Introduction
The effect of motion on time and length has been demonstrated amply in scientific experiments, and made Einstein a superstar overnight. His deep insight and intuitiveness exposed the inner workings of the universe on a vast scale. While Newton's laws still hold good at the low speeds we normally deal with in our daily life, they are only approximations of Einstein's theory. Try using Newton's equations that we learnt in high school for high speed problems like the speed of light and you will get erroneous results. Of course, you can continue to use his equations to calculate the rotational speed of our planets or the escape velocity for firing rockets into space or to find the average velocity of your travel to office, etc.
What Is Gravity?
First of all, what is gravitation? Well, science is still grappling with this entity. Newton showed us that gravity is an attractive force that keeps every celestial object in the universe under its grip. The more massive an object, the greater is the force of its pull. A stone thrown up in the air comes back to earth due to its gravitational force. The Earth and the moon attract each other. If the mass of Earth is mass1 and that of the moon mass2, then the gravitational force between the two is directly proportional to the product - (mass1 x mass2). Further, this gravitational force between the Earth and moon reduces as the distance between them increases. This is the reason why we see low ocean tidal activity during a new moon day, when the moon is farthest from the Earth. The interesting thing about gravity is that the pull of the gravitational force is a universal constant. There are three universal constants - Gravitational constant (G), Speed of light (c) and Planck's constant (h).
Newton invented the equations of gravitation that could be used to accurately predict solar and lunar eclipses or to find the orbits of the planets in the solar system and fire rockets and track their movements accurately, and so on. So, Newton's theory of gravity pretty much explained its effects. But one thing Newton himself could not explain was how gravity actually worked. He wondered how large objects, millions of miles away, like the sun and the stars, could influence the Earth through gravity. How did gravity travel through space without any medium? You need a copper wire as a medium to transmit electrical energy in a circuit, but what about gravity? He could not settle this issue and left it to posterity to figure this out. Two centuries later, Einstein, tormented with the problem of the speed of gravitational force, finally cracked this hard nut through his General Theory of Relativity. But even he did not succeed fully in understanding the role of gravity in theories like Quantum Mechanics. His dream of coming up with a unified theory to explain gravity, which is the weakest of all the four fundamental forces of nature, and how gravity actually worked, along with other fundamental forces like strong nuclear forces (used in nuclear bombs), weak nuclear forces (found in the radioactive decay), and electromagnetic forces (used in mobile communication), remained unfulfilled. However, his intuitive mind and power of imagination unravelled one of the shrouded mysteries of nature. He was able to demystify the true nature of gravity and this brought a paradigm shift in the way science treated gravity once for all.
In order for us to gain a deeper insight into the subtle nature of gravity, let's start with a simple example as before. I promise, you will not only appreciate the true mystery of gravity but also enjoy the moment. Let's get back into our car. When you push the accelerator to race ahead of others in a friendly car race, you feel the car seat pushing you backwards. If you travel with a constant speed, you won't feel a thing. Hence, a sudden change in speed (acceleration or deceleration), results in the pull or force on your body. Now get out of the car and get into your office lift. As the lift starts moving up with acceleration, you feel a force under your feet. So, in both these cases (car and lift), the acceleration imparts some kind of force on you, and your body and mind register this force. Einstein's happy moment came when he deduced that this force due to acceleration, was exactly the same as gravitational force, that ubiquitous force that we all experience while we live every moment of our lives. In a stroke of a genius, Einstein equated gravity to acceleration.
Let me give you another example of how gravity and acceleration are equivalent. Assume that you are travelling in a spaceship into deep space where the influence of gravity is zero. In your accelerating spaceship, if you now drop a ball. It falls to the floor. of your spaceship in exactly the same way as it would while standing on Earth. Even though there is no gravity, the ball falls to the floor of the spaceship as if some force were pulling it down.. In the moving spaceship, acceleration makes the ball drop, while on Earth, it is gravitational force.
Equating gravity to acceleration had a mesmerizing effect on our understanding of the laws of nature. Einstein did not stop there. He looked for a way to understand how gravity really affected its vicinity. He wanted to know how fast the gravitational force travelled. For Newton, gravity was always an instant force of attraction. Newton's understanding of gravity was that gravitational force travelled instantly, and that meant it travelled much faster than the speed of light, which was in a direct conflict with what Einstein had said in his celebrated Special Relativity theory.
What Is General Theory Of Relativity?
In a further development of his theory called General Relativity. Einstein added the effect of gravity on space-time as well. This drastically changed our views of space forever. Space is regarded as an empty, continuously running, smooth fabric that holds all the matter in the universe. In this vast universe, matter fills up only 4% of the space, with the rest being completely void of any matter except what we call Dark Matter and Dark Energy and the radiation from various stars and galaxies. We will dwell more on this later, but for now, let's focus on the effects of gravity on space-time.
All right, we now know that gravity is like acceleration and it affects massive objects all around it, be it a spoon or the moon. But, Einstein's mind did not stop there. He wanted to know if gravity had any effect on space or rather space-time, as well. This spirit of inquiry was one of the greatest moments in the history of science. Einstein in his theory rigorously analyzed the effect of gravitation on space-time. One of the challenges that troubled Einstein was with regard to the speed of the gravitational force. How fast did the gravitational force travel? We all know that the sun is about 150 million kilometers from Earth. It also exerts its gravitational pull on Earth. What if the sun were to vanish one fine day? How long would it take to affect us on earth? According to Newton, it would happen instantaneously. As soon as sun disappeared from the sky, Earth would be thrown out of orbit and start floating around in space. In order for this to happen, the gravitational force has to travel faster than light. Light takes about 8 minutes to reach us from the sun. Does that mean that the gravitational force travels faster than the speed of light? How was this possible? How can anything move faster than light? According to Einstein's well-established theory, the speed limit for any object or energy is the speed of light - 300,000 km per second. This meant that either Einstein or Newton was wrong.
After 10 years of racking his brains, Einstein finally resolved this and came up with his General Theory of Relativity, that also considers the effect of gravity on space-time. What this theory says is that gravity not only pulls at massive objects like the moon and small objects like a football, but affects space-time. But for gravity to pull or act on space-time, it has to be a real quantity like a stone. How can gravitational force pull space-time that has no mass or structure? Until now, space-time was not treated as a real entity. If space or time were some kind of real quantities, we could have moved them around like a stone. But Einstein's theory now gave space-time some identify and structure. We could imagine the space-time fabric as some kind of invisible, colourless and stretched piece of rubber membrane.
Earlier we saw how motion or the speed of our car, affected our sense of time (or space-time). Now gravity also has an influence on space-time, even though it remains invisible. According to General Relativity, what gravity did was that it actually 'bent' or 'curved' or 'warped the space-time fabric. Space-time became warped under the influence of the gravitational force, thus destroying its 'smoothness'. This was one of the revolutionary ideas that changed forever the way we conceived gravity and space-time.
How Gravity Bends The Space Time?
Thus, space-time now becomes a real entity that can be influenced by the presence of gravity. If space-time is real, why is it not experienced by us? When we move our hands slowly in space, we are going through the smooth nothingness of space that is somehow entangled with time. How far down do we have to probe to get to this space-time fabric? Well, the realm of space-time is more complex than we had imagined earlier. Space-time takes on an entirely new avatar and many scientists believe that space-time is itself made up of still smaller atoms of space that have yet to be discovered. While the quest for an ever closer look at space-time continues, there are several theories that try and model space-time to be compliant with the quantum nature of the microscopic world. For us living in a classical world, space-time may be extremely difficult to access, but what about the sub-atomic particles like photons and electrons? Can they actually see or feel the effects of the curvature of the space-time fabric? This idea led to an experiment that confirmed beyond doubt, Einstein's theory of space-time and its warping by gravity.
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