Gravity

Although gravity has been a force at work since even before the earth was formed, until Aristotle, the concept of gravity was hardly acknowledged. Aristotle proposed that all celestial bodies follow a “natural” motion detached from other external agents. In addition, he stated that large planets or planet-like masses have a tendency to move towards the earth’s center.

In the 17^th century, a German astronomer named Johannes Kepler, theorized three laws of motion based on the Copernican perspective that planets orbit the sun. Kepler’s three laws of motion were the following: planets travel in ellipses around the sun, the speed of planets changes, and the time it takes a planet to orbit is relative to it’s distance from the sun. These laws would later guide Newton to propose the theory of gravity.

As the myth goes, Sir Isaac Newton, a scholar of astronomy, math, optics, and physics, was sitting under an apple tree when he was struck by both an apple, and a brilliant idea. In reality, Newton was inspired by the way objects seemed somehow attracted to the earth. Along with his curiosity on the matter, Newton had been solving problems about the moving of planets since 1665. Through this work, he came up with the theory that the moon was falling towards the earth and, at the same time, drawn towards space.

Newton also used the law of inertia, which states that bodies not acted upon by a force move at a constant speed and in a straight line. Using this law, Newton realized that in order to keep the earth and the moon in orbit a separate force needed to be at work. He found that this force was the same force that attracted objects on earth towards the earth. This helped him develop the theory of gravity, that there is a gravitational attraction of one mass for another.

One aspect of the theory is that celestial things and free-falling objects are driven by the same force. This was unknown before Newton. Another aspect of the theory is that the force of gravity is proportional to mass. Although gravity is the weakest force in nature, because it has no role in determining internal properties of everyday matter, due to the great mass of planets, gravity has a significant effect on the universe. Despite this effect, gravity is independent of temperature, electromagnetic fields, shielding by other matter, orientation of crystal axes, etc. Earth’s core allows such a pull because it is composed of Ni and Fe. These minerals are very dense and therefore account for the gravitational pull of the earth. Newton also said, every mass applies equal and opposite force to it’s gravitational field. Gravity is universally attractive and acts over infinite distances.

Since Newton, more additions have been made to the theory of gravity. In 1915, Einstein published his General Theory of Relativity, which relates gravity to space and time, as well as the bending light. In the 20^th century, a physicist named PAM Dirac proposed that the value of gravity may be proportional to the age of the universe. Also interesting, is that the speed of gravity equals the speed of light. Another addition was the discovery that a substance has specific gravity less than a fluid it will float on that fluid. Another interesting aspect of the phenomenon is that light is affected by gravity. In fact, the explanation of a Black Hole may be that light is being attracted to a very dense object rendering it invisible. Although, many of these ideas have been explained by gravity there are still mysteries, such as the wonder that clocks near the sun will rotate more slowly than clocks away from it.

Since gravity is a weak force, measuring it is very difficult. An early and inaccurate form of measuring gravity was with the use of a weight on a string, and measured the weight’s relative pull. Another method of measuring was through the use of a gravimeter. A gravimeter is a sensitive device used to measure the earth’s gravitational pull. Galileo said that gravity makes objects constantly accelerate. The gravimeter utilizes the fact that for every second an object is free-falling it’s speed increases 9.8 meters/second. In 1817, physicist Henry Kater, used a reversible pendulum to measure gravity. This method was inaccurate because it was difficult to measure the space between the swing of the pendulum. In the 1950’s electronic devices were created to measure this space accurately. Now, however, the most accurate way to measure gravity is with the use of lasers and interferometers. Interferometers can measure the fall of an object towards earth. Gravity measurements show that the gravitational pull of earth can vary with position on the earth as well as with the change in moon movement, although the change is very subtle.

author: JH, UCSC