When communism collapsed, people spoke of the "End of History." Some now speak in a similar way of the "End of Physics." They claim that all the fundamental issues are understood; that all the great questions are answered. Others suggest that even if some fundamental issues remain outstanding they are essentially abstractions, irrelevant to human aspirations. So, are there any burning problems left for physicists to tackle or should physicists accept with grace the close of their science?
Questions about the end of physics are not new. In 1890, buoyed by centuries of successful applications of Newtonian mechanics and the electromagnetism of Faraday and Maxwell, eminent voices proclaimed physics at an end. The decade to follow, however, overturned this exuberance. Radioactivity, x-rays and the discovery of the electron opened up a new world. Soon after, physics reached new heights as it developed the two revolutionary pillars of 20th century physics: Quantum Mechanics and General Relativity, another of Einstein's pivotal contributions to modern science.
So, will man's hubris be capsized again? To grasp where physics might go it is necessary to know how far it has come. Physics emerged in the 16th century as a quantitative, mathematically based set of laws through which humans could comprehend the inanimate world. Mysteries that confounded the ancients were replaced by an ever more concise set of principles so that the flight of projectiles, the fall of an apple, the orbit of the moon, and the trajectories of planets could be quantitatively accounted for.
Definitions of space, time, matter and energy became, thanks to physics, precise. Complex phenomena: eclipses, comets, tides, the properties of matter (e.g. solids, liquids and gases), the stability of structures (e.g. bridges, towers, ships), the behavior of light, the processes involving heat flow, temperature, the colors of the rainbow and the more subtle colors emitted by heated substances, electrical charges and magnetism, gravitation and radioactivity were all organized into a small number of "laws of physics."
The twentieth century brought a profound understanding of the properties of the physical world, now considered as "understood" by physics. Stars, galaxies, the evolution of the universe as well as biological entities such as proteins, cells and genes all came under the purview of physics.
With this record of advance behind it, what remains for physics to achieve? Let me focus on a few areas where major advances may take place in the coming decades.
In particle physics and cosmology we are now on the verge of solving problems that have confounded science since antiquity: what are the ultimate blocks of matter? How does the universe work? Our ambition is to find an answer so elegant and simple that it will fit easily on the front of a T-shirt. Today, however, we have only achieved a powerful, but flawed, summary, the so-called "Standard Model," which reduces all reality to a dozen or so particles and four basic forces in nature.
Why is the "Standard Model" flawed? One obvious flaw is aesthetic - ie, it is too complex. The model does not explain why there are so many fundamental particles and why they differ so much. Another flaw in the standard model is that one of the fundamental forces in nature is not included: gravity. We are still searching for a simple, all-encompassing theory that reconciles all these forces.
This effort is specially important for cosmology where peaceful coexistence between relativity, which is the theory of gravity, and quantum theory is needed to understand the universe's beginning. During the earliest moments after the creation of the universe in the Big Bang 12 billion years ago, the universe was extremely small and dense and the laws of physics governed perhaps only one kind of particle and one force. Unification of relativity and quantum theory is therefore necessary in order for us to understand the earliest moments of creation, when other particles and forces came into being.
With that understanding, cosmologists will begin to grasp how this dense universe started to expand and why more than 90% of its mass remains invisible to our instruments. Over time we will begin to reveal all the essential features of the cosmos to explain how a single event billions of years ago not only created galaxies, stars and planets but the atoms that assembled into living beings intricate enough to ponder their origins and purposes.
Of course, some say that physics is unlikely to provide an ultimate explanation for evolution, the event that lies at the basis of biology and medicine. However, I think that application of the principles of physics to chemistry and especially to biology will also assume greater prominence. As biology becomes a quantitative, hard science, physics - in its techniques, computational power and basic laws - will influence biologists more and more.
Indeed, the ultimate role of physics may be to unify all human knowledge. Edmund O. Wilson, the noted Harvard biologist, wrote in his book "Consilience" of the eventual unification of the hard sciences, the social sciences and the humanities. As revolutionary as this sounds, one can - even now - glimpse its possibility through the bridge of human consciousness, because the brain appears to work by the laws of physics and these, utilizing the probabilities of quantum science and the complexity of chaos theory, may be amenable to logical analysis.
Will this knowledge hijack emotion, love, music, poetry, art? Or will this scientific union enrich the great glories of the human spirit? Must we fear, with Keats, to "unweave the rainbow?" Richard Feynman, the great theoretical physicist, has offered a simple yet powerful retort: "Does our understanding of the mechanisms of stellar activity diminish in any way our appreciation of the beauty and splendor of the night sky?"