SPACETIME , LIGHT and GRAVITY in STRING THEORY

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SPACE-TIME , LIGHT and GRAVITY in STRING
THEORY
S. Ramgoolam , Queen Mary, PPARC Wednesday
04/10/2006
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Why theoretical physics ?

• Theoretical Physics, despite its use of
  sophisticated mathematical tools and sometimes
  highly specialized language, reveals surprising
  connections between simple concepts which we use
  in daily life.
• These connections are often easy to state and
  have profound consequences for the measurement of
  simple physical quantities of universal
  relevance.

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• The theory of relativity ( of Einstein )
  provided examples of this kind of novel
  connections between basic concepts
• Time and Space
• String Theory provides equally surprising
  connections between
• Light and Gravity

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TIME and SPACE in RELATIVITY THEORY
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TIME

• Time, as a physical quantity, reminds us of
  duration, measuring instruments sand clocks,
  watches, atomic clocks.

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• It provides signposts along the sequence of
  experiences we live through --- thus providing a
  meter in our personal histories.
• Time is something we feel we know first hand.

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SPACE

• Space reminds us of distance

• and Perspective

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• Of places to get to

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• And Expanses far from our daily experience

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Space-time

• Einstein comes along and teaches us that space
  and time are entwined in a remarkable way, which
  makes it useful to think about space-time as one
  entity.

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THE SURPRISES !!

• Some aspects of the new understanding of
  space-time in Einsteins theory of Relativity.
• Nothing travels faster than light
• A clock travelling at a speed close to that of
  light will be found to be slower by an observer
  at rest !! This is time dilation
• Relativistic dynamics is simple in terms of
  space-time diagrams, where notions of distance in
  space are generalized to distance in space-time.

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• Later, direct observation of time dilation in
  particle physics experiments
• Time dilation Twin travelling to a distant
  galaxy near the speed of light and travelling
  back will be found to be much younger than twin
  who stayed at home.

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LIGHT and GRAVITYin STRING THEORY
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STRING THEORY

• String theory offers us surprises of comparable
  magnitude.
• The elements in Einsteins synthesis were space
  and time.
• The surprising synthesis emerging from String
  theory in the last decade or so involves two
  other familiar elements of our experience Light
  and Gravity.

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• Maxwells Electromagnetic theory of light (
  pre-20th century ) when developed to
  incorporate the insights of quantum mechanics and
  relativity ( early 20th century ) gives rise
  to a new framework
• Quantum field theory ( QFT )
• mid-20th century

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Light and Photons

• In QFT, light can be viewed as an elementary
  particle, called the photon
• QFT provides a new way to understand force
  Force is due to the exchange of elementary
  particles
• Electric Force between particles e.g.
  electrons is due to the exchange of photons (
  light )

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QFT and Strong Force

• Electrons spread to the outskirts of atoms. The
  core of atoms consists of nuclei, made of
  protons and neutrons. Protons and neutrons are
  themselves made of quarks.
• QFT can also account for sub-nuclear forces
  between quarks.
• The mediators of the ( strong ) force between
  quarks are gluons.

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• Hence QFT is the language of the Standard model
  of Particle Physics (discovered by Weinberg
  and Salam ) which has been tested to extremely
  high accuracy in accelerators around the world.

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Photons, Gluons , Large N

• Photons force between electric charge.
• One kind of charge ( one number, positive or
  negative ) . One kind of photon.
• Quarks come in 3 colours. Hence gluons come in
  multiple types, depending on which quarks they
  mediate between.
• A simple mathematical generalization consists of
  imagining theories with N quarks and N2 gluons.

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WHY STRINGS ?

• QFT , while successful in describing
  electromagnetic and sub-nuclear forces, is not so
  successful in describing the force of gravity.
• Rather we need String Theory, which is a new
  paradigm whereby fundamental constituents of the
  world are lines not points.
• A multiplicity of particles arises from different
  vibration modes of the same string.

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STRING gluon and graviton

• Vibrations of an open string give rise to gluons.
• Vibrations of closed string -- graviton ( since
  70s )

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Gravitons and Gravity

• Graviton exchange . force of gravity.
• Charges replaced by masses.
• Photon replaced by graviton.
• By using symmetries of the string
• world-sheet, one can recover the equations
  of general relativity.

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Weak Gravity

• In the limit of weak gravity, we recover Newtons
  gravity, which describes most planetary orbits to
  reasonable accuracy.

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Strong Gravity

• In the limit of strong gravity, the Einstein
  equations describe black holes

• ON very large cosmological scales, it describes
  the expansion of the Universe.

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A String duality between Light and Gravity

• In string theory, light ( and its large N
  generalization ) arises from the same object as
  gravitons. This leads to the unexpected relation
  
• Gravity for space-time in the shape of a solid
  ball in five dimensions is equivalent to a theory
  with no gravitons but with large N-light
  propagating on the four dimensional surface
• of the ball.

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ADS/CFT

• The five dimensional ball is ADS
• ( Anti-de-Sitter ) space a solution to
• Einsteins equations with negative
  cosmological constant.
• The large-N-light theory is a limit of open
  string theory, called a CFT ( conformal field
  theory ).
• This is the celebrated ADS/CFT duality discovered
  in 1996.

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Duality and Probability

• Subject of my current work ---
• Correlators, probabilities, topologies in
  AdS/CFT ( with a student at QM and
  international collaborators )
• Finding probabilities for interactions
• between gravitons and their spacetime
  geometries, using the dual large-N-light theory

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FIVE dimensional space-time geometries
interpolating between gravitons Or between their
corresponding geometries are summed ( in a
Euclidean Path integral ) to compute
probabilities for transition between
gravitons/geometries
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DUALITY to the rescue !

• This sounds complicated. And it is !!
• A direct computation of such probabilities in
  quantum gravity is not possible yet.
• But thanks to the duality between gravity and
  light, we can use calculations with light to get
  the answers

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Probabilities in String Landscape ?

• What are the implications of this new string
  duality between gravitons and gluons ? There are
  bound to be many.
• One aspect of string theory is to build precise
  models of universes. The new picture is that
  there is a huge multiplicity ( string landscape
  ) of models that have to be considered in
  String theory.
• It has been argued that the time evolution of the
  Universe, in the form of eternal inflation ( at
  times close to the Big Bang ) , allows the use of
  a probabilistic scenario in explaining the
  fundamental constants of particle physics from
  String Theory.

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• WHAT are the odds that a universe like ours will
  emerge from cosmological evolution close to the
  big bang, where quantum gravity is important ?

• What does Light-Gravity duality tell us about
  this ?