Everything about Gravitational Waves totally explained
In
physics, a
gravitational wave is a fluctuation in the
curvature of
spacetime which propagates as a
wave, traveling outward from a moving object or system of objects.
Gravitational radiation is the energy transported by these waves. Important examples of systems which emit gravitational waves are
binary star systems, where the two stars in the binary are
white dwarfs,
neutron stars, or
black holes.
Although gravitational radiation hasn't yet been
directly detected, it has been
indirectly shown to exist. This was the basis for the
1993 Nobel Prize in Physics, awarded for measurements of the
Hulse-Taylor binary system.
Introduction
In Einstein's theory of
general relativity, the force of gravity is due to 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 causes, and hence the more intense the gravity. As massive objects move around in spacetime, the curvature will change to reflect the changed locations of those objects. If the objects move around in a certain way, ripples in spacetime can spread outward like ripples on the surface of a pond. These ripples are gravitational waves.
The simplest example of a strong source of gravitational waves is a spinning
neutron star with a small mountain on its surface. The mountain's mass will cause curvature of the spacetime. Its movement will "stir up" spacetime, much like a paddle stirring up water. The waves will spread out through the Universe at the speed of light, never stopping or slowing down.
As these waves pass a distant observer, that observer will find spacetime distorted in a very particular way. Distances between 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. Any gravitational waves expected to be seen on Earth will be quite small; the change in size of any object will never be much more than 1 in 10
20. Still, scientists are attempting to measure the effects of these waves using extraordinarily precise experiments.
By measuring these waves, astrophysicists hope to learn about systems that they couldn't observe with more traditional means such as
optical telescopes,
radio telescopes, etc. Gravitational waves can penetrate regions that the more familiar waves cannot, providing us with a view of black holes and other mysterious objects in the distant Universe. Using precise measurements of gravitational waves in this way will also allow us 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 very high frequency waves have no credible source able to generate detectable waves.
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 10
11 Hz.
Effects of a passing gravitational wave
Imagine a perfectly flat region of spacetime, with a group of motionless test particles lying in a plane. Then, a weak gravitational wave arrives, passing through the particles along a line perpendicular to the plane of the particles. What happens to the test particles? Roughly speaking, that'll oscillate in a "
cruciform" manner, as shown in the animations. The area enclosed by the test particles doesn't change, and there's no motion along the direction of propagation. In the animation at the right, the wave would be passing from you, through the screen, and out the back.
The foregoing animation is the result of a pair of masses that orbit about each other (for example, black holes) on a circular orbit or a rotating rod or dumbbell. In this case the amplitude, A, of the gravitational wave is a constant, but its plane of polarization changes or rotates (at twice the orbital or rotating-rod rate) and so the time-varying gravitational wave size or periodic spacetime strain h, exhibits a variation as shown in the animation. If the orbit is elliptical or the rotating rod’s centrifugal-force change varies during rotation, then the gravitational wave’s amplitude (that is, the amplitude of the periodic spacetime
h), A, actually also varies with time according to an equation called the “quadrupole”.
Like other
waves, there are a few useful numbers describing a gravitational wave:
