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String Theory
Doug Howe
Nimmi Bhatt
PHYS43 Modern Physics, SRJC
Section 5756
Younes Ataiiyan
Spring 2011
Purpose/Goal
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String Theory is a developing theory in particle
physics that attempts to reconcile quantum
mechanics with general relativity.
A contender for the Theory of Everything (TOE)
Fundamental Forces
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There are four fundamental
forces
Each has a force-carrying
Gauge Boson:
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Strong force: gluon
Weak force: W and Z boson
Electromagnetic force: photon
Gravitational force: graviton(?)
The Standard Model
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The Standard Model of particle physics
incorporates the electromagnetic, strong, and
weak nuclear forces
Uses quantum mechanical model; inconsistent
with continuous gravitational fields of relativity
Regarded as Theory of (Almost) Everything
Does not include gravitation; String theory
attempts to fix this and is consistent with
theories of quantum gravity at high energies
History String Theory: Early
Concepts

Some ideas originate from early 20th
century
German Mathematician Theodor Kaluza notes
general relativity in 5 dimensinos describes
both gravity and electromagnatism in 4 (1919)
 Swedish physicist Oskar Klein suggests extra
dimensions can exist if understood as being
“wrapped” into a small circle (1926)
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History String Theory
Earliest string model: The Bosonic String
(1960s)
 Describes all forces, including a quantum
gravity at low energies
 Theory is unstable and only describes
force-carrying bosons, not matter
(fermions)
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Supersymmetry (SUSY)
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Invented in Standard Model and String Theory
A mathematical relation predicting every particle of ½-n
spin (fermions) has a corresponding superpartner of 1-n
spin (bosons) with the same mass and quantum
numbers
Allowed the development of many Superstring Theories
which include fermions (1970s)
Strings and Branes
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Quarks and electrons are not 0dimensional point particles
Instead 1-D lines or “strings”,
oscillating through 10+
dimensions
Also includes interrelated objects
called membranes “branes”
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Ex: D-branes are “endpoints” for
strings
String Properties
On the order of 10E-35m, a Planck length
 Tension, kinetic energy, and uncertainty
control oscillations
 Quantum mechanics of strings cause
discrete vibrational modes, causing flavor,
charge, mass and spin characteristics
 Splitting and recombination of strings
corresponds to particle emission and
absorption
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Open/Closed Strings
Strings can be open or closed (a loop)
 Ex: one closed string mode is the graviton,
one open string mode is the photon
 All string theories include both types, as
the ends of open strings can meet to form
a closed string
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World Sheets
Classical point particles produce world
lines (left) of position over time
 Strings produce world sheets (right)
 String modes (such as photon/graviton)
are surface waves on these world sheets
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String Theory and Gravity
D-branes are of variable dimension (point,
line, plane, volume) and live on the ends
of strings
 D-branes absorb and emit closed strings
of the mode which describe gravitons
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Different String Theories
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Many String and Superstring theories with 10-26
spacetime dimensions
Now thought to each be special case of a more
fundamental theory (M-theory)
Each type linked by mathematical
transformations called dualities
Extra Dimensions
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Many phenomena are better explained
mathematically in more than the usual four
spacetime dimensions
The number of spacetime dimension is not fixed
in string theory
It is best thought of as different under different
circumstances
Where Are The Extra Dimensions
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Explained by Compactification: extra
dimensions are “curled up” and too small to be
experienced in the macroscopic world
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Can be thought of with a garden hose. From a
distance its 1-D, but on a small scale (to an ant) its
2D, the second being the small circular
circumference
Many other theories including “Brane-World
Scenario” in which we are “stuck” in a four
dimensional subspace of the full universe
Compactification
A 3D projection of a
Calabi-Yau
manifold;
sometimes used to
mathematically
describe
compactification
M-Theory
All Superstring theories appear to be
correct in certain circumstances
 M-theory was developed to unify them
 Proposes that strings are really 1D “slices”
of 2D membranes vibrating in 11 rather
than 10 spacetime dimensions
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Problems with String Theory
1.
2.
3.
4.
5.
High energies needed to test quantum gravity.
Large number of solutions.
Lack of uniqueness of predictions as compared with
Quantum mechanics and general relativity, amongst
others.
Lack of background independence.
Lack of a large number of predictions that are
falsifiable, in general.
Is String Theory Science?
To be considered part of
science a theory must make
testable experimental
predictions
 Criticized by prominent
physicists including Feynman
and Glashow
 Mathematical approach rather
than experiment based
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Sources/Works Cited
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http://en.wikipedia.org/wiki/String_theory
Serway, Raymond A., and John W. Jewett, Jr. Physics for Scientists
and Engineers with Modern Physics. 7th ed. Vol. 2.
Thornton, Stephen T., and Andrew Rex. Modern Physics for
Scientists and Engineers. 3rd ed.
Gribbon, John. Schrödinger's Kittens and the Search for Reality.
1996. Print.