Gravity- from the apple to space time

Gravity for us is the force that pulls us towards the largest object near us. Earth being the largest object near us, so we get pulled towards the earth. Also the bigger the object, the greater will be the pull and if the distance between the object increases the gravitational force decreases. If the mass is more, then the force of gravity is also more.

Along with the other two universal constant that is ‘speed of light’ and ‘plank constant’, gravitational force is also a universal constant.

Newton’s falling apple to his further equations helps us accurately predict the lunar and solar eclipse, and also the orbits of planets in our solar system. But what eluded him was that how objects millions of miles away influenced earth through gravity. There is no medium in between, unlike electricity which needs a copper wire to move.

After two centuries Einstein also got pulled by the mystery of gravity. We have noticed that when we accelerate the car we get pulled back and when we hit the brakes we move forward. Therefore acceleration imparts a force on us. The genius in him concluded that gravity can be equated with acceleration. Let us take it a little further. A space ship in outer space has zero gravity. If we drop a ball inside an accelerating spaceship it will fall to the floor the same way as on earth. This is because the acceleration of the space ship pulls the ball down and on the other hand gravity will pull the ball down on earth.

Newton said that the effect of gravitational force is instant. This means that if the Sun which is 150 million kms away from earth vanished in an instant then its effect on earth will be instantaneous. Earth will start floating out of its orbit and pulled by the next big object. But Einstein said nothing can move faster than the speed of light, so one of them must be wrong somewhere.

Einstein delved into this problem and co related it with space time. Up till now space time was thought to be without any mass or structure, and so gravity cannot affect it. But he gave identity to it as some type of elastic rubber surface. Large objects could actually bend or curve the space time around it and mathematically he proved it.

The most common example used for this. Suppose you are holding a stretched rubber sheet of the size of a bed sheet and you drop a bowling ball in its center. The weight of the ball will pull that part of the sheet down and cause a depression. Here the sheet is the space time fabric and the ball is the Sun. Now if we roll a tennis ball in a circular motion then the tennis ball will rotate around the bigger ball making smaller circles until it hits the bigger ball.

He said that this bend surface created by the bigger ball is actually the gravitation. The smaller ball if it moves in a speed that will counter the pull due to the curvature of the surface is just like the earth rotating around the Sun. The speed of the rotation of earth is stopping itself from falling into the Sun. But for that space time has to be something real on which gravity is exerting its force, though apparently it looks as there is nothing there. There must be something out there in space which we haven’t discovered it, on which gravity is exerting itself to bend it.

This can be seen in another way also. If we see the speed of revolution of planets around the sun, then the closest object to the sun will need more speed to stop itself from being pulled into the Sun and the farthest object will require the least speed for it. Let’s check this out.

Revolution speed of planets around Sun.

• Mercury – 47.87 kms/sec

• Venus- 35.02 kms/ sec

• Earth – 29.78 kms/sec

• Mars- 24.07 kms/sec

• Jupiter – 13.07 kms/sec

• Saturn- 9.69 kms/sec

• Uranus – 6.81 kms/sec

• Neptune- 5.43 kms/sec

• Pluto- 4.74 kms/sec

See, it’s just like that; the speed of revolution is decreasing as the distance from Sun is increasing.

Note: The topic has been written as simply as possible for better understanding and also limitation of my knowledge.