To Do Today
œGuided writing
œClass discussion
œOverview: The Quantum World & SUSY
œPeer-led discussion
œReadings for next time
10 minute guided writing
œWhat do you think about dark matter?
What kind of evidence would you like to
see cosmologists present to the public?
to 3:05
1
Let’s make a list . . .
œWhat questions would you like to see
answered about dark matter if you were
visiting a planetarium?
œThink about: parents, children, teachers,
typical adults . . .
to 3:10
Class discussion
œIs it more "radical" to 'invent' a new form of
invisible matter, or is it more radical to replace
Newton's and Einstein's theory of gravity?
œIf you think “dark matter” is more radical, come sit
in the back two rows on the right side of the room
(near the door).
œIf you think “changing gravity” is more radical,
come sit in the front two rows on the right side of
the room.
œOne volunteer from each group
2
Let’s make two lists . . .
œDark matter PRO and CON (including
questions you/we don’t know the answer
to)
œModified gravity PRO and CON
œDiscuss in your groups and then we’ll
share on the board
to 3:20
Overview: The Quantum World
What is a “quantum”?
to 3:40
3
MACHOS
to 3:40
MACHOS
There cannot be
enough stars,
planets, comets,
asteroids, etc. to
make up the dark
matter. We’re
focusing the hunt
for dark matter
on the quantum
world!
to 3:40
4
WIMPs:
Weakly
Interacting
Massive
Particles
Overview: The Quantum World
œThese particles belong to the quantum world,
so they have strange properties. Quantum
“stuff” is both a wave AND a particle.
The Bohr atom vs. the electron cloud model
to 3:40
5
Overview: The Quantum World
œ QM isn’t “deterministic” – instead it deals with probabilities. For
example, a particle has a “wavefunction” that describes the
probability of it being in each possible location. In a sense, the
particle is in all of those locations.
The Bohr atom vs. the electron cloud model
to 3:40
Interpretations of Quantum Mechanics
œThe Copenhagen Interpretation (Bohr
and Heisenberg): the measurement
process “collapses the wavefunction” or
“picks one state.”
to 3:40
6
What is Schrodinger's Cat
Paradox? (in his own words)
"One can even set up quite
ridiculous cases. A cat is penned up
in a steel chamber, along with the
following diabolical device (which
must be secured against direct
interference by the cat): in a Geiger
counter there is a tiny bit of
radioactive substance, so small
that perhaps in the course of one
hour one of the atoms decays, but
also, with equal probability,
perhaps none; if it happens, the
counter tube discharges and
through a relay releases a hammer
which shatters a small flask of
hydrocyanic acid." -- Erwin
Schrodinger
Translation by John D. Trimmer
Interpretations of Quantum Mechanics
œ The Copenhagen Interpretation (Bohr and Heisenberg): the
measurement process “collapses the wavefunction” or “picks one
state.”
œ The Many Worlds Interpretation: for every outcome with a
nonzero probability, there is some world where that outcome
occurred. All the “worlds” exist at the same time, as though they
are in layers.
œ Objective Collapse: wavefunctions will be collapsed by something
objective, i.e. some physical threshold is reached.
œ No tests can tell the difference between these scenarios! So they
are philosophical questions (“metaphysics”), not scientific
theories.
to 3:40
7
QM and Philosophy
œEinstein: “God does not play dice with
the Universe.”
œBohr: “Stop telling God what to do!”
œThe role of the “observer” as actually
telling the Universe what to do is
unsettling for many.
to 3:40
QM and the “Standard Model”
œ When QM was developed, we only knew about:
œ Protons
œ Neutrons
œ Electrons
œ Photons
œ Discoveries of new particles: 1937, 1947, 1948, 1951, 1952 . . .
œ We began to realize that these were not fundamental
particles. They were made of smaller building blocks,
called quarks.
œ By the 1970s, we’d combined this information into the
Standard Model of particle physics
to 3:40
8
Fundamental Particles
œ The Fermions are
matter. Leptons and
quarks cannot be broken
down, but they can be
converted to pure
energy, and the heavy
ones can decay to the
light ones.
œ Bosons are responsible
for “communicating the
forces of nature between
Fermions.”
to 3:40
Fundamental Particles
œ Each Fermion has an
“antiparticle” with the same
properties but the opposite
charge.
œ If antiparticle pairs run into
each other, they annihilate,
releasing their energy to the
Universe.
œ This means that there must
have been fewer anti-quarks
and anti-leptons in the early
Universe than quarks and
leptons. Otherwise the
Universe would be empty!
to 3:40
9
to 3:40
Interactions and Conservation
œ Particles interact when they:
œ Bump into each other, exchanging kinetic energy
œ Gravitate towards one another
œ Exchange photons (interact electromagnetically)
œ Collide to produce new products
to 3:40
10
u
Interactions and Conservation
+⅔
œ After an interaction, the Universe will have
the same:
œ Mass, energy, and electric charge
œ “Colour” charge (quarks can have red, green, or
blue colour charge, and the results must always
have one of each. Example is a proton!)
u
+⅔
d
-⅓
to 3:40
u
Interactions and Conservation
+⅔
œ After an interaction, the Universe will have the
same:
œ Mass, energy, and electric charge
œ “Colour” charge (quarks can have red, green, or
blue colour charge, and the results must always
have one of each. Example is a proton!)
œ The Universe ends up having a nice symmetry
u
+⅔
d
-⅓
œ Every particle has an antiparticle with the same
mass and an opposite charge
œ This “principle of opposites” is important later!
œ The totals always balance out!
to 3:40
11
Fundamental Particles
œ The photon “mediates” the
electromagnetic force.
œ The gluon is responsible for
the strong force (it “glues” the
Universe together)
œ The W,Z bosons are
responsible for the weak force
œ What about gravity? We
don’t know!
œ If the Higgs exists, then
particles that interact with
the Higgs boson are
permitted to have mass
through that interaction.
to 3:40
How does this relate to dark matter?
œThere are three “flavors” of neutrinos, very
low-mass particles that interact with matter
very rarely.
œIf each of them had 1/100,000th the mass of
the electron, that could make a LOT of dark
matter!
œIf you want to find them, you need either a lot
of neutrinos (i.e. a nuclear reactor) or a really
big detector (i.e. SuperKamiokande or the
Solar Neutrino Observatory)
to 3:40
12
Super-K
Neutrinos are WIMPs
œWeakly Interacting Massive Particles
œThere are lots of other possibilities, and they
don’t have to have anything in common. But
neutrinos set us down the path of looking to
“weird” stuff in the Standard Model.
œ“If looking for MACHOs is like looking for a
needle in a haystack, then looking for WIMPs
is like looking for an invisible needle in a
haystack no one has found yet.”
to 3:40
13
Why can’t neutrinos be dark matter?
œ They’re too hot! They move at velocities close to the
speed of light.
œ This “heat” keeps galaxies from forming.
œ It takes much too long for gravity to “overcome” the
heat of neutrinos and form large-scale structure.
to 3:40
Supersymmetry
Supersymmetry is an unproven
theory that postulates a boson for
every fermion and a fermion for
every boson!
It solves some problems deep in the
mathematics of the Standard Model.
In this theory, bosons and fermions are intimately
related, or in other words, matter and force are one
concept, just like space and time.
to 3:40
14
Supersymmetric Pairs
œSupersymmetry is also called SUSY
œElectron/Selectron, Muon/Smuon, Tau/Stau,
Neutrino/Sneutrino, Quark/Squark
œPhoton/Photino, Gluon/Gluino, Z Boson/Zino,
W Boson/Wino, Higgs Boson/Higgsino
to 3:40
Supersymmetric Pairs
œThey’ve never been observed! They must be
much more massive than the non-super
partners, and can’t be created in
accelerators/colliders.
to 3:40
15
Supersymmetry and dark matter
Supersymmetric particles are
prime candidates to be the
dark matter of galactic halos.
But there is as yet no evidence
for supersymmetric particles,
although we are looking very
hard!
to 3:40
Supersymmetry and dark matter
Supersymmetric particles are
prime candidates to be the
dark matter of galactic halos.
The most likely supersymmetry
theories keep protons from
decaying quickly, and also have a
stable lightest supersymmetric
particle. These theories have Rparity, where “even” or
“oddness” is preserved. This
means the lightest particle has
nothing to decay to! These
‘neutralinos’ are good
candidates.
to 3:40
16
Hints of the anthropic principle
œ “Reverse Experiments”
œ Particle & antiparticle numbers can’t cancel out
exactly, or we wouldn’t be here.
œ There can’t be too many hot neutrinos in the early
Universe, or we wouldn’t be here.
œ Protons can’t decay too quickly, or we wouldn’t be
here.
œ These are the best tools astronomers have for
answering cosmological questions (“Can this model
Universe produce the Universe we know to exist?”)
œ . . . But they are also what is known as anthropic
arguments.
to 3:40
Hints of the anthropic principle
œ “Reverse Experiments”
œ Particles & antiparticles can’t cancel out exactly, or we
wouldn’t be here.
œ There can’t be too many hot neutrinos in the early
Universe, or we wouldn’t be here.
œ Protons can’t decay too quickly, or we wouldn’t be
here.
œ The anthropic principle is the idea that we must live
in a Universe capable of forming galaxies, stars,
planets, and life, or else we wouldn’t be here to
observe it. All physical laws and principles must be
consistent with a Universe that produces humans!
to 3:40
17
Peer-guided discussion
œ Work in groups.
1. Group 1: Colleen, Nicki, Dipesh, Yan
2. Group 2: Ryan, Jonathon, Faheem,
Sam
3. Group 3: Dan, Meghan, Rashik, Eli
4. Group 4: John, Justin, DJ, Ethan
Until 3:45
Peer-guided discussion
1.
2.
3.
4.
Group 1: Colleen, Nicki, Dipesh, Yan
Group 2: Ryan, Jonathon, Faheem, Sam
Group 3: Dan, Meghan, Rashik, Eli
Group 4: John, Justin, DJ, Ethan
Question: Quantum theory & SUSY are strange. In what ways do
they relate to some of the themes we thought about and
discussed in relation to Einstein's Dreams? Be prepared to
share your thoughts, ideas, and opinions. Think about free will,
determinism, freedom, social consequences. You can use some
of the ideas we worked on last week; if it helps, you can take the
ideas to bizarre extremes, like Lightman did, and think about
how that would affect the world.
For five minutes, each group should work on coming up with a good
discussion question centering around this topic. Then we’ll
share, and then have small-group discussions on each question.
Until 3:45
18
Peer-guided discussion
1.
2.
3.
4.
5.
Group 1: Colleen, Nicki, Dipesh, Yan. Question:
Group 2: Ryan, Jonathon, Faheem, Sam. Question:
Group 3: Dan, Meghan, Rashik, Eli. Question:
Group 4: John, Justin, DJ, Ethan. Question:
Discuss these in a new set of small groups: [Colleen,
Ryan, Dan, John], [Nicki, Jonathon, Meghan,
Justin], [Dipesh, Faheem, Rashik, Ethan], and [Yan,
Sam, Eli, DJ].
Until 4:05
Readings for next time
œHooper, Chapters 6 & 7
œFerreira, Chapter 11
19
Back to the Higgs boson
œWhen particles interact with the Higgs
boson, it gives them mass.
œParticles that easily interact have a lot of
mass; particles that don’t interact at all (like
the photon!) remain massless.
œBut particles contribute mass to the Higgs
boson too, so SUSY must keep the boson
from obtaining a tremendous mass. It can
do this because a particle and its
superpartner will add/take away mass, and
the Higgs is OK.
to 3:40
20