Inferences and large dosages of imagination actually have allowed the construction of a far more adequate understanding of the cosmic and human past than earlier generations achieved. I believe that this is the central intellectual accomplishment of the twentieth century. Innumerable cosmologists, physicists, mathematicians, anthropologists, sociologists, historians, ecologists, ethologists, and other specialists have played their part; a few swashbuckling intellects led the way, and the outlines of an evolutionary worldview, uniting natural and human history, has begun to emerge. It may be convincing for generations to come—or again may not.

I thought... I had a pretty good idea for what the structure of this... theory that's underneath space and time and so on might be like. ...I thought, "Gosh, in my lifetime... we might be able to figure out what happens in the first 10^-100 seconds of the universe. ...It's pretty far from anything that we can see today and it would be ...hard to test for what's right ...To my huge surprise, although it should have been obvious, ...we managed to get unbelievably much further than that. ...It turns out that even though there's this ...bed of computational irreducibility that ...all these simple rules run into, ...there are ...certain pieces of computational reducibility that ...generically occur for large classes of these rules, and... the big pieces of computational reducibility are ...the pillars of 20th century physics. That's the amazing thing, that general relativity and quantum field theory... turn out to be precisely the stuff you can say. There's a lot you can't say... at this... irreducible level where you.. don't... know what's going to happen. You have to run it [and] you can't run it within our universe... The things you can say turn out to be, very beautifully, exactly the structure that was found in 20th century physics...

What we realized is that... these theories are generic to a huge class of systems that have these particular very unstructured, underlying rules. ...[P]eople have been struggling for a long time... How does general relativity, the theory of gravity, relate to quantum mechanics? They seem to have all kinds of incompatibilities. ...What we realized is at some level they are the same theory!

A new development began for relativity theory after 1925 with its absorption into quantum physics. The first great success was scored by Dirac's quantum mechanical equations of the electron, which introduced a new sort of quantities, the spinors, besides the vectors and tensors into our physical theories. ...But difficulties of the gravest kind turned up when one passed from one electron or photon to the interaction among an indeterminate number of such particles. In spite of several advances a final solution of this problem is not yet in sight and may well require a deep modification of the foundation of quantum mechanics, such as would account in the same basic manner for the elementary electric charge e as relativity theory and our present quantum mechanics account for c and h.

The last century was defined by physics. From the minds of the world’s leading physicists there flowed a river of ideas that would transport man kind to the very pinnacle of wonder and to the very depths of despair. This was a century that began with the certainties of absolute knowledge and ended with the knowledge of absolute uncertainty. It was a century in which physicists developed theories that would deny us the possibility that we can ever properly comprehend the nature of physical reality. It was also a century in which they built weapons with the capacity utterly to destroy this reality.

It has now become clear that the exploration of the Universe, as conducted by physicists, astronomers and cosmologists, is one of the greatest intellectual adventures of the mid-twentieth century.

With the coming of the twentieth century, there came into being a new physics which was especially concerned with phenomenon on the atomic and sub-atomic scale. ...A preliminary glance over the vast territory of this new physics reveals three outstanding landmarks.

Relativity, from one viewpoint the beginning of twentieth-century physics, is from another the capstone of classical physics, the final and most elegant variation of the world view initiated by Galileo and Newton. In a sense, after Einstein, classical physics, just as after Mozart classical music, could go no further.

The development of quantum mechanics in the 1920s was the greatest advance in physical science since the work of Isaac Newton. It was not easy; the ideas of quantum mechanics present a profound departure from ordinary human intuition. Quantum mechanics has won acceptance through its success. It is essential to modern atomic, molecular, nuclear, and elementary particle physics, and to a great deal of chemistry and condensed matter physics as well.

The epithet “revolutionary” is usually reserved for two great modern developments – the Relativity Theory and the Quantum Theory. These are not merely new discoveries as to the content of the world; they involve changes in our mode of thought about the world.

Generally speaking, all the really great ideas of physics are really spin-offs of string theory... Some of them were discovered first, but I consider that a mere accident of the development on planet earth. On planet earth, they were discovered in this order [general relativity, quantum field theory, superstrings, and supersymmetry]... But I don't believe, if there are many civilizations in the universe, that those four ideas were discovered in that order in each civilization.

Einstein's breakthrough was classic in that it sought to unify the elements of a physical analysis, and it placed the older examples and principles within a broader framework. But it was revolutionary in that, ever afterward, we have thought differently about space and time, matter and energy. Space and time-no more absolute-have become forms of intuition that cannot be divorced from perspective or consciousness, anymore than can the colors of the world or the length of a shadow. As the philosopher Ernst Cassirer commented, in relativity, the conception of constancy and absoluteness of the elements is abandoned to give permanence and necessity to the laws instead.

It is odd that the last twenty-five years which have witnessed the greatest progress ever made in physical science—the greatest victories ever achieved by mind over matter—should have produced hardly a volume that will be remembered in 1900.

Today we live in the midst of upheaval and crisis. We do not know where we are going, nor even where we ought to be going. Awareness is spreading that our future cannot be a straight extension of the past or the present … The century now approaching its end has been one of indiscriminate violence, it has been perhaps the most murderous one in Western history of which we have record. Yet I would think that what will strike people most when, hundreds of years from now, they will look back on our days is that this was the age when the exploration of space began, the microchip was invented, revolutions in transport and communication virtually annihilated time and distance, transforming the world into a "global village," and relativity theory, quantum mechanics, and the structure of the atom were discovered, in brief that this has been the century of science and technology.