UNDERSTANDING NOTHINGThe Structure of the VacuumA Talk by Dr Nick EvansA 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 VacuumHistorical perspective on the voidThe discovery of empty spaceSpace timeThe quantum vacuumThe higgs boson searchQuantum gravity and strings

The GreeksNothing = the absence of propertiesExistence is a propertya void can not existSpace is defined by its content Continuous matter Platonic ideals the possibility of matterNature 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 yetGod created the world out of nothingDebate but beware the pyre!

Eastern InfluencesIndian philosophy embraced the idea of voidWe come from nothingWe should seek to return to nothing (Nirvana)The Indians invented the symbol for nothing - 0The idea spread through the medieval Islamic empiresTo the Moors of SpainFinally to Europe Gerbert of Aurillac in ~ 980 AD

Medieval Thought ExperimentsRoger Bacon (~1250 AD) and others were interested in constructing a vacuum They argued about whether you could do it by separating two sheets of glassvacuumMuch argument. Few conclusions

Discovery of the VacuumThe scientific revolution was growth of idea that you should LOOK at nature as well as think about itIn 1643 Galileos student Torricelli created a vacuumSo 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 dont survive in a vacuum they breath air Light can travel in a vacuumPascal 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 headDescartes (1636) had a model of fluid vorticesIt didnt match the observations!Newton (1687) proposed a universal law of gravity F = G m M / r2Which 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 waveWaves are oscillations of a mediumThe vacuum must be full of etherJames 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 earths motion relative to the etherThey saw no time differencebetween 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 )222 x = (1 v /c ) ( x v t)22 x + y + z = (c t)2222Now you have to be careful about speed, momentum, energyE=mc2You can create matter out of energy

Fields are FundamentalSo what is light?An oscillating wave form of electric and magnetic fieldsElectric 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 intrinsicobject that can exist in the absence of a charge experiencing a force!

Relativistic Space-TimeNewtons gravity cant be the whole truth instantaneous action at a distance is forbidden.General Relativity describes gravity in a new wayParticles travel by the shortest path in a space curved by massesSpace-time is like a rubber sheet that can be bentNote: 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 somethingThe energy loss from this emission has been seen for a pair of orbiting neutron starsSearches are now on to see these very weak waves directly at LIGO

The Quantum WorldEnergy comes in lumpsE = h fFields can look like particlesThe photon is the quantum of the electromagnetic field/ light

Quantum DynamicsThe quantum in some sense travels by both paths.There is an uncertainty in the position and momentum of the quantumHeisenbergs Uncertainty Principle x p > ht E > hOr equally

The Quantum VacuumE t > hThe vacuum can borrow energy for short periodsE = mc2The borrowed energy can be used to create particlesThe quantum vacuum is a seething mass of particles appearing and disappearing constantly.(You cant 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 ForceThe 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

ConfinementYou can never pull hard enough to liberate a quark from a protonThe QCD VacuumEvery so often quantum effects create a quark anti-quark pair.The attractive force is so strong that binding energy >> mass energyThe 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 ) !!15The Proton MassThe quark pairs are responsible for the protons massInteraction energy provides proton mass3

Mass Needs Explaining!Massless particles dont exist at rest ( E = 0 ). They must move at the speed of lightMany particles (electrons, quarks) spin on their axesFor 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 thoughThe 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 massTo 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 havent found it so far butThere 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 shouldnt 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!! Theres something big we dont understand about quantum gravity an open problem (much studied!)

Gravity is different to the other forces its only attractive In General Relativity this shows up in that gravitational waves have different polarizations to electromagnetic wavesWhat fundamental theory can encompass both types of fields?

String TheoryA 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 gravityOverviewvacuum