Before embarking upon his ultimately successful quest to discover the laws of planetary motion, Johannes Kepler tried to explain the sizes of the orbits of the planets from first principles: developing a mathematical model of the orbits based upon nested Platonic solids. Since, at the time, the solar system was believed by most to be the entire universe (with the fixed stars on a sphere surrounding it), it seemed plausible that the dimensions of the solar system would be fixed by fundamental principles of science and mathematics.
Even though he eventually rejected his model as inaccurate, he never completely abandoned it — it was for later generations of astronomers to conclude that there is nothing fundamental whatsoever about the structure of the solar system: it is simply a contingent product of the history of its condensation from the solar nebula, and could have been entirely different. With the discovery of planets around other stars in the late twentieth century, we now know that not only do planetary systems vary widely, many are substantially more weird than most astronomers or even science fiction writers would have guessed.
Since the completion of the in the 1970s, a major goal of theoretical physicists has been to derive, from first principles, the values of the more than twenty-five “free parameters” of the Standard Model (such as the masses of particles, relative strengths of forces, and mixing angles). At present, these values have to be measured experimentally and put into the theory “by hand”, and there is no accepted physical explanation for why they have the values they do. of particle physics
Further, many of these values appear to be “fine-tuned” to allow the existence of life in the universe (or at least, life which resembles ourselves) — a tiny change, for example, in the mass ratio of the up and down quarks and the electron would result in a universe with no heavy elements or chemistry; it’s hard to imagine any form of life which could be built out of just protons or neutrons.
The emergence of a Standard Model of cosmology has only deepened the mystery, adding additional apparently fine-tunings to the list. Most stunning is the cosmological constant, which appears to have a nonzero value which is 124 orders of magnitude smaller than predicted from a straightforward calculation from quantum physics.
One might take these fine-tunings as evidence of a benevolent Creator, or of our living in a simulation crafted by a clever programmer intent on optimising its complexity and degree of interestingness. But most physicists shy away from such deus ex machina and “we are in machina” explanations and seek purely physical reasons for the values of the parameters we measure.
Now let’s return for a moment to Kepler’s attempt to derive the orbits of the planets from pure geometry. The orbit of the Earth appears, in fact, fine-tuned to permit the existence of life. Were it more elliptical, or substantially closer to or farther from the Sun, persistent liquid water on the surface would not exist, as seems necessary for terrestrial life.
The apparentcan be explained, however, by the high probability that the galaxy contains a multitude of planetary systems of every possible variety, and such a large ensemble is almost certain to contain a subset (perhaps small, but not void) in which an earthlike planet is in a stable orbit within the habitable zone of its star. Since we can only have evolved and exist in such an environment, we should not be surprised to find ourselves living on one of these rare planets, even though such environments represent an infinitesimal fraction of the volume of the galaxy and universe.
As efforts to explain the particle physics and cosmological parameters have proved frustrating, and theoretical investigations into cosmic inflation and string theory have suggested that the values of the parameters may have simply been chosen at random by some process, theorists have increasingly been tempted to retrace the footsteps of Kepler and step back from trying to explain the values we observe, and instead view them, like the masses and the orbits of the planets, as the result of a historical process which could have produced very different results.
The apparent fine-tuning for life is like the properties of the Earth’s orbit — we can only measure the parameters of a universe which permit us to exist! If they didn’t, we wouldn’t be here to do the measuring.
But note that like the parallel argument for the fine-tuning of the orbit of the Earth, this only makes sense if there are a multitude of actually existing universes with different random settings of the parameters, just as only a large ensemble of planetary systems can contain a few like the one in which we find ourselves. This means that what we think of as our universe (everything we can observe or potentially observe within the Hubble volume) is just one domain in a vastly larger “multiverse”, most or all of which may remain forever beyond the scope of scientific investigation.
Now such a breathtaking concept provides plenty for physicists, cosmologists, philosophers, and theologians to chew upon, and macerate it they do in the book Universe or Multiverse? In this volume, a number of active and eminent researchers in the field address the issues and describe recent developments. The book is far from a cheering section for multiverse theories: both sides are presented and, in fact, the longest chapter is the one which deems the anthropic principle and anthropic arguments entirely nonscientific.
(Source: Reading List: Universe or Multiverse?)