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The 2015 Oskar Klein Memorial Lecture
“Gravitational Waves: The Physics and Astrophysics of LIGO”
Friday, 27. May 2016
Audio-only-Recording as MP3-File (smallest possible size):
- Audio.mp3 (ca.35 Mb)
Video-Recording for any system with MP4-support:
- Video.mp4 (ca.365 Mb)
15:15 – 16:30
Friday 27. May 2016, from 15:15 to 16:30
Speaker : Kip S. Thorne (California Institute of Technology)
Gravitational waves are so radically different from electromagnetic waves that they are
likely to revolutionize our understanding of the universe. LIGO, the Laser Interferometer
Gravitational Wave Observatory, has recently opened up the first of four gravitational-wave
windows onto the universe (the high-frequency window); and over the coming two decades,
three more gravitational-wave windows will be opened.
The astrophysical phenomena that LIGO is likely to explore are remarkable: Already it is
exploring the collision and merger of spinning black holes and the resulting nonlinear
dynamics of curved, empty spacetime. LIGO is likely also to detect and explore spinning
neutron stars, collisions of neutron stars, black holes tearing neutron stars apart, the central
engines of gamma ray bursts, perhaps the cores of supernova explosions, and perhaps
vibrating cosmic strings (thought to have been produced by inflation of fundamental strings
in the earliest moments of our universe).
But most wonderful of all will be completely unexpected phenomena: big surprises.
The physics of LIGO is also remarkable: Gravitational waves stretch and squeeze the
separations of mirrors that hang from overhead supports at the ends of 4 kilometer "arms";
and those mirrors' motions are monitored using light beams and interferometry.
The wavelength of each light beam in LIGO gets stretched and squeezed by the same
fractional amount as the mirror separations, so how can LIGO possibly see the waves?
And the stretch and squeeze is 1/100 the diameter of a proton, or less — which means 10-11
of the wavelength of the light that is used to measure the stretch and squeeze. How can light
possibly reveal such tiny motions? The atoms of which the mirrors are made have sizes 10
million times greater than the stretch and squeeze, and they vibrate, thermally, with amplitudes
that are a million times larger than the stretch and squeeze; why doesn't this produce noise
that masks the waves' signals? Ambient seismic motions in the ground beneath LIGO are ten
million times greater than the waves' stretch and squeeze; why don't these seismic motions
mask the signals? When LIGO reaches its design sensitivity, several years from now, the
Heisenberg uncertainty principle, applied to LIGO's 40 kilogram mirrors, says that the very
act of measuring the mirror motions will perturb the mirrors so much that it may mask
the signal. How can this quantum behavior of human sized mirrors be controlled, so as to
preserve the signals?
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