# Gravity: Newtonian Physics and the Theory of Relativity

## Introduction

For more than two hundred years modern science was dependent upon the theories and assumptions created by Isaac Newton. He was the first to develop scientific laws that help people understand planetary motions as well as how objects behave in relation to other objects. These principles can be described as Newtonian physics and for centuries the concepts and insights gleaned from Newton’s thesis regarding gravity as a force has allowed scientists and inventors to create solutions and develop products that has enhanced human existence.

However, in the second decade of the 20th century Albert Einstein surprised the world when he published a theory that challenged the status quo and revealed the weakness of the Newtonian model. Einstein demonstrated through mathematical calculations that gravity as a force is not just the result of the interaction between two massive bodies but the manifestation of spacetime curvature in response to a massive object in its path. By doing so Einstein provided an alternative explanation as to why smaller bodies seem to be attracted to objects with bigger mass.

## Newton’s Physics

Newtonian physics is based on three absolutes. Newton believed that time was the same all over the universe. He also believed that an object “took up the same amount of space, no matter where it was” (Parker, 2010, p.22). Finally, Newton believed that the mass of an object never changed (Parker, 2010, p.22).

Afterwards, Newton formulated the idea that gravity is the force of attraction between objects with mass and he added that gravity depends on mass and distance (McGlinn, 2003, p.78). This is a crucial idea because it allows other scientists to understand the nature of speed, mass, force and other concepts developed by in the field of Newtonian physics.

One of the most important applications of this idea is to give astronomers and other scientists a basis for understanding planetary motions. It helps explain why the earth revolves around the sun. It also explains why a solar system exists. In the case of the solar system where planet earth belongs there are other planets orbiting the sun. At first glance it seems that these planets are all orbiting the sun for a reason. Newton explained that the force that attracts the earth to the sun and vice versa is called gravity.

Newton’s explanation of gravity as a result of mass and distance gave him the insight that “the speed at which an object moves depends on two things: the force that is affecting it and the mass of the object” (Parker, 2010, p.24). In Newtonian physics the force that acts on the object is gravity.

Thus, Newton pointed out that the force of gravity “grows stronger as the object moves closer to earth’s centre and added that an object with a small mass that is acted upon by a great force will be the fastest” (Parker, 2010, p.24). These scientific principles revolutionized the way people see planet earth and all that it contains.

Due to these findings scientists and inventors were able to develop machines that were designed in relation to Newtonian physics. For example the Wright brothers were wise to work around the idea of gravity and its capability to pull everything down to earth. Thus, they designed an aircraft that has enough lifting power to allow the plane to lift-off and then glide when on air. This made possible commercial flight. Without Newton’s principles concerning gravity it would have been impossible for man to go to the moon.

Two hundred years after Newton, astrodynamicists were able to “devise the trajectories of interplanetary spacecraft… these often include several ‘gravity assist’ manoeuvres in which a probe is helped on its way to a distant target by energy that it gathers from the planets that it encounters en route” (Lambourne, 2010, p.110). There is a wide array of applications from aerodynamic design of cars to the ideal structure of roller coasters. Nevertheless, Newtonian physics leave so much to be desired beginning with the question what is gravity? Newton has no answer.

## Einstein’s Theory of Relativity

Newtonian physics is indeed an important contribution to human history and especially when it comes to scientific endeavours. But in the early part of the 20th century Albert Einstein came along and challenged this view. While Newton asserted that gravity is a type of force, Einstein on the other hand said that “gravity is the manifestation of spacetime curvature” (Lambourne, 2010, p.9).

In Newton gravity is simply the tendency of two objects to be attracted to one another and more importantly it is limited to a “rigid frame of spatial reference” (Henriksen, 2011, p.5). Without this frame of reference Newtonian physics is useless. In space it is possible to experience the manifestation of the force of gravity even without a rigid frame of spatial reference.

It must be pointed out that in Newton’s understanding of gravity, “mass is a conserved quantity that is the ‘source’ of gravitation, however, in Einstein’s relativity theory, the mass m of a particle is no longer conserved but it is related to the energy and momentum magnitude of the particle” (Lambourne, 2010, p.126).

This has serious implications and one of them is the counter-argument to Newtonian physics which is the assertion that “source of gravitation cannot be mass alone but must also involve energy and momentum” (Lambourne, 2010, p.126). Going further Einstein was able to calculate that a smaller object is not attracted to a bigger object because of gravity but due to the fact that spacetime curves in response to an object in its path. In other words a massive object can warp spacetime and smaller objects in the vicinity are affected by this change and behave in relation to the warping of spacetime.

## Conclusion

Newtonian physics provided a framework to understand the mechanics of this world and beyond. But it is only useful to the extent that there is a rigid frame of reference such as the planet earth, the sun and other distant planets. However, in space the impact of gravity is still seen even if Newtonian physics no longer applies.

Albert Einstein supplied an alternative explanation and he pointed out that the core explanation to this problem is not mass that is conserved but to view ‘spacetime’ as an unending stream and curves in response to a massive object in its path (Lambourne, 2010, p.110). The curvature of spacetime is the reason why smaller objects seem to be attracted to larger objects but Einstein said that the smaller objects are simply behaving in relation to the warping of spacetime and not because of some hidden force.

## References

Henriksen, R., 2011. Practical Relativity: From First Principles to the Theory of Gravity. UK: John Wiley & Sons Ltd.

Lambourne, R., 2010. Relativity, Gravitation and Cosmology. UK: Cambridge University Press.

McGlinn, W., 2003. Introduction to Relativity. MD: The Johns Hopkins University Press.

Parker, K., 2010. The Theory of Relativity. New York: Marshall Cavendish.

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