The sound of the Universe: The search for Gravitational Waves Giovanni Santostasi, Ph. D. Baton Rouge Community College, Baton Rouge, LA
constant velocity acceleration Newton and Einstein : theories of Space and Time Special Relativity = Space-Time constant velocity General Relativity = Geometry- Space-Time acceleration
Space and Time Unified Time and Space are not separated quantities but different aspects of a same reality (Space-Time continuum) Light unifies Space and Time Velocity of Light c=300,000 Km/s =3x10^8 m/s=6.7 x 10^8 miles/hour Relativity of the reference system Absoluteness of the laws of Physics
Equivalence Principle: the fundamental principle that unifies inertial and gravitational mass M_inertial=M_gravitational This simple experimental fact is the essential basis for Einsteins theory of Gravity: General Relativity
Acceleration It is not possible to distinguish between gravity and an uniformly accelerated system Gravity can be simulated by an accelerated system
The absence of gravity is equivalent to free fall The presence of gravity can be neutralized in a Reference system in free fall
The observer in free fall with the elevator doesnt see any change in the vertical position of the sphere In the meanwhile the observer on the ground sees an horizontal and vertical change in position and interprets the motion as a curved path
Also light can be bent Gravity = curvature in the Fabric of Space-Time As observed by the observer inside the elevator As observed by external observer The other way around If the acceleration is produced by gravity if acceleration is produced by rocket this is what it will be observed
The essential effects of gravity are of tideal nature The difference is that gravity has GLOBAL Geometric properties (locally just undistinguishable from accelerated frame)
Curvature of Space Time caused by the sun This is a quasi-static situation for what concerns space-time Matter tells space-time how to curve; the curvature tells to matter how to move Einstein s Equation G= G/c^4 T http://www.pbs.org/wgbh/nova/einstein/relativity/ani mations.html
Acceleration of Mass creates Gravitational Waves The waves travel at the velocity of light (3x10^8m/s) and the waves amplitude goes downs with distance
Gravitational radiation has 2 polarizations and the energy is emitted mostly in the quadrupole (football shape distribution of matter required) The wave arrives in the direction perpendicular to the circle Polarization (plus) + Polarization (cross) x
Sources of gravitational waves Supenovae Neutron Stars that rotate (or wobble in space) Coalescent Binary Systems of Black Holes and/or Neutron Stars Cosmic Background caused by the Big Bang
Resonant Bars Cylindrical Bars, typically made of alluminum (about 1 ton. ). They work on the principle of resonance, they are tuned at about 1000 Hz, the resonant frequency of neutron stars. The wave interacts with the bar and the motion is transmitted to a sophisticated microphone that transform the mechanical motion into an electrical impulse: this is our signal.
The noise Problem Noise is bigger than signal in the current detectors (we dont see anything than noise !). Noise at h=10^-20 Signal maybe at h=10^-21 or less Mathemathical tools to extract signal: Filtering. For continuous signals: Integration with long observation times. Sources of noise: How to control Seismic (suspension system) Thermal (low temperature) Eletronic (SQUID)
Interferometers Range of sensitivity on earth 10-1000 Hz In space 10^-4-1 Hz Mirror Semi-transparent Mirror 4 km Laser 10 Watts Photodetector Vacuum Pipes LIGO (USA, Louisiana & Washington) VIRGO (ITALY, Pisa) TAMA (JAPAN) GEO 600 (GERMANy, Postdam) LISA (NASA-ESA, In space, 2016)
Neutron stars Continuous sources. They rotate up to frequencies of 1000 Hz. To emit GW they have to be tri-axial (football shaped). The strain (h=deformation/length measured ) for a star with 3 axis is: They can also wobble: if axis of rotation doesnt coincide with symmetry axis. In this case star doesnt need to be a football to emit Gravitational Waves.
Burst sources: Supernovae and Coalescent Systems Explosion of Supernovae have to be asymmetric to radiate gravitationally. The neutron star that is left over after the explosion vibrate violently (~1000 Hz) Coalescent Systems: compact objects as black holes and neutrons stars. Binary Systems are very football shape like. They emit Gravity Waves so they loose energy. The system is inspiraling until it collides (in time scales of millions of years). Indirect evidence of GW: pulsar 1913+16 (Taylor and Hulse: 1993 Nobel Prize winners).
Cosmic Background The most ancient evidence of the birth of the Universe. Electromagnetic Background 300,000 years after the Big Bang (a young baby, 0.7 day old). Gravitational Background is a polaroid of the birth of the Universe (as it was born !) Gravity doesnt interact a lot with matter. We dont know what to expect but we have some vague ideas from other cosmological observations.
Conclusion: What can we learn from Gravitational Waves? Another, fundamental confirmation of General Relativity (Viva Einstein !) New window on the Universe. Radiation very different from EM and particles. Bulk Motion of mass. GW do not interact well with matter. We can probe very high density region of the universe as neutron stars and the core of black holes. Birth Cry of the Universe.