ASTR-117 Exam Review Vocabulary

The Origin of Life

• How did life begin?

  • • Miller-Urey Experiment: Simulated early Earth conditions to see if organic molecules could form from inorganic ones. They combined water, methane, ammonia, and hydrogen in a closed system, sparked with electricity to mimic lightning, and found that amino acids and other organic compounds formed.

      • This supports the idea that life's building blocks could have arisen abiogenically from simple chemical reactions.

    • Life brought from space: The hypothesis that life originated elsewhere in the universe and migrated to Earth (panspermia).

      • This suggests life did not originate on Earth.

    • Deep-sea vents: Hydrothermal vents on the ocean floor release chemicals from the Earth's interior, providing energy and nutrients for chemosynthetic organisms.

      • These vents could have provided a stable environment for the origin of life.

    • Clay and other minerals: Mineral surfaces can act as catalysts, promoting the polymerization of RNA and other organic molecules.

      • Clay and other minerals acted as catalysts to build RNA strands.

    • Lipid pre-cells: Formation of pre-cells, such as liposomes, which can encapsulate RNA and other molecules.

      • These structures could have been precursors to modern cells.

    • RNA self-replication: Natural selection improves replication until life emerges. RNA molecules can act as both carriers of genetic information and catalysts for their own replication.

      • This may have led to self-replicating systems and the emergence of life.

  • Could life have migrated to Earth?

    • Migration from Venus or Mars: Impact blasts from one world to another can eject rocks into space, potentially carrying microbial life between planets.

      • Suggests possible transfer of life via impact blasts from one world to another.

    • Meteorites from Mars: Some meteorites found on Earth are believed to have originated from Mars. Microbes could potentially survive the journey through space within these rocks.

      • Indicates chance of survival traveling through space.

    • Migration from other star systems highly unlikely: Due to vast distances and the challenges of interstellar travel.

  Cells: The Basic Units of Life

  • What are living cells?

    • Living cells are the basic units of life on Earth. These are the fundamental building blocks of all known organisms.

    • Cell membrane - separates organic material from inorganic material, creating a distinct internal environment that differs from the surroundings.

  Defining Life

  • What are the molecular components of cells?

    • Major Molecular Components of Cells:

      • Carbohydrates: Provide energy and structural support.

      • Lipids: Form cell membranes and store energy.

      • Proteins: Perform a variety of functions, including catalyzing reactions and transporting molecules.

      • Enzymes: Catalysts - speed up biochemical reactions. These are biological catalysts that facilitate chemical reactions within cells.

      • Nucleic acids (DNA, RNA): Carry genetic information.

      • Amino acids - build proteins. Building blocks of proteins, linked together by peptide bonds.

      • Handedness (or Chirality) - Left-handedness and right-handedness. Certain molecules exist in two forms that are mirror images of each other.

        • All currently discovered life on Earth use left-handed proteins and mostly right-handed carbohydrates; the reasons for this homochirality are still under investigation.

  • What are the major groupings of life on Earth?

    • Eukaryotes (cells with nuclei) and prokaryotes (cells without nucleus): Two fundamental cell types.

    • Super-kingdoms (or Domains) - classification based on chemical properties - biochemistry (not appearance or visual distinction):

      • Bacteria: One of the three domains of life, consisting of prokaryotic cells.

      • Archaea: Another domain of prokaryotic cells, often found in extreme environments.

      • Eukarya: The domain of life containing eukaryotic cells, including plants, animals, fungi, and protists.

    • Tree of life (common ancestry): All known life on Earth shares a common ancestor, represented as a branching tree showing evolutionary relationships.

  • Life brought from space: The possibility that life did not originate on Earth.

  • Deep-sea vents: Another possible origin point for life.

  • Clay and other minerals: Acted as catalysts to build RNA strands.

  • Lipid pre-cells: Formation of pre-cells.

  • RNA self-replication: Natural election improving replication until life emerges.

• Could life have migrated to Earth?

  • Migration from Venus or Mars: Impact blasts from one world to another.

  • Meteorites from Mars: Chance of survival traveling through space.

  • Migration from other star systems highly unlikely.

Cells: The Basic Units of Life

• What are living cells?

  • Basic units of life on Earth

  • Cell membrane - separates organic material from inorganic material

Defining Life

• What are the molecular components of cells?

  • Major Molecular Components of Cells

    • Carbohydrates

    • Lipids

    • Proteins

    • Enzymes: Catalysts - speed up biochemical reactions

    • Nucleic acids (DNA, RNA)

    • Amino acids - build proteins

    • Handedness (or Chirality) - Left-handedness and right-handedness

      • All currently discovered life on Earth use left-handed proteins and mostly right-handed carbohydrates

• What are the major groupings of life on Earth?

  • Eukaryotes (cells with nuclei) and prokaryotes (cells without nucleus)

  • Super-kingdoms (or Domains) - classification based on chemical properties - biochemistry (not appearance or visual distinction):

    • Bacteria

    • Archaea

    • Eukarya

  • Tree of life (common ancestry)

The Universal Context of Life

• What major lessons does modern astronomy teach us about our place in the universe?

  • Universe old and vast

  • Elements of life are widespread (chemicals of life found everywhere in the universe - nebula clouds in the universe)

  • Same physical laws that operate on earth also operate in the rest of the universe.

Structure of the Universe

• Cosmic Address

  • Structure of Solar system

    • Planets, moons, asteroids, comets

  • Structure of Milky Way Galaxy and our place in it - over 100 billion stars

  • Galaxies in general - few hundred million to trillions of stars - contains nebulae (gigantic clouds of dust)

  • Local Group

    • Milky Way part of more than 70 other galaxies - in general called galaxy clusters

  • Superclusters (very very large scale)

    • regions where galaxies and chains of galaxy cluster chains grouped together, could be tightly packed or have huge voids between them

    • Our Local Supercluster is called Laniakea - Hawaiian for “immense heaven”

Scale of the Solar System

• How are the planets, moons, asteroids, comets, Kuiper Belt Objects distributed in the solar system

  • Astronomical Unit (AU) - average distance between Earth and Sun

  • Where are the Voyager Space Probes now?

Distances to Stars

• Measured in Lightyears (distance light travels in one year): 1 lightyear approximately equal to 10 trillion kilometers (km)

  • Alpha Centauri (Three-star system) - Closest to the Sun - 4.4 lightyears - Takes 100,000 years to travel there with current spacecraft technology

  • The speed of light (fixed value, not infinite) - 300,000,000 m/s or 300,000 km/s

Scale of the Galaxy

• Over 100 billion stars - would take about 3000 years to try and count each one.

• Milky Way galaxy diameter - approximately 100,000 lightyears

• Center of our Milky Way - approximately 27,000 lightyears for Earth/Sun

• Orion Nebula - Located in Orion Constellation - tip of Orion’s Sword - approximately 1350 lightyears from Earth/Sun

• Flattened disk with central bulge (where most stars in galaxy located) - Earth/Sun located on one of spiral arms

• Milky Way galaxy - is a barred spiral galaxy (Dark Matter believed to surround Milky Way in giant unseen massive halo)

Content of the Universe

• Ordinary (Baryonic) Matter (Makes up ~5% of the Universe)

  • Baryonic Matter - made of three quarks.

  • Quark Flavors: (Up, Down, Charm, Strange, Top, and Bottom)

    • Up, Charm, and Top quarks have +2/3 charge

    • Down, Strange, and Bottom quarks have -1/3 charge

  • Protons made of uud (three quarks): total charge is +1

  • Neutron made of udd (three quarks): total charge is 0

  • Ordinary Matter interacts with light (electromagnetic radiation)

• Dark Matter - does not interact with light (electromagnetic radiation) - literally cannot be see with light, but made of matter - therefore can be observed with gravitational effects (like gravitational lensing & incorrect angular velocities of spinning galaxies).

  • Dark Matter (Makes up ~25% of the Universe) - believed to be key role in formation of galaxies

• Dark Energy (Makes up ~70% of the Universe) - like light / type of pure energy, but not electromagnetic energy.

  • Dark Energy is believed to be cause of expansion of the universe (analogous to how radiation pressure / light pressure from nuclear fusion balancing the size of a star) - stopping gravitational contraction - complete mystery to scientist, so far.

History of the Universe

• Big Bang and Expanding Universe

  • Expanding universe, not an explosion

  • Early universe much smaller and hotter - time of the Quark-Gluon Plasma (made up of u,d,s,c,t,b quarks and gluons)

  • As universe expanded, it cooled & protons and neutrons formed from quark-gluon plasma.

  • Protons slowed down (as temperature cooled) and electrons bound to protons to form Hydrogen atoms

  • Once atoms formed - photons (particles) of light could be produced - light can still be seen today as the Cosmic Microwave Background.

  • Cosmic Microwave Background (CMB) - remnants of the Big Bang - “glow” of radiation that can be seen in every direction of the universe - even in darkest/coldest regions of universe.

  • Early Universe - lacked heavier elements to make like Milky Way Galaxy - likely 12 -13 billion years old

Stellar Lives and Galactic Recycling

• Gravity drives collapse of clouds of gas and dust to form stars and planets (nebular hypothesis)

• Life cycle of stars - cloud of gas compresses due to gravity and center more dense and hot enough to generate energy/light by nuclear fusion

• Nuclear fusion - lighter elements (starts with hydrogen) hot/fast enough to fuse/combine together to form heavier nuclei/elements.

• As giant cloud of gas heats up and spins - it spins faster and faster - flattening into disk of gas and dust.

• Planets form on outer edges of the spinning nebular disk & star in center is formed.

• Star formed - light/energy released due to nuclear fusion in star’s core.

• At least 4 hydrogen required to form helium in core of a star.

• Matter is converted into Energy - Recalls Einstein’s equation; E = m c^2 (where m is mass, E is energy, and c is the speed of light).

• Specific predictions - relative abundance of heavier elements in massive stars at different ages

  • Carbon and oxygen atoms should be more abundant than Nitrogen atoms, and Neon atoms should be more abundant than Fluorine atoms - Theory is consistent with observations.

• We Are Star Stuff

• Stars manufacture heavier elements toward end of their life cycle

• Above a critical mass, massive stars explode through supernova releasing heavier elements

• Recycling process to make new stars and continue the cycle

• Relative abundances of each element can be studied to determine age of stars

Implications for Life in the Universe

• Expect many star systems to be made of similar chemical composition as our solar system - therefore conclude many star systems have the chemical composition to make and support life as we know it.

Scale of Time

• Universe 14 billion years old

• Condense into Cosmic Calendar to fit in one year

  • Midnight January 1st - start of Big Bang

  • Present day would be 11:59 pm December 31st.

  • First humans would have appeared on December 31st at around 9pm.

  • Each month would represent a little more that 1 billion years.

  • Earth would form around September 3rd.

  • Life would have had many chances to potentially star life, before Earth even formed.

How Big is the Universe?

• The universe is thought to be made up of sum total of all matter and energy.

• Could universe be infinite?

  • Limited with what we can observe, based on the age of your universe and limit light can travel in that time frame.

Observable Universe

• Looking back in time when looking at the more distant objects from earth

  • Andromeda galaxy is 2.5 million lightyears way - means light we are receiving from Andromeda left 2.5 million years ago or we are seeing what it looked like 2.5 million years ago.

Worlds Beyond Imagination

• 10^22 stars in the observable universe - equal to number of grains of sand in all the beaches of the world.

• Determined from Hubble Space Telescope - Hubble Extreme Deep Field

Fine-Tuned Universe

• Fine-tuned for life - anthropic principle - conditions were just right for our universe to form

  • Expansion of universe not too fast (to not allow for galaxies to form) or not too slow then gravity would pull all matter back into itself (collapsing the universe).

  • Ratio and strengths of the four fundamental forces balance just right to coexist with the laws of quantum physics.

A Universe of Matter and Energy

• What are the building blocks of matter?

  • Atoms

    • Protons, Neutrons, Electrons

    • Electron energy levels

  • Atoms and molecules - absorbing and emitting light

  • Chemical elements - different number of protons

  • Isotopes, Ions, Molecules

  • Stars of Matter - Solid, Liquid, Gas

What is Energy?

• Making matter move

• Different forms of energy

  • Kinetic energy - energy of motion

  • Potential energy - stored energy

  • Radiative energy - energy of light

• Conservation of energy

What is Light?

• Electromagnetic wave

• Particle vs. wave nature of light

  • photons - pieces of light

• Wavelength, frequency, energy of a wave

• Speed of a wave and speed of light (c = λ f)

• Frequency and wavelength inversely related

• Visible spectrum - ROYGBIV

• Radio waves, microwaves, infrared (IR), visible light, ultraviolet, X-rays, gamma rays

• Spectroscopy - finger-prints of the atoms and molecules

  • Information light carries from object it came from

• Thermal energy (blackbody radiation) - peak wavelength tells astronomers temperature of the star (or blackbody)

• Intensity of light - brightness / number of photons

• Doppler effect - red shift object moving away for observer / blue shift object moving toward observer

The Possibility of Life Beyond Earth

• What are we searching for?

  • Extraterrestrial life

    • Difficulty clearly defining life

    • Search for life similar to life on Earth

    • Single and multicellular life

• Is it reasonable to imagine life beyond Earth?

  • Examining possibility of life beyond Earth using modern science

    • Still no evidence of life beyond Earth

The Scientific Context of the Search

• How does astronomy help us understand the possibilities for extraterrestrial life?

  • Same physical laws operate on Earth as in the universe

    • Universality of Physics, Chemistry, and Biology

• How does planetary science help us understand the possibilities for extraterrestrial life?

  • Expect planets common around other stars

    • Extrasolar planets

    • Habitable worlds

• How does biology help us understand the possibilities for extraterrestrial life?

  • Three lines of evidence suggesting life might be common on habitable worlds:

    1. Life quickly arising quickly on Earth

    2. Meteorites and interstellar clouds observed with organic molecules common through galaxy

    3. Some life on Earth surviving extreme conditions

Places to Search

• Where should we search for life in the universe?

  • Where else can we search in our solar system

    • Direct searches on other planets and moons:

      • Mars

      • Jupiter’s moons:

        • Ganymede, Callisto, and Europa

      • Saturn’s moons:

        • Titan and Enceladus

    • Telescopic searches for life on other stars

• Could aliens be searching for us?

  • If life common, then intelligent life could be common as well

    • Possibility of other civilizations conducting their own searches

      • SETI (Search for Extraterrestrial Intelligence)

The New Science of Astrobiology

• How do we study the possibility of life beyond Earth?

  • Astrobiology

  • Three major areas:

    • Study conditions conducive for origin and ongoing existence of life

    • Finding habitable conditions within our own solar system and other stars

    • Find actual occurrences of life beyond Earth

    • Develop connections between living organisms and places they live

The Ancient Debate About Life Beyond Earth

• How did attempts to understand the sky start us on the road to science?

  • The Sky Observed By Many Civilizations

    • Ancient Chinese

    • Babylonians (near Iraq)

    • Mayans (Central America)

    • Etc,..

  • Early Greek Science

    • Thales (624 - 546 B.C.)

    • Anaximander (610 - 547 B.C.)

    • Pythagoras (560 - 480 B.C.) - mathematical perfection

    • Plato (428 - 348 B.C.)

    • Aristotle (384 - 322 B.C.) - tutor to Alexander the Great (Egypt - Library of Alexandria)

    • Eratosthenes (around 240 B.C.) - Circumference of the Earth

  • Geometry Models - conceptual representations to explain and predict observed phenomena

    • Led to Development of Modern Science

The Geocentric Model

• Celestial Sphere - Anaximander

• Geocentric Model (supporting arguments)

  • Ptolemy (A.D. 100 - 170)

    • Epicycles - circle within circles

The Mystery of Planetary Motion

• Planet - “wanderer” - generally move eastern relative to fixed stars

• Visible with naked eye: Mercury, Venus, Mars, Jupiter, Saturday

• Seven Days in a Week

• Explanation for Apparent Retrograde Motion

  • Ptolemaic Model - geocentric with epicycles (1500 years) - Almagest

    • Heavenly perfection

    • Still no evidence of life beyond Earth

An Alternate Model

• Aristarchus (310 - 230 B.C.)

  • Sun-Centered Model w/ Sun Much Larger Than Earth

  • Arguments Against - inconsistent with observation of stellar positions

    • Stars lie a different distances rather than all on the same spheres

    • Stellar Parallax

    • Stars too far away to be detectable w/ naked eye

The Roots of Modern Science

• Greeks reject idea of Sun-centered Model (Heliocentric Model)

• Modern Science: Reason vs. aesthetics, Models Consistent with Observations

• Nicholas Copernicus (1473 - 1543)

  • Re-visits Aristarchus’ Model

  • Led to development of Modern Science

Why Did the Greeks Argue About the Possibility of Life Beyond Earth?

• Atomists (Democritus - 460 - 370 B.C.)

  • World form by infinite number of indivisible atoms - implies existence of other worlds

  • More associated with Atheism

• Aristotelians

  • all earth must fall to center of universe

  • geocentric universe

  • Earth is unique

  • Heavens fundamentally different from Earth

  • No Other Worlds

  • Four Elements:

    • Earth, Water, Air, Fire

    • Fifth Element: Aether (or Ether) or Quintessence

• Thomas Aquinas (1225 - 1274) - Connection to Christian Theology

The Copernican Revolution

• Europe in Dark Ages

• Baghdad, Iraq - Intellectual Center of World (Middle Ages)

  • Byzantine Empire (Eastern Roman Empire) - Islam sought knowledge

    • Better understand wisdom of Allah

    • Greek works translated into Islam

    • Mathematics & astronomy

    • Algebra developed

  • Capital Constantinople (Modern Day Istanbul, Turkey)

    • Falls (1453)

    • Scholars Leave East & go West (Europe)

    • Start of European Renaissance

How Did the Copernican Revolution Further Develop Science?

• Copernican Revolution

  • Nicholas Copernicus (1473 - 1543)

    • Book published after his death: “Concerning the Revolutions of the Heavenly Spheres”

    • Heliocentric Model of Universe

      • Debated for 50 years / Not any better at prediction than Ptolemaic Model

    • Ideas and Debates - formed foundations of modern science

      • Moved Earth away from center of universe & replaced it with the Sun

      • Kept concept of perfectly circular orbits

    • People did not want to replace 2000 year old (Ptolemaic Model) - that produced similar inaccuracies

• Tycho - A New Standard in Observational Data

  • Lack of Data - could not validate Ptolemaic Model vs. Copernican Model

  • Telescope Not Yet Invented

  • Tycho Brahe (1546 - 1601) - From Dutch Aristocratic Family

    • Built largest naked-eye observatory - giant protractors

    • 30 years of observations (w/in 1 arc minute - less than thickness of thin part of a fingernail)

    • Most accurate data at the time

Kepler - A Successful Model of Planetary Motion

• Johannes Kepler (1571 - 1630) - German Astronomer & Mathematician

  • Used Tycho Brahe’s data to validate Ptolemaic Model vs. Copernican Model

  • Deeply Religious

    • Understanding the Geometry of the Heavens

  • Initially used circular orbits to model Tycho’ data

    • Mars data did not match Kepler’s predictions by 8 arc minutes (with circular orbits)

      • Did not ignore the discrepancy or blame the data

      • Abandoned perfectly circular orbits

      • Used special shape - called an ellipse (type of oval)

Kepler’s Laws of Planetary Motion

• Kepler’s 1st Law of Planetary Motion

  • Each planet orbits the sun in an ellipse with sun at one foci

  • Terms to Know:

    • Perihelion, aphelion, semi-major axis, center-to-focus distance, eccentricity

    • Eccentricity of 1 (straight line) & Eccentricity of 0 (perfect circle)

    • Calculation of Eccentricity: e = (center-to-focus distance) / (semi-major axis)

• Kepler’s 2nd Law of Planetary Motion

  • A planet moves faster in part of its orbit nearer the Sun and slower when farther from the Sun

  • Sweeping our equal areas (A) in equal times (t): A/t = constant

• Kepler’s 3rd Law of Planetary Motion

  • Planets farther from the Sun orbit at slower average speeds than planets closer to the sun

  • Obeying the mathematical relationship:

    • p^2 = a^3

    • p is the orbital period of the planet (time to complete one orbit)

    • a is the semi-major axis (or average distance for the sun)

  • Kepler’s Laws of Planetary Motion - far more accurate than Ptolemaic Model.

    • Describes the motion but does not explain why the planets behave that way.

Galileo - Answering the Remaining Objections

• Still strong objections to the heliocentric model regardless of Kepler’s Success

• Three Basic Objections:

  1. Earth cannot move because objects would be left behind as Earth moved

     • Like feeling a continuous wind on Earth

  2. Non-circular orbits contradicted idea of heavens being perfect

     • Realm of heavens:

       • Sun, Moon, Planets, and Stars

       • Must be perfect and unchanging

  3. No detection of Stellar Parallax

     • Should be observed if Earth revolved around the Sun

     • And the fixed stars were not that far away

Galileo Galilei (1564 - 1642) - Italian Mathematical and Astronomer

• Answers to Three Objections:

  1. Overturned Aristotelian view of physics (heavier objects fall to the ground faster than lighter objects)

     • Create experiments with rolling balls down an ramp (or incline)

     • Argued moving objects would remain in motion unless a force acted to stop it

     • Also known as Newton’s 1st Law of Motion (or the Law of Inertia)

     • Therefore, all objects on Earth share Earth’s motion

     • And stays with the Earth rather than falling behind (or feeling a continuous wind)

  2. Heavenly Perfection

     • Tycho Brahe

       • Observes new star (supernova explosion)

       • Proved comets lie beyond the moon (not earthly objects, but belonged as heavenly objects)

       • Heavens could undergo changes

     • Galileo

       • Built telescope (late 1609) - more powerful than naked-eye

       • Observed sunspots on sun (imperfections)

       • Moon had mountains & valleys

       • Noticed shadows near moon’s Terminator (line separating illuminated and dark sides of moon)

     • Kepler

       • Elliptical orbits and not perfect circles

  3. Absence of Observable Stellar Parallax

     • Tycho’s data (naked eye observations) - precision not enough to observe

     • Needed to prove distances were much, much farther away

     • Galileo’s telescope

       • Resolved individual stars of the Milky Way

       • Number of stars in Milky Way were distant stars

       • Farther than Tycho anticipated

       • Fours moons (companion stars) orbiting Jupiter, and not the Earth (Earth Not Center of Everything)

       • Phases of Venus:

         • Galileo observes phases just like the moon:

           • New Moons, Crescents, Quarter Moons, Gibbous, and Full Moons

           • Ptolemaic Model only allows for Crescents and New Moons

           • Heliocentric Model allows for all phases as observed by Galileo

           • Conclusive evidence supporting the heliocentric model

Newton - The Revolution Concludes

• Kepler’s Laws Described Motion of the Planets

  • Elliptic orbits w/ varying speeds

  • Accepted - But did not know why

• Issac Newton (1642 - 1727) - English Physicist

  • Invented Calculus

  • Published Book (1687): “Principia”

    • Precise mathematical description of motion

    • Explained and discovered many fundamental physics principles

  • Newton’s Three Laws of Motion:

    • 1st Law - Law of Inertia (Galileo)

    • 2nd Law - Net Unbalanced Force on an Object Causes Acceleration of that Object

    • 3rd Law - Action-Reaction Law - Forces Exist in Pairs

  • Kepler’s Laws describe only motion of planets around the sun - Newton’s Laws explain motion of all objects

Newton’s Law of Universal Gravitation - Explains Why Kepler’s Laws Describing Motion of Planets Works

• Created new model for inner workings of the universe

  • Motion governed by clear laws & force of gravity

  • Explained motion of objects on Earth and the Heavens

  • Combined Earth and Heavens as a Universe (One-verse)

  • Destroyed Geocentric Model of the Universe

• Looking Back at Revolutionary Science:

  • Copernicus - De Revolutionibus (1543)

  • Newton - Principia (1687)

    • Revolution ~150 years changed human history

• Radical Change in Human Perspective:

  • Earth shifted its central role to just another planet

  • Changed perspective how to acquire knowledge

  • No longer tolerating inaccuracies in predictions

  • Models of nature reproducing observations

  • Importance to ask why nature works (not just how)

• Past Thinking:

  • Focussed on cultural aesthetics

  • Acceptance even w/o evidence

• After Newton:

  • Guessing no longer good enough

  • Hard evidence / Consistent w/ observations

  • Rigorous mathematics

  • Convincing colleagues through clear logic and debate

  • Closer to understanding nature of universe

How Did the Copernican Revolution Alter the Ancient Debate on Extraterrestrial Life?

• Recall Ancient Greek Debate:

  • Atomists (many worlds) vs. Aristotelians (one unique world / central everything falls toward center)

• Copernican Revolution

  • Aristotle wrong / Heliocentric view of universe

  • Heavens are just other worlds, not just dots in the sky

    • Does not necessarily mean Atomists are right

  • Galileo - speculated moon covered with land and water like Earth

• Kepler

  • Suggested moon had an atmosphere & inhabited by intelligent beings

  • Science fiction story - called Somnium (The Dream”)

• Giordano Bruno (Dominican Friar & Philosopher)

  • Convinced of Extraterrestrial Life

  • Battles with Authorities

  • Burned at the Stake

• William Herschel (1738 - 1822) w/ sister Caroline

  • Discovered Uranus

  • Assumed all planets were inhabited

• Percival Lowell (1855 - 1916)

  • Discovered Pluto (PL may be his initials)

  • Believed saw canals on Mars

    • Popularized idea Martians might want to invade Earth

• Possible to argue endlessly, as long as no evidence to support your arguments

The Fact and Theory of Gravity

• Important to like in universe

  • If no gravity - life would float away

  • Planets could not have formed

  • Life couldn’t form

How Does the Fact of Gravity Differ From the Theory of Gravity?

• Fact of Gravity:

  • Objects do fall to the ground

  • Planets really do orbit the Sun

• Theory of Gravity:

  • Used to explain why gravity acts as it does

• Many Theories Improved Over Time:

  • Aristotle’s Theory of Gravity (claimed heavier objects fell to ground faster that lighter objects)

  • Galileo’s Theory of Gravity (all objects fall at same rate - if no air resistance)

  • Newton’s Law of Universal Gravitation

    • Cause of apple falling from tree - same cause as moon orbiting earth (recall orbital cannon thought experiment)

    • Earth and Heavens governed by a single principle - a universe

    • Newton’s Law of Universal Gravitation

      1. Every mass attracts every other mass through force of gravity

      2. Strength of gravitational force is directly proportional to product of masses

         • FG proportional to m1 and m_2

      3. Strength of gravitational force between two objects increases or decreases as the square of the distances between them

         • F_G proportional to 1/d^2 (Inverse Square Law)

      4. All three statements combined:

         • FG = G m1 m_2/d^2

         • G = 6.67 x 10^{-11} Nm^2/kg^2

  • Explained Galileo’s observations of falling objects

  • Explained Kepler’s Law of Planetary Motion

  • Edmond Halley (British Astronomer)

    • Predicted when a specific comet would return to earth

    • Returns every 76 years (Halley’s comet) using Newton’s Law

  • Urbain Leverrier (French Astronomer)

    • 1846 - strange behavior of Uranus’ orbit

    • Hypothesized an 8th planet - Neptune

    • Johann Galle (German - Berlin Observatory)

      • Observes Neptune w/in 1 degree of where Leverrier predicted (using Newton’s Law)

  • Applies to Not just Sun and Planets, also applies to:

    • Orbits of extrasolar planets around their respective stars

    • Stars around the Milky Way Galaxy

    • Galaxies orbiting each other

    • Moons orbiting planets

    • Artificial satellites orbiting the Earth, etc,…

  • Problems with Newton’s Law of Universal Gravitation

    • Discrepancy with observations of Mercury’s orbit (only observed with Mercury)

    • Leverrier hypothesized another planet closer to the sun - he called Vulcan (never found)

    • Newton never could explain spooky "action at a distance”

      • Distant masses never in contact w/ each other - how is a force then applied?

Einstein’s General Theory of Relativity

• Einstein’s Solution:

  • Action at a Distance - assumed all objects reside in something called spacetime

    • 4-Dimensional Spacetime (x, y, z, ct) - “Fabric”

      • where x, y, and z are space dimensions

      • And t is a time dimension, but ct can also become a spatial dimension.

      • Recall c is the speed of light (~3.00 x 108 m/s) & nothing can travel faster than c.

    • 2-D analogy:

      • Bowling ball on a mattress - bowling ball causes indentation on the mattress

      • Called a Gravitational Well - an object moving near the indentation will be deflected or trapped inside (like a planet orbiting the sun)

      • Action at a Distance - gravity arises from the way mass affects the structure of the spacetime “fabric”

      • Gravity is “curvature of spacetime”

• Newton’s Law of Universal Gravitation still works in general

  • But not with extreme conditions - when gravity is very strong (like being too close to the sun).

  • Otherwise, can still just use Newton’s Laws

• In the future, a better theory than Einstein’s Theory of General Relativity may be discovered - people are currently working to do this

• Need to find out what happens with

  • high density blackholes

  • infinitely small spaces (quantum mechanics)