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ISAAC NEWTON, about 300 years ago, theorized how gravity works. He imagined a man throwing an object from the top of an unusually high mountain. If simply dropped, the object would fall, as would an apple, downward to the ground.
If, however, it was thrown forward, it would follow a curved path in falling to the ground. Newton then reasoned that if thrown fast enough, it would circle the earth in an orbit.
From this theorizing, the link between gravity and the movements of the moon and the planets became apparent to him: the moon bound in an orbit around the earth because of the pull of earth's gravity and the planets kept in their orbits by the sun's gravity.
A Universal Law
After careful study, Newton formulated a precise mathematical description of this universal law. Simply stated, Newton's equations said that all objects, small or large, exert a pull on one another, the strength of that pull being dependent on how massive the objects are and on the distance between them.
With some refinements, scientists still use Newton's basic formulas describing gravity, particularly in planning such space ventures as sending a space probe to encounter Halley's comet in 1985. In fact, English astronomer Edmond Halley, a colleague of Newton, used Newton's theories to predict the year when that comet would next appear.
Newton's discoveries about gravity gave him a glimpse of the order manifest in the universe, an orderliness that arises through intelligent design. But his work was by no means the final word on the subject. At the beginning of this century, scientists came to realize that some aspects of Newton's theories were inadequate, even inconsistent.
Einstein and Gravity
In 1916 Albert Einstein put forward his general theory of relativity. His amazing discovery was that gravity not only shapes the universe but also governs the way we see and measure it. Why, gravity even affects the way time is measured!
Again, an illustration helps clarify matters. Imagine space to be like a boundless rubber sheet. Now, placing an object on this flexible mat will cause a dimple, or depression. According to Einstein's description, the earth, the sun, and the stars are like objects on a flexible mat, causing space to curve. If you roll another object onto the rubber sheet, it will be deflected into a curved path by the depressed area around the first object.
Similarly, the earth, the planets, and the stars move along curved paths, following the natural "depressions" in space. Even a beam of light is deflected when passing near massive objects in the universe. Furthermore, Einstein's equations predicted that light traveling against gravity would lose some of its energy, as noted by a slight shift in color toward the red end of the spectrum. Physicists call this phenomenon gravitational redshift.
Thus, besides clearing up the discrepancies arising from Newton's discoveries, Einstein's theory revealed new secrets of how gravity works in the universe.
The ability of gravity to affect the way light travels gives rise to some astonishing consequences that astronomers have observed.
Desert travelers have long been familiar with mirages optical illusions that have the appearance of water shimmering on the ground. Now, astronomers have photographed cosmic "mirages." How is this?
Light from a distant object, believed to be the active nucleus of a galaxy and called a quasar (or, quasi-stellar object), passes intervening galaxies in the line of sight from the earth. As the light passes the galaxies, it is bent by gravitational forces. The bending of the light forms two or more images of the one quasar. An observer on earth, thinking that light has come straight toward him, concludes that he is seeing more than one object.
Another fascinating aspect arising from Einstein's work concerns black holes. What are they, and what is their connection with gravity? A simple experiment serves to answer.
Try throwing an object above your head. You will notice that it rises to a certain height, stops momentarily, and then falls back to the ground. With light it is different. A beam of light can escape from earth's gravity because it travels fast enough.
Suppose now that the force of gravity was much stronger, strong enough to prevent even light from escaping. From such an object, nothing could escape. The object itself would be invisible because no light could escape its gravity and reach the eyes of an outside observer, hence the name black hole.
The German astronomer Karl Schwarzschild was the first to demonstrate the possibility, in theory, of black holes. Although there is, as yet, no unequivocal proof that black holes really do exist in the universe, astronomers have identified a number of possible candidates. Black holes may also be the hidden powerhouses of quasars.
On the basis of Einstein's work, we can also picture gravity as an invisible web, linking everything and holding the universe together. What happens when that web is disturbed?
Consider again the illustration of the rubber sheet, and suppose that an object on the sheet is suddenly jostled to and fro. The vibrations generated in the sheet will disturb nearby objects. Similarly, if a star were violently "jostled," ripples in space, or gravity waves, might be generated. Planets, stars, or galaxies caught in the path of a gravity wave would experience space itself contracting and expanding like a rubber sheet vibrating.
Since these waves have not as yet been detected, what proof do scientists have that Einstein's theory is correct? One of the best indications comes from a star system known as a binary pulsar. This consists of two neutron stars in orbit about a common center, with an orbital period of about eight hours. One of these stars is also a pulsar it emits a radio pulse as it rotates, like the sweeping light beam from a lighthouse. Thanks to the precise timing of the pulsar, astronomers can map the orbit of the two stars with great precision. They find that the time of orbit is slowly diminishing in exact agreement with Einstein's theory that gravity waves are being emitted.
On the earth, the effects of these waves are infinitesimal. To illustrate: On February 24, 1987, astronomers spotted a supernova a star undergoing a spectacular transformation, blazing forth with the brilliance of millions of suns as it blew off its outer layers. Gravity waves produced by the supernova would cause, on the earth, a shiver in dimension of only a millionth of the diameter of a hydrogen atom. Why so small a change? Because the energy would be spread out over a vast distance by the time the waves reached the earth.
In spite of great advances in knowledge, certain fundamental aspects of gravity still baffle scientists. It has long been assumed that there are basically four forces the electromagnetic force responsible for electricity and magnetism, the weak and the strong forces acting within the nucleus of the atom, and gravity. But why are there four? Could it be that all four are manifestations of a single fundamental force?
Recently it was established that the electromagnetic force and the weak force are manifestations of an underlying phenomenon the electroweak interaction and theories seek to unify the strong force with these two. Gravity, however, is the odd one out it does not seem to fit in with the others.
Scientists hope that clues may come from recent experiments performed in the Greenland ice sheet. Measurements made down a one-and-a-quarter-mile-deep [2,000 m] hole bored in the ice seemed to indicate that the force of gravity differed from what was expected. Previous experiments, performed down mine shafts and up television towers, likewise indicated that something mysterious was causing deviations from the predictions of the Newtonian description of gravity. Meanwhile, some theoreticians are trying to develop a new mathematical approach, the "superstring theory," in order to unify the forces of nature.
Gravity Vital for Life
The discoveries of both Newton and Einstein demonstrate that laws govern the movements of heavenly bodies and that gravity acts as a bond holding the universe together. A professor of physics, writing in New Scientist, drew attention to the evidence of design in these laws and said: "The most minute change in the relative strengths of gravitational and electromagnetic forces would turn stars like the Sun into blue giants or red dwarfs. All around us, we seem to see evidence that nature got it just right."
Without gravity we simply could not exist. Just consider: Gravity holds our sun together, sustaining its nuclear reactions, which supply our needed heat and light. Gravity keeps our spinning earth in orbit around the sun making day and night and seasons and prevents us from being thrown off like mud from a spinning wheel. Earth's atmosphere is held in place by gravity, while the pull of gravity from the moon and the sun generates regular tides that help circulate the waters of our oceans.
Using a tiny organ of our inner ear (otolith), we sense gravity and learn to take it into account from infancy when walking, running, or jumping. How much more difficult it is for astronauts when they have to cope with zero-gravity conditions in spaceflight!
Yes, gravity contributes to making life on earth normal for us. It is, indeed, a fascinating example of our Creator's "wonderful works." Job 37:14, 16.
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