Mr. Bouyer
- Day 3

1. Electromagnetic force:
    
   
   Like charges repel, opposite charges attract.
   
    
   
2. Strong nuclear force:
    
   
   Opposes the electromagnetic force.

   Keeps protons in the nucleus from flying apart.

   The strongest of the four forces, it acts only over the distance of an atomic nucleus (about 10^-13 cm).

   
    
   
3. Weak nuclear force:
    
   
   The force responsible for beta decay.

   During beta decay, a neutron emits an electron and an antineutrino, converting itself into a proton.

   The weak force works over distances so small that it is essentially a "contact" force.

   
    
   
4. Gravity:
   The force of attraction between all objects in the universe.

   Although gravity is the weakest of the four forces, it is additive. Every speck of matter added to an object contributes to the overall gravity of the object. Gravity is a very long range force, falling off at a rate of 1 over the square of the distance between two objects. In principle, gravity extends to infinity.

Grand Unified Theory (GUT):

Any gauge theory (a theory that describes a field that can be measured from different baselines without affecting its properties) that attempts to combine the description of the electromagnetic force, the weak nuclear force, and the strong nuclear force in one mathematical package is called a grand unified theory. All of the candidate theories investigated so far have flaws. No GUT has yet even attempted to include gravity in the package.

Theory of Everything (TOE):

String theories are a class of theories in physics that describe the fundamental particles and their interactions in terms of tiny one-dimensional entities - strings. Unlike GUTs, a true TOE must include gravity as well. Some scientists think string theories may one day bring all particles and forces into one theory, but others think it has little chance of success.

Gravity and Black Holes:

Einstein's "general theory of relativity" explains the force of gravity as a result of the way spacetime is curved in the vicinity of matter. In 1916, Karl Schwarzschild used Einstein's theory to calculate the exact spacetime around a spherical mass. This showed that for any mass there is a critical radius, now called the Schwarzschild radius, which corresponds to such an extreme distorsion of spacetime that, if the mass were to be squeezed inside the critical radius, space would close around and pinch it off from the rest of the Universe. It would, in effect, become a self-contained universe in its own right, from which nothing (not even light) could escape. John Wheeler first called this a black hole in 1967.

The Schwarzschild radius corresponding to any mass M is given by the formula 2GM/c², where G is the constant of gravity and c is the speed of light. The Schwarzschild radius for the Sun is 2.9 kilometers. The Schwarzschild radius for the Earth is 0.88 cubic centimeters.

Theoretical studies show that a black hole has just three properties that define it:

mass
electric charge
rotation (angular momentum)
• An uncharged, non-rotating black hole is described by the Schwarzschild solution to Einstein's equations.
• A charged, non-rotating black hole is described by the Reissner-Nordstrom solution.
• An uncharged, rotating black hole is described by the Kerr solution.
• A charged, rotating black hole is described by the Kerr-Newman solution.

Real black holes are likely to be rotating and uncharged, so the Kerr solution is the one of most interest to astronomers.

As you can imagine, the physics of the natural forces is quite complex. You will learn more about force later this year.

In-class Assignment 083:
This assignment must be turned in by the end of class today to receive credit.
Scoring criteria

1. Perform an internet search for "strong force". Write one paragraph summarzing this force in atoms.
2. Not all stars collapse to form a black hole when they die. Perform an internet search for black holes and write one paragraph explaining why our Sun is one of those.

Research Links:

• Fundamental Interactions - A Wormhole in the Cosmos
• A Unified Physics by 2050? - Scientific American Magazine
• Nuclear Physics - Florida State University
• Nuclear Properties - Los Alamos National Laboratory
• The World's Highest Energy Accelerator - Fermilab
• U. S. Nuclear Data Network - Brookhaven National Laboratory
• Sky Den - Digital Education Network

Physical Science

The graviton, the force-carrying particle for gravity has never been found.
If the current atomic theory is correct, the graviton must exist.