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Announcements

Announcements. Public evening lecture at 7:30 pm Dr. Caty Pilachowski: The Gemini Giants: Twin Telescopes in Chile and Hawaii. Lecture 22 General relativity V: Can we test it ?. Some open problems. How to deal with accelerations ? How to deal with gravity ?

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Announcements

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  1. Announcements • Public evening lecture at 7:30 pmDr. Caty Pilachowski: The Gemini Giants: Twin Telescopes in Chile and Hawaii Astronomy 201 Cosmology - Lecture 22

  2. Lecture 22General relativity V:Can we test it ? Astronomy 201 Cosmology - Lecture 22

  3. Some open problems • How to deal with accelerations ? • How to deal with gravity ? • Newton’s gravity acts instantaneously, i.e. it is inconsistent with special relativity’s conclusion that information cannot be communicated faster than the speed of light. • Distance is relative, so which distance to use in computing the gravitational force ? Astronomy 201 Cosmology - Lecture 22

  4. Strong equivalence principle The laws of physics are precisely the same in all inertial and freely falling frames, there is no experiment that can distinguish them. Weak equivalence principle The laws of mechanics are precisely the same in all inertial and freely falling frames. In particular, gravity is completely indistinguishable from any other acceleration. Astronomy 201 Cosmology - Lecture 22

  5. General relativity • Mass tells space how to curve • Space tells mass how to move Astronomy 201 Cosmology - Lecture 22

  6. Distribution of mass and energy in the universe (stress-energy tensor) Geometry of spacetime (Einstein tensor) The entire Universe in one line Astronomy 201 Cosmology - Lecture 22

  7. Checklist • How to deal with accelerations ? • How to deal with gravity ? • Newton’s gravity acts instantaneously, i.e. it is inconsistent with special relativity’s conclusion that information cannot be communicated faster than the speed of light. • Distance is relative, so which distance to use in computing the gravitational force ?     Astronomy 201 Cosmology - Lecture 22

  8. Some effects predicted by the theory of general relativity • gravity bends light • gravitational redshift • gravitational time dilation • gravitational length contraction • Problem: effects are small, in particular since the gravitational field of most astrophysical objects is weak Astronomy 201 Cosmology - Lecture 22

  9. general relativity: Boost factor • special relativity: Astronomy 201 Cosmology - Lecture 22

  10. First test: bending of light • Star light should be bend as it passes through the gravitational field of the Sun, i.e., it should be possible to see a star behind the Sun Astronomy 201 Cosmology - Lecture 22

  11. First test: bending of light • Star light should be bend as it passes through the gravitational field of the Sun, i.e., it should be possible to see a star behind the Sun • General relativity predicts an angle of 1.75”, twice as big as that predicted by Newtonian gravity • measured by Arthur Eddington in 1919. Key event for Einstein’s elevation to a celebrity. Astronomy 201 Cosmology - Lecture 22

  12. Test 2: Perihelion shift of Mercury • Planets do not move on perfect ellipses, but ellipses are precessing. This effect is due to the gravitational force exerted by the other planets Astronomy 201 Cosmology - Lecture 22

  13. Test 2: Perihelion shift of Mercury • Planets do not move on perfect ellipses, but ellipses are precessing. This effects is caused by the perturbing effect of the other planets gravitational field. • Mercury’s precession amounts to 5600” per century, but only 5557” can be explained by Newtonian gravity, leaves a discrepancy of 43” per century. • General relativity predicts exactly this additional precession Astronomy 201 Cosmology - Lecture 22

  14. Test 3: gravitational time dilation and redshift • Can be measured by experiments on Earth (challenging, but feasible) • Better: White Dwarfs(very compact objects; mass comparable to that of the Sun, radius comparable to that of the Earth), because they have a stronger gravitational field • Even better: Neutron Stars and Pulsars(very compact objects; mass comparable to that of the Sun, radius only 10-100 km), because they have a very strong gravitational field Astronomy 201 Cosmology - Lecture 22

  15. Test 4: Binary pulsar PSR 1913+16 • Pulsar: a rapidly rotating highly magnetized neutron star that emits radio pulses at regular intervals • Discovered by Bell and Hewish in 1967 • Nobel Prize in physics (1974) Astronomy 201 Cosmology - Lecture 22

  16. Test 4: Binary pulsar PSR 1913+16 • Pulsar: Astronomy 201 Cosmology - Lecture 22

  17. Test 4: Binary pulsar PSR 1913+16 • Binary pulsar: two pulsars orbiting each other • Orbital time: 7.75h • Discovered by Hulse and Taylor in 1974 • Nobel Prize in physics (1993) Astronomy 201 Cosmology - Lecture 22

  18. Test 4: Binary pulsar PSR 1913+16 • Precession: 4.2º per year Astronomy 201 Cosmology - Lecture 22

  19. Test 4: Binary pulsar PSR 1913+16 • Time delay: Clocks tick slower in strong gravitational fields Astronomy 201 Cosmology - Lecture 22

  20. Test 4: Binary pulsar PSR 1913+16 • Gravitational Waves: Orbital decay due to emission of gravitational radiation data points Prediction of GR Astronomy 201 Cosmology - Lecture 22

  21. Tests to come: Gravity Probe B Astronomy 201 Cosmology - Lecture 22

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