Gravitational Waves

Introduction to Gravitational Waves

According to Einstein’s general theory of relativity, gravity is the curvature of spacetime which is caused by the presence of mass. The greater the mass of an object is, the greater the curvature of spacetime is at the boundary of the object’s volume. As objects with mass move around in spacetime, the curvature changes to reflect the changed locations of those objects. In certain circumstances, accelerating objects generate changes in this curvature, which propagate outwards at the speed of light in a wave-like manner. These propagating phenomena are known as gravitational waves. In other words, gravitational waves are ripples that distort spacetime, increasing and decreasing distances between objects at a frequency corresponding to that of the wave, stretching and squeezing everything in their path. Much like a stone dropped into a pond, the waves generated get weaker the further they get from the source (Einstein, 1916).

Generation of Gravitational Waves

Powerful gravitational waves are created when gargantuan objects move at high speeds; for instance, two highly dense objects orbiting one another in binary pairs. These pairs can include two black holes, two neutron stars, or a black hole and a neutron star. Detectable gravitational waves can also be generated by a supernova, which is the explosion of a massive star. However, because these events occur so far away, the gravitational waves distort spacetime by an utterly small amount (many times smaller than the width of a proton) by the time they reach Earth. Measuring such minuscule changes is rather impossible for most measuring devices, thus the difficulty of detecting them. That is, until recently (Thorne, 1994).

Detection of Gravitational Waves

The announcement for the first direct detection of gravitational waves was made in 2016, although the actual detection occurred in September 2015 by LIGO (Laser Interferometer Gravitational Wave Observatory). A pair of black holes (29 and 36 times the mass of the Sun) merged, producing distortions in spacetime that LIGO in turn detected. This groundbreaking detection confirmed a major prediction of Einstein's theory of general relativity. Since then, we have had four additional detections, the fourth being significant because it was also detected by the Virgo gravitational wave detector in Italy. Similarly to the first one, the other three detected gravitational waves were also caused by pairs of colliding black holes, while the fifth observation was due to merging neutron stars (Abbott et al., 2016).

The LIGO and Virgo Observatories

The LIGO facility consists of two L-shaped detectors located in Washington State and Louisiana, each equipped with mirrors and lasers to measure tiny changes in spacetime caused by passing gravitational waves. Both of the LIGO observatories have two arms that are more than 4 km long and perpendicular to each other. Two mirrors are installed at each end of the arms, and their change in distance between each other is recorded. A laser bouncing back and forth between the mirrors keeps track of how far apart they are. Because the detectors are sensitive to external disturbances like passing trucks, earthquakes, ocean waves, etc., it is crucial that a signal shows up in both of the detectors for it to be considered real. In addition to LIGO, there is a third observatory, the European Gravitational Observatory’s Virgo detector, which is similarly designed. It is anticipated that India and Japan will soon operate their own similar observatories as well (LIGO Scientific Collaboration, 2015).

The Future of Gravitational Wave Astronomy

Gravitational waves can access areas of space that electromagnetic waves are not able to, making them a powerful tool for astronomical observation. This means gravitational waves can help us observe collisions of black holes and other exotic objects in the far universe, which cannot be seen with the usual instruments like optical telescopes. Gravitational-wave astronomy offers a new means to gain insight into the cosmos, especially the very early years of the Universe. Through gravitational waves, the general theory of relativity can also be tested more thoroughly, potentially leading to new discoveries about the fundamental laws that govern our universe (Sathyaprakash & Schutz, 2009).

References

• Abbott, B. P., Abbott, R., Abbott, T. D., Abernathy, M. R., Acernese, F., Ackley, K., ... & Collaboration, L. S. (2016). Observation of gravitational waves from a binary black hole merger. Physical Review Letters, 116(6), 061102.
• Einstein, A. (1916). The foundation of the general theory of relativity. Annalen der Physik, 49(7), 769-822.
• LIGO Scientific Collaboration. (2015). Advanced LIGO. Classical and Quantum Gravity, 32(7), 074001.
• Sathyaprakash, B. S., & Schutz, B. F. (2009). Physics, astrophysics and cosmology with gravitational waves. Living Reviews in Relativity, 12(1), 2.
• Thorne, K. S. (1994). Black holes and time warps: Einstein's outrageous legacy. WW Norton & Company.