Monday, January 18, 2010

Lecture 1

The primary goal of physics is to determine the nature of the universe: its size, its structure, its beginning. The observable universe is vast and incredible, and certainly there is much more to it than we can observe.

A main idea in physics is that elementary particles constitute the building blocks of matter.

The goal of particle physics is to discover matter's most basic constituents and the most fundamental physical laws obeyed by those constituents.

In a way, understanding nature at this level would mean understanding everything.

I - What are the fundamental particles?
Every time more accurate technological tools are developed, more elementary constituents have emerged.
This is as far as we have been able to break down matter so far.
Everything around you: the blackboard, the windows, your shoelaces, etc. is made up of electrons and up/down quarks.

The Standard Model (well established theory of particle physics) includes many other fundamental particles.
There are additional heavier quarks and heavier election-like particles that are nowhere to be found in ordinary matter. These particles have been discovered in high energy particle colliders that simulate the early universe (immediately after the big bang). The most powerful one, which many of you have probably heard of, is the Large Hadron Collider in Geneva.

Matter Particles

Force-carrying particles

The idea of force carrying particles comes out of quantum physics, which we will go over briefly later in this lecture. Notice that there is no particle associated with the force of gravity.

Questions raised by the Standard Model:
  • Why heavy particles?
  • Why such different masses?
It is very possible that matter may be broken down further; string theory postulates that there is a more fundamental constituent of matter: an elementary string whose vibrations determine the known particles.

II- The fundamental forces
It has been the belief of physicists for some time that all known interactions can be reduced to combinations of four fundamental forces:

1. Gravity
  • A force between objects with mass that depends on the mass of the objects and the distance between those objects
2. Electromagnetic
  • a unification of the electric and magnetic forces
  • Electric charges attract or repel one another with a force inversely proportional to the square of the distance between them: unlike charges attract, like ones repel. An electric current in a wire creates a circular magnetic field around the wire, its direction depending on that of the current.
  • holds electrons and protons together in atoms, and holds atoms together to make molecules.
  • The electromagnetic force is the one responsible for practically all the phenomena one encounters in daily life
  • all the forces involved in interactions between atoms can be traced to the electromagnetic force acting on the electrically charged protons and electrons inside the atoms
3. The Weak Force
  • The weak force is indeed weak; well actually it's about 10^25 times stronger than gravity but it only holds on very very small scales, so it doesn't affect most matter.
  • Explains some forms of nuclear decay, and is essential to many other nuclear processes
  • plays a role in the creation of heavy elements
  • is essential for stars to shine: kicks off the chain of reactions that convert hydrogen to helium
4. The Strong Force
  • extremely powerful force
  • so strong that particles that experience the strong force never exist in isolation: quarks are always bound together in threes.

Notice how much weaker gravity is than the other three forces. This fact is something that intrigues physicists and may come out of string theory.

Questions raised by the Standard Model:
  • Why FOUR forces? Are there others?
  • Why is gravity so much weaker than the other three forces?
III- What do we know about how these forces work?
A- Quantum Physics
Quantum physics comes out of several theories:
  • quantization
  • wavefunction
  • wave-particle duality
  • uncertainty principle
look out for these in the following video:

Quantum mechanics is very bizarre and unintuitive, mainly because quantum effects generally aren't significant at distances greater than the size of an atom. We can generally only see matter at larger scales than this, and further, particles are not isolated, forbidding many quantum effects to be noticed.
When large objects are involved, quantum mechanics agrees with classical physics, but classical physics will never generate quantum predictions.

It turns out that all matter consists of fundamental quanta.
Quanta are the basis of particle physics.

B- Relativity(Theory of Gravity)
Newton's laws allow accurate enough predictions to send men to the moon, satellites into orbit, and was responsible for the discovery of Neptune.
But, Newton's laws are not accurate enough for a functioning GPS system, which relies on General Relativity! Otherwise the system would accumulate errors of more than 10km/day.
(Einstein's theory is proof that abstract mathematical ideas can have incredibly practical applications)
Newton's theory fails for high speed/mass/energy

Newton's law of gravity vs. Relativity

Newton's law is not wrong, it is an approximation; it's still used often.

Special Relativity - comes from two assumptions
  1. Physical laws are the same for all observers (as long as one is not accelerating)
  2. The speed of light is a constant
The theory is described in 4 dimensions - 1 time dimension and 3 spatial dimensions, although relativity works equally well in 3,4,or 10 dimensions.
These 4 dimensions make up a "spacetime fabric" which is curved or "warped" by large masses.
For distances to be equal in spacetime, the more one travels in space, the less one travels in time.

Newton's theory suggests that to calculate the speed of light on a moving train, we would add the two speeds which violates (2.).

Newton's theory is intuitive while many of relativity's implications are unintuitive;
for example, moving and stationary clocks tick at different rates.

videos: simultaneity

General Relativity -- involves Gravity; relativity with acceleration
  • the effects of acceleration cannot be distinguished from gravity
  • the theory predicted how much light should bend because of the sun's influence
  • matter tells spacetime how to curve and spacetime tells matter how to move
Evolution in Theory of Gravity:
Newton formulated an equation giving the strength of gravity
Einstein showed how gravity works
String theory may answer the question of "why gravity?"

IV- Unification
Quantum mechanics allows the unification of all four fundamental forces EXCEPT gravity.
Using the standard model, there seems to be no way to unite all four forces.
String theory has the potential to do so.

V- Introduction to string theory

Primary source: Warped Passages by Lisa Randall

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