Transcript Gravitation KAZAKH NATIONAL UNIVERSITY

Selected Chapter of Nuclear Physics and
Nuclear Astrophysics
N. Takibayev, V. Kurmangaliyeva
Chair of theoretical and nuclear physics
Lectures 1 and 2
Four fundamental interactions are conventionally recognized:
gravitational,
electromagnetic,
strong nuclear,
and weak nuclear.
Everyday phenomena of human experience are mediated via
gravitation and electromagnetism.
Fundamental interactions, also called fundamental forces or
interactive forces, are modeled in fundamental physics as patterns of
relations in physical systems, evolving over time, that appear not
reducible to relations among entities to a more basic relation.
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Lectures Focus
Gravitational Field
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Lectures Focus
Gravitational Field - Historical facts
Heliocentric Theory
Nicholas Copernicus (1473 – 1543)
All planets, including Earth, move in orbits around the
sun
The gravitational force is given by:
F
Gm1m2
r2
where G is a constant called the Universal Gravitational
Constant
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Lectures Focus - Gravitational field strength
• It is also called the field intensity
• The gravitational field strength at a point is the
gravitational force acting on a unit mass placing at
the point
Gravitational field
gravitational force
, g
strength
mass



Unit: N kg-1
On the earth’s surface, g  10 N kg-1
On the moon’s surface, g  1.7 N kg-1
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Lectures Focus : Gravitational potential
• Due to the existence of the field, a net
amount of work has to be done to move a
unit mass from one point to another. We
say that different points in the field have
different gravitational potential
• It represents the work done in taking a unit
mass from one point in a g-field to another
• Unit: J kg-1
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Lectures Focus - Equipotential surfaces
• Surfaces containing points having the same
gravitational potential
• The spacing of the equipotential surface is
an indication of the field strength
• The g-field is pointing in the direction of
decreasing gravitational potential
Mm
• Mathematical form:
FG 2
r
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In modern physics, gravitation is the only fundamental interaction still
modeled as classical/continuous (versus quantum/discrete).
Acting over potentially infinite distance, traversing the known
universe, gravitation is conventionally explained by physicists as a
consequence of space-time's dynamic geometry, "curved" in the
vicinity of mass or energy, via Einstein's general theory of
relativity (GR).
Lectures Focus – Gravitation
In general relativity, the effects of gravitation are ascribed
to spacetime curvature instead of a force. The starting point for
general relativity is the equivalence principle, which equates free
fall with inertial motion, and describes free-falling inertial objects
as being accelerated relative to non-inertial observers on the
ground. In Newtonian physics, however, no such acceleration can
occur unless at least one of the objects is being operated on by a
force.
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Strong nuclear force
The strong interaction, synthesizing chemical elements via
nuclear fusion within stars, holds together the atom's nucleus, and is
released during an atomic bomb's detonation.
•
•
•
•
Strong nuclear
Electromagnetic
Weak nuclear
Gravitational
Strongest
Weakest
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Electromagnetic force
• Causes electric and magnetic effects
– Like charges repel each other
– Opposite charges attract each other
– Interactions between magnets
• Weaker than the strong nuclear force
• Acts over a much longer distance range
than the strong nuclear force
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Weak nuclear force
• Responsible for nuclear decay
• Weak and has a very short distance range
• The weak interaction is involved in radioactive
decay.
Note:
• The weak nuclear force is NOT the weakest of the
fundamental forces.
• GRAVITY is the weakest force, but most important
in understanding how objects in the universe
interact.
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The smallest pieces of matter…
• Nuclear physics and
particle physics study
the smallest known
building blocks of the
physical universe -and the interactions
between them.
• The focus is on single
particles or small
groups of particles, not
the billions of atoms or
molecules making up
an entire planet or
star.
… and their large effects …
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… affect us all.
– History: alchemy,
atomic weapons
– Astronomy: sunshine,
“metals”, cosmology
– Medicine: PET, MRI,
chemotherapy
– Household: smoke
detectors, radon
– Computers: the WorldWide Web
– Archaeology & Earth
Sciences: dating
Atoms, Periodic table
The periodic table lists about 114 atoms with distinct
properties: mass, crystal structure, melting point…
The range and pattern of properties reflects the
internal structure of the atoms themselves.
Inside Atoms: neutrons, protons, electrons
Carbon (C )
Atomic number Z=6 (number
of protons)
Mass number A=12 (number
of protons + neutrons)
# electrons = # protons (count
them!)
(atom is electrically neutral)
Gold (Au)
Atomic number Z = 79
Mass number
A = 197
# electrons = # protons
Properties of nucleons
Name
Spin
Mass
Electric Charge
Proton
1/2
𝑚𝑝 ~ 1 GeV
+1
Neutron 1/2
𝑚𝑛 ~ 1 GeV
but 𝑚𝑛 > 𝑚𝑝
0
• Units:
– The electric charge of an electron is -1 in these units.
– Mass units are “billion electron volts” where 1 eV is a typical
energy spacing of atomic electron energy levels.
• Question: Why are the masses nearly the same
but the electric charges so different?
Further layers of substructure:
u quark:
= 2/3
electric charge
d quark:
electric
charge = -1/3
Proton = uud
charge = 1
electric
Neutron = udd electric
charge = 0
Introducing the neutrino
Международная сеть центров ядерных реакций (NRDC)
Another subatomic
particle, the neutrino,
plays a crucial role in
radioactive decays like
n -> p+ + e- + ve
The ve (electron-neutrino) is closely related to the electron
but has strikingly different properties.
Name
Spin Mass
Electric Charge
electron
1/2
0.0005 GeV
-1
electronneutrino
1/2
< 0.00000001 GeV
0
Exotic Matter Particles
Other subatomic matter particles are heavier copies
of those which make up ordinary atoms (u, d, e, ve)
Sub-atomic interactions
• Two familiar kinds of interactions are
– gravity (masses attract one another)
– electromagnetism (same-sign charges repel, oppositesign charges attract)
More exotic phenomena hint at new interactions
peculiar to the subatomic world:
• What binds protons together into nuclei ?
– Must be a force strong enough to overcome repulsion
due to protons’ electric charge
• What causes radioactive decays of nuclei ?
– Must be a force weak enough to allow most atoms to
be stable.
Force
Strong nuclear
Electromagnetic
Weak nuclear
Gravity
Strength
1
.001
.00001
10-38
Carrier
Gluons
Photon
Z0,W+,WGraviton?
Physical effect
Binds nuclei
Light, electricity
Radioactivity
Gravitation
Subatomic particles interact by exchanging integer-spin
“boson” particles. The varied interactions correspond
to exchange of bosons with different characteristics.
Mass Mysteries
Otherwise similar particles are seen experimentally to
have very different masses (e.g. muon & electron).
Plotting masses in units of the proton mass (1 GeV):
Two "symmetry breaking" mysteries emerge:
• Flavor Whence the diverse fermion masses ?
• Electroweak Why are the W & Z heavy while the g is massless?
Higgs Mechanism
The Standard Model of particle physics postulates a
particle called the Higgs boson, whose interactions
give rise to all mass:
• During an earlier epoch of our universe, all the known
elementary particles were massless.
• The Higgs boson triggered a phase transition
(as when water freezes into ice) which caused all
particles interacting with the Higgs boson to become
massive.
• The W and Z bosons and the fermions are massive
because they interact with the Higgs boson.
• The photon and gluon remain massless because they
do not interact directly with the Higgs boson.
A variety of masses:
The more strongly a particle interacted
with the Higgs, the more mass it
would gain and the more inertia it
would display
The Higgs field would form a uniform
background within the universe. Each
particle would interact with the Higgs
boson to a different degree.
Acceleration & Steering
magnets
Protons will be accelerated
and collided in LHC. Two
beams will travel in
opposite directions.
Electric fields produce
acceleration because
like charges repel and
unlike charges attract
each other.
Magnetic fields steer the
beams of protons
because charged
particles move in circles
when exposed to
magnetic fields.
Detection
At four places around the
LHC ring, protons from
the two counter-rotating
beams will collide.
ATLAS
The collision energy
condenses into
particles (e-, p, p…)
Detectors surrounding
the collision point
are sensitive to the
passage of
energetic particles.
Higgs Detection: H -> gg
A Higgs decaying to 2 energetic
photons would be a striking
event in the LHC detectors.
events
Higgs
signal
ATLAS
The combined energies of the signal photons would
cluster at the mass of the Higgs boson. In contrast,
background events include photon pairs with a
variety of energies.
background
energy
References
• Lectures 1 and 2 (*.pdf)
•
•
•
•
http://en.wikipedia.org/wiki/Standard_Model
http://www2.slac.stanford.edu/vvc/theory/fundamental.html
Google images
http://en.wikipedia.org/wiki/Gravitation