Gravity According to Einstein

Paige Yarker. 21 November, 2016.

The theory proposed by Albert Einstein in 1905 and expanded on in 1915 has since revolutionised science. He suggested that for all bodies in constant motion in our universe the laws of physics are the same, and that the speed of light in a vacuum is constant regardless of the motion of the observer. This became known as his theory of special relativity, special because it required that bodies were not accelerating. In 1915, he managed to incorporate acceleration, giving us what is now known as general relativity. General relativity answered one of the most fundamental questions in physics: what is gravity?

Isaac Newton described gravity as the attractiveness of two objects. He formulated that this attractiveness is related to both the size of each body and the distance between them. If two massive bodies are close to each other the gravity they will feel will be immense, likewise, if two slight bodies are very far from each other the gravity they feel will be negligible. This was Newton’s theory of gravity, and it was heavily incorporated in his laws of motion. This was widely accepted as the correct theory of gravity, however there were two main issues with it. First of all, it didn’t actually explain what gravity is, just what it does. How could the theory be thought of as complete if such a large question is still unanswered? Second of all, there was a slight issue with its accuracy: Mercury. Newton’s theory could be used to accurately predict the motion of all of the planets except the closest planet to our sun. Mercury’s orbit didn’t match up with the orbit predicted by Newton’s calculations, it was too fast. It wasn’t until general relativity that the mystery of Mercury was solved.

General relativity predicts that space is similar to a 4-dimensional rubber sheet. When a mass, a small bowling ball for example, is placed on a rubber sheet the sheet’s fabric is warped. Einstein suggested the same is true for space-time; massive bodies, such as planets or stars, warp the very fabric of space-time around them. Now imagine the rubber sheet and bowling ball again, but the rubber is slippery. If you were to put an ant on the sheet near the ball it would slide towards the ball at an increasing rate until colliding with the ball. This is gravity. More massive objects create a bigger dip in the fabric and so have a larger area where you could fall down. This means that “gravity” is caused by many massive objects constantly falling towards each other.

The Effect of the Earth on Space-Time

This new understanding of gravity helped explain the reason Mercury’s orbit was so fast. The other planets in our solar system are far enough away from the sun that this effect of gravity is inconsequential in Newton’s equations, however Mercury is much too close for that to be the case.

Bibliography:

Einstein’s Theory of General Relativity

The 200-year-old mystery of Mercury’s orbit — solved!

Further Reading:

Einstein Online

The Universe in Your Hand (Christophe Galfard)

The Elegant Universe (Brian Greene)

The Extravagant Universe (Robert Kirshner)