Seven Brief Lessons on Physics Read Online Free Page B

night sky studded with stars.

FIFTH LESSON
    Grains of Space
    Despite certain obscurities, infelicities, and still unanswered questions, the physics I have outlined provide a better description of the world than we have ever had in the past. So we should be quite satisfied. But we are not.
    There’s a paradox at the heart of our understanding of the physical world. The twentieth century gave us the two gems of which I have spoken: general relativity and quantum mechanics. From the first cosmology developed, as well as astrophysics, the study of gravitational waves, of black holes, and much else besides. The second provided the foundation for atomic physics, nuclearphysics, the physics of elementary particles, the physics of condensed matter, and much, much more. Two theories, profligate in their gifts, which are fundamental to today’s technology and have transformed the way we live. And yet the two theories cannot both be right, at least in their current forms, because they contradict each other.
    A university student attending lectures on general relativity in the morning and others on quantum mechanics in the afternoon might be forgiven for concluding that his professors are fools or have neglected to communicate with one another for at least a century. In the morning the world is curved space where everything is continuous; in the afternoon it is a flat space where quanta of energy leap.
    The paradox is that both theories work remarkably well. Nature is behaving with us like that elderly rabbi to whom two men went in order to settle a dispute. Having listened to the first, the rabbi says: “You are in the right.” The second insists on being heard. The rabbi listens to him and says: “You’re also right.” Having overheard from the next room, the rabbi’s wife then calls out, “But they can’t
both
be in the right!” The rabbi reflects and nods before concluding: “And you’re right too.”
    A group of theoretical physicists scattered across the five continents is laboriously trying to settle the issue. Their field of study is called “quantum gravity”: its objective is to find a theory, that is, a set of equations—but above all a coherent vision of the world—with which to resolve the current schizophrenia.
    It is not the first time that physics finds itself faced with two highly successful but apparently contradictory theories. The effort to synthesize has in the past been rewarded with great strides forward in our understanding of the world. Newton discovered universal gravity by combining Galileo’s parabolas with the ellipses of Kepler. Maxwell found the equations of electromagnetism by combining the theories of electricity and of magnetism. Einstein discovered relativity by way of resolving an apparent conflict between electromagnetism and mechanics. A physicist is only too happy when he finds a conflict of this kind between successful theories: it’s an extraordinary opportunity. Can we build a conceptual framework for thinking about the world that is compatible with what we have learned about it from
both
theories?
    Here, in the vanguard, beyond the borders of knowledge, science becomes even more beautiful—incandescent in the forge of nascent ideas, of intuitions, of attempts. Of roads taken and then abandoned, of enthusiasms. In the effort to imagine what has not yet been imagined.
    Twenty years ago the fog was thick. Today paths have appeared that have elicited enthusiasm and optimism. There are more than one of these, so it can’t be said that the problem has been resolved. The multiplicity generates controversy, but the debate is healthy: until the fog has lifted completely, it’s good to have criticism and opposing views. One of the principal attempts to solve the problem is a direction of research called “loop quantum gravity,” pursued by a loose band of researchers working in many countries.
    Loop quantum gravity is an endeavor to combine general relativity and quantum mechanics.