STRING THEORY arises from attempts to answer:
1) What is the world made of?
2) How & Why does the world change in time?
Answers:
1) MATTER!
E.g. Cars, Houses, Buildings, Planets…
All matter has Mass
2) Forms of matter exert Forces on each otherand they move under the influence of these forces
For example: the attraction which causes anapple to fall off a tree.
Matter comes in millions of different forms:
Apples, Oranges…Cars - Hondas BMWs...Garden Hoses, Grass, Trees...Earth, Sun, Moon…Galaxies, Planets, Stars...
All this diversity in fact comes from only a few hundred atoms:
Hydrogen, Oxygen, Nitrogen...
• Each atom has a definite mass.• Each atom has an approximate size of about:
• By going from the large to the tiny, we reduce the diversity of millions to around a hundred!
• Things simplify further if we look deeper into the atom:
+Protons:
Neutrons:
-Electrons:
Electrons orbitthe nucleus whichis composed ofProtons andNeutrons
• The few hundred atoms reduce to only a handful of fundamental objects
• Our Millions of Matter objects have been reduced to just 3 - Great!
Unfortunately, the story starts to get more complicated:
+
updown
strangecharm
bottomtop
Proton&
Neutron
Quarks
redgreen
blue
Colours
•
•
•
• There are 3 ‘generations’ of these quarks & each quark can have one of 3 different ‘colours’
• Likewise there are 3 ‘generations’ of electron-like objects called leptons:
electronelectron-neutrino
muonmuon-neutrino
tautau-neutrino
Leptons
The idea of String Theory is that thisdiversity of quarks and leptons comesfrom 1 String
H O
pne
u d s c t b e
1 String
What about Forces?
“Hammer Force”
“Fuel Force”
“Astronomical Force”
All this diversity of forces comes from:
Gravity&
Electromagnetism (& its 2 cousins)
The Electric Force:
• Rub a pen against your sleeve & pick up pieces of paper.
• Rubbing - Separation of charge.
Matter contains positive & negative charges:
++
--
+ -
Repulsion
Repulsion
Attraction
The connection between
Light & Charge runs deep:
Particle physics teaches
us that “electrons
repel because of the
exchange of particles of
light called photons”
Exchange
Repulsive force
Particle physics reduces
both Matter & Force
to one kind of Entity
i.e. particles.
Matter
Forces
electrons
protons
photons
gravitons
Force of Gravity
The Physics of particles is
governed by
“Quantum Mechanics”
-Which brings about a Radical
Change in our concepts of
space.
E.g. Cannot say electron is
here and going nowhere.
Every position is assigned a
number telling us how likely
the electron is to be
at that point. x
Quantum mechanical
analogue of particle is here:
The particle has maximum
chances of being here
and
electrons in
definite orbits
is replaced by
of finding
electron
High chance
Small chance
Deep inside proton:
In addition to Electricity/Magnetism
we need to know about
Strong & Weak force.
The mediators of strong
forces are gluons.
Quantum theory & particle physics
do a great job of reducing diversity
of matter & Electromagnetism,
S & W forces to a few particles!
Forces
E.M. Strong Weak
γ,gphotons
& gluons
But gravity does not fit nicely
into the Quantum Mechanical world.
Applying Q.M. to gravitation gives infinities.
String Theory combines
all forcesStrong,Weak,
Gravitation,
Electromagnetism
& all matter
electrons,protons,neutrons…
into one object…
…a
Matter Force
e-, p+, n γ, gluon, graviton
How does a string accomplish this?
•All particles have energy.
In classical physics:
I. ; also Rest energy
II. Photons have frequency
(related to colour of light)
Planck’s constant
•Strings can vibrate in different ways
For example:
Guitar string
Different vibration Different sound!
•Fundamental string
Different vibration Different particles!
electron photon graviton
•In ordinary circumstances, we can understand
the world by applying different forces
according to the problem at hand.
•A unified Theory of matter & forces is not just pretty - it is necessary for us to understand:
1. Physics of the Early Universe
2. Black Holes
•Universe is expanding
•Every galaxy is facing away from every other
E.g.
Going back in time,
• All matter was crunched up in a very energetic state within a short distance.
• Under these conditions all forces were equally important.
• Hence String Theory becomes necessary for us to understand what happened at after the Big Bang.