Brian Greene on the Nature of Science

From a reader

The Hidden Realty: Parallel Universes and the Deep Laws of the Cosmos is Brian Greene's third book. Greene has become our most important popularizer of contemporary physics. He takes cutting edge questions that are usually understood only by trained scientists and makes them accessible to a much broader audience. His first book introduced string theory to a popular audience and his second book took on the nature of space. This third book is about the idea of multiple universes, an idea that has gotten a lot of media play and has been the subject of a NOVA special. But the book is more than that. In it, Greene devotes a whole chapter to the question of "what is science." Greene writes from a very nuanced perspective, and gets into some big questions of epistemology and the scientific method.

The book is structured around the various ways in which the idea that there could be more than one universe comes up in modern physics and he uses the generic term "multiverse" to refer to any concept of multiple universes.¹ He explores nine different lines of theoretical inquiry that point to the possible existence of multiple universes. But he stops along the way to ask a very important question: If alternate or additional universes exist beyond our own, is there any way to establish even the plausibility of their actual existence? And if not and additional universes are completely inaccessible to us, is this really science?

Greene addresses this question in a chapter entitled "Science and the Multiverse: On Inference, Explanation, and Prediction." In a footnote at the beginning of this chapter he lays out his philosophical approach to science:

"Because there are differing perspectives regarding the role of scientific theory in the quest to understand nature, the points I'm making are subject to a range of interpretations. Two prominent positions are realists [also known as materialists—ed.], who hold that mathematical theories can provide direct insight into the nature of reality, and instrumentalists [also known as positivists—ed.], who believe that theory provides a means for predicting what our measuring devices should register but tells us nothing about an underlying reality. Over decades of exacting argument, philosophers of science have developed numerous refinements of these and related positions. As no doubt is clear, my perspective, and the approach I take in this book, is decidedly in the realist camp. This chapter in particular, examining the scientific validity of certain types of theories, and assessing what those theories might imply for the nature of reality, is one in which various philosophical orientations would approach the topic with considerable differences."

The first section of this important chapter is entitled "The Soul of Science." In it he addresses the question of whether the notion of a multiverse is testable or falsifiable.² Right off, he states that the answer to both these questions is "no." Here one might think "end of story." If a theory about the material world can neither be verified nor shown to be false, how could this be science?

But that's not the end of the story. Greene continues: "Where you come down on the multiverse also depends on your view of science's core mandate. General summaries often emphasize that science is about finding regularities in the workings of the universe, explaining how the regularities both illuminate and reflect underlying laws of nature, and testing the purported laws by making predictions that can be verified or refuted through further observation. Reasonable though the description may be, it glosses over the fact that the actual process of science is a much messier business, one in which asking the right questions is often as important as finding and testing the proposed answers. And the questions aren't floating in some pre-existing realm in which the role of science is to pick them off, one by one. Instead, today's questions are very often shaped by yesterday's insights. Breakthroughs generally answer some questions but then give rise to a host of others that previously could not be imagined. In judging any development, including multiverse theories, we must take account not only of its capacity for revealing hidden truths but also of its impact on the questions we are led to address. The impact, that is, on the very practice of science. As will become clear, multiverse theories have the capacity to reshape some of the deepest questions scientists have wrestled with for decades. That prospect invigorates some and infuriates others."

I think this is a very important insight: the importance to science of being led to ask the right questions, informed by "yesterday's insights," even if the theory doing the leading ultimately turns out to be wrong.

In another part of this chapter, Greene asks: can it be scientifically justifiable to invoke unobservable universes? In response to this question, he cites a series of examples where science has come to rely on features of the real world, even if they are inaccessible to our direct observation. For example, the curvature of space-time is not directly accessible to our senses, but the predictions of general relativity are quite accurate. In quantum mechanics, the wave equations that are relied upon are unobservable. There are black holes that we cannot see into. Objects that are more than 20 billion light years away cannot be seen because their light could not have reached us in the time since the big bang. But science considers all these to be just as real as things that are tangible to our senses. Thus, he concludes, "a theory's success can be used as an after-the-fact justification for its basic architecture, even when that architecture remains beyond our ability to access directly."

From this he concludes: "So for confidence in a theory to grow we don't require that all of its features be verifiable; a robust and varied assortment of confirmed predictions is enough.... In principle, then—and make no mistake, my point here is one of principle—the mere invocation of inaccessible universes does not consign a proposal to stand outside science."

Again, an important methodological point: Greene is saying, "don't be so quick to reject an idea whose components are not immediately accessible." Or, as he says in the concluding chapter: "I find it parochial to bound our thinking by the arbitrary limits imposed by where we are, when we are, and who we are. Reality transcends these limits, so it's to be expected that sooner or later the search for deep truths will too."

Thus he takes up yet another question: "If the universes constituting a multiverse are inaccessible, can they nevertheless meaningfully contribute to making predictions?" Here he points out that some multiverse theories, while not saying anything about our universe in particular, do make statistical predictions about their range of universes. Suppose a particular theory predicts the existence of certain property in all universes. Then this theory is potentially falsifiable; the property might not be found in our universe, thus falsifying the theory. And if the property is found, then it gives us some reason to believe that the theory might be true.

There is one other interesting question that he raises in this chapter: "Can a multiverse provide explanatory power of which we'd otherwise be deprived?" Here Greene points out that most physics theories give very precise verifiable predictions. If multiverse theories never provide such precise predictions, but rather tell us something like "a certain phenomenon is highly likely," is that still science? Greene says he used to think not, but has gradually changed his mind on this.

At the end of the book, Greene takes up yet another interesting question: "In the absence of compelling experimental or observational results, deciding which mathematics should be taken seriously is as much art as it is science. . . . monumental upheavals in physics have emerged time and again from vigorously following mathematics' lead." This question is quite relevant because most of the multiverse theories emerge from the mathematical implications of existing or proposed theories of matter, space and time.

It is actually true that numerous major discoveries have been made by "following the math"; looking at some seemingly impossible conclusion of existing theories. James Clerk Maxwell's equations pointed to a constant speed of light as seen by all observers, a conclusion that made no sense if light was a wave in some physical "aether." Yet it was verified in practice and finally explained by Albert Einstein. Paul Dirac's mathematical description of the electron led to the discovery of the positron because his equation worked equally well with either a positive or negative charge for the particle. Black holes were a mathematical consequence of Einstein's theory of relativity, but even he didn't believe in them, not taking his own theory seriously enough. So Greene's challenge here is to "follow the math"—but as far as the truth of any new theory is concerned, "[o]nly one standard is relevant: a proposal's ability to explain or predict experimental data and astronomical observations." It has to be testable and verifiable in the real world.

Greene concludes his book on the possibility of multiple universes as follows: "I don't know if this is how things will turn out. No one does. But it's only through fearless engagement that we can learn our own limits. It's only through the rational pursuit of theories, even those that whisk us into strange and unfamiliar domains, that we stand a chance of revealing the expanse of reality."

1. The word universe usually means "everything that exists." The idea of multiple universes is the idea that separate physical realms could exist, each with its own possibly different laws of nature. In the "multiverse" scenario, if intelligent life existed in any one of the separate universes, it would see its own universe as encompassing "everything that exists." [back]

2. To be falsifiable means that the potential exists to prove the theory to be false. The falsifiability criterion means that if a theory is really scientific, then it can be tested in reality to see if its features and predictions are true or not. See the discussion of Karl Popper and the falsifiability criterion in "Making Revolution and Emancipating Humanity" by Bob Avakian. [back]

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