**~Introduction to String Theory**

**The Theory of Everything?**

a. The definition of String Theory:

String theory says that everything in our universe is made of tiny, wiggling, vibrating loops of energy that look like strings.

It is also called the Second Superstring Revolution.

M-Theory unified the 5 versions of superstring theory.

Suggested the 11th dimension, which allows strings to stretch into something bigger, like...membranes.

It explained why gravity is so weak on the subatomic level.

M-THEORY

1995-Edward Witten

Brane Theory

What is String Theory?

**Future**

b. It is currently one of the candidates for ToE, aka the Theory of Everything.

Why Do We Need

String Theory?

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4 Fundamental Interactive Forces

1. Gravitational Interaction:

a. The weakest of the 4 interactions

b. The strength of the gravitational interaction is determined by mass.

2. Electromagnetism Interaction:

a. Responsible for magnets and electricity

b. Determined by charges. Opposite charges attract each other, like protons and electrons.

4 Fundamental Interactive Forces

3. Strong Nuclear Interaction:

a. The strongest of all 4 forces

b. Acts on hadrons. It holds hadrons together like superglue.

c. A pair of hadrons will always attract and not repel each other.

4. Weak Nuclear Interaction:

a. Allows neutrons to turn into protons giving off radiation in the process.

Quantum Mechanics

Einstein and His General Relativity (1916)

1. Space-time

3 dimensions of space

+1 dimension of time

----------------------------

4 dimensional space-time

Dips and curves of the space time

Gravitational Force

Albert Einstein believed that the universe behaves in a consistent, predictable way.

2

Late 1920s

a. Copenhagen Interpretation--Niels Bohr:

The act of measurement immediately and randomly picks an outcome from all the possible ones.

b.Quantum Mechanics states that we could only predict the probability of one outcome or the other.

c.Has never contradicted any observation or experiment.

d. Turbulent, Unpredictable.

I. 4 fundamental interactive forces of our universe can't be unified.

2. 2 sets of theory about our universe don't agree:

Einstein's General Relativity & Quantum Mechanics

"God does not throw dice."--Albert Einstein

General Relativity

Macroscopic

For the VERY BIG, like the stars and the sun.

Predictable

Smooth, calm universe

Einstein's General Relativity

G

3

Quantum Mechanics

Quantum Mechanics

Unpredictable

Turbulent Universe

Microscopic

For the VERY SMALL, like the atoms and particles

EM

4

W

S

1. The particle that scientists think carries gravity, graviton, has not been found yet.

2. Gravity could not be explained at the quantum level yet.

3. The space-time in general relativity is smooth but the quantum universe is turbulent.

The Discovery and Development of

String Theory

1968

1969

1995

1984

Gabriele Veneziano

Discovered that Euler's equation seemed to describe the strong force.

5

5

Leonard Susskind

Discovered that the shape described by the equation was actually a wiggling string.

John Schwarz & Michael Green

Canceled the anomalies (mathematical inconsistency) and gave the equation ability to encompass all 4 forces.

Edward Witten

Combined the various versions of string theory

Proposed M-theory and Brane theory

String theory became the candidate for ToE, meanwhile other versions start to emerge.

A massless particle in the equation appears to be the mysterious 'graviton'.

Everything or Nothing?

Superstring

Theory

Bosonic String Theory

Also...

All string theories require extra dimensions, from 10 up to 26.

The original version of string theory. It requires 26 dimensions.

5 versions of the theory that are developed from the original one.

All include supersymmetry and fermions. 10 dimensions.

However, they don't agree with each other.

Open-ended strands vs. Loops

Strings...?

These strands of energy vibrate like strings. Different types of vibration form different particles.

However......

No experiment or observation can test the theory yet, regardless right or wrong.

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Extra Dimensions

3 dimensions of space + 1 dimension of time

6 extra dimensions

Dimensions are "degrees of freedom". More dimensions provide more capability.

Supersymmetry

It is a major prediction of string theory.

Bosons: Integer-valued spin particles

Fermions: Half-integer spin particles

Every elementary particle must have a "superpartner", which no one has seen yet.

If superpartners exist, they should be really heavy since they haven't been detected by the atoms smashers we have .

But...how?

A big part of M-theory is...

Brane Theory

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Our atoms and particles are strands with endpoints, tied down to our membrane.

Like jam or butter on the bread.

The bulk is the 11 dimensional space and the slice of bread is our membrane universe.

Imagine our universe is only a slice of bread from a big bulk of bread

However...

Graviton (gravity) is close-ended loop, so it can slide off like sugar on the bread.

11 dimensional bulk

Strings can stretch into membranes

Our universe is a membrane

There can be parallel universes in higher dimensions so we can't see or reach out to them.

Our atoms are confined in our brane while gravity/graviton is not.

That is why gravity is so weak on the subatomic level, because it

ESCAPES

Fermilab

Atom smasher

Aiming to catch the moment when graviton escapes.

CERN

Building an more advanced collider

Aiming to find Supersymmetry and graviton before fermilab does.

So...

Is it the Theory of Everything?

To Be Continued...

Presented by...

Andrea Yang

Thank you for listening