Physics, unlike biology or geology, was not considered to be a historical science until now. Physicists have prided themselves on being able to derive the vast bulk of phenomena in the universe from first principles. Biology - and chemistry, as a matter of fact - are different. Chance and contingency play an important role in the evolution of chemical and biological phenomena, so beyond a point scientists in these disciplines have realized that it's pointless to ask questions about origins and first principles.

The overriding "fundamental law" in biology is that of evolution by natural selection. But while the law is fundamental on a macro scale, its details at a micro level don't lend themselves to real explanation in terms of origins. For instance the bacterial flagellum is a product of accident and time, a key structure involved in locomotion, feeding and flight that resulted from gene sharing, recombination and selective survival of certain species spread over billions of years. While one can speculate, it is impossible to know for certain all the details that led to the evolution of this marvelous molecular motor. Thus biologists have accepted history and accident as integral parts of their fundamental laws.

Physics was different until now. Almost everything in the universe could be explained in terms of fundamental laws like Einstein's theory of gravity (general relativity) or the laws of quantum mechanics. If you wanted to explain the shape and structure of a galaxy you could seek the explanation in the precise motion of the various particles governed by the laws of gravity. If you wanted to explain why water is H20 and not H30 you could seek the explanation in the principles of quantum mechanics that in turn dictate the laws of chemical bonding.

But beyond this wildly successful level of explanation seems to lie an impasse. The problem arises when you try to explain one of the most profound facts of nature, the fact that the fundamental constants of nature are fine-tuned to a fault, that the universe as we know it would not exist if these constants had even slightly different values. For instance, it is impossible to imagine life existing had the strength of the strong force binding nuclei together been even a few percent smaller or larger. Scientists have struggled for decades to explain why other numbers like the value of Planck's constant or the electron's mass are what they are. It seems now that they are giving up trying to do this, or at least giving up trying to do it the way they always have.

The point was driven home to me by two books that I read recently. One was Max Tegmark's "The Mathematical Universe". In the book Tegmark takes us on a dizzying journey through modern physics that ends in the fanciful realm of multiple universes. It's hardly the first book to do so. Multiple universes have been invoked to explain many problems in physics, but their most common use is try to explain (or explain away, as some seem to rightly think) the problem of the fundamental constants. The purported "solution" sounds simple; we can stop wondering why the fundamental constants have the precise values that they do if we assume the existence of a potentially infinite number of universes, each of which has a different set of values for the constants. Our universe just happens to have the right combination that allows sentient life to arise and ask such questions in the first place.

Leaving aside the fact that multiple universes still belong to speculation and science fiction rather than science, what is really striking about them to me is that they have finally transported physics into the realm of biology. What physicists are essentially saying is that there have been several universes in the past and there are likely several universes in the present, and our unique universe with its specific combination of fundamental constants is an accident. The multiple universe argument is very much similar to the argument establishing evolution by natural selection as the centerpiece of biology: there have been several species with several genotypic and phenotypic features, and our own human species is a result of contingency and historical accident. This is not so much an explanation as an admission of incomplete knowledge, but biologists are fine with this since it does not obviate any natural law and is still part of a satisfying overarching theory.

It looks like with the postulation of multiple universes physicists too have stepped over from the land of fundamental explanatory laws into the land of historical accident and contingency. This is a radical shift in the way physics has been done until now and a rather painful blow to the physicist's view of nature. One might also say that biology is having the last laugh. In the sixteenth and seventeenth century when biology was still doing the messy job of cataloging data and trying to make sense of the mess, physics was marching on, discovering precise regularities and generalities in nature's offerings. Since then several sciences including biology and economics have suffered from "physics envy". But now it ironically looks like physics's successful run at predicting everything from first principles might have become a victim of its own success. It may be the case that physicists's spectacular findings themselves have illuminated their own limitations. In his latest book "The Accidental Universe", physicist and writer Alan Lightman puts it thus:

"Dramatic developments in cosmological findings and thought have led some of the world's premier physicists to propose that our universe is only one of an enormous number of universes, with wildly varying properties, and that some of the most basic features of our particular universe are mere accidents - random throws of the cosmic dice. In which case, there is no hope of ever explaining these features in terms of fundamental causes and principles."

Lightman also quotes the doyen of physicists, Steven Weinberg, who recognizes this watershed in the history of his discipline:

"We now find ourselves at a historic fork in the road we travel to understand the laws of nature. If the multiverse idea is correct, the style of fundamental physics will be radically changed."

Although Weinberg does not say this, what's depressing about the multiverse is that its existence might always remain postulated and never proven. This is an ever worse scenario because the only thing that a scientist hates even more than an unpleasant answer to a question is no answer at all. It's not inaccurate to say that many physicists - and especially those like Weinberg who have been part of the spectacular revolution in physics during the 60s and 70s - are distressed by this fact. The metamorphosis of physics into a historical science means that many of the facts that have troubled the field's foremost practitioners may be a product of chance and fundamentally unexplainable in terms of more basic laws. I must emphasize that this is not some kind of "end of physics" scenario that I am imagining here (unlike my Scientific American colleague John Horgan); there are still plenty of very challenging questions dealing with the application of the fundamental laws that will keep physicists occupied for decades. Foremost among these may be the conundrum of emergent phenomena which themselves are very fundamental in fields like neuroscience and economics. I am also not implying that physicists should simply give up looking for fundamental laws. But their methodological take on finding these laws may have to change. As far as the deep question of why certain building blocks of the universe seem to exist within very narrow constraints is concerned, physicists might simply have to accept that there is no true causal explanation for the fact.

Are physicists justified in feeling despondent because they seem to be tapping the bottom of the barrel in their search for fundamental laws? I don't think so. Biologists have known about contingency and accident ever since Darwin wrote his great book, but not only has this not made them emotionally unstable but it has also not kept them from making spectacular discoveries in their discipline. Just because a system of laws might have a historical origin based on accident does not mean that there are no great truths about the system still waiting to be discovered. But more importantly, perhaps physicists need to embrace contingency to be as much of a fundamental law as any other. Biologists know this; in fact they know that there would be no evolution in the first place without contingency, and they know that it is thanks to historical accident that they get to study the incredibly rich cornucopia of living structures that the earth has presented to them.

The best thing would be for physicists to realize that just because the ultimate laws of their discipline might have a fundamentally accidental origin, it does not mean that the manifestations of those laws are any less important or useful. The most important page they should lift out of the biologists' playbook is very simple; when ideas about a field evolve, it is best for the practitioners of the field to evolve too.