Transcript Physics of LIGO, lecture 1a

Catching the Gravitational Waves
Dennis Ugolini, Trinity University
Bite of Science Session, TEP 2014
February 13, 2014
Gravity Misconception #1:
Mass Pulls on Mass
On Earth:
F  mg
In general:
Gm1m2
F
2
r
physicscentral.com/coloringbook
Digression: Mirages
not as hot air
hot air
blue sky
Your
1. brain
Light
interprets
travels by
light
fastest
as if possible
it had always
route been
2. Light
traveling
travels
in the
faster
direction
in hotter
it arrived
air
Gravitational Lensing
Image of
quasar
Real
quasar
Galaxy
“Einstein Cross”
The bending of light rays
Warped Space-Time
Einstein’s theory of general relativity says that smaller masses
travel toward larger masses, not because of an attractive force,
but because space is warped toward the larger object.
Gravity Misconception #2:
Instantaneous Action
Einstein also found that changes in the
curvature of space-time will propagate
outwards at the speed of light.
Gravity Misconception #3:
GR Explains Everything
Compact binary mergers
But if we can measure the ripples in space-time curvature
that reach Earth, we can learn about strong-field general
relativity experimentally.
Other Sources
 Supernovae?
 Dark matter?
 Dark energy?
 Background from Big
Bang?
 Who knows what else?
Why are We So Confident?
In 1975, Russell Hulse and Joseph Taylor were
cataloguing pulsars when they found something
unusual – a pulsar that periodically stopped!
Neutron Binary System
PSR 1913 + 16

~ 8 hr
17 / sec

How Big Are They?
 Size in terms of strain, h = ΔL/L
 Imagine two neutron stars, each with
the mass of our sun, but as big as San
Antonio’s Loop 410 freeway
 They are in the Virgo cluster of
galaxies, about 60 million light
years away
 They are about to collide, and are
spinning around each other 400 times
per second (!?!)
km
That’s REALLY Small!
L = 1.5 × 1011 m
Sun
ΔL = 10-10 m
Earth
h ~ 10-21 is equivalent to the distance from the Earth to the
Sun changing by the width of a single atom!
Interferometry
 Uses light interference to
measure length difference
between the two arms
 Each arm is a reflective
cavity, effectively
increasing arm length
The LIGO Project
LIGO: Laser Interferometer
Gravitational-Wave Observatory
 US project to build GW observatories
 Detection, followed by astronomy of
gravitational waves
 830 collaborators from dozens of
institutions
 Funded by US National Science
Foundation
LIGO-G1000214
SMU Physics Seminar, March 29, 2010
14
The LIGO Observatories
LIGO Hanford Observatory (LHO)
(4km and 2km in same vacuum)
LIGO Livingston Observatory (LLO)
Vacuum System
The LIGO beam tubes must be kept at one trillionth of atmospheric pressure!
Seismic
LIGO-G1000214
Internal thermal
SMU Physics Seminar, March 29, 2010
Shot noise and
pole frequency
17
Suspended Test Masses
Optics are 25 cm diameter, 10 cm thick, 10.7 kg, of high purity fused silica.
They must have less than 1 part per million absorption losses.
The optics are suspended to
attenuate seismic motion above
the pendulum frequency.
f  f0
Length Sensing and Control
 Each optic has five magnet and coil
assemblies (four on the back, one on the side)
 The magnet blocks light from
the LED, giving position
 Current through the coil
creates a magnetic field,
allowing mirror control
New Optic Suspensions
• 40 kg fused silica optics
• Quadruple suspension with reaction mass
• Last stage suspended by fused silica ribbons
The Need for Advanced LIGO
Initial LIGO
 Goal: factor of ten
improvement in sensitivity
at all frequencies
 x10 increase in sensitivity
= x1000 volume of sky
searched
E-LIGO
 Inspiral event rate from one
every few years to one
every few days!
 Resolution improved for
astronomy
 Transition began Oct. 2010
Future Timeline
S4
S5
Virgo
GEO
LIGO
’05 ’06 ’07 ’08 ’09 ’10 ’11 ’12 ’13 ’14 ’15 ’16
AstroWatch
VSR1
S6
VSR2 VSR3
Advanced
LIGO →
GEO-HF
Advanced
Virgo →
??
S7
S8
VSR4
VSR5
Interferometry Video
http://www.trinity.edu/physics/dugolini/
Interferometer/Interferometer.html
http://www.ligo.org
We have a magazine!
http://www.ligo.org/magazine