But Why is Gravity? | Part-III
Introduction
We still haven’t answered our original question. The reason behind the very existence of gravity. As we read in the last blog, special relativity introduces a new framework for all physics, by proposing a new concept of spacetime. For sure Newton’s law of Gravitation was inconsistent with special relativity. Einstein has to come up with a more robust theory that could correct the flaws in Newton’s theory and lays its foundation on his earlier theory. We will now progress towards understanding general relativity(or GR in short) using our existing understanding of special relativity.
4D Universe
Consider a thin flat pipe. Two particles are moving away from each other. Is it possible for them to meet? No from their perspective. However, if we can bend that pipe to create a circle, join its endpoints then they both will meet at some point later. Those two particles can’t comprehend how that happened, as according to them moving in opposite directions will only increase the distance between them.
Let’s consider another example. Two ants confined to a 2D space, walk parallel to each other. Can they eventually meet again? It sounds impossible, however, it is indeed possible. Imagine they are walking parallelly along the longitude of the globe; continuing the journey they will reach one of the poles. Two their whole world is a flat space, like what earth appears to us. However, since the earth is 3D the parallel lines in 2D can meet. The point I’m trying to make is, a low-dimensional object cannot interpret phenomena occurring in higher dimension as the object is limited to its dimension. The two ants walking parallelly along a sphere can’t appreciate the existence of the third dimension.
Let’s understand it further by something easy. Everything you see on your page of paper is confined to a 2D space. You open your math’s textbook showing that the distance between two points on either side of the page is 10 cm. However, if you fold the textbook, you can connect those ends, and thus the distance between those two points will be just 0 cm. This is bizarre for all characters confined to the 2D space of your textbook. However, as you are a high-dimensional being, you were able to do something which was not comprehendible for them. They will wonder how these two points can come so close which were earlier separated by some distance?
That’s what happening to us when we try to understand gravitation. As we know, Universe is 4D. There are 3 axes of space and one axis of time. Together we call this spacetime, as we discussed in the second blog. We also sometimes refer to this as the “fabric of spacetime”. For matter to exist, it must occupy some space. To do so, it must distort the spacetime around it to free up some region to exist. This distortion makes the lines of spacetime fabric curved around any mass. This is simply one part of the answer that why gravitational force depends upon the mass.
You may be surprised to know that everything, including you is moving at the speed of light at every time in spacetime. Recall spacetime diagrams, which had a space axis and a time axis. If you are stationary, you are moving along the time axis at a speed of c. Light moves along the space axis at c, while stationary on the time axis. This is the consequence of the fact that time slows down for objects moving at high speed. At light speed, time doesn’t move at all. We also know that unless an external force acts on an object, it continues to move in a straight line. An apple falls on the earth. Both earth and the apple are moving in a straight line. However, due to distortion of spacetime fabric itself caused by matter, there is a curvature in the spacetime. Thus the apple and the earth meet each other at some time later. This is the same as two ants travelling parallelly along straight lines on a globe meet each other, due to curvature of the surface. The same thing is happening here, the exception being we are 3D creatures and we cannot interpret why the apple and earth were able to meet each other unless we think of gravity as a force.
Thus it turns out gravity is not a force at all! This is the most important realization from all of my blog and this is also the gist of general relativity. If you understand it, you can appreciate the fact that no objects the other in the universe. They all are travelling on the straight path, only due to the curvature in these paths, they end up meeting each other. General Relativity is very non-intuitive for beginners. Our human minds haven’t evolved themselves to comprehend the phenomena of the 4D world. We assume the world is 3D, so all our theories were earlier based on only this assumption. Discarding this assumption indeed opened up new venues for physicists.
Gravitational Time Dilation
Near a black hole, spacetime is curved, hence light has to travel greater distances. To an observer near a black hole, light is still travelling with the speed of light. We know speed is the ratio of distance and time. If the speed remains the same and distance increases, time should increase proportionately. This simply explains gravitational time dilation. Since the curvature happens around every entity possessing mass, time is dilated everywhere. The extend of dilation depends upon the mass of the body. Scientists have to take into account this effect while designing GPS satellites. Time runs slower on the satellite as a result of being at a larger distance from earth, and thus correct adjustment had to be made in position measurements. There was a deviation of few kilometres if we ignored the impact of GR. We use GPS in our daily lives, and it won’t work if we didn’t know about Einstein’s theory.
A Crazy Idea
I will end this blog with a bizarre concept in my mind. As you have now understood general relativity and its consequences like gravitational time dilation, I can talk about some idea which is based on using these phenomena for human benefit. As the sun ages, it will become a red giant. Those who don’t know why this will happen, I recommend you to study stellar evolution. In short, it is a process of ageing of the star. Sun produces energy using nuclear fusion reaction happening in its core.
The fusion is powered by Hydrogen atoms which get converted to Helium atoms under high pressure and temperature of the core. This hydrogen is not unlimited, and thus have to extinguish after some time. The powerful nuclear reaction happening at the core is the only force responsible to hold against the star’s collapse under its own gravity. As hydrogen exhausts, fusion stops and the star collapses rapidly. However, this collapse again increases pressure at the core, the helium atoms began to fuse into carbon and in turn producing even greater energy. This energy not only stops the collapse of the star but also pushes the outer layers of the stars out, making it bigger with time. It is estimated that our sun’s outer layer will at least engulf the earth. Even life on mars won’t be possible in that case, living in such close proximity to the hot outer layers of the sun. The sun won’t be able to support life, and thus if humanity survives, it must be living near any other star instead of the sun, as all life forms require energy to live and stars are one of the most prominent sources of such energy in our universe.
But for how long we can live near those stars? They will also die someday and thus we have to search for a home that can provide energy for an astronomically longer duration. Small stars are good targets for supporting life, as they can live a lot longer than small stars. It takes time for them to exhaust their hydrogen because they have less pressure inside the core owing to its low mass. As the entropy of the Universe is increasing always, there it won’t be possible to reach such orderliness in the nebula to create new stars. We are in the age where most of the stars that will ever exist in our universe are already formed. This can make us gloomy. But we forgot one thing. The thing which we always think of as monsters of the universe, the hostile one, the killer, and so on.
Yes, you guessed it right, black holes. Everything is very extreme with these members of the universe. Gravity is astonishingly strong. They have no normal ways of dying. Their death can happen only by Hawking radiation, which is black body radiation released by black holes due to quantum effects happening near its event horizon. However, this process is very slow. A supermassive black hole that is generally at the center of a galaxy can last for 1⁰¹⁰⁰ years! That is 100 zeros after 1. Humans cannot imagine such large numbers. Our universe age is around 13.8 billion years, which is just 10 zeros after 1. Black holes can outlive everything in the universe. They will be the best sites to support human life of the far future since the matter in the accretion disk around it can lead to the production of energy.
Computers are catching up with everything. We can easily assume computation will be the most important thing for those humans, as we can see the same trend now. Algorithms are fixed, we just need time to solve problems using computation. Can time be a currency? Yes, and I will explain to you how. You know that time runs faster near a massive object due to gravitational time dilation. As time will be a very valuable resource, humans would have engineered specialized space vehicles that can carry a processor performing a computation in a slingshot motion near a black hole. The time will pass very fast when the spaceship is near the event horizon of the black hole and thus the computation will take exponentially less time for computation. On returning to the human space station, the processor would have saved many years of time and the trained computation can be put to use for humanity. Time can be traded by future firms and sold by exploiting the consequences of relativity.
We have come too far in this journey of gravity. I hope you enjoyed this journey of understanding the very fabric of spacetime with me. This is the largest blog I have ever wrote and spent a lot of time collecting references for facts stated here. I am thankful to you for spending your time reading 3 large blogs. It was a great experience for me to put my crazy idea in front of you all. So I end here, I will meet you with another crazy idea as I have one, bye and take care!
