Gravitational Wave Astronomy: Story until the GW150914 Observation

Notes from Lecture ‘First five years of Gravitational wave astronomy’ by Prof. Archana Pai, Department of Physics, IIT Bombay. World Space Week, Webinar organized by National Space Society(USA)-Mumbai.

A broad overview

Gravitational-Wave astronomy is an emerging branch of observational astronomy. A distant cousin, we can say, of optical (electromagnetic-wave) astronomy. It’s a young field – just five years old. Since then it has progressed tremendously. Nature of observations and study under gravitational-wave astronomy is quite different.

What are gravitational-waves?

Gelileo’s Pisa Tower Experiment:  Between 1589 and 1592 the Italian scientist Galileo Galilei (then professor of mathematics at the University of Pisa) is said to have dropped two spheres of different masses from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass. The actual cause behind this observation was unknown.

According to Newton’s Law of Gravitation: Force attractive between two objects is directly proportional to product of their masses. Using this Law, orbits of all planets was explained correctly, except that of Mercury. This is where Gravitational-waves and Einstein’s general relativity comes in.

Under Einstein’s General Theory of Relativity:

1. Equivalence principle: Motion under gravity == Motion of object in curved geometry.
2. Matter defines geometry and the geometry decides the trajectory
3. Newton’s Law of gravitation is only applicable for weak gravity and small velocities.

Both, orbit of Mercury and Gelileo’s Pisa Tower experiment can be explained by Theory of Relativity.

Gravitational Wave: Gravitational Waves are ripples on the space-time pond generated by accelerated masses. When two mass objects revolve around each other, they produce gravitational waves outward traveling away from their center of mass. Gravitational waves were proposed by Henri Poincaré in 1905 and subsequently predicted in 1916 by Albert Einstein on the basis of his general theory of relativity.

Sudden explosions can also produce gravitational-waves. If the object/system is not symmetric and its under acceleration then only gravitational-waves will produced.

Properties of gravitational-waves:

1. Travels with speed of light
2. They are transverse waves – disturbance happen perpendicular to the direction of motion.
3. Radiates away from its source.

Both terrestrial and astrophysical sources can produce gravitational-waves. Amplitude ‘h’ of gravitational wave produced by a terrestrial source will be of order 10^-44 meters, whereas any astrophysical source will produce amplitude of around 10^-21 meters. To bring the order of amplitude in perspective, the size of atom is ~10^-10 meters.

Astrophysical sources of gravitational-waves

1. Single spinning massive object like neutron star
2. Black-hole attracting matter
3. Binary black-hole
4. Binary neutron star
5. Interacting galaxies
6. Exploding star – supernova

Effect of gravitational-waves

In the above GIF, you can see the transverse nature of gravitational waves. Disturbance in space is perpendicular the direction of motion of the waves (follow the motion of a node).

Steps toward building gravitational-wave detectors

In 1965, Joseph Weber proposed metal bars for detection of gravitational-waves. According to him, passing gravitational waves will excite the resonant mode of the metal bar. He announced gravitational wave detection in 1968. Several resonant bar detectors were built at MIT, IBM, Glasgow, Germany, Italy etc. to confirm the results.

Although in the 1970s, the results of these gravitational wave experiments were largely discredited (results were due to some error/noise in the instrument itself), Weber is widely regarded as the father of gravitational wave detection efforts.

Concept of Laser Interferometric Gravitational-Wave Observatory (LIGO)

• Felix Pirani (1957): Light signal can be used to measure distance between the free test masses.
• Reiner Weiss (1972): Laser interferometer can be used for the gravitational-wave detection.
• 1989 – Full LIGO proposal was submitted to NSF.

Advanced LIGO detectors are an engineering and technological marvel. Interferometers with 4 km vacuum chambers to measure a motion of 10000 times smaller than atomic nucleus caused due to violent events in the Universe located at millions/billions of light years away.

Inside the interferometer, light from the laser passes through the beam splitter, through which half the light passes through and half is reflected, resulting in the splitting of light equally to two arms of the interferometer. mirrors at the end of both arms returns the light back. Now, the light from both the arms falls on photodetector.

Now, if there happens to be gravitational-waves passing through our interferometer, there would be changes in the length of the arms. If you recall the GIF (under ‘Effects of gravitational-waves), and imagine our interferometer placed along the cross-section then alteration in the length of both arms will be alike. Which will result in out-of-phase light beam on the photodetector.

Technological challenges

1. Mirrors are suspended to isolate from terrestrial interference
2. Kept in vacuum
3. High reflectivity of mirrors to lower the noise

Experts from many different fields work together in LIGO like: astrophysicist and astronomers to understand and study sources of gravitational wave, engineers from different fields for building sensitive instruments in LIGO observatories. LIGO collects data continuously – we also need data scientists for handling massive amount of data.

First observation of gravitational waves

In 2015, LIGO achieved sensitivity of 10^-23 (well smaller than needed for detecting astrophysical sources).

The first direct observation of gravitational waves, GW150914, was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016. The signal just two-tenths of a second long. The video below shows the evolution of the signal in the instrument along with the chirp of our first black hole merger detection (the signal is played several times, repeating the chirp first in its natural frequency–the low ‘thump’–and then increased to make it easier to hear).

Event happened at a distance of ~1.5 billion light years when two black holes, with masses 35 and 30 times of the Sun, merged to form a single black hole of mass 62 times that of the Sun. Mass energy of 3 Solar masses was radiated away in the form of gravitational waves.

~AK