Mississauga Centre RASC
Day: Friday February 8, 2008
Chair: Randy Attwood
Speaker: Amanda Peet
A Gentle Introduction to Modern String Theory
Amanda Peet was born in Great Britain, did her undergraduate training in New Zealand,
studied at Stanford in California, and now is at the Physics Department at the University
In science, there are two groups of people: the measurers and experimentalists. As a
subatomic physicist, Amanda Peet wishes to determine patterns and forces. She
described powers of 10 for the universe from 10-33 cm being the Planck length, to the
Hubble scale of 1028 cm. The smallest distance that we are currently able to look at is
10-18 cm. We want a description of the physics that covers the scale of 60 powers of 10.
It is an audacious idea that there may be a set of equations that can describe the entire
universe at any scale. The universe is a dynamic place with the Big Bang happening 14.3
billion years ago and the expansion continuing.
Physics of the micro-world are uncertain. Quantum weirdness gives a jitter to things with
uncertainty at small distance scales. Even at absolute zero there is still some wiggling.
Cell phones and blackberries make use of this. The Heisenberg Uncertainty Principal
allows us to be quantitative so that ∆t∆E≥h (time vs energy) or ∆x∆p≥h (space vs
momentum). If one quantity is specified with greater precision, the other is specified
with less. Even at absolute zero we can never turn off the uncertainty. Classical physics
isn’t quite correct but with a large mass it is not noticeable. Any particle with a
momentum has a wavelength as per the de Broglie equation of λ=h/p (wavelength
equals the Planck constant divided by the momentum). Particles can be described as
There are only 2 invariants that enable us to tell particles apart: mass and spin.
Consequently, physicists spend a lot of time measuring mass, spin, gravity and electric
charge. The fermions, or matter, with a spin of ½ obey the Pauli exclusion principle
whereas the boson, or interaction-transmitters have a spin of 0, 1, 2. Amanda then
described the four forces: strong and weak nuclear, gravity and electromagnetic with their
carrier particles. Gravity affects everything in the universe that has energy and this
affects the evolution of the universe, and yet gravity is the weakest but has infinite range.
The “constants” describing the strength of the forces are not really constant but change as
the energy increases so that eventually the strengths of the forces become equal. To test
high energy physics atom “smashers” accelerate small particles in opposite directions
near the speed of light, and the debris is measured. Now the biggest particle accelerator
is being built near Geneva (Large Hadron Collider). 6,000 physicists from 84 countries
are involved in this major effort.
What does the physics of the very small have to do with the very large? If the universe
started from a tiny fireball, it connects the universe of the very big 14 billion years later
in an interplay between micro and macro. The WMAP satellite mapped the microwave
radiation from the Big Bang – radiation that shows up as noise of TV channels which
have no other signal, and looked at differences in appearance, or ripples, in various
places. When we survey “stuff” in the universe, we find that heavy elements make up
0.03% of the mass, neutrinos make up 0.3%, stars 0.5%, hydrogen and helium 4%. Dark
matter composes 25% and dark energy 70%. What make up 95%? We don’t know what
Strings allow us to bring together quantum mechanics and relativity. Einstein’s theory of
gravity breaks down at the Big Bang and in black holes coming up with the answer of
“infinity”. In addition, quantum mechanics cannot deal with the scattering of gravitons.
Gravity at short distances or high energy gives nonsensical answers.
Now, particles can be described as vibrating strings. Neutrons look different than
electrons because their strings vibrate in a different way. If vibrating strings rather than
dot particles are used in calculations, the equations make more sense. The string
hypothesis posits that strings, with open ends or joined together, vibrate in various ways
and this determines what they describe. They can describe all four forces including
gravity, and particles that exist. Strings are postulated to be between 10-18 cm which is
the smallest that we can observe and 10-33 cm, which is at the Planck scale.
Finally, Amanda described the possibility of the Big Bang starting when two “branes”
crashed into each other and dumped a lot of energy into our universe.
Submitted by Chris Malicki, Secretary