Concepts about the universe By Stephen Hawking

Stephen Hawking gave a series of lectures on the history of the universe from the Big Bang to the Black Hole. In this speech he gives an outline of his thoughts. In the first speech, he briefly described the ideas of the past about the universe. This series of lectures has been published in the book The Theory of Everything by Stephen Hawking, published from the first edition.

Stephen Hawking. Photo.

The earth is not like a flat dish, but a round object. Long before this is believed, Aristotle was able to make at least two good arguments in 340 BC. He made these arguments in a book entitled On the Heaven. First, he realized that the lunar eclipse was due to the earth coming between the sun and the moon. During a lunar eclipse, the shadow of the earth falling on the moon always looks round. This can only be true if the earth's own shape is round. On the other hand, if the earth were flat like a disc, its shadow on the moon should be delayed and oval. However, in that case, if the sun had ever been directly above this flat disc, then only the shadow of the earth would have appeared on the moon.

Second, because of travel at home and abroad, the Greeks knew that the polar stars from the south were slightly lower in the sky than in the north. Aristotle calculated the difference in the relative positions of the pole stars in Egypt and Greece and mentioned that the circumference of the earth is approximately four thousand stadia. The exact length of one stadia is not known. However, it is thought to measure 200 yards. As such, Aristotle received a number twice the length of the Earth's current recognition.

The Greeks, of course, had a third reason why the earth was round. If the earth were not round, why would a ship sailing from the horizon to the coast be seen first and then the mast? Aristotle thought that the earth was stationary and that the sun, moon, planets, and other stars revolved in a circular orbit around the earth. The reason behind his belief was spiritual. That is why he realized that the earth is the center of the universe and in this case the circular motion is the most perfect.

This idea of ​​Aristotle was developed in more detail by Ptolemy in the first century AD on a completely cosmic model. In this model the position of the earth was at the very center and there were eight spheres around it. These spheres carried the moon, sun, other stars, and the five known planets বুধ Mercury, Venus, Mars, Jupiter, and Saturn. In this model, the planets themselves revolve in a small circle next to their respective spherical orbits. This was done to explain the complex path of the planets found in the sky observation. The very last sphere of Ptolemy's model carried the so-called stationary stars. These fixed stars are always fixed in the same place as the others and always revolve in the sky at the same time. Of course what is after this last sphere, this model has never said anything clearly. But there was no doubt that there was no part of the universe seen through the skin of mankind.

This model of Ptolemy was able to suggest a fairly accurate method of predicting the position of celestial objects in the sky. However, Ptolemy had to guess some things in order to accurately predict the position of objects. For example, he had to assume that the moon follows an orbit that sometimes brings the moon closer to Earth than at any other time. That is, according to the calculations, the moon should sometimes appear twice as large as the normal time. However, Ptolemy was aware of this flaw in his model. But even then his model was not universally accepted by everyone in general. The Christian Church, meanwhile, supported the model, finding scriptural similarities with the model as a picture of the universe. Because the biggest advantage of the model was that there was quite a bit of space to place heaven and hell outside the sphere of fixed stars.

However, in 1514, a simpler model was proposed by the Polish priest Nicolas Copernicus. Initially Copernicus published his model under a pseudonym. Because he was afraid of being accused of being against the conventional religion. In his model, the Sun was stationary at the center of the universe, and the Earth and other planets revolved around the Sun in various circular orbits. Unfortunately for Copernicus, almost a century later, no one at the time valued his idea. Then two astronomers began to openly support Copernicus' doctrine. One of them is the German astronomer Johannes Kepler and the other is the Italian astronomer Galileo Galilei. However, the predictions of orbit made by Copernicus doctrine did not exactly match the observations. But even then Kepler and Galileo supported it. Thus died the Aristotelian-Ptolemy theory in 1809. That same year Galileo began observing the night sky through binoculars. Then the telescope has just been discovered.

Galileo looked up at the planet Jupiter and saw several small satellites or moons orbiting the planet. This is the first proof of service that not all objects in the sky revolve directly around the earth. But Aristotle and Ptolemy had the same idea. However, after Galileo's discovery, it was possible to believe that the earth was at the center of the universe. But the problem is, Jupiter's moons would have to orbit the earth in some extremely complex orbit. However, it seems that they are revolving around Jupiter. Rather, Copernicus' theory was much simpler than this complexity.

About the same time, Kepler adapted Copernicus' theory. By this he proposed that the planets revolve in an elliptical way, not in a circular way. Eventually, his observations matched his observations perfectly. Kepler took this elliptical orbit of the planet as a temporary hypothesis for some special advantage. But this hypothesis was his dislike. Because, according to him, elliptical orbits are actually less perfect than circular orbits. Much by accident he discovered that elliptical orbits coincided well with observations. His idea was that the planets revolved around the sun because of the magnetic field. But he could not reconcile the concept of elliptical orbits with his own.

An explanation was found long after this incident. That was 18 years. That year, scientist Newton published a book entitled Principia Mathematica Naturalis Cause. This book is probably the most important single research publication in physics. In the book, Newton not only developed the theory of how objects move in space and time, but also developed the mathematics necessary to interpret and analyze these motions. In addition to these, Newton established a law of universal gravitation. According to this formula, every object in this universe is attracting each other with a force. The larger the objects and the closer they are to each other, the stronger the force will be. The same force is the reason behind the fall of an object on the surface. It is natural to doubt the veracity of the story of Newton reading an apple in his head. Newton's own commentary on this subject is that once he was sitting in a meditative state. Just then he saw an apple read. Just then the thought of gravity came to his mind.

With his formula, Newton showed that the moon revolves around the earth in an elliptical orbit due to gravity. For the same reason, the earth and other planets are following an elliptical path around the sun. Copernicus's model canceled Ptolemy's B-sphere long ago. At the same time, his model was able to disprove the idea of ​​a natural boundary of the universe. Apparently, fixed stars did not change their relative position as the earth revolved around the sun. So it is natural to assume that stationary stars are objects like the sun, but that they are far away from us. But there was also a problem. Newton realized that according to his law of gravitation, these stars were supposed to attract each other. So they can't be stable in any way. So shouldn't they collapse or break at some point?

In a letter to Richard Bentley, another leading thinker of the time, in 1891, Newton made an argument in this regard. In the letter, he said the phenomenon could only be true if there were a finite number of stars. He also made another argument. He said that if there were a fairly evenly distributed infinite number of stars in an infinite space, they would not break at any one point. Because then there would be no focal point for them to break down. When it comes to talking about infinity, you have to face the problem of stealth. This argument is one such example.

All the points of an infinite universe can be considered as centers. Because then there will be an infinite number of stars around each point. So the right way to do this is to consider a finite situation, where all the stars will fall on top of each other. Of course, this was understood much later. In this case, the question may arise, what would happen if more fairly evenly expanded stars were added outside this region. According to Newton, these extra stars would not be able to change the position of the previous stars very much. So the stars will continue to fall as fast as before. We can add as many stars as we like, but they will always fall apart on their own. Now we know that it is impossible to have an infinitely fixed model of a universe where gravity is always attracting.

The most striking aspect of the general picture of conventional thinking before the twentieth century is that no one at the time could have imagined that the universe was expanding or shrinking. It was then generally assumed that the universe was either unchanged for eternity or that it had been created in the past a finite period of time, which has not changed much at present. There was also a tendency among people to believe in an eternal truth. So people would grow old and die someday, but they would be relieved to think that the universe would not change.

Newton's theory of gravitation proves that the universe can never be static. But even those who understood this could never think of proposing that the universe could expand. Instead, they tried to transform that theory. In this theory, they repulsed the gravitational force at a great distance. Of course, their predictions did not help the planets. This, however, was arranged to balance an infinite distribution of stars. In this case, the gravitational force of nearby stars collided with the repulsive force of distant stars to balance.

In any case, we now believe that such a balance is bound to be shaky. If the stars in a region move a little closer to each other, the force of attraction inside them will become much stronger and it will be stronger than the force of repulsion. This means that the stars will continue to fall on each other. On the other hand, if the stars move a little farther away from each other, the repulsive force will begin to dominate the force of attraction. Even then, they will continue to move away from each other.

There is also another charge against an infinitely stable universe. The German philosopher Henrik Albers is generally blamed for this. In fact, many of Newton's contemporaries raised the issue. But many still think that the first acceptable argument was made in an article published by Albers in 1823. But that is not true. In fact, this article was first noticed by many. The problem is, in an infinitely stable universe, almost all the lines or sides will end up on the surface of a star. In that case it can be expected that the whole sky will be as bright as the sun even at night. Albers argued that light from distant stars would be dimmed by absorbing interstitial objects.