little or no evidence to back them up; in fact, for many of these theories we will probably never have sufficient evidence.  I am referring here to cosmological theories about the origins of the universe that propose a “multiverse,” that is, a large or even infinite collection of universes that exist alongside our own observable universe.

There are several different types of multiverse theories.  The first type, which many if not most cosmologists accept, proposes multiple universes with the same physical laws and constants as ours, but with different distributions of matter.  A second type, which is more controversial, proposes an infinite number of universes with different physical laws and constants.  A third type, also controversial, arises out of the “many worlds” interpretation of quantum physics — in this view, every time an indeterminate event occurs (say, a six-sided die comes up a “four”), an entirely new universe splits off from our own.  Thus, the most extreme multiverse theories claim that all possibilities exist in some universe, somewhere.  There are even an infinite number of people like you, each with a slight variation in life history (i.e., turning left instead of turning right when leaving the house this morning).

The problem with these theories, however, is that is impossible to obtain solid evidence on the existence of other universes through observation — the universes either exist far beyond the limits of our observable universe, or they reside on a different branch of reality that we cannot reach.  Now it’s not unusual for a scientific theory to predict the existence of particles or forces or worlds that we cannot yet observe; historically, a number of such predictions have proved true when the particle or force or world was finally observed.  But many other predictions have not been proved true.  With the multiverse, it is unlikely that we will have definitive evidence one way or the other.  And a number of scientists have revolted at this development, arguing that cosmology at this level is no longer scientific.  According to physicist Paul Davies,

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.

Likewise, Freeman Dyson insists:

[T]he multiverse is philosophy and not science. Science is about facts that can be tested and mysteries that can be explored, and I see no way of testing hypotheses of the multiverse. Philosophy is about ideas that can be imagined and stories that can be told. I put narrow limits on science, but I recognize other sources of human wisdom going beyond science. Other sources of wisdom are literature, art, history, religion, and philosophy. The multiverse has its place in philosophy and in literature.

Cosmologist George F.R. Ellis, in the August 2011 issue of Scientific American, notes that there are several ways of indirectly testing for the existence of multiple universes, but none are likely to be definitive.  He concludes: “Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are.  But we should name it for what it is.”

Given the thinness of the evidence for extreme multiverse theories, one might ask why modern day atheists do not seem to attack and mock such theorists for believing in something for which they cannot provide solid evidence.  At the very least, Christopher Hitchens’s claim that “exceptional claims require exceptional evidence” would seem to invalidate belief in any multiverse theory.  At best, at some future point we may have indirect or circumstantial evidence for the existence of some other universes; but we are never going to have exceptional evidence for an infinite number of universes consisting of all possibilities.  So why do we not hear of insulting analogies involving orbiting teapots and flying spaghetti monsters when some scientists propose an infinite number of universes based on different physical laws or an infinite number of versions of you?  I think it’s because scientists are respected authority figures in a modern, secular society.  If a scientist says there are multiple universes, we are inclined to believe them even in the absence of solid evidence, because scientists have social prestige, especially among atheists.

Ultimately, there is no solid evidence for the existence of God, no solid evidence for the existence of an infinite variety of universes, and no solid evidence for the existence of other versions of me.  Whether or not one chooses to believe any of these propositions depends on whether one decides to leap into the dark, and which direction one decides to  leap.  This does not mean that any religious belief is permissible — on issues which have abundant evidence, beliefs cannot go against evidence.  Evolution has abundant evidence, as does modern medical science, chemistry, and rocket science.  But where evidence is scarce, and a variety of beliefs are compatible with existing evidence, holding a particular belief cannot be regarded as wholly unjustified and irrational.