Hi, so you want a quick overview of string theory?
Great! You've come to the right place!
Standard Model of particle physics.
When we think of an atom we generally think of a nucleus made up of protons and neutrons
surrounded by a cloud of electrons. If we look deeper into the heart of the nucleus
we realise that the protons and neutrons are made of other particles called quarks.
Quarks, like electrons, are thought to be elementary particles that make up our universe.
The Standard Model categorises all the elementary particles into either matter particles, otherwise
known as fermions or force carrier particles otherwise known as gauge bosons.
Fermions are divided into quarks and leptons. There are six quarks; up, down, charm, strange,
top and bottom. There are six leptons; electron, muon, tau,
electron neutrino, muon neutrino and tau neutrino.
The gauge bosons are gluons, photons and the W and Z bosons.
There are four forces that we know of, the strong force, the electromagnetic force, the
weak force and gravity.
Gluons mediate the strong force between the quarks, holding them together within the protons
and neutrons. Gluons also mediate the strong force between the protons and the neutrons
holding them together within the nucleus of the atom.
Photons are particles of light which carry the electromagnetic force
that holds the electrons in atoms.
W and Z bosons carry the weak nuclear force which is involved in some forms of radioactivity
and plays a role in how the sun burns.
The Standard Model also describes the Higgs field, which is a quantum field that permeates
all of space. Particle interactions with the Higgs field imparts other particles with their mass.
For example, the top quark is interacting with the Higgs field and this is what is giving
the top quark its mass.
The Higgs boson is the local excitation of the Higgs field.
And there's one final boson to talk about, the graviton. This is a hypothetical particle
that mediates the gravitational force.
What is string theory?
String theory proposes that rather than the elementary particles the fundamental building
block of the universe are tiny vibrating strands of energy from which all the matter and all
the forces in the universe emanate.
Why string theory?
The general theory of relativity and quantum mechanics are
two of our most successful theories in physics.
The general theory of relativity describes how the force of gravity works and quantum
mechanics describes how the other three forces work at the smallest of scales, but, the two
theories are incompatible.
General relativity describes space and time as a smooth continuum whereas quantum mechanics
describes space and time as being quantized.
String theory attempted to resolve the incompatibility and explain how all the four forces work,
which made it popular but it had a couple of issues:
1). The theory originally dealt with only bosons and did not include a description for
the fermions and 2). It predicted the existence of hypothetical
faster than light particles - tachyons. These particles would travel back in time and destabilise space.
Adding the principle of supersymmetry solved both the fermion and tachyon problem.
This leads us to ask what is supersymmetry?
Supersymmetry and superstring theory
The principle of supersymmetry says that nature is symmetric and as a consequence every boson
has a fermion superpartner and every fermion has a boson superpartner.
When supersymmetry is combined with String theory, superstring theory emerges. When physicists
refer to string theory they generally means superstring theory which describes all the
fermions and bosons as being the result of vibrating superstrings of energy.
Higher dimensions.
We live in a three dimensional world, meaning there are three directions we can move in,
forward and backward, up and down and left and right.
Einstein added a fourth dimension, time.
When describing where we are at any given time we combine our location in space with time.
So for example, if I wanted to meet my friend Janet for a coffee there is no point
in telling her I'll meet her outside Covent Garden station if I don't also give her
a specific time of when I will be there.
Einstein combined the three dimensions of space and the one dimension of time and termed
it spacetime.
In the theory of general relativity spacetime is described as the fabric of our cosmos.
String theory proposes the existence of additional spatial dimensions plus the three familiar
ones we experience in our everyday lives.
In fact string theory proposes six additional spatial dimensions that are curled up or compactified
around every point in our three dimensional space. We can't experience them as these
higher dimensions are so small, we don't have access to them, but superstrings do.
These superstrings oscillate in these dimensions in vibrating patterns that go on to create
the matter and forces in the Universe we see around us today.
M theory.
String theory was a popular area for study owing to the elegance of its mathematics and
the fact that it appeared to be a theory of everything but there existed five versions of it.
M theory illustrated that these five versions were ways of looking
at a more fundamental theory.
M theory also introduced one additional spatial dimension, the 11th dimension.
This extra dimension allows a string to stretch into sheet like membranes and M theory went
onto describe a hyperspace within which membranes exist with universes attached to them.
These membrane or so-called brane universes make up the multiverse. And one of these branes
is our very own universe.
String theory answers some big questions.
1). The weakness of gravity. Compared to the other forces in the universe,
gravity is incredibly weak and scientists have long questioned why.
The weakness of gravity can be illustrated by the fact that we are able to pick up a
set of keys off the ground using only a magnet. Even with the entire gravitational pull of
the earth being exerted on the set of keys, one simple magnet is able to overcome it.
In fact, gravity is a staggering 10 to the power 40 times weaker than electromagnetism.
String theory explains the weakness of gravity by proposing that gravity seeps off our Universe
and into the higher dimensions.
Superstrings can be either open strings or closed strings. Open strings are attached
to the branes and have no way of leaving them, closed strings however are able to seep off
the branes. So if we imagine gravity to be closed strings that would explain its relative
weakness in comparison to the other forces.
2). The second thing that string theory answers is the origin of our universe -
how our universe came to be.
General relativity and quantum mechanics can't go back to the very beginning of time and
reveal how our universe was born.
The mathematics of both break down when we attempt to go back to the very beginning and
starts offering up nonsensical results.
String theory gives us its own unique explanation of how universes are born, they are born out
of membrane collisions.
How can we test string theory?
We imagine that the compactified additional dimensions of space twist and turn to form
a Calabi–Yau shape.
It is supposed that the particular geometry of our Calabi–Yau shape allows strings to
vibrate and oscillate to give us the exact requirements for our universe to have formed.
So in essence it is because of our Calabi–Yau shape that the mass of the elementary
particles and the strength of the different forces are what they are.
If we were to know what the exact Calabi–Yau shape of
our universe we should be able to work backward and calculate the exact masses of the different
particles and the strengths of the different forces, thus proving string theory in a roundabout way.
But the Calabi–Yau shape could have many many variations which would create many different
string vibrations and oscillations giving rise to many different types of universes,
not just our own. This once again hints at the existence of a multiverse.
We can test string theory in a more practical way by particle collisions.
Particle collisions allow all sorts of extraordinary particles to be released and string theory
believes that these collisions could generate the elusive graviton.
If String theory is right about gravity seeping off our membrane universe into higher dimensions,
these collisions should show that the amount of energy after the collisions are less than
the amount of energy before the collisions as gravitons have seeped into higher dimensions.
But, before we get too excited we have to note that because gravitons are so weak a
detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a
neutron star, would only be expected to observe one graviton every 10 years, even under the
most favorable conditions.
And so, that in a nutshell is our guide to string theory.
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