Updated: Mar 9, 2021
'Gravity' is a term you could have come across in and outside the boundaries of Physics. 'What is gravity?', this question among scientists has helped us understand the world with astounding precision but also pushed us further into mysteries over the past couple of centuries. Gravity is a pretty enormous topic of discussion, but here's what we will cover today for understanding its underlying nature. What is gravity? Why is understanding it so important? What is Newton's view of gravity? What is Einstein's theory of gravity? What is quantum gravity? And Getting into the weird part, we will talk about, Why does gravity bend light? Why does gravity slow-down time? And Why is gravity so weak?
What is Gravity?
Gravity is one of the four fundamental forces (gravity, electromagnetism, strong nuclear force, and weak nuclear force) of nature, it is the most fragile among the four, but that is all we know. We only know how gravity operates; we don't understand gravity itself (Yet!).
It is crucial to understand gravity because if there is no gravity, we won't be here. We learned in star formation how gravity is necessary for holding stars together as they burn their insides out. There will be no stars, no planets, no moons, no us, and no pizza without gravity.
Because there is a law such as gravity, the universe can and will create itself from nothing.
Newton's gravitational law is a part of physics we are familiar with since elementary school. We all know the classic story of the apple falling on the great Isaac Newton's head, prompting him to the brilliant discovery. The apple part is bogus, but it is a fine story that helps you remember the events.
Newton believed gravity is the force of attraction between two or more objects with mass. Everything exerts an attractive gravitational force on absolutely everything else.
Gravity depends on the mass of the object and the distance between them. Newton's law, with great accuracy, explained the orbit of planets around stars and helped us put rockets in space, yet there is one significant problem. Newton did not explain what gravity is or how it transmits between two objects in space.
To quote the genius himself,
"It is inconceivable, that inanimate brute matter, should, without the mediation of something else, which is not material, operate upon and affect other matter without mutual contact.
-Sir Isaac Newton's Mathematical Principle of Natural Philosophy and His System of the World.
The man who discovered orbital mechanics and integral calculus might seem confused here; but then came Albert Einstein to the rescue.
Einstein's view of gravity:
The way we view the Universe changed when Einstein proposed his theory of Special relativity in 1905. Einstein took the great leap no one ever took before and declared gravity results from warping space-time.
Wait, what do you mean by warping of space-time? What does that even mean? Let us go through this step by step!
For understanding's sake, forget time and consider space. In the absence of any matter or energy space, it could be flat like the smooth surface of a table-when a massive body like the Sun emerges, it causes space to warp. It acts like a rubber sheet that curves when a bowling ball falls on it.
A massive object (like the Sun) in space warps its surroundings; this affects other stuff (like planets) in its vicinity and forces them to follow a curved path.
Einstein explained how gravity is nothing but a geometric consequence of curved space. The more massive the object, the more distortion it causes; more distortion means more gravity. Remember that Einstein's version of gravity is an upgraded view of Newton's gravity; in a way, both are correct.
Einstein's space is poetic. It is not just a platform for events but a live arena that responds to matter and energy; it explains how gravity transmits from the Sun to earth through spatial distortion.
Special relativity has led us to fascinating discoveries like the multi-dimensional view of space, black holes, gravitational waves, etcetera, yet our understanding of gravity remains incomplete.
The problem is, the theory of relativity fails when it meets Quantum mechanics. To be precise, we do not know how gravity affects particles at the microscopic scale.
Quantum mechanics is a concept for understanding the microscopic properties of the Universe. Probing into quantum mechanics has led us to one conclusion so far; 'Understanding the familiar every day fails in the microscopic realm'.
Nature offers us four fundamental forces. They are gravity, electromagnetism, and strong and weak nuclear forces. Among the four, three have a quantum description; that is- we know how they operate at subatomic levels.
For example, consider a massive body like Jupiter; it is relatively easier to calculate how space curves around such a giant and its gravity.
Now imagine an electron; how does such a microscopic particle curve space around it? It is not just hard to calculate; it is impossible as of now.
Physicists also suggest that at quantum scales, gravitons exist. They are the hypothetical elementary particle that mediates the force of gravity. We haven't detected them, or we have no proof of their existence.
Scientists are struggling to crack this problem and produce a unified theory of gravity that brings General relativity and Quantum mechanics together.
Loop quantum gravity and string theory are strong candidates but lack experimental evidence. The answer to this problem will provide a much better understanding of reality itself; until then, May the force be with us!
Gravity and its weirdness:
Gravity has pretty wild tricks up its sleeve; it is not weird as we think it is. Gravity does not behave like the other forces we know well. Gravity is unique, and it has properties that we are just beginning to grasp; here is a few of them,
Gravity slows time:
Gravity not only warps space but also warps time; the stronger the gravitational field, the severe the warp. This effect has a mind-bending consequence called time-dilation. The closer you are to a massive object, the slower your time flows.
For example, if your friend is living on Mount Everest and you are living in the mountain valley, your clock ticks slower than his clock because you are closer to the earth's gravitational field. It is the same reason gravity decreases with altitude.
Whenever scientists calculate the amount of gravity in the universe, there is always more than what there should be. That means we don't know where it comes from. We know gravity arises from the mass; we couldn't detect the actual matter; scientists call this unknown matter Dark matter.
Dark matter does not interact with the electromagnetic force; therefore, it does not absorb, reflect or emit light, making it extremely hard to detect. In fact, researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter.
Dark matter seems to outweigh visible matter on a six to one ratio. For every one gram of visible matter, there are 6 grams of dark matter out there. The stuff we know and see (that makes up all stars and galaxies) only account for 5% of the universe's content! Scientists also suggest that dark matter is not some exotic matter; it's just our lack of understanding of gravity itself.
Gravity bends light:
We know mass and gravity go hand in hand, but how does gravity affect massless particles like photons? (Light is composed of elementary particles or packets of light called photons)
The answer is, it doesn't directly affect light but distorts its path.
When light travels from Sun to Earth, it doesn't travel in a straight line all the time. Massive planets (Mercury, Venus) interfere in their path and distort space around them; as the light passes through these distortions, it follows the curved path making it bent.
Gravity is the weakest force:
Gravity is the weakest among the four fundamental forces; we don't know why. It is 10^24 times weaker than weak nuclear force and 10^40 times weaker than electromagnetism. We call this the hierarchy problem in Physics. String theorists suggest gravity seems weak because it travels through extra dimensions in space; we haven't unleashed its full potential, again it leaves us with no proof.
Gravity increases with density:
Gravity not only increases with mass but also increases with density. Black holes are excellent examples of this phenomenon. When massive stars collapse on themselves, their mass becomes concentrated on a relatively smaller area and infinitely dense. These highly dense areas form black holes; they have an enormous gravitational pull that not even light can escape them!
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