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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
