# Understanding Forces

We know today of 4 different forces which rule the world around us. These are; gravity, the weak force, the strong force and electromagnetic force. The last of these we usually think of in terms of light. Light and gravity have been known about for a very long time. In astronomy, we use light to measure most of what we see. But gravity plays a large part in the Universe we study.

It has been known for a long time that objects which are dropped fall to the ground. The force causing this is called gravity. It took a lot longer to understand how gravity worked. In the 7th Century an Indian astronomer, Brahmagupta, saw that gravity was an attractive force. It pulled things together. In the late 1500s, Galileo found that if there was no air or water, objects would fall at the same speed.

Newton used maths to explain how gravity worked in 1687. His work linked gravity to astronomy for the first time. It said that gravity can be felt through the whole Universe. Although gravity is the weakest of all the forces, its effects can be seen at huge distances.

Newton's work seemed to work most of the time, but not always. For example, he could not explain the orbit of the planet Mercury. So Einstein came up with a new theory called 'general relativity' in 1915. This linked space and time to gravity. In 1919 Eddington took observations during a solar eclipse to test Einstein's theory. His results showed that light from stars was bent by the Sun on its way to Earth. This supported Einstein's work. General relativity is still the best way of explaining gravity today.

Einstein's work also made predictions. It said that black holes and gravitational waves should exist. We have since found both of these. The first black hole was found in 1964 within our own Milky Way galaxy. In 2015 the telescopes, LIGO and Virgo found the first gravitational wave.

There is still a lot that we do not know about gravity. Scientists are working to find out whether there is a particle which carries the force of gravity. These carrier particles are found in all other forces that we see. Bringing gravity in line with what we know about the other forces is called the Theory of Everything. It is an important part of physics research today.