JOIN USDeep inside the gravitational wave phenomenon
In the light of the detection of gravitational waves, C Sivaram analyses what it could possibly mean to the astronomical world
On February 11, the Laser Interferometer Gravitational-Wave Observatory (LIGO) group announced the landmark discovery of the first direct detection of gravitational waves (predicted by Albert Einstein a hundred years ago in 1916). The waves came from two black holes circling closer and closer to each other till they finally collided and coalesced. Most of the radiation was released in the final orbit, which had a period of a millisecond. The waves had a frequency from 0.6 to 1.2 kHz, typical of such stellar events. The final black hole had the mass of 60 solar masses. About three solar masses were converted to energy of gravitational waves, i.e. around 5x1047 joules, with peak power of about 1051 watts.
When gravitational waves pass through an object (detector), they alternately stretch and contract it. This is parameterised by the strain, which is the ratio of the change in separation between two points of the detector to the initial separation or the change in length scale of the detector to the initial length. This strain (h) was about 10-21 for this event, which occurred, more than a billion light years away.
So the laser arm (i.e. the high vacuum tubes with mirrors to reflect the laser beams back and forth), which is three to four km long, was distorted by barely one per cent of the diameter of an atomic nucleus. The earth would have changed its diameter (alternately expanding and contracting) by a hundred thousand of a nanometre. The tallest tree would have stretched an attometre (nano-nanometre).
Relationship
The h value is related inversely to the distance to the source. We have in our galaxy several binary pairs of massive stars which are potential sources for such events. One well known example is the Plaskett star, about 5,000 light years away, which is a massive binary of stars of 50 solar masses each.
These stars could evolve to black holes (after supernova explosions) after about 10 million years. Their remnant black holes could take a few billion years to merge. Their coalescence would release comparable energy (power) to the present LIGO event. The strain would be million times more, i.e. 10-15. Earth would now stretch to just a nanometre and the tallest trees to a less than a picometre, a negligible effect.
Imagine if the event had occurred just 10 light years away. The strain in the detector would be then, h ~ 10-12, our tree would have ‘strained’ by ~ 10-10 m, the whole Earth would have been distorted (for a millisecond) by ~ 10-6 m (a micron). The interaction of gravitational waves with matter is extremely weak, even weaker than neutrinos.
Hypothesis
If Betelgeuse, a red supergiant, 400 light years away explodes as a supernova in a few thousand years time (it is a strong candidate for what is called a Type II supernova), the UV and X-ray radiation, could damage our Ozone layer (and cause short duration climatic changes), but the neutrinos would mostly pass right through (a flux of hundred trillion neutrinos per cm2 per second, for about 10 seconds). So also would the gravitational wave radiation from such an explosion, the h would be just ~ 10-20.
Even if the 2.4-hour period binary pulsar, 2000 light years away, merges in a few million years, i.e. two neutron stars merge, we would get a short duration gamma ray burst which would fry the Earth (in gamma rays of high energy) but the gravitational waves would cause an h of around 10-14 and have no effect on life. It is sometimes thought that the Devonian mass extinction, 400 million years ago, could have been due to nearby supernova or gamma ray burst, but certainly not because of its neutrino or gravitational wave emission.
Even if the gravitational wave flux from a binary neutron star merger, 2000 light years away is ~ 106 W/cm2 (for a second or two), the interaction of gravitational waves with matter is very weak. LIGO in its preliminary runs from 2002-2012 (when it was shut down for upgrading) expected to see a few events from such neutron star merges, especially considering the ‘nearby’ Virgo cluster (hardly 15 megaparsec away) chockfull of thousands of galaxies, hosting many such merging binaries.
It would be good if LIGO (like in the case of SN1987A, where neutrinos were detected before the optical emission) detects gravitational waves from sources which can also be simultaneously observed in gamma rays, optical and other wavelengths.