Introduction To String Theory

In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings.  String theory describes how these strings propagate through space and interact with each other.  We live in a wonderfully complex universe, and we are curious about it by nature.  Time and again people have wondered, why are we here?   Where did we and the world come from?  What is the world made of?  It is our privilege to live in a time when enormous progress has been made towards finding some of the answers.  String theory is our most recent attempt to answer where the world come from and what is the world made of.  String theory describes how one-dimensional objects called strings propagate through space and interact with each other.

Physics is the branch of science concerned with the nature and properties of matter and energy.  Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena and String Theory is a part of theoretical physics.  Sir Isaac Newton was the first theoretical physicist, although in his own time his profession was called ‘natural philosophy’.  One of the goals of Physics is to find a single theory that unites all of the four forces of nature which are, electromagnetism, gravity, and the strong and weak nuclear forces.  Concerning the two most familiar forces, Electromagnetism is the force that holds a fridge magnet to a refrigerator, while gravity is trying to pull it off towards the earth.

One crucial idea that has driven physics since Newton’s time is that of unification: the attempt to explain seemingly different phenomena by a single overarching concept.  Perhaps the first example of this came from Newton himself, who in his 1687 work Principia Mathematicae explained that the motion of the planets in the solar system, the motion of the Moon around the Earth, and the force that holds us to the Earth are all part of the same thing: the force of gravity.  We take this for granted today, but pre-Newton the connection between a falling apple and the orbit of the Moon would have been far from obvious and quite amazing.

180 years after Newton the Scottish mathematician James Clerk Maxwell showed that electrostatics and magnetism, by no means similar phenomena at first sight, are just different aspects of a single thing called electromagnetism.  In the process Maxwell discovered electromagnetic waves, which are in fact light, Maxwell had inadvertently explained a further seemingly different aspect of nature.

Each of the four fundamental forces in the universe: gravity, electromagnetism, and the weak and strong nuclear forces is produced by fundamental particles that act as carriers of the force.  The most familiar of these is the photon, a particle of light, which is the mediator of electromagnetic forces.  This means that, for instance, a magnet attracts a nail because both objects exchange photons.  The graviton is the particle associated with gravity.  The strong force is carried by eight particles known as gluons.  Finally, the weak force is transmitted by three particles, the W+, the W , and the Z.

There have been two great breakthroughs in the 20th century physics.  Perhaps the most famous is Einstein’s theory of general relativity.  The other equally impressive theory is quantum mechanics.  In 1984, the Pakistani Abdus Salam and the American Steven Weinberg showed that the electromagnetic force and the weak nuclear force, which causes radioactive decay, are both just different aspects of a single force called the electroweak force.  This left us with three fundamental forces of nature being gravity, the electroweak force and the strong nuclear force which holds protons together.

Having dealt with the forces, physicists became concerned to know about matter.  Many ancient belief systems have postulated that matter, and reality itself, is made from a finite number of elements and modern physics confirms this idea.  Experiments performed with the particle accelerator at CERN in Geneva have shown that there are just twelve basic building blocks of matter.  These are known as the elementary particles.  Everything we’ve ever seen in any experiment, here or in distant stars, is made of just these twelve elementary particles.  This is truly impressive, the entire Universe, its matter and dynamics explained by just three forces and twelve elementary objects.  It’s a good start, but questions still abound, and this is where string theory enters as an attempt to unify further.

Particle physics has spent a long time searching for the Higgs boson particle.  On July 4, 2012, scientists at CERN announced that they had found a particle that behaved the way they expect the Higgs boson to behave.  Maybe the famed boson’s grand and controversial nickname, the “God Particle”, has kept media outlets buzzing.  Then again, the intriguing possibility that the Higgs boson is responsible for all the mass in the universe rather captures the imagination, too.  Or perhaps we’re simply excited to learn more about our world, and we know that if the Higgs boson does exist, we’ll unravel the mystery a little more.

In order to truly understand what the Higgs boson is, however, we need to examine one of the most prominent theories describing the way the cosmos works and that is called the standard model.  The model comes to us by way of particle physics, a field filled with physicists dedicated to reducing our complicated universe to its most basic building blocks.  It’s a challenge we’ve been tackling for centuries, and we’ve made a lot of progress.  First we discovered atoms, then protons, neutrons and electrons, and finally quarks and leptons, which will be discussed more later on.  But the universe doesn’t only contain matter; it also contains forces that act upon that matter.  The standard model has given us more insight into the types of matter and forces than perhaps any other theory we have.

As theories go, the standard model has been very effective, aside from its failure to fit in gravity.  As it turns out, scientists think each one of the fundamental forces has a corresponding carrier particle, or boson, that acts upon matter.  That’s a hard concept to grasp.  We tend to think of forces as mysterious, ethereal things that straddle the line between existence and nothingness, but in reality, they’’re as real as matter itself.

Some physicists have described bosons as weights anchored by mysterious rubber bands to the matter particles that generate them.  Using this analogy, we can think of the particles constantly snapping back out of existence in an instant and yet equally capable of getting entangled with other rubber bands attached to other bosons (and imparting force in the process).

Scientists think each of the fundamental forces has its own specific bosons.  Electromagnetic fields, for instance, depend on the photon to transit electromagnetic force to matter.  Physicists think the Higgs boson might have a similar function, but transferring mass itself.

The behavior of all of these particles and forces is described with impeccable precision by the Standard Model, with one notable exception, the one we call gravity.  For technical reasons, the gravitational force, the most familiar in our everyday lives, has proven very difficult to describe microscopically.  This has been for many years one of the most important problems in theoretical physics, to formulate a quantum theory of gravity.

So, what is the world made of?  Ordinary matter is made of atoms, which are in turn made of just three basic components being, electrons that are whirling around a nucleus which is composed of neutrons and protons.  The electron is a truly fundamental particle (it is one of a family of particles known as leptons), but neutrons and protons are made of smaller particles, known as quarks.  Quarks are, as far as we know, truly elementary.

Our current knowledge about the subatomic composition of the universe is summarized in what is known as the Standard Model of particle physics.  It describes both the fundamental building blocks out of which the world is made, and the forces through which these blocks interact.  There are twelve basic building blocks.  Six of these are quarks, they go by the interesting names of up, down, charm, strange, bottom and top.  A proton, for instance, is made of two up quarks and one down quark.  The other six are leptons, these include the electron and its two heavier siblings, the muon and the tauon, as well as three neutrinos.

In the last few decades, string theory has emerged as the most promising candidate for a microscopic theory of gravity.  And it is infinitely more ambitious than that: it attempts to provide a complete, unified, and consistent description of the fundamental structure of our universe.  For this reason it is sometimes, quite arrogantly, called a ‘Theory of Everything’.

The essential idea behind string theory is that all of the different ‘fundamental’ particles of the Standard Model are really just different manifestations of one basic object that is termed a string.  How can that be?  Well, we would ordinarily picture an electron, for instance, as a point with no internal structure.  A point cannot do anything but move.  But, if string theory is correct, then under an extremely powerful microscope we would realize that the electron is not really a point, but a tiny loop of string.  A string can do something aside from moving, it can oscillate in different ways.  If it oscillates a certain way, then from a distance, unable to tell it is really a string, we see an electron.  But if it oscillates some other way, well, then we call it a photon, or a quark, hopefully you get the idea.  Thus, if string theory is correct, the entire world is made of strings!

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