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Lecture
6: The Nature of Light
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| Astronomy
101/103 |
Terry
Herter, Cornell University
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Lecture
Goals
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Cosmic
forces (forces of Nature)
- The
four fundamental forces (Gravity,
E-M, Strong, and Weak)
- GUTs
and TOEs
Light
- Wavelength
and Frequency
- The
E-M Spectrum
- Light
and Energy
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Forces of
Nature
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The four fundamental forces of nature are:
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Force
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Relative
Strength
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Comment
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Nuclear
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1
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Holds
nuclei together
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Electromagnetic
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1/137
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Holds
atoms, molecules, our bodies, etc. together
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Weak
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10-13
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Responsible
for radioactive decay of certain particles
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Gravitational
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10-39
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Holds
planets together, governs evolution of the
universe, etc.
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Your
everyday experience is with Electromagnetic and Gravitational
Forces.
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E-M
Forces
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- Allows
us to exist (among other things).
- Hold
us together, make blood flow, make molecules form, etc.
- Very
strong
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EM force can be either attractive or repulsive |
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Unlike
charges attract
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(+)==>
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<==(-) |
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Like
charges repel
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<==(+)
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(+)==> |
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<==(-)
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(-)==> |
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Light
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What
is light?
Light
is an electromagnetic (EM) wave propagating through space.
EM
waves (photons) include:
- ultraviolet
radiation
- visual
light
- infrared
radiation
- radio
waves, etc.
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Aside
[In case you are interested]:
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Light
is a transverse wave. This means that the
oscillation is perpendicular to the direction
of motion (much like a water wave).
So
what causes light to exist and move? In
physics it is known that a changing magnetic
field causes a changing electric field (and
visa versa). A photon consists of a time
varying electric and magnetic field which
regenerate each other! As far as we know
this will go on forever unless the photon
gets absorbed (by an atom or collection
of atoms).
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The Colors
of Light
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Isaac
Newton determined that when light passes through a PRISM
(see below), it disperses into various "colors".
In
essence, he used a PRISM to make a rainbow, and invented
spectroscopy.
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"Nature
and nature's laws lay hid in the night:
God said, 'Let Newton be!' and all was light."
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...
Alexander Pope
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Spectroscopy
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Spectroscopy
breaks the light into different (wavelengths) colors.
Rain drops act as prisms to cause a rainbow.
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Wavelength
of Light
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Light
is characterized by a wavelength, which is the distance
between two similar points in the wave, e.g. the valleys
or peaks.
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Frequency
of Light
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In
addition to wavelength, we sometimes talk about the frequency
of a photon.
Let
"c" be the speed of light (3x108 m/s), then
we have
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Relationship
between frequency and wavelength
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l
f = c or f
= c/l
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where
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f
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=
frequency of radiation |
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l
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=
wavelength of radiation |
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c
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=
speed of light |
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What
is "frequency"?
- If
a wave goes by while you are sitting in a boat, you go
up and down at a given rate.
- This
is the frequency of the wave.
- The
analogy carries over to light waves.
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The E-M
Spectrum
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The
electromagnetic spectrum is our "window" to
the Universe. Historically various parts of the E-M spectrum
have been given labels. Some of the most common names
are listed below.
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Radiation
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Approximate
Wavelength
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Angstroms
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Microns
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Gamma
rays
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0.01
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10-6
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X-rays
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1
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10-4
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Ultraviolet
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1000
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0.1
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Visible
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5000
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0.5
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Infrared
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105
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10
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Microwave
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107
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103
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Radio
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108
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104
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Radio
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109
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106
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Why the
Labels?
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The
use of labels is partly for convenience and partly historical.
Photons
of different energy are usually detected with different
instruments.
- our
eyes detect visible light
- radio
detect radio waves
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Light
and
Energy
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Just
like particles, photons carry energy. Each
wavelength has a unique energy given by the expression below
(Einstein again!)
Let
"c" be the speed of light (3x108 m/s), then
we have
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Relationship
between energy and frequency/wavelength
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where
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h
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=
Planck's constant |
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=
6.626x10-34 |
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f
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=
frequency of radiation |
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l
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=
wavelength of radiation |
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c
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=
speed of light |
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Long
waves, such as radio waves, carry little energy.
Short
waves, like X-rays, carry lots of energy.
This
is why ultraviolet radiation can give you a sunburn and
cause cancer (see the discussion on the ozone hole below).
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Atmospheric
Windows
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Atmospheric
windows are locations in the EM spectrum where the atmospheric
is transparent.
The
atmosphere only allows certain wavelengths to pass through
it.
- Visible:
3500 - 10000 A
- Radio:
1 mm - 100 m
This
is one reason why astronomers want observatories in space.
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The
Ozone
Hole
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Ozone
(O3) occurs at an altitude that varies with latitude
and season. The peak concentration in the ozone layer occurs
at about 16 km. However, ozone is present up to about 80
km.
Because
it has an absorption band near 3000 A, ozone blocks the
ultraviolet (UV) rays from the sun. Due to their shorter
wavelength (higher frequency), UV photons have more energy
than their optical counterparts. This means that they can
damage our skin, potentially resulting in cancer.
A
breakdown in the ozone layer can not only be harmful to
humans but plant and animal life as well.
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Aside
[In case you are interested]:
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At
shorter wavelengths absorption due to water,
atomic and molecular oxygen, and atomic
and molecular nitrogen blocks UV and X-rays
from reaching the earth.
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Forces of
Nature
(Again)
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The four fundamental forces of nature are:
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Force
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Relative
Strength
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Comment
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Nuclear
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1
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Holds
nuclei together
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Electromagnetic
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1/137
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Holds
atoms, molecules, our bodies, etc. together
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Weak
|
10-13
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Responsible
for radioactive decay of certain particles
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Gravitational
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10-39
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Holds
planets together, governs evolution of the
universe, etc.
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Your
everyday experience is with Electromagnetic and Gravitational
Forces.
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Gravity
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| Newton
formulated his theory of gravity in 1667
More
mass => stronger attraction
Newton's
law of gravity is given by:
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F
=
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Force
of attraction (Newtons)
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G
=
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6.67
x 10-11 m3kg-1sec-2
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m1
, m2 =
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masses
(kg)
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d
=
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distance
between objects (m)
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In
words, this means that the attractive force between
to
bodies is proportional to the product of their masses.
The more massive a body the stronger the attraction.
In
addition the force gets weaker as you move the bodies
further apart. It decreases as the square of the distance.
Doubling the distance means the force is four times weaker.
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Gravity
Examples
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The
table below (column 4) shows how much more you would weigh
if you were standing on the "surface" of the various
astronomical bodies.
Examples
of Surface Gravity
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Object
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Mass
(Earth=1)
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Diameter
(Earth=1)
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Gravity
(Earth=1)
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Moon
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0.0123
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0.27
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0.17
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Venus
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0.81
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0.95
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0.91
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Mars
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0.11
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0.53
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0.38
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Jupiter
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317
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11.2
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2.54
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Sun
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333,000
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109
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27.9
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Aside
[In case you are interested]:
These
numbers are derived from Newton's law of
gravity and the force equation F = ma. We
typically designate the acceleration due
to gravity with the letter g, thus a = g.
Let your mass (or what every object you
want) be m and the mass of the planet or
star be M. Then g = F/m = GM/d2
gives the gravity. Rather than looking up
the value of G, the gravitational constant,
you can compute everything relative to the
earth (mass, diameter, and gravity). This
is done for the table above.
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Nuclear
Force
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- Holds
atomic nuclei together
- Glues
protons and neutrons together
- Very
short range
- Can
release tremendous amount of energy via fusion or fission
- Nuclear
energy
- Atomic
bomb
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Energy
and
Matter
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Einstein
showed there is an equivalence between mass and energy.
1000
kg of matter converted into energy
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Energy
from
Fusion
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 Fusion
is the combination of two or nuclei to form a heavier nucleus.
For
example, fusing Hydrogen into Helium releases energy by
the conversion of mass (since He is less massive than H).
Example
chemical reaction
Fusion
releases about a million times more energy per reaction.
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Aside
[Total conversion]:
Suppose
you had a car which you could go
20 miles using one gallon of gasoline.
A gallon is ~4
liters or ~4 kg. Burning one gallon
of gasoline in your car produces approximately
1.2x108 Joules. How far could
you go on one gallon of gasoline if you
could convert all the atoms directly to
energy?
From
E = mc2 we have E = 4 kg x (3x108
m/sec)2 = 3.6x1017 J.
This is 3x109 times more energy
than that released by normal combustion
of gasoline.
Thus you could travel over 60 billion miles
on one gallon! Of course, this exercise
is only hypothetical since we cannot convert
all
the mass to
energy. |
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Nuclear
Fission
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Fission
is the breaking up of nuclei
Uranium
fission:
Hiroshima
Atomic Bomb
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Energy
Release
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Energy
from Fusion and Fission
Energy
is released when elements lighter than iron are fused together.
Likewise energy is released when element heavier than iron
are split apart (fission). Conversely, to fuse heavier elements
or split light elements requires extra energy.
The
plot below shows the "binding energy" per nucleon
versus mass number. Iron is the most tightly bound nucleus.
This means that moving towards iron releases energy. It
is like a ball rolling to the lowest point.
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Weak
Force
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The
Weak Force is responsible
for the radioactive decay of certain kinds of particles.
The
decay of the neutron is an example of the weak interaction.
n
--> p + e- + n
The
neutron decays into a proton, electron, and a neutrino (actually
an antineutrino). The neutrino is very low (perhaps zero)
mass particle that interacts very little with matter.
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Unified
Theories
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The
"standard model" of particle physics brings together
quantum chromodynamics (QCD) and electroweak theory. QCD
describes the strong (force) interactions under the hypothesis
that all strongly interacting particles are made of quarks.
The electroweak theory unifies the weak and electromagnetic
interactions. Together general relativity and the "standard
model" appear to explain all known physics.
Unified
Theories attempt to bring together our understanding of
physics one step further.
Grand
Unified Theories (GUTs)
- The
strong, weak, and electromagnetic interactions are unified.
- GUTs
have the basic idea that these three forces are really
manifestations of one force. [If we could do experiments
at very high energies (1014 to 1015
GeV) then there would be one force, not three.]
- However,
even the simplest GUTs have over 20 free parameters (such
as the charge of the electron) that must be determined
experimentally.
- The
ultimate GUT would have only a few or perhaps no adjustable
parameters.
Theory
of Everything (TOE)
- The
idea behind a TOE to include gravity as well.
- This
would unifies all 4 forces, and perhaps create a theory
in which there are fewer adjustable parameters.
- This
hasn't been done yet
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TOEs
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The
tree below shows the hiearchical structure for GUTs and
TOEs.
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