GRAVITY & SPACE

What is gravity?

Gravity is a force pulling together all matter (which is anything you can physically touch). The more matter, the more gravity, so things that have a lot of matter such as planets and moons and stars pull more strongly.
Mass is how we measure the amount of matter in something. The more massive something is, the more of a gravitational pull it exerts. As we walk on the surface of the Earth, it pulls on us, and we pull back. But since the Earth is so much more massive than we are, the pull from us is not strong enough to move the Earth, while the pull from the Earth can make us fall flat on our faces.
In addition to depending on the amount of mass, gravity also depends on how far you are from something. This is why we are stuck to the surface of the Earth instead of being pulled off into the Sun, which has many more times the gravity of the Earth. 

Is there gravity in space?

There is gravity everywhere. It gives shape to the orbits of the planets, the solar system, and even galaxies. Gravity from the Sun reaches throughout the solar system and beyond, keeping the planets in their orbits. Gravity from Earth keeps the Moon and human-made satellites in orbit.
It is true that gravity decreases with distance, so it is possible to be far away from a planet or star and feel less gravity. But that doesn't account for the weightless feeling that astronauts experience in space. The reason that astronauts feel weightless actually has to do with their position compared to their spaceship. We feel weight on Earth because gravity is pulling us down, while the floor or ground stop us from falling. We are pressed against it. Any ship in orbit around the Earth is falling slowly to Earth. Since the ship and the astronauts are falling at the same speed, the astronauts don't press against anything, so they feel weightless.

You can feel something very like what the astronauts feel for a moment in a fast-moving elevator going down or in a roller coaster, when you start going down a big hill. You are going down rapidly, but so is the roller coaster or the elevator so for a second you feel weightless.

Is there energy in space?

There is abundant energy in space. Even though most of deep space (the vast stretches of empty area between planets, stars and moons) is cold and dark, space is flooded constantly by electromagnetic energy. All stars in the universe produce energy and send it out into space. Planets, asteroids and moons reflect that energy back, glowing in the darkness. They can also release energy themselves in the form of heat from volcanoes or other processes. Virtually everything in space is in motion, so there is also kinetic or motion energy in space

IN MY TERM:-
gravity is a result it is not a thing. points of energy interact and form sub atomic particles - these particles interact and form atoms and atoms interact and form... As particles interact and they form energy which warps the lattice work of points of energy causing dips within the framework. The bigger the force of complicated particles (like our earth) the more it warps space and the more things slide towards it. If smaller forms are rocking by they fall in to an orbit. 


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What is Gravity

All of us know the effects of the mysterious force called gravity. However, the question 'what is gravity' is not easy to answer at all. The reason is that we don't really understand what this force actually is (if it is a force at all).
The 'Giants'
It would have been nice if we could have popped the question 'what is gravity' to the 'gravity-giants' like Kepler, Newton and Einstein. Maybe they could explain the characteristics and effects of this phenomenon properly and we could then (perhaps) answer the question.
Kepler could not explain gravity, but amazingly, he worked out the details of how the orbits of the moon and planets can be described mathematically. This is known as the Kepler laws of planetary motion, as described later, but it does not answer the question 'what is gravity'.
Newton, reportedly while observing an apple falling from a tree, got an inspiration that allowed him to work out how the force of gravity can be described mathematically. It later became apparent that there are some scenarios where Newton's mathematical description does not quite hold, but it still the simplest way of describing gravity. It does however also not answer the 'what is' question.
Einstein later worked out how the force of gravity is not quite a force, but rather an artifact of the natural movement of objects through curved four-dimensional spacetime. Einstein reportedly got the inspiration for this imaginative leap in understanding of gravity by contemplating a man falling off a building. Such a falling man would not experience any force while he is falling, at least not before hitting the ground and suffering severe forces.
Kepler's Gravity (1605)
Johannes Kepler's noted his three laws of planetary motion in 1605, by studying the precise measurement of the orbits of the planets by Tycho Brahe. He found that these observations followed three relatively simple mathematical laws, i.e.
1. The orbit of every planet is an ellipse with the Sun at one of the two focus points.
2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.
3. The squares of the orbital periods of planets are directly proportional to the cubes of the major axis (the "length" of the ellipse) of the orbits.
However, the physical explanation of this behaviour of the planets came almost a century later when Sir Isaac Newton was able to deduce Kepler's laws from his laws of motion and his law of universal gravity, using his prior invention of calculus.
Newton's Gravity (1687)
In his 'Principia' of 1687, Isaac Newton included his famous three laws of motion and the law of 'universal gravitation', which can be briefly stated as:
1. An object in motion will remain in motion unless acted upon by a net force.
2. Force equals mass multiplied by acceleration.
3. To every action there is an equal and opposite reaction.
4. The force of gravity is proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses.
Double one of the two masses and the force of gravity will also double. Double the distance between the masses and the force of gravity will be four times weaker.
Newton was uncomfortable with his own theory of gravity and in his words, never "assigned the cause of this power. He was unable to experimentally identify what produces the force of gravity and he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was not sound science.
It is now known that Newton's universal gravitation does not fully describe the effects of gravity when the gravitational field is very strong, or when objects move at very high speed in the field. This is where Einstein's general theory of relativity rules.
Einstein's Gravity (1916)
In his monumental 1916 work 'The Foundation of the General Theory of Relativity', Albert Einstein unified his own Special relativity, Newton's law of universal gravitation, and the crucial insight that the effects of gravity can be described by the curvature of space and time, usually just called 'space-time' curvature.
It is reasonably easy to accept that space can be curved – after all, we all know that a disk has a curved edge, but how can time be 'curved'? The secret lurks in the way that space and time is combined into space-time. Normally, a space-time diagram is drawn with a straight horizontal spatial axis and a straight vertical time axis. Just bend the two straight axes a little and we have curved space-time.
What is Gravity? (Newton) 

The horizontal axis of the diagram represents space and the vertical axis time (actually time multiplied by the speed of light) - hence it is a spacetime diagram. The mass M disturbs the spacetime in such a way that it causes the spacetime path of a particle P to be curved towards the mass.
At a particular radial distance r from the mass, the particle P follows a curved path that has a center at a distance R from the particle, defining a point called the center of spacetime curvature.
Although it may look like it, this diagram does not represent a particle in orbit around the mass, or around the center of curvature. Because it is a spacetime diagram, it represents the flow of time PLUS the movement of particle P towards the mass M - i.e., the particle is starting to fall directly towards the mass.
The radius of spacetime curvature is indicated on the diagram as R. As you will spot, the radius of curvature has something to do with the acceleration that the particle will suffer - the centripetal acceleration towards the center of curvature.
If we plug in real values, like Earth's mass as M, the gravitational constant G and the radius of Earth as r (with c the speed of light, what else?), we find that the centripetal acceleration is just about the acceleration of 1g that keeps us firmly on the surface of Earth. The tiny difference is due to Earth's rotation, Earth's uneven density and the fact that Earth's is not a perfect sphere.
The above holds well for weak gravity fields and low speed movement, i.e., the Newtonian limit of general relativity. In strong gravity fields, the curvature of spacetime and the effect of velocity must be catered for. They both have the effect of lengthening the radius of curvature of the path of the particle. The diagram below illustrates this shift in the position of the center of curvature in an exaggerated fashion.
What is gravity? (Einstein) 

 Essentially, the center of curvature drops below the x-axis, firstly due to curved space-time and then also due to velocity. The resultant radius of curvature is hence modified by a relativistic factor, which is rather difficult to express in simple terms.
In essence, the original (quasi-Newtonian gravity) radius of curvature is shortened - first by a gravitational time dilation term end then by a velocity time dilation term. This causes the acceleration of a radially falling object, as experienced by the free falling object to be larger than what Newton predicted.
Einstein came the closest of the three 'giants' in answering the question 'what is gravity?' 

Summary
So, what is gravity? The truth is that at the most fundamental level, no one really knows. This page covered the basics of Newton's and Einstein's gravity in terms of the gravitational acceleration that is caused by curved space-time and velocity. We may have to wait for 'quantum gravity' to be completed before we will know a better answer to the topical question: 'what is gravity?'. 



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BALBEER SINGH DANU

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