String theory
String theory is based on the idea, that elementary particles are not point like particles,
but vibrations of strings.
If it is right, it would be profound.
String theory is still evolving.
We will discuss some lines of thought in its evolution.
Protons and neutrons are made up of quarks.
These are strongly interacting particles, and are governed by the strong nuclear force.
Scientists have been studying, what happened when strongly interacting particles,
collide and scatter off one another.
Based on these experiments, scientist proposed that particles were like strings.
They existed only in a single dimension.
They could be stretched like rubber bands.
When they gain energy they stretched.
When they lost energy they contracted.
And like rubber bands they vibrated.
Strongly interacting particles were all rubber bands.
They vibrated as they travelled, colliding with one another and exchanging energy.
The various states of vibrations corresponded to various kinds of particles,
produced in proton smashing experiments, in particle accelerators.
The force between quarks is described most fundamentally, by a gauge field.
The basic law seems to be given by quantum chromodynamics, QCD.
This is part of the standard model.
In some circumstances, this force is like having a rubber band between the quarks.
The strong nuclear force is very unlike the electro magnetic force.
The electro magnetic force becomes weaker with distance.
The force between two quarks, approaches a constant strength, as we pull the quarks apart.
It remains constant no matter how far they are apart.
This is the reason we do not ever see free quarks, in particle accelerators.
But when quarks are very close to each other,
the force between them weakens.
The string picture works only when the quarks are at a sufficient distance from each other.
Some scientist found that the new theory would be consistent only if it had a new symmetry.
String theory let to the discovery of supersymmetry.
Supersymmetric string theory addressed some of the problems of earlier string theories.
It had nine dimensions.
With time added, the new supersymmetric string theory lives in 10 dimensions.
There was one puzzle.
Supersymmetric theory is associated with strong interactions.
It had bosons with no mass.
One is a photon.
A photon never stands still, and moves at the speed of light.
It has energy and no mass.
Scientists found that superstrings, had states of vibrations corresponding to gauge bosons,
including the photon.
Scientist speculated that some of the massless particles could be gravitons.
Photon like and graviton like particles, arise from strings.
Strings can be both closed an open.
A closed string is a loop.
An open string is a line, and it has ends.
Massless particles like photons come from vibrations of either open or closed strings.
Gravitons come only from vibration of closed strings, or loops.
The ends of an open string, can be seen as charged particles.
For example, one end could be a negatively charged particle, such as an electron.
The other end would be its anti particle, the positron, which is positively charged.
The massless vibration of a string between them describes the photon,
which carries the electrical force between the particle and their antiparticle.
We get particles and forces from the open strings.
Nature produces collision between particles and antiparticles.
They annihilate creating a photon.
The two ends of the string come together and join to form a closed loop.
The difference between gravity and other forces is explained in terms,
of the difference between open and closed strings.
Is there a law, that requires the ends of the string to meet and join?
In string theory, the law of motion dictates the laws of the forces.
All forces in string theory come from the simple origin, - that comes from breaking and joining of strings -.
When a force is added a string can break into two strings.
Two strings can join and give out a force.
The law of breaking and joining seem to be consistent with special relativity and quantum theory.
In string theory there can be only two fundamental constants.
One is called string tension.
It describes how much energy is contained per unit length of string.
The other is called string coupling constant.
It is the probability of a string breaking into two strings,
thus giving rise to a force.
All other constants have to be related to these two constants.
For example, Newton's gravitational constant, relates to the product of their values.
As a one dimensional string moves through time, it makes a two dimensional surface in spacetime.
The surface has a certain area.
It is defined roughly as the product of the length and the duration in time.
The strings move to minimise this area.
This simple concept is similar to the law of bubbles,
which states that a bubble takes up the minimal area it can,
given the constraints of forces acting on it.
String theory explains the motion of strings.
When strings are allowed to break and join, it explains all the forces.
It unifies all the forces we know, with the description of the particles.
It is simpler than the laws describing any of the things it unifies.
Faraday had imagined electrical and magnetic fields, in terms of field lines.
These are the lines running between the poles of the magnet,
or between the positive and negative electric charges.
They conveyed the forces between magnets and charges.
In Maxwell's theory the field lines become secondary to the fields.
We can imagine that the field lines really exist.
Forces between particles are the field lines stretching between them.
This is possible in quantum theory.
One can think of the field lines as a kind of atom, of the magnetic field.
Some scientists proposed that the same thing was true of the lines of force in QCD,
which are analogs to field lines of electro magnetism.
They visualised strings as quantised lines of electric flux.
Scientists wished to solve QCD, by re-expressing it as a theory of strings.
The strings mean the lines of quantised electrical flux.
According to these scientists, the primary objects in a gauge theory are only field lines.
They satisfy simple laws, which dictate how they stretch between charges.
The fields themselves are only a alternate description.
This fits into string theory, because the field lines can be taken to be strings.
This suggests a kind of duality.
One can think of field lines as the primary object, and the basic laws as describing,
how they stretch and move.
We can think of the field as primary,
and the field lines just as a convenient way to describe the field.
This gives rise to the principle of duality of strings and fields.
Either can be taken to be fundamental.
Scientist thought about the puzzles that would be solved,
if elementary particles were vibrations of strings.
1. String theory can give us an automatic unification of all elementary particles.
It can also unify all the forces with one another.
All come from vibrations of one fundamental kind of object.
2. String theory automatically gives us gauge fields,
which are responsible for electro magnetism and the nuclear forces.
These naturally arise from the vibrations of open strings.
3. String theory automatically gives us gravitons,
which come from vibrations of closed strings.
Any quantum theory of strings must involve closed strings.
As a consequence we get an automatic unification of gravity with other forces.
4. A supersymmetric string theory unified the bosons and fermions,
which are both us oscillations of strings, thus unifying all the forces with particles.
Supersymmetry may be true even if string theory is not true.
Supersymmetric string theories are deeply elegant objects.
String theory achieved a natural unification of the laws of motion,
with the laws that governed forces.
The whole standard model with its 12 kinds of quarks and leptons,
and its three forces, plus gravity could be unified.
All these phenomena arise from the vibrations of strings,
stretched in space time.
It follows the simple law, that the area is minimised.
All the constants of the standard model,
can be reduced to a combination of Newton's gravitational constant,
and one simple number which is the probability for a string to break into two and join.
The second number is not fundamental but a property of the environment.