UNDERSTANDING NOTHINGThe 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 NOTHINGThe Structure of the Vacuum

Historical perspective on the void

The discovery of empty space

Space time

The quantum vacuum

The higgs boson search

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 PhilosophyGreeks 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 InfluencesIndian 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 VacuumThe 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 NothingBoyle 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!)

SpaceThe 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

F = G m M / r2

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 MorleyIn 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

RelativityA 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 ‘ = (1 – v /c ) ( t – v x / c )2 22

x ‘ = (1 – v /c ) ( x – v t)2 2

x + y + z = (c t)2 2 2 2

Now you have to be careful about speed, momentum, energy…

E=mc2

You can create matter out of energy

Fields are FundamentalSo 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-TimeNewton’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 WavesA 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 = h f

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 x p > ht E > hOr equally

The Quantum Vacuum

E t > h

The vacuum can borrow energy for short periods

E = mc2

The borrowed energy can be used to create particles

The quantum vacuum is a seething mass of particles appearing and disappearing constantly….

(You can’t just create an electron because of charge conservation - but can create electron positron pair)

How Can You Tell?

The effective charge seen in two electron scattering depends on the separation of the electrons.

The “virtual” particle pairs interfere in electron scattering processes.

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 anti-quark pairs!

The vacuum is really full of quark anti-quark pairs with a density

like that of an atomic nucleus (10 grams/cm ) !!15

The Proton Mass

The quark pairs are responsible for the proton’s mass

Interaction energy provides proton mass

3

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 MassThe strong nuclear force cannot explain the mass of the electron though…

The Higgs BosonWe suspect the vacuum is full of another sort of matter that is responsible – the higgs….

Or very heavy quarks such as the top quark

top mass = 175 proton mass

To explain the top mass the higgs vacuum must be 100 times denser than nuclear matter!!

The Search for the HiggsTo 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….

The Large Hadron Collider in Switzerland will switch on in 2008…

We haven’t found it so far but…

There are many versions of the “higgs theory” – when we find it we can study its properties in detail….

Quantum GravityIf 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!

• Science in part emerged from philosophical discussion of the 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…

Overview

vacuum