Quanta, Quarks, Qubits
A few thoughts on 100 years of ‘modern’ physics’
Blackbody radiation and the ‘ultraviolet
catastrophe’ — classical physics does not
agree with the observed world
• atoms are harmonic oscillators that only
absorb and emit energy in ‘bundles’ of hν.
• Bolzmann distribution of energies
Nn = N0 e(-nhν/kT)
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
Early ‘modern’ physics
Syllabus talks about:
• Heisenberg and Pauli, Fermi and his piles,
Pauli and the neutrino
• Investigations: Hydrogen spectrum, cloud
• 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
• nuclear physics with ping-pong balls or scrunched-up
• So where are we now?
• Everything we experience* can be described
very well** as matter (atoms) and forces
• 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
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
Crystals/solids/gases/liquids — molecules — atoms — electrons,
protons and neutrons
Ignoring gravity, the only force we ‘feel’ is Electromagnetism.
QED, quantum electrodynamics — quantum theory of light
interacting with electrically-charged particles (electrons, protons …)
QED: forces are treated as
exchanges of photons
Our greatest invention
QED: Quantum Electrodynamics
Theory & Experiment agree to 13 decimal places
Almost everything you ever experience is QED
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
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?
Protons and neutrons have structure
Colliding high-speed electrons with atoms uncovered
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?
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 …
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)
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:
3H1 Stable 3He2
Neutrino — must exist to
account for electron’s energy
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
The List of ‘fundamental’ particles
That’s it — aside from Gravity, that’s everything in the universe …
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
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?
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?
Some unanswered questions
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.
Why three generations of particles?