GRAVITY

What is Space Time Continuum?

Time is not completely separate from and independent of space, but is combined with it to form an object called space-time.

It is a matter of common experience that one can describe the position of a point in space by three numbers, or coordinates. For instance, one can say that a point in a room is 7 feet from 1 wall, 3 feet from another and 5 feet above the floor. Or one could specify that a point was at a certain latitude and longitude and a certain height above sea level. One would not specify the position of the moon in terms of kilometres north and kilometres west of a building on Earth and some feet above sea level. Instead, one might describe it in terms of distance from the Sun, distance from the plane of the orbits of the planets, and the angle between the line joining the Moon to the Sun and the line joining the Sun to a nearby star such as Alpha Centauri. Even these coordinates would not be of much use in describing the position of the Sun in our galaxy for the position of a galaxy in the local group of galaxies. In fact, one may describe the whole universe in terms of a collection of overlapping patches. In each patch, one can use a different set of three coordinates to specify the position of a point.

 

The Space-Time continuum is a mathematical model that joins space and time into single idea. The space-time is represented by a model where space is three-dimensional and time has the role of the fourth dimension.

Combining these two ideas helped to understand cosmology, and to explain how the universe works on the big level. Whenever an important quantity of matter exists, it bends the geometry of space-time. The results in a curved shape of space-time that can be understood as gravity. The white lines on the picture above do not represent the curvature of space, but instead represent the co-ordinate system imposed on the curved space-time which would be rectilinear (straight and uncurved) in a flat space-time where there is no mass.

Newton's Gravity

Sir Isaac Newton discovered a law to describe the force of gravity, which states that every body attracts every other body with a force that is proportional to the mass of each body. Thus the force between two bodies would be twice as strong if one of the bodies (say, body A) had its mass doubled. This is what you might expect because one could think of the new body A as being made of two bodies with the original mass. Each would attract body B with the original force. Thus the total force between A and B would be twice the original force. And if, say, one of the bodies has twice the mass, and other has three times the mass, then the force would be six times as strong. One can now see why all bodies fall at the same rate: a body of twice the weight will have twice the force of gravity pulling it down, but it will also have twice the mass. According to Newton's second law these two effects will exactly cancel each other so the acceleration will be same in all cases. 

Newton's law Of gravity also tells us that the farther apart the bodies, the smaller the force. Newton's law Of gravity says that the gravitational attraction of a star is exactly one quarter that of a similar star at half the distance. This law protects the orbit of the Earth, the Moon, and the planets with great accuracy. If the law were that the gravitational attraction of a star wind down faster or increased more rapidly with distance, the orbit of the planets would not be elliptical, they would either spiral into the sun or escape from the Sun.

Einstein's Gravity

Einstein hypnotized that space and time were one and the same and served as the fabric of the Universe. He never believed that Gravity was a force at all. He stated that Gravity was simply a curvature in space-time created by a mass object pretty much in the same way a piece of cloth would be called if it was stretched out and a heft object was placed on it. This curvarture in space created by an object with greater mass than the object surrounding it would cause these objects of lesser mass to fall toward the more massive object. 

Basic physics state that if there are no external forces at work, an object will always travel in the straight possible line. Accordingly, without an external force, two objects travelling along parallel paths will always remain parallel. They will never meet. 

But the fact is, they do meet. Particles that start off on parallel path sometimes and of colliding. Newton's theory says this can occur because of gravity, a force attracting those objects to one another or to a single, third object. Einstein also says this occurs due to gravity – but in his theory, gravity is not a force. It's a curve in space-time. 

According to Einstein, those objects are still travelling along the straightest possible line, but due to a distortion in space-time the straightest possible line is now a long a spherical path.

So two objects that were moving along a flat plane are now moving along a spherical plain. And two straight path along that sphere end in a single point.

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