Gravitation

Gravitation

Gravitation is a universal attractive force exerted by any two physical bodies on each other, even though they may be separated by a large distance. Gravitation is responsible for making objects fall to the surface of the Earth (gravitational attraction of the object by the Earth), for the nearly circular motions of the planets around the sun (gravitational attraction of the planets by the sun), for the structure of stars and planets (gravitational attraction balanced by pressure forces of constituent particles towards each other), and for the structure of star clusters and galaxies (hundreds of millions of stars would fly apart from each other if not held together by gravity). Gravitation also controls the rate at which the universe expands, and is responsible for the growth of small inhomogeneities in the expanding universe into galaxies and clusters of galaxies.

Gravity is the weakest of the four fundamental forces known to physics, but it dominates on large scales because it is a long-distance force that is locally always attractive (in contrast to the far stronger electromagnetic force, which can both attract and repel, and cancels itself out on large scales). Thus gravitation is a dominant force in every day life, as well as in the motions of stars and planets and in the evolution of the cosmos. Indeed, it is one of the forces that makes our existence possible by enabling the formation and stability of plants like Earth that are hospitable to life. Without gravity (at approximately the strength it has on Earth) evolution of life would be difficult if not impossible. This fact can naturally lead to speculation that the existence and specific nature of gravitation could be part of a grand design allowing self-assembling structures to come into existence and lead to intelligent life. In this way, gravity can have theological significance.

Classical physics

Italian astronomer Galileo Galilei (1564–1642) first recognized in the early seventeenth century that when air resistance can be neglected, objects accelerate at the same rate towards the surface of the earth, irrespective of their physical composition. Thus a feather and a cannon ball will arrive at the same time at the earth's surface if simultaneously released from rest at the same height in a vacuum chamber. This means there is a universal rate of acceleration downwards caused by the earth's gravitational field—approximately 32 feet per second squared—irrespective of the nature of the object considered. Gravitational potential energy can be converted to kinetic energy, with total energy conserved, as for example in a roller coaster or a pole vaulter. This enables gravity to do useful work, as in a clock driven by weights or a water mill, but it also means people must work to go uphill. Gravity can also be a danger to people, who can fall or be hurt by falling objects. Despite this danger, gravity is an essential part of the stability of every day life—it is the reason that objects stay firmly rooted on the ground rather than floating into the air.

In the late seventeenth century, Isaac Newton (1642–1727) showed that the gravitational attraction of objects towards the earth and the motion of the planets around the sun could be described accurately by assuming a universal attractive force between any two bodies, proportional to each of their masses and to the inverse of the square of the distance between them. The attractive nature of gravity results because masses are always positive. On this basis he was able to explain both the universal acceleration towards the surface of the earth observed by Galileo and the laws of motion of planets around the sun that had been observationally established earlier in the century by Johannes Kepler (1571–1630). This was the first major unification of explanation attained in theoretical physics, showing that two phenomena that initially appeared completely unrelated had a unified origin. Newton's account of gravitation also explained why the direction of gravity varies at different places on the surface of the earth (always being directed towards its center), allowing "up" to be different directions at different places on the earth's surface (Australia and England, for example).

In conformity with the rest of theoretical physics, Newton's theory of gravity can be reformulated as a variational principle (Hamilton's principle or Lagrange's equations) based on minimisation of particular combinations of kinetic energy and gravitational potential energy along the trajectory followed by a particle. Gravity by itself is a conservative theory (energy is conserved), so there is no friction associated with the motion of stars and planets in the sky, and their motion is fully reversible; the past and future directions of time are indistinguishable, as far as gravity is concerned. Newton was puzzled as to how the force of gravity, as described by his equations, could succeed in acting at a distance when there was no apparent contact between the bodied concerned. Pierre Laplace (1749-1827), a French physicist and mathematician, essentially resolved this puzzle by introducing the idea of a gravitational force field that fills the empty space between massive bodies and mediates the gravitational force between them. The concept of such fields became one of the major features of classical physics, particularly in the case of electromagnetism. In quantum theory the idea gravitational fields is revised and understood as a force mediated by the interchange of force-carrying particles.

Einstein and after

In the early twentieth century, Albert Einstein (1879–1955) radically reshaped the understanding of gravity through his proposal of the general theory of relativity, based on the idea that space-time is curved, with the space-time curvature determined by the matter in it. This theory predicts the motion of planets round the sun more accurately than Newtonian theory can, and also predicts radically new phenomena, in particular, black holes and gravitational radiation. Insofar as science has been able to test these predictions, they are correct. A problem with the theory is that it predicts that under many conditions (for example, at the start of the universe and at the end of gravitational collapse to form a black hole), space-time singularities will occur. Scientists still do not properly understand this phenomenon, but presumably it means that they will have to take the effect of quantum theory on gravity into account. General Relativity does not do so; it is a purely classical theory.

Quantum gravity theories try to develop a theory of gravity that generalizes Einstein's theory and is also compatible with quantum theory. Even the way to start such a project is unclear. Approaches include twistor theory, lattice theories, noncommutative geometries, loop variable theories, and superstring theories. None has reached a satisfactorily developed state, however, much less been tested and shown to be correct. Indeed, in many ways such theories are likely to be untestable. The most ambitious are the superstring theories, now extended into a metatheory of uncertain nature known as M-theory, which promises to provide a unified theory of all fundamental forces and particles. M-theory still has far to go before making good on that promise.

Despite the lack of a definite quantum theory of gravity, various attempts have been made to develop quantum theories of cosmology. These theories also face considerable conceptual and calculational problems. The satisfactory unification of quantum theory and general relativity theory, perhaps in some unified theory of all the fundamental forces, remains one of the most significant outstanding problems of theoretical physics.

The desire to develop a practical antigravity machine remains one of humanity's outstanding wishes. No present theory offers a way to such a machine, but the negative gravitational effect of the vacuum energy will continue to inspire some to hope that one day such a machine might exist.

See also Black Holes; Cosmology, Physical Aspects; Forces of Nature; Galileo Galilei; Newton, Isaac; Physics, Quantum; Quantum Theory; Relativity, General Theory of; Singularity; String Theory; Superstrings

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george f. r. ellis