14 Black Holes and General Relativity Stuff that warps your mind

14 Goals Why should light be affected by gravity? Why should time be affected by gravity? What proof is there that they are?

14 Are Black Holes Black? Why should light be affected by gravity? Newton - “Whenever you have mass you have gravity.” But light has no mass! Einstein’s General Theory of Relativity (1915): –New way of looking at gravity. –Allows gravitational effects on light.

14 General Relativity 2 Main Postulates: 1. The speed of light is always c. Thou shalt not add your speed to the speed of light!

14 General Relativity 2. Accelerating reference frames are indistinguishable from a gravitational force. The Star Tours ride at Disneyland is awesome!

14 Gravity Bends Light

14 Is there Proof? Scientific Method: 1.Hypothesis: predict gravity bends light. 2.Test: The Sun has a lot of gravity and should deflect star light.

14 Solar Eclipse of 1919 Hard to see stars near the Sun. Can see stars near the Sun during an eclipse. Can predict time and position of Sun during eclipse years in advance. Measure the positions of stars 6 months before when they are up at night. During eclipse, measure their positions again. SOHO spacecraft Result: Theory of Relativity confirmed!

14 Gravitational Lensing If geometry right: gravity cause multiple images.

14 Einstein Lenses Copyright – J. Rhoads, WIYN Copyright – Charles R Evans (UNC) Einstein Ring Copyright – L. J. King (U. Manchester)

14 Copyright – A. Fruchter (STScI) Abell 2218

14 Space is Curved Einstein said to picture gravity as a warp in space. –Sagan : A “pucker” in fabric of space. Kepler’s Laws explained by movement around “puckers.” Everything moving is affected, regardless of mass.

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14 Gravity makes time slow S F S F

14 Proof? GPS satellites use atomic clocks. Position determined by timing radio signals. In orbit F g = ¼ F g at Earth surface. Clock on satellite runs fast by 45  sec per day. D = v t D = c t  D = c  t  D = 3 x 10 5 km/s * 45 x s/day  D = 13.5 km/day

14 Concept Test We think the Theory of General Relativity is correct because: a.We have performed thought experiments showing that accelerating rockets bend light and slow time. b.We know that the speed of light is constant and that what happens in an accelerating frame is also what happens in a gravitational field. c.We observe phenomena that confirm the results of our thought experiments. d.Our thought experiments confirm the predictions made by General Relativity. e.Einstein is always right; that’s why he’s called Einstein.

14 Homework #14 For Friday: Read articles