Chapter 19: Life in the Universe
Chapter 1
How Science Works
- The Scientific Method
- Evidence
- Measurements
- Units and the Metric System
- Measurement Errors
- Estimation
- Dimensions
- Mass, Length, and Time
- Observations and Uncertainty
- Precision and Significant Figures
- Errors and Statistics
- Scientific Notation
- Ways of Representing Data
- Logic
- Mathematics
- Geometry
- Algebra
- Logarithms
- Testing a Hypothesis
- Case Study of Life on Mars
- Theories
- Systems of Knowledge
- The Culture of Science
- Computer Simulations
- Modern Scientific Research
- The Scope of Astronomy
- Astronomy as a Science
- A Scale Model of Space
- A Scale Model of Time
- Questions
Chapter 2
Early Astronomy
- The Night Sky
- Motions in the Sky
- Navigation
- Constellations and Seasons
- Cause of the Seasons
- The Magnitude System
- Angular Size and Linear Size
- Phases of the Moon
- Eclipses
- Auroras
- Dividing Time
- Solar and Lunar Calendars
- History of Astronomy
- Stonehenge
- Ancient Observatories
- Counting and Measurement
- Astrology
- Greek Astronomy
- Aristotle and Geocentric Cosmology
- Aristarchus and Heliocentric Cosmology
- The Dark Ages
- Arab Astronomy
- Indian Astronomy
- Chinese Astronomy
- Mayan Astronomy
- Questions
Chapter 3
The Copernican Revolution
- Ptolemy and the Geocentric Model
- The Renaissance
- Copernicus and the Heliocentric Model
- Tycho Brahe
- Johannes Kepler
- Elliptical Orbits
- Kepler's Laws
- Galileo Galilei
- The Trial of Galileo
- Isaac Newton
- Newton's Law of Gravity
- The Plurality of Worlds
- The Birth of Modern Science
- Layout of the Solar System
- Scale of the Solar System
- The Idea of Space Exploration
- Orbits
- History of Space Exploration
- Moon Landings
- International Space Station
- Manned versus Robotic Missions
- Commercial Space Flight
- Future of Space Exploration
- Living in Space
- Moon, Mars, and Beyond
- Societies in Space
- Questions
Chapter 4
Matter and Energy in the Universe
- Matter and Energy
- Rutherford and Atomic Structure
- Early Greek Physics
- Dalton and Atoms
- The Periodic Table
- Structure of the Atom
- Energy
- Heat and Temperature
- Potential and Kinetic Energy
- Conservation of Energy
- Velocity of Gas Particles
- States of Matter
- Thermodynamics
- Entropy
- Laws of Thermodynamics
- Heat Transfer
- Thermal Radiation
- Wien's Law
- Radiation from Planets and Stars
- Internal Heat in Planets and Stars
- Periodic Processes
- Random Processes
- Questions
Chapter 5
The Earth-Moon System
- Earth and Moon
- Early Estimates of Earth's Age
- How the Earth Cooled
- Ages Using Radioactivity
- Radioactive Half-Life
- Ages of the Earth and Moon
- Geological Activity
- Internal Structure of the Earth and Moon
- Basic Rock Types
- Layers of the Earth and Moon
- Origin of Water on Earth
- The Evolving Earth
- Plate Tectonics
- Volcanoes
- Geological Processes
- Impact Craters
- The Geological Timescale
- Mass Extinctions
- Evolution and the Cosmic Environment
- Earth's Atmosphere and Oceans
- Weather Circulation
- Environmental Change on Earth
- The Earth-Moon System
- Geological History of the Moon
- Tidal Forces
- Effects of Tidal Forces
- Historical Studies of the Moon
- Lunar Surface
- Ice on the Moon
- Origin of the Moon
- Humans on the Moon
- Questions
Chapter 6
The Terrestrial Planets
- Studying Other Planets
- The Planets
- The Terrestrial Planets
- Mercury
- Mercury's Orbit
- Mercury's Surface
- Venus
- Volcanism on Venus
- Venus and the Greenhouse Effect
- Tectonics on Venus
- Exploring Venus
- Mars in Myth and Legend
- Early Studies of Mars
- Mars Close-Up
- Modern Views of Mars
- Missions to Mars
- Geology of Mars
- Water on Mars
- Polar Caps of Mars
- Climate Change on Mars
- Terraforming Mars
- Life on Mars
- The Moons of Mars
- Martian Meteorites
- Comparative Planetology
- Incidence of Craters
- Counting Craters
- Counting Statistics
- Internal Heat and Geological Activity
- Magnetic Fields of the Terrestrial Planets
- Mountains and Rifts
- Radar Studies of Planetary Surfaces
- Laser Ranging and Altimetry
- Gravity and Atmospheres
- Normal Atmospheric Composition
- The Significance of Oxygen
- Questions
Chapter 7
The Giant Planets and Their Moons
- The Gas Giant Planets
- Atmospheres of the Gas Giant Planets
- Clouds and Weather on Gas Giant Planets
- Internal Structure of the Gas Giant Planets
- Thermal Radiation from Gas Giant Planets
- Life on Gas Giant Planets?
- Why Giant Planets are Giant
- Gas Laws
- Ring Systems of the Giant Planets
- Structure Within Ring Systems
- The Origin of Ring Particles
- The Roche Limit
- Resonance and Harmonics
- Tidal Forces in the Solar System
- Moons of Gas Giant Planets
- Geology of Large Moons
- The Voyager Missions
- Jupiter
- Jupiter's Galilean Moons
- Jupiter's Ganymede
- Jupiter's Europa
- Jupiter's Callisto
- Jupiter's Io
- Volcanoes on Io
- Saturn
- Cassini Mission to Saturn
- Saturn's Titan
- Saturn's Enceladus
- Discovery of Uranus and Neptune
- Uranus
- Uranus' Miranda
- Neptune
- Neptune's Triton
- Pluto
- The Discovery of Pluto
- Pluto as a Dwarf Planet
- Dwarf Planets
- Questions
Chapter 8
Interplanetary Bodies
- Interplanetary Bodies
- Comets
- Early Observations of Comets
- Structure of the Comet Nucleus
- Comet Chemistry
- Oort Cloud and Kuiper Belt
- Kuiper Belt
- Comet Orbits
- Life Story of Comets
- The Largest Kuiper Belt Objects
- Meteors and Meteor Showers
- Gravitational Perturbations
- Asteroids
- Surveys for Earth Crossing Asteroids
- Asteroid Shapes
- Composition of Asteroids
- Introduction to Meteorites
- Origin of Meteorites
- Types of Meteorites
- The Tunguska Event
- The Threat from Space
- Probability and Impacts
- Impact on Jupiter
- Interplanetary Opportunity
- Questions
Chapter 9
Planet Formation and Exoplanets
- Formation of the Solar System
- Early History of the Solar System
- Conservation of Angular Momentum
- Angular Momentum in a Collapsing Cloud
- Helmholtz Contraction
- Safronov and Planet Formation
- Collapse of the Solar Nebula
- Why the Solar System Collapsed
- From Planetesimals to Planets
- Accretion and Solar System Bodies
- Differentiation
- Planetary Magnetic Fields
- The Origin of Satellites
- Solar System Debris and Formation
- Gradual Evolution and a Few Catastrophies
- Chaos and Determinism
- Extrasolar Planets
- Discoveries of Exoplanets
- Doppler Detection of Exoplanets
- Transit Detection of Exoplanets
- The Kepler Mission
- Direct Detection of Exoplanets
- Properties of Exoplanets
- Implications of Exoplanet Surveys
- Future Detection of Exoplanets
- Questions
Chapter 10
Detecting Radiation from Space
- Observing the Universe
- Radiation and the Universe
- The Nature of Light
- The Electromagnetic Spectrum
- Properties of Waves
- Waves and Particles
- How Radiation Travels
- Properties of Electromagnetic Radiation
- The Doppler Effect
- Invisible Radiation
- Thermal Spectra
- The Quantum Theory
- The Uncertainty Principle
- Spectral Lines
- Emission Lines and Bands
- Absorption and Emission Spectra
- Kirchoff's Laws
- Astronomical Detection of Radiation
- The Telescope
- Optical Telescopes
- Optical Detectors
- Adaptive Optics
- Image Processing
- Digital Information
- Radio Telescopes
- Telescopes in Space
- Hubble Space Telescope
- Interferometry
- Collecting Area and Resolution
- Frontier Observatories
- Questions
Chapter 11
Our Sun: The Nearest Star
- The Sun
- The Nearest Star
- Properties of the Sun
- Kelvin and the Sun's Age
- The Sun's Composition
- Energy From Atomic Nuclei
- Mass-Energy Conversion
- Examples of Mass-Energy Conversion
- Energy From Nuclear Fission
- Energy From Nuclear Fusion
- Nuclear Reactions in the Sun
- The Sun's Interior
- Energy Flow in the Sun
- Collisions and Opacity
- Solar Neutrinos
- Solar Oscillations
- The Sun's Atmosphere
- Solar Chromosphere and Corona
- Sunspots
- The Solar Cycle
- The Solar Wind
- Effects of the Sun on the Earth
- Cosmic Energy Sources
- Questions
Chapter 12
Properties of Stars
- Stars
- Star Names
- Star Properties
- The Distance to Stars
- Apparent Brightness
- Absolute Brightness
- Measuring Star Distances
- Stellar Parallax
- Spectra of Stars
- Spectral Classification
- Temperature and Spectral Class
- Stellar Composition
- Stellar Motion
- Stellar Luminosity
- The Size of Stars
- Stefan-Boltzmann Law
- Stellar Mass
- Hydrostatic Equilibrium
- Stellar Classification
- The Hertzsprung-Russell Diagram
- Volume and Brightness Selected Samples
- Stars of Different Sizes
- Understanding the Main Sequence
- Stellar Structure
- Stellar Evolution
- Questions
Chapter 13
Star Birth and Death
- Star Birth and Death
- Understanding Star Birth and Death
- Cosmic Abundance of Elements
- Star Formation
- Molecular Clouds
- Young Stars
- T Tauri Stars
- Mass Limits for Stars
- Brown Dwarfs
- Young Star Clusters
- Cauldron of the Elements
- Main Sequence Stars
- Nuclear Reactions in Main Sequence Stars
- Main Sequence Lifetimes
- Evolved Stars
- Cycles of Star Life and Death
- The Creation of Heavy Elements
- Red Giants
- Horizontal Branch and Asymptotic Giant Branch Stars
- Variable Stars
- Magnetic Stars
- Stellar Mass Loss
- White Dwarfs
- Supernovae
- Seeing the Death of a Star
- Supernova 1987A
- Neutron Stars and Pulsars
- Special Theory of Relativity
- General Theory of Relativity
- Black Holes
- Properties of Black Holes
- Questions
Chapter 14
The Milky Way
- The Distribution of Stars in Space
- Stellar Companions
- Binary Star Systems
- Binary and Multiple Stars
- Mass Transfer in Binaries
- Binaries and Stellar Mass
- Nova and Supernova
- Exotic Binary Systems
- Gamma Ray Bursts
- How Multiple Stars Form
- Environments of Stars
- The Interstellar Medium
- Effects of Interstellar Material on Starlight
- Structure of the Interstellar Medium
- Dust Extinction and Reddening
- Groups of Stars
- Open Star Clusters
- Globular Star Clusters
- Distances to Groups of Stars
- Ages of Groups of Stars
- Layout of the Milky Way
- William Herschel
- Isotropy and Anisotropy
- Mapping the Milky Way
- Questions
Chapter 15
Galaxies
- The Milky Way Galaxy
- Mapping the Galaxy Disk
- Spiral Structure in Galaxies
- Mass of the Milky Way
- Dark Matter in the Milky Way
- Galaxy Mass
- The Galactic Center
- Black Hole in the Galactic Center
- Stellar Populations
- Formation of the Milky Way
- Galaxies
- The Shapley-Curtis Debate
- Edwin Hubble
- Distances to Galaxies
- Classifying Galaxies
- Spiral Galaxies
- Elliptical Galaxies
- Lenticular Galaxies
- Dwarf and Irregular Galaxies
- Overview of Galaxy Structures
- The Local Group
- Light Travel Time
- Galaxy Size and Luminosity
- Mass to Light Ratios
- Dark Matter in Galaxies
- Gravity of Many Bodies
- Galaxy Evolution
- Galaxy Interactions
- Galaxy Formation
- Questions
Chapter 16
The Expanding Universe
- Galaxy Redshifts
- The Expanding Universe
- Cosmological Redshifts
- The Hubble Relation
- Relating Redshift and Distance
- Galaxy Distance Indicators
- Size and Age of the Universe
- The Hubble Constant
- Large Scale Structure
- Galaxy Clustering
- Clusters of Galaxies
- Overview of Large Scale Structure
- Dark Matter on the Largest Scales
- The Most Distant Galaxies
- Black Holes in Nearby Galaxies
- Active Galaxies
- Radio Galaxies
- The Discovery of Quasars
- Quasars
- Types of Gravitational Lensing
- Properties of Quasars
- The Quasar Power Source
- Quasars as Probes of the Universe
- Star Formation History of the Universe
- Expansion History of the Universe
- Questions
Chapter 17
Cosmology
- Cosmology
- Early Cosmologies
- Relativity and Cosmology
- The Big Bang Model
- The Cosmological Principle
- Universal Expansion
- Cosmic Nucleosynthesis
- Cosmic Microwave Background Radiation
- Discovery of the Microwave Background Radiation
- Measuring Space Curvature
- Cosmic Evolution
- Evolution of Structure
- Mean Cosmic Density
- Critical Density
- Dark Matter and Dark Energy
- Age of the Universe
- Precision Cosmology
- The Future of the Contents of the Universe
- Fate of the Universe
- Alternatives to the Big Bang Model
- Space-Time
- Particles and Radiation
- The Very Early Universe
- Mass and Energy in the Early Universe
- Matter and Antimatter
- The Forces of Nature
- Fine-Tuning in Cosmology
- The Anthropic Principle in Cosmology
- String Theory and Cosmology
- The Multiverse
- The Limits of Knowledge
- Questions
Chapter 18
Life On Earth
- Nature of Life
- Chemistry of Life
- Molecules of Life
- The Origin of Life on Earth
- Origin of Complex Molecules
- Miller-Urey Experiment
- Pre-RNA World
- RNA World
- From Molecules to Cells
- Metabolism
- Anaerobes
- Extremophiles
- Thermophiles
- Psychrophiles
- Xerophiles
- Halophiles
- Barophiles
- Acidophiles
- Alkaliphiles
- Radiation Resistant Biology
- Importance of Water for Life
- Hydrothermal Systems
- Silicon Versus Carbon
- DNA and Heredity
- Life as Digital Information
- Synthetic Biology
- Life in a Computer
- Natural Selection
- Tree Of Life
- Evolution and Intelligence
- Culture and Technology
- The Gaia Hypothesis
- Life and the Cosmic Environment
The Best Way to Communicate
The search for extraterrestrial intelligence (SETI) is based entirely on speculation. The only way to move beyond mere speculation is to obtain some real data, conduct an experiment, and gather supporting evidence. We have already been listening for messages and we have even sent our own on space probes and with radio waves. So what is our strategy? What is the best message to send? What type of communication should we use? There are millions of possible targets in the galaxy, where should we beam our signal?
It is important to be aware that, unlike other scientific endeavors, we cannot adequately justify a search on strictly rational grounds. The case for SETI cannot be made in the same way as the case for building a particle accelerator or for sequencing a gene. To confront this reality, SETI optimists, such as Carl Sagan, have argued that the results of a search for extraterrestrial intelligence will be significant, no matter what the result is. However, a failure to detect intelligent life will not prove that we are alone. The truth is that SETI is a scientific gamble. The odds are stacked against us, but the stakes are incredibly high. Our role as sentient beings in this expansive universe will be profoundly affected by whether or not we are alone. And unless we conduct an experiment, we will never know.
In 1972, the Pioneer 10 spacecraft was launched toward Jupiter, and it has since become the first human artifact to leave the Solar System. With it, we attached our first message, a plaque carrying a greeting to any civilization that might find it. The plaque shows a naked man and woman next to a silhouette of the Pioneer spacecraft. The top of the drawing shows the spin transition of the hydrogen atom, and the bottom shows the trajectory of the spacecraft within the Solar System. The radial pattern represents the position of the Solar System within the Milky Way, by triangulation among a set of 14 pulsars. At the time it was launched, the plaque created quite a stir. Many complained about the government-sanctioned nudity, and feminists objected to the fact that the man's hand was raised in greeting, but not the woman's. Also, some people found the pulsar map to be obscure, wondering how aliens would decipher it if they could not. The plaque illustrates our first difficulties in trying to encapsulate the essence of human beings in a short, concise message.
Five years later, in 1977, the Voyager spacecraft was slated for launch. A team headed by Carl Sagan and Frank Drake designed a new message to be sent into space with it. This time they included music in addition to images. As Lewis Thomas has said, "I would vote for Bach, all of Bach, streamed out into space, over and over again. We would be bragging, of course, but... we can tell the harder truths later." Instead of a plaque, this time a gold-anodized record was attached to the spacecraft. Both images and sounds were encoded onto the record and instructions were etched on the cover. Now, just a few decades later and well before reaching another planetary system, the recording technology used to encode the message is obsolete on Earth. Well, nearly, vinyl LPs are currently making a resurgence. Also, the team who designed it pointed out that modern digital storage devices like CDs and DVDs are already degrading, whereas the sturdy analog technology attached to Pioneer will endure for millennia.
In an attempt to represent the diversity of humans and the natural environment, a wide variety of images were included on the record. Unfortunately, perhaps in response to the complaints of the naked pair on Pioneer's plaque, censorship prevailed and NASA vetoed one picture involving nudity. The sound selections included many natural sounds such as those made by whales, rain, footsteps, and a kiss. They also included music such as Bach, jazz, and rock and roll, but also much non-Western music. Spoken greetings in 55 different languages are followed by a message from the United Nations Secretary General. Despite the efforts, it is difficult not to see the record as a message to us, rather than to an alien civilization. It is our "message in a bottle," tossed hopefully into the void of space.
Since the primary purpose of the spacecraft was exploration and not communication with an extraterrestrial civilization, it's important to contemplate the time and effort put into creating the messages. Like bottles tossed into the ocean, no one knows how long they will be lost at sea before being picked up by an unsuspecting seafarer. Almost five decades have passed, and Pioneer 10 is now over 20 billion kilometers (12 billion miles) away from us, far beyond the orbit of Pluto. Even traveling at about 6 miles per second, it will take 100,000 years to reach the nearest star to the Sun.
How can we build an interstellar probe that travels at more than the current limit of 10 miles per second, which is only ½0,000 of the speed of light? Unfortunately, the laws of physics work against us. The energy required to accelerate a small payload of 100 kg to one-tenth the speed of light exceeds one year's output from all the power plants on the Earth! To reach 99% of the speed of light using the most efficient energy source imaginable, the annihilation of matter and antimatter, requires a spacecraft with 40,000 times as much mass in fuel as in payload. The energy requirements for transmitting electromagnetic radiation are far less restrictive. The kinetic energy of a radio wave photon is 1012 times less than the kinetic energy of an electron traveling at 99% of the speed of light.
Rather than create the technology to launch something that can accelerate to the speed of light, why not use something that naturally achieves that velocity? Electromagnetic waves (or photons) are the preferred carriers of information for just this reason, and they are easy to transmit, modulate, and receive. The only drawback is that photons range in frequency and wavelength over a factor of 1020 from radio waves to gamma rays. How do we choose a single optimum frequency for communication from such a large range? Luckily, nature has provided us with some guidance. First, radio waves contain the least amount of energy per photon, and so are the most efficient to produce. Secondly, photons with optical frequencies or higher suffer absorption and scattering by gas and dust in the interstellar medium. The best penetration through these barriers is achieved by radio waves. Finally, when we measure the spectrum of cosmic radio "noise", it shows that the quietest region on the dial is the zone around 1000 MHz (a thousand million Hertz, or 109 Hz). At lower radio frequencies, radiation from high-energy electrons in the Milky Way contaminates the signal. At higher radio frequencies, there is a rising noise source due to the cosmic background radiation. The quiet zone — in other words, the frequency range where naturally occurring cosmic noise is low — also contains the frequency of the spin transition of cold hydrogen, the most abundant element in the universe.
Even if we accept the arguments for radio communication, we still face a challenge. Due to the nature of the beast, we have a classic "needle in a haystack" problem. There is a range of thousands of MHz in the zone in which cosmic noise is low. We do not know in advance what the bandwidth or range of frequencies of a signal should be. To send a large amount of information, the bandwidth should be large. A single TV channel has a range of 6 MHz, and an FM radio station transmits over a range of 200 kHz (200,000 Hertz). On the other hand, to transmit the information as efficiently as possible, a narrow bandwidth should be used. It doesn't matter how narrow the signal is originally, radio waves will scatter in interstellar space that smears the signal to a width of about 0.1 Hz. Smearing is analogous to the blurring of an optical image as it passes through the Earth's atmosphere. Therefore, when searching for a message from space, we must examine thousands or perhaps millions of targets, each of which must be searched over billions of separate frequency channels! And when composing a message, we must decide what are the most efficient channels and targets to use.
Communicating via radio waves presents a unique set of challenges when compared to similar messages in pictorial form on plaques. How do we convey a picture over radio waves? How will the radio waves be interpreted? We haven't spent much time designing a message. In fact, many people are aware that for fifty years we have been inadvertently leaking radio, radar, and television signals into space, creating a bubble of radio energy expanding outward from the Earth at the speed of light. Dilute signals of I Love Lucy have crossed the paths of approximately 1000 stars, and The Brady Bunch has reached several hundred. Only a few dozen star systems have been treated to episodes of Seinfeld. The joke goes that aliens are not visiting us because they have received our radio and television broadcasting and have so far seen no signs of intelligent life. In fact, these signals weaken so rapidly by the inverse square law that they fall below the radio "hiss" of the microwave background from the big bang before they even leave the Solar System. The spinning Earth does send out radio waves that rise and fall several times per day due to the concentration of transmitters in the United States and Europe, but the two largest sources of our radio leakage have diminished. Powerful early warning radar has been dismantled due to the end of the Cold War, and TV transmissions are moving towards fiber and cable.
What is the best way to communicate? Scientists at the SETI Institute contemplate this very question every day. They have moved on from listening only to radio waves and have added a new program called Optical SETI, which will look for single, short-lived bursts of optical light from our neighbors. The search is vast, the stakes are high, but the results of success would be profound.