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Saturday Morning Physics

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Saturday Morning Physics

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    1. 4/2/2012 Saturday Morning Physics 1 Lectures on String Theory Music of the spheres – Lectures on string theory; Lecture 2 : A World Made of StringsMusic of the spheres – Lectures on string theory; Lecture 2 : A World Made of Strings

    2. 4/2/2012 Saturday Morning Physics 2 Outline of the series Outline of Saturday morning lecturesOutline of Saturday morning lectures

    3. 4/2/2012 Saturday Morning Physics 3 Summary last week

    4. 4/2/2012 Saturday Morning Physics 4 String theory vs. quantum field theory

    5. 4/2/2012 Saturday Morning Physics 5 String vibration modes Strings have to wiggle Classically: to avoid collapse under its tension QM: nothing can sit still Discrete spectrum of waves that “fit” Classically: non-zero, positive energy QM: “vacuum energy” can lead to negative or zero internal energy Different string modes correspond to “particles” with different properties!

    6. 4/2/2012 Saturday Morning Physics 6 Determining the string length Among massless modes of a closed string The graviton particle! Tension of string has to be huge to describe the weakness of gravity Huge tension implies tiny string length ~ Planck length = 1.6 x 10-33 cm Second excited state masses are Planck mass A single string can vibrate in infinitely many ways; infinite stair of particles with Planck mass steps Explain this in pictures : Harmonic oscillator pictureExplain this in pictures : Harmonic oscillator picture

    7. 4/2/2012 Saturday Morning Physics 7 String scales

    8. 4/2/2012 Saturday Morning Physics 8 Supersymmetry Matter (fermions) and forces (bosons) behave very differently Half integer spin ( ˝ h ) are fermions and integer spin ( 1 h ) are bosons Quantum version of “rotation”, which is connected to rotational symmetry transformations Space-time symmetries can be uniquely extended with supersymmetries that connect fermions and bosons Add pictures perhaps: Wess/Zumino ?Add pictures perhaps: Wess/Zumino ?

    9. 4/2/2012 Saturday Morning Physics 9 Supersymmetry has to be broken No superpartners have been observed High temperature or high energy : supersymmetry would be restored Superpartner masses high but perhaps will be observed in the near future in accelerators Perhaps add a picture explaining broken symmetriesPerhaps add a picture explaining broken symmetries

    10. 4/2/2012 Saturday Morning Physics 10 Strings and supersymmetry Bosonic string theory only contains boson particles Sure enough our world contains fermion particles (matter=fermions, forces=bosons) Superstring theory: Contains fermion particles Predicts supersymmetry ! Add some illustrations here: Add some illustrations here:

    11. 4/2/2012 Saturday Morning Physics 11 Superstring theories Ten space-time dimensions ! Closed superstrings and closed + open superstrings Different kinds of supersymmetries Five different superstring theories

    12. 4/2/2012 Saturday Morning Physics 12 Superstrings overview Heterotic SO(32) : closed superstrings Heterotic E8 x E8 : closed superstrings Type I : open and closed superstrings Type IIA : closed superstrings Type IIB : closed superstrings

    13. 4/2/2012 Saturday Morning Physics 13 Extra space-time dimensions Obviously are universe is 4-dimensional: time + up/down + left/right + forward/backward However imagine our universe was 2-dimensional: time + left/right Now imagine that our world actually contains a small extra 3rd dimension that is curled up If the extra 3rd dimension is small enough 2-dimensional observers will not be able to detect it ! Add photos of Kaluza-Klein somewhereAdd photos of Kaluza-Klein somewhere

    14. 4/2/2012 Saturday Morning Physics 14 Signatures of extra dimensions Extra dimensions : corrections to gravitational law Gravity at millimeter scales Current limit : R < 0.01 cm Particles can travel in the extra dimensions and because of the wavelike character of particles this motion in the extra dimensions is not arbitrary Kaluza-Klein modes with masses M ~ 1/R Current limit : R < 10-18 cm Add cylinder dimensions with Kaluza-Klein modes. Check limits on extra dimensions size; gravitational and particle physics limits.Add cylinder dimensions with Kaluza-Klein modes. Check limits on extra dimensions size; gravitational and particle physics limits.

    15. 4/2/2012 Saturday Morning Physics 15 Extra dimensions in string theory Amazingly, consistent quantum string theories can only be defined in a unique number of space-time dimensions ! Superstrings require D=10 So six dimensions have to be curled up and very small The spectrum of particles of the string depends crucially on the shape and size of the extra 6 dimensions Good news: some of these look a lot like our world (Calabi-Yau)! Perhaps add a picture of a Calabi-Yau space (Greene’s)Perhaps add a picture of a Calabi-Yau space (Greene’s)

    16. 4/2/2012 Saturday Morning Physics 16 Setting the stage in string theory General relativity tells us that the space-time stage is a treacherous swamp (space-time is dynamical) String theory includes quantum gravity Studying string theory (until recently) meant studying 2d tube string diagrams in a hand-picked, but not completely arbitrary, space-time stage (also called the string vacuum) Bad news: possibilities to pick space-time stages seem to be endless and worse, without preference !

    17. 4/2/2012 Saturday Morning Physics 17 Interacting strings Only approximately known through “tube” diagrams Strength of interactions determined by the string coupling and only when this number is smaller than 1 can we use our approximate description in terms of “tube” diagrams When the string coupling is strong we have no control; not just quantitatively, but also qualitatively (until recently)

    18. 4/2/2012 Saturday Morning Physics 18 String signatures of extra dimensions Kaluza-Klein particles indicate extra dimensions Strings can wrap the compact extra dimensions Strings: KK-tower and winding mode tower Minimal radius : Rmin = string length

    19. 4/2/2012 Saturday Morning Physics 19 To summarize today’s lecture: String vibration modes correspond to particles To get fermions we need superstrings Superstrings predict supersymmetry Superstrings require 10 space-time dimensions Strings “see” space-time differently Our understanding is only approximate

    20. 4/2/2012 Saturday Morning Physics 20 Come back next week Strongly coupled strings D-branes Black holes Dualities and M-theory

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