- Posted by Oem Trivedi
- June 7, 2016 12:37 AM PDT
- 0 comments
- 777 views

This Article is a read for anyone who wants to know about what is the state of development in quantum gravity today. The beauty of tiny gravity for a non technical account. Enjoy the read .

*Gravity- The Cute Force:-*

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Gravity, the force we feel grateful to when we step our foot on the ground, when we sit near a waterfall to witness the beauty of nature. The tantalizing and remarkable features of gravity and it's far reaching role in constructing the beauty of the cosmos, it's influence on the movement of marvels of the cosmos and it's hands in making some lovely structures of the cosmos is truly remarkable. Gravity is truly stunning and seems so straightforward isn't it? Well apparently it seems that gravity has been taken way ** too much for granted**. As we will uncover shortly, it has come to our notice through the recent century that alike many new understandings of the cosmos we have come to unravel in recent times, gravity too seems to have a hidden structure. The force with which we have had a long affinity with, has stunning properties that have enriched the true beauty of this force even more to us. The problems with gravity we today have, are centralized on the structure and properties of

*The Story of Gravity:- *

*Part 1 :*

Well it's not that when we discovered gravity we actually then discovered it, it had been there before us, is still now and will hopefully be for a very long time even after we go. Well lets rewind it's history with us and see through. Back in earlier times, not much just 2000 years ago, people read the words of ** Aristotle** and someone asked him once that why does celestial objects,

*Part 2:-*

So this from where things get seriously serious. The year was 1665, Cambridge being closed down to a plague, forcing students to leave the campus for an indefinite period. While many I suppose would have rubbed their heads in anxiousness over their degree and future being affected by this, one particularly carefree and a bit short tempered student by the name of Isaac Newton went back to his native home in Wools thorpe in England with slightly different concerns awaiting him, the concerns which were going to change our view of the cosmos. So as the highly distinguished story goes, Newton was sitting on a fine bright day in the vicinity of an apple tree when, yes you guessed it right, an apple fell down. The details of whether the apple hit his head or fell somewhere more safe for Newton's case is not a thing for much worry here, the thing that impacted Newton was a question on the motion of the apple itself, ** why in the heck did the apple go down? **And of an even greater virtue, if

*Part 3:-*

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The question we encountered on the cause of gravity just in the last part, became a question of a degree of importance for one young teenager in Germany, a question he kept in mind and wondered upon all the way through to his youth ages. That young fellow in 1905 while working as a clerk in an office in Bern, Switzerland, published some tantalizing papers on the dynamics and properties of bodies when traveling near the speed of light and some papers on the motion of molecules in gases, the latter set of papers became pivotal for a theoretical proof of the existence of atoms. And for an exciting and pretty energetic importance, the former set of papers became what is now known as the "** Theory of Special Relativity**", giving out one particularly famous formula

Think of ** space-time as a cosmic sheet **on which all the different stuff of the cosmos take place. This space-time is the stage of all the events of our cosmos. Einstein in his papers at the age of 23 in 1905 from a professional point of view would easily guarantee a great career in theoretical physics but Einstein wasn't done yet. His Childhood wonder about the cause of gravity had still kept flame somewhere in his heart and so persuaded it again and with a new found friend in the concept of space-time helping him, it was about time that humans got to see a new face of gravity.

Einstein now asked himself, a question on space-time- ** What would happen if space-time gets curved or is made to curve ?**" This question held the key to understand the cause of gravity as Einstein found out. He discovered that gravity isn't an intrinsic property of everything in the cosmos. Rather, as he anticipated it, was caused by something, Contrary to Newtonian Gravity. Einstein discovered,

He basically shown that the ** trampoline like thing on the cosmological scale is space-time**, the heavy ball and light ball or balls , being

Some time before in our talk, I had stated that Gravity has turned out to be a lucky charm whenever a stunning new realization about it has been unraveled, and so this time too the same occurred. General Relativity revolutionized physics with a new view of the cosmos but also turned out to be of great importance in mathematics , ** particularly three dimensional geometry and Tensor Calculus**. General Relativity provided many stunning realizations about Gravity that were truly remarkable. It shown that , as not something we usually see in daily experiences, that

*So,What's the Problem ?:-*

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The problem that physicists face today with gravity is it's ** working and properties on the tinniest of scales**. In physics, the laws according to which matter behaves varies . At our normal everyday life experiences, the

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*The Quantum Nature of Reality:-*

Imagine you're at the bowling alley, so naturally you have a ball and now consider you have 5 targets. Common Sense tells us that we can hit one target with the amount of balls we currently have and that is 1. Now, if this situation were to be in the quantum realm, the theory tells us that the ** 1 ball we have can hit all the targets simultaneously at once too! **So I think you might have got a feel of this. At the start of the 20

Talking of light , there was a debate at the start of 20^{th} century about the nature of light , some thought it was a wave, others thought it was particulate or made by particles. With the introduction of the photon , the particle of light, points were put towards particle nature. An effect observed by Heinrich Hertz around 1885 now called as the ** Photoelectric effect**, in which by the emission of light on a metal surface electrons were ejected, could now be explained by photon. But, light could show wavy nature too in many effects. Then, Einstein proposed that light is of

Quantum Mechanics differs from our everyday reality as due to the completely different features of the tiny bits of matter in particles behold.

*Weird Stuff in Quantum Realm:-*

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So far we have seen how the field of Quantum Mechanics developed over the years, let's now see what kind of weirdness has developed. Imagine you're sitting in an international airport in New Delhi, and your friend in London is watching a football game in a stadium. Now, if you want to get the feel about how the live match is, you contact your friend. But he didn't respond to you while watching the match ? But ** Quantum Mechanics tells us that you can get every ounce of information** about your friend in the stadium, when he is shouting for the team, when he is dancing etc. and in the some way he can get it all about you and features you both see about each other will be opposite. For that to be true, you two need to be entangled and need to be the antiparticle ( every particle has an antiparticle, a particle with opposite charge and opposite properties but same kind) of one another. This is called

Another weird property of quantum mechanics is one called ** Quantum Tunneling**, where even though if a particle does not have enough energy to cross a barrier, still it has a probability to be found at the other side of the barrier. A property called

*Fitting the particles in:-*

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The concept of the standard model developed around the 1960's. It was based on the need that due to the huge band of particles, one had a tiresome time for their group analysis. And so in around middle 1960's ** Sheldon Lee Glashow** ,

*The Particle of Gravity - The Graviton :-*

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Gravity, alongside the other 3 fundamental forces of nature has been sought to be carried by a particle, more precisely it should have a gauge boson (the force carrying particle) too. Electromagnetism has the Photon, The strong interaction or the Strong nuclear force operative in the nucleus of the atom has something called the ** Gluon**. The weak Interaction or the Weak Nuclear force concerned with radioactivity has

Another problem is that we don't really know what model describing the properties of gravity could or should be true. The thing is that when we created the classical and cosmological views of gravity, we had observed something and the theory agreed with it. Now, in this case ** we haven't observed the stuff **and so we don't know which approach should be true. So that might give you a sense of how important the discovery of gravitational waves is for quantum gravity and for physics as whole. The key to a theory of quantum gravity lies in incorporating the laws of general relativity and quantum mechanics on equal footing in the theory and in a way

*The Possible Roads towards Quantum Gravity:-*

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*String Theory:-*

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String Theory is one of the hottest topics of talk , research and amazement in physics today. String Theory is a radical theory not only operative on tiny scales but it's range of applications go to cosmology, our everyday classical mechanics, quantum mechanics, because the basic idea of this theory encompasses on all these topics. Consider playing a guitar. The guitar has chords, and by playing different vibrations on the chords, different musical tones are generated.

String theory says that in the cosmos, ** every different kind of energy is just a different vibration of strings**. The strings, they are extremely small in size, smaller that any particle we'll see and String Theorists claim that this radical realization holds the

String Theory has done a good share in ** describing the dynamics**, the physical properties concerned with energy, of the graviton. String Theory has also described the

String theory predicts the entropy of the black holes as it predicts the extent of the event horizon, the outer layer of a black hole, correctly. String Theory has succeeded in measuring the entropy of a special kind of black holes known as ** Extremal Black Holes**. Extremal black holes are black holes which have a minimum amount of mass and are compatible with a given amount of charge basically. But researchers like Andrew Strominger have shown that the method for extremal black holes can be extended for normal astrophysical black holes too. And last but not the least , String Theory has shown that

*Loop** Quan**tum** Gravity:-*

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As we saw in the case of String Theory, something extremely radical in itself seems to have the potential to describe gravity at the smallest scales. And the same feature beholds our second approach. Loop Quantum Gravity's basic concept is that it does not consider a background realm for itself , i.e. ** it does not consider a background space-time**. Instead, Loop Quantum Gravity's biggest breakthrough has been that it shows that space-time could be interpreted as being formed due to

The initial and fundamental work on loop quantum gravity was done by** Abhay Ashtekar** in 1986 when he rewrote general relativity in a more familiar physics language. The background independent framework was developed by

*String Theory vs Loop Quantum Gravity:-*

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If I have to say a straightforward difference between both String Theory and Loop Quantum Gravity, ** String Theory starts from the smallest scales and moves up to the largest** but

*So,What else does Gravity have in store ? :-*

The answer to the above question, only time will tell. Maybe Greene's prediction might turn out true. And a possibility what I think is maybe there might turn up a ** third theory** in future, of Quantum Gravity explaining the 2 previous approaches, even merging them and expanding our view of the cosmos even more. According to