But Why is Gravity? | Part-I
Introduction
You must have heard this story. A young genius named Isaac Newton, sitting beneath an Apple Tree; and contemplating the mysteries of the universe. All of sudden — boink! — an apple hits him on the head. “Eureka!”, he shouts, or something like that and in a flash he discovered the very same force that brought apple towards him and is responsible for the moon not falling and spinning around the earth.
This is one of the most popular historical stories related to scientific discovery. But now you can see what Newton said. It is the manuscript for what would become a biography of Newton entitled Memoirs of Sir Isaac Newton’s Life written by William Stukeley, an archaeologist and one of Newton’s first biographers, and published in 1752. Newton told the apple story to Stukeley, who relayed it as such:
“After dinner, the weather being warm, we went into the garden and drank tea, under the shade of some apple trees…he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. It was occasioned by the fall of an apple, as he sat in contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself…”
So it turns out, for the most part, the apple story is true. Meanwhile, three and a half centuries and Albert Einstein later, physicists still don’t understand gravity. We’re gonna need a bigger apple. These apples are so huge that their vastness is beyond your imagination, and their power too.
The Idea
Imagine you went back in time; near the 16th century. Forget F=Gm1m2/r² and all classical gravitational theory you jolted within your high school and just think for a while; think with your instinct. What on the earth caused the automatic descend of that apple towards the ground?
Note, we are thinking about something very fundamental. This type of philosophical thinking is sometimes also called metaphysics. We generally don’t tend to get very verifiable reasons for such questions. You can insert Why after everything. Continuously asking “why” for consecutive answers lead us to a point where “why” can not be answered and we reply to it by stating that it is nature.
For our question, can reply with a statement that the Earth is bound to attract any falling body as it is a fundamental property of earth. This reason just won’t work since there are many other planets, asteroids, and stars that are observed to attract each other. We know astronomical objects are very huge. We may now think that gravity has to do something with size. But what size is the threshold for gravity to work?
We also observe that how an object is attracted to gravity also depends upon the object’s mass. We know, massive objects are heavier to lift. Turns out that gravity should depend upon the masses of both bodies. This rejects the existence of a threshold. Astronauts have experienced weighing less on the moon, confirming our analogy that the more massive the object is, the stronger is the force. Sun is more massive than the earth, so shouldn’t we attracted to the Sun instead of falling to earth every time we rise? It gives us a hint that gravity between two objects should also depend upon the distance between them. The more away from the body is, the less will be its gravitational pull.
Similar thoughts might have also come to mind of Newton, and he gave the famous the universal law of gravitation. The law states:
very particle attracts every other particle in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
Was F = G*Mm/R² all about gravity? It was for many years. Newton gave many groundbreaking laws and formulations about physics and mathematics. His laws of motion, calculus, and then gravity made him probably the most renowned mathematician and scientist of all time. Although he was accused of plagiarism multiple times. It was difficult to disagree with his theories, as they worked very well; explaining most of the phenomena that earlier had non-scientific explanations.
As I wrote earlier, you can add “Why” after any statement. Doing the same here, why do a particle’s mass causes attraction between another ? Luckily, physicists were able to explain it using fundamental forces. There is a total of 4 fundamental forces in the universe. These are weak and strong nuclear force(responsible for the stability of atoms), electromagnetic force, and gravitational force. The gravity of weakest of all. There are few theories about the cause of such forces. In the theory of quantum gravity, a hypothetical particle that goes by the name graviton is responsible for mediating the force of gravity. It is still hypothetical since it hasn’t been so far as possible to detect an isolated graviton.
Limitations of Newton’s law of Gravitation
Newton’s gravity is sufficiently accurate for many practical purposes. This led to its wide acceptance and use. However, there were three prominent phenomena which Newton’s theory can not explain:
- Newton’s theory wasn’t able to explain precession in the perihelion of Mercury’s orbit. There was a discrepancy of 43 arcseconds between what was predicted according to his theory with the observation from 19th-century telescopes. Mercury is the closest planet to the sun, so it gave a hint that Newton’s theory would conflict in regions of the high gravitational potential. This was the first major blow to the theory which was earlier perceived as perfect. Such conflicts usually guide upcoming scientists to think in different directions in order to fix the flaws in the old theory.
- Newton treated light as made of particles. His theory was used to predict the deflection in the path of light rays when passing near a heavy body due to gravitation. It was found that deflections predicted using his theory was only half of the actual deflection. Einstein’s theory of General Relativity was able to fix these two limitations and calculations made using his formula agreed a lot with the astronomical observations.
- The orbit of stars around a spiral galaxy disobeys Newton’s theory of gravity. It is astonishing to learn that it also disobeys Einstein’s General Relativity. Astrophysicists nowadays explain this phenomenon by believing in the existence of a different kind of matter, also called dark matter. It doesn’t interact with the conventional matter which we are made up of, and thus it is very difficult to study. I will talk about dark matter in another blog. Let’s bring back our focus to gravity now.
Fixing these limitations
Before I end this blog, I would just want you to know what to expect in the next blogs of this series. We are now aware there are many shortcomings of newton’s theory of gravitation and thus it needs correction. However, this theory is so close to our intuition that it would be very difficult to think of some other theories that can explain gravitation.
It turns out that General Relativity is very far from intuition and thus difficult to understand. To comprehend Einstein’s theory, we would first need to understand Special Relativity, which is a special case of General Relativity. Therefore in the second blog, I will build up concepts from scratch which will let you question your existing understanding of the Universe. The transition from classical physics to modern physics is a bumpy trip, so I will try to make it as easy as possible and don’t use mathematical equations or complications that come along with it. In the last blog, I will at last talk about the existence of gravity, general relativity (GR), how it solves classical theory’s limitations, and what are the limitations of GR. This gives you a little hope that not everything in the universe is explained so far and you have a chance to be the next big scientist!
