Henri Poincaré
Henri Poincaré; drawing by David Levine


Today the name of Einstein is known to almost everybody, the name of Poincaré to almost nobody. A hundred years ago the opposite was true. Then, Albert Einstein was a newly appointed technical expert, third class, examining patent applications in the Swiss patent office in Bern, having failed in his efforts to find an academic job, while Henri Poincaré was one of the leading figures of the French scientific establishment, famous not only as a great scientist but as the author of popular books that were translated into many languages and kept the public informed about the dramatic progress of science during the early years of the twentieth century. A hundred years ago, Einstein and Poincaré were both working hard at one of the central problems of science, trying to find a correct theory to describe how fast particles behave in electric and magnetic fields. Poincaré had published several papers on the subject which Einstein may or may not have read. Einstein had published nothing.

Two years later, in 1905, Poincaré and Einstein simultaneously arrived at a solution to the problem. Poincaré presented a summary of his results to the French Academy of Sciences in Paris, and in the same month Einstein mailed his classic paper, “Electrodynamics of Moving Bodies,” to the German journal Annalen der Physik. The two versions of the solution were in substance almost identical. Both were based on the principle of relativity, which says that the laws of nature are the same for a moving observer as they are for an observer standing still. Both agreed with the experimentally observed behavior of fast particles, and made the same predictions for the results of future experiments. How then did it happen that Einstein became world-famous as the discoverer of relativity, while Poincaré did not? Poincaré’s lasting fame, such as it is, derives from his discoveries in other areas of science and not from his work on relativity. Is the verdict of posterity, giving all the credit for relativity to Einstein and none to Poincaré, fair or unfair? I will return to these questions later.

Peter Galison is a historian and not a judge. His purpose is to understand the way in which Poincaré and Einstein arrived at their insights, not to hand out praise or blame. His book is an extended double portrait, describing their lives and times in detail. At the beginning, he complains of the unequal treatment given to them by biographers: “There are, to be sure, too many biographies of Einstein and not enough of Poincaré.” Poincaré was a great man who lived a full and many-sided life, and he deserves at least a fraction of the attention that has been lavished on Einstein. For readers who may be interested in learning more about Poincaré, I recommend a short biography by Benjamin Yandell that Galison does not mention. Yandell’s book, The Honors Class: Hilbert’s Problems and Their Solvers,* is a collection of biographies of mathematicians who solved a famous list of twenty-three problems propounded by David Hilbert at the International Congress of Mathematicians in Paris in 1900. Poincaré solved problem number 22. The Poincaré biography in Yandell’s book is one of the best. It gives us in thirty pages a vivid picture of Poincaré’s life as a mathematician, and overlaps very little with Galison’s account.

Galison’s book does not contain a single equation. Anybody with an interest in history can read it. The stories that it tells are mostly about the applications of science and not about science itself. The applications are things that everybody can understand, maps in the case of Poincaré, clocks in the case of Einstein. Poincaré and Einstein were both deeply involved in the practical world of electrical communication and machines at the same time as they were working out the theory of relativity. The scientific theories that emerged from their practical concerns are described briefly, without any mathematical or technical jargon. The book consists of six chapters, four long chapters in the middle and two short chapters at the beginning and the end. The two short chapters set the stage and summarize the conclusions. The little that is said about the details of the theory of relativity is mostly contained in the two short chapters. The main conclusion is that, in the events that led to the discovery of the theory, philosophical speculation and technological invention were inextricably mixed.

The four long chapters consist of a collection of stories, describing how electricity transformed the world in the second half of the nineteenth century. Here is a typical story. A political battle was fought in Hartford, Connecticut, in 1880, to decide whether the trains in Connecticut should run on Boston time or New York time. The battle was fought between the Harvard astronomer Leonard Waldo and the New York to Hartford railroad. Waldo was not only an astronomer but also an entrepreneur. His observatory ran a business, selling accurate time signals that were distributed to customers by electric telegraph. The customers were railroads and city fire departments, manufacturers and retailers of clocks and watches, and private citizens who possessed fine watches and wished to check their accuracy. Waldo made a big pitch to the Hartford city council, emphasizing the superior precision of Boston time. But the New York to Hartford railroad ran on New York time and would not be moved. The railroad won the battle.


Another story tells how the United States led the world in adopting a unified system of time zones in 1883, with the times in neighboring zones differing by exactly one hour. This was another victory for the railroads. The crucial convention to decide the fate of the time-zone system was held in St. Louis, Missouri. The voting was measured not by the number of delegates voting but by the miles of railroad track that they represented. The final vote was 79,041 miles of track in favor and 1,714 miles opposed. After that, cities were compelled to set their clocks by railroad time and not by local time. Even New York City gave up its local time and set its clocks to astronomical time at longitude seventy-five west.

A third story tells how the French Bureau of Longitude established a commission in the year 1897 to extend the metric system to the measurement of time. The idea was to abolish the antiquated division of the day into hours, minutes, and seconds, and replace it by a division into tenths, thousandths, and hundred thousandths of a day. The new second would be a hundred thousandth of a day, and the new hour would be ten kiloseconds. The new units of time could then be handled as conveniently as grams and kilograms, meters and kilometers. To convert time from days to hours or minutes or seconds, we would only need to move a decimal point. We would no longer need to struggle with multiplications and divisions by twenty-four and sixty. This was a revival of a dream that was in the minds of the creators of the metric system at the time of the French Revolution a hundred years earlier. Some members of the Bureau of Longitude commission introduced a compromise proposal, retaining the old-fashioned hour as the basic unit of time and dividing it into hundredths and ten thousandths. Poincaré served as secretary of the commission and took its work very seriously, writing several of its reports. He was a fervent believer in a universal metric system. But he lost the battle. The rest of the world outside France gave no support to the commission’s proposals, and the French government was not prepared to go it alone. After three years of hard work, the commission was dissolved in 1900.

These stories, and many others, are told to illustrate Galison’s thesis that the coordination of time signals was a central concern of people and governments in the later part of the nineteenth century. So it was no accident that the coordination of time signals also has a central role in the theory of relativity. Poincaré and Einstein lived in a period of history when the transmission of time signals was a growth industry, and both of them were professionally involved in it. Poincaré worked for the Bureau of Longitude, which was responsible for the mapping of French territories around the globe. To make accurate maps, the bureau needed to determine accurate longitudes. To determine longitude at a remote place such as Dakar or Haiphong, it was necessary to compare the local time, obtained from local astronomical measurements, with Paris time, obtained by receiving an accurate time signal from Paris. The accuracy of maps therefore depended on the accuracy of long-distance transmission of time signals. The transmission of time signals, first by overland telegraph lines and undersea cables, and later by radio, was a difficult technical problem. Signals were attenuated by transmission losses and corrupted by ambient noise. The transmission introduced delays which had to be accurately calculated, so that Paris time could be correctly deduced from the observed time of reception of signals. The recording apparatus introduced other delays which had to be measured and compensated. The transmission of time signals with high accuracy required a mastery of both theory and practical engineering.

Poincaré was well versed in practice as well as theory. He started his professional career as a mining engineer, inspecting mines in the coal fields of northern France. One of his first jobs was to investigate a disastrous explosion that killed eighteen miners. He descended into the mine looking for clues while the corpses were still warm. He found a miner’s lamp which had a rectangular hole in its wire mesh, apparently caused by a blow from a pickax. The wire mesh, a device invented by Humphrey Davy sixty-five years earlier, prevents the flame inside the lamp from igniting explosive gases in the mine outside the lamp. The mesh allows air to pass through but stops flame from propagating from inside to outside. When the mesh was broken, the flame could propagate freely and all hell broke loose. Poincaré risked his life to discover what had happened, and wrote a report analyzing in detail the flow of gases in the mine.


Einstein grew up in a family of electrical engineers. His father and uncle ran a business in Munich, manufacturing and selling electrical measuring equipment. One of his uncle Jakob’s patents dealt with equipment for the electrical control of clocks. Einstein’s early familiarity with electrical machinery helped him to get his job at the Swiss patent office, and helped him to do the job well. As soon as he started work, he was confronted with numerous applications for patents concerned with electric clocks and with their coordination by distribution of electric time signals. In the year 1904, when the theory of relativity was in process of gestation, fourteen such patents were approved by the Bern office. The number of applications that were disapproved is not recorded.

At that time, Switzerland was becoming a world leader in the manufacture of precision clocks, and applications for Swiss patents were pouring in from hopeful inventors all over the world. For Einstein, analyzing and understanding these inventions was not just a convenient way to pay the rent. He enjoyed the work at the patent office and found it intellectually challenging. Later in his life, he remarked that the formulation of technological patents had been an important stimulus to his thinking about physics.


Among historians of science during the last half-century, there have been two predominant schools of thought. The leaders of the two schools were Thomas Kuhn and Peter Galison. Kuhn, in his classic work The Structure of Scientific Revolutions, published in 1962, portrays the progress of science as a kind of punctuated equilibrium, like the evolution of species in the history of life. Most of the time, evolution is slow or stagnant, the species are well adapted to their environments, and natural selection keeps them from changing fast. Then, when the environment is disturbed and new ecological niches are opened, selection favors rapid change, and small populations of lucky individuals change rapidly enough to form new species. So in science, the normal state of affairs is a slowly changing equilibrium, with a dominant orthodox theory that explains observed phenomena and is not seriously questioned. So long as normal science prevails, the job of the scientist is to solve unimportant puzzles that arise within the accepted dogma. But at rare moments, new discoveries or new ideas arise that call the accepted dogma into question, and then a scientific revolution may occur. To cause a scientific revolution, the new discoveries must be powerful enough to overthrow the prevailing theory, and a new set of ideas must be ready to replace it. In Kuhn’s view, it is new ideas that drive scientific revolutions. The big steps forward in the progress of science are idea-driven.

In contrast to Kuhn, Galison in his classic work, Image and Logic, published in 1997, describes the history of particle physics as a history of tools rather than ideas. According to Image and Logic, the progress of science is tool-driven. The tools of particle physics are of two kinds, optical and electronic. The optical tools are devices such as cloud chambers, bubble chambers, and photographic emulsions, which display particle interactions visually by means of images. The images record the tracks of particles. An experienced experimenter can see at once from the image when a particle is doing something unexpected. Optical tools are more likely to lead to discoveries that are qualitatively new.

On the other hand, electronic tools are better for answering quantitative questions. Electronic detectors such as the Geiger counters that measure radioactivity in the cellars of old houses are based on logic. They are programmed to ask simple questions each time they detect a particle, and to record whether the answers to the questions are yes or no. They can detect particle collisions at rates of millions per second, sort them into yeses and noes, and count the number that answered yes and the number that answered no. The history of particle physics may be divided into two periods, the earlier period ending about 1980 when optical detectors and images were dominant, and the later period when electronic detectors and logic were dominant. Before the transition, science advanced by making qualitative discoveries of new particles and new relationships between particles. After the transition, with the zoo of known particles more or less complete, the science advanced by measuring their interactions with greater and greater precision. In both periods, before and after the transition, tools were the driving force of progress.

The arguments between Kuhnian historians emphasizing ideas and Galisonian historians emphasizing tools have continued to be vigorous. Historians trained in theoretical science tend to be Kuhnians, while those trained in experimental science tend to be Galisonians. Whether one chooses to emphasize ideas or tools is to some extent a matter of taste. I myself tend to be a Galisonian, although I am a theoretician by training. But in this debate, as often happens when academic scholars engage in disputes, the disciples of each leader are more dogmatic than the leaders. I once attended a meeting of historians at which the disciples of Kuhn were presenting an extreme and exaggerated version of his views. Kuhn interrupted them by shouting from the back of the hall with overwhelming volume, “One thing you people need to understand: I am not a Kuhnian.”

Kuhn believed in the primacy of ideas, but not to the exclusion of everything else. And in his new book, Galison is telling us that he still believes in the primacy of tools, but not to the exclusion of everything else. As I came to the final chapter of the book, I could almost hear him shouting, “One thing you people need to understand: I am not a Galisonian.”

Galison uses the phrase “critical opalescence” to sum up the story of what happened in 1905 when relativity was discovered. Critical opalescence is a strikingly beautiful effect that is seen when water is heated to a temperature of 374 degrees Celsius under high pressure. 374 degrees is called the critical temperature of water. It is the temperature at which water turns continuously into steam without boiling. At the critical temperature and pressure, water and steam are indistinguishable. They are a single fluid, unable to make up its mind whether to be a gas or a liquid. In that critical state, the fluid is continually fluctuating between gas and liquid, and the fluctuations are seen visually as a multicolored sparkling. The sparkling is called opalescence because it is also seen in opal jewels which have a similar multicolored radiance.

Galison uses critical opalescence as a metaphor for the merging of technology, science, and philosophy that happened in the minds of Poincaré and Einstein in the spring of 1905. Poincaré and Einstein were immersed in the technical tools of time signaling, but the tools by themselves did not lead them to their discoveries. They were immersed in the mathematical ideas of electrodynamics, but the ideas by themselves did not lead them to their discoveries. They were also immersed in the philosophy of space and time. Poincaré had written a philosophical book, Science and Hypothesis, which Einstein studied, digging deep into the foundations of knowledge and criticizing the Newtonian notions of absolute space and time. But the philosophy by itself did not lead them to their discoveries. What was needed to give birth to the theory of relativity was a critical moment, when tools, ideas, and philosophical reflections jostled together and merged into a new way of thinking. Galison would like to put an end to the argument between Kuhnians and Galisonians. In this book he takes his position squarely in the middle: “Attending to moments of critical opalescence offers a way out of this endless oscillation between thinking of history as ultimately about ideas or fundamentally about material objects.”

The one question that Galison’s metaphor of critical opalescence does not answer is why Einstein discovered the theory of relativity as we know it and Poincaré did not. The theories discovered by Poincaré and Einstein were operationally equivalent, with identical experimental consequences, but there was one crucial difference. The difference was the use of the word “ether.” The wave theory of light, and the theories of electric and magnetic forces that were developed in the nineteenth century, were all based on the idea of ether. James Clerk Maxwell, who unified the theories of light and electromagnetism in 1865, was a firm believer in ether. Electric and magnetic forces behaved like mechanical stresses in a solid medium with suitable properties of rigidity and elasticity. Therefore, it was believed, a solid medium must exist, pervading the whole of space and carrying the electric and magnetic stresses. Light waves must be shear waves in the same elastic medium. The all-pervading solid medium was given the name “ether.”

The essential difference between Poincaré and Einstein was that Poincaré was by temperament conservative and Einstein was by temperament revolutionary. When Poincaré looked for a new theory of electromagnetism, he tried to preserve as much as he could of the old. He loved the ether and continued to believe in it, even when his own theory showed that it was unobservable. His version of relativity theory was a patchwork quilt. The new idea of local time, depending on the motion of the observer, was patched onto the old framework of absolute space and time defined by a rigid and immovable ether. Einstein, on the other hand, saw the old framework as cumbersome and unnecessary and was delighted to be rid of it. His version of the theory was simpler and more elegant. There was no absolute space and time and there was no ether. All the complicated explanations of electric and magnetic forces as elastic stresses in the ether could be swept into the dustbin of history, together with the famous old professors who still believed in them. All local times were equally valid. In order to calculate with Einstein’s version of relativity, all you needed to know was the rule for transforming from one local time to another. In the competition for public recognition, the clarity and simplicity of Einstein’s argument gave him an overwhelming advantage.

Poincaré and Einstein only met once, at a conference in Brussels in 1911. The meeting did not go well. Einstein afterward reported his impression of Poincaré: “Poincaré was simply negative in general, and, all his acumen notwithstanding, he showed little grasp of the situation.” So far as Einstein was concerned, Poincaré belonged with the ether in the dustbin of history. But Einstein underestimated Poincaré. Einstein did not know that Poincaré had just then written a letter recommending him for a professorship at the Swiss Federal Institute of Technology in Zürich. Here is what Poincaré had to say about Einstein:

What we must above all admire in him, is the facility with which he has adapted to new conceptions and from which he knows how to draw the consequences. He does not remain attached to classical principles, and, in the presence of a problem of physics, is prompt to envision all the possibilities…. The future will show more and more the value of Mr. Einstein, and the university that finds a way to secure this young master is assured of drawing from it great honor.

Poincaré bore no grudge against his young rival. He was still driven by the same generous impulse that made him rush into the coal mine at Magny thirty-two years earlier. A year after the meeting with Einstein in Brussels, Poincaré was dead. Einstein never saw Poincaré’s letter and never knew that he had misjudged him.

Looking back upon this history, I disagree with Galison’s conclusion. I do not see critical opalescence as a decisive factor in Einstein’s victory. I see Poincaré and Einstein equal in their grasp of contemporary technology, equal in their love of philosophical speculation, unequal only in their receptiveness to new ideas. Ideas were the decisive factor. Einstein made the big jump into the world of relativity because he was eager to throw out old ideas and bring in new ones. Poincaré hesitated on the brink and never made the big jump. In this instance at least, Kuhn was right. The scientific revolution of 1905 was driven by ideas and not by tools.

This Issue

November 6, 2003