Modern Physics (PC300) Class #2 Review Einstein and Newton The Ether question How to draw Events on a Spacetime diagram

Maxwells Equations ->"Light must travel at 3*10 8 ms -1 relative to….er….er….ether, yes that's it! In all other reference frames light would be observed to travel at different speeds." - Maxwells private moment of astonishment, sometime last century.

Maxwells Four Equations predicted waves of oscillating electric and magnetic fields travel through empty space at a speed 310,740,000 m/s. "This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws. " 1. Electric charges produce electric fields (Gauss's law), 2. Experimental absence of magnetic charges (Gauss's Magnetism Law), 3. Currents produce magnetic fields (Ampère's law) 4. Changing magnetic fields produce electric fields (Faradays Law + Maxwells Extension)

Ether Is it real? Lab Extension Idea: Assess literature claiming to still support ether

Michelson Morley Experiment MORE IN LAB NULL RESULT

Michelson Explanation Downwind cv c + v vc c - v Upwind √(c 2 - v 2 )c v Acrosswind Resultants

Kennedy Thorndike Experiment (1932)

More accurate experiments Brillet-Hall expt

More Evidence

Einsteins Logical Thinking Ether is "too problematic" -> throw it out! Maxwell's Equations are frame independent. (Why is this necessary?)

Einsteins Logical Thinking 2 Light travels at c in a "vacuum" - no special frame. Light travels at c in every frame; hence is frame independent Maxwell's equations fit with principle of relativity.

Galilean Transformation Equations must be wrong! Einstein Postulates: 1: All laws of physics, and all numerical constants that appear in them, are exactly the same in all inertial reference frames. 2: The speed of light in empty space always has the same value c.

What is the consequence of this hypothesis? " Time is NOT absolute. Two people will measure different times of light moving from one place to another, and disagree on the distance traveled BUT would agree on velocity."

Spacetime Diagrams x t Slope= xAxA tAtA Event A y tAtA xAxA yAyA inverse velocity Worldline

Spacetime Movies x t * E

Radar Method x t tAtA tBtB master clock worldline EventtEtE xExE

Moore Chapter 2 2 Minute Questions

Wow! This course is moving! Homework Set #1 Due tomorrow morning Lab class today…assignment for next week in lab and notebooks… Simulation #1 Due on Friday The next week you have a second Problems Set…and Lab… Read the text…are you finished your first read through yet? Thank goodness this is 6 credits! (For those in PC300)

Homework Questions Problems from Moore. See problem assignment sheet! Reading! Albert Michelson Next Wed by 8:00 am

Concept of Time Coordinate Time (  t) Time between events in given inertial frame (frame dependent) Proper Time (  ) One clock present at both events (frame independent) Spacetime Interval (  s) One clock present at both events whose worldline is inertial (frame independent) One clock present at both events whose worldline is inertial (frame independent) 1. Proper Time? 2. Spacetime? 3. Coordinate Time? Who measures:

Newton & Einsteins view of time Isaac Newton was right about some of his ideas about time but he was wrong about others. For instance, he said that time is absolute. His theory for this was that if a pulse of light is sent from one place to another, different observers would agree on the time it took for the light to get from one place to the next (since time is absolute), but the two observers would not always agree on the distance the light traveled (since space is not absolute). Since the speed of light is simply the distance divided by time, different observers would measure different speeds of light. This is where Einstein disagreed with Newton, because in Einstein's relativity all observers must agree on the speed of light because it never changes. In Einstein's theory, since the two observers don't agree on the distance the light has traveled, they must disagree on the time it took for the light to go between the two places. The time taken is the distance the light traveled, which the observers don't agree on, divided by the speed of light, which they do agree on. In other words, Einstein put an end to the idea that time is absolute. Since time is relative, space must be relative too.

From Newton to Einstein Newton: Time is absolute. Two people will measure the same time between events, but disagree on distance and thus velocity. Einstein: Time is NOT absolute. Two people will measure different times of light moving from one place to another, and disagree on the distance travelled BUT would agree on velocity.