Though less well known by far than Einstein, at the turn of the century Poincaré’s popular philosophical books, Science and Hypothesis and Science and Values, were bestsellers in France. They had a profound effect on modern philosophy of science, and today are still read in philosophy courses. They were also translated into many other languages very early on, including German and English, and were widely distributed. He opened up whole new areas of mathematics, including the mathematics of topology. He helped invent the science of chaos, and all that we understand of the science of complexity owes an enormous amount to him. He contributed enormously to what became relativity theory, and is important in many other branches of physics. He was truly a polymath and went on to do things in engineering. He was one of the people who rescued the Eiffel Tower from being taken down after the International Exhibition for which it was built, because he saw a way of using it as a military antenna. In fact, in large measure under Poincaré’s direction, the Eiffel Tower itself became an enormous antenna that would send time signals all over the world, allowing longitude finders from Canada to the tip of Africa to do their work. Moving back and forth smoothly between high engineering and abstract mathematics, he left an enormous legacy across many fields, always reasoning concretely, visually—as an abstract engineer so to speak. His thoughts on time were no exception.

After learning more about Poincaré, I tried to understand how he and Einstein could have radically reformulated our ideas of time and space by looking at the way that philosophically abstract concerns, physics concerns, and these technological problems of keeping trains from bashing into each other and coordinating mapmaking across the empires might fit into a single story. It begins with an extraordinarily simple idea: that two events are simultaneous if I can make clocks at the two events say the same thing. How do I coordinate these clocks? I send a signal from one to the other and take into account the time it takes for the signal to get there. That’s the basic idea, but all of relativity theory, E = mc2, and so much of what Einstein does follows from it. The question is, where did this idea come from? Albert Einstein and Henri Poincaré were the two people who worked out this practical, almost operational idea of simultaneity, and I want to see them as occupying points of intersection of technological, philosophical, and physical reasoning. They were the two people who stood dead center in those triple crossing points.

Sometimes people ask me, what is really at the base of Einstein’s and Poincaré’s account of simultaneity? Is it really physics, or fundamentally technology, or does it come down to philosophy? I think that those are wrong ways of putting the question. That is to say, to me it’s like asking if the Place de l’Etoile is truly in the Avenue Foch or the Avenue Victor Hugo. The Place de l’Etoile is a place because it is at the intersection of those great avenues. And that’s what happens here. We’re looking at an extraordinary moment when philosophy, physics and technology cross, precisely because of the intersection of three very powerful streams of action and reasoning at the turn of the century. It is like having a triple spotlight focussed on one position in an enormous theater; it’s triply illuminated. It was important to railroad engineers and map makers that they knew how to define simultaneity. It was important to philosophers to figure out what time is, what a clock is, and how to think about what defined time: mechanical clocks or astronomical phenomena or some sort of abstract time that lay behind all appearances. And it was important to physicists to understand what simultaneity was in order to know how to interpret the most important equations of physics: Maxwell’s equations concerning electricity and magnetism. Poincaré and Einstein were the two people—more than anyone else—who were concerned with all three parts of that intersection, and that is why they need to be understood together. Of course clocks did not cause relativity any more than relativity caused the transformation of modern clock synchronization.

In human terms, Einstein and Poincaré are fascinating because in some ways you couldn’t imagine two people closer. They had common friends, published in many of the same places, and were working intensively on many of the same problems. They were both at the top of their professions, both enjoyed writing for broader audiences, both were taken very seriously by philosophers, and both had serious technological-engineering interests and training. Yet they couldn’t have been farther apart. In a certain way they remind me of Freud, for whom it was almost unbearable to read Nietzsche, because (as Freud said on several occasions) Nietzsche’s ideas were too close, and yet organized around a different approach.

Poincaré and Einstein, who had two of the largest scientific correspondences of the 19th and 20th centuries, including thousands of letters to and from other people, never exchanged a single postcard over the entirety of their overlapping lives. They met once, towards the end of Poincaré’s life, when Poincaré presided over a session at a vitally important physics conference where Einstein was talking about his new ideas about the quantum of light. At the end of this session, Poincaré said that Einstein’s presentation was so different from what physics should be — namely that it could be represented with causal interactions, with good differential equations, with clear presentations of principles and consequences — that he simply found it unbearable, and ended by making it clear that what Einstein was saying was so contradictory that anything could follow from it. It was a disaster for science, he thought. Einstein for his part went home and scribbled a note to a friend in which he recounted the wonderful work that had been done by various colleagues, how much he admired, even loved, the physicist Hendrik Lorentz, but disparaged Poincaré who simply seemed to understand nothing. The passed like ships in the night, each, on relativity, unable to acknowledge the other’s existence. Yet a few weeks after their ill-starred meeting, Poincaré wrote a letter of recommendation for Einstein for a job that was very important to him. It was a stunning letter that said, essentially, that this young man may well be up to some of the greatest things, and even if only a few of his wild ideas turn out to be true he’s a person of extraordinary importance. It was a letter of enormous grace and generosity. They never directly exchanged another word and never met again.

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