ISAACSON: To get it back to the history: In the period from 1900 to 1915, which is to me—maybe because I’m a bit prejudiced by having worked on Einstein so much—a period of great explosive creativity. It’s a period in which both quantum theory and relativity theory were developed. Much of that development was driven by philosophy and philosophers. If you ask Einstein who his most important influences were, he would get to Michelson and Morley at about number 3,500, if at all. Every now and then he would say, “Yes, I don’t know if I ever read them. But Ernst Mach and David Hume—those are the people we were discussing all the time, and those are the people who led us to make the creative leaps we had to make in that period.”
It’s an interesting question to ask why it is that between 1900 and 1915 we have such creative leaps. Obviously in science, but even Stravinsky and Schoenberg saying, “OK, we don’t have to stick to the classical bonds,” or Proust and Joyce, or Picasso and Kandinsky—breaking the classical bonds. But especially in science, the leaps seemed to have been pushed by people like Ernst Mach, who are almost more philosophers than they are physicists.
STEINHARDT: My impression is that this began to break down with the developments of quantum mechanics in the 1920s, where after a certain effort to struggle with the interpretation of it, there did come this attitude that said, “OK, let’s stop worrying about its interpretation; what we know we can do with it is to calculate and make predictions of new phenomena.” And there were so many new phenomena to examine that it occupied generations of theorists to pursue this line of attack, ignoring interpretation and philosophy and just going forward in a straight line with these calculations. Once that historical connection with philosophy was broken, it became disparaged. It was considered that philosophy might even distract you from discovering something interesting. But now it might be that it’s time to return to it.
You see, great progress had been occurring, because physicists were asking questions that could be tested almost immediately—the rapid interplay between experiment and theory was going back and forth for nearly a century. Every time a new observation would come in, or a new experiment would be performed, there would be a new question; you could do a new calculation; and someone else might do the next experiment. New physics was flowing from theory to experiment and back again very rapidly. But now we’ve reached the stage where the time between major experimental breakthroughs in fundamental physics is very long—decades, in the case of particle physics. We don’t have experimental guidance, and we don’t have the philosophical underpinnings, either. Maybe we don’t just need new experiments. Maybe we need to look back to philosophy for guidance.
GREENE: In the PBS special on string theory that aired some time ago, there were a number of people who were interviewed about the fact that string theory had not yet made predictions that could be tested. And the framing of that fact by a number of physicists interviewed was, if string theory can’t make, or doesn’t make, those kinds of predictions, it becomes philosophy, not physics.
As I watched the series, I kept saying to myself, “the poor philosophers.” Philosophy is not bad physics, it’s not physics that hasn’t reached its goal. It’s just a way of analyzing pathways toward truth that perhaps don’t use as much mathematics as the physicists and mathematicians typically do. There’s a lot of insight yet to be tapped from the philosophical community, and I imagine we’ll go through a cycle where that kind of interaction happens more and more.
STEINHARDT: Yes. In fact one of the interesting turns of events in string theory we’ve been talking about is the idea that there’s a multiplicity of possibilities, and one of the approaches for dealing with it is the anthropic reasoning—to use the fact that we exist as a kind of selection principle. That turns out to be territory that philosophers have thought about quite a bit. They’re far ahead of the physicists in terms of realizing the flaws and the trapdoors.
GREENE: I agree with you completely. Could we just come back to the assessment you gave of string theory a little while ago in terms of having crashed? That seems to me a pretty strong negative assessment, and I wonder if I’m hearing you fully, or if it’s more nuanced.
When one looks at the history of string theory, the achievements have been manifold, as you are familiar with—the insights on spacetime singularities, mirror symmetry, topology change, the ability to understand certain symmetry structures, the ability to give insight into the possibility of having a generation structure in the families of matter particles, the insights that it’s given on gauge theories as a general structure and in particular being able to realize gauge theories that we’re familiar with—and all of these features, on top of its putting together general relativity and quantum mechanics. Now, I agree—and I’m actually all too happy to say—that we have a ways to go, because we’ve not made that direct contact with experimental observation. But to me, we have a road ahead of us that we still need to travel. Whether it will ultimately take us to those predictions or not, the future will tell. I don’t think we can judge now and say the program has crashed. I can say the program has gone spectacularly far but we definitely have further to go until we know whether what we’re doing is right or wrong. Is that not a good assessment in your view?
STEINHARDT: My view is more nuanced. This multiplicity, if that were to be the endpoint of the theory, is a crash. There’s one of several possibilities—
GREENE: The only reason I interrupt you is because I’ve heard a number of people take a similar perspective, which is to listen to a couple of string theorists who are pushing one particular point of view: that maybe this is the endpoint of string theory—that there are many many universes, we’re one of that many and there’s no further explanation to be had. It may be right, it may be wrong, but I certainly don’t at this point say that that’s the endpoint of string theory. That’s a way-station that some people are exploring, and others are pushing on other pathways.
STEINHARDT: You have a more reasonable attitude on this than others I’ve heard. I’d like it to be firmly recognized that googol possibilities is a crash, and that it’s not an acceptable—
GREENE: But the many-universe version of string theory—is that what you mean?
STEINHARDT: Yes. But let me hasten to add that I can envision several ways to escape from this crash. One is to discover some new ideas in string theory showing that the multiplicity isn’t really predicted by string theory. After all, the mathematical case is not firm. Or, second, even if there’s a multiplicity, perhaps one can find some reason why almost everywhere in the universe should correspond to just one of these possibilities—namely, the one we actually observe. Or maybe string theory in its present form is fine but you have to change the cosmology, and that change removes the multiplicity. All these rescues are conceivable to me.
What I can’t accept is the current view, which simply accepts the multiplicity. Not only is it a crash but it’s a particularly nefarious kind of crash, because if you accept the idea of having a theory which allows an infinite number of possibilities, of which our observable universe is one, then there’s really no way within science of disproving this idea. Whether a new observation or experiment comes out one way or the other, you can always claim afterward that we happen to live in a sector of the universe where that’s so. In fact, this reasoning has already been applied recently, as theorists tried to explain the unexpected discovery of dark energy. The problem is that you can never disprove such a theory—nor can you prove it.
GREENE: You can imagine there are features that are consistent across all of these universes—
STEINHARDT: You could imagine it, but you could never prove it, experimentally.
GREENE: No, my point is, mathematically you could find that in each of these universes, property X always holds.
STEINHARDT: Do you mean, as derived from string theory? I don’t believe that’s true. I don’t believe it’s possible.
GREENE: Right, well, that’s a belief—it’s not based on any cal
culation.
STEINHARDT: Well, I believe that if you came to me with such a theory, I could probably turn around within twenty-four hours and come up with an alternative theory in which property X wasn’t universal after all. In fact, you almost know that’s true from the conversation that’s been happening in the field already, where someone says, “These properties are universal and these others are not.” The next day, another theorist will write a paper saying, no, other properties are universal. There are simply no strong guidelines for deciding.
GREENE: I agree that that’s definitely been the way things have unfolded. But I thought I heard you say that you couldn’t imagine being able to disprove a theory that had this kind of framework, and I’m just setting up a way in which one could disprove it.
STEINHARDT: That may be true in principle, but in practice I don’t think this would ever occur. If a version of string theory with a googolfold multiplicity of physical laws were to be disproved one day, I don’t think proponents would give up on string theory. I suspect a clever theorist would come up with a variation that would evade the conflict. In fact, this has already been our experience with multiverse theories to date. In practice, there are never enough experiments or observations, or enough mathematical constraints, to rule out a multiverse of possibilities. By the same token, this means that there are no firm predictions that can definitively decide whether this multiplicity beyond our horizon is true or not.
GREENE: I agree with you. But just so I understand; you’re saying that this one particular way in which one may think about string theory—for which the endpoint is many many universes—is unacceptable.
STEINHARDT: Right. I claim it needs to be fixed.
GREENE: But you also agree—just so it’s clear—that that’s not a crash in string theory per se; that’s a particular way of approaching the theory that you would not advocate because the endpoint would be unacceptable. You need to go further—
STEINHARDT: That’s right. So it’s just what you were saying: Some people say that’s the endpoint, and I’m saying that’s not acceptable. If you believe that, it’s time to abandon it.
GREENE: But it’s those people who’ve crashed.
STEINHARDT: Yes, it’s that point of view which is a crash and needs a fix. I’m not arguing that string theory should be abandoned. I think it holds too much promise. I’m arguing that it’s in trouble and needs new ideas to save it.
But let’s get back to Einstein. One interesting question to consider about Einstein is how his generation of physicists were radicals and were replaced by a generation of physicists that would be considered conservative.
ISAACSON: What’s particularly interesting to me is that Einstein was a radical who in 1925 becomes replaced with Einstein who’s a conservative. That’s overstating it a bit, but right as he makes his last contribution of greatness to quantum theory—basically the whole Bose-Einstein statistics—he almost instantaneously is spinning around into a defensive crouch and resisting everything from the lack of realism to the lack of rigid causality in quantum mechanics, and he’s calling them the young Turks at the Solvay conference 1927-1931, and they’re calling him ridiculously conservative and saying he abandoned his radicalism, when he used to challenge everything.
It’s a theme that goes well beyond physics, which is, Why is it that you used to think of yourself as a radical and then you become age fifty, and whether you’re editing Time magazine or doing theoretical physics, you start saying things like, “No, we can’t do that, we’ve tried that before and it doesn’t work.”
If I were to give a real reason for Einstein’s basic conservatism, it would go back to what Paul was talking about, which is the philosophical, which is just that the concept of realism is so at the core. There are three or four reasons he doesn’t like quantum mechanics, and if you had to pick one, it’s not probabilities or the end of strict causality, even though he says strict causality is the greatest enduring gift that Newton gave us. It’s the abandonment of realism, and to him that becomes a pillar of classical physics. If you have to define conservatism, I assume the definition would be defending the classical order as opposed to radically throwing out the old order. That’s what he quits doing in 1925.
15
Einstein and Poincaré
Peter Galison
Joseph Pellegrino University Professor, director Collection of Historical Scientific Instruments, Harvard University; author, Einstein’s Clocks and Poincaré’s Maps
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’m 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’re about ordinary scientific practice and yet fundamentally philosophical. When I choose to work on a problem, it’s 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 a much larger domain of study. I’m 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 the practices of science, sometimes material, sometimes abstract. And I suppose I’m always interested in blasting away the mid-level generalizations, and exploring, as in Einstein’s Clocks, Poincaré’s Maps, 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 we
re 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 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 subculture 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 subculture 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, in Einstein’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.”
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