From Einstein to Gravitational Waves
and Beyond

Barry C Barish
Caltech/UC Riverside
12-Jan-2023
LIGO-G1602199

1

A Little History2

General Relativity and Gravitational Waves
Newton’s Theory ofGravity (1687)

Universal Gravity: force between
massive objects is directly proportional
to the product of their masses, andinversely proportional to the square of
LIGO-G1602199
the distance
between them.

Einstein’s Theory ofGravity (1915)

Space and Time are unified in afour dimensional

spacetime3

4

The Only Observed Problem with Newton’s Gravity
fixed in Einstein’s Theory

Mercury's elliptical path around the Sun. Perihelion shifts forward with eachpass. (Newton 532 arc-sec/century vs Observed 575 arc-sec/century)(1 arc-sec = 1/3600 degree).

5Einstein Explains WHY the apple falls!

Earth distorts spacetime
6Einstein Solves a Conceptual Problem with
Newton’s Theory of Gravity

In Newton’s Theory: “Instantaneous Action at a Distance”

It takes finite time for information
to travel from the sun to the earth7Einstein Theory
Makes a ‘New’ Prediction

EinsteinEddington"Not only is the universe stranger than weimagine, it is stranger than we can imagine.
Sir Arthur Eddington

8In Modern Astronomy: Gravitational Lensing

Einstein Cross

9

GPS: General Relativity in Everyday LifeSpecial Relativity(Satellites v = 14,000 km/hour“moving clocks tick more slowly”
Correction = - 7 microsec/day
General Relativity
Gravity: Satellites = 1/4 x Earth
Clocks faster = + 45 microsec/day

GPS Correction = + 38 microsec/day(Accuracy required ~ 30 nanoseconds
to give 10 meter resolution
10Einstein Predicted Gravitational Waves in 1916

• 1st publication indicating the existence of gravitational
waves by Einstein in 1916

• Contained errors relating wave amplitude to source motions

LIGO-G1602199•1918 paper corrected earlier errors (factor of 2), and it contains thequadrupole formula for radiating source
11Einstein’s Theory Contains Gravitational Waves
A necessary consequence ofSpecial Relativity with its finite
speed for information transfer

Gravitational waves come fromthe acceleration of masses andpropagate away from theirsources as a space-time
warpage at the speed of lightgravitational radiation
binary inspiralofcompact objects
12

The Chapel Hill Conference
Could the waves be a coordinate effect only, with nophysical reality? Einstein didn’t live long enough tolearn the answer.
In January 1957, the U.S. Air Force sponsored theConference on the Role of Gravitation in Physics, a.k.a.
the Chapel Hill Conference, a.k.a. GR1.
The organizers were Bryce and Cecile DeWitt. 44 of theworld’s leading relativists attended.
The “gravitational wave problem” was solved there,
and the quest to detect gravitational waves was born.
(Pirani, Feynman and Babson)
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Agreement: Gravitational Waves are Real•

Felix Pirani presentation: relative
acceleraton of particle pairs can beassociated with the Riemann tensor. Theinterpretation of the attendees was that
non-zero components of the Riemanntensor were due to gravitational waves.

•

Sticky bead argument (Feynman)o Gravitational waves can transfer
energy?

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How to Detect Gravitational Waves

15

Now the problem is for experimentalists
Try it in your own lab!
M = 1000 kg
R=1m
f = 1000 Hz
r = 300 m

1000 kgh ~ 10-35-24-April-2018

1000 kg16

Astrophysical Sources
signatures•

Compact binary inspiral: “chirps”– NS-NS waveforms are well described– BH-BH need better waveforms– search technique: matched templates•Supernovae / GRBs:“bursts”– burst signals in coincidence with signals inelectromagnetic radiation– prompt alarm (~ one hour) with neutrino detectors•

Pulsars in our galaxy:“periodic”– search for observed neutron stars (frequency,doppler shift)– all sky search (computing challenge)– r-modes•

Cosmological Signal “stochastic background”
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Gravitational Waves• Ripples of spacetime that stretch andcompress spacetime itselfL• The amplitude of the wave is h ≈ 10-21• Change the distance between masses
that are free to move by ΔL = h x L• Spacetime is “stiff” so changes indistance are very small

ΔL18

Suspended Mass Interferometry~~

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Credit: LIGO/T. Pyle

Interferometry – The scheme

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LIGO: Interferometry
photodiode21

Many innovations to make theinstrument so sensitive

Passive / Active Multi-Stage Isolation
Advanced LIGO

22

LIGOLIGO
Sites
SitingProject Approved 1994

23

LIGO Interferometers

Hanford, WALivingston, LA

24

What Limits
LIGO Sensitivity?
 Seismic noise limits lowfrequencies
 Thermal Noise limits middle
frequencies
 Quantum nature of light(Shot Noise) limits highfrequencies
 Technical issues - alignment,electronics, acoustics, etc
limit us before we reachthese design goals-25Evolution of LIGO Sensitivity

26

Advanced LIGO GOALS
At ~40 Hz,
Factor~100
improvem
enth

Broadband,Factor ~3
improvement

Betterseismic
isolation

Better test
massesLIGO-G1602199
andsuspension

Higher
power
laser

27

Gravitational Wave Detections

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Compact Binary Collisions– Neutron Star – Neutron Star• waveforms are well described– Black Hole – Black Hole
• Numerical Relativity waveforms– Search: matched templates“chirps”

29The Discovery of Gravitational Waves
Black Hole Merger: GW150914

30

Black Hole Merger Parameters for GW150914


Use numerical simulations fits of black holemerger to determine parameters; determine totalenergy radiated in gravitational waves is 3.0±0.5
Mo c2 . The system reached a peak ~3.6 x1056
ergs, and the spin of the final black hole < 0.7(not maximal spin)

Phys. Rev. Lett. 116, 061102 (2016)

Phys. Rev. Lett. 116, 241102 (2016)31

Testing General Relativity – Dispersion Term?
• In GR, there is no dispersion!
Add dispersion term of form
𝐸𝐸 2 = p2c2 +Apαcα, α ≥ 0(E, p are energy, momenturm of GW, A isamplitude of dispersion)• Plot shows 90% upper bounds• Limit on graviton mass
Mg ≤ 7.7 × 10−23 eV/c2• Null tests to quantify genericdeviations from GR
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Observed Binary Mergers to Date

33

34

Searching for Electromagnetic Counterparts

35Fermi Satellite GRB detection 2 seconds later

170817A” Astrophys. J. Lett., 848:L13, (2017)

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Light Curves

38

NS Mergers are Incredible Gold Factories

LIGO observed Neutron Star
Merger produced~ 100 Earth Masses of Gold

39

The Pandemic Pause

40

41

O4 will include the two LIGO Observatories, the Virgo Observatory,
and the KAGRA Observatory
 the first LIGO-Virgo-KAGRA 4-detector runTarget sensitivities (binary neutron star inspiral range):» LIGO: 160-190 Mpc (520 - 620 Mly)
» Virgo: 90 MPc (200 Mly)
» KAGRA: 25 - 130 MPc (80 - 425 Mly)
A 2X to 3X increase in GW event rateO4 will start no earlier than March 2023



O4 run duration is still not set, but likely somewhere in the 12 – 18month range
42

O4, N=170(simulated)O3, N=56

LIGO and Virgo are currently engaged in an extended upgrade
period in advance of the next O4 observing run» Advanced LIGO ‘A+’ and Advanced Virgo + upgrade program
will implement frequency-dependent squeezing to reduce
low frequency noise» Also, LIGO will replace many of the primary ‘test mass mirrors

O1, N=3

O2, N=8O4 Run and Upgrade Plans

Astrophysical Sourcessignatures•

Compact binary inspiral: “chirps”– NS-NS waveforms are well described– BH-BH need better waveforms– search technique: matched templates•Supernovae / GRBs:“bursts”– burst signals in coincidence with signals inelectromagnetic radiation– prompt alarm (~ one hour) with neutrino detectors•

Pulsars in our galaxy:“periodic”– search for observed neutron stars (frequency,doppler shift)– all sky search (computing challenge)– r-modes•

Cosmological Signal “stochastic background”
43

LISA: Laser Interferometer Space Array
ThreeInterferometers
2.5 106 km arms

44

Gravitational Wave Frequency Coverage

45Thanks!!

LIGO Hanford46