TheThe StrangeStrange WorldWorld ofof thethe Marc Scott StrongStrong ForceForce STAG Research Centre & Physics and Astronomy UnderstandingUnderstanding whywhy thethe vacuumvacuum isn'tisn't soso emptyempty Supervisor: Prof. Nick Evans

AA goodgood wayway toto trytry andand understandunderstand thethe QCDQCD vacuumvacuum isis toto simplifysimplify thethe FourFour forcesforces ofof naturenature UnderstandingUnderstanding situationsituation toto aa twotwo quarkquark QCD,QCD, i.e.i.e. thethe twotwo lightestlightest -- upup (u)(u) andand downdown (d).(d). SinceSince thethe vacuumvacuum inin QCDQCD isis aa seethingseething realmrealm ofof quark-anti-quarkquark-anti-quark (( )) Image: JPL/NASA time thethe QCDQCD vacuumvacuum pairspairs –– withwith onlyonly twotwo quarksquarks toto choosechoose fromfrom (u(u oror d),d), therethere areare onlyonly 44 BIG combinationscombinations toto pick;pick; BANG GRAVITYGRAVITY Orbits. Downhill Movement.

Binding of quarks inside nucleons. EachEach ofof thethe fourfour possiblepossible combinationscombinations hashas aa certaincertain probabilityprobability givengiven byby thethe vacuum,vacuum, Binding of nuclei. whichwhich variesvaries inin space.space. WeWe cancan representrepresent thisthis onon 4-dimensional4-dimensional grid,grid, eacheach axisaxis beingbeing aa Were all the Nuclear power. forces of nature STRONGSTRONG combination.combination. EachEach locationlocation inin spacespace isis givengiven anan arrowarrow whichwhich isis placedplaced inin thethe 4-D4-D grid,grid, united at the thethe moremore thethe arrowarrow pointspoints inin thethe directiondirection ofof aa givengiven combinationcombination thethe moremore probableprobable beginning of the itit isis (see(see diagram).diagram). universe? Beta Decay. In fact the vacuum is a little more WEAKWEAK The usual method for analysing interacting systems is constrained. In order to obtain the lowest TheThe usualusual methodmethod forfor analysinganalysing interactinginteracting systemssystems isis perturbation theory, but this relies on the strength of Magnetism. energy state, i.e. the vacuum, all of the perturbationperturbation theorytheory,, butbut thisthis reliesrelies onon thethe strengthstrength ofof We know at large enough Electricity. the interaction to be small. The strength of the strong Chemical reactions. arrows must point in exactly the same the interaction to be small. The strength of the strong temperatures the weak Heat Radiation. force is too large to consider this method. EM Heat Radiation. way, which is called a condensate. forceforce isis tootoo largelarge toto considerconsider thisthis method.method. interaction and EM Radio. Understanding this condensate is very electromagnetism are one An alternative approach is to use the concept of difficult and its origin is poorly AnAn alternativealternative approachapproach isis toto useuse thethe conceptconcept ofof and the same... HOLOGRAPHY. understood. HOLOGRAPHY.HOLOGRAPHY. ELECTROWEAK FORCE HowHow strongstrong isis thethe strongstrong force?force? HolographyHolography Consider the electric potential of a point charge. Coulomb's law tells us it goes like Consider the electric potential of a point charge. Coulomb's law tells us it goes like Consider a hologram (as you might see on a bank card, ThatThat is,is, thethe furtherfurther oneone getsgets fromfrom thethe source,source, thethe weakerweaker thethe interaction.interaction. NowNow considerconsider aa banknote or DVD). It is a three dimensional image 'strong'strong charge'charge' (called(called colourcolour charge).charge). TheThe potentialpotential forfor thethe strongstrong interactioninteraction lookslooks veryvery represented on a two dimensional, i.e. flat, plane. similarsimilar atat smallsmall distancesdistances toto thethe CoulombicCoulombic potentialpotential ofof thethe electricelectric chargecharge BUTBUT atat largelarge Holography (as the name suggests) is based on a similar idea; that certain physics in n-dimensions may be represented by another distancesdistances thethe potentialpotential isis dominateddominated byby aa linearlinear term,term, akinakin toto thethe CornellCornell potential:potential: Image: type of physical system in (n-1)-dimensions. Flickr/FFCU This potential tells us that the further the distance, r, between the strongly interaction objects, the GREATER the strength of the strong interaction! Such physics is difficult to model and calculate with. ItIt turnsturns outout thatthat thethe physicsphysics ofof thethe strongstrong interactioninteraction QCDQCD (QCD)(QCD) inin ourour usualusual 4-dimensions4-dimensions (3(3 spacespace ++ time)time) isis equivalentequivalent toto thethe physicsphysics ofof aa systemsystem containingcontaining somesome TheThe vacuumvacuum isn'tisn't emptyempty formform ofof gravitygravity ofof smallsmall interactioninteraction strengthstrength inin fivefive A proton, the nucleus of a hydrogen atom, is comprised of three quarks, GRAVITYGRAVITY Q:Q:two up and one down, which give it its properties. Experiments have shown dimensions.dimensions. that the masses of the up and down quarks are about 1000 times lighter than the proton... but how can this be, if the proton is made of three of It turns out that the one extra dimension that we The holographic principle described It turns out that the one extra dimension that we these quarks? have in on the gravity side, may be representative of above allows us to uncover the above allows us to uncover the an energy scale. So in fact we may come to think of A:A:The answer lies with the nature of the vacuum related to the elusive properties of the strong the gravity 'bulk' (the black area on the diagram) as a physics of the strong force. Intuition tells us that the vacuum is just interaction by studying a mathematical loaf of bread, with each slice empty space. When we say vacuum, however, what we really mean is gravitational model. pertaining to a certain energy. the lowest energy state of a given system. Usually, this is the state with nothing in it, i.e. zero energy. But in QCD (the physics of the Contact: strong force) the vacuum isn't empty but a bubbling broth of quarks, We therefore can look at the antiquarks and gluons (particles that, as the name suggests, glue We therefore can look at the [email protected] properties of the complex QCD the quarks together inside protons and neutrons). So the majority of properties of the complex QCD vacuum by looking at its simpler the mass of the proton arises from the usual quarks interacting with vacuum by looking at its simpler southampton.ac.uk/stag counterpart on the gravity side. this rather busy QCD vacuum! counterpart on the gravity side.