This is one of the most interesting theory in the science world. Mainly, this theory was proposed after building the Standard Model of Particle Physics. This model explains about the particles which are responsible for the building block and the four fundamental forces of nature. Before, reading this, I highly recommend to read the Standard Model of Particle Physics to understand the properties of the particles.
I hope you have read the article on Standard Model of Particle Physics. In that model, we have seen that, there are two types of particles – Fermions, those are responsible for the building block of the universe and Bosons, those are responsible for the four fundamental forces of nature. We have discussed briefly about the three forces – electromagnetic force (carried by photons), weak nuclear force (carried by W/Z bosons) and strong nuclear force (carried by gluons). There is also a force “Gravitation” which we did not discuss earlier.
For understanding the behaviour of the universe, we use two theories. One is Quantum Mechanics and other one is General Theory of Relativity. Earlier, when quantum mechanics was not so popular, then the scientists did not need to study about the atoms. That is why, they used classical physics to solve the general problems. But after revolution of quantum mechanics, scientists started to observe the whole universe exactly how quantum mechanics shows us. All predictions of outcomes in atomic level was proved “Correct” by quantum mechanics. So, it is a scientifically proved theory and this theory is correct.
But when Einstein proposed the General Theory of Relativity, it broke the whole world. With the help of his theory, he proposed about 4 dimensional space-time at the very first. According to the theory, space-time bounds themselves by forming a network of space and time. Any massive object warps space-time according to their mass. That is why when a small object comes in the range of the warping object, then the small one follows the curve path made by that large object and we call it Gravitation (exactly like when a massive ball is placed on a stretched carpet or rubber sheet and make a small ball to surround it, then the ball will follow the curve path of carpet or rubber sheet made by the massive ball). This theory was proved many times and most of the outcomes predicted by this theory was “correct”. Recently, LIGO (Large Interferometer Gravitational wave Observatory) proved again the general theory of relativity by detecting the gravitational waves. That means, this theory is correct.
But, when we want to superimpose the two theories, we always fail. Because according to quantum mechanics, an electron can stay in different positions in the same time. But, are the stars (or any matters including us) not made up of such tiny particles? Then they should also show such properties right? That means, there can be many suns or planets in our solar system in the same time. But, if there are so many suns in our solar system then will we exist? No, the planets will be destroyed by the temperature of the stars (suns). That is why, the two theories cannot be combined. But, the important thing is, they are 100% correct in their own domains. The problem is, we cannot combine them. But why? Why two different theories? Why have we not a unified theory? This question gave birth the Theory of Everything.
Now what is Quantum Gravity? As the name suggests, it is the combination of quantum mechanics and general theory of relativity. This theory explains gravity with the help of quantum mechanics. But how? We already heard that general theory of relativity and quantum mechanics cannot be superimposed. But this theory superimposes these two separate theories without violating any laws. Quantum Gravity talks about a particle which carries the characteristics of gravitation, hence named Graviton.
We already discussed about the three fundamental forces of nature with the help of Standard Model of Particle Physics. We learnt that, there are four fundamental forces of nature and each force is carried by different bosons. The electromagnetic force is carried by photons, weak nuclear force is carried by W/Z Bosons and strong nuclear force is carried by gluons. There is also a force which we call Gravitation. But practically, till yet, scientists have not got any particle in quantum level which can explain the gravitational force. But scientists have imagined a hypothetical particle which carries the characteristics of gravitation and they named it Graviton. Scientists still believe that graviton should exist, because the Standard Model of Particle Physics explains almost everything about the nature. Then why gravity would be an exception? We have not detected any particle in Large Hadron Collider (LHC) which carries the characteristics of gravitation but that does not mean that it does not exist. According to the scientists it was not detected because gravity is so weak compared to the other three forces. It is so weak that it is almost negligible in the quantum world.
By mathematical analysis, the theoretical physicists proved that graviton should be massless, because the range of gravitational force is infinite. But, here a question should arise,” How could it be possible? Because we know that earth’s gravitational force acts to a certain level, after that it vanishes.” But actually, it is not true. The gravity is infinite in range. It binds the whole universe and it is everywhere. The space-time bounds the whole celestial objects in a manner and gravitation of an object really affects to the others. We may not feel that but it really affects every celestial objects. As, the gravity does not interact with any charged particles in space, it is also clear that gravitons should be electrically neutral. Again, here is a big deal about graviton which is called Spin Value. The spin value of graviton is 2, whereas photons have spin value 1 and electrons have 1/2.
Thus, if we get such particle then, the whole Standard Model of Particle Physics will be completed and we can explain all forces with the help of the unified theory. That unified theory was named as Theory of Everything.
Theory of Everything:
The Theory of Everything is that unified theory which explains almost everything about the universe. It was originated from a theory which is called String Theory.
According to Standard Model of Particle Physics, the quarks and leptons are the smallest thing inside the atoms which cannot be broken beyond that. But this model is quite incomplete because it excludes the gravitational force. Again, when the string theory was not so popular, the physicists mathematically predicted that there may be a massless particle inside an atom. But at that time there were not any chances to get such particles. That is why this concept was rejected by the scientists.
But later, due to increase of particles, they were divided into many categories with the help of Standard Model of Particle Physics. This model explained clearly about the matters and fundamental forces of nature accept gravitation. Then, scientists started to imagine a unified theory which could explain also about gravitation. Here, string theory concept was born.
String theory explains the universe in a different way. According to this theory, every matter in this universe is made of tiny strings which exist inside the quarks. That means, quarks are also made of small one-dimensional strings which vibrate in different patterns. That vibration of the string decides about the nature of the matter. As different wavelengths of light produce different lights, different frequencies produce different sounds, like that different vibrations of strings produce different matters. This can be understood easily by a guitar, in where vibrations of different strings produce different sounds. A water molecule is very different from an oxygen atom, because their vibration patterns of different strings are different from each other.
String theory explains about different dimensions. According to this theory, there are 10 dimensions in space. But if this theory is true, then where are the rest dimensions? We all know about 4 dimension; the three dimension of space and one dimension of time. Then where are the rest 6 dimensions? Actually, some dimensions are too small to observe. For example, if we see a cable wire from a distance then we will see it as one dimension, because we can only see its length. But if we observe that wire as an ant, then the wire will be like a circular cylinder, that means it will be three dimension. Because, the ant can travel the whole wire in left, right or even can rotate around the cylindrical wire. Like that, some dimensions of our universe is hidden from us. If we can compress ourselves to a microscopic level then we will not only be able to see those dimensions but also travel to those higher dimensions.
According to string theory, like the other boson particles (photons, W/Z bosons, gluons), graviton has also a property which carries the characteristics of gravitation. The string inside graviton, vibrates in a particular manner which gives the ability of gravitation to that particle. String theory also talks about a particle which travels faster than light, and that hypothetical particle was named as Tachyon. But according to special theory of relativity, nothing can exceed the speed of light. That is why some scientists do not agree with this concept. However, string theory provides us some interesting concepts about the universe.
String theory also explains about the origin of big bang. There is another version of string theory which is called M-theory (Membrane theory). According to this theory the whole universe is contained in a membrane made of infinite strings and that is why it is referred as a different dimension called 11 dimension. There are infinite numbers of membranes in space which are parallel to each other and each membrane contains one universe. These are also referred as Parallel Universe. As, these membranes are made of such tiny strings that is why they also vibrate in a particular pattern. Sometimes, this vibration of the membranes cause collision between the two membranes parallel to each other. Then an explosion (big bang) occurs and the matters coming out due to explosion from the two membranes, create another membrane which we call creation of a new universe. Thus, string theory explains that big bang was not a unique event. It happened earlier and will happen in future, even it may be possible that big bang is still happening anywhere outside this universe, we cannot feel it because we are too far away from that.
String theory also explains why gravity is the weakest force among all other forces. Actually there are different types of strings. Some strings are open, that means its edge does not meet each other and some strings are closed loop, that means their edge meet each other. The strings of some subatomic particles like photons, W/Z bosons, gluons are open and that is why the edge of the strings are connected to the membrane. As a result they cannot move to the other dimensions (as strings are one-dimensional, they can vibrate to the tenth dimension) from their place and that is why they are detectable. But the strings of gravitons are closed loop, that is why they can move to the other dimensions. As they travel to the other dimensions that is why very few amount of gravitons are detectable. This is the reason why gravity is so weak compared to the other forces.
Thus, string theory explains almost everything about the nature. That is why this theory is called Theory of Everything, that means, this is the theory which explains everything about this universe. But still, there is a big problem with this theory that, it cannot be proved practically. String theory is only exists in mathematics world. All the above explanations were predicted by only mathematics. The main reason of not proving this theory practically is that, the strings are quite invisible for us. Because, we can see an object only when light falls on it and come back to our eyes. That means the object should be larger than the wavelength of photons. But strings are so small that even wavelength of photons are greater than these strings. The length of these strings are almost comparable with the Planck’s length which is 1.616229(38)×10−35 meter. That is why there is no way to detect these strings.
There are still some questions that string theory does not give us, such as, “What is dark matter or dark energy?” But physicists are trying to solve these questions with different mathematical models. However, till now, string theory is the only theory which explains almost everything about the universe.