Astronomy 182: Origin

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Astronomy 182 Origin Evolution of the Universe
Lecture 13


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Assignments
For today Essay due on Ferris, Chapter 4 For
May 14 Essay due on Ferris, Chapter 9 May 23
Final essay due (please discuss your proposed
topic with me in advance) May 30 Final
exam in class

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Today
Quantum Mechanics Particle Physics Symmetries
Unified Theories
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Quantum Fields, Particle Physics, Symmetries
Resources Ferris, Ch. 8 Greene, Chapters 4 5
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Waves Particles
Einsteins view of light as composed of
particles, while originating with Newton, went
against 100 years of standard lore and
experimental results which said that light is a
wave. Light is a Wave Early 1800s Youngs
double-slit experiment showed that light
waves interfere. This view of light as a wave was
subsequently incorporated into Maxwells
theory of light as Electromagnetic Waves.
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Shine light on a barrier in which 2 slits have
been cut record the pattern of light intensity
on a photographic plate.
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First cover up one or the other of the slits
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Now open both slits if light consisted of
particles (or if one was shooting bullets), the
resulting pattern should look like this
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Intensity pattern on the screen
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Waves emerging from the 2 slits overlap
interfere with each other
Light areas wave peaks Dark areas wave
troughs Gray areas wave peak overlaps with
another wave trough,
canceling each other out
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Waves Particles
How to reconcile the established wave nature of
light with Einsteins view of light as
particles? Turn down the beam intensity in the
double slit experiment the interference
pattern remains (if you wait long enough),
even when you fire one photon at a time through
the barrier! How can photons in the
low-intensity beam interfere with each
other? According to classical physics, each
photon particle should pass through either
the left or the right slit, producing the
bullets intensity pattern. How can the photon
passing through one of the slits care
about whether the other slit is open or
closed?
Wave-particle duality Quantum
Mechanics light has both wave-like
particle-like properties.
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Matter particles are also waves
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Probability
(both wavelike particlelike properties)
Waves
Interpret in terms of Probability Amplitude of
the wave is highest where the particle is more
likely to be found. This is a fundamental
departure from classical physics.
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Classical vs. Quantum Mechanics
Classical Mechanics describes the
trajectories of particles through space
over time. Given the positions and velocities of
all particles at some time, we can
predict their positions and velocities at
all future times. Quantum Mechanics
describes the probability of finding particles at
given positions the wave function. This
probability evolves deterministically
(knowing it at some time, we can predict
its future behavior), but we cannot predict with
absolute precision the future positions
and velocities of the particles (and we
cannot measure them with absolute
precision now).
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Implications Quantum tunneling quantum vacuum
fluctuations
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Function of space time
Quantum Electrodynamics Quantum Theory of the
Electromagnetic field
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Elementary Particle Physics
Quantum numbers
(colorless)
(fermions)
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(Bosons integral spin particles)
Electromagnetic
Weak
Strong
Gravity
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of Matter
Photon Gluon
Protons and neutrons each composed of 3 up (u)
and down (d) quarks, held together by gluons
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Experimental proliferation of heavy mesons
baryons
1950s and 1960s
resonances
strange particle (1953) decays slowly
Later interpreted as bound states of more
fundamental particles quarks antiquarks
(1963)
up
down
strange
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Symmetry Particles
1961 Gellman and Neeman the plethora of
baryons mesons can be grouped (arranged)
into patterns according to their properties
(quantum numbers) form octet (groups of
8) representations of the SU(3) symmetry
(an internal rotation symmetry). This
Eightfold Way imposed a rational order on the
particle zoo and led to the prediction of a
new particle, the ?- , which was
subsequently discovered. Another
representation of SU(3) involves 3 particles,
not 8 Gellman and Zweig postulated that
there must be associated fractionally
charged particles, the 3 quarks u,d,s, of which
the baryons mesons are composed.
Three quarks for Muster Marks. --James Joyce,
Finnegans Wake
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Mesons Baryons
Strongly interacting particles (called hadrons),
come in two observed forms Mesons
(e.g., pions, kaons,) with baryon B 0
Baryons (e.g., protons, neutrons) which carry B
? 0 Following SU(3) symmetry meson
composed of a quark-antiquark pair baryon
composed of 3 quarks Quarks are permanently
confined inside hadrons by the strong force
(gluons)
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(1974 discovery of first charmed (c) meson)
Discovery of bottom (b) quark at Fermilab
Upsilon
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mid-1990s experiments at
Fermilab