How can we understand the world we find ourselves in? 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 torch bearers of discovery in our quest for knowledge. - StephenHawking and Leonard Mlodinow.

This passage from the 2012 book "The Grand Design" set off a firestorm (or at least a bonfire) of controversy. Has philosophy been eclipsed by science in the quest to understand reality? Has philosophy merely dressed itself up as mysticism, disconnected from scientific understanding?

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 into account relativity theory and quantum theory. Philosophers accept this. Indeed, several leading philosophers of physics hold doctorates in physics. However, they have chosen to affiliate themselves withphilosophy departments rather than physics departments because many physicists strongly discourage questions about the nature of reality. The reigning attitude in physics was "shut up and calculate": solve the equations and don't ask questions about what they mean.

But putting computation before conceptual clarity can lead to confusion. Take, for example, relativity's iconic "twin paradox." Identical twins separate and then reunite. When they meet again, one twin is biologically older than the other. (Twin astronauts Scott and Mark Kelly are about to perform this experiment: when Scott returns from a year in orbit in2016, he will be about 28 microseconds shorter than Mark, who is on Earth.)¹ No competent physicist would make a mistake in computing the magnitude of this effect.

But even the great Richard Feynman cannot always get the explanation right. In "The Feynman Lectures on Physics," he attributes the difference in ages to the acceleration of a twin experiment: the twin that accelerates ends up younger. But it is easy to describe the cases where the opposite is true, and even the cases where neither twin accelerates but end up at different ages. The calculation may be correctand the explanation of the incorrect explanation.

If your goal is just to calculate, that may be enough. But understanding existing theories and formulating new ones requires more. Einstein arrived at the theory of relativity by reflecting on conceptual rather than empirical problems. He was mainly bothered by the explanatory asymmetries in classical electromagnetic theory. Physicists before Einstein knew, for example, that moving amagnet 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 completely different when the motion was attributed 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 accountrequiring the theory of relativity.

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

The problems surrounding quantum theory are not mathematical. They take instead from the unacceptable terminology that appears in presentations of the theory. Physical theories should be stated in precise terminology, without ambiguity and imprecision. John Bell provides a list of insufficiently clear concepts in his essay "Against 'measurement'":

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

Textbook expositions of quantum theory make gratuitous 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 attention to language is the outward expression of his concern with concepts. Well-established physical theories cannot bebuilt on 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 to understanding what a mathematical formalism might suggest about the real world. Philosophers also learn to detect the gaps and elisions in everyday arguments. These gaps provideentry points for conceptual wedges: nooks where ignored alternatives can take root and grow. The ethos² "shut up and calculate" does not promote this critical attitude towards arguments; philosophy does.

What philosophy offers science, then, is not mystical ideas but meticulous method. Philosophical skepticism focuses attention on conceptual weaknesses in theories and arguments. It encourages exploration of alternative explanations and new theoretical approaches. Philosophers have an obsession with subtle ambiguities of language and about what follows from such. When the foundations of adiscipline are secure, this can be counterproductive: just get on with the work to be done! But where secure foundations (or new foundations) are needed, critical scrutiny can suggest the way forward. The search for ways to bring quantum theory and general relativity together would certainly benefit from precisely articulated accounts of the fundamental concepts of these theories,even if it only suggests what should be changed or abandoned.

Philosophical skepticism arises from the theory of knowledge, the branch of philosophy called "epistemology." Epistemology studies the foundations of our beliefs and the sources of our concepts. It often reveals tacit assumptions that may turn out to be wrong, sources of doubt about how much we really know. Having started with Hawking, let's let Einstein have the last word:

How does a properly gifted natural scientist come to care about epistemology? Is there no more valuable work in his specialty? I hear many of my colleagues say, and I sense it from many more, that they feel this way. I cannot share that feeling....

Concepts that have proved useful in ordering things, easily achieve such authority over us that we forget their earthly origins and accept them as unalterable data. Thus, they come to be stamped as "necessities of thought," "a priori data," etc. The path of scientific advancement is often long infallible by these errors. For this reason, it is by no meansan idle game if we are forewarned in analyzing the extensive common concepts, and exhibiting the circumstances on which their justification and utility depend, as they have grown, individually, from the data of experience. By this means, their so great authority will be broken.

References and notes:

[1] Mission conducted from March 27, 2015 to March 2, 2016.

[2] Ethos: set of habits or beliefs that define a community or nation.

Written by Tim Maudlin with the title Why Physics Needs Philosophy for the //www.pbs.org/wgbh/nova/blogs/physics/ website.

Translated and adapted by Carlos Germano.