String Theory (Overview)

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String Theory (Overview)

• by
•  Robert J. Nemiroff 
•  Michigan Technological University

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Physics X About This Course

• Officially "Extraordinary Concepts in Physics"
• Being taught for credit at Michigan Tech
• Light on math, heavy on concepts
• Anyone anywhere is welcome
• No textbook required
• Wikipedia, web links, and lectures only
• Find all the lectures with Google at
• "Starship Asterisk" then "Physics X" 
• http//bb.nightskylive.net/asterisk/viewforum.php?
  f39

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String Theory

• attempts to explain both gravitational and
  quantum effects in a single theory.
• models electrons and quarks not as one
  dimensional points but two dimensional strings.
• Strings posses only length, not height or width. 
• Usually posit the existence of several extra
  spatial dimensions, besides the well known 3 and
  time (a fourth).
• Now considered as part of 11-dimensional
  M-theory.
• RJN Note I am not a string theorist.  
• My understanding of this particularly deep topic
  may be flawed.

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String Theory Strings of what?

• What are these strings (and branes) made out of?
• This is rarely addressed.  Formally, only gross
  attributes are considered length, spin, energy,
  tension, background, and boundary attributes.  
• One possibility is that strings are made out of a
  confined form of dark energy, not unlike
  miniature versions of cosmic strings or domain
  walls. Cosmic strings might then be stretched
  versions of particle strings from the early
  universe.

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String Theory Branes

• Branes (short for membranes) are more general
  than strings.
• D-brane string end with Dirchlet boundary
  conditions
• P-brane (black) a black hole solution
  generalized to 11 dimensions
• NS5-brane a five dimensional object that can
  mimic a magnetic monopole

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String Theory Open and closed strings

• Strings can be open or closed.  Examples
• closed string graviton
• open string photon
• Strings have
• size
• tension
• vibrations
• translation 
• the ability to split and reconnect
• Designed to have many more degrees of freedom
  than a point particle

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String theory Different types
Type Spacetime dimensions Details
Bosonic 26 Only describes bosons tachyons destabilizing
I 10 Supersymmetry between forces and matter
IIA 10 Supersymmetry
IIB 10 Supersymmetry
HO 10 Supersymmetry
HE 10 Supersymmetry
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String Theory Actions

• Define how strings move through space.  Strings
  will move to minimize an "action" integral.
•  
• Two types 
• Nambu-Goto action
• simplest invariant action
• not all that useful
• Polyakov action
• involves string tension 
• better constrained by boundary conditions

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String theory Number of spatial dimensions

• In classical physics, EM, and GR, there are
  three space dimensions and one time dimension.
   These are empirical.
• In string theories, the number of dimensions is
  determined by things like potential energy and
  the need to make force-carrying particles like
  the photon massless.
• All of these new dimensions are in space, not in
  time.

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String Theory Compact Dimensions

• Extra space dimensions are not observed and so
  are hypothesized to be either very small or very
  large.
• Very small dimensions
• as small as the Planck scale (10-35 meters)
• termed "compactified"
• need small wavelength particles to fit in and
  "see" them
• typically "rolled up" meaning circular
• move in this dimension and you quickly return to
  your starting point
• analogy a garden hose viewed from far away
  appears as a two-dimensional string.

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String Theory Compact Dimensions

• Very large dimensions
• Called the "bulk"
• We live on smaller-dimensional boundaries
• termed the "brane"
• Prediction gravity much stronger on very small
  scales
• because one sees "leaking" from the bulk

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String Theory Compact Dimensions

• Light and most particles are constrained to the
  brane.  Gravity, however, is not, and can "spread
  out" over all of the dimensions of the bulk.
• Acceleration due to gravity falls off as 1/r(D-1)
  where D is the number of spatial dimensions
• This is one reason that gravity is weaker than
  electricity and magnetism.

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Calabi-Yau Manifold

• A three dimensional projection of a
  multi-dimensional object having properties and
  symmetries inherent in a Calabi-Yau manifold.

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String Theory Falsifiable?

• Does string theory make falsifiable predictions?
• Historically, some famous physicists say no
• Feynman, Glashow, Smolin  
•  
• Some string theories predict that gravity will
  much stronger at very short distances.  But this
  is not an easily falsifiable prediction.

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String Theory Falsifiable?

• "I wouldn't have thought that a wrong theory
  should lead us to understand better the ordinary
  quantum field theories or to have new insights
  about the quantum states of black holes."  --
  Edward Witten Princeton IAS, Cite
• "For more than a generation, physicists have been
  chasing a will-o-the-wisp called string theory.
  The beginning of this chase marked the end of
  what had been three-quarters of a century of
  progress. Dozens of string-theory conferences
  have been held, hundreds of new Ph.D.s have been
  minted, and thousands of papers have been
  written. Yet, for all this activity, not a single
  new testable prediction has been made, not a
  single theoretical puzzle has been solved. In
  fact, there is no theory so farjust a set of
  hunches and calculations suggesting that a theory
  might exist. And, even if it does, this theory
  will come in such a bewildering number of
  versions that it will be of no practical use a
  Theory of Nothing." -- Jim Holt  New Yorker,
  2006