Gravitational Waves, How Close Are We?

Topics: General relativity, Gravitational wave, LIGO Pages: 13 (3840 words) Published: December 8, 2008



The Detection of Gravitational Waves,
How Close Are We?

Since the realization that the general theory of relativity predicts gravitational waves, there have been attempts to actually detect these waves. Indirect observations have been made that support their existence but no direct measurement. This paper gives a brief explanation of gravitational waves and discusses the current condition of the experimental search for gravitational waves. It deals with the newest techniques that will enable their detection. The focus of the paper is on three experimental groups: LIGO, VIRGO, and LISA. From our research of these groups we believe that the detection of gravitational waves will occur within the next decade.


The Detection of Gravitational Waves,
How Close Are We?

Einstein's general theory of relativity was published in 1915.1 Since that time many of the predictions derived from the theory have been experimentally observed. Three main examples are the bending of light by gravity, the red-shift of light traveling in a gravitational field, and the precession of Mercury. Einstein's theory has been credibly established because of observations like these. There are still other predictions that have yet to be observed. The detection of gravitational waves is one of these predictions.

It was discovered in 1916 that the general theory of relativity predicts the existence of gravitational waves. “Gravitational waves are perturbations in the curvature of spacetime propagating with the velocity of light. They are caused by accelerating masses.”2 In order to understand the concept of a gravitational wave it is helpful to understand gravity as explained by the general theory of relativity. Relativity does not analyze gravity in terms of forces and acceleration as in Newtonian physics. Instead it explains gravity in terms of the geometry of spacetime.

Space time is a very difficult concept to visualize. It is made up of the three positionaxes, x, y and z, but also includes the dimension of time. It is the fourth axis of time that makes spacetime difficult to conceptualize. Spacetime is all around us. It maybe helpful to think of it as a medium that encompasses everything: earth, our galaxy, the universe, etc. All planets, suns, moons and celestial bodies are “submersed” in this medium called spacetime. 4

According to the general theory of relativity mass bends spacetime. Larger masses bend space-time more than smaller masses, just as a more massive object would bend a trampoline more than a less massive object. If the gridlines in Figure 1 represent spacetime it can be seen how the Earth bends it. Objects that approach the Earth will be affected by this curvature around it. Specifically, an object will be moved towards the Earth. This is how general relativity pictures gravity.

As mentioned gravitational waves are perturbations in the curvature of spacetime, and are created by accelerated masses. A similar occurrence can be observed with water. As a fish in a bowl moves around underwater it produces movements, or waves, in the water that spread throughout the bowl. In this same way accelerated masses produce waves in spacetime. These waves travel throughout the universe affecting spacetime and other masses within it. The magnitude, or strength, of the gravitational waves is directly proportional to both the mass and the acceleration of that mass. The magnitude of the wave also depends on the distance it travels before it reaches us. The further it travels the smaller its magnitude will be. It is this fact that has made detection of the gravitational waves unsuccessful in the past. It is difficult to understand how a gravitational wave affects matter. It is best to consider the wave's effect on the spacetime around the matter. As...

References: 1 John Taylor and C.D. Zafiratos Modem Physics for Scientists and Engineers. (Prentice-Hall,
Inc., Englewood, NJ, 1991), p
2 (1999)
3 John Yaukey, “Hey Mr
May 7, 1998.
4 Oxford Dictionary and Thesaurus, The American Edition
1996), p.1212.
5 (1999)
10 Barry C. Barish and R Weiss, Phys. Today. 52 (10), 44-50 (1999).
11 Ibid
12 (1999)
13 See #10
18 See #12
19 (1999)
20 (1999)
21 Ibid.
22 (1999)
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