The Bay of Cassis

Dear John,

Here we are in a little village on the Mediterranean, where I am working for a few weeks with my friend Carlo Rovelli, who is a Professor nearby in Marseille. Carlo and I discovered the main ideas that went into loop quantum gravity working together in a setting like this, in Verona, getting together each day to talk, and then going home to calculate and check on our own, and it is wonderful to be back working with him. We understand each other easily, and from long experience know how to compensate for each other's strengths and weaknesses and so we work quickly. We are making fast progress on understanding the implications of the existence of a cosmological constant for quantum gravity. I had taken a detour of a few years to apply what we had learned about quantum spacetimes to string theory, but there is so much about nature, not the least the apparent fact that there is a cosmological constant, that string theory seems not to incorporate. Now it is wonderful to be back in reality, four dimensional and non-supersymmetric as it seems to be after all.

From the patio where I work I have a view of the bay of Cassis and the beautiful cliffs that rise to the east of it. Today there is little wind on the bay and the sailboats hardly move. Yesterday was windy and we took Carlo's boat out. He has bought an old wooden boat, built a century ago in this harbour, five meters, open, symmetric front to back, with gentle curves such as one sees in old paintings. Working from drawings in 19th century books Carlo has restored it to what might have been its original design, adding a mast, sail and rigging of the style used in the Mediterranean from the middle ages to the advent of modern, triangular sails. The sail hangs from a pole, which in turn is hung by a complicated organization of ropes from the top of the mast. Carlo has a lot of fun watching me try to sail his boat. Downwind we do get some speed, but the boat will hardly go upwind, and coming about takes practice. In such a boat one understands why it took Ulysses so long to get home and one wonders, watching the modern fiberglass sloops speeding by, whether it was a matter of materials or imagination that it took more than 20 centuries for people to realize it's much better to attach the sails directly to the mast.

Afterwards we have a meal which, with the exception of the tomatoes and a bit of chocolate, has been enjoyed in the Mediterranean since the advent of agriculture: olives, cheese, bread, fish, a good local wine. One cannot but think of Jared Diamond and wonder if it was really the case that the small scale organization required to reach a high standard of living on such food made possible small independent city states and hence the Phoenicians, the Greeks, the Romans, the notion of mathematical proof, the monotheistic religions, the idea of democracy and hence so much of modern civilization.

Before this I was in London for a few days, catching up with old friends, and working with Joao Maguiejo on our version of an idea that has sprung up from a number of friends and colleagues in the last two years: that perhaps special relativity is modified at very short distances. The idea is that not only can the speed of light be a constant, but one can have also an invariant length, that all observers agree on, in spite of the fact that usually lengths contract in relativity. The invariant length can be the Planck length, below which loop quantum gravity predicts the geometry of space and time is atomic and discrete. But what is really wonderful is that some people, particularly Giovanni Amelino-Camelia, have realized that such a theory has consequences that are experimentally testable not in 500 years, but now. By observing light or cosmic rays which have traveled for most of the history of the universe, one can detect such modifications of relativity. The result is that it appears possible to test experimentally which if any of the quantum theories of gravity are correct. So physical theory will not be a matter of sociology, as it has sometimes seemed over the last twenty-five years when experiment played little role. Instead nature will let us know if string theory, loop quantum gravity or something else, is the right description of space and time at the shortest distances.

For me the Mediterranean has always been a good place to reflect on where science is going, both the community as a whole and my own work. Science, at least theoretical physics, is very social, and it is good to get away from universities, conferences and research groups, and remember that it is all about nature.


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