# What is String Theory?: The Superstrings ## What is String Theory particle in String Theory?

What is String Theory? The essence of string theory is that it can explain the nature of both matter and space-time – that is, the nature of wood and marble.  String Theory answers many confusing questions about particles, such as why there are so many in natures.  The deeper we investigate the nature of subatomic particles, the more particles we find.

### Area of String Theory

String theory answers this question because the string, about 100 billion times smaller than a proton, is vibrating; each mode of vibration represents a specific resonance or particle.  The string is so incredibly short that, from a distance, the resonance of a string and a particle is indistinguishable.  Only when we somehow enlarge the particle can we see that it is not a point, but a mode of a vibrating string.

### String Theory and Sub atomic Particles

Each sub-atomic particle corresponds to a specific resonance that only vibrates at a specific frequency.  In theory, the string can vibrate at any of an infinite number of different frequencies.  We know that what is fundamental is the string itself.

According to string theory, if we could make a point particle bigger in some way, we would actually see a smaller vibrating string.  In fact, according to this theory, matter is nothing but the harmony created by this vibrational string.  Since there can be an infinite number of harmonies for a violin, an infinite number of forms of a material can be formed from vibrating strings.  This explains the richness of particles in nature.  Similarly, the laws of physics can be compared to the laws of harmony allowed on a string.

String theory can explain not only the nature of particles, but also space-time.  As a string moves in space-time, it performs a complex set of motions.  The string may, in turn, break into shorter strings or collide with other strings to form longer strings.  The key point is that all these quantum correction or loop diagrams are finite and computable.  It is the first quantum theory of gravity in the history of physics with finite quantum corrections.

### Space-Time and String

To perform these complex motions, a string must obey a large set of self-accompaniment conditions.  These self-consistency conditions are so stringent that they place extraordinarily restrictive terms on space-time.  In other words, the string cannot travel self-perpetually in any arbitrary space-time like a point particle.

When the constraints of string locations on space-time were first calculated, physicists were shocked to see Einstein’s equations emerge from the string.  It was remarkable; without assuming any of Einstein’s equations, physicists found that they came out of string theory, such as by magic.  Einstein’s equations were no longer fundamental; they can be derived from string theory.  If true, string theory solves a long-standing mystery about the nature of wood and marble.

Einstein predicted that one day only marble would explain all the properties of wood.  To Einstein, wood was just a kink or vibration of space-time, nothing more or less.

### Quantum Concepts

What is String Theory? Can quantum physics answer this question? Answer is YES. However, quantum physicists thought the opposite.  He thought that marble could be turned into wood – that is, Einstein’s metric tensor could be turned into gravity, the discrete packet of energy that the force of gravity carries.  These are two completely opposite points of view, and it was long thought that a compromise between them was impossible.

The string, however, is precisely the “missing link” between the wood and the marble.  String theory can obtain particles of matter in the form of resonances vibrating on the string.  And string theory can also derive Einstein’s equations from it by demanding that the string itself moves in a self-perpetuating way in space-time.  In this way, we have a comprehensive theory of both matter-energy and space-time.

### Existence of Higher Dimensions

These self-consistency barriers are surprisingly rigid.  For example, they refuse to move strings in three or four dimensions.  We will see that these self-consistency conditions force the string to move in specific dimensions.  In fact, the only “magic number” allowed by string theory are ten and 26 dimensions.

Fortunately, a string theory has enough “room” to unite all the fundamental forces inherent in these dimensions.  Therefore, string theory is rich enough to explain all the fundamental laws of nature.  Starting with a simple theory of vibrating string, one can deduce Einstein’s theory, supergravity, standard model, and even GUT theory.  It seems no less than a miracle that, starting with some purely geometric arguments from a string, has been able to recapture the complete progress of physics for the last 2 millennia.

## Development of String Theory

Current interest in string theory stems from the work of John Schwarz of the California Institute of Technology and his colleague Michael Green of Queen Mary College in London.  Previously, it was thought that the string could contain defects that would prevent a completely self-contained theory.  Then in 1984 these two physicists proved that the self-consistency conditions could be satisfied on all strings.  This, in turn, ignited the current stampede among young physicists to solve the theory and gain potential recognition.  By the end of the 1980s, a real “gold rush” began among physicists.

## Why Strings?

The distinguishing feature of a string is that it is one of the most compact ways of storing large amounts of data in which information can be replicated.  For living things, nature uses double strands of DNA molecule, which copy each other and make duplicate copies.  In addition, our body consists of billions of protein strings, which are made up of amino acid building blocks.  Our bodies can, in some sense, be seen as a huge collection of strings – protein molecules wrapped around our bones.

## String Quartet

Currently, the most successful version of string theory has been created by Princeton physicists David Gross, Emil Martinek, Jeffrey Harvey and Ryan Röhm, sometimes referred to as the Princeton String Quartet.

Today, of all the various Kaluza-Kleintype theories that have been proposed in the past, this is indeed the heterotic string, which has the greatest ability to integrate all the laws of nature into one theory.  Gross believes that string theory solves the problem of turning wood into marble: “To create matter from the geometry itself—in a sense what string theory does. It can be thought of as such, especially with heterotic strings.”  A theory of gravitation naturally as in the theory in which particles of matter as well as other forces of nature emerge in the same way that gravity emerges from geometry.”  The most notable feature of string theory, as we have emphasized, is that Einstein’s theory of gravity automatically merges into it.

In fact, gravity (the amount of gravity) emerges as the smallest vibration of a closed string.  While GUT avoided any mention of Einstein’s theory of gravity, superstring theories demand that Einstein’s theory be included.  For example, if we leave out Einstein’s theory of gravitation as a vibration of a string, the theory becomes inconsistent and useless.

In fact, this is why Witten was attracted to string theory at first.  In 1982, he read a review article by John Schwarz and was stunned to learn that gravity derives only from self-consistency requirements from superstring theory.  He recalls that it was “the greatest intellectual thrill of my life.”  Witten says, “String theory is extremely attractive because gravity is forced upon us. All known coherent string theories involve gravity, so gravity is

## Compactification and beauty

String theory is such a promising candidate for physics because it gives a simple origin of the symmetries found in particle physics as well as in general relativity.  The supergravity was too small to accommodate the symmetry of both non-generalizable and standard models.  Therefore, it was not self-contained and did not begin to realistically describe known particles.

### In concern with Standard Model

However, string theory does both.  As we will soon see, this overcomes the infinity found in quantum gravity, giving rise to a finite theory of quantum gravity.  Would only guarantee that string theory should be taken as a serious candidate for a theory of the universe?  However, there is an additional bonus.  When we compress some dimensions of the string, we find that there is “enough space” to accommodate the symmetry of the standard model and even the GUT.

### Heterotic String

A heterotic string is a closed string with two types of vibrations, clockwise and counter clockwise, which are treated differently.  Clockwise vibrations reside in ten-dimensional space.  Counter clockwise occupies a 26-dimensional space, of which 16 dimensions have been compressed.  (We remember that in Kaluza’s original five-dimensional theory, the fifth dimension was compressed by wrapping it into a circle.)

The heterotic string is named due to the fact that the clockwise and counter clockwise vibrations reside in two different dimensions but are combined to produce a superstring principle.  That is why it is named after the Greek word heterosis, which means “hybrid vigor.”

### Compatibility with Higher Dimensions

The 16-dimensional compactified space is by far the most interesting.  In the Kaluza–Klein principle, we recall that the compressed IV-dimensional space can have a symmetry,

Then all vibrations (or fields) located on 26 dimensional space automatically acquire these symmetries.  If the symmetry is SU(JV), then all vibrations on space must obey the SU(N) symmetry (in the same way that clay inherits the symmetry of the mold).  In this way, the Kaluza – Klein theory can adjust the symmetry of the standard model.

### Supergravity

It can also be determined in this way that the supergravity was “too small” to contain all particles of symmetry found in the standard model.

This was enough to kill the supergravity theory as a realistic theory of matter and space-time.  But when the Princeton String Quartet analyzed the symmetry of the 16-dimensional space, they found that it had a monstrously large symmetry, called E (8) x E (8), more than any GUT symmetry.  Very large that has ever been tried.  This was an unexpected bonus.  This meant that all vibrations of the string would achieve the symmetry of 16-dimensional space, which was more than enough to accommodate the symmetry of the Standard Model.

### Geometry of Higher Dimensions

The 26-dimensional space of counter clockwise vibrations has enough room to explain all the symmetries found in both Einstein’s theory and quantum theory.  Therefore, for the first time, pure geometry has given a simple explanation as to why the sub-atomic world must exhibit some of the symmetries emerging from the curling of the higher-dimensional space: the symmetries of the sub-atomic sphere are remnants of the symmetry of the higher-dimensional space.  This means that the beauty and symmetry found in nature may eventually be discovered in higher-dimensional space.

### Examples of Higher Dimensions

For example, snowflakes create beautiful, hexagonal patterns, neither of which are exactly the same.  These ice cubes and crystals, in turn, inherited their structure in the way their molecules are geometrically arranged.  This arrangement is mainly determined by the electron shells of the molecule, which in turn takes us back to the rotational symmetry of the quantum theory given by O (3).  All the symmetries of the low-energy universe that we observe in chemical elements are due to the symmetries listed by the Standard Model, which in turn can be obtained by compressing the heterotic string.

Ultimately, the symmetries we see around us, from rainbows to blooming flowers to crystals, can ultimately be seen as manifestations of fragments of the original ten-dimensional principle.  Riemann and Einstein hoped to find a geometric understanding of why forces determine the motion and nature of matter.  But they were missing a major component in showing the relationship between wood and marble.

This missing link is most likely the superstring principle.  With ten dimensional string theory, we see that the geometry of a string can ultimately be responsible for both the force and the structure of matter.

## Conclusion

A piece of Twenty-First-Century Physics Given the immense power of its symmetry, it should come as no surprise that superstring theory is fundamentally different from any other form of physics.

In fact, it was discovered quite by accident.  Many physicists have remarked that if this sudden accident had never happened, the theory would not have been discovered until the twenty-first century.

This is because it is such a sharp departure from all the ideas tried in this century.  This is not a continuous extension of trends and theories prevalent in this century;  It stands apart.  In contrast, there was a “general” and logical development of the theory of general relativity.  First, Einstein propounded the equivalence principle.  He then reformulated this physical theory into the mathematics of the field theory of gravity based on Faraday’s fields and Riemann’s metric tensor.  Later came “classical solutions” such as black holes and the Big Bang.  Finally, the final stage is the current attempt to formulate the quantum theory of gravity.