Option H: Relativity H8 Evidence to support general relativity

1. Option H: RelativityH8 Evidence to support general relativity H.8.1 Outline an experiment for the bending of electromagnetic waves by a massive object. H.8.2 Describe gravitational lensing. H.8.3 Outline an experiment that provides evidence for gravitational red-shift. The Pound-Rebka experiment will suffice.

2. Option H: RelativityH8 Evidence to support general relativity Outline an experiment for the bending of electromagnetic waves by a massive object. ●In 1919, Sir Arthur Eddington conduct- ed the first experiment to determine the validity of Einstein’s General Theory of Relativity, published in 1915. ●General relativity’s principle of equivalence predicts the bending of light rays in a gravitational field. ●During a total eclipse, the moon blocks out the sun entirely on certain parts of the Earth to such an extent that you can see stars near the disk of the sun. ●In fact, stars that were behind the sun were expected to be seen, according to the general theory, because of the bending of light by its strong gravitational field.

3. Option H: RelativityH8 Evidence to support general relativity Outline an experiment for the bending of electromagnetic waves by a massive object. ●Here’s how the experiment worked: ●The sun, moon, Earth and star would all be lined up for an eclipse. ●The apparent position of the star would be measured during the eclipse. This position would be affected by the bending of light by the sun. ●The true position of the star would be determined 6 months after the apparent position of the star was recorded. Why 6 months? Apparent star Apparent position Actual star True position Earth /moon Sun

4. Option H: RelativityH8 Evidence to support general relativity Outline an experiment for the bending of electromagnetic waves by a massive object. ●Relativity predicted a deflection of 1.75 arc seconds for starlight passing close to the edge of the sun. ●Although Eddington’s results were in agreement with relativity, his uncertainty was comparable to the measurement itself. ●Furthermore, his results were disputed by another team doing the same thing in Brazil. FYI The Eddington-Einstein collaboration continued through the First World War, even though the two scientists were on opposite sides. The results of Eddington’s observations have sometimes been criticized as a “self-fulfilling” desire to prove general relativity correct.

5. Option H: RelativityH8 Evidence to support general relativity Describe gravitational lensing. ●If a sufficiently massive object is between an observer and a distant object, because of the bending of the light by gravity, the mass acts like a lens. ●A gravitational lens will present to the observer numerous images of the same distant object. EXAMPLE: Here are images of so-called Einstein rings (gravita- tional lens images).

6. This is an animation of gravitational lensing. ●The dark object doing the lensing is a massive black hole between us and the band of stars. ●The band of stars is a galaxy on edge, farther away than the black hole. ●Note the Einstein rings.

7. Option H: RelativityH8 Evidence to support general relativity Outline an experiment that provides evidence for gravitational red-shift. ●We have already done a sample problem modeled on the Pound-Rebka experiment. ●This experiment, conducted in the Jefferson Laboratory at Harvard in 1959, verified the gravitational red shift by detecting the change in frequency of gamma rays emitted by 57Fe. 22.6 m The “Tower”