EINSTEIN AND POINCARÉ 

[PETER GALISON:] When the Einstein centenary was celebrated in 1979 the speakers at all of these great events spoke about physics only as theory. It seemed odd to me that somebody like Einstein, who had begun as a patent officer and who had been profoundly interested in experiments, had left such a thoroughly abstract image of himself. My interest in Einstein began in that period, but beyond Einstein I was intrigued by the startling way that experiment and theory worked together, fascinated by the abutting of craft knowledge hard against the great abstractions of theoretical physics.

For quite a number of years I have been guided in my work by the odd confrontation of abstract ideas and extremely concrete objects. Science history, sociology, and epistemology are for me very connected, and the kind of work that I do in the history of science is always propelled and illuminated through philosophical questions. For example, I am interested in what counts as a demonstration. What does it mean to be done with a demonstration? How do experimenters distinguish between a real effect and artifacts of the apparatus or the environment? We think we know what it means to conclude a mathematical deduction, but what does it mean when I’ve shown something with a computer simulation? If I do a simulation and show that the tail of a comet forms into islands, have I demonstrated that, or is my result just the beginning of an explanation that then needs a more analytic mathematical derivation? These are questions that even today puzzle across a myriad of fields. They are questions that are, inevitably both historical and epistemological — that is they are about ordinary scientific practice and yet fundamentally philosophical. When I choose to work on a problem it is usually because it is illuminated by these different beams of light, so to speak.

When I and a few other historians, sociologists, and philosophers began looking at instruments and laboratories back in the late 1970s, emphasizing experimentation in the history of science seemed rather odd. Most historians and philosophers were keen (in the aftermath of Thomas Kuhn’s work) to show that all of science issued from theory. I suppose it was a kind of reaction against all those years of positivism from the 1920s through the 1950s when philosophers insisted that all knowledge came down to perception and observation. In any case, there wasn’t really a body of serious work on what a laboratory was, where the lab came from, or how it functioned. Since then, inquiry into the history and dynamics of experimental practice has grown into much larger domain of study. I am interested not just in the laboratory itself, however, but also in the most abstract kinds of theories. Recently, for example, I’ve been writing about string theory—specifically the confrontation between physicists and mathematicians as they try to sort out what ought to be a demonstration—in what is without doubt the most abstract form of science ever pursued.

But in every instance I’m above all intrigued by how philosophical questions illuminate and are illuminated by very the practices of science, sometimes material, sometimes abstract. And I suppose that I am always interested in blasting away the mid-level generalizations, and to explore, as in Einstein’s Clocks, Poincaré's Maps, at the way the most abstract and the most concrete come together. Instead of thinking of a kind of smooth spectrum that goes from ultraviolet to infrared with everything in between, I’m interested in bending the edges of the spectrum to make the abstract and the concrete hit one another more directly.

When I began my work quite a number of years ago, the history of science was focused almost exclusively on the history of ideas and theories. Experiments and instruments, to the extent that they were of interest to anybody, were peripheral helpmates to the production of theory. I began by being interested in the way that certain kinds of instruments, or the way that instruments were used, shaped the way knowledge worked and the kinds of questions that people were asking. My first book, How Experiments End, was about how experimentalists decide that they’re looking at something real, whether it’s using a small scale table-top device or a huge experiment involving hundreds of people. 

Then I turned to another subculture of physics, if you will, a sub-culture of people who are really interested in the machines themselves, not just in experimentation. I wanted to know how certain kinds of devices have carried a philosophy with them. For example, how did machines like cloud chambers and bubble chambers, which produce pictures, become the standard of evidence for a whole group of physicists across most of the 20th century? Or how did funny little objects like Geiger counters, which click when they’re near something radioactive, produce a kind of statistical argument for new effects? What interested me was the contrast between the tradition of scientists who wanted to take pictures — who wanted to see in order to know — and another computing group who wanted to combine information more quantitatively — digitally, if you will — to produce a logic of demonstration. My second book,Image and Logic is about these two huge, long-standing traditions within modern physics.

More recently I’ve been looking at what I consider to be the third sub-culture of physics: the theorists. I want to get at how theorists in the production of the most abstract ideas of physics, whether it’s quantum field theory, relativity theory, or any other branch of theory, come to their concerns in relationship to very specific kinds of machines and devices in the world. Specifically, inEinstein’s Clocks, Poincaré's Maps I pursue the vast concern about simultaneity in the late 19th century — what time was, and what clocks were. This had a crucial dimension that was abstract -and philosophical, but it also sprang from purely technological concerns. How, for example, do you make maps or send signals across undersea cables? How do you coordinate and shunt trains so they don’t smash into each other while going in opposite directions on the same track? Finally, my interest in theorists led me to look at the physics concerning the most pressing problem of the late 19th century, which was how electricity and magnetism work when an object moves through that all-pervasive entity people called “the ether.”

My interest in the materiality of science goes back to my childhood. My great-grandfather, who lived until his mid-90s, trained in Berlin and worked in Thomas Edison’s laboratory as an electrical engineer, and I spent a great amount of time with him in his basement laboratory. I was completely riveted by what he did. It was the kind of laboratory that you could imagine in a film about Dr. Frankenstein, with giant double throw switches, arcs of electricity in the dark space, and bottles of mercury lining the shelves. I loved every bit of it. I left high school when I was 17 to study physics and mathematics at the Ecole Polytechnique in Paris for a year. I had a chance to learn from one of the great mathematicians, Laurent Schwartz. I’d been to France a fair amount, spoke French, and wanted to go there because I was very interested in European politics—these were wild times politically—towards the end of the Vietnam War. I thought that the only chance I had of working in an interesting place would involve pursuing something in physics, so I wrote to various physics laboratories, and they must have taken me out of amusement at the idea of this American 17-year-old writing to the Polytechnique.

When I began I was interested in philosophical questions, and thought that studying physics was a way to get at some of these problems. I worked in a laboratory on plasma physics, which is now done in gigantic machines in huge laboratories, although at the time it was still possible to do small-scale experiments on devices not much bigger than a table. I became quite fascinated with the machinery, the signal generators, recording devides, oscilloscopes, and how theoretical knowledge about the world could come out of such material objects. In college at Harvard I found a way, having done a fair amount of physics, to combine it with history and philosophy.

This brings me back to Einstein.

The Einstein we know today is mostly based on Einstein’s later years, when he prided himself on his alienation from practically everything sociable and human, projecting an image of himself as a distracted, other-worldly character. We remember that Einstein who said that the best thing for a theoretical physicist would be to tend a lighthouse in quiet isolation from the world in order to be able to think pure thoughts. We have this picture of the theoretical physicist, and project it backwards to Einstein’s miraculous year, 1905. It is easy enough to think of him as working a day job in a patent office merely to keep body and soul together, while in actuality his real work was purely cerebral. Such a split existence never made sense to me; I wondered how his work in the details of machines and objects might connect to these abstract ideas, and began thinking about how relativity itself might have been lodged in the time, place, and machinery in which it was created.

Years later—one day in the summer of 1997—I was in a train station in northern Europe, looking down the platforms at these beautifully arranged clocks. The minute hands were all the same. I thought, “God, they made these extraordinary clocks back then. What an extraordinarily wonderful piece of machinery!” But I then noticed that the second hands were also all clicking along in sync. That meant the clocks were too good. So I thought that maybe they’re not good clocks — maybe they’re synchronized clocks bound together by electrical signals that advanced them together, in lockstep. Maybe Einstein had seen such clocks when he was writing his paper on relativity. 

When I came back to the United States I started poking around old Swiss, British, German, and American patents and industrial records, and it turns out that there was an enormous industry in coordinated clocks in the late 19th century. Suddenly the famous metaphor with which Einstein begins his 1905 paper began to look not so peculiar. Einstein asks us to interrogate what we mean by simultaneity. He says, imagine a train comes into a station where you are standing. If the hour hand of your watch just touches 7:00 as the train pulls in front of your nose, then you would say that the train’s arrival and your watch showing 7:00 were simultaneous. But what does it mean to say that your clock ticks 7:00 at just the moment that a train arrives at a distant station? Einstein goes on to develop a technique for saying what it would mean to coordinate clocks, and explains that this is what simultaneity is. This quasi-operational definition of simultaneity becomes the foundation of his theory and leads to his startling conclusions that simultaneity depends on frame of reference, that therefore length measurements are different in different frames of reference, and to all of the other famous and amazing results of relativity theory. Suddenly I could see that Einstein’s seemingly abstract metaphor about trains and stations was actually both entirely metaphorical and yet altogether literal. Far from being the only person worried about the meaning of simultaneity—a lighthouse keeper in splendid isolation--there was a vast industry of people worrying about what it meant to say that a train was arriving at a distant train station. And they were determining simultaneity by sending electrical signals down telegraph lines to distant stations in ways very much like the way Einstein was describing in that fateful paper.

So I began to look further, wondering who else would have been worrying about simultaneity in the late 19th century. It turns out that the great French philosopher, mathematician, and physicist, Henri Poincaré, had much the same idea as Einstein. He also wanted to criticize the idea of absolute simultaneity and to make it something that could be measured. Instead of trains and stations, Poincaré chose for his key metaphor the exchange of a telegrapher’s signal down a line. In his famous philosophical article of January 1898, Poincaré says that simultaneity is really just the exchange of signals, like two telegraphers trying to determine how much longitudinal difference there is between them. You see, if the earth were stationary, we could find our longitude simply by looking up to see which stars were straight above us. But the earth turns, so to compare two longitudes, that is the stars above two different sites, you have to make the measurement at the same time. Consequently, for centuries map-makers have worried about simultaneity and how to determine it. By the late 19th century people were exchanging electrical time signals across the oceans via undersea cables, and what is interesting is that Poincaré was right in the middle of it — in 1899 he was elected president of the Bureau of Longitude in Paris. Then, in December 1900, he brought his new definition of time from philosophy and technology into the heartland of physics. He showed that if the telegraphers coordinated their clocks when moving through the ether, their clocks would “appear” to be simultaneous even though from the “true” ether-rest system they were not. But now the new definition of simultaneity stood central for Poincaré in all three domains: philosophy, technology, and physics.

Though Poincaré was as famous as any mathematician or philosopher of his time, he was also a man of enormous engineering skills, trained as a sophisticated engineer at the Polytechnique and Ecole des Mines in Paris and later becoming one of Polytechnique’s most illustrious professors. It is Poincaré’s situatedness that intrigues me: like Einstein, when Poincaré invoked the longitude-finding telegraphers, he was speaking both metaphorically and literally. He was changing a central concept for all physics and at the same time addressing the real practices of map-makers.

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.

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 placebecause 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.

The contrast between Einstein and Poincaré, and their different understandings of what they were doing represent two grand competing visions of modern science for the 20th century. Although the equations that Poincaré and Einstein come up with around relativity theory are very similar — essentially identical — Poincaré always thought of what he was doing as fixing, repairing, or continuing the past by applying reason to it by. As one of his relatives once put it, he was filling in the blank spots on the map of the world. Einstein was willing to do things rather differently, to say that the old way of proceeding is too complicated, too filled with piecemeal solutions, and that what we need is something that starts over again with the classical purity of a few stark principles. Poincaré saw himself in some ways as saving an empire—the empire of France, no doubt, but also the empire of nineteenth century physics. His was a grand ambition, but it’s a different kind of modernism from Einstein’s. It’s a reparative, ameliorative modernism, a modernism with all the rational hopefulness of a third Republic Frenchman. Einstein’s is a much more disruptive, classifying, purifying modernism. It is only by understanding this triple intersection of philosophy, physics, and technology that one can really grasp what each of these alternative visions of the new century is about.

You might ask, and I’ve often wondered, how to think about this kind of event in the present. That is to say, is there an analogy now to this kind of triple intersection? Here is how I think about it: When you consider Poincaré and Einstein you’re dealing with an attempt to understand time coordination and the synchronization of clocks at a huge variety of scales. In some ways they’re trying to figure out how to coordinate clocks inside a single room or observatory, or a block, or a whole city, at the same time that the people who are worried about these things are also sending cables across the Pacific and Atlantic Oceans. Einstein and Poincaré are not just worrying about such planetary scales, but also about how to coordinate clocks in different reference frames in the universe as a whole. They are asking, what does synchronization mean? What does simultaneity mean? These are questions that occur at everyscale, from the smallest to the largest, from philosophy and physics all the way down to electrical wiring along train tracks. In that sense it is unlike most questions that we ask in science, since it doesn’t have the character of starting out as something purely abstract that then becomes applied physics and engineering, eventually ending up on the factory floor. It’s also not a platonic ascension, or a naive version of Marxism, in which machines and machine shop relations are slowly abstracted to ever wider spheres until they become a theory of the universe. 

Questions of the conventionality of time, of how it becomes equated with physical processes and procedures are key to all of the domains considered. And the metaphor we need for this back-and-forth between the practical and the philosophical not just one of condensation from the abstract to the concrete. Nor is it one of evaporation, in which water grows less dense as it passes into a vapor. Instead, more helpful is that phenomenon physicists call critical opalescence. Ordinary opalescence is that oyster-shell color in which you see all colors reflected, that remarkable surface of a pearl or the inner surface of certain shells in which you can see red, green and white, all at once. Critical opalescence in matter occurs under very particular circumstances—for example in a system of water and vapor held under just the right combination of temperature. At this critical point something quite extraordinary happens. The liquid starts to evaporate and condense at every scale, from a couple of molecules to a whole system. Suddenly, because droplets form of every size—from couple of molecules coming together to the whole of the system—light of every wavelength reflects back. If you shine in blue you see blue, if you shine in red you see red, and if you shine in yellow you see yellow.

That’s the kind of metaphor that we need to look at a situation like this. Poincaré and Einstein are flipping back and forth between philosophical questions, physics questions, and practical questions. At the end of the 1890s Poincaré was publishing in journals for map makers and longitude finders at the same time he was publishing in physics journals and in the Journal of Metaphysics and Morals. In his thinking he was and flipping back and forth extraordinarily quickly between these three domains of philosophy, physics, and technology. 

Now one can ask how this might compare to the present. What kind of critical opalescence marks the science of recent times? It seems to me fairly rare, but one place you might see it is in the collection of sciences that have grown up around computation. Here, ideas about the mind, about how computers function, and about science, codes, and mathematical physics all come together. Von Neumann thinks about the mind and its organs (memory, input-output, processing) as a way of designing a programmed computer. The programmed computer then becomes a model for the mind. The ideas of information, which are encoded into the development of computation, also become ways to understand language and communication more generally, and again feed back into devices. Information, entropy, and computation become metaphors for us at a much broader level. Such opalescent moments are not that common, surely rarer than whatever it is that we mean by scientific revolutions. They’re something else. No, points of critical opalescence in this sense point to science in times and places where we’re starting to think with and through machines at radically different scales—Where we are flipping back and forth between abstraction and concreteness so intensively that they illuminate each other in fundamentally novel ways, in ways not captured by models of simple evaporation or condensation. When we see such opalescence, we should dig into them, and deeply, for they are transformative moments of our cultures.