Gravitational waves, space-time deformation, quantum gravity etc are now the buzz words in the scientific as well as non-scientific parlance. People tend to show off their academic pedigree by frequently and quite often inappropriately using these terms and baffling their audience, but now they can have some inkling of the implications of these terms. These terms ushered in a new frontier of scientific discipline in the general area of cosmology. A new scientific toolkit has been invented for the cosmologists and astrophysicists to use.
If the early part of the 20th century could be categorised as the age of theory of relativity (produced by Einstein in 1905 and in 1916) and then the age of quantum mechanics (produced by Max Planck, Niels Bohr, Heisenberg, Schrodinger and others), then this part of the 21st century can surely be ascribed as the age of gravitational waves.
But the invention of gravitational wave now is not a fortuitous event. Back in 1916, when Einstein produced the general theory of relativity, he stipulated space-time continuum. But nobody, except a few elites in theoretical physics, had the faintest idea what it really means.
According to general relativity, gravity is a manifestation of the curvature of space-time. Space-time becomes curved and time slows down in the presence of material body. The more the mass, the greater is the curvature and slower is the time. When a massive body moves, the curvature moves with it to a new position stretching the space in one direction and compressing it in another direction. This deformation in space-time produces ripples, called gravitational waves, which travel outward from the gravitational source at the speed of light. But there is no light involved, only gravitational energy travels at the speed of light.
In a more mundane way one can say that when a body is attracted and moves towards another body, it follows the curved space-time fabric. When there are a number of bodies, the space-time curvature becomes quite jagged and a body travelling in that space follows a tortuous path.
It may, however, be mentioned here that there is no scientific reason why bodies should always attract each other, as they do in our observable universe. In another parallel universe, it may be that bodies repel each other or some bodies repel each other, while others attract each other. When these attracting and repelling bodies are scattered randomly in the vast space, they may stabilise and create a stable universe – all attracting bodies may not collapse and all repelling bodies may not push each other into infinity.
How does this gravitational attraction get propagated between the bodies? Drawing analogy between the electromagnetic energy and gravitational energy, physicists coined the term ‘graviton’ for gravitational energy, as ‘photon’ for electromagnetic energy. The photon is the smallest packet of energy (the quantum of energy) that came into the jargon of physics following Einstein’s theory of photoelectric effect in 1905. It may be mentioned that Einstein received Nobel prize for his theory of photoelectric effect, not for the theory of relativity; although both of these theories are of immense importance and monumental in physical sciences.
Whereas photon had been detected experimentally, graviton had never been detected. How come, despite all the predictions of the general theory of relativity being found to be meticulously accurate, the central plank of this theory involving gravitational waves had been left undetected? Now that gap has been filled. This recent discovery of gravitational waves may eventually lead to the identification of graviton. But whether graviton comes into the scientific arena or not, gravitational wave is now a reality.
When the LIGO (Laser Interferometer Gravitational-wave Observatory) collaborative experiments in Baton Rouge, Louisiana and in Hanford, Washington State first detected the gravitational wave, they hesitated to make it public. It was so earth shattering that the researchers had to make it absolutely certain that the results are genuine, not spurious. After all, no scientist wants egg on his face! On September 14, 2015 they disclosed the detection of gravitational waves.
When two super massive black holes some 1.3 billion light years away, one 36 times and the other 29 times of the mass of the Sun, spiralled around each another and eventually merged, a gigantic massive black hole was created. And in the final half a second or so in this cataclysmic event, a massive amount of energy was produced. It had been estimated that the final black hole, instead of having 65 times the mass of Sun was 62 times and the remaining three solar masses had been converted to gravitational energy. This energy was so massive that it rippled through the entire universe at the speed of light and deformed the space-time fabric in the form of gravitational waves. It may be noted that there was no emission of light at all from the collapse of those spiralling black holes – the energy was all gravitational energy. The generated energy from the conversion of three solar masses (each having a mass of 2×1030kg) was absolutely staggering (~ 5×1047 J).
At the LIGO facility, a monochromatic laser beam was produced and then split in two directions at the point of origin. The facility comprises two arms at right angles to each other and each arm is 4 km long through which the laser beams travel. At the edge of these arms, precisely positioned reflecting mirrors are placed. The reflected beams are allowed to interfere back at the source. If there is no space distortion due to ripples in the space time, the reflected beams combining in anti-phases will cancel each other and there will be no interference pattern. On the other hand, if the arms of the facility are deformed due to stretching and compression by the incoming gravitational wave, there would be interference. This physical deformation is extremely small – trillion times smaller than the width of a human hair (~ 10-18 m) – and even with the emission of astounding amount of energy (~ 5×1047 J, an extreme precision in detection by laser interferometer was needed.
Since that discovery in 2015, more black hole mergers of smaller sizes had been detected. That shows that the technique is becoming more refined and sensitive. Even smaller masses than black holes can create ripples in gravitational waves and they may be detected by this technique. In September 2017 LIGO had been closed for about a year for upgrade. But before shutdown, on August 17, 2017, LIGO detected gravitational waves produced by the collision of two neutron stars in the galaxy Hydra, 130 million light years away. This is the first time that collision between stars had been detected. In August, a new facility in Italy, called the Virgo interferometer, had joined in. India also is going to build a facility in Maharashtra, which is similar to LIGO facility in Washington State and plan to start its operation in 2024.
This gravitational wave may be a cosmic messenger, which may lead human beings to probe right into the origin of the universe. This is the exciting time not only for cosmologists and astrophysicists but also for humanity as a whole; no longer we will have to rely on myths and mysteries passed down from generations to generations of the creation of universe.
- Dr A Rahman is an author and a columnist.