lecture 1 bh32 copy - insight cruises · rich wolfson prof of physics middlebury college scientific...

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Simply EinsteinA Mini-Course in Relativity

Rich WolfsonProf of Physics

Middlebury CollegeScientific American Travel

Bright Horizons 32July-August 2017





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Physics 101

1) A Brief History of Physics2) Einstein Asserts Relativity3) Time & Space in Relativity4) Space, Time, & Causality5) Black Holes & Gravitational Waves:

The General Theory


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A Brief History of Physics● ~1600 – 1750: Galileo, Newton,

and others develop a mechanical understanding of physical reality, based in laws of motion

● ~1750 – 1900: Maxwell and others develop an understanding of electromagnetic phenomena, including light

● ~1900 – Einstein, Bohr, and others develop modern understanding of physical reality

Motion!●Why study motion?

➧ Because without motion nothing would happen➧ Because to move is to movethrough space➧ Because to move takes time

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Questions about Motion

● What causes motion?

● Is there a “natural” state of motion?

Aristotle’s Answers● What causes motion?➧ On Earth, pushes and pulls (forces)➧ In the heavens, a “prime mover”

● Is there a “natural” state of motion?➧ On Earth, at rest as close as possible to Earth’s center

➧ In the heavens, motion in perfect circles● Different laws for different realms!

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The Universe According toAristotle & Ptolemy

Earth

Sun Planet

Earth

Sun Planet

The Universe According toAristotle & Ptolemy

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The Universe According to Copernicus (~1540)

Earth

Sun

PlanetEarth now less special!

Kepler & Galileo (~1600)Imperfection in the Heavens!

Sun

Orbit (highly exaggerated)

Planet

Orbits not circular

Blemished Sun

Jupiter’s moons Phases of Venus

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Galileo: Thoughts on Motion


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Galileo’s Answers● What causes motion?➧ No cause needed!➧ Force causes change in motion

● Is there a “natural” state of motion?➧ Uniform motion(straight line, constant speed)

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Isaac Newton (b. 1643)● 3 laws of motion: predictability!

➧ How forces cause change in motion

● Universal gravitation➧ Every object in the universe attracts every other object

● Celestial motion explained➧ But first invent calculus!

● One set of laws for all realms!● And…a relativity principle

Galilean/NewtonianPrinciple of Relativity● The laws of motion are the same

for anyone in uniform motion● You can play tennis on a cruise ship● Only relative motion matters● You can eat dinner on an airplane● “I am moving” is a meaningless statement● You can juggle on a moving cart● There is no experiment involving the laws

of motion that will answer the question “Am I moving?”

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RelativityPrincipleat thePalace Hotel,Copenhagen

Tennis: A Physics Experiment

www.virginia.edu/ims/photogallery/images/rip-tennis-ladies.jpg

On Earth

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On a Cruise Ship



On Venus

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On an Earthlikeplanet in a distantgalaxy, moving rapidlyaway from Earth

History of Physics…So Far● Newtonian Physics (~1600 – 1750)

➧ Unites the celestial and terrestrial realms under a common set of physical laws–Newton’s laws of motion–Universal gravitation–“Natural” state of motion is uniform motion

➧ Obeys a relativity principle: The laws of Newtonian physics (the laws governing motion) are the same for anyone who’s in uniform motion. Statements like “I am moving” and “I am at rest” are meaningless.

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What about Light?

● Newton (1600s): Light consists of Particles

● Huygens (1678): Light consists of Waves

● Young (1801): Experimental proof: It’s Waves

Something Waves DoThat Particles Don’t…● They undergo interference

Constructive interference(Waves reinforce)

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Something Waves DoThat Particles Don’t…● They undergo interference

Destructive interference(Waves cancel)

Young’s Proof that LightConsists of Waves● Passed light through two slits

to create two light sources● Observed interference of light

from the two slits, thus proving conclusively that light consists of waves

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What about Light?

● Newton (1600s): Light consists of Particles

● Huygens (1678): Light consists of Waves

● Young (1801): Experimental proof: It’s Waves

● But…waves of what?

A New Realmof Physics

● Electricity

●Magnetism

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Electric Charge● A fundamental property of matter● Comes in two kinds (B. Franklin)

➧ Positive (+)➧ Negative (–)

● Likes repel, opposites attract➧ Force weakens with distance

+ –Strong force (attractive)

+ +Weaker force (repulsive)

A New Realmof Physics

● Electricity ✔

●Magnetism

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Magnetism● Originates from moving

electric charge

● Results in forces on other movingcharges

The Field Concept● The old view: “action at a distance”

➧ Earth reaches out across empty space, pulls on Moon

● The new view:➧ Earth creates a “gravitational field” throughout space➧ Moon responds to the gravitational field at its location

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Electric and Magnetic Fields● Electric field

➧ Proton creates electric field➧ electron responds to field

Electric and Magnetic Fields● Electric field

➧ Proton creates electric field➧ Electron responds to field

● Magnetic field➧ Magnet or moving charge creates magnetic field➧ Magnet or moving charge responds to the field

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Electricity and Magnetism– So Far● Electricity

➧ Electric charges produce electric fields➧ Other charges respond to those fields

● Magnetism➧ Moving electric charges produce magnetic fields➧ Other moving charges respond to those fields

● With both electricity and magnetism➧ Electric charge is the source of the fields (so

far…)

Changing E&M Fields● Faraday (1831): Changing magnetic field

creates electric field➧ Basis of electric generators, computer disks,

credit cards…➧ Electromagnetic induction

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Electric & Magnetic Fields● Changing magnetic field creates

electric field

● Question: Does changing electric field create magnetic field?➧ Maxwell (1860s): It should!

● Then each field is a source of the other!

Maxwell’s Insight● Changing magnetic field creates

electric field● Changing electric field creates magnetic field● So: Changing electric and magnetic fields

regenerate each other and propagate through space—electromagnetic waves

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How Fast?● How fast do electromagnetic waves go? ● Maxwell’s theory says:➧ 186,000 miles/second, 300,000 km/sthe speed of light

So What is Light?● Light consists of

electromagnetic waves➧ Includes not only visible light but also radio

waves, microwaves, infrared, ultraviolet, x-rays, gamma rays

➧ All these waves are essentially the same; they differ only in wavelength

➧ All propagate (in vacuum) at c, the speed of light

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So What is Light?● Light consists of

electromagnetic waves➧ Includes not only visible light but also radio

waves, microwaves, infrared, ultraviolet, x-rays, gamma rays

➧ All these waves are essentially the same; they differ only in wavelength

Maxwell’s Equations

Equation What it says

∇⋅E = ρ / ε0 How charges attract/repel

∇⋅B = 0 No isolated magnetic poles

∇×E = − ∂

B∂t Changing magnetic field

produces electric field

∇×B = µ0

J + µ0ε0

∂E∂t

Changing electric field produces magnetic field

And God said…

…and there was light

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A Brief History of Physics● ~1600 – 1750: Galileo, Newton,

and others develop a mechanical understanding of physical reality, based in laws of motion �

● ~1750 – 1900: Maxwell and others develop an understanding of electromagnetic phenomena, including light �

A Nagging Question…

● With respect to what does light go at speed c?

➧ Possible answer:–With respect to its source

–Ruled out by observations, especially of double stars

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A Nagging Question…

● With respect to what does light go at speed c?➧ Possible answer:

–With respect to its source–Ruled out by observations, especially of double stars

● Equivalent question:In what frame of reference are the laws of electromagnetism (Maxwell’s equations) valid?

Corresponding Answersfor Newtonian Physics● With respect to what does a wave (sound,

etc.) go at its characteristic speed?➧ Answer: with respect to the medium in which the wave

is a disturbance–air, for sound waves–water, for water waves–etc.

● In what frame of reference are the laws of motion valid?➧ Answer: In any uniformly moving frame of reference

(Principle of Galilean relativity)

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Light & Electromagnetism:19th Century Answers● With respect to what does light go at

speed c?➧ Answer: With respect to the ether, a substance that

permeates the universe and which is the medium in which electromagnetic waves (light) are disturbances

● Equivalent question:In what frame of reference are the laws of electromagnetism (Maxwell’s equations) valid?➧ Answer: in a frame of reference at rest with respect to

the ether

Properties of the Ether● Very tenuous

➧ Planets move through it without resistance

● Able to penetrate everywhere➧ Light propagates through otherwise empty space

● Very stiff➧ Speed of light is fast!

● Not a gas or liquid; more like Jello➧ Electromagnetic waves are transverse

(disturbance is perpendicular to the wave’s motion)

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Don’t like the Ether concept?

● Then you answer the question:

“With respect to what does light go at speed c?”

A Dichotomy in Physics

● Laws of motion (mechanics; Newtonian physics) obey a relativity principle➧ The laws of mechanics work just the same in all

uniformly moving frames of reference➧ “Absolute motion” and “absolute rest” are meaningless

in mechanics

● Laws of electromagnetism (Maxwell’s equations, including the prediction of EM waves with speed c) are valid only in one frame of reference: the ether frame➧ “Absolute motion” and “absolute rest” are meaningful in

electromagnetism

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Is Earth Moving Relative to the Ether?

● One possibility: IT ISN’T

● The other possibility: IT IS

Is Earth Moving Relative to the Ether?

● One possibility: IT ISN’T. Then either:➧ Earth alone among the cosmos is at rest

relative to the ether➧ Or, Earth drags with it the ether in its

vicinity(and so do other astronomical bodies)

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Aberration of Starlight: Umbrella Analogy

(a) Stand still (b) Run

(c) In runner's frame (d) With "air drag"

Aberration of Starlight:The Real Thing

March

With ether drag

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September

With ether drag


March

No ether drag

SeptemberJames Bradley,1727:angular position of starg-Draconis


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Is Earth Moving Relative to the Ether?● One possibility: IT ISN’T. Then

either:➧ Earth alone among the cosmos is at rest relative

to the ether➧ Or, Earth drags with it the ether in its vicinity

(Copernican paradigm)

(Aberration of starlight)

● The other possibility: IT IS➧ Then there should be an “ether wind” blowing past Earth➧ We should be able to detect this “wind” and determine

the speed and direction of Earth’s motion through the ether

Where We Are Now(late 1800s)● Two branches of knowledge that describe

all known physical reality➧ Newtonian mechanics

–Phenomena involving motion–Relativity principle holds

➧ Electromagnetism–Phenomena involving electricity & magnetism–Includes light–Seems not to obey a relativity principle–Valid in ether frame only–Question: What’s Earth’s motion relative to ether?

lecture 1 bh32 copy - insight cruises · rich wolfson prof of physics middlebury college scientific...

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