INVASIONS
IN
PARTICLE PHYSICS
Compton Lectures Autumn 2001
Lecture 1 October 6
M. Spiropulu
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LECTURE 1
outline
What is Particle Physics about - The scientific method - Photons:
the quanta of light - Electrons - The Electromagnetic Spectrum High energies and elementary particles
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PARTICLE PHYSICS (PP)
or
Elementary Particle Physics (EPP)
or
High Energy Physics (HEP)
3
One of the physical sciences that attempts to answer the
following questions:
What is the world made of?
How it did come about?
How does it work?
We seek fundamental answers to these questions in that
the answers are given in terms of physical principles
that cannot be explained by “deeper” principles
4
What will we know if we find the fundamental answers
to these questions?
The dynamics of space and time
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PP questions (in other words)
Is there a small number of distinct objects from which
the universe is made?
Is there a small number of simple rules to explain how
these objects mesh together to make up everything?
What kind of observations in what kind of laboratories
can we make to study the above?
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In more words
Particle physics tries to answer questions
about (the origin and nature of) the universe
by
studying the objects that are found in it,
it
by
studying their interactions
by means of
arranging experimentally the objects (and their interactions)
in such ways that the ultimately “primitive”, ultimately elementary,
smaller objects (and ultimately simple interactions)
are discovered or revealed or deduced.
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This is almost like someone trying to learn
how to play chess, by studying the shapes of
the pieces and the ways in which they move
across the board !
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How do we go about getting the answers?
1 or 2:
2 Make a guess and compute physical consequences
2 or 1:
1 Observe/measure
3:
Compare the consequences of the guess with the
measurements
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Breakdown:
What is the world made of?
The “visible” matter in the universe is made of
electrons (e)
up (u) quarks
down (d) quarks
5-10 times more than this, is we don’t know
what; we call it
“dark matter”
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Timeline: What is the world made of?
Anaxagoras : changes in matter are due to different orderings of
indivisible particles.
Empedocles : reduced these indivisible particles into four
elements: earth, air, fire, and water.
Democritus : developed the theory that the universe consists of
empty space and an (almost) infinite number of indivisible
particles which differ from each other in form, position, and
arrangement. All matter is made of indivisible particles called
atoms.
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Timeline: How does it work?
Archimedes (287-212 BC) provided the foundations of
hydrostatics.
Eratosthenis (276-196 BC) measured the circumference of the
earth.
Roger Bacon (1214-92) taught that in order to learn the secrets
of nature we must first observe the method by which people can
develop deductive theories is observation, i.e. using evidence
from the natural world.
Nicolaus Copernicus (1473-1543) claimed that the Sun and not
the Earth is at rest in the center of the Universe.
1550-1898: Galileo Galilei, Johannes Kepler, Isaac Newton,
Thomas Young, Michael Faraday, Joseph Henry, James Clerk
Maxwell, William Röntgen, Marie Curie, Joseph Thompson
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Timeline:
The first elementary particles
The particle of light (or photon*, or γ (gamma)) & the electron e-
* “phos” = light ( in greek)
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Characterize the photon (γ)
It does not stand still
It goes with the speed of light c (300,000Km/s)
It does not have mass
It does not have electric charge but it is responsible for charged
particles “feeling” each other (good or bad feelings, attract or repel)
It has spin* one (s=1, and is called a boson* because of this)
“interaction”
“mediates”
Photon
Photonisis the
the“quantum”
“quantum”ofoflight
light: :Photons
Photonsofofaaparticular
particularcolor
color(frequency)
(frequency)
come
with
energy:
come with energy:
Photon
PhotonEnergy
Energy==Planck’s
Planck’s quantum
quantumconstant
constant(h)
(h)**frequency
frequency
Is it a particle or is it a wave?
* commit the words to memory; we will come back to spin
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Waves Interfere
Photoelectric Effect
Double Slit Experiment
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Characterize the eIt is a massive elementary particle
It has negative electric charge
It has spin* 1/2 (s=1/2, and is called a fermion* because of this)
It is found in the atom
An accelerating electron emits electromagnetic radiation
Is it a particle or is it a wave?
* commit the words to memory; we will come back to spin
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Bohr’s atom and de Broglie’s electron waves
The
Theatoms
atomscan
canexist
existonly
only
in
discrete
quantum
states
in discrete quantum states
separated
separatedby
byfinite
finiteenergy
energy
differences;
when
in
differences; when inthese
these
quantum
quantumstates
statesthe
theatoms
atoms
do
not
radiate
!
set
of
do not radiate ! set of
allowed
allowedorbits
orbitsfor
forthe
the
electrons
around
the
electrons around the
nucleous.
nucleous.
WHEN
WHENAN
ANELECTRON
ELECTRON
MAKES
A
TRANSITION
MAKES A TRANSITION
FROM
FROMONE
ONEORBIT
ORBITTO
TOTHE
THE
OTHER
OTHERIT
ITEMITS
EMITSLIGHT
LIGHT
The
Thewavelength
wavelengthof
ofaaparticle
particlewave
waveisisinversely
inversely
proportional
to
its
momentum,
the
constant
proportional to its momentum, the constantof
of
proportionality
proportionalitybeing
beingPlanck’s
Planck’scontant
contant(h):
(h):
Allowed
orbits
are
explained
as
containing
Allowed orbits are explained as containingan
an
integral
number
of
the
de
Broglie’s
wavelengths.
integral number of the de Broglie’s wavelengths.
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To all practical intents and purpose a physics system such as a single
electron, or a photon or an atom exists by some form of observation.
[“Concerning that of which we can not speak we must pass over in
silence” Wittgenstein. “If you don’t see, don’t say” Feynman ]
Any act of observation on a quantum system will disturb it.
Electrons and photons are neither particles nor waves; they are
“quantum fields”. Depending on the experiment, we see them
either as waves or as particles.
There is a wave-function that reflects a particle’s localization
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Aspects of light : the Electromagnetic Spectrum
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Radio waves
Lowest
Lowestfrequencies
frequencies(longest
(longestwavelengths)
wavelengths)of
ofthe
the
electromagnetic
electromagneticspectrum.
spectrum.
An
Aninteresting
interestingthought
thoughtisisthat
thatsome
someform
formof
of
extra-terrestrial
extra-terrestrialintelligence
intelligencecould
couldsend
sendus
us
messages
messageswith
withradio
radiowaves
wavesthat
thatcan
canbe
bereceived
received
by
byradio
radiotelescopes.
telescopes.
Look
Lookatatthe
theSETI
SETIproject
project
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Microwaves
Microwaves
Microwaveshave
have
wavelength
wavelength between
between
roughly
roughly30
30centimeters
centimeters
and
and11millimeter
millimeter
The
Thecosmic
cosmicmicrowave
microwave
background
backgroundisisthe
theafterglow
afterglow
radiation
radiationleft
leftover
overfrom
fromthe
the
hot
hotBig
BigBang.
Bang.Its
Itstemperature
temperature
isisextremely
extremelyuniform
uniformall
allover
over
the
thesky.
sky.However,
However,tiny
tiny
temperature
temperaturevariations
variationsor
or
fluctuations
fluctuations(at
(atthe
thepart
partper
per
million
millionlevel)
level)can
canoffer
offergreat
great
insight
insightinto
intothe
theorigin,
origin,
evolution,
evolution,and
andcontent
contentof
ofthe
the
universe.
universe.
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Infrared and Visible
Infrared
Infraredradiation
radiationcomprises
comprises
wavelengths
wavelengthsbetween
between11
millimeter
millimeterand
and750
750nanometers
nanometers
(750
(750millionths
millionthsof
ofaamillimeter).
millimeter).
We
Wecannot
cannotsee
seeit,
it,but
butwe
wecan
canfeel
feel
ititas
asheat.
heat.
Infrared
Infraredlight
lightisisemitted
emittedwhen
when
the
theatoms
atomsand
andmolecules
moleculesin
inan
an
object
objectoscillate.
oscillate.IR
IRobservations
observations
have
haveto
tobe
bemade
madefrom
fromtelescopes
telescopes
in
inhigh,
high,dry
dryplaces,
places,away
awayfrom
from
clouds
cloudsthat
thatblock
blockthe
theIR.
IR.
How
Howyou
youcan
canmeasure
measureIR
IR
temperature
at
home
temperature at home
The
Theatmosheree
atmoshereefilters
filtersout
outmost
mostof
of
the
theelectromagnetic
electromagneticradiation
radiation
from
fromthe
thesun
sunexcept
exceptfor
forportion
portionof
of
the
theradio
radiowaves
wavesand
andthe
theregion
region
around
aroundvisible
visiblelight.
light.The
Thesun
sunemits
emits
almost
almosthalf
halfof
ofits
itsenergy
energyin
injust
just
that
thatsegment
segmentof
ofthe
thespectrum
spectrumthat
that
we
weare
areable
ableto
tosee.
see.
Water
Waterisistransparent
transparentto
tovisible
visible
light
lighttoo.
too.That
Thatisisthe
thereason
reasonwhy
why
light
lightcan
canpass
passthrough
throughour
our
eyeballs,
isis
eyeballs,whose
whosechief
chiefconstituent
constituent22
water,
water,to
toreach
reachthe
theretina.
retina.
Ultaviolate X-rays Gamma Rays
Ultraviolate
radiationcovers
coverswavelengths
wavelengths
Ultraviolateradiation
between
between400
400nanometers
nanometers(4
(4millionths
millionthsof
ofaa
millimeter)
millimeter)and
and10
10nanometers.
nanometers.
X-rays
X-rayshave
havewavelengths
wavelengths
between
between10
10nanometers*
nanometers*and
and
0.01
0.01nanometers.
nanometers.They
Theyare
are
created
createdwhen
whenthe
theinner
inner
electrons
electronsin
inan
anatom
atomchange
change
their
theirorbit
orbitaround
aroundthe
thenucleus
nucleus
and
andlose
loseenergy
energyin
inthe
theprocess.
process.
*1nm = 10-9 m
Gamma
Gammarays
raysform
formthe
theshort-wavelength
short-wavelengthend
end
of
ofthe
theelectromagnetic
electromagneticspectrum.
spectrum.They
Theyhave
have
the
thehighest
highestfrequencies
frequenciesand
andenergies.
energies.
Gamma
Gammarays
raysare
arecreated
createdin
inradioactive
radioactive
processes
processesin
inatomic
atomicnuclei
nucleiand
andwhen
whenmatter
matter
and
andantimatter
antimatterannihilate
annihilateeach
eachother
otherand
and
create
createpure
pureenergy.
energy.
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radio
X-ray
infrared
optical
X-ray+optical
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Eta Carinae Pictures
Particle Physics - High Energy Physics
The smaller the wavelength the smaller the structure the wave
will resolve (compare the optical microscope to the X-ray).
Particles with very large momentum (high energy particles)
have extremely small wavelengths and can probe subatomic
distances: High Energy Particle Accelerators serve as supermicroscopes.
The higher the energy the closer particles can come to each other
and thus the smaller details of their structure can become
important in determining how they bounce off each other.
The higher energy available, the heaviest particles can be
produced in a collision (E=mc 2).
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