TURNING DATA INTO EVIDENCE Three Lectures on the Role of Theory in Science 1. CLOSING THE LOOP Testing Newtonian Gravity, Then and Now 2. GETTING STARTED Building Theories from Working Hypotheses 3. GAINING ACCESS Using Seismology to Probe the EarthвЂ™s Insides George E. Smith Tufts University THEORY-MEDIATED ACCESS vs. Theory-mediated measurement vs. вЂњTheory-mediated observationвЂќ Areas of science in which theory is indispensable to having empirical access to the subject matter at all Microphysics: atomic and subatomic Internal structure of the Earth THE QUESTION OF CORROBORATION вЂў Some historians and philosophers contend that science is a construct constrained on its boundaries by observation вЂў What evidence is there then that unobserved вЂњtheoretical entitiesвЂќ like electrons really exist вЂ“ vs. mere constructs? вЂў Questions of this sort gain their maximum force when the evidence for theory has to come from вЂњdataвЂќ that presuppose the very theory in question вЂў Seismological research over the last century is no less an example of this than research since 1850 in microphysics вЂў What sort of corroboration has there been for the conclusions from seismology about the internal structure of the Earth? OUTLINE I. Introduction: the issue II. Seismological research from 1900 to 1960 III. Seismological research since 1960 A. From 1960 to вЂњPreliminary Reference Earth ModelвЂќ B. The years since вЂњPREMвЂќ IV. Concluding remarks NewtonвЂ™s question: How does density vary below the EarthвЂ™s surface? вЂњAll these things will be so on the hypothesis that the earth consists of uniform matterвЂ¦. If [, however,] the excess of gravity in these northern places over the gravity at the equator is finally determined exactly by experiments conducted with greater diligence, and then its excess is everywhere taken in the ratio of the versed sine of twice the latitude, then there will be determined вЂ¦ the proportion of the diameters of the earth and its density at the center, on the hypothesis that the density, as one goes to the circumference, decreases uniformly.вЂќ Isaac Newton, Principia, 1687 Gravity Measurements Underdetermine Deviation of surface gravity from NewtonвЂ™s ideal variation implies the value of (C-A)/Ma2 and hence a correction to the difference (C-A) in the EarthвЂ™s moments of inertia, and the lunar-solar precession implies the value of (C-A)/C and hence a correction to the polar moment C; these two corrected values constrain the variation пЃІ(r) of density inside the Earth by implying it is notably greater toward the center, but they do not suffice to determine the variation пЃІ(r) . Hypothetical models of пЃІ(r): вЂў Legendre (1793) вЂў Laplace (1825) вЂў Roche (1848) вЂў G. Darwin (1884) вЂў Radau (1885) вЂў Wiechert (1897) вЂў Georg Kreisel (1949): Gravity measurements at or above the surface of the Earth can never uniquely determine the variation of density below the surface. NINETEENTH CENTURY BACKGROUND Observational advances Early pendulum seismometers e.g. Palmieri (1856) e.g. Ewing (1881) Networks of observing stations Italy Japan Increasing sensitivity Milne (1892) Wiechert (1903) RICHARD DIXON OLDHAM 1899: Report on the great earthquake of 12 June 1897 1900: On the propagation of earthquake motion to great distances 1906: The constitution of the earth as revealed by earthquakes NINETEENTH CENTURY BACKGROUND Theoretical foundations Transmission of compression (p) and transverse shear (s) waves Poisson (1829, 1831) Stokes (1849) Surface waves Rayleigh (1885) Love (1911) Free oscillation modes of a sphere Lamb (1882) Love (1911) Assumptions elastic linear isotropic 2 stress-strain parameters vs. as many as 21 in the general case of anisotropy homogeneous вЂ¦. EVIDENCE FOR THE THEORY OF p AND s WAVES? вЂў Poisson: Addition to MГ©moire sur lвЂ™Г©quilibre des corps Г©lastiques MГ©moire a classic in continuum mechanics Mathematical consequences of Navier-Stokes equation вЂў Basic equations of continuum mechanics Fundamental principles of physics, e.g. F=ma вЂў Constitituve equations for individual media Solid vs. fluid, elastic vs. plastic, isotropic vs. вЂ¦. вЂў The question of evidence: Do the proposed constitutive equations hold for the medium? SEISMIC WAVES AT ONE LOCATION SEISMIC WAVE PROPAGATION OLDHAMвЂ™S вЂњBREAKTHROUGHвЂќ вЂњOf all regions of the earth none invites speculation more than that which lies beneath our feet, and in none is speculation more dangerous; yet, apart from speculation, it is little that we can say regarding the constitution of the interior of the earthвЂ¦.The object of this paper is not to introduce another speculation, but to point out that the subject is, at least partly, removed from the realm of speculation into that of knowledge by the instrument of research which the modern seismograph has put in our hands.вЂќ DISCONTINUITIES: A BRIEF HISTORY вЂў Crust-mantle boundary MohoroviДЌiД‡ 1909 вЂў Core (Oldham 1906) Gutenberg 1914 at 2900 km below surface вЂў Core is liquid Jeffreys 1926 вЂў Inner Core Lehman 1936 THE PROJECT: 1900-1940 вЂ¦ вЂў from Arrival times of seismic waves from earthquakes at many locations around the Earth вЂў to Travel times (О”t vs. О”Оё) for a spherically symmetric Earth for p and s waves вЂ“ reflected and diffracted as well as refracted within a medium of varying density вЂў to Velocity variation of p and s waves in a spherically symmetric Earth, via ray theory and the HerglotzWiechart integral (1907) for an isotropic medium DIFFICULTIES Need to identify phases (different pathways) of waves reaching a single point at different times THE JEFFREYS-BULLEN TABLES, 1940 Assumptions: вЂў Arrival times of principal phases distinguished from each other вЂў Times and source locations of wave-origin identified, including focal depth вЂў Systematic errors corrected for вЂ“ Ellipticity of Earth вЂ“ Double quakes вЂ“ Late readings due to weak p, pkp вЂў Averaging for spherical symmetry makes sense THE JEFFREYS VELOCITIES, 1939 Assumptions: вЂў Fractional change in v gradient over one wavelength small compared to v вЂў Velocity increases slowly with depth or вЂ“ Decreasing velocity zones identified and provided for вЂў Numerical derivatives of О”t vs. О”Оё are well behaved вЂў (Isotropic, linear elasticity with continuous properties except at identified discontinuities) A FURTHER PROJECT: INFER DENSITY vs. RADIUS P velocity in isotropic elastic medium п‚µ пѓ–[(bulk-mod+4shear-mod/3)/density] S velocity in isotropic elastic medium п‚µ пѓ–(shear-mod/density) Two equations in three unknowns: (bulk-modulus/density) (shear-modulus/density) From gravity constraints, lab experiments at high pressure, and assumptions (equations of state), infer density vs. radius in symmetric Earth Bullen, 1940-42 THE QUESTION OF EVIDENCE вЂў вЂў вЂў вЂў Precision: error bands? Resolution: scale of detail? Idealization: uniqueness? Corroboration: assumptions? Form of evidence: coherence, as judged by magnitudes and absence of systematicity in residual discrepancies Inference to best explanation OUTLINE I. Introduction: the issue II. Seismological research from 1900 to 1960 III. Seismological research since 1960 A. From 1960 to вЂњPreliminary Reference Earth ModelвЂќ B. The years since вЂњPREMвЂќ IV. Concluding remarks THE FIELD TRANSFORMS: 1950-1970 вЂў Nuclear testing yields evidence supporting travel times вЂў Nuclear detection в†’ U.S. finances open-data network вЂ“ World Wide Standardized Seismographic Network (1960) вЂ“ International Seismological Centre (1964) вЂў Advent of digital computers, of increasing power вЂў Satellites в†’ improved values of mass, moments of inertia вЂў Improved and new instrumentation вЂ“ Including long period, electronic strain-based seismometers вЂ“ Fast Fourier transform: spectra (Cooley & Tukey, 1965) вЂў Burgeoning number of people entering the field вЂў Detection of natural modes of vibration of the Earth вЂ“ Proposed 1958, confirmed following Chile (1960), Alaska (1964) вЂ“ Initiating advanced efforts on вЂњinverse methodsвЂќ (late 1960s) DETECTING FREE OSCILLATIONS AN EXAMPLE: COLOMBIA, 1970 FREE OSCILLATIONS OF THE EARTH Why so important п‚§ New data, independent of travel times (& ray theory) п‚§ Each mode of oscillation samples the whole Earth, but differently п‚§ Long period modes give direct information about density variations п‚§ Conclusive evidence for solid inner core п‚§ Differing amplitudes give information about action in individual earthquakes вЂњINVERSE-THEORYвЂќ Initial Earth model: densities & material properties Calculate natural frequencies for model Find array of discrepancies vs. observed frequencies Use array of discrepancies to revise Earth model FREE-OSCILLATION-BASED MODELS вЂњ1066вЂќ inverse solution: Start from two prior models Use 1064 natural modes + mass, moments of inertia Obtain new Earth models Results: вЂў Reconstruct two quakes вЂў Systematic discrepancies between calculated and traditional travel times EMPIRICALLY DRIVEN REVISIONS TO THE CONSTITUTIVE EQUATIONS Low frequency waves more highly attenuated, producing anelastic wave dispersion Outer mantle is anisotropic, with different velocities horizontally and vertically PREM: Preliminary Reference Earth Model (Dziewonski & Anderson, 1981) вЂў 1000 normal mode periods, 500 summary travel times, 100 normal mode Q-factors, mass, moment of inertia вЂў Mantle includes anelastic dispersion and anisotropy (transversely isotropic, yielding two velocities) вЂў In spite of other models and known shortcomings, still preferred as textbook model WHY STILL вЂњPRELIMINARYвЂќ? вЂў Multiple spherically symmetric models вЂў Question: What exactly do they represent? вЂў Interest turns to details, including tomography using compact arrays of seismometers to identify lateral density variations A QUESTION ANSWERED вЂњThe early satellite results yielded anomalies that exceeded expectations and led to the conclusion that significant lateral variations in the density of the mantle occurred. These departures from isostatic and hydrostatic equilibrium imply either a finite strength for the mantle or convection within it. With the finite strength interpretation, the gravity field reflects a long-past condition of the planet, while the convection interpretation implies an on-going evolutionary process. The inability to distinguish between two extreme alternative hypotheses emphasizes once again that Earth models based on gravity observations alone are no better than the assumptions made to render a non-unique problem tractable.вЂќ Lambeck, Geophysical Geodesy: The Slow Deformations of the Earth, 1988 Van der Hilst et al., 1997 TWO MORE RECENT EXAMPLES Inner Core Differential Motion Confirmed by Earthquake Waveform Doublets, Zhang et al., 2005 Crustal Dilatation Observed by GRACE After the 2004 Sumatra-Andaman Earthquake, Han, et al., 2006 Gravity changes in Ојgal SOURCES OF CORROBORATION вЂў The highly redundant data have been sufficiently well-behaved to be yielding reasonably unequivocal answers to questions вЂў Systematic discrepancies between observation and theoretical models have proved informative, e.g. in answering questions вЂў Complementary sources of data have converged on the same conclusions rather than opposing one another вЂў Theoretical models have enabled advanced research to develop evidence for details that reach well beyond those models PRIMARY CONCLUSIONS вЂў Without the theoretical basis supplied by continuum mechanics, seismology would not have given us empirical access to the interior of the Earth вЂў While this theoretical basis has been indispensable to turning seismographic data into evidence, that basis has itself been tested in the process, providing corroborative evidence вЂў Seismology has given us, in particular, an enormously more strongly confirmed answer to NewtonвЂ™s question about the density variation than we had in 1900 вЂў Seismology has done this even though the constitutive equations it used throughout much of the last century were over-simplified and hence were made вЂњmore exact or liable to exceptions.вЂќ THE QUESTION OF THEORETICAL ENTITIES вЂў Theory-mediated measurements vs. theoretical entities вЂ“ Do electrons really exist? вЂ“ Does the Earth really have a liquid outer core 2891 km below its surface and an anisotropic solid inner core of radius 1221.5 km? вЂў The evidence for these entities consists of gross differences we have concluded that they make in our measurements вЂў For which is the evidence stronger, that we should take electrons to exist or that we should take the liquid outer and solid inner core to exist? The nature, scope, and limits of the knowledge attained in individual sciences when they at least seem to be most successful in marshaling evidence вЂў Science viewed from inside is an endeavor to turn data into compelling evidence, something that is difficult to do and for which theory is invariably needed вЂў Success in doing so has generally presupposed theoretical claims that were first adopted when little evidence was available for their truth вЂў Knowledge pursued is not merely theory, but also, even more so, which details in the domain make a difference and what differences they make вЂў How, if at all, has the theory presupposed in turning data into evidence while establishing such details itself been tested in the process?