- 10 May 2010 by Marcelo Gleiser
FIFTEEN years ago, I was a physicist hard at work hunting for a theory of nature that would unify the very big and the very small. There was good reason to hope. The great and the good were committed. Even Einstein, who recognised that our understanding of reality is necessarily incomplete, had spent the last 20 years of his life searching for a unified field theory that would describe the two main forces we see acting around us - gravity and electromagnetism - as manifestations of a single force. For him, such a mathematical theory represented the purest and most elegant expression of nature and the highest achievement of the human intellect.
Fifty-five years after Einstein's death, the hunt for this elusive unified field theory continues. To physicist Stephen Hawking and many others, finding the "theory of everything" would be equivalent to knowing the "mind of God". The metaphor is not accidental.
Modern critics say that Einstein and other giants of 20th-century physics (including Wolfgang Pauli, Erwin Schrödinger and Werner Heisenberg) failed because their models didn't include all particles of matter and their fundamental interactions. Factor them in, they argue, and we stand a much better chance of success. Dreams of a final theory (as a book on the subject, by Nobel laureate Steven Weinberg, was titled) live on, stronger than ever.
But are we really getting any closer? Do we dare ask whether the search is fundamentally misguided? Could belief in a physical theory that unifies the secrets of the material world - a "hidden code" of nature - be the scientific equivalent of the religious belief in oneness held by the billions who go to churches, mosques and synagogues every day?
Even before what we now call physics existed, ancient Greek philosophers pondered whether the diversity of nature could radiate from a single source, a primal substance. Thales, regarded by Aristotle as the first philosopher in the Greek tradition, proposed that everything was made of water, a substance he believed represented nature's dynamic essence. Later, Pythagoras and his followers believed that nature was a mathematical puzzle, constructed through ratios and patterns that combine integers, and that geometry was the key to
The idea of mathematics as a fundamental gateway to nature's secrets re-emerged during the late Renaissance. Galileo Galilei, René Descartes, Johannes Kepler and Isaac Newton made it clear that the mathematical description of nature succeeds only through the painstaking application of the scientific method, where hypotheses are tested by experiments and observations and then accepted or rejected. Physics became the science of the "how", leaving the "why" for philosophy and religion. When Newton was asked why matter attracts matter with a strength that weakens with the square of the distance, he answered that he "feigned no hypotheses"; it was enough to provide a quantitative description of the phenomenon.
That, however, is only half the story. To Newton, God was the supreme mathematician and the mathematical laws of nature were Creation's blueprint. As science advanced, the notion that god interfered explicitly with natural phenomena faded away, but not the idea that nature's hidden code lay in an all-encompassing mathematical theory. Einstein's "God" was far removed from Newton's, as he famously said: "I believe in Spinoza's God who reveals himself in the orderly harmony of what exists." His search for a unified field theory was very much a search for the essence of this natural god.
Modern incarnations of unified field theories come in two flavours. The more traditional version, the so-called Grand Unified Theory (GUT), seeks to describe electromagnetism and the weak and strong nuclear forces as a single force. The first of these theories was proposed in 1974 by Howard Georgi, of Harvard University, and Sheldon Glashow, now at Boston University. The more ambitious version seeks to include gravity in the unification framework. Superstring theory tries to do this by abandoning the age-old paradigm that matter is made of small, indivisible blocks, substituting them with vibrating strings that live in higher-dimensional spaces.
Like all good physical theories, GUTs make predictions. One is that the proton, the particle that inhabits all atomic nuclei, is unstable. For decades, experiments of increasing sensitivity have looked for decaying protons and failed to find them. As a consequence, the models have been tweaked so that protons decay so rarely as to be outside the current reach of detection. Another prediction fared no better: bundled-up interacting fields called magnetic monopoles have never been found.
For superstrings, the situation is even worse. In spite of its mathematical elegance, the theory is so detached from physical reality that it is exceedingly difficult to determine what a measurable string effect might be.
I now think that the very notion of a final theory is faulty. Even if we succeed in unifying the forces we know, we can only claim to have achieved partial unification. Our instruments have limits. Since knowledge of physical reality depends on what we can measure, we will never know all there is to know. Who is to say there are only four fundamental forces? Science is full of surprises. Much better to accept that our knowledge of physical reality is necessarily incomplete. This way, science is understood as a human enterprise and the "mind of God" is exorcised once and for all.
I now think that the very notion of a final theory is faulty
Ever since the discovery of parity violation in the weak interaction over 50 years ago, experiments in particle physics have shown us that our hopes for perfection are just that - hopes. Symmetries are violated left and right; in nature, unlike in John Keats's famous poem, beauty isn't always truth.
But there's more. I propose that fundamental asymmetries are a necessary part of our universe, that they determine our very existence. Consider the following. The universe had to have special properties to keep on expanding for 14 billion years. And particles of matter had to dominate those of antimatter soon after the big bang, or the universe would consist mostly of radiation.
Life itself is a product of imperfections, from the spatial asymmetry of amino acids to mutations during reproduction. Asymmetries forged the long, complex and erratic path from particles to atoms to cells, from simple prokaryotic cells without nuclei to more sophisticated eukaryotic cells, and then from unicellular to multicellular organisms.
The history of life is deeply enmeshed with the earth's environmental changes, from the increase of oxygen availability, to the advent of plate tectonics that help regulate carbon dioxide. Life (not to mention intelligence) in the extraordinarily complex forms we have come to know is probably quite rare, a product of asymmetries, imperfections and accidents.
In the end, giving up on a final theory won't make doing physics - or science - less exciting. Nature has plenty of mysteries to keep us busy for a very long time.