Download Superstrings: The “Ultimate Theory of Everything”? Sera Cremonini

Superstrings:
The “Ultimate Theory of Everything”?
Sera Cremonini
Michigan Society of Fellows
Physics Department, University of Michigan
International Year of Astronomy Lecture Series
WVU, April 16 2009
Outline
¾
The Structure of Matter:
¾
the Building Blocks of Nature
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Why Quantum Gravity?
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String Theory Basics
¾
Recent String Theory Developments:
¾
¾
Black Holes in String Theory
Towards Cosmology
Some Old Questions…
¾
What are we made of?
¾ Basic building block of nature?
¾ What holds us together?
Earth
Air
Fire
Water
Empedocles
(490 BC – 430 BC)
Science has found smaller and smaller building blocks
Chemistry reduces all matter to known atoms (elements)
but atoms are not fundamental
A lot of structure inside the atom:
Smaller distance
(higher energies)
Probing higher and higher energies we have found
a large collection of elementary particles (“particle zoo”)
Fundamental
building block?
Where is this done?
¾ particle accelerators: our best microscopes!
Alps
LHC@CERN
What holds it all together?
Four known interactions:
¾
Gravity
¾ Electromagnetism
more familiar
¾
Nuclear weak
¾ Nuclear strong
only visible if we
probe deep inside the nucleus
Gravity does not fit in!
Described by
the SAME theory
(STANDARD MODEL)
BUT the Standard Model is an INCOMPLETE theory:
¾
¾
¾
How do particles get their masses?
Nature of dark matter?
Are quarks (and leptons) fundamental?
Why doesn’t gravity fit in?
Standard Model
General relativity
¾
¾
based on quantum mechanics
(deterministic)
(probabilistic)
¾
works well at small scales
(atoms, electrons, quarks)
classical theory of gravity
¾
works well at large scales
(planets, falling apples)
work well in opposite regimes
Why de we insist on combining QM with GR?
In General Relativity (Einstein):
¾ matter causes space to curve
¾ curvature tells matter how to move
Geometry is dynamical !
The heavier the object, the more curved the spacetime
Very massive objects can tear the fabric of spacetime
give rise to black holes
General relativity
breaks down here
Quantum effects are equally important
Why Quantum Gravity?
When
¾ size is small
¾ curvature very large
general relativity is not a good description, and
quantum effects can’t be ignored
Examples:
¾ very early universe (after “Big Bang”)
¾ interior of black holes (which exist!)
To understand these, we need
fundamental theory of quantum gravity
PART II
Why Strings?
¾
String theory incorporates naturally
general relativity and quantum mechanics:
z leading candidate for a consistent theory of
quantum gravity
Idea remarkably simple:
fundamental unit:
tiny vibrating string !
How Small is a String? (very)
1018
=
Corresponds to
VERY HIGH ENERGIES!
x proton mass (1018 GeV )
10000000000000000000000000000000
times larger than a string
Can We Make Strings in the Lab?
No!
¾
Energies are too high to produce them at accelerators
Estrings ~ 1014 ELHC
LHC ~ 104 GeV
Strings ~ 1018 GeV
¾
Indirect testing is where efforts are concentrated
z Footprints of string theory ingredients
at lower energies?
¾
Cosmology may help
How do we get protons, electrons, etc?
different vibrations
modes of string
(harmonics)
different particles
(quarks, electrons, etc)
Analogy with
standing waves
on a string
Graviton is one
of vibration modes
Why do we live in 3+1 dimensions?
Strings like to live in 10 dimensions
Why do we see only four (space-time) ?
The other 6 can be very small and curled up
into tiny balls
Small enough that we
can’t see them
Compact dimensions can have different shapes:
Their size and shape determine the properties of
elementary particles and the “constants of nature”
Not Just Strings
Other ingredients are allowed in string theory:
membranes or “branes”
Example I: Large Extra Dimensions?
¾
The extra dimensions may be large
z we may live on a brane:
a 3-dimensional surface in 10 dimensions
Standard
Model
Parallel
Universe?
Example II: Cyclic Universe?
Time may not have a beginning:
the Big Bang could have been preceded by a collapsing phase
Big Bang:
collision between two brane worlds
Punchline
We have a tool box:
¾ Open strings
¾ Closed strings
¾ Branes
Goal:
use these tools to make realistic models
of our universe
Next:
“Applications”
(current research in the field)
I. Branes and Black Holes
General relativity breaks down
Quantum Mechanics tells us :
black holes are NOT COMPLETELY BLACK
¾
emit radiation (Hawking radiation)
(they lose mass and evaporate)
¾
have a temperature and entropy
Why should we care about the entropy?
Entropy counts number of
microscopic configurations of a system
Black Hole Entropy:
¾
gives insight into microscopic structure of the
black hole
Old Puzzles:
¾
What happens to what falls in?
¾
Is information lost?
¾
Can we calculate entropy?
1970’s:
approximate calculations for entropy
(Bekenstein-Hawking)
Need full quantum gravity theory to get right answer!
String theory comes in:
Black hole: system of branes and strings !
Entropy calculations match!
and more…
Counts correctly microstates
that make up the black hole
¾
Big achievement for string theory:
¾
¾
¾
microscopic quantum description of a GR system
(in terms of strings and branes)
Framework for working on “information loss”:
¾
in the brane picture, there seems to be no
information loss
Cartoon Summary:
String theory can
describe black holes
and Hawking
Radiation
II. Testing String Theory with Cosmology?
The universe started out very small, hot and dense
As the universe expanded, it cooled.
If we look back in time, temperatures become higher
In very early stages of the universe, energies were
high enough for stringy effects to become important
Here
space was very, very small
quantum gravity (or string theory) regime!
Quantum mechanics tells us space is not smooth and
does not sit still – it fluctuates
ripples of spacetime
These small ripples are stretched by “inflation”
period of rapid
expansion
ripples
getting bigger
universe
inflating
Universe Expansion:
Picture of the early universe:
Remnant heat left over from Big Bang (CMB)
Today:
Picture of universe ~ 380,000 years after the Big Bang
(light takes time to reach us):
Temperature
Fluctuations
Density fluctuations in
primordial plasma
Bottom Line?
“picture” of the
early universe
Ripples (quantum fluctuations) contain info
about time when they were tiny
Æ stringy effects?
String theory and Cosmology?
¾
Stringy effects might leave imprint
on cosmological observations
¾
Cosmology may rule out some string
models:
z too much gravitational waves?
z inflation?
¾
Cosmology gives us picture of early
times
very high energies !
(our strongest microscope)
Big challenge: universe that changes over time
Summary
¾
Quantum gravity is needed, string theory does the job
¾
Recent developments:
insight into gravity/quantum connection
(structure of black holes, singularities)
¾
Many challenges:
• we need more realistic models
(particle physics and cosmology)
• stringy imprints we may measure?
¾
Much more one can say…
Thank You!