Transcript No Slide Title

Modern picture of space-time, matter &
forces
• What is reality in relatively and QM? (Twin
paradox and Schrodinger Cat)
• How about concept of vacuum as a
“medium”?
• How are space, time, matter and force
related to each other in General Relativity
or Quantum Physics?
Space & Time in Relativity
• Reality in relativity - everyone has “same”
observation on what has happened except
they can disagree on duration, timing
ordering and distance (space) between
events!
Space & Time in Relativity
• Space-time = a “strange” medium that can
be deformed by matter (General Relativity)
Motion in General Relativity
• A mass creates space-time curvature, and
moves according to how space-time is
deformed.
Reality in Quantum Physics
• Particle-wave duality - particle is wave
(obeys principle of superposition) and wave
is particle,  Feynman’s sum-over history
interpretation
• Space-time = a “strange” medium that
supports this picture
Quantum Field Theory-The
origin of particles(matter &
force) in Quantum World
e-
n
p
ve
First of all, the vacuum of our
universe is a kind of “field”(medium)
obeying (special) relativity ( ether)
There are motions of field = waves
Quantum Mechanics
( wave-particle duality) 
Corresponding particles exist!
There are motions of field = waves
Notice that the nature of particles are
determined by the media in quantum mechanics
For example: photon = particle
from electromagnetic medium
Electron/positron = particle
from a medium called Dirac
Fermion Field
When a particle “travels”, it’s actually a
disturbance of the field;
Force = interaction between different fields
For example: photon = particle
from electromagnetic medium
Electron/positron = particle
from a medium called Dirac
Fermion Field
 Matter, Forces (= field ~ disturbance
of our space-time) become all
inseparable!
Unified-Field Theory
Einstein:
Can one understand
Space, time, Force, Matter
in one unified framework?
Unified-Field Theory
Can one understand
The origin and fate of our universe
with this unified framework?
Unified-Field Theory
Complication:
(1)There are more kinds of particles
and forces discovered!
(2)General Relativity and Quantum
Physics dislike each other!
A brief history of particles & forces
• At early 20th century, people believe that
there are only 2 fundamental forces, gravity
& electromagnetism.
• Moreover, all matters are composed of
protons, neutrons, electrons, photons and
(perhaps) gravitons.
Problems:
• A lot of “new” particles are discovered
when people probe into the structure of
nuclei - e.g. muon, tau, pion, kaon, eta,
lambda, sigma, omega,…….
• Moreover, these particles interact with new
forces
• More fundamental problem: QM seems to
dislike GR and vice versa.
Experiments at Accelerators :
New particles are discovered!
After 30+ yrs….
• with great effort, scientists are able to
“group” all particles into different classes:
• (Like grouping different atoms into a
periodic table)
• First of all, there are
• Leptons (electron, neutrino, etc.)
(electromagnetic & weak interaction)
The Story cont...
• Hadrons (neutrons, protons, mesons, etc)
which themselves are composed of quarks
(strong and electromagnetic interaction)
• (there are 6 kinds of quarks, u,d,s,c,b,t)
• Vector bosons (force mediator), including
photons, gluons, z- and w-bosons, etc.
Summary (the Standard Model)
There are 3 kinds of forces
(Strong, weak and electromagnetic)
governing our microscopic world
&
particles grouped in “generations”
In addition there is gravitational force
The Standard Model (fermionic)
elementary particles
1st
generation
Particle
Mass/mp
2nd
generation
Particle
Mass/mp
3rd
generation
Particle
Mass/mp
electron
0.00054
Muon
0.11
Tau-on
1.9
Electron
neutrino
< 10
Muon
neutrino
< 0.0003
Tau-on
neutrino
< 0.033
Up
quark
0.0047
Charm
quark
1.6
Top
quark
1.89
Down
quark
0.0074
Strange
quark
0.16
Bottom
quark
5.2
-8
Question: can this “grouping” be
further simplified?
e
-
Yes (?)Grand Unified Theory.
e
Nowadays it is believe that
the 3 “microscopic” forces (except
gravity) are so called Yang-Mills Gauge
forces and have a common origin.
n pv
Yang-Mills Gauge force
e
-
n pv
e
Yang-Mills Gauge forces
are tied with a particular symmetry called Gauge Symmetry in Quantum
Physics
(Standard Model + GUT)
• We can unified electromagnetic, weak and
strong interactions!
• (But) We don’t understand why there are 3
(or more) generations of “fundamental”
particles ) characterize by 19 constants in
our universe
• Gravity does not fit in this picture
Directions where physicists are working!
(1)The origin of so many species of
particles - supersymmetry and string
theory, others
(2)Unification of Gravitational force with
other forces - string theory, others.
… They are related to Cosmology!
What is supersymmetry?
What is symmetry?
Special relativity: the physical laws
are the same in all inertial reference
frames.
General relativity: the physical
laws are the same in all reference
frames
Symmetry: physical law is the same
under certain transformation
Simpler Example: translational invariance 
Kinetic energy cannot dependent on position.
It can depend only on velocity.
Potential energy between 2 particles depends
only on the distance between them and
nothing else.
Symmetry: physical law is the same
under certain transformation
Example:Lorentz invariance (special
relativity) 
E2 = (pc)2 + (rest energy)2
More symmetry more restrictions on the
plausible form of physical law
When special relativity is combined with
quantum field theory, the form of physical
law is pretty much fixed. Besides Gauge
Symmetry, it allows only 1 more symmetry
Supersymmetry
What are they?
• Roughly speaking:
• Gauge symmetry is a symmetry associated
complex numbers  a.ei used in quantum
mechanics
• Supersymmetry is associated with the
presence of 2 forms of matters in our
universe: fermions (hadrons & leptons) and
bosons(force mediators)
String + supersymmetry
• One way to explain the “hierarchy” of
particles by modifying the concept of
“field” in Quantum physics.
• The field (medium) at small length scale is
replaced by loops of strings!
• Different mode of oscillations of string +
Supersymmetry Different particles
String + supersymmetry
• Advantage: Can also make gravity existing
peacefully with Quantum Mechanics(very
difficult otherwise)
• At small distances (Planck Scale ~ 10-33cm)
space & time looks very different (7
dimensional “curled” (Calabi-Yau) space +
3 dimensional flat space + time)
Others:
• Lorentz invariance has to be modified in
small distance (less restriction placed on the
form of physical law)
•  NEW possibilities!
• Space-time looks different at small (Planck
scale) distances
New possibilities:
• Condensed Matter Physics =
• Creating our own universe and physical
law! (e.g. Quantum Hall effect, Strongly
Correlated electrons, Nanotubes, etc.)
• Lots of new phenomena
Cosmology
• We believe that our universe started from a
“Big Bang”
• Much of the Big Bang process can be
explained by Standard Model or Grand
Unified Theory
• There are a few things which needs more..
Cosmology
• (1) To describe very early Bang Bang when
universe size is of order of Planck scale we need Quantum Gravity
• (2) We also need Quantum Gravity to
understand Black Hole
• (3)Cosmological Constant problem
• (4)Others….
Cosmology
• (1) String theory proposed that the universe
has a MINIMUM size ~ Planck scale.
• It’s not clear what happens “before”
universe is of that size.Many possibilities
are suggested...
Cosmology
• (2)- String theory suggests that black holes
may have internal structure. Some theories
suggest that they are similar to elementary
particles. String theory also provide the
possibility of baby universes within black
hole.
Cosmology
• (3)Cosmological constant
• It’s believed to be a small (nonzero) number
from observation - most plausible
explanation is supersymmetry. Without
supersymmetry the cosmological constant
should be ~10100 times larger than
observed.
~ End ~
(for now)
Many more new things to learn if you
want to understand this.