WALKING INTO THE SPACE

INTRODUCTION:

EARTH ROTATES AROUND THE SUN, RIGHT! BUT, HOW DID WE COME TO KNOW?

Curiosity, an incredible gift of nature to species on earth known as homo-sapiens (humans) in scientific language.  It is this curiosity that perhaps differentiates us from every other species on the planet. The most wonderful thing about us is that we wonder. We seek answers.

Humans from the very beginning are curious about their origin. The first question that arises is how life begins on earth. Ongoing farther the question arises how earth comes into the existence. Then the problem gets bigger as we want to know, where did the universe came from? What phases the universe had gone through before taking its present shape? What is the future of universe?  Did the universe have a beginning? What is the nature of time? Why does it always go in forward direction? Can we go back in time? Are we alone in this incredible, infinite, endless universe? Was it a coincidence that life started on earth or was it driven by some supernatural power? Does God really exist? Do we need to consider the existence of God, for explaining the origin of universe? Or can it be explained by the simple laws of nature?

Man has been searching the answers of these questions for a long time. Some of them had been solved, some still remain mystery. Till today, whatever man had concluded, he threaded it into a single branch of science known as astrophysics (physics of stars and planet).  It is one field where aptitude matter more than academic achievements. So, we will try to discuss about the origin, evolution of the universe. Also, we will discuss about the contributions made by some of the greatest physicists of all times.

Knowing about the motion of earth around the sun was perhaps the first major milestone achieved by the human civilization. Indian and Greek philosophers were the first to take interest in knowing the motion of planets and stars. The great Indian mathematician Aryabhata was the first who concluded that it is earth that rotates around the sun. He once said, “As for a sailor sailing in a sea, outside objects seems to be moving in opposite direction, for a person on earth, sun seems to be moving around the earth”. But unfortunately, his work remains unknown to the most of the world because lack of communication at that time. Thus, until 17^th century sun was supposed to be moving around the earth.

Around 340 BC the Greek philosopher Aristotle, in his book ON THE HEAVENS, suggested that earth was spherical but not a flat plate. In support of his conclusion he put up two good arguments. 1^st, he realized that eclipses of the moon were caused by the earth coming b/w sun and moon. Since the shadow was round shaped, it meant that earth must be round. If it had been a flat disk, shadow would have been elliptical (ellipse is the elongated form of the circle) for most of the time. Secondly, North Star appeared lower in the sky when viewed in the south then it did in the northerly regions. It was because North Star lies over North Pole, it appears to be directly above an observer at the North Pole but not to an observer at the southern region. Greek also argued that since we first see the sails of a ship coming over the horizon and later see the hull, it means that earth must be spherical.

There were some false assumptions made by the Aristotle. He thought that the earth is stationery. He believed that earth was the center of the universe and everything else moved around it in circular orbits. The cosmological model of this idea was given by Ptolemy. He suggested that earth was at the center of the universe and was surrounded by eight spheres. The 1^st sphere carried the moon. The next six carried Mercury, Venus, Sun, Mars, Jupiter and Saturn respectively. The eighth and the last sphere carried the whole of the universe where stars supposed to be fixed relative to each other but rotate about the sky.

But he was unable to explain what was beyond that eighth sphere. He also did not explain the continuously changing size of moon as it sometime appears twice as big as at other times. Regardless of these defects the model became popular and was widely accepted.

Nicholas Copernicus, a Polish priest, suggested that earth was stationery and planets move around the sun in stationery circular orbits. But his idea was not taken seriously for the next hundred years.

About a century later, an Italian astronomer, Sir Galileo Galilee, widely regarded as the father of modern science, started to observe the night sky with a telescope. Galileo found that a number of moons or satellites orbit the planet Jupiter. Thus, he concluded that everything did not have to revolve around the earth directly. Johannes Kepler, another astronomer from Germany, at the same time found the laws of planetary motion. He suggested that planets moved in ellipses but not in circles. The prediction of elliptical orbits matches the observations perfectly. The several facts like change in apparent size of moon now can be described by taking earth as one of the foci of its elliptical orbit. But Kepler was still unable to explain the nature of the force that was responsible for the motion of stars and planets.

 The explanation was provided in the 17^th century by one of the greatest physicists ever, Sir Isaac Newton, in his book “PHILOSOPHIAE NATURALIS PRINCIPIA MATHEMATICA”. In his book, he not only put forward the Laws of Motion and Gravitation (laws that determines how bodies moves in space and time) but also, almost single handedly, developed a new branch of mathematics known as Calculus which was needed to analyze those motions.

  According to Newton’s law of Gravitation, “Each body is attracted by every other body in the universe with a force that depends upon their mass and distance b/w them. More massive the bodies were more will be the force and more will be the distance b/w them lesser will be the force”. Then after, Newton went to show that gravity causes the moon to move around the earth and the earth and the other planets to move around the earth in an elliptical orbit.

According to Newton’s theory of gravity, the star should attract each other, so they should remain in motion. But if they keep attracting each other they should ultimately fall together at some point. But this did not happen? WHY?

According to Newton’s law, ‘the stars, the planets, the galaxies and ultimately everything should fall together at a point’. But this did not happen.

Newton realized that since the universe is infinite, it has no center. Therefore, the stars have no point or center to fall in. If there would have been a finite number of stars in the universe then they indeed have possessed a center and must have fallen together at that point.     

In an infinite model of universe, every point could be considered as its center. If we consider a finite model of universe, in which the stars all fall in on each other. Suppose we increase the number of stars in that region. Then believing the Newton’s law, these stars too would fall in at the same point as it did not affect the already existing stars. Similarly, we can add as many stars as we want to. They all would still collapse at the assumed center. If we add an infinite number of stars outside this region, it would become an infinite static universe with a center, where according to Newton’s law of gravitation every star would fall to its center. Thus, we can conclude that, “it is impossible to have an infinite static model of universe where gravity is always attractive”.

NATURE OF GRAVITY:

Newton argued that universe could not be static and nature of gravity is always attractive. Also, we see that universe could not be static where gravity is always attractive. At that point of time people had two types of thought in their mind. The first one was that the universe could not be static and gravity was not always attractive. Since, it was easy to believe in having a creator of universe, some started believing that universe was static and unchangeable. Those who started believing in Newton’s theory started to modify the theory by taking gravity as repulsive in nature at very large distances.

But this assumption was logically wrong. HOW?

If we consider gravity as attractive at small distances and repulsive at large distances then b/w the planets or stars which are relatively closer to each other attractive forces will dominate and they will keep moving towards each other. Thus, ultimately they will fall together at some point. Similarly, b/w stars which are farther from each other repulsive forces will dominate and they will keep moving away from each other. So, this idea was not acceptable.

The predictions were still not matching the observations correctly. We still were far away from a theory that would explain the behavior of universe. The questions that were not explained till the beginning of 20th century were:

1. What is the nature of gravity?

2. Is universe static or not?  

In 1929, an astronomer, Edwin Hubble made a remarkable observation which just revolutionized our journey of understanding our origin and evolution of universe.

But till that time we had completely changed our view and the way we see the world. Our understanding of physics and nature was completely changed by some of the great discoveries.

What were the discoveries that had helped and still are helping through our journey of understanding the origin and evolution of universe?