1. Introduction Classical Physics Laws: Mechanics (Newton), Electromagnetism (Maxwell), Optics, Fluids,.. Etc. Modern Physics: What do we mean? Are the laws of physics different? Sizes of Objects: Radius of earth, distance between two cities, height of a boy, size of the class room, length of a pen, length of an ant, Speeds of Objects: turtle, man, car, plane, electron, light. Technology of the future: Nanotechnlogy.

2. History Before 1900: Newton’s mechanics was accepted for over 200 years. Along with Maxwell’s law of electricity and magnetism, scientist thought they know everything they needed to know about physics. Problems: Is light wave or particles? Does it need a medium to propagate? After 1900: Planck put the basic ideas that lead to the formulation of quantum mechanics. Einstein formulated the special theory of relativity Results: All contradiction were solved. Impact: Transistor, Nuclear bomb, radioactive medicine

3. Principle of Relativity Newton’s Laws and early ideas on space and time were formulated for big objects that move at speeds much less than the speed of light. Experimentally, it was found that Newton’s laws fail when the speed of the object approaches the speed of light. Example: if an electron is moving at a speed 0.99c then if you increase its kinetic energy by 4, you expect its speed to double (according to classical laws), but it does not. It does not exceed c. In 1905 Einstein wrote: The relativity theory arose from necessity, from serious and deep contradictions in the old theory from which there seemed no escape. Basic Postulates: 1.1. The laws of physics are the same in all reference systems which move uniformly with respect to one another 2.2. The speed of light in vacuum is always measured to be, and this value is independent of the motion of the observer or the source of light.

4. Newtonian Relativity Inertial frame of reference or inertial system is a frame in which a free body is not accelerating. Any system moving with constant velocity with respect to an inertial system must also be an inertial system. The laws of mechanics must be the same in all inertial frames of reference. Example: A ball thrown upwards in a moving truck. The event takes the same time in the laboratory frame and in the truck frame by using Newton’s second law. Although the experiment look different to different observers, the observers agree on the validity of Newton’s laws, conservation of energy and momentum. The only thing that differs is the RELATIVE motion between the frames. Absolute motion in space through meaningless. Galilean Transformation of coordinates and velocities

5. Galilean Transformation and the speed of light Do the concepts of Newtonian mechanics and the Galilean addition of velocities apply to light? In 1986, Maxwell showed that the speed of light in free space is 3.0x10 8 m/s. What does free space mean? People at that time thought that light, like all mechanical waves need a medium to propagate. They called the medium “ether”. The value calculated for c was in an absolute frame at rest with respect to “ether”. If it was calculated in a frame that is moving with respect to “ether” with velocity v, the value of c will change. Experiment: Use earth as a moving frame (30 Km/s) or “ether” is moving and earth is stationary. If v is the speed of “ether”, then the maximum speed of light will be c+v, if they are in the same direction and the minimum is c- v if they are moving in opposite directions.

6. Michelson-Morley Experiment Done in 1887 to prove the existence of “ether”, and to detect the shift in the speed of light caused by the motion of the earth through “ether”. The outcome of the experiment was negative. The experiment has been repeated many times and by various scientists under various conditions but still the outcome is negative. This means that no “ether” exists and classical Galilean transformation is not valid. In 1890, Fitzgerald and Lorentz proposed that the length of the object at a speed v would contract along the direction of travel.