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Superstrings: The “Ultimate Theory of Everything”? Sera Cremonini Michigan Society of Fellows Physics Department, University of Michigan International Year of Astronomy Lecture Series WVU, April 16 2009 Outline ¾ The Structure of Matter: ¾ the Building Blocks of Nature ¾ Why Quantum Gravity? ¾ String Theory Basics ¾ Recent String Theory Developments: ¾ ¾ Black Holes in String Theory Towards Cosmology Some Old Questions… ¾ What are we made of? ¾ Basic building block of nature? ¾ What holds us together? Earth Air Fire Water Empedocles (490 BC – 430 BC) Science has found smaller and smaller building blocks Chemistry reduces all matter to known atoms (elements) but atoms are not fundamental A lot of structure inside the atom: Smaller distance (higher energies) Probing higher and higher energies we have found a large collection of elementary particles (“particle zoo”) Fundamental building block? Where is this done? ¾ particle accelerators: our best microscopes! Alps LHC@CERN What holds it all together? Four known interactions: ¾ Gravity ¾ Electromagnetism more familiar ¾ Nuclear weak ¾ Nuclear strong only visible if we probe deep inside the nucleus Gravity does not fit in! Described by the SAME theory (STANDARD MODEL) BUT the Standard Model is an INCOMPLETE theory: ¾ ¾ ¾ How do particles get their masses? Nature of dark matter? Are quarks (and leptons) fundamental? Why doesn’t gravity fit in? Standard Model General relativity ¾ ¾ based on quantum mechanics (deterministic) (probabilistic) ¾ works well at small scales (atoms, electrons, quarks) classical theory of gravity ¾ works well at large scales (planets, falling apples) work well in opposite regimes Why de we insist on combining QM with GR? In General Relativity (Einstein): ¾ matter causes space to curve ¾ curvature tells matter how to move Geometry is dynamical ! The heavier the object, the more curved the spacetime Very massive objects can tear the fabric of spacetime give rise to black holes General relativity breaks down here Quantum effects are equally important Why Quantum Gravity? When ¾ size is small ¾ curvature very large general relativity is not a good description, and quantum effects can’t be ignored Examples: ¾ very early universe (after “Big Bang”) ¾ interior of black holes (which exist!) To understand these, we need fundamental theory of quantum gravity PART II Why Strings? ¾ String theory incorporates naturally general relativity and quantum mechanics: z leading candidate for a consistent theory of quantum gravity Idea remarkably simple: fundamental unit: tiny vibrating string ! How Small is a String? (very) 1018 = Corresponds to VERY HIGH ENERGIES! x proton mass (1018 GeV ) 10000000000000000000000000000000 times larger than a string Can We Make Strings in the Lab? No! ¾ Energies are too high to produce them at accelerators Estrings ~ 1014 ELHC LHC ~ 104 GeV Strings ~ 1018 GeV ¾ Indirect testing is where efforts are concentrated z Footprints of string theory ingredients at lower energies? ¾ Cosmology may help How do we get protons, electrons, etc? different vibrations modes of string (harmonics) different particles (quarks, electrons, etc) Analogy with standing waves on a string Graviton is one of vibration modes Why do we live in 3+1 dimensions? Strings like to live in 10 dimensions Why do we see only four (space-time) ? The other 6 can be very small and curled up into tiny balls Small enough that we can’t see them Compact dimensions can have different shapes: Their size and shape determine the properties of elementary particles and the “constants of nature” Not Just Strings Other ingredients are allowed in string theory: membranes or “branes” Example I: Large Extra Dimensions? ¾ The extra dimensions may be large z we may live on a brane: a 3-dimensional surface in 10 dimensions Standard Model Parallel Universe? Example II: Cyclic Universe? Time may not have a beginning: the Big Bang could have been preceded by a collapsing phase Big Bang: collision between two brane worlds Punchline We have a tool box: ¾ Open strings ¾ Closed strings ¾ Branes Goal: use these tools to make realistic models of our universe Next: “Applications” (current research in the field) I. Branes and Black Holes General relativity breaks down Quantum Mechanics tells us : black holes are NOT COMPLETELY BLACK ¾ emit radiation (Hawking radiation) (they lose mass and evaporate) ¾ have a temperature and entropy Why should we care about the entropy? Entropy counts number of microscopic configurations of a system Black Hole Entropy: ¾ gives insight into microscopic structure of the black hole Old Puzzles: ¾ What happens to what falls in? ¾ Is information lost? ¾ Can we calculate entropy? 1970’s: approximate calculations for entropy (Bekenstein-Hawking) Need full quantum gravity theory to get right answer! String theory comes in: Black hole: system of branes and strings ! Entropy calculations match! and more… Counts correctly microstates that make up the black hole ¾ Big achievement for string theory: ¾ ¾ ¾ microscopic quantum description of a GR system (in terms of strings and branes) Framework for working on “information loss”: ¾ in the brane picture, there seems to be no information loss Cartoon Summary: String theory can describe black holes and Hawking Radiation II. Testing String Theory with Cosmology? The universe started out very small, hot and dense As the universe expanded, it cooled. If we look back in time, temperatures become higher In very early stages of the universe, energies were high enough for stringy effects to become important Here space was very, very small quantum gravity (or string theory) regime! Quantum mechanics tells us space is not smooth and does not sit still – it fluctuates ripples of spacetime These small ripples are stretched by “inflation” period of rapid expansion ripples getting bigger universe inflating Universe Expansion: Picture of the early universe: Remnant heat left over from Big Bang (CMB) Today: Picture of universe ~ 380,000 years after the Big Bang (light takes time to reach us): Temperature Fluctuations Density fluctuations in primordial plasma Bottom Line? “picture” of the early universe Ripples (quantum fluctuations) contain info about time when they were tiny Æ stringy effects? String theory and Cosmology? ¾ Stringy effects might leave imprint on cosmological observations ¾ Cosmology may rule out some string models: z too much gravitational waves? z inflation? ¾ Cosmology gives us picture of early times very high energies ! (our strongest microscope) Big challenge: universe that changes over time Summary ¾ Quantum gravity is needed, string theory does the job ¾ Recent developments: insight into gravity/quantum connection (structure of black holes, singularities) ¾ Many challenges: • we need more realistic models (particle physics and cosmology) • stringy imprints we may measure? ¾ Much more one can say… Thank You!