Grand Unification and a Summer Day

11 September 2010


When we look up into the night sky what we see are stars. If it is very clear and very dark, we will see one of the spiral arms of the Milky Way, therefore glimpsing a small part of the large scale structure of our galaxy. The large scale structure of the universe is nearly beyond our comprehension, and beyond the immediate experience of all except a few astronomers. But we all have an immediate experience of the stars, and we have a personal sense of what the stars are like from our acquaintance with our own sun. This makes stars immediately comprehensible to us.

For all the scientific progress of recent decades, almost no real progress has been made in reconciling general relativity with quantum theory. The difference of approach between the two was evident from the start, and the founders of the two theories knew it right from the outset. Little has changed since the emergence of both relativity and quantum theory in the early part of the twentieth century in terms of the different philosophical approach that each has to the natural world.

Relativity is a “classical” theory in so far as it assumes an ideal continuity to space; it is concerned with the big picture of the world, with how gravity has shaped the large scale structure of the cosmos. Quantum theory begins from the other extreme, starting with the smallest possible constituents of the world and building up from there. It is also a “non-classical” theory because it assumes an ultimate graininess to the structure of the world, an atomicity that is nevertheless distinct from classical atomism.

While the best efforts of the best scientists have not yet bridged the gap between relativity and quantum theory, we can see with our own eyes, every time we look at the stars, the practical consequences of the actual unity of relativity and quantum theory in nature. For stars are the nexus of that unity. Stars stand in gravitational relationships to each other. Some stars get big enough that they collapse into black holes, and some black holes become so large that they drag vast quantities of matter around with them. These spinning agglomerations of matter are galaxies, and the largest scale structure of the cosmos is described by the gravitational interactions of galaxies, of clusters of galaxies, and of superclusters of galaxies.

All of this vast structure, so well described by relativity, is born of stars. But stars themselves are born of an astonishing complexity of quantum interactions in the superheated cores of stars. When supernova SN 1987A exploded not far from the Milky Way in the Large Magellanic Cloud and its light finally reached Earth in 1987, scientists were able to check neutrino detectors buried deep within the earth, and when they found that neutrinos had streamed out of the collapsing core of the doomed star before any visible evidence of the supernova appeared, they confirmed an important prediction of theories about stellar structure and evolution. Such theories can only be formulated, and can only be understood, in the context of quantum theory.

It is in the very cores of stars that general relativity and quantum theory are unified, and the light that the stars give off, the warmth that we can feel upon our faces when to turn to look up at the sun on a summer day, is an immediate and concrete experience of that unity.

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