Gravitation

One of the four fundamental forces, gravitation is defined as the force of mutual attraction that is exerted between massive bodies and between particles that have mass. Although gravitation is much weaker than the other three forces (the electromagnetic force and the weak and strong nuclear forces) over short ranges, it is the dominant force on large scales because its range of influence is far greater than that of the nuclear forces and because, unlike electrical charges (which can be positive or negative), all mass is positive (and mutually attractive). Because like charges repel and opposite charges attract, and matter on the large scale is electrically neutral (there are equal numbers of negative and positive charges in the universe), the electromagnetic force does not play a major role in the overall dynamics of the universe. Consequently, gravitation alone determines the motions of, and mutual interactions between, planets, stars and galaxies, and dominates the overall dynamics of the universe.

In 1687 Isaac Newton published his law of universal gravitation in his book, De Philosophiae Naturalis Principia Mathematica (The Mathematical Principles of Natural Philosophy). It stated that the force of attraction between any two masses is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. For two bodies, with masses m1 and m2, separated by distance, r, the mutual force of attraction, F, is given by F = Gm1m2/r2, where G is the universal gravitational constant (=6.670 × 10⁻¹¹ Nm² kg⁻²). An equal and opposite force acts on each of the masses; thus the force exerted on m1 is equal in magnitude but opposite in direction to the force exerted on m2.

According to Newtonian theory, gravity is a force that acts, instantaneously, directly between individual bodies and particles (a process that is called ‘action at a distance’). Although, for most purposes, Newtonian gravitation gives a perfectly satisfactory description of the motions of, for example, projectiles, planets, stars and galaxies, in certain situations (for example, where gravitation is very strong, as in the vicinity of a black hole, or when dealing with the structure and dynamics of the universe as a whole), the theory is inadequate. The best current theory of gravitation is Einstein’s general theory of relativity, according to which mass distorts the geometry of space (or, strictly, four-dimensional space-time) and the paths followed by material particles, or rays of light, in the neighborhood of massive bodies are determined by these local distortions of space.

Just as an accelerating or oscillating charged particle radiates electromagnetic waves, so an accelerating, oscillating or violently disturbed mass, or system of masses, is expected to radiate wave-like gravitational disturbances. These waves are called Gravitational waves. Currently, research is underway to determine the existence of the hypothetical force-carrying particle for gravitation – it is termed ‘Graviton’. Although there is no generally accepted quantum theory of gravity as yet, it is widely believed that gravitation, like the other fundamental forces, should be amenable to being formulated in quantum terms and that the gravitational interaction between particles of matter should be conveyed by gravitons.

Questions to Ponder

• What is the speed of gravity?
• What would happen if the gravity on Earth was suddenly turned off?