Gravity- The Cute Force:-
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 gravity at tiny scales as we shall unravel later in our talk. But hold on, first let's see when did our story with gravity began.
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, the Sun, planets, stars etc. revolve around the Earth ? ( the fact that people earlier believed the Ptolemaic view that Earth is in the Centre of the universe ) and the great philosopher famously answered that they yearn to be united with the Earth, they love the Earth or more easily they are attracted towards the Earth. That, though being indirect, was the first indication of some kind of force, in this case an attractive one that existed between bodies being observed in the modern history of humans. These ideas held firm until coming of the age of modern science with people like Galilee Galileo and Sir Isaac Newton coming along.
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 Apples fall towards Earth, do the Moon fall towards the Earth too? Newton came to a game changing realization for science that there is a force that pulls the apple, the moon and every object that we see towards each other and he named that lovely force as gravity. Exploring it even further, the stunning new realization of gravity worked as a lucky charm for not only science but also Mathematics and we shall see further on, it wasn't to be the last time that gravity would act as a lucky charm. For studying gravity in details Newton came to realize that the mathematics of his time was incapable for describing the motion of falling bodies, and so Newton embarked on the discovery of the branch of mathematics today, known as Calculus, that too in under 2 weeks! To add to the incredible note, a great mathematician of modern times Pierre De Fermat of France died January of the same 1665 when Newton got the idea of Calculus !And this also enabled him to also make the classical laws of motion and their equations and to make a revolution in Science. This great work of his is summarized in his Book "Principia Mathematica" or "The Mathematical Principles of Natural Philosophy". So, this is pretty much how in modern history we humans had our first interaction with gravity. But the questions now rises, on scales as large as the cosmos, does gravity always keep doing the job of attraction? Will it always do the work for the motion of bodies in a way in which we see it in our daily experience and a bit more fundamental question, What causes gravity?
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 E=MC2. For the name of that brilliant young fellow, as you might've guessed now, was Albert Einstein. In the years to follow, Einstein got recognized throughout the radical ideas about space and light his theory had put forward and new realizations developing on his theory was also being pursued heavily. And it turned out that one of Einstein's teachers at the Polytechnic University in Bern, Hermann Minkowski put out a radical idea in 1907, that was, in that time, of equal magnitude to the set of radical ideas put forward by his distinguished student in 1905. Minkowski proposed that Space and Time, long thought of as 2 separate entities, in order to be understood by the application of special relativity should be united into one, and so the great birth of "Space-Time" took place.
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, Gravity is caused due to bending of space-time. To get a feel about this, think about a trampoline. Now consider a heavy ball being kept on that trampoline (for convenience let it be around the centre ) with it bending a sufficient part. Now roll a relatively light weight ball on that trampoline towards the heavy ball, and you'll notice that the light ball will start revolving around the heavy ball in the bent part. That was how Einstein explained gravity.
He basically shown that the trampoline like thing on the cosmological scale is space-time, the heavy ball and light ball or balls , being the sun and our planets in the case of our solar system. Einstein discovered that massive bodies like stars, planets, galaxies etc. bend space-time around them and the objects coming in that bent space-time revolved around the massive bodies as if being pulled by a force, and that force is gravity. This is the radical fundamental idea put forward in his theory explaining large scale gravity as never known before, This theory came to be known as "The General Theory of Relativity".
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 gravity could be repulsive too. It also shown that Gravity depends not only on mass of the object as according to classical gravity, but it also depends upon the energy and pressure of the object. The theory went on to show ecstatically that Gravity and Time are related too, the more gravity around an object the more slowly time ticks for that object. Among all the marvels, this theory have also shown the expansion of the universe theoretically, years before it was experimentally discovered. Solutions from this theory shown the existence of black holes, Space-Time Singularities and also shown a new realization about the birth of the cosmos with existence of big bang singularity. Einstein also showed that some kind of waves, transport energy as gravitational radiation , a form similar to electromagnetic radiation, those waves are called the gravitational waves, which were just recently experimentally confirmed by the LIGO (Laser Interferometer Gravitational Wave Observatory)experiments. So far, not so good, but absolutely Great. So now the question turns out , what problem do modern theorists have with gravity? Let's See to that.
So,What's the Problem ?:-
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 laws of classical mechanics, the theory developed from the 17th to the late 19th century, work. The features of this theory account for the basic understanding of nature around us we have observed through the ages like classical gravity, as we discussed above, the laws of motion, basics of electromagnetism etc. When we work out stuff on the largest scales i.e. we work on the scales of the stars, planets and galaxies , our understanding takes a sufficient reshuffling as then the classical laws don't hold on much as we apply General Relativity for understanding these large friends of ours, as our views of the large scale force of gravity has changed. For studying the movement of Galaxies , we apply some laws developed on the fundamentals of general relativity made by people like Edwin Hubble, George Lemaitre etc. On the cosmological and the classical fronts, things don't become somewhat completely different or I should more precisely say weird but when we study at the smallest scales, heads turn fast. On the smallest scales, a radical revision of our basic sense of nature is made. The Common sense notion goes away and weird things are observed with totally stunning realizations about the fabric of our cosmos. This theory today is known as Quantum Mechanics.
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 20th Century, German Physicist Max Planck showed that Light was formed and travelled due to packets of energy, and he called those packets Quanta. In the Special Theory of Relativity, Einstein showed that light was formed from a fundamental particle and he named it as "Photon".
Talking of light , there was a debate at the start of 20th 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 dual nature, particle and wave both. Some time later around 1913, Neils Bohr showed his model of an atom and it held sway for around 15 years without a question but then in 1924 , Louis De Broglie showed that alike Light, all the matter is of dual nature. Then in 1926 , German Werner Heisenberg showed that we cannot determine some properties of the electron exactly, called as the uncertainty principle and it pulled out Bohr's deterministic theory of the atom a bit. Then in 1927, another German physicist, Erwin Schrodinger showed 2 equations describing the properties of the electron probabilistically , maintaining consistency with Heisenberg's Uncertainty principle in doing so and then Another German Max Born showed that Electron are Probabilistic waves. Then in 1928, English Physicist Paul Dirac in his book on Quantum Mechanics developed almost all the essential mathematics needed to describe particles and he showed in that book a variety of new particles besides the fundamental ones known for some time. He firstly named some classes of particles as fermions and bosons (Fermions named in honor of Italian Enrico Fermi and Bosons in honour of Indian physicist Satyendra Nath Bose). Bosons are the particles who are most fundamental to the construction of matter, the likes of photon. Fermions basically mean the elementary particles who compose the matter around us, the likes of electrons. Bosons are broadly talked about on 2 of its kinds , Gauge and Scalar bosons. Gauge bosons are the force carrying particles like photons for electromagnetism and Scalar bosons are a more technically defined kind of boson to be honest. But in practical simplicity, It's one and only example is what is known as the higgs boson or the so called god particle which exists everywhere in our universe and is responsible for the mass of an object. A more basic difference between fermions and bosons is that, any fermion in space cannot occupy a same quantum state, a kind of an energy level, as any other of its kind while the same isn't true for bosons , they can occupy the same quantum states.
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:-
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 Quantum Entanglement.
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 Quantum Superposition says that a particle can be at multiple positions at one time. And so I think you might have now got a good feel of how the notions of reality change at the smallest scales. Earlier we remarked upon the fact that in Dirac's work, he showed that a huge variety of particles exist and over the years more and more particles have been discovered, experimentally and theoretically too. And so with a sea of particles, physicists wanted to study them systematically and so a Model called the Standard Model was formed.
Fitting the particles in:-
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 , Abdus Salam and Steven Weinberg presented a systematic tabular form of keeping the particles and from observing the table, particle properties could be worked out by seeing their family. It's importance in particle physics is much like that of the periodic table of elements in chemistry. The standard Model Trio as mentioned above were awarded the Nobel Prize in 1968. Over the years many particles have been added in the standard model, with one of the most important inclusions in recent times being that of the higgs boson in 2012 . but the model lacks one thing, which is Gravity.
The Particle of Gravity - The Graviton :-
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 the W and the Z Bosons as carrier particles and Gravity's gauge boson has been speculated to be the Graviton. The former three gauge Bosons have been observed experimentally over the course of 20th Century but the graviton has still not been experimentally observed till date. And so, the standard model completely looses out on gravity.
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 unifying both the theories. It's not that we humans hadn't made progress towards a theory of quantum gravity. Today, 2 approaches to lead towards a theory of quantum gravity lead the way in our understanding of tiny gravity and let's now see the beauty they behold.
The Possible Roads towards Quantum Gravity:-
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 key to many mysteries of the cosmos. The initial focus of String theory at the start of its hardcore research, carried out prominently by John Schwarz and Michael Green, was unification of all the 4 fundamental forces of nature. In so doing, it describes the graviton as just another string vibration. Speaking of the extent to which the theory has progressed in describing gravity, it has made quite some rounds.
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 entropy of special kind of black holes. Black holes are basically objects in the cosmos formed when a sufficiently massive star dies in a supernova and that explosion leads to the making of singularity which leads to the formation of a black hole , having a monstrously enormous gravity. Entropy ? well it is just the randomness or the disorder there is in stuff. Or entropy can be just basically thought of as how much the stuff is scattered around.
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 every fermionic particle has a partner bosonic particle and vice versa, like a photon has a fermionic partner by the name of photino and an electron has one bosonic by the name of selectron ( don't go on the naming, physicists just do it for fun!) this concept goes by the name of supersymmetry. And String Theory shows that general relativity and supersymmetry could be related in an idea by the name of supergravity , where both the concepts above are ought to be merged into one. So that's prominently and basically much of what String Theory's progress has been in the field of Quantum Gravity.
Loop Quantum Gravity:-
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 loop or bands and being interwoven in such a way that we see space-time as it is, and those loops are formed by gravitons.
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 Lee Smolin, Carlo Rovelli and Ted Jacobson. The background independent framework has been able to calculate the areas of space-time too and that is a massive plus point for the theory. Loop Quantum Gravity has also been applied to measure the entropy of a black hole's event horizon. The correct prediction of the entropy for astrophysical black holes can be made by this theory with the application of a special technical parameter called as the Immirizi Parameter.
String Theory vs Loop Quantum Gravity:-
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 Loop Quantum Gravity (LQG) starts from the largest scales as the base and moves towards the smallest scales. For String Theory, Unification has been the main focus with gravity being secondary but in LQG's case, gravity has been the main focus all the way long. While both the theories have their ups and downs, the harmonious possibility as quoted by Brian Greene in his book " The Fabric of the Cosmos" is that, maybe sometime in the future, both the theories will lead to the same result and show the same nature of reality.
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 Shrimad Bhagwad Geeta, whatever will happen will take place for good so hold on to your hats folks, some delicious and lovely concepts await us. So people, always keep doing work for good and work for creativity and believe, Some stunning realizations are there.