1. The First Ever Detection of Gravity Waves
R. D. Gehrz
ASTRO 2001, Spring Semester 2016
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

2. The Minnesota LIGO Group
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University of Minnesota, ASTRO 2001, Spring 2016

3. Note that a spherically symmetric spinning object
Note that a spherically symmetric spinning object
produces no gravity waves.
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

4. Predictions of General Relativity
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University of Minnesota, ASTRO 2001, Spring 2016

5. University of Minnesota, ASTRO 2001, Spring 2016
Gravitational Waves
Gravitational waves are ripples in the curvature of spacetime that propagate as waves, travelling outward from the source.
Predicted in 1916 by Albert Einstein on the basis of his theory of general relativity
gravitational waves transport energy as gravitational radiation
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

6. University of Minnesota, ASTRO 2001, Spring 2016
Gravity Wave Theory
 is the flat space metric
g is the metric tensor that describes the curvature of space
h is the radiation field
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

7. Indirect Evidence for the Existence of Gravity Waves
Joseph Taylor and Russell Hulse in 1974:
The Decay rate of pulses from pulsars in
binary pulsars is consistent with loss of
orbital energy by gravitational radiation
1993 Nobel Prize in Physics
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

8. What are Prospects for the Direct Detection of Gravity Waves?
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University of Minnesota, ASTRO 2001, Spring 2016

9. Gravity Waves Have a Very Small Amplitude
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University of Minnesota, ASTRO 2001, Spring 2016

10. What Happens When a Gravitational Wave Passes by Earth?
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University of Minnesota, ASTRO 2001, Spring 2016

11. University of Minnesota, ASTRO 2001, Spring 2016
Early Experiments
Joseph Weber: 1960’s Experiments with large metal bars covered with strain gauges
Results were claimed but never confirmed
Sensitivity was way too low to detect predicted signals
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

12. University of Minnesota, ASTRO 2001, Spring 2016
LIGO
Laser Interferometer Gravitational-Wave Observatory (LIGO)
Two widely separated installations
Hanford Washington
Livingston, Louisiana
Operated in unison as a single observatory
See more at:
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

13. The LIGO Interferometers
Livingston, LA
Hanford, WA
4 Km legs
2000 miles apart (light travel time = 11 ms)
N/S Separation is 1100 miles (light travel time = 6 ms)
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

14. About LIGO’s Interferometer
Fringes:
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University of Minnesota, ASTRO 2001, Spring 2016

15. What Happens When a Gravitational Wave Passes a LIGO Interferometer?
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University of Minnesota, ASTRO 2001, Spring 2016

16. University of Minnesota, ASTRO 2001, Spring 2016
The Size of a Proton
Classical Radius of the proton:
rH = 𝑒 2 𝐻 𝑐 2 = x cm
LIGO sensitivity:
Mirror motions as small as ~ 1/500 rH !!!
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

17. University of Minnesota, ASTRO 2001, Spring 2016
The Data
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University of Minnesota, ASTRO 2001, Spring 2016

18. University of Minnesota, ASTRO 2001, Spring 2016
The Chirp Signal
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University of Minnesota, ASTRO 2001, Spring 2016

19. What We Think Produced the Signal
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University of Minnesota, ASTRO 2001, Spring 2016

20. What We Conclude from Models of the Chirp
The wave form of the chirp is consistent with
the in-spiral and merger of two massive black
holes in a binary system
The black hole masses are 36 M⊙ and 29 M⊙
The final black hole mass is 62 M⊙
3.0 M⊙c2 was radiated in gravitational waves
The event happened 1.3 Gyr ago
The event happened in the southern hemisphere
(7 ms delay, signal first detected in Louisiana)
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016

21. The Astrophysical Implications of this Detection
We now have a new window on the Universe.
Astrophysicists now have “ears” as well as eyes!
4/29/2019
University of Minnesota, ASTRO 2001, Spring 2016