Gravitatiol waves: Einstein's legacy

By Vinod Kumar

Gravitatiol Waves (GW) were the last prediction of Albert Einstein’s Theory of Relativity. The first direct detection of gravitatiol waves was announced on 11 February by the Advanced Laser Interferometer Gravitatiol-Wave Observatory (LIGO). Using LIGO’s twin giant detectors — one in Livingston, Louisia, and the other in Hanford, Washington — researchers measured ripples in space-time produced by a collision between two black holes. This is the first major detection by LIGO experiments after more than a decade in operation. The new discovery is truly incredible science and marks three milestones for physics:

1. Direct detection of gravitatiol waves.

2. The first detection of a biry black hole system.

3. The most convincing evidence to date that ture’s black holes are the objects as predicted by Einstein’s theory.

According to Newtonian physics, gravity is a force which makes two bodies with mass attracts each other. Einstein in the year 1915, with his radical General Theory of Relativity, gave a completely new perspective of gravity. The concept is mathematical and quite sophisticated, but can be defined simply as “Matter curves space and objects responds to that curvature.” i.e. Gravity is not a force as such, but a curvature caused in spacetime fabric due to the presence of an object with mass. To visualize it, imagine a ball in a stretched rubber sheet — the presence of the ball causes a dip to form and the sheet curves depending on the weight of the ball — now another smaller ball, if rolled to the bigger ball follows the curved space. Roughly this is how earth revolves around the sun .Even though this alogy is not true cent percent, for visualization this will suffice.

The General Theory of Relativity makes prediction far beyond the familiar gravity. A few of them are:

Time Dilation, in distinct but similar manner mass distorts time too. Flow of time changes with respect to the proximity of mass. The time that one experience near to sun is vastly different from that one do on earth.

Deflection of light that we see - Gravitatiol Lensing.  

Dragging of spacetime by spinning objects - Frame Dragging.

The beauty of Einstein’s work lies in the fact that each one of his predictions are physically tested and verified. There was one last incredible prediction which was never directly observed, i.e. Gravitatiol waves. Now this is history!

The denser the mass is, the greater the curvature of spacetime. As objects with mass move around in spacetime, the curvature also changes to the trail of the moving masses. At times, accelerating objects generate changes in this curvature, which propagate outwards in a wave-like manner. These propagating phenome are known as gravitatiol waves. These are outflowing fluctuations of expanding and contracting space time. Not all movements create gravitatiol waves; you need to change the quadrupole moment of a mass distribution. Simply put, it means motion should not be perfectly spherically symmetric or cylindrically symmetric.

* A spinning disk will not radiate. This can be regarded as a consequence of the principle of conservation of angular momentum.

* An isolated non-spinning solid object moving at a constant velocity will not radiate. This can be regarded as a consequence of the principle of conservation of linear momentum.

But, two objects orbiting each other in a plar orbit such as a planet orbiting the Sun or a biry star system or the  merging of two black holes will radiate gravitatiol waves!

LIGO have detected sigls of gravitatiol waves from two merging black holes 1.3 billion years ago. These G-waves propagate at the speed of light, unlike the Newtonian gravity which propagates at infinite speed. This speed limit comes from the fact that speed of light is built into Einstein’s field equations. Therefore, for all massless things this is the ultimate speed limit.

Since G-waves are distortions in spacetime, if it passes through a free standing body, the body may experience rhythmic stretching or shortening, without an unbalanced exterl force. But you may not even notice this stretching, one reason being that these stretchings are negligibly miniscule, other reason, everything gets stretched equally, nullifying the effect.

But there is one entity that is absolute, an universal constant - the speed of light ‘c’, LIGO uses this property of Light (using LASERs) to measure minute changes in the spacetime continuum. The precision that LIGO requires for this kind of detection can be compared to measuring the distance between Milky Way and Andromeda galaxy (2.5 million Light years away) to the scale of the width of a hair.    

Gravity, being a very very weak force, when compared to nuclear force or electrostatic force, one needs really really massive object or something accelerating at a high rate to have G-waves capable of being detected. Gravitatiol waves carry energy away from their sources, and in the case of orbiting bodies, this is associated with an inspiral or decrease in orbit.

Higher acceleration generates powerful gravitatiol waves, since these huge systems generating G-waves are millions of light years away; the power that reaches on earth is minuscule. Orbital lifetimes of biry systems which have high acceleration are one of the most important properties of gravitatiol radiation sources. It determines the average number of biry stars in the universe that are close enough to be detected.

Short lifetime biries, i.e. systems which have imminent merger/collapse are strong sources of gravitatiol radiation but are few in number. Long lifetime biries are more plentiful but they are weak sources of gravitatiol waves. LIGO is most sensitive in the frequency band where biry systems are about to inspiral, i.e. about to merge. Inspirals are very important sources of gravitatiol waves.

Any time two objects such as white dwarfs, neutron stars, or black holes are in close orbits, they send out intense gravitatiol waves. As they spiral closer to each other, these waves become intense. At some point they should become so intense that direct detection by their effect on objects on Earth is possible.  This time frame is only a few seconds. LIGO achieved sensitivities to detect such a microscopic blip in the cosmic noise after a multiyear shut down and an upgrade.

G-waves can pass through any intervening matter without being scattered significantly. While light from distant stars may be blocked out by interstellar dust, gravitatiol waves will pass through essentially unimpeded. This feature allows G-waves to carry information about astronomical phenome never before observed by humans. With this detection we will be able to turn the Universe into our own laboratory!

There are many things that we cannot replicate here on Earth, like the dense cores of neutron stars, strangeness of a black hole singularity or an event horizon.  Under extreme conditions like this, nuclear physics and thermodymics can theoretically do some interesting things.  However, we can’t investigate those directly because we cannot create these environments ourselves. The rules of quantum mechanics say that there ought to be a particle counterpart to G-waves, they are hypothetical particles called gravitons. Now we stand a higher probability in finding those exotic particles.

Gravitatiol wave astronomy’s finest moment is also India’s. The Indian scientific community has made semil contributions to gravitatiol-wave physics over the last couple of decades. The group at RRI, Bangalore led by Bala R. Iyer (currently at ICTS-TIFR) in collaboration with a group of French scientists pioneered the theoretical calculations used to model gravitatiol-wave sigls from orbiting black holes. In parallel, the group of at IUCAA, Pune did foundatiol work on developing the data-alysis techniques used to detect these weak sigls buried in the detector noise.

Over the last decade, the Indian gravitatiol-wave community had expanded to a number of institutions. The Indian participation in the LIGO Scientific Collaboration, under the umbrella of the Indian Initiative in Gravitatiol-Wave Observations (IndIGO), includes scientists from Cheni Mathematical Institute, ICTS-TIFR Bangalore, IISER Kolkata, IISER Trivandrum, IIT Gandhigar, Institute for Plasma Research Gandhigar, IUCAA Pune, Raja Raman Centre for Advanced Technology Indore and TIFR Mumbai.

The ICTS-TIFR group made significant, direct contributions in obtaining estimates of the mass and spin of the fil black hole, and the energy and peak power radiated in gravitatiol waves. The group has also contributed to the astrophysical interpretation of the biry black hole merger. The ICTS-TIFR group designed and implemented one of the tests of general relativity that have shown that the current observation is completely consistent with a biry black hole collision in Einstein’s theory.

Researchers from CMI Cheni, IISER Trivandrum and IISER Kolkata were actively involved in the implementation of this test.

Prime Minister rendra Modi  praised the role of Indian scientists who were part of the team that discovered gravitatiol waves. In a series of tweets, Prime minister added, “The historic detection of gravitatiol waves will open up new frontier for understanding of universe. Hope to move forward to make even bigger contribution with an advanced gravitatiol wave detector in the country.”

This discovery is sure to usher a new era in gravitatiol wave astronomy and will eble us in finding answers to fundamental questions on the origin of the universe, on how a primordial singularity Big Banged into the vast vistas of the cosmos.(PIB)