PowerPoint PresentationSlide 2Is There a Real (i.e. measurable) Distinction between General Relativity and the Newtonian Viewpoint?The Scientific Method at WorkThe 3 “Classic” Tests of General RelativityPrecession of Mercury’s OrbitMercury PrecessionDeflection of StarlightSlide 9Think of light as ants trying to go straightSlide 11Deflection of Starlight During an EclipseEddington’s Eclipse Expedition Experience, 1919As GR predicts, starlight is deflectedImprovements over TimeGravitational RedshiftLunar Laser RangingLunar Laser Ranging, continuedOther Consequences of GRLIGOLISA: A space-based grav. wave observatoryIs GR the last word on gravity?References and AssignmentsUCSDPhysics 10Spring 2008 2UCSDPhysics 10There is a big difference between the Newtonian and There is a big difference between the Newtonian and the Relativistic frameworks:the Relativistic frameworks:Newtonian:Rigid flat geometry, universal clocksGravitational force between objects“Magic” dependence on massRelativistic: Objects interact with distorted spacetime “Natural” reference frames are in free fallSpring 2008 3UCSDPhysics 10Is There a Is There a RealReal (i.e. measurable) Distinction between (i.e. measurable) Distinction between General Relativity and the Newtonian Viewpoint?General Relativity and the Newtonian Viewpoint?•Absolutely! Absolutely! •Discriminating among different contending theories Discriminating among different contending theories is one of the tasks of experimental physics.is one of the tasks of experimental physics.•Probing the basic foundations of gravity continues to Probing the basic foundations of gravity continues to be a forefront issue be a forefront issue •Was Einstein RightWas Einstein Right by Clifford Will is a nice by Clifford Will is a nice reference for this topic...reference for this topic...Spring 2008 4UCSDPhysics 10The Scientific Method at WorkThe Scientific Method at Work•Newtonian world view was challenged by GR Newtonian world view was challenged by GR •Both theories made concrete predictions for Both theories made concrete predictions for physical phenomenaphysical phenomena•Nature is the final arbiter – carry out experimentsNature is the final arbiter – carry out experimentsSpring 2008 5UCSDPhysics 10The 3 “Classic” Tests of General RelativityThe 3 “Classic” Tests of General Relativity•Precession of Mercury’s orbit Precession of Mercury’s orbit •Deflection of starlight (gravitational lensing)Deflection of starlight (gravitational lensing)•Gravitational RedshiftGravitational RedshiftSpring 2008 6UCSDPhysics 10Precession of Mercury’s OrbitPrecession of Mercury’s Orbit Newtonian General RelativitySpring 2008 7UCSDPhysics 10Mercury PrecessionMercury Precession•Known since 1850’s not to match Newtonian Known since 1850’s not to match Newtonian theorytheory–Perihelion precessed by 43 arcseconds per century•Would take 30,000 years to go full-circle•While putting finishing touches on GR in 1915, While putting finishing touches on GR in 1915, Einstein computed expected perihelion precession Einstein computed expected perihelion precession of Mercuryof Mercury–When he got out 43 arcsec/century, his heart fluttered!Spring 2008 8UCSDPhysics 10Deflection of StarlightDeflection of Starlight•Light is deflected by Light is deflected by gravitational fieldgravitational field–called “gravitational lensing”•Much like ball deflected by Much like ball deflected by divotdivotSpring 2008 9UCSDPhysics 10Spring 2008 10UCSDPhysics 10Think of light as ants Think of light as ants tryingtrying to go straight to go straightIn each case, the ants do their best to pick out thestraightest path they can. Unless space is flat, theydon’t stay on parallel lines forever, and eitherconverge or diverge.Spring 2008 11UCSDPhysics 10Deflection of StarlightDeflection of Starlight•During an eclipse, the sky around the sun is dark During an eclipse, the sky around the sun is dark enough to see distant stars. enough to see distant stars. •Stars close to the sun have their light deflected and Stars close to the sun have their light deflected and so appear at a shifted position (farther from sun)so appear at a shifted position (farther from sun)•Comparing stellar locations with and without the Comparing stellar locations with and without the presence of the sun along the line of sight allows presence of the sun along the line of sight allows for a measurement of the deflection of light rays.for a measurement of the deflection of light rays.Spring 2008 12UCSDPhysics 10Deflection of Starlight During an EclipseDeflection of Starlight During an EclipseIf deflection = 1.74 arcseconds General Relativity PredictionIf deflection = 0.87 arcseconds Newtonian PredictionIf deflection = 0.0 arcseconds Both wrong!!Seen at a distance of 4 km, a quarter (25 cents) spans about an arcsecond of angleSpring 2008 13UCSDPhysics 10Eddington’s Eclipse Expedition Experience, Eddington’s Eclipse Expedition Experience, 19191919•Eddington was a British astronomer (and arrogant!)Eddington was a British astronomer (and arrogant!)•Decided to go to Principe Island in the Gulf of GuineaDecided to go to Principe Island in the Gulf of Guinea•After months of drought, it was pouring rain on the day of After months of drought, it was pouring rain on the day of the eclipsethe eclipse•Clouds parted just in time, they took photographic plates Clouds parted just in time, they took photographic plates showing the location of stars around the limb of the sunshowing the location of stars around the limb of the sun•Analysis of the photographs back in the UK produced a Analysis of the photographs back in the UK produced a deflection in agreement with the GR predictiondeflection in agreement with the GR prediction•Gravitational Lensing is now a powerful tool in Gravitational Lensing is now a powerful tool in astrophysicsastrophysicsSpring 2008 14UCSDPhysics 10As GR predicts, starlight is deflectedAs GR predicts, starlight is deflectedSpring 2008 15UCSDPhysics 10Improvements over TimeImprovements over TimeSpring 2008 16UCSDPhysics 10Gravitational RedshiftGravitational Redshift•Clocks that are deeper in a gravitational potential Clocks that are deeper in a gravitational potential well (spacetime divot) run more slowly!well (spacetime divot) run more slowly!–A clock on earth’s surface runs 20 milliseconds slow