SPU-22: The Unity of Science from the Big Bang to the Brontosaurus and Beyond

1. SPU-22: The Unity of Science from the Big Bang to the Brontosaurus and Beyond Lecture 10 3 March 2014 Science Center Lecture Hall A

2. Banishing Another Blemish Forgot demo set up last Wednesday to address question: Why is equatorial radius of earth > polar radius (by about 22 km)? Answer: Earth’s spin causes bulge at equator – that’s the way Nature behaves (see demo analogy)! Note: Radii difference varies by meters, due to seasonal shifts of mass, tides, earthquakes …

3. Day Of Demos(Outline of Lecture 10) Conflict over age of earth continues: words only Major digression into advances in physics that allowed resolution of conflict: - Interaction of light with matter - Structure and behavior of matter

4. Irresistible Force & Immovable Body During last third 19th century, controversy on earth’s age simmered as quotations on following slides imply Neither side yielded: Kelvin remained (mostly) irresistible force and geologists remained immovable body How could conflict be resolved?

5. Lord Kelvin on Lord Kelvin It seems, therefore, on the whole most probable that the sun has not illuminated the earth for 100 million years, and almost certain that he (sic!) has not done so for 500 million years. As for the future, we may say, with equal certainty, that inhabitants of the earth cannot continue to enjoy the light and heat essential to their life, for many million years longer, unless sources now unknown to us are prepared in the great storehouse of creation. 1862

6. One Famous Comment on Kelvin’s Calculation of Age of Earth Mathematics may be compared to a mill of exquisite workmanship, which grinds you stuff of any degree of fineness; but, nevertheless, what you get out depends on what you put in; and as the grandest mill in the world will not extract wheat-flour from peascods, so pages of formulae will not get a definite result out of loose data. Thomas Huxley, 1869

7. Geologist’s Reaction To Lord Kelvin’s Claims On Earth’s Age The fascinating impressiveness of rigorous mathematical analyses, with its atmosphere of precision and elegance, should not blind us to the defects of the premises that condition the whole process. There is perhaps no beguilement more insidious and dangerous than an elaborate and elegant mathematical process built upon on unfortified premises. Chamberlain, 1899

8. Reaction to Kelvin’s Results (Concluded) Is present knowledge relative to the behavior of matter under such extraordinary conditions as obtain in the interior of the sun sufficiently exhaustive to warrant the assertion that no unrecognized sources of heat reside there? What the internal constitution of the atoms may be is yet an open question. It is not improbable that they are complex organizations and the seats of enormous energies. Chamberlain, 1899

9. Kelvin’s Clever Caveat Kelvin was very sure of himself, rather arrogant in fact, but did leave door ajar with his comment in last sentence Matters remained in unresolved state (geologists versus physicists) for about half century. We largely skip this interregnum, save for preceding quotes and for laying groundwork for conflict resolution via course’s largest digression, starting with perhaps humanity’s most important advance in knowledge of how world works: probing structure of matter, its behavior, and its interaction with light

10. Unity Of Science & Resolution Of Age Conflict Back in physics laboratory, amazing properties of Nature were being unfolded, building on earlier work. We now discuss them, first for light then for matter Very fruitful - but long! – digression (a little repetitive…)

11. Interaction Of Light With Matter Back to Newton: He discovered, c. 1666, that white light composed of colors (see demo); tested his conclusion in two clever ways (listen and ye shall hear) Fraunhofer (1813) re-discovered, and elaborated upon, decade earlier discovery of dark lines in solar spectrum (see next two slides)

12. Joseph von Fraunhofer (1787-1826)

13. Spectrum Of Sun ShowingFraunhofer Lines

14. Emission Lines Scientists discovered first half 19th century: elements subjected to electric “discharges” gave off light, which showed line structure – emission lines, unique for each element (see demo; use gratings distributed for purpose) See next slide for both absorption and emission lines for hydrogen. Note match. To repeat: spectra provide unique fingerprints for elements

15. Emission And Absorption Spectra OfHydrogen (Angstrom Units: 10-8 cm)

16. Complications Uncovering structure of matter, radioactivity, and x-rays were tightly intermingled How to tell story coherently? Not obvious We chose order: structure of matter, x-rays, and radioactivity (still have minor ordering problems within topics)

17. What Is Structure Of Matter? Know different kinds of matter, called elements, from chemistry. But what is their structure? Democritus, c. 480 – 390 BCE, credited with being first to postulate existence of smallest particles of matter not further divisible How do our observations give us clue about structure?

18. Simple Experiments Liquids, like water, seem incompressible. Yet pour in colored dye or sugar and it spreads quickly throughout liquid (see demos). How possible? Liquid penetrable, made up of smaller parts with space between Brownian motion (discovered 1827; analyzed 1905 by Albert Einstein): Put small particles in water and they are moved around (see next slide for Brown and see demo). What’s pushing them around? Smaller parts

19. Robert Brown (1773-1858)

20. In Search Of Atoms: Truly Fundamental Detective Story Resolving riddle of atom (basic questions to answer): • What are structures of atoms? • How do atoms behave [e.g., how do we account for their unique “signatures” signed through frequencies of spectral lines they emit (or absorb) from “white” light, as we demonstrated with hydrogen and other elements]?

21. Rapid Unveiling Of World Of Small: Probing Atoms (c.1890 - c. 1910) Remarkable new tools were being discovered and developed, and their properties explored in physics laboratories, soon resoundingly defeating then prevalent notion that nothing new and important was left to discover about Nature at its fundamental level

22. Discovery Of Electron Equipment used: Crookes tube (see example in class and next slide) Conditions: High voltage created between cathode and anode (see tube); high vacuum created within tube (~10-7 atmospheres) Procedure: Studied emerging (“beta”) rays by changing ray trajectories with electric and magnetic effects Result: Particle (“electron”) had ~ 1/2,000 mass of hydrogen atom

23. Crookes Tube

24. J.J. Thomson’s Model Of Atom Based on discovery of electron, Thomson devised “plum pudding” model of atom: distribution of negatively charged electrons (plums) in substance of pudding (uniform distribution of positive charge); atom, overall, was neutral: no net charge Model had no explanatory power; e.g., incapable of explaining spectra, even of hydrogen – presumably simplest of all

25. New Models And New Experiments: Intertwining Of Science And Technology In Operation Concept of scattering experiments led to Ernest Rutherford’s path-breaking determination of structure of atom Result proved Thomson’s raisin- pudding model of atom sorely needed replacement (foreshadowing)

26. Scattering Experiments Scattering experiments involve particles being deliberately aimed, usually at high speed, at other particles. Higher energy of collision more deeply probe structure of colliding particles Rutherford pioneered such experiments in original deduction of structure of atoms. In probes of ever smaller constituents of matter, scattering experiments at ever higher energies of collision continue to dominate. (Why?) Discovery of Higgs boson is latest prominent example

27. Ernest Rutherford’s Experiment Performed clever experiment of type discussed in previous slide. His group, in particular Geiger and Marsden, directed fast-moving “alpha” particles head-on into thin gold foil (see next slide)

28. Discovery Of Atomic Nucleus Equipment used: Source of alpha (positively charged) particles Procedure: Directed stream of particles towards thin gold foil (next slide) and observed what happened via particle detector (human at first) that could be swiveled around foil (see next slide plus one) Observed: Most particles passed right through, hardly changed direction; some few directed back at or (relatively) near source Conclusion by Rutherford, after careful analysis: All positive charge of gold atom resides in central, very small nucleus (<10-4 diameter of whole atom!). Plum-pudding model needed to be replaced

29. Schematic Of Experimental Set-up

30. Detection Of Scattered Alpha Particles(View From Above)

31. Rutherford On Atomic Nucleus “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge”

32. Ernest Rutherford (1871-1937)

33. Prelude To Bohr’s Model Of Atoms Atoms had positively charged, exceedingly small nuclei, with negatively charged electrons making up outer part of atom. How did it “work”? Classical physics predicted that such atoms would be unstable and soon disappear Enter Niels Bohr and his assumption-breaking model (see next slide).

34. NielsBohr’s (1913) Model Of Atom A miniature planetary system, but with very profound differences: - Electrons can move only in specific orbits, with nucleus at center - Electrons can (somehow!) “jump” from one orbit to another, either by absorbing or emitting photon (what is that?), with jump to lower energy orbit accompanying emission of photon to conserve energy, and similarly for absorption of photon (see next slide) - Quite inconsistent with “classical” theory, as noted (last slide)

35. Bohr’s Model Of Atom(Schematic; NOT To Scale)

36. Bohr’s Model Of Atom:Explanatory Power Bohr’s model of atom represented dramatic new approach. How well did it address key questions? Model appropriately incorporated results of scattering of alpha particles by gold foil Model yielded proper spectrum of lines observed for hydrogen atoms Model failed to predict spectra of atoms of heavier elements. Better model needed

37. Quantum Mechanics (In Name Only) Far better model of atom than Bohr’s appeared only dozen years later: quantum mechanics. It incorporates some features of Bohr’s model This model, which is “beyond scope of course” has proven amazingly accurate in its prediction of all atomic and molecular phenomena, even when tested at levels of one part in trillion! Note: Roughly speaking, molecule is collection of more than one atom bound together as unit by means we won’t explore