Gravitational waves a new way to observe the spacetime

Figure 1 Gravitational-wave (Credit: R. Hurt, Caltech-JPL)

Gravitational waves (GW) are curvatures in space and time caused by the movement of massive objects. It propagates outward from its source at the speed of light. GW was first proposed by Henri Poincaré in 1905 and afterward postulated in 1916 by Albert Einstein on the basis of his general theory of relativity. These waves can be explained by analogies with sound waves through a medium or electromagnetic wave in space. Like sound waves in the air or the ripples made on a pond’s surface when someone throws a rock in the water, gravitational wave spreads through space-time. But unlike ripples of sound waves in water, GWs are vibrations in space-time itself, which means they move just fine through the vacuum of space. Gentle vibration in a pond can create a rippling wave but to trigger a gravitational wave, it is required to have an interaction between compact stellar-mass objects like colliding black holes, pulsars, or neutron stars.

In the path of GW, the object feels a tidal gravitational force and it acts perpendicular to the wave’s direction of propagation. It alters the distance between points, and the magnitude of the changes is proportional to the distance between the points. Gravitational waves can be detected by devices that measure the induced length changes. The frequencies and the amplitudes of the waves are related to the motion of the masses involved. Thus, the analysis of gravitational waveforms allows us to learn about their source. If there are more than two detectors for the observation, we can estimate the distance and position of their source on the sky.

A laser interferometer is used to measure the distance between two well-separated masses. This consisting of three mirrors placed in an L-shaped configuration. The laser beams are reflected back and forth between the mirrors attached to the three masses, the mirrors lying several kilometers away from each other. A gravitational wave passing-by will cause the lengths of the two arms to oscillate with time. When one arm contracts, the other expands, and this pattern alternates. The result is that the interference pattern of the two laser beams changes with time. With this technique, it is possible to detect gravitational waves.

Figure 2 Russell A. Hulse and Joseph H. Taylor, Jr (Left to Right)

Alternatively, this can be proved by monitoring pulsars using radio telescopes. In 1974, Russell A. Hulse and Joseph H. Taylor, Jr discovered twine Pulsars using the 300-m radiotelescope at Arecibo, Puerto Rico, West Indies. The Pulsers were named after them as Hulse-Taylor Pulsars and both of them were awarded the Nobel Prize in Physics for 1993. This discovery opened up new possibilities for the study of gravitation.

Figure 3 (a) LIGO Livingston and (b) LIGO Hanford establishment sites

The first success of the use of laser interferometer came on September 14, 2015, when the GW signal was detected by the twin laser interferometers, namely LIGO Livingston and LIGO Hanford of Washington. In the same year in the month of December and January 2017, again the waves were detected by the twin detectors. These successes started the beginning of gravitational-wave astronomy. We are now able to observe the universe in an entirely new way, by capturing the signals of spacetime. The three key players in the development and ultimate success of the Laser Interferometer Gravitational-wave were awarded the 2017 Nobel Prize. They were Barry Barish and Kip Thorne of Caltech and Rainer Weiss of MIT.

Figure 4 Barry Barish and Kip Thorne of Caltech and Rainer Weiss of MIT (Left to Right).

Gravitational-wave has a deep impact on our understanding of the structure and evaluation of the universe. Its detection and use are important for two reasons. Primarily, their detection will open up a new way for observational astronomy as the information carried by gravitational waves is very different from that carried by electromagnetic waves. This alternative approach will provide a different view of the cosmos and will help us unveil the fabric of spacetime around black-holes. With GW we can observe directly the formation of black holes or the merging of binary systems consisting of black holes or neutron stars. This will also help us to search rapidly spinning neutron stars, dig deep into the very early moments of the origin of the universe, and look at the very center of the galaxies where supermassive black holes weighing millions of solar masses are hidden.

Second, the detection of gravitational waves will help us to understand the fundamental laws of physics; will verify the fundamentals of the 85-year-old prediction of General Relativity (GR).

Also, by comparing the arrival times of light and gravitational waves from spinning neutron stars and other sources, Einstein’s prediction that light and gravitational waves travel at the same speed could be checked.

Finally, we could also verify that they have the two polarizations as predicted by General Relativity theory.

As mention earlier, GW can be explained using analogies with electromagnetic waves. The mass and mass currents as sources of GW waves can be compared with the electric charge and electric currents as sources of electromagnetic waves in classical electrodynamics. The calculated waveform are very similar to the GW signals as observed by LIGO experiments. However, polarization, angular distributions, and overall power results are different from those of GR. This analogical treatment can produce most of the GR features of GWs but not all, as GR is based on tensor theory whereas electromagnetism is basically vector-based mathematics.

When a gravitational field of a source changes with time, those changes propagate out from the source at speed c. These changing ripple fields form gravitational radiation. If the changes are continuous or oscillatory, they form a transverse gravitational waveform. Static fields have both radial and transverse components, whereas the radiative fields are purely transverse and vary as the inverse of the distance. Like the equation of light, the speed, wavelength, and frequency of a gravitational wave are related by the equation c = λ f. So, for a frequency (expected to have frequencies 10−16 Hz < f < 104 Hz) of 0.1 Hz wavelength will be about 300 000 km.

GWs from their sources at great distances are continually passing Earth. Since the effect of even an extreme GW is very small, it requires an extremely sophisticated device to record it while it passes through the detector. For example, the waves given off by a pair of binary stars at a billion light-years away as a ripple in spacetime change the 4 Km arm of LIGO by a thousandth of the width of a proton. This minuscule effect from even extreme GW makes them observable on Earth only with the most sophisticated detectors. When a GW passes through a plane containing the test particles along a line perpendicular to the plane of particles, particles follow the distortion in spacetime and oscillate in a “+” or “x” direction. The area bounded by the test particles does not change and there is no motion along the direction of propagation.

Dipankar Ray dipankarray@ieee.org

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