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OUR PICTURE OF THE UNIVERSE
Hawking, a distinguished theoretical physicist, explores the big picture of the universe. Excerpts have been taken from Hawking’s material, and sometimes adapted, in these notes.
Recent breakthroughs in physics, made possible in part by fantastic new technologies, suggest answers to some of humankind’s longstanding questions:
· What do we know about the universe, and how do we know it?
· Where did the universe come from, and where is it going?
· Did the universe have a beginning, and if so, what happened before then?
· What is the nature of time, and will it ever come to an end?
As long ago as 340 B.C., the Greek philosopher, Aristotle, in his book On the Heavens, believed that the earth was round sphere rather than a flat plate. He thought that the earth was stationary, and that the sun, moon, planets, and stars moved in circular orbits around the earth – that the earth was the centre of the universe.
In 1514, a Polish priest, Nicholas Copernicus, proposed that the sun was stationary at the centre, and that the earth and planets moved in circular orbits around the sun. Nearly a century later, two astronomers – the German, Johannes Kepler, and the Italian, Galileo Galilei – supported the Copernican theory. Kepler suggested that the planets moved not in circles, but in ellipses (elongated circles).
In the 17th century, Sir Isaac Newton postulated a law of universal gravitation – according to which each body in the universe was attracted toward every other body by a force that was stronger the more massive the bodies and the closer they were to each other. It was this same force that caused objects to fall to the ground. Also, gravity causes the moon to move in an elliptical orbit around the earth, and causes the earth and the planets to follow elliptical paths around the sun.
Newton realized that, according to his theory of gravity, the stars should attract each other, so it seemed they could not remain essentially motionless. We now know it is impossible to have an infinite static model of the universe in which gravity is always attractive.
The question always arises as to the beginning of the universe. According to a number of early cosmologies, and the Jewish/Christian/Muslim tradition, the universe started at a finite, and not very distant, time in the past. One argument for such a beginning was the feeling that it was necessary to have a “First Cause” to explain the existence of the universe. Also, the concept of time has no meaning before the beginning of the universe. (This was first pointed out by Augustine.)
In 1929, Edwin Hubble made the landmark observation that wherever you look, distant galaxies are moving rapidly away from us. In other words, the universe is expanding. This means that at earlier times, objects would have been closer together. This discovery finally brought the question of the beginning of the universe into the realm of science. Hubble’s observations suggested there was a time, called the big bang, when the universe was infinitesimally small and infinitely dense. One may say that time had a beginning at the big bang.
In an unchanging universe, a beginning in time is something that has to be imposed by some being outside the universe; there is no physical necessity for a beginning. One can imagine that God created the universe at literally any time in the past. On the other hand, if the universe is expanding, there may be physical reasons why there had to be a beginning.
Today, scientists describe the universe in terms of two basic partial theories – the general theory of relativity and quantum mechanics. The general theory of relativity describes the force of gravity and the large-scale structure of the universe. Quantum mechanics, on the other hand, deals with phenomena on extremely small scales. Unfortunately, however, these two theories are known to be inconsistent with each other. One of the major endeavours in physics today is the search for a new theory that will incorporate them both – a quantum theory of gravity.
Now, if one believes that the universe is not arbitrary, but is governed by definite laws, one has to ultimately have to combine the partial theories into a complete unified theory that will describe everything in the universe.
Today we still yearn to know why we are here, and where we came from. Our goal is nothing less than a complete description of the universe we live in.
Source: Stephen Hawking, “Our Picture of the Universe”, in A Brief History of Time (New York: Bantam, 1991), 1-13.
Photo credit: Intellimon Ltd.