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Particles and Strings
“The search for a
fundamental theory of the
building blocks of nature
and their interactions”
Nick Evans
University of Southampton
Reality Check!
Particles
Hard Science
&
Strings
Speculative
Mathematically well defined
A theory of quantum gravity
Experimentally verified
Maths rules still up for grabs
New experiments - LHC
No experimental verification
No clear experiment to do!
Search for a missing piece
– the higgs particle
Throws up fascinating
possibilities!
The Particle World
Forces
• Electromagnetism
• Strong nuclear
• Weak nuclear
• Gravity
Particles
Three families (copies) of
2 quarks
1 electron
plus their antiparticles
1 neutrinos
Relativity
The speed of light is the
same for any observer
This means nothing can travel with light –
nothing can reach v=c!
E=
1
mc 2
1-v2/c2
Rest mass = energy
Space - Time
A flash of light causes a spherical wave front even if you
move relative to source
This only makes sense if
space and time mix!
*
t‘ =
x‘=
2
2
2
(1 – v /c ) ( t – v x / c )
2
2
(1 – v /c )
( x – v t)
Quantum Rules of Motion
Energy comes in lumps
E=hf
Fields can look like particles
The photon is the quantum of the electromagnetic field/ light
Quantum Dynamics
The quantum in
some sense travels
by both paths….
There is an uncertainty in the position and momentum of the quantum
Heisenberg’s Uncertainty Principle
Or equally
x
t
p >h
E >h
Dirac’s Legacy
Electrons can absorb photons
But in relativity observers do not agree on
time ordering of events… so can we have
What does it mean for an electron to travel
backwards in time? We only measure charge…
It looks like a +ve charge electron moving
forward in time
We have discovered anti-particles!
Accelerator Physics
Electron positron annihilation to a photon
allows us to convert their energy to look for
all the particles that make up nature
The Quantum Vacuum
E
t >h
The vacuum can borrow energy for short periods
E = mc 2
The borrowed energy can be used to create particles
(You can’t just create an electron because of charge
conservation - but can create electron positron pair)
The quantum vacuum is a seething mass of particles appearing and
disappearing constantly….
How Can You Tell?
The “virtual” particle
pairs interfere in
electron scattering
processes.
The effective charge
seen in two electron
scattering depends on
the separation of the
electrons.
The Strong Nuclear Force
The strong nuclear force is
described by a theory that is
similar to electromagnetism…
except that the fields carry
(colour) charge…..
This difference changes the
way in which the vacuum is
polarized so that…
Confinement
You can never pull hard enough to
liberate a quark from a proton…
The Quantum Vacuum
Every so often quantum effects create a quark anti-quark pair.
The attractive force is so strong that
binding energy >> mass energy
The vacuum has lower energy if it fills itself with quark antiquark pairs!
The vacuum is really full of quark anti-quark pairs with a density
15
3
like that of an atomic nucleus (10 grams/cm ) !!
The Proton Mass
The quark pairs are responsible for the proton’s mass
Interaction
energy provides
proton mass
The Origin of Mass
The strong nuclear force cannot explain the mass of the electron
though…
Or very heavy quarks such as the top quark
top mass = 175 proton mass
The Higgs Boson
We suspect the vacuum is full of another sort of matter that
is responsible – the higgs….
To explain the top mass the higgs vacuum must be 100 times
denser than nuclear matter!!
The Search for the Higgs
To find the higgs we must “excite” the vacuum – produce a
higgs particle… we collide electrons, protons etc so there is 100
times nuclear energy density in some region….
We haven’t found it
so far but…
The Large Hadron
Collider in
Switzerland will
switch on in
2008…
There are many versions of the “higgs theory” – when we find it
we can study its properties in detail….
Quantum Gravity
If the vacuum is full of all this stuff shouldn’t we be pulled
gravitationally by it?
Since it is uniformily distributed there is no net pull (equal
space to all sides)
But General Relativity says the energy should
uniformily curve space-time… the Universe
should be the size of a grapefruit!!
There’s something big we don’t
understand about quantum gravity
Gravity is so weak that we’ve never measured
it’s effects between individual particles or on
distance scales smaller than 1mm!
Gravity is different to the other forces – it’s only attractive…
In General Relativity this shows up in that gravitational
waves have different polarizations to electromagnetic waves
What fundamental theory can encompass both types of fields?
String Theory
String theory unifies gravity and other forces
String theory is A quantum theory of gravity.. Is it THE quantum
theory of gravity? - entering realm of speculation!
What are these strings?
We describe them only in terms of a fundamental
tension – as for a rubber band
T
How big are they?
A particle of energy E has a wavelength
E=hc/l
So can probe down to scales of order l…. So far we’re down to
1/1000th size of atomic nucleus… strings could be 10 19 times
smaller!
Extra Dimensions
Surprisingly the mathematics of string theory only makes
sense in 9 spatial dimensions and 1 time dimension!
A prediction…. But wrong!!
Compactification
We can imagine a space where
directions are curled up
We study A string in this space
not ALL the strings that make
the space…. Why are 6
dimensions compact… by what
mechanism? UNKNOWN!
The radius must be at
least 1000 times
smaller than an atomic
nucleus!
Membranes too…
Recently we have discovered that open strings may be restricted
to a sub-space or membrane
Electromagnetic
particles live on a
sub-space or
“brane”
Gravitons live in a higher
dimension “bulk”
“Existence proof” for
such a world
Could Our Universe Be A Brane?
S
U
2 pR
The strength of gravity is
determined by the number of
spatial dimensions (gravitons
spread out around mass)
F=GMm
r2
D=3+1
But…. we don’t know anything about gravity on length scales
below 1mm… R could = 1mm… and we wouldn’t know it!
If so we’ve miscalculated the strength of gravity (G) – it could
become strong in our particle accelerators at any new energy!!
Matter Loss To The Bulk
Our Universe will slowly lose matter into the bulk…
In particle physics
gravity is too weak to
ever see these events
In fact the energy loss
from the centre of stars
is too low to see too!
Other Branes
Why should there be only one brane in the higher dimension
spacetime?
There could be entire Universes only mm away!
W S
E U
N
The matter on the other
brane will only interact
with our world
gravitationally – it’s
dark matter….
Until we can produce high energy gravitons that are strongly
interacting there’s little way to directly probe this idea though
Brane Collisions
There’s no reason branes should be static in the extra
dimensions…. So they could collide!
This would be catastrophic!
Huge amounts of energy
would be dumped into our
Universe….
Could that have triggered the currently observed expansion of the
Universe??!!
Overview
• Particle physics has a concise description of matter and
forces
• The missing element is the higgs that generates mass (plus
explanation of why the building blocks are what they are)
• Our theories of particles don’t fit with theories of gravity
• String Theory is an attempt to construct a sensible theory
of both
• We are realizing it predicts wild possibilities
• Extra dimensions and other Universes
• Many exciting possibilities but very hard to
experimentally test because gravity is very very weak….