Physics and Philosophy

Summary

Physics and Philosophy: The Revolution in Modern Science is Werner Heisenberg's account of what quantum theory means for our understanding of nature, knowledge and language, set within the history of Western thought from the Greek atomists to the twentieth century. It grew from the Gifford Lectures he gave at the University of St Andrews in the winter of 1955-56 and was published in New York in 1958 by Harper and Brothers in the World Perspectives series, with an introduction by the Yale philosopher F. S. C. Northrop; a German edition, Physik und Philosophie, followed in 1959. The book is under copyright, so this library describes it in its own words and gives no quotations. Heisenberg begins by contrasting the old tradition of natural philosophy with the new situation forced on physics by experiment. He then tells the history of quantum theory from Planck's quantum of action in 1900 through Einstein's light quanta, Bohr's atom and the matrix and wave mechanics of 1925-26. The central chapter sets out the Copenhagen interpretation: the results of experiments must be described in the concepts of classical physics; these concepts limit one another, as the uncertainty relations show for position and velocity; the wave function expresses not a definite process but a tendency or possibility, close to Aristotle's potentia; and the act of observation, the interaction of the system with the measuring device, changes this possibility into a fact. He then traces the roots of atomic science to Leucippus and Democritus, contrasts their indivisible atoms with Plato's mathematical forms in the Timaeus, and argues that modern elementary particles resemble Plato's symmetries more than Democritus's hard bodies, and that energy is like the fire of Heraclitus, the substance from which all particles are made. A chapter on philosophy since Descartes argues that quantum theory unsettles the sharp division between thinking subject and extended world, and qualifies Kant's a priori categories of space, time and causality. Further chapters relate quantum theory to chemistry, biology and relativity, review and answer critics of the Copenhagen view, among them Einstein, de Broglie, Bohm and Soviet philosophers, and treat the structure of matter and the limits of language. Heisenberg concludes that plain language, though imprecise, remains indispensable, that science cannot reach absolute truth by pure reason, and that modern physics may help different cultures and traditions understand one another.

Plot or Argument

• An old and a new tradition

  Experiment has forced physics beyond the concepts of earlier natural philosophy; the lectures will ask what the change means for thought as a whole.

• The history of quantum theory

  From Planck's quantum of 1900 through Einstein, Bohr and the new mechanics of 1925-26, with the long discussions in Copenhagen that led to the uncertainty principle and complementarity.

• The Copenhagen interpretation

  Classical concepts describe experiments; they limit one another; the probability function represents potentiality; observation turns the possible into the actual.

• Quantum theory and the roots of atomic science

  Greek atomism from Leucippus and Democritus, Plato's mathematical solids, and Heraclitus's fire, compared with elementary particles and energy.

• Philosophy since Descartes

  The Cartesian split of mind and matter, the materialist picture of the nineteenth century, and Kant's categories reconsidered in light of quantum theory.

• Quantum theory, other sciences and relativity

  Quantum theory in chemistry and its limits in biology; the theory of relativity and its changes to the concepts of space and time.

• Criticism and counterproposals

  Objections from Einstein, de Broglie, Bohm, and others who sought an objective realism or hidden variables are reviewed and answered.

• The structure of matter, language and modern thought

  Elementary particles as forms of energy; the necessity and imprecision of ordinary language; physics as a meeting ground of cultures.

Characters

• Niels Bohr, Danish physicist

  Heisenberg's mentor and partner in the debates that produced the Copenhagen interpretation.

• Albert Einstein, physicist and critic

  Founder of light quanta and relativity who rejected the Copenhagen view of chance.

• Democritus, Greek atomist

  Proposed indivisible atoms moving in the void, the beginning of atomic science.

• Plato, Greek philosopher

  His mathematical solids in the Timaeus are compared with symmetries of modern particle physics.

• Aristotle, Greek philosopher

  His concept of potentia is used to explain the meaning of the probability function.

• René Descartes, French philosopher

  His division of thinking and extended substance is examined in light of quantum observation.

• Immanuel Kant, German philosopher

  His a priori categories of space, time and causality are qualified by the new physics.

Lessons

• Observation as participation

  In the Copenhagen interpretation the experimenter does not simply look at an atom; the measurement is an interaction that turns a range of possibilities into one actual result. Heisenberg concludes that physics describes nature as it responds to our questions. The lesson is that knowing is an activity with its own conditions, not a passive copying.

  • Why can an electron's position and velocity not both be measured exactly?
  • What does Heisenberg mean when he compares the probability function to Aristotle's potentia?
  • Does his view make the world less real, or only our knowledge more modest?

• The ancients and the moderns

  Heisenberg reads the Greek philosophers as partners in present inquiry. Democritus gave physics the atom, but Plato's idea that the smallest units are mathematical forms seemed to him closer to the truth of modern particles. The history of ideas becomes a resource for current science.

  • Why does Heisenberg prefer Plato's view of the smallest parts of matter to Democritus's?
  • In what sense is energy like Heraclitus's fire?

• The limits of language

  The concepts of classical physics and ordinary speech were formed for everyday experience, and they fit the atomic world only partly. Yet Heisenberg insists that experiments must be described in those concepts, and that plain language remains the test of understanding. Science must speak clearly while knowing its words are approximate.

  • Why must experiments be described in classical terms, according to the Copenhagen view?
  • What is lost if a science can be expressed only in mathematics?

• A disputed interpretation

  Heisenberg devotes a chapter to the critics of the Copenhagen view, from Einstein's hope for a complete objective description to Bohm's hidden variables, and answers them. The debate over how to interpret quantum mechanics remains open among physicists and philosophers. Reading him well means hearing both sides.

  • What did Einstein find unsatisfactory in the Copenhagen interpretation?
  • How does Heisenberg answer critics who want an objective description independent of observation?

Context

Quantum mechanics took shape in 1925-27 in the work of Heisenberg, Born, Jordan, Schrodinger, Dirac and Bohr, and the interpretation worked out in Copenhagen became the most widely taught account. By the 1950s it faced challenges from Einstein, from David Bohm's hidden-variables theory of 1952, and from Soviet philosophers who judged it idealist. Heisenberg's lectures restate the Copenhagen position for a general audience and set it within the history of philosophy.

Composition

The Gifford Lectures were endowed by Adam Gifford for the study of natural theology in the Scottish universities. Heisenberg gave his series at St Andrews in the winter of 1955-56. The English text appeared in 1958 from Harper and Brothers, New York, as volume 19 of World Perspectives edited by Ruth Nanda Anshen, with an introduction by F. S. C. Northrop; a British edition followed from Allen and Unwin, and a German edition, Physik und Philosophie, in 1959.

Reception

The book became one of the most read statements of the Copenhagen interpretation by one of its founders and has been reprinted many times. Philosophers of physics have both drawn on it and criticized its claims, for instance over the role of the observer and the Aristotelian reading of probability. Readers also note that the Copenhagen interpretation was not a single doctrine, and that Bohr's and Heisenberg's views differed in emphasis.

Comparisons

• einstein-relativity

  Heisenberg devotes a chapter to relativity and answers Einstein's objections to quantum theory; Einstein's own exposition shows the classical ideal of an observer-independent description that Heisenberg thinks the atom forbids.

• on-the-nature-of-the-universe

  Lucretius's poem on Epicurean atoms is the fullest ancient statement of the atomism whose roots Heisenberg traces, and whose hard indivisible bodies he thinks modern physics has abandoned.

• eddington-the-nature-of-the-physical-world

  An earlier Gifford Lectures series by a physicist, given in 1927 as the uncertainty principle appeared; Eddington draws idealist and religious conclusions, Heisenberg more cautious philosophical ones.

• meditations-on-first-philosophy

  Heisenberg argues that quantum observation unsettles the sharp division between thinking subject and extended world that Descartes established.

• aristotle-posterior-analytics-and-physics

  Aristotle's distinction of potentiality and actuality is the concept Heisenberg borrows to explain the quantum probability function.

Quotes

For Students

A reading companion for classical Christian high school students. See the student guide.

Reading Guide

Watch for

• How the history of quantum theory leads to the late-night discussions in Copenhagen and the uncertainty principle.
• The steps of the Copenhagen interpretation: classical concepts, their mutual limitation, probability as potentiality, and the act of observation.
• The comparison of Democritus, Plato and Heraclitus with modern particle physics.
• The argument about Descartes and Kant in the chapter on philosophy since Descartes.
• How Heisenberg answers his critics in the chapter of counterproposals.

Key terms and names

Uncertainty principle

Heisenberg's 1927 result that position and momentum of a particle cannot both be fixed with unlimited precision.

Copenhagen interpretation

The account of quantum mechanics developed by Bohr and Heisenberg, in which observation turns possibilities into actual events.

Complementarity

Bohr's idea that wave and particle pictures exclude each other in a single experiment yet are both needed.

Potentia

Aristotle's term for what can be but is not yet, which Heisenberg uses to explain the probability function.

Gifford Lectures

Lectures on natural theology in Scottish universities; Heisenberg gave his at St Andrews in 1955-56.

Hidden variables

Proposed underlying quantities, as in Bohm's 1952 theory, that would restore a determinate description; Heisenberg rejects them.

Seminar Questions

1. Comprehension What are the main steps of the Copenhagen interpretation as Heisenberg presents it?
2. Comprehension How does Heisenberg compare Democritus's atoms with Plato's account of the smallest parts of matter?
3. Interpretation Why does Heisenberg insist that experiments be described in the language of classical physics even though that physics is incomplete?
4. Interpretation What does Heisenberg think quantum theory does to Descartes's division between mind and matter?
5. Interpretation Is Heisenberg's use of Aristotle's potentia an explanation or a metaphor?
6. Big Question If our questions shape what nature shows us, can science still claim to know the world as it is?
7. Big Question Can modern physics, as Heisenberg hoped, help people of different traditions understand one another?

Faith Connections

• Proverbs 25:2 (Scripture)

  It is the glory of God to conceal a thing, but the honour of kings is to search out a matter: Heisenberg's account of nature answering only the questions put to it fits this picture of a creation that rewards patient search.

• 1 Corinthians 13:12 (Scripture)

  Now we see through a glass, darkly: the limits that the uncertainty relations set on knowledge echo Paul's description of partial human knowing.

• Thomas Aquinas, Summa Theologica I, q. 2, a. 3 (Church Fathers)

  Aquinas's first way argues from the passage of things from potentiality to actuality, the same Aristotelian distinction Heisenberg revives to describe quantum measurement.

Logic and Rhetoric

Central argument: Experiments on atoms must be described in classical concepts; those concepts cannot all apply at once, as the uncertainty relations show; the mathematical formalism therefore describes possibilities rather than definite events; observation, an interaction with the measuring device, actualizes one possibility. Therefore physics describes not nature apart from us but nature as it answers our questions, and the older ideals of materialism and of a wholly detached observer must be given up.

Rhetorical device: historical analogy. In the chapter on the roots of atomic science Heisenberg sets Democritus, Plato and Heraclitus beside elementary particles and energy, making the strange new physics intelligible as a turn in an old conversation.

Debate: Resolved: that the Copenhagen interpretation shows nature itself to be indeterminate.

Writing Prompts

Essay: Explain Heisenberg's claim that elementary particles are closer to Plato's forms than to Democritus's atoms, and argue whether the comparison helps or misleads.

Comparison with On the Nature of the Universe: Lucretius thought atoms freed people from fear of the gods; Heisenberg thought the new physics undid materialism. Compare what each believes the study of atoms teaches about human life.

Medical Lens

The book does not concern medicine. Its chapter relating quantum theory to other sciences asks whether physics and chemistry can fully account for living organisms, and Heisenberg suggests that biology may require concepts of its own, as quantum theory required concepts beyond classical mechanics. His discussion of the observer and of the Cartesian split between mind and matter touches the mind-body problem in a general philosophical way; any application to psychiatry is an interpretation beyond the text.

• Chapter on the relation of quantum theory to other parts of natural science

  Heisenberg discusses whether physics and chemistry suffice to explain living organisms.

  Mind Body
• Chapter on philosophical ideas since Descartes

  The Cartesian split between thinking and extended substance reconsidered after quantum observation.

  Mind Body

Creative Prompts

• Classroom

  Make a two-column chart of Democritus's atoms and Plato's mathematical solids, and add a third column for what Heisenberg says about elementary particles.

• Dialogue

  Write a conversation between Heisenberg and Einstein on whether God plays dice, keeping each to his own position as described in the book.

• Essay

  Does the uncertainty principle show that nature is indeterminate, or only that our knowledge of it is limited? Present Heisenberg's view and one critic's.

Further Reading

• The Physical Principles of the Quantum Theory, Werner Heisenberg

• Physics and Beyond, Werner Heisenberg

  His memoir of conversations with Bohr, Einstein and others.

• Atomic Physics and Human Knowledge, Niels Bohr

• Uncertainty: The Life and Science of Werner Heisenberg, David C. Cassidy