ஐன்ஸ்டீனின் "எம் கோட்பாடு" - M-theory, the theory formerly known as Strings

M-theory, the theory formerly known as Strings

The Standard Model

In the standard model of particle physics, particles are considered to be points moving through space, tracing out a line called the World Line. To take into account the different interactions observed in Nature one has to provide particles with more degrees of freedom than only their position and velocity, such as mass, electric charge, color (which is the "charge" associated with the strong interaction) or spin.

The standard model was designed within a framework known as Quantum Field Theory (QFT), which gives us the tools to build theories consistent both with quantum mechanics and the special theory of relativity. With these tools, theories were built which describe with great success three of the four known interactions in Nature: Electromagnetism, and the Strong and Weak nuclear forces. Furthermore, a very successful unification between Electromagnetism and the Weak force was achieved (Electroweak Theory), and promising ideas put forward to try to include the Strong force. But unfortunately the fourth interaction, gravity, beautifully described by Einstein's General Relativity (GR), does not seem to fit into this scheme. Whenever one tries to apply the rules of QFT to GR one gets results which make no sense. For instance, the force between two gravitons (the particles that mediate gravitational interactions), becomes infinite and we do not know how to get rid of these infinities to get physically sensible results.

String Theory

In String Theory, the myriad of particle types is replaced by a single fundamental building block, a `string'. These strings can be closed, like loops, or open, like a hair. As the string moves through time it traces out a tube or a sheet, according to whether it is closed or open. Furthermore, the string is free to vibrate, and different vibrational modes of the string represent the different particle types, since different modes are seen as different masses or spins.

One mode of vibration, or `note', makes the string appear as an electron, another as a photon. There is even a mode describing the graviton, the particle carrying the force of gravity, which is an important reason why String Theory has received so much attention. The point is that we can make sense of the interaction of two gravitons in String theory in a way we could not in QFT. There are no infinities! And gravity is not something we put in by hand. It has to be there in a theory of strings. So, the first great achievement of String Theory was to give a consistent theory of quantum gravity, which resembles GR at macroscopic distances. Moreover String Theory also possesses the necessary degrees of freedom to describe the other interactions! At this point a great hope was created that String Theory would be able to unify all the known forces and particles together into a single `Theory of Everything'.

From Strings to Superstrings

The particles known in nature are classified according to their spin into bosons (integer spin) or fermions (odd half integer spin). The former are the ones that carry forces, for example, the photon, which carries electromagnetic force, the gluon, which carries the strong nuclear force, and the graviton, which carries gravitational force. The latter make up the matter we are made of, like the electron or the quark. The original String Theory only described particles that were bosons, hence Bosonic String Theory. It did not describe Fermions. So quarks and electrons, for instance, were not included in Bosonic String Theory.

By introducing Supersymmetry to Bosonic String Theory, we can obtain a new theory that describes both the forces and the matter which make up the Universe. This is the theory of superstrings. There are three different superstring theories which make sense, i.e. display no mathematical inconsistencies. In two of them the fundamental object is a closed string, while in the third, open strings are the building blocks. Furthermore, mixing the best features of the bosonic string and the superstring, we can create two other consistent theories of strings, Heterotic String Theories.

However, this abundance of theories of strings was a puzzle: If we are searching for the theory of everything, to have five of them is an embarrassment of riches! Fortunately, M-theory came to save us.

Extra dimensions...

One of the most remarkable predictions of String Theory is that space-time has ten dimensions! At first sight, this may be seen as a reason to dismiss the theory altogether, as we obviously have only three dimensions of space and one of time. However, if we assume that six of these dimensions are curled up very tightly, then we may never be aware of their existence. Furthermore, having these so-called compact dimensions is very beneficial if String Theory is to describe a Theory of Everything. The idea is that degrees of freedom like the electric charge of an electron will then arise simply as motion in the extra compact directions! The principle that compact dimensions may lead to unifying theories is not new, but dates from the 1920's, since the theory of Kaluza and Klein. In a sense, String Theory is the ultimate Kaluza-Klein theory.

For simplicity, it is usually assumed that the extra dimensions are wrapped up on six circles. For realistic results they are treated as being wrapped up on mathematical elaborations known as Calabi-Yau Manifolds and Orbifolds.

M-theory

Apart from the fact that instead of one there are five different, healthy theories of strings (three superstrings and two heterotic strings) there was another difficulty in studying these theories: we did not have tools to explore the theory over all possible values of the parameters in the theory. Each theory was like a large planet of which we only knew a small island somewhere on the planet. But over the last four years, techniques were developed to explore the theories more thoroughly, in other words, to travel around the seas in each of those planets and find new islands. And only then it was realized that those five string theories are actually islands on the same planet, not different ones! Thus there is an underlying theory of which all string theories are only different aspects. This was called M-theory. The M might stand for Mother of all theories or Mystery, because the planet we call M-theory is still largely unexplored.

There is still a third possibility for the M in M-theory. One of the islands that was found on the M-theory planet corresponds to a theory that lives not in 10 but in 11 dimensions. This seems to be telling us that M-theory should be viewed as an 11 dimensional theory that looks 10 dimensional at some points in its space of parameters. Such a theory could have as a fundamental object a Membrane, as opposed to a string. Like a drinking straw seen at a distance, the membranes would look like strings when we curl the 11th dimension into a small circle.

Black Holes in M-theory

Black Holes have been studied for many years as configurations of spacetime in General Relativity, corresponding to very strong gravitational fields. But since we cannot build a consistent quantum theory from GR, several puzzles were raised concerning the microscopic physics of black holes. One of the most intriguing was related to the entropy of Black Holes. In thermodynamics, entropy is the quantity that measures the number of states of a system that look the same. A very untidy room has a large entropy, since one can move something on the floor from one side of the room to the other and no one will notice because of the mess - they are equivalent states. In a very tidy room, if you change anything it will be noticeable, since everything has its own place. So we associate entropy to disorder. Black Holes have a huge disorder. However, no one knew what the states associated to the entropy of the black hole were. The last four years brought great excitement in this area. Similar techniques to the ones used to find the islands of M-theory, allowed us to explain exactly what states correspond to the disorder of some black holes, and to explain using fundamental theory the thermodynamic properties that had been deduced previously using less direct arguments.

Many other problems are still open, but the application of string theory to the study of Black Holes promises to be one of the most interesting topics for the next few years.

ஐன்ஸ்டைனின் ஒருங்கமைக் கோட்பாட்டின் (Theory of Everything) அடிப்படையில், அதன் நீட்சியாக
உருவாக்கப்பட்டது 'அதிர்விழைக்
கோட்பாடு' (String Theory) . இந்த
அதிர்விழைக் கோட்பாட்டின் நீட்சியாக 'எம் கோட்பாடு' (M Theory) உருவாகியது.

அதிர்விழைக் கோட்பாட்டையும், எம்

கோட்பாட்டையும் நிரூபிப்பதற்கான

வாய்ப்புகள் அதை

உருவாக்கியவர்களுக்குக்

கிடைக்கவில்லை. அந்தக் கோட்பாடுகள்

பல விஞ்ஞானிகளால்

விமர்சனத்துக்குள்ளாகியது.

கற்பனாவாதிகள் என்று சிலரால்

ஏளனமும் செய்யப்பட்டது. ஆனால், அந்தக் கோட்பாட்டைப் பலர் ஏற்றுக்

கொண்டுமிருக்கிறார்கள்.

அதிர்விழைக் கோட்பாடு, எம்

கோட்பாடுகளின்படி, நாம் வாழும்

அண்டத்திற்கு மொத்தமாகப் பனினொரு

பரிமாணங்கள் உள்ளன என்று

நிறுவப்பட்டிருக்கின்றது. மனிதர்களான

நம்மால் அதிகபட்சமாக மூன்றே மூன்று

பரிமாணங்களைத்தான் புரிந்து கொள்ள

முடியும். நீளம், அகலம், உயரம் (X,Y,Z,)

என்பவையே அந்த மூன்று

பரிமாணங்களுமாகும். நம் கண்ணால்

பார்க்கும் காட்சிகளெல்லாம்

முப்பரிமாண வடிவங்களாலானவை.

மனிதன் மூன்று பரிமாணத்தில்

வாழ்ந்து கொண்டிருக்கிறான். ஆனால்,

ஒரு பரிமாணத்தில் வாழும்

உயிரினங்களும், இரு பரிமாணங்களில்

வாழும் உயிரினங்குகளும் பூமியில்

இருக்கின்றன. உதாரணமாக நிலத்தில்

அசையாமல் இருக்கும் ஒரு சிறிய புல்

ஒரு பரிமாணத்தில் வாழ்வதாகவே நாம்

எடுத்துக் கொள்ளலாம். அதற்கு, இடம்

வலமாகவோ, முன் பின்னாகவோ

அசைவதற்கான பரிமாணம் இல்லை.

ஆனால் கீழ் மேலாக அது தன்னை

வளர்த்துக் கொள்கிறது. அதனால் அதை

ஒரு பரிமாணத்தில் வாழும்

உயிரினமாகக் கொள்ளலாம்.

இது ஒரு உதாரணத்திற்கு மட்டுமே.

அதுபோல, நிலத்தில் ஊர்ந்து செல்லும்

எறும்பொன்றை இரு பரிமாணங்களில்

வாழும் உயிரினமாகக் கொள்ளலாம்.

எறும்பு, முன் பின்னாகவும், இடம்

வலமாகவும் அசைகிறது. ஆனால்,

அதனால், மேல் கீழாக அசைய

முடிவதில்லை. ஆனால், ஒரு

பறவையால் இடம் வலம், முன் பின், மேல் கீழ் ஆகிய மூன்று பரிமாணங்களில்

பயணிக்க முடிகிறது.

ஒரு எறும்பிடம்"மூன்றாவது பரிமாணமென்ற ஒன்று

உண்டென்றும், அங்கு மனிதன்

என்பவனும், பறவைகளும் வாழ்ந்து

கொண்டிருக்கின்றன என்றும்

சொன்னால், அதனால் அதைக்

கற்பனையே செய்து பார்க்க

முடியாது. அதுபோல, நான்காவது

பரிமாணம் உண்டென்று மனிதனிடம்

சொன்னால், அவனாலும் அதை நம்ப

முடிவதில்லை. கற்பனை செய்யவும்

முடிவதில்லை.

எம் கோட்பாடு, மொத்தமாகப் பதினோரு பரிமாணங்கள் உள்ளன என்று சொல்கின்றது. இதுஉண்மையாக இருக்கும் பட்சத்தில் நான்காவது, ஐந்தாவது, ஆறாவது, ஏழாவது, எட்டாவது, ஒன்பதாவது, பத்தாவது, பதினோராவது பரிமாணங்களிலும் வெவ்வேறு வகையான உயிரினங்கள்

வாழ்ந்துகொண்டிருக்கலாம். அவையே

ஆவிகளாகவும், பேய்களாகவும்,

ஏலியன்களாகவும், அசுரர்களாகவும்,

தேவர்களாகவும், கடவுள்களாகவும்

இருக்கலாம்.

ஒவ்வொரு பரிமாணங்களாக

அதிகரிக்க அதிகரிக்க, அதில் வாழும்

உயிரினங்களின் சக்திகளும்

அதிகரித்துச் செல்லலாம். மூன்று

பரிமாணங்களைவிட அதிகப்

பரிமாணங்கள் உண்டென்று

நிரூபிக்கப்பட்டால், அதில் வேறு

வகையான உயிரினங்களும்

வாழ்வதற்குச் சாத்தியங்களும் உண்டு

என்பதும் உறுதியாகும்....