Modern PhysicsModern Physics(PC300)(PC300)

Class #2 Class #2 Review Einstein and NewtonReview Einstein and Newton

The Ether questionThe Ether question

How to draw Events on a Spacetime diagramHow to draw Events on a Spacetime diagram

Maxwells EquationsMaxwells Equations

->"Light must travel at 3*10->"Light must travel at 3*1088 ms ms-1-1 relative to….er….er….ether, yes that's it! In relative to….er….er….ether, yes that's it! In all other reference frames light would be observed to travel at different all other reference frames light would be observed to travel at different speeds."speeds."

- Maxwells private moment of astonishment, sometime last century.- Maxwells private moment of astonishment, sometime last century.

Maxwells Four EquationsMaxwells Four Equations

1865 - 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)

EtherEther

Is it real?Is it real?

Lab Extension Idea: Assess literature claiming to still support ether

http://pages.sbcglobal.net/webster.kehr/

http://ca.geocities.com/rayredbourne/

Michelson Morley ExperimentMichelson Morley Experiment

MORE IN LAB

NULL RESULT

Michelson ExplanationMichelson Explanation

Downwind

c v

c + v

v c

c - v

Upwind

√(c2 - v2) c

v

Acrosswind

Resultants

http://www.germanheritage.com/biographies/mtoz/michelson.html

Kennedy Thorndike Experiment Kennedy Thorndike Experiment (1932)(1932)

More accurate experimentsMore accurate experiments

Brillet-Hall expt - 1979

More EvidenceMore Evidence

Einsteins Logical ThinkingEinsteins Logical Thinking

Ether is "too problematic" -> throw it out!Ether is "too problematic" -> throw it out!

Maxwell's Equations are frame independent. Maxwell's Equations are frame independent. (Why is this necessary?)(Why is this necessary?)

Einsteins Logical Thinking 2Einsteins Logical Thinking 2

Light travels at Light travels at cc in a "vacuum" - no special in a "vacuum" - no special frame.frame.

Light travels at Light travels at cc in every frame; hence is in every frame; hence is frame independentframe independent

Maxwell's equations fit with principle of Maxwell's equations fit with principle of relativity.relativity.

Galilean Transformation Galilean Transformation Equations must be wrong!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 What is the consequence of this hypothesis?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 DiagramsSpacetime Diagrams

x

t

Slope=

xA

tA

Event A

y

tA

xA

Event A

yA

inverse velocity

Worldline

Spacetime MoviesSpacetime Movies

x

t

*E

Radar MethodRadar Method

x

t

tA

tB

masterclock

worldline

)t-(t2

1x ABE

))t(t2

1t ABE

EventtE

xE

MooreMooreChapter 2 Chapter 2

2 Minute Questions2 Minute Questions

Wow! This course is moving!Wow! This course is moving!Homework Set #1 Due tomorrow morningHomework Set #1 Due tomorrow morning

Lab class today…assignment for next week Lab class today…assignment for next week in lab and notebooks…in lab and notebooks…

Simulation #1 Due on FridaySimulation #1 Due on Friday

The next week you have a second The next week you have a second Problems Set…and Lab…Problems Set…and Lab…

Read the text…are you finished your first Read the text…are you finished your first read through yet?read through yet?

Thank goodness this is 6 credits! (For Thank goodness this is 6 credits! (For those in PC300)those in PC300)

Homework QuestionsHomework Questions

Problems from Moore. See Problems from Moore. See problem assignment problem assignment sheet!sheet!

Reading!Reading!

Albert Michelson

Next Wed by 8:00 amNext Wed by 8:00 am

Concept of TimeConcept of Time

Coordinate Time (Coordinate Time (t)t)Time between events in given Time between events in given

inertial frame (frame dependent)inertial frame (frame dependent)

Proper Time (Proper Time ())One clock present at both One clock present at both

events (frame independent)events (frame independent)

Spacetime Interval (Spacetime Interval (s)s) One clock present at both One clock present at both events whose worldline is inertial events whose worldline is inertial (frame independent)(frame independent) 1. Proper Time?

2. Spacetime?3. Coordinate Time?

Who measures:

Newton & Einsteins view of timeNewton & 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 EinsteinFrom 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.