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Gravitational Waves

Gravitational Waves. Ripples in spacetime Ex. Black Hole Binary Propagate like rotating “lawn sprinkler” Each bit of curvature flies radially outward Together they form a spiral pattern According to Einstein, curvature = gravity Gravitational Waves

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Gravitational Waves

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  1. Gravitational Waves • Ripples in spacetime • Ex. Black Hole Binary • Propagate like rotating “lawn sprinkler” • Each bit of curvature flies radially outward • Together they form a spiral pattern • According to Einstein, curvature = gravity • Gravitational Waves • Created whenever a massive object accelerates! • But incredibly weak! What makes two black holes spiral closer to one another and eventually merge?

  2. Gravitational Waves and the “No Hair” Theorem • Black Holes' orbits become closer and faster • Horizons touch and merge near speed of light • Single, dumbell-shaped horizon • Why is this not a possible final state for the system?

  3. Gravitational Waves and the “No Hair” Theorem • Black Holes' orbits become closer and faster • Horizons touch and merge near speed of light • Single, dumbell-shaped horizon • Why is this not a possible final state for the system? • Shape gives information about history • Violates the “No Hair” theorem Gravitational radiation pushes back on the protrusions, leaving a perfectly smooth horizon behind.

  4. Information in Gravitational Waves • No Hair Theorem only applies to the Event Horizon • Cannot tell whether black hole formed from: • Merger of two smaller holes • Direct implosion of an ordinary star, antimatter star, etc. • But, a record is kept by gravitational waves • Travel outward => we can detect them! • Intervening matter does not distort or absorb waves • Impervious to noise and degradation • What other types of information might gravitational waves encode?

  5. Information in Gravitational Waves (cont.) • Gravitational waves may contain information about: • Supernovae • Other mergers (Masses, locations, distances) • Big Bang origin of the universe • Pulsations of young neutron stars

  6. Tidal Effects of Gravitational Waves • Moon raises tides on Earth • Gravitational wave should have a similar effect • But, curvature due to Moon is static and unchanging • Only orientation changes • Gravitational waves propagate at speed of light, oscillating as they travel • How do the strengths of the moon’s tides compare with those caused by gravitational waves?

  7. Tidal Effects of Gravitational Waves • Moon raises tides on Earth • Gravitational wave should have a similar effect • But, curvature due to Moon is static and unchanging • Only orientation changes • Gravitational waves propagate at speed of light, oscillating as they travel • Tides raised by grav. waves are ~ 1014 smaller than those raised by the Moon (~ 1 meter) • Undectable on the Earth's turbulent oceans

  8. Tidal Effects (cont.) • Moon => tidal forces in all directions • Stretches longitudinally • Compresses in the transverse directions • Grav. Waves - no tides along direction of motion • Alternate stretching and squeezing along transverse directions (Up-Down) and (Front-Back)

  9. Detecting Gravitational Waves • Weakness of gravitational waves => difficult to detect • 1st Attempt: Weber Bar • How is the natural frequency of the bar important? • Benefits? • Drawbacks? Weber bar: 1 ton aluminum cylinder + piezoelectric crystals.

  10. Detecting Gravitational Waves • Weakness of gravitational waves => difficult to detect • 1st Attempt: Weber Bar • Resonance amplifies weak signals ☻ • Sensitive to only small range of frequencies X • Need lots of bars • Bigger Problem • Uncertainty Principle? Weber bar: 1 ton aluminum cylinder + piezoelectric crystals.

  11. Detecting Gravitational Waves • Weakness of gravitational waves => difficult to detect • 1st Attempt: Weber Bar • Resonance amplifies weak signals ☻ • Sensitive to only small range of frequencies X • Need lots of bars • Required accuracy => treat one ton bar as quantum object! • Measurement disturbance can mask signal. Weber bar: 1 ton aluminum cylinder + piezoelectric crystals.

  12. QND Measurements • Preserves signal, even in the presence of unavoidable kicks • Measure positions of vibrating ends “on resonance” • Amplitude disturbed, but • One period later, end returns to same position, unless disturbed by gravitational wave. • Still requires major advances and thousands of bars

  13. Laser Interferometer • Relies on interference of light • Benefits • Responds at all frequencies • 1,000 times longer => 1,000 times better sensitivity • No need for QND measurements • Can't scale up bar w/out changing natural frequency • Drawbacks • Precise laser alignment, power fluctuations • Outside vibrations 40-meter-long prototype.

  14. Laser Interferometer Gravitational-Wave Observatory • Two interferometers in Washington and Louisiana • Started taking data in 2003 • Link to LIGO site • Each detector has 4 km arms • Two needed for verification • Disregard false signals due to: • Power fluctuations • Mini-earthquakes, etc. • Enable detection from different orientations LIGO detector in Hanford, Washington.

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