How do we judge the reality of a physical entity? We commonly know something is real when we can detect it with our senses. The sense which convinces us most is the sense of sight - seeing is believing.
How do we know that gravity is real? We can note the effect of gravity on the body, pulling us down to earth, but we cannot see gravity or detect it directly with any of our senses. However, the latter observation is no longer strictly true. It is now possible for astronomers to "see" gravity by observing its effect on light from distant objects.
Isaac Newton (1642-1727) formulated the first scientific description of gravity as "action at a distance". All material objects (masses) attract each other through the force of gravity regardless of the medium between them or the distance between them.
Newton formulated a law of universal gravitation which says that the force of attraction operating between two masses is equal to the two masses multiplied together, all divided by the square of the distance between them, and then multiplied by a constant known as the constant of gravity, which is the same everywhere in the universe and at all times. The constant of gravity is a measure of the basic strength of the force of gravity.
A force field is the region in which one object exerts a force on other objects. The universe is, therefore, pervaded by a gravitational field whose local strength depends on the masses which are present.
In Newton's time the gravitational field was a mathematical abstraction. It could be used to predict natural events with great accuracy but this did not erase the abstract nature of the gravitational field.
Albert Einstein (1879-1955) radically changed our conception of the gravitational field when he identified it with space-time in his general theory of relativity. Einstein's view of the universe so mingled space and time that either concept alone becomes meaningless.
Einstein's universe is four-dimensional, with time as one of the dimensions, in addition to length, breadth and height. This four-dimensional fusion is referred to as space-time. Einstein explained gravity as a property of spacetime rather than as a force.
As a result of the presence of matter space becomes curved and anything moving through space-time, including light, will, as it were, trail along the line of least resistance on these curves. This is summarised in the saying: "Matter tells space-time how to bend, space-time tells matter and light how to move."
Einstein's concept is more striking and easier to visualise than Newton's but it remains an abstraction because both space and time are abstractions which only get their meaning from the concrete rulers and clocks embedded in their definitions. Space-time and the gravitational field it gives rise to remained inaccessible to the senses - at least initially.
In 1911, Einstein predicted that light rays passing close to a massive body, such as the sun, would be noticeably deflected by the powerful gravitational field of the star. Actually this had previously been predicted in 1804 by the German scientist Johann Georg von Soldner, who calculated that light grazing the sun would be deflected by a small angle. This deflection was too small to be detected by the telescopes of the day and, in any event, the sun is far too bright to allow observations close to its edge.
Einstein recalculated the amount of deflection to be expected as light grazed the sun, using his new theory of general relativity, and came up with a deflection exactly twice von Soldner's calculation. But, more importantly, Einstein cleverly proposed that this deflection could be detected during a solar eclipse when the sun's brightness is blocked from reaching earth, and stars near the rim of the sun become visible.
Einstein's prediction was tested in 1919 during a solar eclipse. Many stars, whose true position was known, appeared displaced from their positions by the predicted amounts when their light was measured while passing close to the sun. This spectacular confirmation of Einstein's prediction made him an international hero and the terms space-time and relativity entered the popular consciousness, if not the popular understanding.
The bending, focusing and distorting of light by gravitational fields throughout the cosmos, a phenomenon known as gravitational lensing, now allows astronomers to "see" the gravitational field as a moving rippling continuum which extends throughout the universe.
As a rough analogy, imagine you have a pint tumbler made of perfectly clear glass. The world viewed through the thick base of the tumbler will look grossly distorted because of the random ways the uneven glass bottom refracts (bends) light which passes through it. Even if the glass is perfectly clear, and in that sense invisible, you can still "see" it because of its distorting effect on images.
Gravitational lensing can produce strange effects. For example in 1970 a double quasar was imaged. Quasars are extremely distant sources of powerful radiation thought to be young versions of active galaxies. It turned out that the double quasar was an optical illusion. There is, in this instance, only a single quasar but light from it passed simultaneously around each side of a huge intervening galaxy to produce two images of the same object.
The gravitational field of the earth holds and enfolds us just as surely as the ocean does when we swim underwater and science now allows us to apprehend gravity just as we can apprehend the water we swim in.
This article was inspired by a piece written by Hans Christian van Baeyer in the April 1990 of The Sciences.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC