Announcements. Reading:

Transcription

1 Announcements Reading: Gregory, Chapter 27, to p. 576, pp. 581 to end - Erazim Kohak, A Philosophy of Personalism (coursepack) - Steven Weinberg, The more comprehensible the universe is the more pointless it seems (online) Online evaluations open Final Exam Thursday, Dec. 17 7:30am - 9:30am, NPB 1002.

2 Final exam Thursday, Dec. 17 7:30am - 9:30am, NPB 1002, aka here Comprehensive: covers entire course but with emphasis (2/3) on material since midterm Bring: ID, scratch paper (no calculator necessary). One cheat sheet with formulas allowed, handwritten front and back of 1 page Format: 30% mult. choice 40% short answer 30% essay (choice of 2) Review session 2 nd part of class Tues next week, + during reading week TBA

3 Last time Photoelectric effect: minimum energy required for 1 photon to remove electron from a metal minimum frequency for photon (Einstein 1905) Bohr atom model (1913) accounted for Balmer spectrum (set of discrete frequencies) of hydrogen gas de Broglie: a particle is a wave λ=h/p Schroedinger and Heisenberg (1920s) constructed general framework of quantum mechanics Born: wave function ψ has probabilistic interpretation: ψ(x,t) is probability of finding particle at x,t Wave-particle duality and role of observer in measurement

4 Bohr s atomic model was criticized by classical physicists, who believed 1. protons and electrons both had positive charge 2. atoms did not exist 3. the proton should orbit the electron, not vice versa 4. electrons did not exist 5. the atom would collapse because the accelerating electrons would radiate energy

5 In the film Bronowski said that Heisenberg's Uncertainty Principle was badly named. He said we should instead call it the Principle of 1. Certainty 2. Tolerance 3. Unknowability 4. Taste 5. Deviance

6 Guest Michael Shüring, UF Center for European Studies Worked at Max Planck Institute for the History of Science in Berlin and the Institute for the History of Science and Technology at the Deutsches Museum in Munich, and taught at the University of California, Berkeley.

7 Einstein theory of general relativity Einstein felt there should be a way to formulate laws of nature in every kind of frame, including accelerated ones He began with the Principle of Equivalence : There s no way to tell the difference between acceleration and gravity

8 Equivalence principle a Can t tell the difference Rocket far from any gravitating body (star, planet ), at rest. Acceleration=0, no gravity Rocket far from any gravitating body, accelerating a = g, no gravity Rocket at rest on Earth a=0, gravity

9 Equivalence principle cont d t a a a Rock falls according to you as though It were under the influence of gravity

10 Equivalence principle cont d t Rock thrown horizontally in frame of outside observer How rock looks to you in rocket

11 A strange consequence of equivalence t Light shone horizontally in frame of outside observer How light looks to you in rocket Light bends in accelerating frame light bends in gravitational field

12 Eddington 1919 expt. confirms bending of light Einstein theory of general relativity says any mass curves spacetime around it: Light travels on a geodesic the shortest path between 2 pts. on the curved surface

13 Cosmology in 20 th century

14 Measuring large distances Henrietta Leavitt: In 1908 studied Cepheid variables stars that oscillate in brightness. She found intrinsic relation between brightness and period compare stars with same period. If they are dimmer, then further away because brightness 1/r 2

15 Composition of Stars Cecelia Payne-Gaposchkin: Ph.D thesis (1925) showed that spectral lines observed in stars showed overwhelmingly evidence for hydrogen and helium, unlike Earth Hans Bethe 1939: shows energy of sun comes from nuclear fusion of hydrogen E=mc 2 Mass of fusion products less than inputs

16 Doppler shift/ Redshift Doppler shift (usually for sound) occurs when emmitter is moving with respect to the observer: change in apparent frequency Redshift is Doppler shift for light

17 Hubble s Discovery Edwin Hubble s observations of remote galaxies, and the redshift of their spectral lines (1924). Hubble noticed that the further away the galaxy, the greater the redshift of its spectral lines. This linear relationship is called Hubble s Law.

18 Hubble s Law v = H 0 d v = recessional velocity of the galaxy H 0 = Hubble constant* d = distance of galaxy to earth Galaxies are getting farther apart as time progresses, therefore the universe is expanding. Note 1/H 0 is a time meaning?

19 Hubble s law: data

20 Hubble s Constant Expansion rate measured using Type 1A Supernovae. The age of the universe can be derived from Hubble s constant by working backwards d 0 T 0 = 1 /H 0 Current measurements H 0 = 73 km/s*mpc, then T 0 = 13.4 billion years old (age of universe)

21 Einstein doesn t like it. All the solutions to E s equations which give Hubbard s law have singularities Einstein thought that singularities such as these indicated that there were important physical effects not accounted for in his equations. He also thought that the right answer would involve static behavior: large-scale structure should not change with time. He also saw how he could fix the field equation to eliminate singular and dynamic solutions: introduce an additional constant term, which became known as the cosmological constant, to represent the missing, unknown, physical effects.

22 Einstein gives up. The Universe is observed to be expanding; it is not static. Thus the real Universe may be described by one of the dynamic solutions to the original Einstein field equation. Thus there appeared to be no point in Einstein s cosmological constant, so he let it drop, calling it my greatest blunder. Thereafter he began trying to show that the singularities in the dynamic solutions simply wouldn t be realized. His effort resulted in the steady-state model of the Universe, which we ll describe later. Cosmological constant, will come back, though.

23 How the Universe Expands The space between galaxies expands, not the galaxies themselves Example: raisins in a loaf of bread. As the dough rises, the overall loaf of bread expands; the space between raisins increases but the raisins themselves do not expand.

24 Center of Universe? There is NO CENTER to the universe Expansion looks the same regardless of where you are in the universe. Every point appears to be the center of the expansion, therefore no point is the center. The universe is infinite.

25 Evidence for Expansion The light from remote galaxies and other objects is redshifted. This redshift is called cosmological redshift because it is caused by the expansion of the universe, not by the actual movement of the object (Doppler redshift).

26 Questions unanswered or unasked What was there before the big bang? What caused the big bang? Is the universe open or closed or marginal? What is the meaning of life? What is the airspeed of an unladen swallow? One of these questions will be answered on Thursday