A few thoughts on 100 years of modern physics. Quanta, Quarks, Qubits

Transcription

1 A few thoughts on 100 years of modern physics Quanta, Quarks, Qubits

2 Quanta Blackbody radiation and the ultraviolet catastrophe classical physics does not agree with the observed world Planck s idea: atoms are harmonic oscillators that only absorb and emit energy in bundles of hν. Bolzmann distribution of energies Nn = N0 e (-nhν/kt)

3 What does it mean? Mathematical trick? Planck probably thought so, certainly thought light was classical Quantum nature of physical reality? Einstein in 1905 photoelectric effect

4 Early modern physics Syllabus talks about: Heisenberg and Pauli, Fermi and his piles, Pauli and the neutrino Investigations: Hydrogen spectrum, cloud chamber

5 Other investigations Thanks to Mark Butler, Gosford HS Model the size of an atom Moderators in nuclear reactors Flame spectra Planck s constant with LEDs Bragg, diffraction and interference nuclear physics with match-sticks: plotting half-life, chain reactions nuclear physics with ping-pong balls or scrunched-up paper

6 Quanta: Quarks So where are we now? Everything we experience* can be described very well** as matter (atoms) and forces (EM, gravity) If we look hard, we see other stuff: nuclear physics (decaying atoms, radiation), new matter (weird, short-lived particle zoo ) * except for emotions, consciousness, dreams... ** except for * and a few things we ll get to in a minute

7 The Standard Model Explains everything*** in the universe in terms of a small number of ideas 12 particles of matter quarks and leptons (electrons, neutrinos,...) 2 fundamental forces (electro-weak force and strong colour force) *** gravity and some fringe observations not included

8 Matter: Crystals/solids/gases/liquids molecules atoms electrons, protons and neutrons Forces: Ignoring gravity, the only force we feel is Electromagnetism. QED, quantum electrodynamics quantum theory of light interacting with electrically-charged particles (electrons, protons ) electron proton photon QED: forces are treated as exchanges of photons

9 Our greatest invention electron photon proton QED: Quantum Electrodynamics Theory & Experiment agree to 13 decimal places Almost everything you ever experience is QED

10 Problems Helium nucleus has two positively-charged protons. Why don t the protons fly apart due to Coulomb force? Some heavy atoms are unstable they decay spontaneously, releasing beta-radiation (electrons) and changing into different atoms. Somehow a neutron becomes a proton by ejecting an electron how does this happen? Protons and neutrons don t seem to be simple point particles or little spheres they have structure. What could it be? Strange new particles appear when you bang electrons or protons together at high energies. What are they, and why do they disappear so quickly?

11 Protons and neutrons have structure Colliding high-speed electrons with atoms uncovered the nucleus Higher energy collisions showed the nucleus has structure protons and neutrons Even higher energy collisions show that Ps and Ns have structure but what could it be? Around the same time, many new particles were showing up too many fundamental particles (similar situation to the periodic table a century before) Models developed based on new particles called quarks explained the masses and charges of P, N and some of the new particles What held the quarks together? What was the theory?

12 Quarks and the Colour Force There are six flavours of quark: up, down, strange, charm, beauty and truth (bottom and top) Protons and neutrons are uud and udd The other four quarks appear (briefly) in accelerator experiments: the D is an up + anti-charm, the Σ + is a strange and two ups Quarks always combine in threes or as a quark-antiquark pair, because they feel a new kind of force

13 Quarks and the Colour Force QCD, Quantum Chromodynamics, the Colour Force, was proposed in the 1970s to explain how quarks stick together. Quarks have a new kind of charge colour that comes in three types: red, blue and green In EM, + and make an electrically neutral object (atom) In QCD, r, g and b make a colour neutral object (baryon) Also, colour + anti-colour make a neutral object (meson) The HADRONS:

14 Leptons: The Other Particle Family Hadrons (protons, neutrons ) are made of quarks What about electrons what are they? Particle accelerator experiments: some new particles appear that are NOT made of quarks μ (muon): charge 1, no colour, m ~ 200 x electron τ (tau): charge 1, no colour, m ~ 3500 x electron Other than mass, exactly the same three generations of electron? 1931: Wolfgang Pauli predicted a new particle the neutrino because the energy didn t add up in some radioactive decays: Unstable Tritium decays to become: electron 3 H 1 Stable 3 He 2 Neutrino must exist to account for electron s energy distribution

15 Leptons There is a kind of neutrino associated with each electron-type lepton: an e-neutrino, μ-neutrino and τ-neutrino No electric or colour charge don t feel the EM or QCD forces. So what do they feel? Must be some other force The Weak Force quarks and leptons can change type by exchanging new force-carrier particles, the W and Z u ν W - d e

16 The List of fundamental particles That s it aside from Gravity, that s everything in the universe

17 Some unanswered questions Unification and GUTs Electricity, magnetism parts of same underlying thing electromagnetism (Maxwell, 1868) EM, weak force parts of same underlying thing electroweak force (Glashow Weignberg Salam, 1970s) Can this be continued? Are electroweak force and QCD part of some underlying Grand Unified Theory? What about gravity? Is there just one force a SUPERFORCE to describe everything? Why mass? There are many fundamental particles, all with very different masses why? Why do they have masses at all? A bit like the periodic table and the 1960s particle zoo is there another layer to the onion?

18 Some unanswered questions Why three generations of particles? Before top quark was found, physicists found good mathematical reasons for it to exist three generations make the SM work Also very good mathematical arguments and experimental results to expect no more than three generations. Why would nature decide to create three copies of itself in this way? 2002: Neutrinos can change type and they have mass! That s not Standard Model physics.