What Is The World Made Of? Ken Krane Academy for Lifelong
Learning January 23, 2013 March 12, 2013
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Program 1.To the very small Atoms Nuclei, protons and neutrons
Probing inside the nucleus the nuclear glue Families of particles:
baryons, mesons, leptons Quarks and gluons The Standard Model The
Higgs particle BREAK 2.To the very large The large-scale structure
of the universe The expansion of the universe Big Bang vs. Steady
State theories The Big Bang Theory The formation of the chemical
elements Is the expansion accelerating?
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Properties of Atoms 1.Fundamental building blocks of matter
(but not indivisible) 2.They are small 100,000,000 laid end-to-end
would make about one centimeter 3.They are electrically neutral
they contain equal amounts of positive and negative charges 4.They
are stable they dont spontaneously collapse 5.They emit and absorb
various types of electromagnetic radiation light, x rays, etc.
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The Periodic Table of the Elements
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Constituents of Atoms 1897 J. J. Thomson (England) observed
corpuscles of negative charge later called electrons that were
emitted from atoms and assumed to be constituents of atoms but only
0.1% of the mass of the atoms. 1911 Ernest Rutherford (England)
showed that the positive charge in an atom provided most (99.9%) of
its mass and was concentrated in a very small region at the center
of the atom called its nucleus. Thomson model Rutherford model
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Bohrs Model (1913) 1913 Niels Bohr (Danish, but working in
England with Rutherford) developed a planetary model of the atom in
which the electrons circulated about the central nucleus like
planets about the sun. Instead of gravity, the binding is provided
by the electrical attraction of positive and negative charges for
one another. Open questions (1913): 1.Why dont electrons fall into
nucleus? 2.Why dont all electrons choose lowest orbit (least
energy)? 3.What are constituents (if any) of nucleus?
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Atomic Number Henry Moseley (British, 1913) from measuring x
rays emitted by atoms deduced number of positive charges in
nucleus, called the atomic number Z. If atoms are electrically
neutral, an atom of atomic number Z with Z positive charges must
also contain Z negative charges (electrons).
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Rutherfords Transmutation Experiments (1917) N H Alpha
particles striking nitrogen release particle with one unit of
positive charge. Identical to nucleus of hydrogen Named by
Rutherford proton
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Constituents of Nucleus The elementary positive charge is
identical to that of the simplest nucleus, hydrogen. The mass or
weight of an atom is very nearly an integer multiple of that of the
lightest atom, hydrogen. This integer is called A, the mass number.
This was originally assumed (incorrectly) to be the number of
protons in the nucleus. If a nucleus contained A protons, it would
have too much positive charge because A is larger than Z. For
example, helium has a mass of 4 but a charge of only 2. The protons
in the nucleus repel one another. What keeps the nucleus
together?
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The Neutron In 1920, in order to explain the disparity between
atomic mass and atomic number, Rutherford proposed that the nucleus
also contained a neutral particle of about the same mass as the
proton. Discovered in 1932 by Rutherfords former student, James
Chadwick. Being neutral, neutrons experience no electrical force
but provide the extra nuclear force to overcome the electrical
repulsion of the protons. p p pp p p n n nn X X nn p
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Characteristics of Atom Mass of A units (A = total number of
protons + neutrons) At its center, a nucleus containing Z
positively charged protons and N neutrons (Z + N = A). Nucleus
accounts for 99.97% of the mass of the atom. Nucleus is very small
if an atom were a large as a football field, the nucleus would be a
pea on the 50 yard line. The nucleus is surrounded by a cloud of Z
negatively charged electrons.
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What Force Holds the Nucleus Together? Yukawa (1934) exchange
force A particle (the force carrier) is exchanged between p and n
Postulated exchanged particle: meson Observed in 1947
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Nuclear Beta Decay (1930s) Joliot-Curie, Fermi Neutron proton
Proton neutron Relatively long time scale (hours years) Caused by
weak interaction
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The Forces of Nature (1800) Magnetism Gilbert (1600)
Gravitation Newton (1687) Electrical Coulomb (1785)
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The First Great Synthesis Electrical forceMagnetic force
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The First Great Synthesis Electromagnetic force Faraday (1800s)
Maxwell (1876) Electric generators (rotating magnet produces
electricity) Electric motors (electricity produces rotating
magnet)
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The Four Forces (1930s) Strong nuclear force Electromagnetism
Weak nuclear force Gravitation 1 0.01 0.00000001 0.00000000000001
Strong and weak nuclear forces: short range Electromagnetism:
neutralized by shielding Gravitation: cumulative 1/1,000,000,000 of
the electrons in a penny would provide enough electrical force to
launch another penny to the Moon.
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The Particle Universe in 1950 Electron (e) Proton (p) 1836 x
electron mass Neutron (n) 1839 x electron mass Meson () 274 x
electron mass Mu () 207 x electron mass
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Digression 1 The Neutrino: Natures Little Joke Beta decay of
neutron: neutron proton neutron proton + electron (conservation of
electric charge) Conservation of energy: energy of neutron = energy
of proton + energy of electron All electrons should emerge from
decay with same energy, equal to energy of neutron minus energy of
proton Instead, all electrons were observed (1920s) to emerge with
less than this energy What happens to the missing energy?
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Digression 1 The Neutrino: Natures Little Joke Wolfgang Pauli
(1930): suppose there is another particle emitted in the beta
decay: neutron proton + electron + x x particle must be
electrically neutral (because charge conservation is already taken
care of) Electron and x particle share the available energy the
missing energy is then simply the energy carried by the unobserved
x particle The x particle is now known as the neutrino Neutrinos
are elusive and extremely hard to detect not done until 1950s
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Digression 2 Antiparticles: Natures Symmetry Every particle has
a corresponding antiparticle: electron antielectron or positron
(1932) proton antiproton (1955) neutron antineutron (1955) Same
mass but opposite electric charge Annihilation: particle +
antiparticle energy Pair production: energy particle +
antiparticle
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The Particle Universe in 1950 Electron (e) Proton (p) 1836 x
electron mass Neutron (n) 1839 x electron mass Meson () 274 x
electron mass Mu () 207 x electron mass Neutrino () mass zero
Antiparticles Now begins era of nuclear accelerators (atom
smashers)
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The Particle Accelerator Era (1950s-1970s) Beam of high-energy
protons directed against a target (usually also protons) Multitude
of new, short-lived particles produced Berkeley, Brookhaven,
Fermilab, CERN (Geneva), Stanford (electrons), etc.
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Experiments in Particle Physics
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3 Families of Particles Hundreds of proton-like particles
(called baryons, meaning heavy particles) with increasing mass. All
have short lifetimes and decay eventually to protons. Hundreds of
meson-like particles with increasing mass. All have short lifetimes
and decay eventually to electrons and antielectrons (positrons).
One electron-like particle (in addition to ) called , unstable and
decaying eventually to electrons. Each of the three (e, , ) has a
distinct associated type of neutrino. These 6 particles are known
as leptons (meaning light particles) and they appear to be point
particles with no internal structure. What are the mesons and
baryons made of? Are there really hundreds of elementary particles
or do they have a simpler set of constituents?
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Deep Inelastic Scattering (Stanford, 1969) electron proton
High-speed electrons penetrate inside the proton where they
encounter something massive that causes them to recoil backward,
exactly analogous to Rutherfords discovery of the nucleus. The
interior of the proton contains 3 of these massive objects, which
are the internal constituents of protons (and neutrons). They are
called quarks.
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The Quark Model of Baryons and Mesons up quark: charge = +2/3
down quark: charge = -1/3 proton neutron charge = +2/3 + 2/3 1/3 =
1 charge = +2/3 1/3 1/3 = 0
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The Quark Model of Baryons and Mesons All of the hundreds of
baryons and mesons can be accounted for in terms of 6 elementary
and indivisible particles called quarks. Baryons are composed of
three quarks, mesons of a quark and an antiquark. Quarks are the
fundamental particles of the strong interaction. The force between
quarks is based on the exchange of particles called gluons.
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The Standard Model Matter is composed of 3 pairs of quarks and
3 pairs of leptons (and their antiparticles). quarks: (u,d) (c,s)
(t,b) leptons: (e,) (,) (,) Decay lifetimes of certain particles
limit the leptons to 3 types: electron + antielectron mu + antimu
tau + antitau Lifetime is determined by number of decay paths.
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No Freedom for Quarks! Electrical or gravitational force
decreases with increasing separation Strong force between quarks
increases with separation (like rubber band) Large energy needed to
free a quark appears as the creation of new particles, which form
jets of tracks in detector
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The Second Great Synthesis Electromagnetic forceWeak force
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The Second Great Synthesis Electroweak force Weinberg, Glashow,
Salam (1967; Nobel 1979) Force carriers: photon, weak bosons W and
Z The unification is not quite perfect: the photon is massless but
the W and Z are very massive about 100 times the mass of the
proton. Why are the masses so different?
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Another Puzzle The 3 charged leptons: electron mu 207 x
electron mass tau 3500 x electron mass A similar seeming
arbitrariness exists for the masses of the 6 quarks. Why these
particular mass values???
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Solution: The Higgs Force (Peter Higgs, 1964) A force field
(the Higgs field) pervades the universe. Particles get their masses
by interacting with the Higgs field. The interaction occurs through
the exchange of a particle called the Higgs particle or Higgs
boson. (Boson: different way of classifying particles includes
mesons and field particles, excludes baryons, leptons, and quarks)
Observation of the Higgs boson would not only confirm the theory,
but would provide a way to study how particles acquire mass. Higgs
particle is expected to be very short-lived and to decay rapidly
into 2 other particles, which in turn might either decay or leave
visible tracks in the detector. One possible final result: 2
photons or 2 electrons and 2 positrons. Previously unknown boson
discovered in 2012 at LHC in CERN; seems consistent with Higgs but
so far other explanations cannot be excluded.
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Finding the Higgs (2012)
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The Standard Model with Higgs
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The Next Great Synthesis Electroweak forceStrong force
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The Next Great Synthesis Grand Unified Theory (GUT) Many
different candidate theories no consensus yet Predictions proton
decay, etc. Might explain neutrino oscillations (electron mu or
tau)
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BREAK
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The Large-Scale Structure of the Universe (1900s) Universe
believed to be eternal, static, and infinite Subject to Newtons law
of gravity No structure known beyond our galaxy Solar system at
center of galaxy Size of galaxy 10,000 light-years Beginning of era
of large optical telescopes Einsteins General Theory of Relativity
(1916) Requires cosmological constant
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The Expanding Universe (1920s) Evidence from observations by
Edwin Hubble Nebulae are galaxies with individual stars Galaxies
are receding in all directions Speed increases with distance
Universe is expanding (therefore not static) No need for Einsteins
cosmological constant
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Two Competing Theories Steady State Theory Universe is eternal,
same from every vantage point and at all times New matter is
created to fill voids resulting from expansion Big Bang Theory If
galaxies are separating, they must previously have been closer
together; therefore earlier universe was denser and hotter Run
cosmic clock backward to a single point of creation with infinite
density
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The Cosmic Microwave Background Radiation (1965) Arno Penzias
and Robert Wilson set up microwave receiver at Bell Labs to observe
signals from Bells new Telstar communications satellite Annoying
background hum in receiver coming from all directions, day and
night Deduced to be remnant heat radiation from early hot universe,
now cooled to 2.7 degrees above absolute zero Propels Big Bang
Theory to forefront
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WMAP Satellite (2001-2010) In solar orbit 1 million miles from
Earth Temperature map of universe at age 380,000 y (variations by
0.0002 degree)
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Dark Matter Light from stars in arms moving toward observer is
Doppler shifted toward shorter wavelengths (blue), while light from
stars moving away is shifted toward the red Permits measurement of
variation of rotation speed with distance from center of
galaxy
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Dark Matter Galaxies are surrounded by a halo of dark matter
not visible in telescopes, not made from ordinary stuff (protons
and neutrons)
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Dark Matter and Gravitational Lensing
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What is the Geometry of the Universe? General Theory of
Relativity (Einstein, 1916) Gravity = geometry: matter tells space
how to curve, curved space tells matter how to move Curvature
depends on relationship between actual density of matter and energy
in space and a particular critical density As universe expands,
closed universe becomes more closed, open universe becomes more
open, but flat universe remains flat closed open flat
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The Expanding Universe Closed Flat Open
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Two Problems The Horizon Problem: Why do the farthest limits of
the observable universe in different directions look the same? How
can they achieve an apparent equilibrium? The Flatness Problem: The
present universe is flat to within about 1%. Because the drift with
time is away from flatness, the early universe must have been
within 0.000000..00001% of flatness. Why? Solution: Inflation
Hypotheses Just after the Big Bang, the universe underwent a brief
period of extreme expansion.
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A New Surprise (1998-2000) Observations of supernovas
(exploding stars) in the most distant galaxies showed them to be
moving away from us much faster than would be predicted by the
Hubble expansion The expansion of the universe must be
accelerating! What force could be driving this acceleration?
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The Expanding Accelerating Universe
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Results from Cosmological Experiments Age of universe: 13.77
billion years ( 0.5%) Universe is flat ( 0.4%) Ordinary matter is
5% of universe Dark matter is 23% (unknown form) Remaining 72% is
dark energy, a mysterious force that is driving the accelerated
expansion (very similar to Einsteins cosmological constant) The
number of lepton/quark generations cannot be greater than 3
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The Big Bang Cosmology
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The Big Bang Chronology Big Bang occurs 13.77 billion years ago
Hot dense universe consists of quarks and leptons (+ antiparticles)
and radiation Inflation drives possibly non-flat universe to
flatness and causally connected regions (in equilibrium) disconnect
1,000,000,001 particles + 1,000,000,000 antiparticles 1 particle +
radiation 3 quarks protons and neutrons Protons and neutrons +
electrons form hydrogen (75%) and helium (25%) atoms at about
400,000 y (3000 degrees) The universe continues to expand and cool,
so that the radiation eventually reaches its present temperature of
2.7 degrees Atoms preferentially condense in low-temperature
regions of the radiation field and eventually form stars and
galaxies First generation stars explode and die, casting out the
heavier elements forged from hydrogen and helium in their interiors
From the debris are formed second generation stars and their
planets, some of which provide hospitable locations from which
physicists can ask fundamental questions about their origins
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Open Questions Are there more generations of quarks and
leptons? Why dont free quarks exist? How does the Higgs mechanism
work? Are there more Higgs particles? What is the correct Grand
Unified Theory (strong + weak + electromagnetic)? Is there a Theory
of Everything (GUT + gravity)? What is the mass of the neutrinos?
What is dark matter? What is dark energy? What causes the
accelerating expansion of the universe? What caused inflation? What
caused the Big Bang? Was it a quantum fluctuation? Is our universe
unique?