The Physical Construction of the World

1. The Physical Construction of the World Leiden: March 24th, 2010 Richard Healey, University of Arizona rhealey@email.Arizona.edu

6. Atomic Structure • Rutherford(1911) discovers the nucleus: atoms are neither indivisible nor immutable! • Bohr’s(1912) model of the hydrogen atom • De Broglie’s(1924) matter waves

7. Quantum theory • Heisenberg(1925): No electronic orbits! • Schrödinger(1926): Not electronic orbits, but wave-functions. • Born’s rule(1926) relates wave-function to probabilities of measurement outcomes. • Example: the two-slit experiment

12. Feynman on the 2-slit experiment “if one has a piece of apparatus which is capable of determining whether the electrons go through slit A or slit B, then one can say it goes through either slit A or slit B. [otherwise] one may not say that an electron goes through either slit A or slit B. If one does say that, and starts to make any deductions from the statement, he will make errors in the analysis. This is the logical tight rope on which we must walk if we wish to describe nature successfully.”

13. “Elementary” Particles • Electrons, protons, neutrons: • Positrons, mesons, neutrinos: • Baryons, leptons; photons and other bosons: • None are indestructible---so neither are atoms! • The fundamental interactions: electromagnetic, strong, weak, gravitational • Each “carried by” corresponding bosons, including the photon associated with electric and magnetic fields whose variations constitute light and other forms of electromagnetic radiation

14. Quantum field theories • How is the photon associated with light? • As a minimal amount (a quantum) of energy that may be exchanged when light interacts with something else: • The size of this minimal amount depends on the “color” (wavelength) of the light. • A photon is a quantum of a quantum field theory of electromagnetism, while • electrons (and positrons) are quanta associated with a second quantum field: • The forces between them are manifestations of the interaction of these two quantum fields...

16. Interacting quantum field theories • Quantum electrodynamics (QED—the title of Feynman’s lovely little book): the quantum field theory of the electromagnetic interaction of electrons and photons. • Quantum chromodynamics (QCD): the quantum field theory of the strong interaction of hadrons. (Protons and neutrons are now taken to be composed of quarks, held together by gluons!) • Unified electroweak theory: a quantum field theory incorporating both electromagnetism and the weak interaction.

17. The Standard Model • Quantum field theories for all the basic interactions (except gravity). • Elementary particles: quarks and leptons, including the electron (“matter”) gauge bosons, including photons and gluons (“forces”) the Higgs boson (for which the LHC will soon search).

18. Particles or Fields? Richard Feynman (QED): “I want to emphasize that light comes in this form: particles.” “Newton thought light was made up of particles—he called them ‘corpuscles’—and he was right…” Robert Wald (Quantum Field Theory…): “Quantum field theory is a quantum theory of fields, not particles. Although in appropriate circumstances a particle interpretation of the theory may be available, the notion of ‘particles’ plays no fundamental role either in the formulation or interpretation of the theory.”

19. Do quantum field theories describe particles? Doreen Fraser (2008): “Quantum field theory (QFT) is the basis of the branch of physics known as ‘particle physics’. However the philosophical question of whether quantum field theories genuinely describe particles is not straightforward to answer. What is at stake is whether QFT, one of our current best physical theories, supports the inclusion of particles in our ontology.” “because systems which interact cannot be given a particle interpretation, QFT does not describe particles.”

20. Do quantum field theories describe fields? David Baker (2009): “The notion that QFT can be understood as describing systems of point particles has been all but refuted by recent work in the philosophy of physics.” however “the most popular extant proposal for fleshing out a field interpretation is problematic. ..two of the most powerful arguments against particles are also arguments against such a field interpretation. … If the particle concept cannot be applied to QFT, it seems that the field concept must break down as well.”

21. Do quantum field theories describe anything? Paul Davies (“Particles do not Exist”, 1984): “There are quantum states and there are particle detectors. Quantum field theory enables us to predict probabilistically how a particular detector will respond to that state. That is all. That is all there can ever be in physics, because physics is about the observations and measurements that we can make in the world. We can’t talk meaningfully about whether such-and-such a state contains particles except in the context of a specified particle detector measurement.”

22. A monism suited to quantum field theory Horgan and Potrč (Austere Realism: 2008): • There is only one object---”the blobject”. • It has no parts: neither particles, nor localized field-values, events nor spatiotemporal locations (space-time points). • But the world (i.e. the blobject) has a very rich structure • Everything we think of as an ordinary or extraordinary object emerges as an aspect of this structure. • The quantum field theories of the Standard Model give us our only ways of modeling certain details of this structure. Other scientific theories, as well as everyday beliefs, model other aspects of the world-structure. • But none of these systems of thought and experience ultimately refer to any objects except the blobjectitself.

23. …with a “top-down” metaphysics • All nontrivial claims of composition are literally false, since the blobjecthas no parts: there are no building blocks to compose the world • But aspects of the world-structure themselves display a structure that warrants us in making compositional claims that we can agree are true, such as • A proton is composed of two up quarks and one down quark or • Your nose is part of your face • So a derivative composition relation then emerges as higher-order structure.

24. The Standard Model as Metaphysics? • The quarks, leptons, gauge bosons and the (as yet undiscovered) Higgs boson of the Standard Model cannot play the same metaphysical role as Democritean atoms • If anything, the Standard Model comports better with a monistic metaphysics than with an atomistic metaphysics • But…

25. The Standard Model is Physics, not Metaphysics! • …and there is more to physics than the Standard Model • There is gravity, whose description by the theory of general relativity has yet to be reconciled with quantum theory. • Only about 4% of the total energy density in the universe can be understood in terms of the Standard Model. • About 22% is thought to be composed of so-called dark matter, whose nature remains unknown. • The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. • If this is right, then most of the stuff in the universe is neither composed of atoms, nor of the “elementary particles” of the Standard Model.

26. Some different parts of matter • To understand how atoms behave in the aggregate, condensed matter physicists have introduced many different kinds of quasiparticle constituents---phonons, excitons, magnons, polaritons, rotons, etc. • These are not simply composed of atoms or their constituents, and play an independent explanatory role in theoretical models. • One recent Nobel prize-winner (Robert Laughlin) wrote “The prefix ‘quasi-’ turns out to be a vestige of historical battles over the physical meaning of these objects and conveys no meaning. In private conversations one drops pretense and refers to the objects as particles.”

27. The decompositional strategy • Decomposing a system into its parts is one of many strategies physics employs in its quest to explain as well as predict the behaviour of the natural world. • This strategy may be locally successful whether or not it furthers the global metaphysical goal of displaying the world as a single, unified, compositional hierarchy.

28. Physics is an unfinished quest • Since Democritus, the search for more elementary constituents has repeatedly played a successful role in that quest • Atomistic metaphysics has often proved a remarkably useful heuristic in the history of science • But our current best theories do not straightforwardly describe ultimate building blocks---neither particles nor fields • They merely permit us to talk that way, with care, in certain contexts (remember Feynman’s caution about saying which slit an electron goes through!) • We can only guess whether atomistic metaphysics will inspire successful new physics beyond the Standard Model

29. Pragmatism, not Metaphysics • This comports well with a philosophy that views metaphysics not as a systematic search for truth, but as a fruitful source of ideas and motivation for the scientist. • This is pragmatism • For the pragmatist, contemporary physics neither supports nor presupposes atomistic metaphysics: • Rather, atomistic metaphysics is an intellectual tool that has proved, and may again prove, useful in science’s continuing attempt to come to grips with the natural world.