Underdetermination and the Phenomena of Physics

1. Underdeterminationand the Phenomena of Physics • Introduction • The Phenomena of Physics • How to Determine Particle Phenomena • Underdetermination in Astroparticle Physics • Conclusions brigitte.falkenburg@tu-dortmund.de

2. Underdeterminationand the Phenomena of Physicsbrigitte.falkenburg@tu-dortmund.de 1. Introduction

3. 1. Introduction Underdetermination: An Empiricist Story • Phenomena  Empirical Structures • Same empirical structure many theories • Fewexamples from“real physics” Classical Mechanics: • With/without Absolute Space? (van Fraassen’s example) • Continuous particle trajectories (given up in QM) Quantum Mechanics (probabilistic view of ): • Bohm’s QM (hidden variables & particle trajectories) • Many Worlds (splitting wave functions )

4. 1. Introduction But, what are the Phenomena? • philosophical tradition (phenomena noumena): • phenomena = appearances(Leibniz, Kant) • phenomenology = logic of what seems to be (Lambert) • tradition of physics (“saving the phenomena”): • motions of celestial bodies (Ptolemy – Kepler - Newton) • law-like, non-miraculous appearances (Hacking) • events predicted by a theory & explanandaof theories (Bogen & Woodward; most physicists)  no unambiguous meaning of “phenomenon”

5. 1. Introduction Tradition of Physics: • motions of celestial bodies: • description of apparent motions? (Ptolemy) • explanation of true motions? (Kepler& Newton) • observation of the appearances: • by sensory experience only? (Aristotle & empiricism) • by technological devices? (Galileo &scientific realism) • events predicted by a theory: • sense data only? (Mach, Carnap, van Fraassen) • physical effects? (physicists & principle of causality)  “phenomena” debated since Galileo!

6. 1. Introduction Phenomena of Physics = Theory-Laden • theory  phenomena: fuzzy distinction • Phenomena of physics have always been theory-laden • To talk of “empirical substructure” is naïve! • Example: Particle tracks of subatomic physics phenomenon of CM = full-fledged trajectory of QM = sequence of measurement points Hence: The Phenomena of Science are Underdetermined, too!

7. 1. Introduction Phenomena of Physics: Underdetermined • No Problem - Only for Empiricists! “Myth of the Given” • Underdetermination of Phenomena: • Not strong, but weak = transient • Demarcation theory/phenomena: time-dependent • Evidence depends on well-established knowledge! Goal of Physics: To Determine the Phenomena in terms of Laws of Nature!

8. Underdetermination and the Phenomena of Physicsbrigitte.falkenburg@tu-dortmund.de 2.The Phenomena of Physics

9. 2. The Phenomena of Physics Two prominent examples: • Newton’s “Phenomena” ? • Principia: motions of celestial bodies • Opticks: observations in experiments • Bohr‘s “quantum phenomena” • Claim: there are no quantum objects • Quantum Phenomena are “individual” (=indivisible) • They correspond to classical pictures of physical reality Current physics/science: phenomena “evidence”

10. 2. The Phenomena of Physics Newton’s “phenomena” ? • ambiguous concept: • “Principia” : motions of celestial bodies • planetary motions described by Kepler’slaws • phenomenological laws • Optics: observations in experiments • light propagation • light colors • light diffraction  but common features: • regularity, predictability, law-likeness • phenomena are typical & connected

11. 2. The Phenomena of Physics Newton’s Phenomena: Two examples • Connection of the Phenomena: • from Galileo‘s to Kepler‘s motions • 2. Analysis & Synthesis of Phenomena: • spectral decomposition & re-composition of light Principia: connection between Galileo‘s and Kepler‘s motions Opticks: spectral decomposition & re-composition of light

12. 2. The Phenomena of Physics Newton’s Phenomena: • phenomena =what is given • at any stage of research : appearances, phenomenological laws, experimental results, measurement outcomes • phenomena =subject to causal analysis • mathematical analysis  forces & universal laws • experimental analysis atoms of matter & light • phenomena = connected by laws • trust in law-likeness & unity of nature “Nature will be very conformable to herself and very simple.” (Newton 1704)

13. 2. The Phenomena of Physics Niels Bohr (Nobel lecture, 1922): Phenomena are: • explanandaof theories • observations Explanation is: • classification in terms of analogies “By a theoretical explanation of natural phenomena we understand in general a classification of the observations of a certain domain with the help of analogiespertaining to other domains [...], where one has presumably to do with simpler phenomena.”  scientific realism of phenomena & weakened account of explanation!

14. 2. The Phenomena of Physics Niels Bohr (Como lecture, 1927): There are noquantum objects, only quantum phenomena Physical objects are: •defined in terms of spatio-temporal & causalproperties • these properties can beobserved simultaneously For quantum “objects”: definition observation Quantum phenomena are: • individual (=indivisible) • complementary(=mutually exclusive) • in correspondence to classical models of wave or particle

15. 2. The Phenomena of Physics Bohr’s Quantum Phenomena: Wave-Particle Duality • Particle Tracks: • Proton tracks in nuclear emulsions • 2. Wave interference: • Diffraction of (a) electrons & (b) photons at a crystal

16. 2. The Phenomena of Physics Newton & Bohr: Phenomena are • concrete, intuitive facts of Nature  spatio-temporally individuatedobjects &events • empirical, observable, given  given by some kind of observation or measurement • typical, class constructs, connected by laws  explainable in terms of laws & causal stories Phenomena can be found at many levels of observation & measurement!

17. Underdetermination and the Phenomena of Physicsbrigitte.falkenburg@tu-dortmund.de 3. How to Determine Particle Phenomena

18. 3. How to Determine Particle Phenomena Phenomena of Particle Physics: • Quantum Phenomena  Particle Picture • empirical, observable, given  Particle Tracks • typical, class constructs, connected by laws  Causal Analysis of Particle Tracks  Classified in terms of mass, charge, spin, ...  many kinds of particles  How are they identified?

19. 3. How to Determine Particle Phenomena The Track of the Positron (Anderson 1932): Electron mass, but opposite charge? • Experimental device: • Bubble chamber: •  tracks from cosmic rays • Magnetic field: •  curvature of charged particle • 3. Lead plate as stopping device: • direction of particle Without the lead plate, the phenomenon remained underdetermined!

20. 3. How to Determine Particle Phenomena Particle Identification in the 1940s: Puzzle of “mesons” Later particle accelerator measurement of energy loss dE/dx -   p • Problem: • Particle tracks in bubble chamber • melectron < mass < mproton • No trust in QED • no safe calculation of energy loss • no mass measurement from particle range • 3. No particle identification possible •  vague concept of “mesons” e- Without safe measurement method, the phenomenon remained underdetermined!

21. 3. How to Determine Particle Phenomena Particle Identification in the 1940s: Puzzle of “mesons” • Resolution: • Development of nuclear emulsions •  better resolution of measurement points • 2. mass estimation from density of points •  QED-independent mass measurement • 2. Particle identification possible • distinction of - and • independent test of QED Without safe measurement method, the phenomenon remained underdetermined!

22. 3. How to Determine Particle Phenomena Particle Identification in the 1930s-1960s: Many Puzzles! • Problems: • Many kinds of particle tracks •  classified in terms of mass & charge • 2. Many kinds of particle reactions •  classified in terms of conserved quantities • 3. Many kinds of particle resonances • classifiedin terms of unstable particles •  wide range of phenomena • at different levels of observation & measurement! •  How are they established? •  Independent measurement methods

23. 3. How to Determine Particle Phenomena Particle Identification in the 1930s-1960s:  Particle phenomenaare identified in terms of particle types (mass, charge, spin, parity, …)  ifindependent measurement methods available Identification of the Particle Phenomena aims at their Theoretical Explanation! stage 1: classif. in terms of particle types stage 2: classif. in terms of symmetries stage 3: quantum dynamics

24. Underdetermination and the Phenomena of Physicsbrigitte.falkenburg@tu-dortmund.de 4. Underdetermination in Astroparticle Physics

25. 4. Underdetermination in APP Phenomena of APP: Cosmic Rays (CRs) • Discovered in 1912 (ViktorHess, Vienna) • (Institutfür Radiumforschung) • Extraterrestrial Origin • „primary“ & „secondary“ CRs • scattering in the atmosphere

26. 4. Underdetermination in Astroparticle Physics Primary CRs: Satellites & Space telescopes Phenomena of APP: Cosmic Rays (CRs) Secondary CRs: Earthbound experiments neutrino  muon detected: Cherenkov light high-energy photon  e+e- detected: Cherenkov light MAGIC Cherenkov neutrino&gamma ray telescopes ICECUBE

27. 4. Underdetermination in Astroparticle Physics Phenomena of APP: Cosmic Rays (CRs) • Power law decrease • two „kinks“: • „knee“ & „ankle“ • Today, the CR phenomena are known! But, what is their explanation? •  Goal: Model determination!

28. 4. Underdetermination in Astroparticle Physics Phenomena of APP All Particle Spectrum particle content of primary CRs: charged particles: 90 % protons 9 % -particles 1 % electrons (uncharged particles: photons & neutrinos)

29. 4. Underdeterminationin Astroparticle Physics Concepts of APP Messenger particles CRs carry Information from Cosmic Sources Where do they come from? What did happen to them?

30. 4. Underdetermination in Astroparticle Physics Concepts of APP Messenger particles Information = signal transmisson from emitter to receiver to read it out, you must know 2 of: • cosmic source • nature of signal • interactions • during transfer • Explanatory Model • Determined?

31. 4. Underdetermination in Astroparticle Physics Concepts of APP Messenger particles Information =particle propagation from source to detector reading it out, you will learn about 1 of: • cosmic source • messenger particles • interactions • during propagation • Explanatory Model • Determined?

32. 4. Underdetermination in Astroparticle Physics Concepts of APP Messenger particles Only uncharged particles point to their source! They allow „direct observation“ of source, (if !) no interactions during transfer (D.Shapere 1982)

33. 4. Underdetermination in Astroparticle Physics Concepts of APP Messenger particles Photon & neutrino telescopes observe extragalactic sources, like Galileo observed Jupiter moons Proton detection does not!

34. 4. Underdetermination in Astroparticle Physics Concepts of APP Messenger particles But, be cautious! Shapere‘s example: Observation of sunwith solar neutrinos Solar neutrino experiments  neutrino oszillations (information about messenger particles, notsource!) Observation of cosmic sources depends on knowledge of messenger particles & their interactions ! „theory-ladenness“  underdetermination!

35. 4. Underdetermination in Astroparticle Physics Explanations Extragalactic sources Sources & their activities astrophysical data: luminosity & spectra & temporal evolutionof AGNs, GRBs, SNRs

36. 4. Underdetermination in Astroparticle Physics Explanations Extragalactic sources • But, what goes on in between? astrophysical data: Dark matter & gravitational lensing & other effects How reliable are the models?

37. 4. Underdetermination in Astroparticle Physics Phenomena & Concepts & Models of APP:  Cosmic Ray phenomenaare identified in terms of messenger particles  andinterpreted in terms of extragalactic sources Goal of APP: Investigation of Cosmic Rays in order to Identify their Sources! stage 1: measurement of messenger particles stage 2: identification of cosmic origin stage 3: dynamics of source

38. Underdeterminationand the Phenomena of Physicsbrigitte.falkenburg@tu-dortmund.de 5. Conclusions

39. 5. Conclusions 1. Goal of Physics: ToDeterminethePhenomena interms of Laws of Nature 2. Concrete & stablePhenomena can be found at Many Levels of Observation &Measurement 3. To Identifythe Particle Phenomena aims at their Theoretical Explanation. 4. To Investigate Cosmic Raysnow aims at Identifying Galactic & Extragalactic Sources. Typical for physics: Transient Underdetermination of Phenomena & Explanatory Models

40. 5. Conclusions 1. Goal of Physics: ToDeterminethePhenomena interms of Laws of Nature 2. Concrete & stablePhenomena can be found at Many Levels of Observation &Measurement 3. To Identifythe Particle Phenomena aims at their Theoretical Explanation. 4. To Investigate Cosmic Raysnow aims at Identifying Galactic & Extragalactic Sources. So: All the Worse for Scientific Realism? No: Physicists are struggling very hard for independent measurement methods.

41. Literature • Brigitte Falkenburg: • - Particle Metaphysics. A Critical Account of Subatomic Reality. Springer 2007. • - What are the Phenomena of Physics? In: Synthese (forthcoming) • - Incommensurability and Measurement. In: Theoria, Vol. 12 Numero 30 (1997), 467-491.