By the time you read this article, the LIGO Science Collaboration — a worldwide consortium of over 1,000 scientists at about 90 institutions — is, from all accounts, expected to have made a dramatic announcement, of the detection and analysis of gravitational waves. Why has the prospect of such an event generated so much excitement among scientists and even in the popular press?
It is because this would enable mankind to now “see” the universe in a new light. To appreciate this, we need to recall how the scientific understanding of gravity — a force we feel every day — has evolved over the last few centuries.
The narrative of the ubiquitous force of gravity begins with Galileo’s discovery that all bodies fall in the same way. The next step was Newton’s law of universal gravitation, which has the characteristic that the force acts instantaneously: If an object moves, the other responds immediately to this motion. This law explains both objects falling on earth as well as the motion of planets. However, Albert Einstein’s special theory of relativity came into conflict with this instantaneous law of gravity.
The resolution of this puzzle led to the General Theory of Relativity (GTR), whose complete equations were announced to the Prussian Academy in Berlin on November 25, 1915. GTR is a revolutionary theory because it changed our conception of space and time. In Newton’s theory, space and time are passive spectators. Special relativity mixed up space and time depending on the state of motion. If this motion is slow compared to the speed of light, then the mixing is hardly noticeable! In general relativity, space-time has a life of its own.
Einstein’s equations say that matter (or energy in general) stresses and curves space-time as if it were elastic. Hence, the motion of the Earth around the Sun is described by the Sun curving space-time around it and the earth responding to that curvature very much like a heavy ball distorts the surface of a trampoline and a smaller ball moves guided by the contours of the distortion. Einstein’s theory could correctly account for the bending of starlight by the Sun.
In the years that followed, Einstein’s theory predicted the existence of exotic objects like black holes. In later years, the astrophysical significance of these solutions appeared in the work of Indian astrophysicist S. Chandrasekhar, who predicted that beyond the Chandrasekhar Limit, a massive star becomes a black hole. Black holes range from a few solar masses to a few million solar masses. Einstein’s equations also form the framework for cosmology and our present understanding of the evolution and largescale structure of the universe. Another prediction of Einstein’s equations is that time ticks faster the farther you are from a massive body. Accounting for this time delay is crucial for the accuracy of the GPS in our smartphones.
Einstein also predicted gravitational waves as solutions of his equations. These are small distortions of the “elastic or fluid” space-time that travel at the speed of light, set off by the motion of massive objects in space-time. A good analogy is ripples in water set off by a pebble thrown into a still pond. Space-time is, however, very “stiff” and it needs large masses and violent motions to bend it and set off gravitational waves. Nevertheless, such events abound in the cosmos, such as the cataclysmic mergers of binary black holes or neutron stars. For example, the merger of two black holes of sizes of several solar masses would generate an enormous amount of energy in a fraction of a second acting like a huge gravitational dynamo. The power radiated would easily dwarf that of the visible universe.
We are familiar with light, X-rays, radio waves that are all part of the electromagnetic spectrum. These waves are produced by the motions of electrically charged particles as predicted by James Maxwell. All hitherto knowledge of our universe is from these types of waves. Astronomers “see” the sky using various telescopes, which are sensitive to radio waves, light, X-rays, gamma rays, etc. However, electromagnetic waves can be blocked. A familiar effect is microwaves being screened from emerging out of the microwave oven. For this reason, the primordial universe, right after the “big bang”, is not electromagnetically visible to us. But gravitational waves can see far back in time and far out in space unhindered.
Russell Hulse and Joseph Taylor had indirectly inferred the existence of gravitational waves from the energy loss in a binary pulsar system. But the outstanding challenge was to directly detect these waves. In order to achieve this gravitational wave, observatories have been set up in various parts of the world, notably LIGO (Laser Interferometer Gravitational-Wave Observatory), which has two observatories in the US and the VIRGO observatory in Italy. The instruments at these observatories are Michelson-type laser interferometers. Gravitational waves from realistic astrophysical sources are very weak and their wavelengths are from a few hundred to a few thousand kilometres. When they pass through the Earth, they distort the geometry of space-time and, in particular, affect the lengths of the two arms of the interferometer differently. This effect is so tiny that for an arm of length four kilometres, the distortion would be of the size of an atomic nucleus. Efforts in perfecting the high precision instruments have been on for nearly two decades. Because these detectors are omnidirectional, it is hard to localise the sources of the gravitational waves. This requires the setting up of at least a third LIGO detector somewhere in the eastern hemisphere for gravitational wave astronomy.
Scientists in various institutions in India have been members of the LIGO Scientific Collaboration. They are involved in various aspects of gravitational wave detection, especially involving black hole mergers that are governed solely by Einstein’s equations.
This would be a first step towards the beginning of what promises to be a turning point in astronomy that will enable us to study hitherto inaccessible phenomena, and in future “see” the universe in the remote past — all the way to the moment of its birth.