Gravitational WavesChristina McConville

Thursday, April 28 • Modern Physics PHYS 3305 • Southern Methodist University

OutlineI. General Relativity

● Predictions

II. Gravitational waves

● Characteristics

● Sources

III. LIGO

● Interferometry

IV. Detection

V. Conclusions

VI. Significance

1

Timeline

NSF asks

MIT,

Caltech to

lead LIGO

project

1984

LIGO

receives

funding

1994

Search

begins

2002

Advanced

LIGO

begins

2015

1962

Paper

published

on how to use

interferometry

to detect g-

waves

GR

1915

Feynman: g-

waves are

detectable

1957

LIGO

conceived

1968

1916

Albert Einstein

predicts existence

of g-waves

1970s

Indirect

confirmation

of g-waves

1997

LSC &

GWIC

formed

2004

Upgrade

approved

2016

Detection

announced

2

General Relativity● Albert Einstein, 1915

● Massive objects warp space and time

● Spacetime is a “fabric”

● Newtonian gravity failed to make

accurate predictions in all cases

● GR transformed idea of gravity

3

http://sci.esa.int/science-e-media/img/72/ESA_LISA-Pathfinder_spacetime_curvature_above_orig.jpg

Einstein’s Predictions● Direction of light propagation should be changed in a gravitational field (lensing)

● Light coming from a strong gravitational field should have a wavelength shifted to a larger

value (redshift)

● Gravitational fields can have waves that carry energy (gravitational waves)

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Verifications of GR

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Solar eclipse of 1919

Principle of equivalence

5

Verifications of GR

Gravitational lensingPerihelion precession of Mercury

Gravitational redshifting Gravitational waves 6

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What is a Gravitational Wave?Gravitational waves: “ripples in the

curvature of spacetime”

● Transport energy as

gravitational radiation

● Waves radiated at speed of light

● Changes in spacetime curvature

caused by accelerating massive

objects

● Penetrate regions of space that

electromagnetic waves cannot

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http://daily.jstor.org/wp-content/uploads/2016/02/GravitationalWaves_1050x700.jpg

Gravitational WavesSources:

● Continuous (single spinning object)

○ Neutron star

● Compact binary inspiral (orbiting pairs of massive and dense objects)

○ Binary neutron star

○ Binary black hole

○ Neutron star-black hole binary

● Stochastic (small gravitational waves from every direction mixed at random)

● Burst (short-duration from unknown or unanticipated sources)

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A Brief Lesson: Interferometry● Invented in the late 19th

century by Albert Michelson

● Michelson-Morley Experiment

(1887)

● Light can make very small

measurements

○ Use superimposed beams of light

to create interference patterns

● Laser, beam splitter, mirrors,

photodetector

9

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Gravitational Waves● Stretching/compressing of arms

from gravitational waves causes

interference in light waves

● h (fraction of

stretching/compressing) of

gravitational waves is ~10

-20

h ~0.5 (50%)

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http://www.ligo.org/images/faq-interferometer.gif

https://www.youtube.com/watch?v=U_hLM1WPDqM

Laser Interferometer Gravitational-Wave Observatory (LIGO)

LIGO Gravitational wave detector in Livingston, Louisiana

11

LIGO● Two interferometers separated by

1,856 miles

○ LIGO Livingston (LA)

○ LIGO Hanford (WA)

● Primary research centers at Caltech

and MIT

● Two 4-km vacuum chambers

(arms)

● Laser split into two identical beams,

reflected back by mirrors at end of

arms

● Arrival at same time causes

cancellation12

https://www.ligo.caltech.edu/system/pages/images/27/page/basic_ifo_diagram.jpg?1432340449

LIGO● Initial LIGO (2002-2010) did not detect any gravitational waves

● Increased sensitivity and power of instruments: Advanced LIGO

● Distance detectors are able to look out increased from 15 Mpc (50 million light

years) to 200 Mpc

● Factor of over 10 times increase in distance → factor of over 1000 times more

volume

● Can measure gravitational wave strain of 1 part in 10

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● Laser power increased from 10 W to 200 W

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DetectionGW150914

● Detects a change in distance in the

arm 1/10,000th the width of a proton

● Detector validation: test response to

environmental disturbances

(generated magnetic, radio-

frequency, acoustic, and vibration

excitations)

● 16 days of coincident observations

between both detectors

14

“Observation of Gravitational Waves from a Binary Black Hole Merger” Published in PRL116, 061102 (2016).

The “Chirp”

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Conclusion

“The LIGO detectors have observed gravitational waves from the merger of two

stellar-mass black holes. The detected waveform matches the predictions of general

relativity for the inspiral and merger of a pair of black holes and the ringdown of the

resulting single black hole. These observations demonstrate the existence of binary

stellar-mass black hole systems. This is the first direct detection of gravitational waves

and the first observation of a binary black hole merger.”

-”Observation of Gravitational Waves from a Binary Black Hole Merger”

Published in PRL 116, 061102 (2016).

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Significance● Previously used electromagnetic radiation to observe phenomena

● Gravitational waves give a different, complementary perspective of the universe

● Further confirms General Relativity

● Increase in instrument precision promotes further discovery

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ReferencesAbout. (n.d.). In Laser Interferometer Gravitational-Wave Observatory. Retrieved from LIGO Caltech.

Facilities. (n.d.). In Laser Interferometer Gravitational-Wave Observatory. Retrieved from LIGO Caltech.

Gravitational Wave. (n.d.). In Wikipedia. Retrieved April 25, 2016, from Wikipedia

Harris, R. (2008). Modern Physics. San Francisco: Pearson/Addison Wesley.

Introduction to General Relativity. (n.d.). In Wikipedia. Retrieved April 25, 2016, from Wikipedia.

“Observation of Gravitational Waves from a Binary Black Hole Merger”

Published in PRL116, 061102 (2016).

Sources and Types of Gravitational Waves. (n.d.). In Laser Interferometer Gravitational-Wave Observatory. Retrieved from LIGO

Caltech

Tests of General Relativity. (n.d.). In Wikipedia. Retrieved April 25, 2016, from Wikipedia.

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Binary Star Systems● Evidence first deduced through motion

of double neutron star system

● Pulsar emits radio frequencies as it

rotates

● Over time frequency shortened, stars

lost energy

● Agreed closely with prediction of energy

loss from gravitational waves

● Hulse and Taylor awarded Nobel Prize

in Physics (1993) http://www.atnf.csiro.au/outreach//education/everyone/radio-astronomy/whatis_images/binary_pulsars.jpg

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https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.116.061102

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Data

https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.116.061102

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Coincident Observations

https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.116.06110222

Other Detectors

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LIGO one of many ground-based detectors:

● GEO - British/German (600 m)

● VIRGO - Italian/French (3 km)

● TAMA - Japanese (300m)

● AIGO - Australian (80m)

Space-based proposed detector: LISA - NASA/ESA

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https://xkcd.com/1642/

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