Prior to the groundbreaking scientific revelations of Sir Isaac Newton and Albert Einstein, Gravity was an elusive concept. Greek philosophers believed the planets followed a natural motion due to being a part of the gods’ realm, as they did not realize that gravity was involved. These ideas stayed around until the 1500s, when astronomers Galileo and Brahe discovered that the earth and other planets revolved around the sun. Astronomer Kepler found that they moved in an elliptical orbit, not a circle. Kepler’s discoveries provided a great depiction of the planets’ motion, however they did not explain how or why they did so. Newton and Einstein groundbreaking discoveries of gravity provided the answers.

Mathematician Sir Isaac Newton was troubled by the lack of explanations for the planets’ orbits. One day, as he was sitting under an apple tree, pondering about the forces of nature, an apple fell from the tree. According to Newton’s laws of motion, an object will stay at rest and those in motion will continue moving along a straight line unless acted upon by a force. Newton believed there had to be some force acting on the apple to make it fall because an object cannot move from rest without the presence of a force. He also wondered why the Apple fell down, towards the earth, not upward. He also wondered if the force that pulled the apple from the tree was the same one which kept the moon in orbit around Earth. Newton realized that unless that force was causing the moon to move towards Earth, the moon would fly off in a straight line away from Earth because according to the laws of motion, a straight line defines an object’s natural state of motion. Since the planets move in ellipses and not straight lines, Newton assumed that there must be a force attracting objects towards the earth, bending their path of motion. He reasoned that this force was the same force that acted upon the apple that fell towards the earth. This theory coincided with two sets of natural laws for earthly events and for motion in space. This revolutionary union of terrestrial and cosmic laws was known as the Newtonian Synthesis. Prior to Newton, Keller had established the three laws of planetary motion. The first being: planets orbit around the sun. Second: the time it takes for them to orbit around the sun depends on the vector radius. Lastly: the farther away a planet is from the sun, the slower it orbits. Newton’s discoveries about the force acting on the planets provided explanation for these laws. The discoveries of such proved that the Earth is not at the center of the solar system, which was the previously popular held belief and that Earth orbits around the sun. Attempting to prove the connection between the force acting on the apple, and the forces acting on the planets, in the 1680s Newton published his theory of gravity, “The Universal Law of Gravitation.” Which theorized that the force of gravity was a “universal” force. It said that everything attracts, or pulls everything. Newton knew that gravitational forces must have to behave like any net force on an object. He calculated that gravity among objects close to Earth’s surface, makes the object accelerate approximately by ten meters per second squared. Newton applied the Inverse Square law to gravity, stating that the gravitational force between objects is directly proportional to the product of the objects’ masses and is inversely proportional to the square distance between the objects’ centers. Meaning, the greater the masses of objects, the greater the force of attraction between them and therefore the smaller the masses, the weaker the attraction. Meanwhile, greater the distance between the object’ centers, the weaker the force of attraction, and therefore the closer the objects are to each other, the stronger the gravitational force between them. Stated mathematically, the force of gravity between two bodies is: F=GM1M2/d^2. Where F equaling the force due to gravity, Ml being the mass of the first body, M2 being the mass of the second body, d being distance between their centers and G being the Gravitational Force Constant. The Inverse Square law of gravitation is in accordance with Kepler’s law that the square of the orbital period equals the cube of the orbital radius. Newton’s insights were groundbreaking at the time, as they revealed that there was a mathematical order to even the most mysterious workings of the world. Newton’s Universal Law of Gravity is what governs instances as small as the fall of an apple on the Earth to instances as large the fall of the Moon around the Earth to the fall of the Earth/Moon system around the Sun. Despite Newton’ momentous findings about gravity, he failed to completely explain how it worked. The question of what gravity actually is remained unanswered for hundreds of years, until an extraordinarily bright physicist answered it.

In 1905, Albert Einstein created the “Quantum Theory of Light,” which was the idea that light exists as particles which he coined as “photons.” He then applied this theory to everything, proposing that we live in a “quantum universe, one built out of tiny chunks of energy and matter.” Months later, Einstein introduced special relativity, describing light as as a continuous field of waves, contradicting his prior theory of light as particles. Later in 1905 he extended this theory of relativity. The Special Theory of Relativity explains how time and space are linked for objects moving at a consistent speed along a straight line, implying that time and space are intertwined. Explicated by Einstein’s mass-energy equivalence equation which states that energy equals mass times the speed of light squared, or E= mc^2. Einstein expressed that mass and energy are different forms of the same entity and thus interchangeable and that all objects inherently fall the same way, no matter their size or what they’re made of. This equation put a speed limit on everything moving in the universe as it showed that speed increases with mass. Having spent much time studying light, Einstein concluded the speed of light (around 186,000 miles per second) is unchanging no matter what and the speed of light physically cannot be exceeded by anything that has mass. This poked a hole in Newton’s theory of gravity, which described gravity as a force that acted instantaneously across any distance. However, not

even light travels instantaneously and the “force” of gravity cannot surpass the speed light so therefore, gravity cannot simply be a force, such as Newton believed. After years of conducting further research and a lot of thinking. In 1916, Einstein published the General theory of Relativity, which concluded that what we perceive as gravity is really the result of curvatures in space- time. The force of gravity is the sensation of following the shortest path through curved, four-dimensional space-time. This idea was revolutionary as it described space dramatically different then it had been before. Einstein described space as something much more complex than a stage, or an unchanging box that surrounds the universe, but rather space and time as well as matter and energy are locked together. A change in one aspect, affects the others. Space evolves with the presence of matter and energy, stretching and warping, causing bodies moving through it to zigzag and curve. The more heavy an object is, the more it warps space and time around it. So when something heavy is in space, such as a planet i.e. Earth, the space around it gives way, causing a dent in space-time. When something else moves close to an object such as the Moon, it feels the dent in space and rolls around it like a marble rolling in a bowl, which is what Einstein concluded to be gravity. So although Earth appears to be pulled towards the sun by gravity, as Newton surmised, Einstein’s theory of general relativity proves there is no such

force. The Earth’s orbit around the sun is simply the geometry of space-time around the sun guiding Earth’s movement.

Almost 300 years apart in their publishing, Newton’s and Einstein’s theory of gravity share similarities as well as fundamental differences. Although much of Einstein’s findings disprove Newton’s theories, it can be agreed that without Newton laying the ground work, Einstein may have never been able to answer the questions of gravity. Newton’s physics were based off the understanding that the universe was an unchanging, constant, place which coincided with his belief that objects travel straight because they experience no force. However, Einstein found the opposite to be true. Einstein’s physics were centered around the flexibility of spacetime, so he believed that an object travels in a straight line because it is traveling in flat space-time. According to Newton, gravity is the mysterious force through which masses are attracted towards each other, concluding that gravity has no effect on light since light doesn’t have mass. Einstein’s theory of gravity concludes that mass warps the fabric of space-time and therefore matter and space-time mutually interact with each other. Based in Geometrical concepts such as curvature, Einstein’s theory proved that gravity not a force, but rather the consequence of warps in space-time. Therefore light bends as it travels through curved space-time, and is subject to gravity just as objects with mass. Both theories involve principles of proportionality regarding the objects and gravity. Newton’s theory of gravity states that the closer objects are, the greater force of attraction between them and the further they are, the weaker. The greater the mass of he objects, the greater the force of attraction between them and the smaller they are, the weaker the force. Einstein’s theory concludes that the greater the mass of the object, the greater the warp in space time. The closer an object is to another, the faster it travels along space time. The further from the center of an object, the less warping of space-time. Although the variables between each relationship differ, both Newtonian and Einsteinian principles include proportions. In Newton’s theory, gravity being an outside force causes particles to leave their inherent straight paths. However in Einstein’s theory, particles still follow the straightest possible paths but since space-time is now warped, even on those paths, gravity accelerates particles as if they were under the influence of what Newton would call the gravitational force. Both theories were those of unification- attempting to explain seemingly vehemently different, unrelated, phenomena through an overarching concept.

Aspects of Newton’s and Einstein’s theory are still used to predict everything from the existence of black holes, to the behavior of the planet Mercury in its orbit. Without such revolutionary advances in the understanding of gravity, inventions such as the GPS, satellites and many more would have never been able to be created. The atomic clocks in GPS systems are so accurate because they apply Einstein’s theories of time, space, and relativity. “Since GPS satellites experience less gravity and move at higher velocity in space, their clocks operate at a different rate than those on Earth. Since all the clocks in the system must be synchronized, a net correction of 38 millionths of a second per day must be added to the satellite clocks’ time signals.” – Smithsonian. Newton’s formulas are still used to calculate the answers to mathematical questions such as how fast an object travels after being dropped from a certain height. By using the Law of Universal Gravitation, the behavior of objects under the influence of gravitational force can be accurately predicted. Newton’s mathematical model of gravitation paved the way for subsequent scientists to build a greater grasp of the relationships between acceleration, the force of attraction, and mass.

Currently, we know almost everything pertaining to the physical aspects can be explained through either general relativity or quantum mechanics. Relativity explaining activity on a large scale, while quantum mechanics explaining the activity on a microscopic scale. However these two theories are incompatible, which has prompted the search for the ever so elusive “theory of everything.” A theory that could explain everything regarding space and time one every scale. As such a theory could provide possible explanations for the mysteries of nature such as black holes, dark matter, dark energy, and other cosmic objects, researchers are currently in hot pursuit. The future of gravity research lies in the linkage of quantum mechanics and general relativity

So far, the most promising has been known as the String Theory. This theory describes all particles as strings which vibrate in different modes, producing different particles. It also requires 10 dimensions to be explained and thus has not yet made any testable predictions. However, the future of gravity research may possibly hold validation of this theory and thus finally complete physics’ quest of a true, theory of everything.

Sir Isaac Newton and Albert Einstein are two of history’s most influential forefathers of science. Their genius was in their innovation, as each of their theories broke entirely new ground, not only differing, but contradicting those accepted by society during their times. Their ideas about gravity are responsible for our understanding of the universe today. Without Newton’s universal law of gravity and Einstein’s theories of Relativity, the technological advances derived from these theories would have never been born, and thus society would not be where it is

today. Thus, we can thank these two physicists for the current world we live in, for without them, it would be very different than as we know.

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