Is Gravity Really a Force?

Gravity was first thought to be a force. A force between objects with its mass and was called the gravitational force because Newton, the scientist who discovered it, took it as a force. 

This is the force that gives us weight on earth, if we go to another planet, our weight will be different. So the question arises here, Is gravity really a force? As we call it gravity and place it in the category of force. But did Newton think he was going to discover a force? As we know it is the force that gives us weight on the earth. So when we discuss the gravity in the context of the entire universe, we must keep in mind the Sun’s gravity that keep all the planets in their orbits, and we all know well about the gravitational pull near the black hole. Gravity is a simple thing and very easy to understand. As we currently know, gravity is only a small part of much complex phenomenon, the theory of relativity.

Before we get into that first let's try to understand a little physics. A common story about the discovery of gravity is that Newton, a famous English physicist, was sitting under an apple tree in his house, an apple fell from above and hit him on the head. Which made him wonder why this apple fell straight towards the ground or not sideways or some other direction? And in search of an answer to this question, Newton discovered gravity.

It can't be said with certainty whether the above story about the discovery of gravity by Newton is true or not. But it is certain that Newton repeatedly observed objects falling towards the earth, and he wrote in his writings that he also saw a comet revolving around the sun, of which he also made a picture. In addition, he observed that the moon revolves around the earth, comets revolve around the sun and objects on the earth fall towards the earth. In his quest to find the cause of all these factors, he discovered that there is a force that makes them all to do this. But he did not have the mathematics to find this force, so he created calculus. Although some people believe that Leibniz also had contribution in inventing calculus. But Newton was the first mathematician who created the famous calculus. However, without getting into this discussion, we continue our discussion on gravity. Newton probably thought that he had not done justice with the force of gravity and there is something missing from his theory because he thought it was something bigger than he thought, but he still gave the law of gravitation.

And then when we started using Newton's law of gravity in different experiments, we saw very positive results. For example, if you look at a rocket, it also moves under Newton's law. This and for many other reasons, scientists after Newton was believed under Newton's law that gravity was nothing else but really a force. But unfortunately this theory obscured the truth for the next four hundred years that was until in 1905, Albert Einstein, while working as a patent clerk in Sweden, gave the theory of relativity. He not only ignored Newton's law of gravitation, but mocked it. The key to understanding the theory of relativity is that all objects falling in a gravitational field may fall at the same rate, but Einstein was not the first person to state this fact. Earlier, Galileo had also stated that if bodies of different masses were dropped together in the absence of air, would fall at the same rate regardless of their mass. While Einstein created a sheet-like model of space-time and any object in space warps this space-time Continuum. 

Space-time is the three dimensions’ space, length, width and height. Einstein said that if these three dimensions of space are combined with the fourth dimension ‘time’, we have a model that is a very good representation of space. He said that whenever we place mass or energy in the sheet of the space-time model, for example the Sun, Moon, planets or black holes, a curve or bend forms in the sheet of the space-time model. Means a curvature is formed in this sheet of space-time model due to this mass or energy and the surrounding objects will follow this curvature. Due to this, planets or comets are seen orbiting the Sun. Although this is exactly the same as we go up on a slide and it’s obvious that when we slide down we always slide along with curve of the slide. In the same way, the planets also take a small orbit around the sun. So Einstein believed that gravity is not a force that falls under the category of pushing or pulling. Rather a property of space-time, a curvature that we see and apparently perceived it as a force.

This phenomenon can be understood by a simple example, assuming or visualize the earth to be placed on a grid of space-time. You can observe that the mass of the Earth made a bend or curvature in space-time and thus creating a gravitational well or field on space-time. In this way, whatever the matter or bodies come around it, you, me or even the moon, will be drawn towards this well. The moon itself has a gravitational well or gravity. But the attraction or gravitational field between the Earth and the Moon is not strong enough to pull the Moon towards our Earth. Similarly, the Sun has a huge gravitational well or field that keeps everything in our solar system like floating in space. We can also understand these planetary gravity wells as by knowing that how spacecraft launches. Engineers use wrapped space-time or gravity around other planets to move spacecraft in different directions and increase their speed. In this way, they launch the spacecraft into space like a slingshot. The closer the spacecraft is to the next planet's gravity well, the faster it will accelerate. The bodies in the universe are attracted to each other because of this space-time curve or well. The closer they are to the gravity well, the greater their gravitational pull towards each other.

So, what about this so-called phenomenon of gravitational field that we are discussing about earlier? Is not a force? This phenomenon of gravity is actually a gravitational field that surrounds everything having mass in space. This gravitational field of the Moon will be smaller than that of any other planet due to the mass of the Moon and this field of the Earth is larger than the Moon due to mass of the Earth. This gravitational field exists almost everywhere in space. The International Space Station also feels Earth's gravity. Surprisingly, the gravity in the outer orbit of the planet and the gravity on the surface of the planet are 90% the same. Earlier it was believed that gravity becomes zero when we reach into space. However, today we know that gravity is not zero at any place in space but decreases. Because everywhere in space, there are some celestial bodies, like solar system, stars, planets, black holes, other celestial bodies and we will feel their gravity. So we can say that there is microgravity, but it’s wrong to say zero gravity. This was the point that puzzled experts as to why we start floating in spacecraft when we go into space. Eventually, many observations and experiments showed that in space we are actually in microgravity and freely falling towards Earth. So all the astronauts you see floating in space are actually freely-falling towards Earth at a certain speed. Because they accelerate towards the earth under the influence of gravity.

There are a few ways to prove Einstein's hypothesis, according to which, bodies having mass made bend or curve in space-time. The value of gravity at the Earth's surface is 9.81 meters per second. So all bodies that have mass will make a curve or bend in space-time and this curve or bend is actually the gravity. So if somehow you may reach the center of the earth, there will be no gravity. Because you yourself is having weight and will be right in the middle of the big body from where no more bend or curve will be possible and you will start floating weightless. But as soon as you start moving towards the surface, you would start to feel the curvature of space-time from the mass of the earth and the effect of that curve ‘gravity’ would start to get stronger. There is another way to prove space-time warp or gravity, called gravitational lensing. This happens when a massive celestial body forms a large or proper curve in space-time and light appears to curve around such body. Like a galaxy creates fascinating scenes of light around it. This feature helps us find other galaxies, without this we would never be able to find them.

Einstein's cross is a famous example of this, the ‘Quasar’. Four images of Quasar from the very back of the galaxy prove gravitational lensing. It might seem that Einstein has proved the theory of relativity by freezing the concept of gravity in one point and there is lot of evidence to support general relativity. But the problem is that its current form is not compatible with quantum mechanics.

Quantum gravity is a theoretical physics that seeks to define gravity in terms of quantum mechanics. In this regard, no such theory has been presented so far, which is universally acceptable to all schools of thought of physics and which can be tested with the help of experiments. But that’s not all, researchers know that at a point in a black hole Einstein's theory breaks down and stops working. Scientists have installed three large telescopes in Hawaii which are observing a star called Blue Star SO-2, make it closest approach to the black hole Sagittarius A star in the middle of the milky way galaxy in its 16 years’ orbit. If Einstein was right, then the black hole would follow the space-time and increase the wavelength of light coming from the star, because the black hole's gravitational force pulls its energy and should have turns the star's light from blue to red just as Einstein predicted. Had it been another colour it would have hinted a completely different model of gravity altogether. Scientists are currently exploring this curve of space-time extensively and think that in the next ten years, the theory of relativity will be confined to certain limits. Then a genius come along and tell us that where the Einstein wrong? Hopefully we don't need to wait another three hundred years for this.