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The Origin of Modern Astronomy

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1 The Origin of Modern Astronomy

2 Why did people look up? Religion Navigation
Time keeping (calendar, clock) Food (planting, hunting, breeding)

3 The Roots of Astronomy Already in the stone and bronze ages, human cultures realized the cyclic nature of motions in the sky. Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance. Those monuments were probably used as calendars or even to predict eclipses.

4 Stonehenge Constructed: 3000 – 1800 B.C. Summer solstice Heelstone Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices Probably used as calendar.

5 Other Examples All Over the World
Big Horn Medicine Wheel (Wyoming)

6 Newgrange, Ireland, built around 3200 B.C.:
The Roots of Astronomy Newgrange, Ireland, built around 3200 B.C.: Sunlight shining down a passageway into the central chamber of the mount indicates the day of winter solstice.

7 Other Examples All Over the World (2)
Caracol (Maya culture, approx. A.D. 1000)

8 Other Examples All Around the World
Chaco Canyon, New Mexico Slit in the rock formation produces a sunlit “dagger” shape, indicating the day of summer solstice

9 Other Examples All Around the World (2)
Mammoth tusk found at Gontzi, Ukraine: Inscriptions probably describing astronomical events

10 Ancient Greek Astronomers (1)
Unfortunately, there are no written documents about the significance of stone and bronze age monuments. First preserved written documents about ancient astronomy are from ancient Greek philosophy. Greeks tried to understand the motions of the sky and describe them in terms of mathematical (not physical!) models.

11 Ancient Greek Astronomers (2)
Models were generally wrong because they were based on wrong “first principles”, believed to be “obvious” and not questioned: Geocentric Universe: Earth at the Center of the Universe. “Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles.

12 lived from about 624 BC to about 547 BC
Thales of Miletus lived from about 624 BC to about 547 BC Founder of Greek Science “What is the universe made of?” without resorting to supernatural explanations Suggested that the world was inherently understandable and not just the result of arbitrary or incomprehensible events. Founder of Greek Science Suggested that supernatural explanations were not necessary to understand what the universe was made of. Suggested that the world was inherently understandable and not just the result of arbitrary or incomprehensible events.

13 This was not widely accepted.
Thales’ Cosmos Air water earth Universe fundamentally water with Earth as a flat disk on an infinite ocean. Not widely accepted. Thales’ believed the universe consisted fundamentally of water with Earth as a flat disk on an infinite ocean. This was not widely accepted.

14 Anaximander of Miletus 610-c. 547 BC
Student of Thales. Suggested that the heavens must form a complete sphere around Earth (to explain the sky turning around the north star). Based on how the sky changes with travel north and south, he concluded that Earth must not be flat. Because the sky didn’t change with east-west travel, he guessed that Earth might be a cylinder curved only in the north-south direction. Student of Thales. Suggested that the heavens must form a complete sphere around Earth (to explain the sky turning around the north star). Based on how the sky changes with travel north and south, he concluded that Earth must not be flat.

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16 Underground home of the heavenly bodies
Anaximander’s Cosmos air and clouds Earth (a cylinder) Ring of Fire Horizon Underground home of the heavenly bodies Because the sky didn’t change with east-west travel, he guessed that Earth might be a cylinder curved only in the north-south direction. Because the sky didn’t change with east-west travel, he guessed that Earth might be a cylinder curved only in the north-south direction.

17 lived from about 569 BC to about 475 BC
Taught that Earth was a sphere. Pythagoras of Samos lived from about 569 BC to about 475 BC Taught that Earth was a sphere.

18 Pythagorean Theorem A C B a2 + b2 = c2

19 lived from about 460 BC to about 370 BC
“Nothing exists but atoms and empty space. Everything else is opinion.” Democritus of Abdera lived from about 460 BC to about 370 BC

20 Democritus was a student of Leucippus
Democritus was a student of Leucippus. Together they are considered “co-originators” of the belief that all matter is made up of atoms. He said that atoms were eternal, invisible, indivisible, and incompressible. Democritus believed the universe was made of an infinite number of atoms of the four elements. He claimed the moon had mountains and valleys, the Milky Way was a vast group of individual stars, and that Earth and other worlds were created by random motions of infinite atoms. Other philosophers, including later Aristotle, argued against this. Democritus was among the first to propose that the universe contains many worlds, some of them inhabited: "In some worlds there is no Sun and Moon while in others they are larger than in our world and in others more numerous. In some parts there are more worlds, in others fewer (...); in some parts they are arising, in others failing. There are some worlds devoid of living creatures or plants or any moisture." Because his theories do not give credit to a Creator, atomism became linked with atheism. This persisted into the mid-1800s. (In 17th century France you could be burned at the stake for believing in atoms.)

21 Plato lived from 427 BC to 347 BC

22 Plato in a Small Nutshell
The world cannot not be known through the senses (world view presented by the senses are like shadows on a cave wall) The philosopher, through pure thought, can see through surface appearances to the ideal forms underneath. The heavens, for example, are perfect and, therefore, move in uniform, circular motion because a circle is the perfect form. Question for students: If the heavens move uniformly in perfect circles, then why do planets appear to make loops in the sky and speed up, then slow down?

23 Ancient Greek Astronomers (3)
Eudoxus (409 – 356 B.C.): Model of 27 nested spheres Aristotle (384 – 322 B.C.), major authority of philosophy until the late middle ages: Universe can be divided in 2 parts: 1. Imperfect, changeable Earth, 2. Perfect Heavens (described by spheres) He expanded Eudoxus’ Model to use 55 spheres.

24 Eudoxus’ Cosmos (simplified)
Earth Sphere of the Sun Sphere of the stars Axis of stellar sphere Axis of sun sphere Eudoxus’ Cosmos (simplified)

25 Aristotle’s Cosmos (simplified)
Earth Sun Venus Mercury Moon Saturn Jupiter Mars Sphere of Fixed Stars Aristotle’s Cosmos (simplified)

26 Aristotle’s Physics What is the world made of? How do things move?
Earth, Water, Air, and Fire How do things move? Natural Motion (towards the Earth) Violent Motion (requires a force) The Heavens are different from the Earth Made of fifth substance (Quintessence) Experience only circular motion Other than repetitive circular motion, heavens experience no change The Earth is Round The moon revolves around Earth, giving us lunar phases

27 N A B S Aristotle’s argument (2): Observer at A never sees star B. However, if he travels south to position B, star B becomes visible. Therefore, the earth’s surface must be curved.

28 Aristotle’s argument (1): Shadow cast by earth on the moon during an eclipse is always curved. The only geometric shape which always casts a circular shadow is a sphere

29 Aristarchus of Samos c. 310 – 230 BC
Proposed a heliocentric system Distance and size of the moon Distance and size of the sun Geometry of eclipses

30 Aristarchus of Samos Proposed a heliocentric system -
His belief in a heliocentric system was not popular. Many argued against it. Arguments included: If Earth is moving, why don’t we feel it? If Earth is moving, why don’t we leave the moon behind? If Earth is moving around the sun, why don’t we see stellar parallax?

31 Parallax

32

33

34 This imaginary triangle extends from Earth to a nearby object in space (such as a planet). The group of stars at the top represents a background field of very distant stars. Hypothetical photographs of the same star field showing the nearby object’s apparent displacement, or shift, relative to the distant, undisplaced stars.

35 Aristarchus of Samos If Earth is moving around the sun, why don’t we see stellar parallax? Philosophers who did not believe in a heliocentric system argued that no stellar parallax meant Earth didn’t move and Aristarchus was wrong. Now we know Earth does move, so why don’t we see stellar parallax? Try putting your finger in front of your nose and looking at it with one eye and then the other. Now move your finger farther from your face and try again. Move it farther still, and try again. What do you see? We don’t see stellar parallax because the stars are so far from us. The Greeks did not consider this answer as their version of the universe was smaller than our solar system. We know can measure stellar parallax for a handful of stars that are close to us.

36 Relative Distances of Sun and Moon
Earth Moon at first quarter Sun Angular separation between sun and moon when moon is at first quarter Method of Aristarchus Right angle Relative Distances of Sun and Moon

37 Problem with Aristarchus’ Method
Earth Moon at first quarter To Sun Angular separation between sun and moon when moon is at first quarter is so close to 90 (89.5) that it could not be reliably measured in ancient times Right angle Problem with Aristarchus’ Method

38 Eratosthenes Circumference of Earth Tilt of Earth
Lived from 276 B.C. to 195 B.C.

39 How big is Earth? Start with a circle.
He heard tell of a town named Syene, where on a particular day of the year at noon there were no shadows on the water in the water well. To Sun The Sun was overhead. To Sun Alexandria He was in Alexandria. Syene The sun was not overhead for him, but he could measure the angle between overhead and the Sun.

40 How big is Earth? He calculated the angle using shadows. It was approximately 7°. We have 2 parallel lines, bisected by a third line What can we say about this angle? ? To Sun It is the same! 7°! To Sun Alexandria Syene

41 How big is Earth? Now we have a 7° “pie piece” of Earth.
A circle has 360°. We can keep adding “pie pieces” until we get to 360°. Then you can calculate the circumference. If you know the distance between the two towns, you just keep adding that distance all the way around the circle. To Sun To Sun Alexandria D Syene 360°

42 How big is Earth? Did he get the right answer?
That depends on how well he measured the distance between towns (without an odometer or a GPS). He measured the distance between Syene and Alexandria as ~ 5,000 stadia To Sun Syene Alexandria D We think he was off by a bit. Depending on the length of a stadia, he was off by 3-14%. Best estimate yet!

43 Tilt of Earth measured difference between noontime elevation of Sun in winter and summer deduced that Earth's equator is tilted by 23.5 degrees difference in height of Sun at different times gives latitude

44 Tilt of Earth We now know that the tilt (obliquity) varies over a 400,000 year cycle. It ranges from 22.1 to 24.5.

45 Hipparchus of Rhodes lived from 190 BC to 120 BC

46 Achievements of Hipparchus
Trigonometry 1st Large Star Catalog (about 3000 stars) Invented latitude and longitude Discovered Precession Measured the length of a year to 6 minutes Used Eccentrics to explain retrograde motion (moved Earth off exact center)

47 The Sun’s gravity is doing the same to Earth.
Precession (1) At left, gravity is pulling on a slanted top. => Wobbling around the vertical. The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.

48 Precession The pole will be closest to Polaris ~ A.D. 2100.
As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. Precession is caused by the gravitational effects of the sun and moon. The pole will be closest to Polaris ~ A.D There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, the celestial north pole will be close to Vega in the constellation Lyra. Earth’s axis currently points nearly toward the star Polaris. About 12,000 years from now—almost halfway through one cycle of precession—Earth’s axis will point toward a star called Vega, which will then be the “North Star.” Five thousand years ago, the North Star was a star named Thuban in the constellation Draco. The circle shows the precessional path of the north celestial pole among some prominent northern stars. Tick marks indicate intervals of a thousand years.

49 North and South Precession

50 Later refinements (2nd century B.C.)
Hipparchus: Placing the Earth away from the centers of the “perfect spheres” Ptolemy: Further refinements, including epicycles

51 Claudius Ptolemy lived from 85 to 165

52 Ptolemy – The Almagest Ptolemy's greatest work was the Almagest. It was a combination textbook, encyclopedia, and astronomical almanac. It was a remarkable piece of work, despite the errors. The Greek title was "Great Syntaxis" or "Great Compilation" but the European title comes from the Arabic Al Majisti, the same root word as majestic and majesty, essentially "The Greatest." It was essentially a collection and compilation of data, calculations, methods of observations and calculation, and tables of planetary locations. Basically, a compendium of six hundred years of Greek astronomy as well as new results of his own work on planetary motion. It also contained an updated star catalog, with several hundred new stars discovered and located by himself and others since Hipparchus's time nearly two hundred and fifty years before. The book defined the basis of mathematical astronomy and remained the best and simplest until Copernicus described his heliocentric methods in the sixteenth century

53 Ptolemy’s System Deferent Earth planet Epicycle
Equant Earth planet For this scheme to work, Earth has to be offset from the center Epicycle Ptolemy’s System

54                    Here are the retrograde loops as formed by a single epicycle on the deferent. Notice how the motion creates a single and symmetrical set of loops.

55 Introduced to explain retrograde (westward) motion of planets
Epicycles Introduced to explain retrograde (westward) motion of planets The Ptolemaic system was considered the “standard model” of the Universe until the Copernican Revolution.

56 Distance to the Moon Ptolemy also calculated the distance to the moon.
R 60 x Radius of Earth Not to scale.

57 Medieval Times Time period in Western Europe from the fall of Rome (476) to around 1500 Characteristics: Europe divided into a multitude of warring principalities Relatively little intellectual activity due to turbulent social conditions Learning (mostly religious) carried on in Monasteries

58 Medieval Times “900 years without a bath” Monty Python

59 The Dark Ages Other Parts of the world were flourishing
Islamic world (Spain to India) China Mesoamerica (Maya, Inca) Islamic sought knowledge of mathematics and astronomy in hopes of better understanding the wisdom of Allah. They translated (and thereby saved) many of the ancient Greek works. Developed algebra and new instruments and techniques for astronomical observation. Intellectual center in Baghdad of ancient Greek, Indian, and Chinese contributions, along with its own scholars

60 Islamic achievements during the medieval period
Mathematics Arabic numerals algebra trigonometry Optics (Al-Hazen of Basara invents the camera obscura) Astronomy commentaries and improvements on Ptolemy more accurate almanacs development of the astrolabe Preservation of ancient texts

61 Medieval Times Medieval European cosmology was always from a Christian perspective. Realistic physical and mathematical models of the universe were not of great interest to most Christian scholars (virtually all of whom were priests or monks). In the later Middle Ages (after 1200) Aristotle’s cosmos was cast in a Christian form.

62

63 The Renaissance ( ) By 1500 Western Europe was experiencing a Renaissance (“rebirth”) of scholarship. Problems with Ptolemy were viewed with greater seriousness than in previous centuries. Discovery of lands unknown to the Greeks cast doubt on the “wisdom of the ancients”. Time was ripe for fresh thinking about celestial motions

64 The Renaissance Big Four
Nicolas Copernicus (Revived heliocentric theory) Tycho Brahe (Last great naked eye observer) Johannes Kepler (Elliptical orbits, three laws of motion) Galileo Galilei (Telescopic observations, dynamics of motion)

65 Nicholas Copernicus (1473 – 1543)
Polish, born near Torun Earned his living as a cathedral Cannon (un- ordained church official) Proposed a heliocentric system with the Earth as a planet rotating and moving along a circular orbit around the central Sun just like all the other planets. Published brief versions of the model during his lifetime, but waited until he was near death to publish “On the Revolutions of the Heavenly Spheres”, the complete theory Nicholas Copernicus (1473 – 1543)

66 Copernicus’ Heliocentric System
Earth Mars Venus Mercury Sun Jupiter Saturn Copernicus’ Heliocentric System

67 Copernicus’ new (and correct) explanation for retrograde motion of the planets
Retrograde (westward) motion of a planet occurs when the Earth passes the planet. This made Ptolemy’s epicycles unnecessary.

68 Retrograde Motion in the Copernican System
Mars Earth Apparent path of Mars Background stars

69 Retrograde Motion in the Copernican System
Mars Earth Apparent path of Mars Background stars Retrograde Motion in the Copernican System

70 Retrograde Motion in the Copernican System
Mars Earth Apparent path of Mars Background stars Retrograde Motion in the Copernican System

71 Retrograde Motion in the Copernican System
Mars Earth Apparent path of Mars Background stars Retrograde Motion in the Copernican System

72 Retrograde Motion in the Copernican System
Mars Earth Apparent path of Mars Background stars Retrograde Motion in the Copernican System

73 Retrograde Motion in the Copernican System
Mars Earth Apparent path of Mars Background stars Retrograde Motion in the Copernican System

74 Retrograde Motion in the Copernican System
Mars Earth Apparent path of Mars Background stars Retrograde Motion in the Copernican System

75 Retrograde Motion in the Copernican System
Mars Earth Apparent path of Mars Background stars Retrograde Motion in the Copernican System

76 The Heliocentric Solar System of Copernicus
Highlights of Copernicus’ system Earth is a planet Day and night are due to the rotation of the Earth The year is due to the revolution of the Earth around the Sun The Moon is the only celestial body which orbits the Earth Explained retrograde motion in an elegant manner Explained why Venus and Mercury are always near the Sun Provided a straightforward way of determining the scale of the solar system Problems with Copernicus’ system Predictions of planetary positions no better than Ptolemy If the Earth is moving why don’t we feel it? If the Earth is a planet, the other planets must be like Earth. Are they? Why don’t the stars appear to shift as the Earth changes position This system is physically impossible according to Aristotle’s physics.

77 Tycho Brahe Danish, born in Skaane (now in Sweden)
A nobleman who established an observatory on an island near Copenhagen Devised new and improved existing instruments which were used to produce the most accurate star maps ever made Not a Copernican, but demonstrated that, contrary to Aristotle, the heavens are changeable (comets are celestial and new stars, “novae,” appear) These data were invaluable to Johannes Kepler who used them to formulate his orbital model Tycho Brahe (1546 – 1601)

78

79 High precision observations of the positions of stars and planets
Tycho Brahe (1546 – 1601) High precision observations of the positions of stars and planets Measurement of the nightly motion of a “new star” (a supernova) showed no parallax Evidence against Aristotelian belief of “perfect”, unchangeable heavens

80 Tycho Brahe’s Legacy New World model
New World model Still geocentric (Earth in the center of the sphere of stars) Sun and Moon orbit Earth; Planets orbit the sun.

81 Tycho’s Compromise System
Jupiter Mars Venus Mercury Sun Earth Saturn Tycho’s Compromise System

82

83 Johannes Kepler German, born near Stuttgart
Lived in near poverty most of his life, usually earning a living as a teacher of mathematics Became convinced of the truth of the Copernican model and was determined to make its predictions more accurate Went to work for Tycho Discovered that if Copernicus’ circular orbits were replaced by ellipses, then predicted positions of the planets were more accurate than Ptolemy. His work is summarized in his Three Laws of Motion Johannes Kepler ( )

84 Kepler’s Laws of Planetary Motion
The orbits of the planets are ellipses with the sun at one focus. c Semimajor axis Eccentricity e = c/a

85 Eccentricities of Ellipses
1) 2) 3) e = 0.02 e = 0.1 e = 0.2 5) 4) e = 0.4 e = 0.6

86 Eccentricities of Planetary Orbits
Orbits of planets are virtually indistinguishable from circles: Most extreme example: dwarf planet Pluto: e = 0.248 Earth: e = varies over period of ~ 100,000 years, from e= to e=0.0607

87 Planetary Orbits (2) 2. A line from a planet to the sun sweeps over equal areas in equal intervals of time.

88 Planetary Orbits (3) 3. A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed: (Py = period in years; aAU = distance in AU) Py2 = aAU3

89 Galileo Galilei Italian, born in Pisa
Studied medicine, but excelled in mathematics and physics Taught at Pisa, Padua and Florence First physicist in the modern sense. Fundamental work on moving bodies Heard about the telescope invented in Holland and built an improved version Used the telescope to discover craters on the moon, spots on the sun, phases of Venus and the moons of Jupiter Became a convinced Copernican and wrote “Dialogue Concerning the Two Chief World Systems”, a treatise expounding his views. Condemned by the Church for teaching Copernicanism a proven fact Galileo Galilei ( )

90 Galileo's Greatest HIts
Surface of the Moon Showed that the moon is not a smooth sphere; appears to be a “landscape”, thus it is a “world” Moons of Jupiter A “miniature solar system”; bodies can orbit something other than Earth (contrary to Aristotle) Phases of Venus Venus exhibits all phases, just like the moon; thus, Venus must orbit the sun, and its orbit must be closer to the sun than the Earth’s. Mars and Saturn Planets show disks something like the moon, implying they are also “worlds”. Saturn’s puzzling shape implies that we don’t everything in the universe. Sunspots Showed that the sun’s surface is not “perfect”, a position advocated by Aristotle and widely accepted. Milky Way The telescope revealed many more stars not visible to the naked eye. Implied a three dimensional universal.

91 Major Discoveries of Galileo
Moons of Jupiter (4 Galilean moons) (What he really saw) Rings of Saturn (What he really saw)

92 Major Discoveries of Galileo (2)
Surface structures on the moon; first estimates of the height of mountains on the moon

93 Major Discoveries of Galileo (3)
Sun spots (proving that the sun is not perfect!)

94 Major Discoveries of Galileo (4)
Phases of Venus (including “full Venus”), proving that Venus orbits the sun, not the Earth!

95 Phases of Venus

96 Galileo's finger is on display at the Museo di Storia del Scienza in Florence, Italy.
The finger was detached from Galileo's body by Anton Francesco Gori (Florence, , literate and antiquary) on 12 March 1737 when Galileo's remains were transferred from a small closet next to the chapel of Saints Cosmas and Damian to the main body of the church of Santa Croce where a mausoleum had been built by Vincenzo Viviani. This is a normal sized finger in a small cup. Galileo may have approved…. It is his middle finger!

97 Historical Overview

98 Issues Raised by the Copernican System
Why do planets move in elliptical orbits? If the earth is moving, why don’t we feel it? What keeps the earth, moon and planets moving? Why don’t we see parallax as the earth moves around the sun?

99 Isaac Newton (1642 – 1727)

100 Newton Summarized Law of Universal Gravitation F = G(m1 x m2 )/r2
Three Laws of Motion A body stays at rest or moving uniformly until acted upon by an external force The force acting on a body is proportional to its mass and its change in velocity (acceleration) F = m x a For every action there is an equal and and opposite reaction

101 The Universal Law of Gravity
Any two bodies are attracting each other through gravitation, with a force proportional to the product of their masses and inversely proportional to the square of their distance: Mm F = - G r2 (G is the Universal constant of gravity.)

102

103 First Law (Law of Inertia)

104 Second Law (F=ma)

105 Third Law (action – reaction)

106 Third Law: Action - Reaction
NASA photo Third Law: Action - Reaction

107 The first law of motion and the universal law of gravity explains why the Earth orbits the Sun
F=G(Ms x Me)/r2 The only force acting on the Earth is the mutual gravitational attraction of the Sun. Without this force, Earth would continue in a straight line at velocity V. Instead, it is pulled in a path around the Sun. If an external force somehow brought V to zero, Earth would collide with the Sun.

108 The Scientific Revolution
Generally, the period between the publication of Copernicus’ On the Revolutions of the Heavenly Orbs (1543) and the publication of Newton’s Mathematical Principles of Natural Philosophy (1687) During this period the scholarly outlook changed from a static Earth in a geocentric universe to a dynamic heliocentric solar system with a moving Earth as one of the planets. The success of Newton’s laws in explaining this new universe with mathematical precision encouraged scholars to believe that all natural phenomena could be explained following the scientific method (experiment and theory) rather than by deductive logic based on authority.

109 Historical Overview

110 William Herschel

111 William Herschel’ Legacy
Discovery of Uranus “Father” of Stellar Astronomy First Serious Use of Reflecting Telescope First Model of the Universe Based on Systematic Observation (Disk of Stars) Binary Stars (physically connected double stars) shows Newton’s laws are universal shows stars have different luminosities Extensive Catalog of Nebulae Discovery of “Invisible” (Infrared) Radiation

112 Herschel’s Conclusion from Star Counts
Stars appear to be concentrated here (Milky Way) Fewer stars with some nearby bright ones away from Milky Way Herschel’s Conclusion from Star Counts Photo of Milky Way

113 Implied Structure of Stellar System
Fewer stars seen More stars seen Apparent Celestial Sphere Sun

114 Herschel’s Actual Plot from Data

115 Einstein and Relativity
Einstein (1879 – 1955) noticed that Newton’s laws of motion are only correct in the limit of low velocities, much less than the speed of light. Theory of Special Relativity (1905) Photoelectric Effect (1905) Nobel Prize in 1921.  Theory of General Relativity (1916)

116 Two Postulates Leading to Special Relativity (1)
Observers can never detect their uniform motion, except relative to other objects. This is equivalent to: The laws of physics are the same for all observers, no matter what their motion, as long as they are not accelerated.

117 Two Postulates Leading to Special Relativity (2)
The velocity of light, c, is constant and will be the same for all observers, independent of their motion relative to the light source.

118 Basics of Special Relativity
The two postulates of special relativity have some amazing consequences. Time dilation: The faster something moves, the slower time goes for it. Length contraction: Length scales on a rapidly moving object appear shortened Relativistic aberration: Distortion of angles The energy of a body at rest is not 0. Instead, we find E0 = m c2

119 General Relativity A new description of gravity Postulate:
A new description of gravity Postulate: Equivalence Principle: “Observers can not distinguish locally between inertial forces due to acceleration and uniform gravitational forces due to the presence of massive bodies.”

120 Einstein’s Theories of Relativity
General relativity It is impossible to tell, from within a closed system, whether one is in a gravitational field, or accelerating:

121 Einstein’s Theories of Relativity
Matter tends to warp spacetime, and in doing so redefines straight lines (the path a light beam would take): A black hole occurs when the “indentation” caused by the mass of the hole becomes infinitely deep.

122 Thought Experiment (Conclusion)
This bending of light by the gravitation of massive bodies has indeed been observed: During total solar eclipses: The positions of stars apparently close to the sun are shifted away from the position of the sun.  New description of gravity as curvature of space-time!

123 Photoelectric effect Photoelectric effect can be understood only if light behaves like particles

124 The Structure of the Cosmos
600 BC (Thales) 300 BC (Aristotle) 1543 AD (Copernicus) 1600 (Kepler) 1600 (Digges) 1800 (Herschel) 1920 (Shapley) 1930 (Hubble) 2000 (Geller) The Structure of the Cosmos


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