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 17th
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. 1st,
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 1st 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 17th
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?
The modern stirling engine is a clean and efficient enfine. This is because the heat driving the pistons is supplied from outside the engine and transfered through heat exchanges to the piston volume.
