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How can we understand the world in which we find ourselves? How does the universe behave? What is the nature of reality?….Traditionally these are questions for philosophy, but philosophy is dead. Philosophy has not kept up with modern developments in science, particularly physics. Scientists have become the bearers of the torch of discovery in our quest for knowledge. —Stephen Hawking and Leonard Mlodinow

This passage from the 2012 book “The Grand Design” set off a firestorm (or at least a brushfire) of controversy. Has philosophy been eclipsed by science in the quest for understanding reality? Is philosophy just dressed-up mysticism, disconnected from scientific understanding?

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Many questions about the nature of reality cannot be properly pursued without contemporary physics. Inquiry into the fundamental structure of space, time and matter must take account of the theory of relativity and quantum theory. Philosophers accept this. In fact, several leading philosophers of physics hold doctorates in physics. Yet they chose to affiliate with philosophy departments rather than physics departments because so many physicists strongly discourage questions about the nature of reality. The reigning attitude in physics has been “shut up and calculate”: solve the equations, and do not ask questions about what they mean.

But putting computation ahead of conceptual clarity can lead to confusion. Take, for example, relativity’s iconic “twin paradox.” Identical twins separate from each other and later reunite. When they meet again, one twin is biologically older than the other. (Astronaut twins Scott and Mark Kelly are about to realize this experiment: when Scott returns from a year in orbit in 2016 he will be about 28 microseconds younger than Mark, who is staying on Earth.) No competent physicist would make an error in computing the magnitude of this effect.

But even the great Richard Feynman did not always get the explanation right. In “The Feynman Lectures on Physics,” he attributes the difference in ages to the acceleration one twin experiences: the twin who accelerates ends up younger. But it is easy to describe cases where the opposite is true, and even cases where neither twin accelerates but they end up different ages. The calculation can be right and the accompanying explanation wrong.

If your goal is only to calculate, this might be sufficient. But understanding existing theories and formulating new ones requires more. Einstein arrived at the theory of relativity by reflecting on conceptual problems rather than on empirical ones. He was primarily bothered by explanatory asymmetries in classical electromagnetic theory. Physicists before Einstein knew, for instance, that moving a magnet in or near a coil of wire would induce an electric current in the coil. But the classical explanation for this effect appeared to be entirely different when the motion was ascribed to the magnet as opposed to the coil; the reality is that the effect depends only on the relative motion of the two. Resolving the explanatory asymmetry required rethinking the notion of simultaneity and rejecting the classical account of space and time. It required the theory of relativity.

Comprehending quantum theory is an even deeper challenge. What does quantum theory imply about “the nature of reality?” Scientists do not agree about the answer; they even disagree about whether it is a sensible question.

The problems surrounding quantum theory are not mathematical. They stem instead from the unacceptable terminology that appears in presentations of the theory. Physical theories ought to be stated in precise terminology, free of ambiguity and vagueness. John Bell provides a list of insufficiently clear concepts in his essay “Against ‘measurement’”:

Here are some words which, however legitimate and necessary in application, have no place in a formulation with any pretension to physical precision: system, apparatus, environment, microscopic, macroscopic, reversible, irreversible, observable, information, measurement .

Textbook expositions of quantum theory make free use of these forbidden terms. But how, in the end, are we to determine whether something is a “system”, or is large enough to count as “macroscopic,” or whether an interaction constitutes a “measurement?” Bell’s fastidiousness about language is the outward expression of his concern about concepts. Sharp physical theories cannot be built out of vague notions.

Philosophers strive for conceptual clarity. Their training instills certain habits of thought—sensitivity to ambiguity, precision of expression, attention to theoretical detail—that are essential for understanding what a mathematical formalism might suggest about the actual world. Philosophers also learn to spot the gaps and elisions in everyday arguments. These gaps provide entry points for conceptual wedges: nooks where overlooked alternatives can take root and grow. The “shut up and calculate” ethos does not promote this critical attitude toward arguments; philosophy does.

What philosophy offers to science, then, is not mystical ideas but meticulous method. Philosophical skepticism focuses attention on the conceptual weak points in theories and in arguments. It encourages exploration of alternative explanations and new theoretical approaches. Philosophers obsess over subtle ambiguities of language and over what follows from what. When the foundations of a discipline are secure this may be counter-productive: just get on with the job to be done! But where secure foundations (or new foundations) are needed, critical scrutiny can suggest the way forward. The search for ways to marry quantum theory with general relativity would surely benefit from precisely articulated accounts of the foundational concepts of these theories, even if only to suggest what must be altered or abandoned.

Philosophical skepticism arises from the theory of knowledge, the branch of philosophy called “epistemology.” Epistemology studies the grounds for our beliefs and the sources of our concepts. It often reveals tacit presuppositions that may prove wrong, sources of doubt about how much we really know. Having started with Hawking, let’s let Einstein have the last word :

How does it happen that a properly endowed natural scientist comes to concern himself with epistemology? Is there no more valuable work in his specialty? I hear many of my colleagues saying, and I sense it from many more, that they feel this way. I cannot share this sentiment…. Concepts that have proven useful in ordering things easily achieve such an authority over us that we forget their earthly origins and accept them as unalterable givens. Thus they come to be stamped as “necessities of thought,” “a priori givens,” etc. The path of scientific advance is often made impassable for a long time through such errors. For that reason, it is by no means an idle game if we become practiced in analyzing the long commonplace concepts and exhibiting those circumstances upon which their justification and usefulness depend, how they have grown up, individually, out of the givens of experience. By this means, their all-too-great authority will be broken.

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