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Lecture 20

Lecture 20

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Laura Parker

Lecture 20Friday November 25 20111230 PMWatching something fall into a BHAnything that crosses the event horizon will never come backWe dont see anything cross the event horizon the last photon from the object will reach and we will never see it againDue to time dilation you dont see anything fall into the black holeLast photon signal are extremely red shifted to very long wavelengthsSmall black holes have stronger tidal forces Black holes are only a few km wide cant see even horizon due to their distance awayWe can only see materials close to the black hole Do black holes existWe cant see them directly so look for their gravitational effects on the surroundingsCandidatesBinary star systemCenter of galaxiesCentre of galaxies show the biggest black holesIf the stars are orbiting around nothing then this nothing must be a black hole as it has high massIn a binary system if we see a star being stripped of its material with xrays this is good evidence for black holesSupermassive BH in the center of galaxiesLook at stars near the galactic centerMeasure their speeds as they orbit the centerFrom the speeds calculate the mass of the central objectWith an
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