Light. Physics Gymnasium 15/06/00. Outline. Einstein s Theory of Relativity Outrageous, but true! What about Galileo and Newton? What about Einstein?

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1 Outline Einstein s Theory of Relativity Outrageous, but true! Galileo and Newton The start of eletromagnetism Einstein - The Theory of Speial Relativity Einstein - The Theory of General Relativity Physis Gymnasium, June 2000 by David N. Jamieson, PhD, FAIP Shool of Physis University of Melbourne Light What about Galileo and Newton? Light real light radio waves (x-rays, gamma rays, mirowaves, or any other sort of eletromagneti field) Speed is = 299,792,458 m/s (1 billion km/hr) fast but finite Alternative pitures of light: waves or partiles Galileo: The laws of Physis do not depend on absolute motion (does this inlude eletromagnetism?) Newton: An objet one set in motion remains in motion until ated upon by an external fore The universe is governed by a majesti lokwork where all loks everywhere at all times tik in perfet synhronisation. What about Einstein? Time, spae and gravity The theory of Speial Relativity for fast objets The theory of General Relativity for high objets Essential and entral to the funtioning of the Global Positioning System!! How fast are we going? Relative to surfae of Earth? Relative to the Sun?...entre of Galaxy?...origin of Big Bang? Can t use ordinary Mehanis to test this! What about eletromagnetism? Can we use properties of light? by Dr D.N. Jamieson 1

2 Galileo and Newton knew that, in the absene of frition, a ball would roll down a hill and up the same height on the other side. Galileo and Newton knew that, in the absene of frition, a ball would roll down a hill and up the same height on the other side. Galileo and Newton knew that, in the absene of frition, a ball would roll down a hill and up the same height on the other side. A flat bit in the middle makes no differene. A flat bit in the middle makes no differene. A flat bit in the middle makes no differene. Coasting at onstant speed by Dr D.N. Jamieson 2

3 What about if you get rid of the seond hill? What about if you get rid of the seond hill? To infinity at onstant speed How fast are we going anyway? Galileo (and Newton) knew that an objet, one set in motion, ontinues indefinitely at onstant speed unless ated upon by an external fore. Cannot detet this motion from inside Sun s Orbit around galaxy: 250 km/s Twie the speed of sound and not a drop spilled! Lifestyle of a Conorde passenger...or parked at the gate? Milky Way Galaxy trajetory towards Great Attrator: 7000 km/s Earth s Orbit around Sun: 30 km/s But eletromagnetism (and light) may be different Eletrostati fore: F = kqq/r 2 q r Q Magneti fore: F = qvb N S q...magneti fore depends on the speed, v! Light is an eletromagneti wave So experiments with light might allow us to detet our absolute speed through the osmos! So how does light behave? Possible Answers: Like Tennis Balls? Like Sound Waves? Like Something Else? by Dr D.N. Jamieson 3

4 Like tennis balls? Like tennis balls? 100 km/hr 0 km/hr 100 km/hr Like tennis balls? Like tennis balls? NO! v v? v Light signals would get out of syn!! True situation +v + v? +v Like sound waves? Like sound waves? Speed of sound in air always Mah 1 Mah 1 Mah 2 by Dr D.N. Jamieson 4

5 Like sound waves Like sound waves? Aether wind! Sound in Air Air has pressure Made of O, N, Ar Has mass You breath it... Light in Aether? Aether is insubstantial Very elasti Other properties??? 30 km/s? 30 km/s Like sound waves? NO!! Mihelson -Morley experiment found no differene! 30 km/s Summary Objet Us Them Us 0 v Them v 0 Tennis Ball 0 100km/hr Sound Wave speed of sound speed of sound v Light v Enter Einstein The Speial Theory of Relativity Light annot be used to measure an absolute speed! This is entirely onsistent with the rest of the laws of physis whih apply to mehanis And light does not behave like tennis balls light does not behave like sound waves What does this mean? Laws of Physis the same for everybody The speed Sun s of light the same for everybody moves at Let us look at speed a startling onsequene of these fats Sun s light moves at speed by Dr D.N. Jamieson 5

6 The Light Clok The Light Clok A photon bouning between mirrors may be used as a lok A photon bouning between mirrors may be used as a lok The Light Clok A photon bouning between mirrors may be used as a lok Moving Clok is slow! Speed Short distane Long distane Speed tik Moving loks run slow! The Twin Phenomenon Cosmi Rays reate fast muons in outer atmosphere Idential muons reated in the laboratory live for 2.2 millionths of a seond Even at speed of light, this is not long enough to reah sea level! Fast muons live longer than slow muons Fast muons reated here Expet to deay here See plenty here! by Dr D.N. Jamieson 6

7 Summary Galaxy rossing protons Constant speed of light leads to: Time dilation: 1 t = t t = γ. t whereγ v = 1 v Galaxy view proton speed v < (just) γ = Cross galaxy in t=10 5 years Proton view galaxy speed v < (just) L = L/γ = 10 5 /10 10 = 10-5 l.y. = 95 million km = 0.7 A.U. Hene t = L / = 10-5 years = 315 seonds Gamma is always greater than 1! (The bigger the better) Galaxy View L = 10 5 light years Proton View SS433 - The strangest objet in our galaxy Relativity and Spae Travel The entral engine produes jets of matter moving at 25% the speed of light! Neutron star Before Roket and fuel at rest Fuel After Fuel thrown away Roket goes forward by onservation of momentum Normal star Fuel Chemial Rokets Gravitational Slingshot Can use gravity to boune off a planet Pik up twie the orbital speed for a head-on ollision! v 2 = 29.6 km/s v o = 9.6 km/s NB: Essential that trajetory does not interset planet! v 1 = 10.4 km/s by Dr D.N. Jamieson 7

8 High Speed Exhaust - Ion Engine Use Hg +, Xe + or Cs + as reation mass Aelerate ions to 1% by eletri fields Low thrust (not many ions per seond) But be patient! Laboratory test In flight To the Stars Prelaunh High temperature exhaust - Nulear Engines High temperature exhaust: Nulear Engines NERVA nulear roket prototype Built in USA in 1960 s Suffered from failure of the nulear ore 1 2 For the future: high temperature fusion or even fission engines Extremely high exhaust temperatures 3 Nulear Pulse Jet Antimatter rokets Momentum from nulear explosions Conept tested with hemial explosives by Dr D.N. Jamieson 8

9 Ultra fast exhaust - Light Ramsoop: Burn the interstellar medium Starwisp interstellar probe Burn interstellar hydrogen Density 1 atom/m 3 DNJ Seal of Approval 1000 km 2 mylar with 20nm Si oating Ramsoop Maths Kineti Energy of Relativisti Spaeraft Speed (v/) γ = 1/(1-v 2 / 2 ) 1/2 Kineti Energy (J/kg) Compare: 100 km/hr USA annual eletri power (1982) 8.5x10 18 J World annual eletri power (1982) 4.5x10 3.1x10 10 J Solar flux on Earth for 1 year 5.3x10 24 J x x x x x , x Gev eletron in a storage ring at CERN General Relativity Equivalene Priniple... remained, for forty years after its disovery (by Einstein), an austere intelletual monument - a somewhat sterile topi isolated from the mainstream of physis and astronomy - whose pratitioners were magnifient ultural ornaments In the late 20thC, GR is an engineering subjet! Inertial mass: F = m i a Gravitational mass: F = GMm g /r 2 Newton knew m i = m g Why? by Dr D.N. Jamieson 9

10 Equivalene Priniple Experimental test of C=2πr Sine m i = m g these are equivalent (same laws of Physis, everything) Radius, r Measured C/r Cirumfrene, C 10 m 62.8 m m m m m m m ,000 km 40,0000 km 4.00 Yikes!! C<2πr Why?? Drawing the Big Cirle (on Earth) C<2πr for irles drawn on urved surfaes Drawing the Big Cirle (off Earth) Apply Equivalene Priniple Apply Equivalene Priniple Twenty 1 metre rulers laid down around rim by observer on ground Cirumfrene is 20 m = 2πr Veloity of rim View from rotating disk Artifiial gravity in the ontrol room of the Disovery (2001- A spae Odyssey) by Dr D.N. Jamieson 10

11 Apply Equivalene Priniple Apply Equivalene Priniple Observer on rim sees rulers going past at high speed I am standing on the bottom of a stationary ylinder wathing lots of rulers go past Cirumfrene now measued by ontrated rulers I am standing on the bottom of a stationary ylinder wathing lots of rulers go past They are shrunken to less than 1 m by Lorentz ontration! Fast moving shrunken ruler Same number as before, but shorter Cirumfrene is less than 2πr! Apply Equivalene Priniple Conlusion Gravity points outwards on rim of rotating disk C < 2πR there Gravity points inwards on Earth C > 2πr there Earth s gravity makes more spae! The Speed of Light is the same for everybody The Laws of Physis are the same for everybody Moving loks run slow Moving objets ontrat Gravity and aeleration are the same thing Spae is warped Experimental status: All OK! by Dr D.N. Jamieson 11