A long journey of Gravitational theory
In this modern era, everyone is busy with their lifestyle. Some are busy in their job, and some are busy on getting a degree and finding a job and some are just wasting their time on useless stuff. This world has become nothing just a rat race where people grow and is influenced by this rat race and at the end, they throw himself into this rat race. Here in this world people in quest of the real happiness got trapped in the type of work which bring joy but only for short period of time. There are only few people in this world who know that the real happiness lies in exploring the beauty of this nature and for exploring this beauty we must have only one tool. That tool is " A strong desire to know everything" or in single word "CURIOSITY". In today world this tool is completely missing in the current era youth. Thanks to the poor education system and teachers who don't know how to teach children to use this tool. Do you know this tool is present inside us from that day when we begin speaking. But gradually it gets lost in somewhere as we become adult. But you don't have to worry. if you get fascinated by looking at the night sky and floods of weird questions are coming in your mind even now. Then congratulate you, you are belonging to category of those guys who does not lose their curiosity and now you just have to explore like now you are reading this article. Now without making any more delay let's begin our topic. When you read in your science textbook about our solar system in which the sun is present in the center and all other planets are revolving around sun. A common question must strike your mind. why does they not go toward the sky. what made Earth to revolve around the sun. Is there something present between earth and sun or leaves and ground that make earth revolves around sun and leaves to fall down. Today we're going to explain the reason behind this phenomenon. The quest for finding the answer to this question had begun many years ago. Our history has given birth to many great thinkers who given their numerous theories to explain these phenomena. First of all, in 4th century BC a Greek thinker Aristotle gave his own theory. According to him sense perceptions and quantitively properties of bodies are seem far more real and plausible than atomist theory and mathematical model. In his model he divided the universe into two separate spheres, the celestial sphere and the heavens above where unchanging perfection reside; and the terrestrial sphere here below, where all five tattva of nature Fire, Water, Earth, Air and Sky reside. In his model of universe, he divided the five tattva and their place on the basis of their weight and the thing which is lightest will remain above of the things which are heaviest. In his view when we drop stone from higher place. it falls because it tends toward below due to it gravitas (heaviness). As you can see, Aristotle's explanation is all common sensibly , plausible and reasonable, if you don't ask too many questions For Aristotle, a theory was acceptable if it was logical sound, if all of the ideas were consistent with each other and if the result was plausible. That is fine as far as it goes, and it is found in all the stories today. But he did not take necessary step further. He couldn't provide precise, perhaps even quantitative, explanations of the observed events that could be tested and confirmed and therefore due to it inconsistency his theory was later modified by John philoponus with the theory of impetus. Further in the 14th century at first time, Great Indian astronomer Brahmagupta spoke of that something as an attractive force and then in the early 17th century, Galileo Galilei give his major contribution in the understanding of concept of speed, acceleration and free falling. Galileo prove that Aristotle theory wrong and concluded that. " All freely falling object whether heavy or light, will cover the same distance in the same amount of time, moving with the same constant acceleration. They will hit the ground together if they are dropped together from the same height." But he does not explain why object fall and move as they do. he was providing only a description of these motions. In technical terms, Galileo provided what is known as Kinematics. After these our champion of science Isaac Newton came and published his greatest work, the Philosophiae Naturalis Principia Mathematics after forth-five year of Galileo death in 1642. Newton theory can explain every phenomenon that are happening around us. But there are many phenomena observed by astronomer like Mercury peculiar orbit around the sun which seek the demand of new gravitational theory. Then in the 19th century, our era's greatest physicist Albert Einstein gave his General theory of Relativity in which he offered a different view of gravity, one that made sense of Mercury. This is the entire story till 19th century.
Newton's law of gravitation
Newton had begun to think about planetary force while living at home during the Black Plague. There an idea came to him (perhaps when he saw an apple fall). Newton’s idea was that the force between the Sun and the planets was the same as the force that caused objects near the Earth’s surface to fall. He first tested this idea on the Earth’s attraction for the Moon. It was an ingenious argument. he already knows that distance between the center of the Earth and the center of the Moon is nearly equal to 60 times the radius of the Earth. Newton believed that the attractive force varies as 1/R2. Therefore, the gravitational acceleration the Earth exerts on the Moon should be only 1/ (60)2 or 1/3600 of that exerted upon objects at the Earth’s surface. Observations of falling bodies had long established gravitational acceleration at the Earth’s surface as about 9.80 m/s/s. Therefore, the Moon should fall at 1/3600 of that acceleration value: 9.80 m/s2 * (1/3600) = 2.72 103 m/s2. It was also common knowledge that the orbital period of the Moon is very nearly 27.33 days. The centripetal acceleration ac of a body moving uniformly with period T in a circle of radius r is ac = 4(pie)2R/T2, as indicated earlier. When you insert the observed values for the quantities R and T (in meters and seconds) for the case of the Moon orbiting the Earth and do the arithmetic, you find that the centripetal acceleration of the Moon, based upon observed quantities, should be centripetal acceleration = 2.74 103 m/s2. This is in very good agreement with the value of 2.72 103 m/s2 predicted above. From the values available to Newton, he concluded that the force by which the moon is retained in its orbit becomes, at the very surface of the earth, equal to the force of gravity which we observe in heavy bodies there. And, therefore, the force by which the moon is retained in its orbit is that very same force which we commonly call gravitational force. This was really a triumph. The same gravity that brings apples down from trees also keeps the Moon in its orbit, i.e., making it constantly fall away from a straight-line motion into space, just enough to make it move around the Earth. This assertion is an aspect of what is known as the law of universal gravitation: Every object in the Universe attracts every other object with a gravitational force.
If this is so, there must be gravitational forces not only between a rock and the Earth, but also between the Earth and the Moon, between Jupiter and its satellites, and between the Sun and each of the planets. And by Newton’s third law, we should expect the Moon to exert an equal (but oppositely directed) gravitational force on the Earth, and the planets on the Sun.
NOTE - Because the Earth is so much more massive than the Moon, the “wobble” of the Earth caused by the pull of the Moon is tiny. Similarly, our Sun, enormously more massive than the planets of our solar system, is acted on by them, but this action produces only very small motions of the Sun.
Conclusion: Newton's law of universal gravitation states that every particle attracts every other particle in the universe with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
Gravity in the perception of Albert Einstein:
Newton's law of universal gravitational force was nearly successful in giving explain for every phenomenon which are happening around us like falling of object and motion of planet. Even that Newton' law of gravitation was capable of predicting the motion of planet and star. But unfortunately, it failed to explain phenomena that happen in black holes, mercury peculiar behavior around sun and bending of light near heavy mass such as black hole. Newton also did not say anything about the cause of gravity. He said that there is attractive force between every masses. but he did not explain the origin of that attractive force. Later, the explanation for this phenomenon was given by most beautiful theory developed by great scientist Albert Einstein. Everyone know his so I think there is no need of the introduction of him.so let jump directly in his theory.
THE RELATIVITY PRINCIPLE
Compared with other theories discussed so far in this book, Einstein’s theory of relativity is more like Copernicus’s heliocentric theory than Newton’s universal gravitation. Einstein’s work on relativity comprises two parts: a “special theory” and a “general theory.” Einstein’s special theory of relativity involve the explanation of all that phenomena which happen in inertial frame (Frame of reference in which motions of observers and events that do not exhibit any accelerations) and Einstein’s theory of general relativity take into the account of event that involve acceleration. Einstein's publish his special theory of relativity in 1950 and then after 10 year he published theory of general relativity. we will talk about special theory of relativity in another article. Here, in this article we will only talk about Einstein theory of general relativity.
Einstein's theory of general relativity
To understand the Einstein gravitational theory, we first have to do some thought experiment. As we know gravitational force is attractive in nature and everything on the Earth fall due to gravitational force. Let imagine a boy named Kartik standing on the roof of supernova. Then at that moment a strong blow of air came and hit the Kartik. Due to this he falls from the roof. Now I have a question for you.
In the middle of air during falling what will Kartik feel. Yes, he will feel weightlessness. Now the question is What is weightlessness? (Assume you standing on the spring balance. At this moment gravitational force pulling you downward and in counter of it spring of balance exert equal force upward which we read as a weight on balance and feel as our weight. Now imagine a situation when there is only downward force. There is no upward perpendicular force. At this stage you will not feel your weight and you are under Free Fall and this phenomenon is usually called the phenomena of weightlessness.) In the middle of air, there is two things are happening. First there is a downward attractive force pulling toward the center of Earth and second, he was accelerating downward. Do you notice something weird?
A downward force is acting on the Kartik, but he does not feel anything even your weight. This situation is exactly like phenomena that person feel when he was in space shuttle. Now we are going to perform experiment for Kartik. After falling from the roof, he was sent to hospital. but we have to do experiment. so, we will kidnap Kartik and put him in the room. we make him to stand on weighting machine. let assume we read his weight 70 kg. Now if we send this room with Kartik in the space with the help of rocket. In space Kartik will began to float in the room. Now if we accelerate the room upward with same value equal to g and then we observe the Kartik standing on weighting machine. yes, we will get same value of weight which did we observe on the Earth. Now if we ask Kartik. Where is he? He could be able to figure where is he? No, he can't until someone tell him. do you get it? what am i want to tell you? Yes, according to Einstein Gravity is not a force but actually an acceleration. Jeans that acceleration and gravity are the same things. This can be understood more clearly if we do one more experiment. so let go back to our space room, what Kartik has died and froze due to low temperature of space. Ooh shit. Don't worry we will use robot for our further experiment. Now if we make robot to produce a beam of photon on the wall of room. Now we will observe trajectory of beam in two cases. When room is stationary and second when room is accelerating with value g. In case of stationary, the trajectory of photon is straight but in case of acceleration trajectory of photon is parabolic which mean path of light ids bending due to acceleration of room. This same phenomenon we observe in the case of Black hole and other heavy celestial object where trajectory of light gets bended due to gravity of them. From this experiment we understand that the working principle of gravity is same as acceleration. But here question arise in our mind that according to newton force of gravity only act on mass, but photon is massless so how does it affecting photon and second is that what cause this acceleration or gravity? and what is the source of gravity and connection between space, time and gravity? Relation between space and gravity. From above we know that when does light pass near the celestial bodies, it gets bended. Now Einstein's know the reason for bending of light but one question even now giving stress to Einstein's that how does that thing happen? From previous study we know that acceleration and gravity are same thing. If that is true, then light must show same behavior in both cases Earth and space room. Because if this will not true then principal equivalent become wrong, and it proved true then it will make proved many physics law wrong. Then a thought came in the mind of Einstein that light always cover possible smallest distance which curve. if we assume that true then it will mean that space can be curve and this curve nature of space fabric make light bend instead of gravity (attractive force). Yes, you got it right. Gravity is just a of space fabric due to mass of body. Greater the mass of object, greater will be the curve in the space. Now we know the real source of gravity which is the curve nature of space. Now we also got the answer to our question about the justification of bending of light, Greater the mass of body, greater the curve in the space, grater will the strength of gravity and greater will be bending of light observed. These same phenomena happen in the case of black hole. As the mass of black hole is infinite at its singularity point and therefore it produces enormous amount of curve in space which is powerful enough to cancel the return ticket of arriving light.
Time dilation:
Assume a boy who lived on the summit of mount Everest and his living on the surface of Earth. then the time for boy living on the surface will pass slower than the boy who on the peak of Mt Everest. Have you ever heard that someday talking that you can do time travel if you go near the event horizon of Black hole. Does you know what make GPS technology possible and what is relation between space and time. Yes, answer to all those questions is given by single theory Einstein's theory of general relativity. So now we know the importance of Einstein theory but how does this theory do it. Let's check it out. For answering all of this questions Einstein's take the help of his own postulate which state that speed of light will remain constant in all cases in the spaces. nothing can cross this limit of speed, and nothing can decrease the speed of light in space. Remember the case of bending of light when it passed near the celestial object. we know that this bending is caused by curve nature of space. But speed of light must be in both cases either in empty space or curved space. As we know speed = distance / time and in curve space distance travelled by light increased. But we have to maintain the speed of light constant. For this we conclude that light must travel the distance in less time. which means time become slowed down for light in curved space.
Conclusions : General Relativity states mass and energy in space will produce curve in space and this curvature of space make light to travel greater distance and consequently time will be slowed for the light to maintain constancy of speed of light. In other word, space and time are related to each other. Greater the curve in space due to mass and energy, slower will be time passed for person in this curved region and that is called Time dilation.
Einstein's space-time:
Above as we read that a curve in space cause time to slow down at that curved region. In other word, we can say that greater the curve in space, slower will be time in that region which means space and time are related.
From this we can prove Newton's theory wrong which state that space and time are different thing, and both are absolute. Here, it's clear that time and space are not more different thing but are deeply related to each other and we can say it's a space-time instead of only space.
space- time become gravitational field.
Finally, we can prove that space is just a gravitational field. Because in above article we read that space, time and gravity are interrelated so we can say that space is also a kind of field which produce gravitational waves ripples. We can also observe this ripple with the help of antenna on Earth. When wave of two black hole falling into each other.
Gravitational Lensing:
As we read above that distortion in space created due to mass can bend the upcoming light. This same phenomenon we observe in the space when large cluster if matter such as galaxy comes in the way of photon emitted from a distant galaxy which is present right behind the cluster galaxy. Gravitational field due to this cluster distort and magnify the light of distant galaxy. when watch this effect then it will look that we are looking through giant magnifier telescope. This phenomenon is very useful for those astronomers who want to study further galaxies whose observation are not possible with our current technology.
Quest for unified theory:
Einstein's relativity explains a large number of macroscopic phenomena in our universe single Handley . General relativity gives exact explanation to space-time, postulating that large mass distort both space and time, creating the effect of gravity. Einstein also give a theory on quantum mechanics which describe the behavior of light as a particle and wave. Both theories have many practical applications in our modern. However, his quantum model does not completely align with his general relativity. If we try to combine theory, then it brings disaster. Relativity treat object as point particle that exists independent masses in time and space. While Quantum mechanics treat matter as a wave function, but we can't find their exact position. we can find their probabilistic distribution in space only. Both theories are successfully able to predict in their own domain. But when we try to apply the principle of one on other then we always got to see disaster. For example, let's assume an electron revolving around nucleic. Since electron the has certain amount of mass. so according to relativity, it must curve space-time and as we know electron exit in superposition state means it exist in every possible wave function at same time, Now the question is that where it creates curve. Einstein Relativity also not capable of explaining gravity inside black hole and before the big bang. So, to solve this problem every physicist began quest for unify relativity and quantum mechanics even Einstein himself spend 30 years in solving the inconsistency of his theory. This unified theory is later called theory of everything. For this we have to unify all fundamental force of nature. As we know there are four fundamental forces in nature which are strong nuclear force, weak nuclear force, electro- magnetic force and gravitational force. The work of unifying first three fundamental force is already done by quantum mechanics. Now there is only one force remaining which is gravitational force. But due to inconsistency of gravity microscopic world, we are still unable to give a unified theory. So, in seeking of theory of everything physicist began the quest for quantum Gravity.
Quantum Gravity:
It's that theory which tell how gravity is connected with the principle of quantum mechanics or in simple word it is that theory that combine general theory and quantum mechanics. so that we can understand gravity in the quantum level like other three fundamental force in our universe, there are some phenomena which we can observe in our daily life and their behavior can be easily understood with the help of classical physics. Now we have two types of phenomena remaining. one is macroscopic phenomena and microscopic phenomena. Macroscopic phenomena are that phenomena in which object are very massive like motion of planet and star. even now we can easily understand their behavior with the help of Einstein's relativity. but this work only when mass of object is enough to neglect their quantum effect. last, we have microscopic phenomena are that phenomena in which size of objects are very small like atomic phenomena and nuclear phenomena. Their behavior can also easily understand with the help of quantum mechanics until the size of object is small enough so that we can neglect gravitational effect. But there are some objects in our universe which are tiny in size and heavy at same time as center of black hole and before the big bang. but we have not any unified theory therefore we do not know what inside black hole is and what was happening before the big bang. However, this all can be explained with the help of quantum gravity theory. if you look before the big bang then we will find that at that time all fundamental force is unified at that momenta and we found that remaining three forces have their own carrier which means gravity must also have its own carrier and if we succeeded in discovering this carrier then we will also succeed in developing unified theory. and if this happen then we will get our all answer to our question. let's we know what the things are required for quantum gravity. The carriers for three force electro-magnetic, strong nuclear and weak nuclear force are given as photon for electro-magnetic force, W/Z boson for weak- nuclear force and gluon for strong nuclear force. Now if talk about gravity, we still do not find anything in quantum level which is responsible for gravity. But physicist believed that there must be some hypothetical particle which is the carrier of gravity, and they named this hypothetical particle a Graviton. Now a question arises that how we can identify this new particle if we will find this in near future. For this we have some properties of this particles on the basis of nature of gravitational force.
Properties of Graviton: -
1. Massless (because range of gravity is infante)
2. Spin value = 2 (because gravity is a second rank denser field)
3. Neutrality
Because gravity is very weak force therefore, we are not still able to detect it carrier.
There are many theories which to try to unify this force. One of them is String theory.
String theory:
According to string theory, this universe is made up of one-dimensional vibrating string. For example, if we look inside quark, we will find this dancing string. we were know that we are living in three-dimensional world, but this theory tells that there is total ten dimensions in this world.According to this thory, If we assumed that our three dimensional world as a membrane then this theory tell us that elementary particle such as quark are made up of that string whose roots are always connected with this membrane and therefore, they can't travel in another dimension. Whereas graviton is the type of closed ring string due to this there are not connected with membrane and therefore they can easily travel from one to another dimension. Thus, all gravitons are not always present in our dimension therefore we can only experience only little part of gravity and this is the reason that gravity is the weakest force. But we can detect the graviton in Large Hadron Collider by collision of particles and if this happen then we can observe graviton in our three-dimensional world. Other than string theory there are more theory which are given below:
1. String theory
2. Supergravity Theory
3. Loop Quantum Gravity Theory
4. Twister Theory
5. Noncommutative Geometry
Among of all of them only first and second theory gain most importance in the quantum world. Now, this Article end here. I hope that you got to learn many things from this article. Let's meet again in next article.
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