Transcript Understanding Nothing - University of Southampton

UNDERSTANDING NOTHING
The Structure of the Vacuum
A Talk by
Dr Nick Evans
A tour of our understanding
of empty space in modern
physics from Relativity to
Quantum Mechanics to the
frontiers of Particle Physics
and String Theory
UNDERSTANDING NOTHING
The Structure of the Vacuum
Historical perspective on the void
The quantum vacuum
The discovery of empty space
The higgs boson search
Space time
Quantum gravity and strings
The Greeks
Nothing = the absence of properties
Existence is a property
a void can not exist
Space is defined by its content
• Continuous matter
• Platonic ideals – the possibility of matter
Nature abhors a vacuum!
The Greeks even declined to invent the essential symbol 0
Medieval Christian Philosophy
Greeks were closer to creation so remembered more of
Eden – Aristotle ruled!
Nothing = no God!
There can not be empty space… yet…
God created the world out of nothing
Debate… but beware the pyre!
Eastern Influences
Indian philosophy embraced the idea of void
We come from nothing
We should seek to return to nothing (Nirvana)
The Indians invented the symbol for nothing - 0
The idea spread through the medieval Islamic empires
To the Moors of Spain
Finally to Europe – Gerbert of Aurillac in ~ 980 AD
Medieval Thought Experiments
Roger Bacon (~1250 AD) and others were
interested in constructing a vacuum
They argued about whether you could do it by
separating two sheets of glass
vacuum
Much argument…. Few conclusions
Discovery of the Vacuum
The scientific revolution was growth of idea that you should
LOOK at nature as well as think about it…
In 1643 Galileo’s student Torricelli created
a vacuum
So Simple!
Otto Von Guerick revelled in the
discovery!
(Magdeburg 1654)
Nature protects a vacuum!
Studying Nothing
Boyle created a vacuum by pumping air from a flask then studied
• Sound does not travel in a vacuum – air waves
• Animals don’t survive in a vacuum – they breath air
• Light can travel in a vacuum…
Pascal showed the atmosphere had limited extent
(The Tibetans believed air
was poisonous at the top
of mountains – just
thinner!)
Space
The problem of understanding planetary motion also came to a head
Descartes (1636) had a model
of fluid vortices
It didn’t match the observations!
Newton (1687) proposed a universal law of gravity
2
F= GmM/r
Which acted instantaneously at a distance on planets in an empty
space – it worked!
More confirmation that the atmosphere ends
The Ethereal Realm
Light is a wave
Waves are oscillations of a medium
The vacuum must be full of “ether”
James Clarke Maxwell (who pioneered our
understanding of light as an electro-magnetic wave)
envision a mechanical vacuum of etheric vortices.
Science fantasy: Space 1889
Michelson and Morley
In 1887 Michelson and Morley did an experiment to detect the ether
using the earth’s motion relative to the ether
They saw no time difference
between the paths….
There is no ether!
Amazingly light travels at the same speed relative to any observer!
Thus nothing can move faster than light
Relativity
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!
*
x 2 + y 2 + z 2 = (c t)
2
2
2
t‘ =
(1 – v /c ) ( t – v x / c )
x‘=
(1 – v /c )
2
2
( x – v t)
2
Now you have to be careful about speed, momentum, energy…
E=mc
2
You can create matter out of energy
Fields are Fundamental
So what is light?
An oscillating wave form of electric and magnetic fields
Electric field was introduced as a short hand for recording the force a
particle would feel at a point…. Now we believe that it is an intrinsic
object that can exist in the absence of a charge experiencing a force!
Relativistic Space-Time
Newton’s gravity can’t be the whole truth… instantaneous action at a
distance is forbidden….
General Relativity describes gravity in a new way
Particles travel by the shortest path in a space curved by
masses
Space-time is like a rubber sheet that can be bent
Note: only the surface exists!
Note: change to gravity and F=ma sets gravit. mass = inertial mass
Gravitational Waves
A very heavy mass, like a star, distorts the space time
sheet if it oscillates or collides with something…
The energy loss from this emission has
been seen for a pair of orbiting neutron
stars
Searches are now on to see these
very weak waves directly at LIGO
The Quantum World
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
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 QCD 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
Mass Needs Explaining!
Massless particles “don’t exist” at rest ( E = 0 ). They must
move at the speed of light…
Many particles (electrons, quarks…) spin on their axes
For massless particles these are
different sorts of particles!
(The weak nuclear force only
acts on left-handed spinning
particles!)
Why do they get tied up into massive particles? The strong
nuclear force is the answer for quarks….
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
– an open problem (much studied!)
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
A rapidly developing area of study – producing many fresh
ideas… but all speculation to date!
Overview
• Science in part emerged from philosophical discussion of the
vacuum
vacuum
• Empty space does appear to exist (what is it though?)
• Space and time mix into a curved surface in GR
• Quantum theory fills space with virtual particles
• Nuclear force lowers vacuum energy by filling with quarks
• Is the vacuum full of higgs particles too?
• Problems remain in understanding quantum gravity…