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