1. 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

2. UNDERSTANDING NOTHING The 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

3. The Greeks Nothing = the absence of properties Existence is a propertya 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

4. 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!

5. 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

6. 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

7. 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!

8. 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!)

9. 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 F = G m M / r 2 Which acted instantaneously at a distance on planets in an empty space – it worked! More confirmation that the atmosphere ends

10. 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

11. 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

12. 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! t ‘ = (1 – v /c ) ( t – v x / c ) 222 x ‘ = (1 – v /c ) ( x – v t) 22 x + y + z = (c t) 222 2 Now you have to be careful about speed, momentum, energy… E=mc 2 You can create matter out of energy

13. 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!

14. 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

15. 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

16. 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

17. 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 > h t E > h Or equally

18. 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 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)

19. 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.

20. 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…

21. 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!

22. 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

23. 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….

24. The Origin of Mass The strong nuclear force cannot explain the mass of the electron though… The Higgs Boson We 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!!

25. 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…. 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….

26. 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!)

27. 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?

28. String Theory A rapidly developing area of study – producing many fresh ideas… but all speculation to date!

29. 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