One should not get the impression that all (or even most) physicists believe that the solution to the puzzle of the energy of empty space will come from anthropic reasoning. The mere mention of the “multiverse” and “anthropics” tends to raise the blood pressure of some physicists. There are two main reasons for this adverse reaction. First, as already mentioned in chapter 9, ever since the seminal work of philosopher of science Karl Popper, for a scientific theory to be worthy of its name, it has to be falsifiable by experiments or observations. This requirement has become the foundation of the “scientific method.” An assumption about the existence of an ensemble of potentially unobservable universes appears, at first glance at least, to be in conflict with this prerequisite and therefore in the realm of metaphysics rather than physics. Note, however, that the boundary between what we define as observable and what is not is unclear. Consider, for instance, the “particle horizon”: that surface around us from which radiation emitted at the big bang is just reaching us. In the Einstein–de Sitter model—the model for a homogeneous, isotropic, constant curvature universe, with no cosmological constant—the cosmic expansion decelerates, and one could safely expect that all the objects currently lying beyond the horizon will eventually become observable in the distant future. But since 1998, we know that we don’t live in an Einstein–de Sitter cosmos: our universe is accelerating. In this universe any object now beyond the horizon will stay beyond the horizon forever. Moreover, if the accelerating expansion continues, as anticipated from a cosmological constant, even galaxies that we can now see will become invisible to us! As their recession speed approaches the speed of light, their radiation will stretch (redshift) to the point where its wavelength will exceed the size of the universe. (There is no limit on how fast space-time can stretch, since no mass is really moving.) So even our own accelerating universe contains objects that neither we nor future generations of astronomers will ever be able to observe. Yet we would not consider such objects as belonging to metaphysics. What could then give us confidence in potentially unobservable universes? The answer is a natural extension of the scientific method: We can believe in their existence if they are predicted by a theory that gains credibility because it is corroborated in other ways. We believe in the properties of black holes because their existence is predicted by general relativity—a theory that has been tested in numerous experiments. The rules should be a straightforward extrapolation of Popper’s ideas: If a theory makes testable and falsifiable predictions in the observable parts of the universe, we should be prepared to accept its predictions in those parts of the universe (or multiverse) that are not accessible to direct observations.

The second main reason for the hostile passions that anthropic reasoning provokes is that to some scientists it signals the “end of physics.” Following Descartes, most physicists dream, above all, of a uniquely self-consistent mathematical theory that explains and determines all the microphysical constants, as well as the entire cosmic evolution. Consequently, they would like to pursue, in the words of cosmologist Edward Milne, “a single path towards the understanding of this unique entity, the universe.” There is very little doubt that this was Einstein’s hope too. In a lecture delivered at Oxford in 1933, Einstein said, “It is my conviction that pure mathematical construction enables us to discover the concepts and the laws connecting them which give us the key to the understanding of the phenomena of Nature.” As is well known, Einstein was uncomfortable even with the probabilistic nature of quantum mechanics, though he appreciated fully its successes. In a letter he wrote on December 4, 1926, to Max Born, one of the founding fathers of quantum mechanics, Einstein expressed his opinion:

Quantum mechanics is certainly imposing. But an inner voice tells me that this is not yet the real thing [emphasis added]. The theory yields much, but it hardly brings us closer to the Old One’s secrets. I, in any case, am convinced that He does not play dice.

The concept of accidental variables in a potentially unobservable multiverse would have probably distressed Einstein even more. Note, however, that Einstein’s reservations about quantum mechanics stemmed more from psychology—his belief that he knew the direction in which to look—than from hard-core physics. The same may turn out to be the case with the objections to anthropic reasoning. In spite of the experience in the past few centuries, there are no assurances that physical reality will indeed oblige and render itself in its entirety to first-principles explanations. The quest for such descriptions may prove as futile as Kepler’s quest for a beautiful geometrical model for the solar system. What we have traditionally called fundamental constants and maybe even laws of nature could turn out to be mere accidental variables and parochial bylaws in our universe. The anthropic principle could perhaps eventually play a similar role to that assigned by philosopher Bertrand Russell to philosophy: “The point of philosophy is to start with something so simple as to seem not worth stating, and to end with something so paradoxical that no one will believe it.”

The anthropic thinking on the nature of the cosmological constant demonstrates the profound impact that Einstein’s seemingly innocent attempt at a static universe continues to have on cutting-edge physics. How then do we appraise Einstein’s “biggest blunder” today?