1) Gravitational waves are ripples in spacetime caused by accelerating massive objects like neutron stars or black holes. 2) Einstein predicted gravitational waves in his theory of general relativity and scientists have been working to detect them using detectors like LIGO. 3) In 2016, LIGO directly detected gravitational waves for the first time, confirming another prediction of general relativity and opening an new window to observe the universe.

1. Gravitational wave
Safal Bhattarai
Kishor phuyal
Anubhab Regmi
Debu magar
Manish Poudel
Alir Tamang

2. What is gravity & who proposed it?
Newton
• “Action at a distance”
• Newton’s Law describes effect of gravity but does explain it

3. Who explained Gravity?
• Einstein’s General theory of relativity :
• Gravity is a manifestation of curvature of 4-dimensional (3 space + 1
time) space-time produced by matter.
• Freely falling objects follow the local background curvature
• If the curvature is weak, it produces the familiar Newtonian gravity:
• F = G M1.M2/r2

4. Gravitational-waves
• When the curvature varies rapidly due to motion of the object(s),
curvature ripples are produced. These ripples of the space-time are
Gravitational-waves.
• Gravitational-waves propagate at the speed of light.

5. General Relativity
• Einstein predicted that objects cause the fabric of space-time around
them to curve.
• Moving objects should therefore create ripples in space-time
• Einstein predicted that the more massive the object the larger the
gracitatioal waves it ould create.

6. Tests of General Relativity
• Einstein’s formulas explained Mercury’s perihelion changes.
• Observations of starlight passing the sun during a solar eclipse in
1919 confirmed that light is bent by the curved space-time
surrounding the sun.

7. Electromagnetic (versus) Gravitational-
waves
• EM waves are produced by accelerated charges, whereas
GWs are produced by accelerated “masses”.
• EM waves propogate through space-time, GWs are
oscillations of space-time itself.
• Typical frequencies of EM waves range from (107 Hz –
1020 Hz) whereas GW frequencies range from ~ (10-9 Hz
– 104 Hz). They are more like sound waves.

8. Cassini Test
• Test took place in September 2002, when the sun was between the
cassini spacecraft and Earth
• Cassini confirmed the theory with 50x greater precision than previous
tests.
• Researchers observed a frequescy shift of radio waves traveling to
and from Cassisn when passed near the sun.
• The extra distance that the radio waves traveled was measured bu th
time were delayed in reaching Earth.

9. What exactly are gravitational waves?
• Ripples pr ascillations I space-time itself, unlike
electromagnetic radiation, which passes through space-time
• They travel at the speed of light.
• Their strength weakens proportionally to the distance
travelled from the source.
• By the time the waves reach Earth, they are weak and difficult
to detect – comparable to detectiong a change the size of an
atom in the distance between the sun and the earth.
• Neutrons star : an extermly dense burnt-out core left behind
after a star explodes.
• Can have as much mass as the sun in a smaller space ( a few
miles wide).

10. • Imagine two neutron stars orbiting each other. Their motion couses
space time to be ‘stirred’ and gravitational waves are generated and
send outward from the stars.
• In 1974, Joseph Taylor and Russel Hulse found a pair of neutron stars,
one of the starts was found to be a pulsar.
• After 20 years of measuring these pulses, the shift in their timing
indicated that the pulsar’s orbital period decreased by 75µ per year
• Taylor and Hulse won the Nobel prize in 1993 for their work.
• More binary pulsar systems have since been detected.

11. What else causes gravitational waves?
• Supernovae and star’s collapse into neutron stars
• Two black holes colliding or orbiting each other
• Neutron star orbitind a black hole
• Rotating neutron star- continuous sources of waves
• Collinging galaxies
• Stochastis background of gravitational waves emitted in the early
stages of the universe- comparable to microwave background.
• Other new and exciting objects.

12. Detection of Gravitational-waves
• Ground based detectors:
• LIGO (U.S.A), VIRGO (Italy), GEO (Germany), TAMA (Japan),
AURIGA (Australia)
• (Proposed) Space-based detectors:
• LISA (NASA-ESA)

13. • LIGO is a collaboration between the California Institute of
Technology (Caltech) and the Massachusetts Insitute of
Technology(MIT)
• It is funded by the National Science Foundation.
• It will function as a national resource for both physics and
astrophysics, and universities and institutions around the would
will be involved.
• Two locations:

14. Goals of LIGO
• Prove the existence of gravitational waves by direct measurements
• Confirm that gravitational waves travel at the speed of light
• Verify that gravitational waves cause disturbances of predicted
amounts in the matter thay pass through.
• Learn more about black hles by provig their existence and study their
behavior.
• Gain other knowledge about the universe, including more information
about supernovae and the big bang.

15. Other Detectors
• Allegro (Us)
• Auriga (Italy)
• Explorer (Italy)
• Niobe(Australia)
• MiniGrail(Denmark)
• Graviton

16. Properties of Gravitational waves
• They do not shows reflection or refraction.
• The gravitational waves are tranverse waves
• They show polarization like electromagnetic waves( For eg. Light)
• A powerful properties of gravitational wave is that the universe is
essentially transparent to them. This maked gravitational wwaves a
unique messenger with which to observe the universe, there is
nothing that can block the observation of gravitational wave.

17. Why should we care about gravitational
waves?
• Learning about gravitational wave will expand our knowledge of the
universe.
• They are thought to remain unchanged bu passing through material –
can carry unaltered information about their source.
• Could gain insight into wy the universe is the way it is and its fate will
be.
• Can accurately determine cosmological distances.
• Searching for existence of gravitational waves may uncover new
phenomena.

18. • Scientists can detect a black hole using gravitational wave – and how
big and how the black hole is spinning.
• The gravitational waves emitted from each binary system – inspiral
waves – have characteristic frequencies and amplitutde.
• There characteristics depend on properties of the system ( mass,
orbital period, etc).
• When waves emitted during the merging of two neutron stars are
detected, we will be able to learn more about their structure and
equation of state.
• Eventually, we will be able to use the information from inspiral waves
to perform more precise tests of general relativity, measure the
Hubble constant, and understand the geometry of the space-time
black holes and other objects.

19. Conclusion
• Gravitational waves are one of the most interesting predictions of
general relativity, and provide an unprecedented probe of extreme
gravity environments in the universe.
• There are many potential sources of gravitational wave for out
dectectors, ranging from binary star system to supermassive black hole
mergers to cosmic string cusps.
• We are on the verge of making our first derect gravitational wave
detection. This should happen within 5-10 years, probablt using
Advance LIGO.
• Once gravitational wave dections become routine, we stand to learn a
great deal about systm that are inaccessible to electromagnetic
telescopes.