Extracting Energy from a BH

•  Rotational energy stored in ergosphere – Can be extracted

•  Penrose process: – mass falls in, splits in two – Half falls in, half falls out

with more energy

Extracting Energy from a BH

•  Blandford-Znajek process: – Magnetic field threads ergosphere – Plasma travels along field lines, is accelerated

in ergosphere

Gravitational Waves

•  Gravitational radiation •  Binary inspiral •  Evidence for gravity waves •  Methods for direct detection

Gravitational Radiation

•  Gravity moves at the speed of light •  Change is how mass is arranged

communicated out at speed of light

Electromagnetic Radiation

•  When charge is accelerated, it produces light

Gravitational Radiation

•  Only one charge in gravity, so no dipole radiation

•  But, quadrupole moment can change and produce gravitational radiation

Gravitational Radiation

•  Only one charge in gravity, so no dipole radiation

•  But, quadrupole moment can change and produce gravitational radiation

•  Mass distribution must be asymetric

Sources of gravitational waves

•  Supernovae – asymmetric explosion

Sources of gravitational waves

•  Supernovae – asymmetric explosion •  Binary stars, neutron stars, black holes

Sources of gravitational waves

•  Supernovae – asymmetric explosion •  Binary stars, neutron stars, black holes •  Merging neutron stars, black holes

Sources of gravitational waves

•  Merging neutron stars

Sources of gravitational waves

•  Merging neutron stars

Sources of gravitational waves

•  Merging black holes

Sources of gravitational waves

•  Merging black holes

Gravitational Waves

•  Gravitational waves carry away energy and angular momentum

•  Eventually gravitational radiation will cause a binary to merge

Evidence for gravitational waves

•  Pulsar binaries •  Pulsar in orbit with a neutron star of white

dwarf •  Pulse period changes due to doppler shift as

it orbits

Evidence for gravitational waves

•  Pulsar binaries •  Pulsar in orbit with a neutron star of white

dwarf •  Pulse period changes due to doppler shift as

it orbits

•  Over time, orbit gets closer, orbital period decreases

Evidence for gravitational waves

•  PSR B1913+16

•  Discovered by Russell Hulse and Joseph Taylor in 1974

•  Won Nobel Prize in 1993

Direct detection

•  As gravitational waves goes by, is stretches space

•  Distance between points changes

Methods for direct detection

•  Need something of fixed size you can measure changes in

Methods for direct detection

•  Need something of fixed size you can measure changes in

•  Gravitational waves will set off vibrations in a solid mass

Methods for direct detection

•  Need something of fixed size you can measure changes in

•  Laser interferometer: –  2 beams of fixed length, at 90 degrees to each

other –  Interference pattern will change if length of

arms changes

LIGO

LIGO

•  2 detector – Louisiana and Washington –  3000 km apart

•  4 km arms

LISA

•  Space interferometer

LISA

LISA