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Black Holes

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Underlying principles of General Relativity The Equivalence Principle No difference between a steady acceleration and a gravitational field

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Gravity and Acceleration cannot be distinguished

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h V = a h/c

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h Gravitational field Equivalence principle – this situation should be the same

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Principe May 1919 Eddington tests General Relativity and spacetime curvature GR predicts light- bending of order 1 arcsecond near the limb of the Sun

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Lensing of distant galaxies by a foreground cluster

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QSO The Einstein cross

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Curved Space: A 2-dimensional analogy Flat space Angles of a triangle add up to 180 degrees Radius r Circumference of a circle is 2πr

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Positive and Negative Curvature Triangle angles >180 degrees Circle circumference < 2πr Triangle angles <180 degrees Circle circumference > 2πr

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The effects of curvature only become noticeable on scales comparable to the radius of curvature. Locally, space is flat.

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A geodesic – the “shortest possible path”** a body can take between two points in spacetime (with no external forces). Particles with mass follow timelike geodesics. Light follows “null” geodesics. ** This is actually the path that takes the maximum “proper” time. Space Time Spacelike Timelike Curved geodesic caused by acceleration OR gravity Matter tells space(time) how to curve Spacetime curvature tells matter how to move

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Mass (and energy, pressure, momentum) tell spacetime how to curve; Curved spacetime tells matter how to move A formidable problem to solve, except in symmetric cases – “chicken and egg”

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Curvature of space in spherical symmetry – e.g. around the Sun

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h V = (2ah) 1/2 Special Relativity A moving clock runs slow

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Observer ON TRAINObserver BY TRACKSIDE Speed of light is c=300,000 km/s t’ = 2d / c Width of carriage Is d meters Train speed v d vt/2 s t = 2s / c So t’ is smaller than t Observers don’t agree! Smaller by a factor Where v 2 /c 2 )

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h V = (2ah) 1/2 Special Relativity A moving ruler is shorter

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h Gravitational field According to the equivalence principle, this is the same as

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Curvature of space in spherical symmetry – e.g. around the Sun

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Spacetime curvature near a black hole

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A black hole forms when a mass is squashed inside it’s Schwarzschild Radius R S = 3 (M/Msun) km Time dilation factor 1/(1 – R S /r) 1/2 Becomes infinite when r=R S

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Remnant < 1.4 M Progenitor < 8 M White Dwarf Planetary Nebula A cooling C/O core, supported by quantum mechanics! Electron degeneracy pressure. The Chandrasekhar limit Cools forever – gravity loses!

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Progenitor > 8 M Black Hole – gravity wins! Supernova Neutron star, supported by quantum mechanics! Neutron degeneracy pressure. Cools forever – gravity loses! Remnant < 2.5 M Remnant > 2.5 M 20 km

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Black Holes in binary systems

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M 3 sin 3 i = 0.25 (M + m) 2 Period = 6 days Cygnus X-1 M > 5 Msun

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Ellipsoidal light curve variations Depend on mass ratio and orbit inclination

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BH mass Black hole mass 10 –15 x Msun Combine ellipsoidal model with radial velocity curve

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Spinning black holes – the Kerr metric

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Spaghettification A 10g stretching force felt at 3700 km (>R S ) from a 10 Msun black hole Force increases as 1/r 3

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Supermassive Black Holes Jets propelled by twisted magnetic field lines attached to gas spiralling around a central black hole

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Supermassive Black Hole in the Galactic Centre Mass is 4 millions times that of the Sun Schwarzschild radius 12 million km = 0.08 au

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Falling into a black hole

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