Adventures in String Theory

Science One, April 7th, 2011

What are the basic building

blocks of nature?

Old Answer

Air Earth FireWater

Modern Answer

Modern physics: understand in detail how nature works at distances 100,000,000,000,000 times smaller than we can see!

The Standard Model of Particle PhysicsAll matter made of elementary particles

-different masses and spins

Forces arise from basic interactions of these particles

Rules for interactions determined by quantum mechanics and special relativity ~20 parameters

electromagnetic strong

weak

The Basic Building Blocks?

Air Earth

Fire Water

Proton Neutron

Electron Photon

Questions

Why this set of particles and interactions?

Are these particles really elementary?

Are there more particles or forces?

What about gravity?

General Relativity

•Accurately predicts gravitational effects from solar system dynamics to the evolution of the whole universe.

•Predicts new phenomena, e.g. black holes, gravity waves, expansion of the universe

How did the universe begin?

Old Answer

Modern Answer: the Big Bang

General relativity

universe has expanded and cooled for 13.6 billion years from extremely hot, dense state

Questions

What happened in the first instant of the big bang (or before)?

What is inside a black hole?

We need a quantum mechanical theory of gravity!

String Theory: the Basics

Basic building block: string (1 dimensional loop of energy)

Quantum mechanics

Specific vibrational modes with definite energy and angular momentum

We see: particles with different mass and spin

m = E/c2

String interactions

time

Particle interactions arise from splitting and joining of strings

String interactions

Particle interactions arise from splitting and joining of strings

String interactions

Particle interactions arise from splitting and joining of strings

String interactions

Particle interactions arise from splitting and joining of strings

String interactions

Particle interactions arise from splitting and joining of strings

String interactions

Basic law of string physics:

Strings move to minimize the area of their worldsheet

String physics: strings vibrate, join, split via basic law + quantum mechanics

We see: various types of particles with forces arising from basic interactions INCLUDING gravity

String theory automatically gives a theory of quantum gravity and particle physics with no parameters!

Extra DimensionsSurprise: even the number of dimensions is fixed!

We see: 4

String theory predicts: 10

BUT: recall that space can be curved

Garden-variety extra dimensions

Our universe:

Hose universe:

The shape of the extra dimensions1 extra dimension:

2 extra dimensions:

6 extra dimensions:

Many possibilities for the shape / size / topology of extra dimensions

...

...

Physics from geometry

The Landscape of String TheoryExtra dimensions dynamical:Only special shapes are stable

Different possibilities for 4D physics

Big question: does one of these stable shapes give the physics we observe?

Currently: just starting to understand the possibilities

Can we make new predictions that can be tested experimentally?

The End of the Universe?