Gravitational Wave - Introduction

Introduction

In Einstein's theory of general relativity, gravity is treated as a phenomenon resulting from the curvature of spacetime. This curvature is caused by the presence of massive objects. Roughly speaking, the more massive the object is, the greater the curvature it produces and hence the more intense the gravity. As massive objects move around in spacetime, the curvature changes to reflect the changed locations of those objects. In certain circumstances, moving objects generate a disturbance in spacetime which spreads, as the metaphor goes, "like ripples on the surface of a pond", although perhaps a better analogy would be electromagnetic waves. This disturbance is known as a gravitational wave. According to general relativity, gravitational waves travel through the universe at the speed of light.

As a gravitational wave passes a distant observer, that observer will find spacetime distorted by the effects of strain. Distances between free objects will increase and decrease rhythmically as the wave passes. The magnitude of this effect will decrease the farther the observer is from the source. Binary neutron stars are predicted to be a strong source of such waves owing to the acceleration of their enormous masses as they orbit each other and yet even those waves are expected to be very weak by the time they reach the Earth, resulting in strains of less than 1 part in 1020. Scientists are attempting to demonstrate the existence of these waves with ever more sensitive detectors. The current most sensitive measurement is about one part in 5×1022 (as of 2012) provided by the LIGO and VIRGO observatories. The lack of detection in these observatories provides an upper limit on the frequency of such powerful sources. A space based observatory, the Laser Interferometer Space Antenna, is currently under development by ESA.

Gravitational waves should penetrate regions of space that electromagnetic waves cannot. It is hypothesized that they will be able to provide observers on Earth with information about black holes and other mysterious objects in the distant Universe. Such systems cannot be observed with more traditional means such as optical telescopes and radio telescopes. In particular, gravitational waves could be of interest to cosmologists as they offer a possible way of observing the very early universe. This is not possible with conventional astronomy, since before recombination the universe was opaque to electromagnetic radiation. Precise measurements of gravitational waves will also allow scientists to test the general theory of relativity more thoroughly.

In principle, gravitational waves could exist at any frequency. However, very low frequency waves would be impossible to detect and there is no credible source for detectable waves of very high frequency. Stephen W. Hawking and Werner Israel list different frequency bands for gravitational waves that could be plausibly detected, ranging from 10−7 Hz up to 1011 Hz.

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