ASTR 1210 (O'Connell) Study Guide

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1. INTRODUCTION: SCIENTIFIC DISCOVERY
AND THE SCALE OF THE UNIVERSE

The European Southern Observatory Very Large Telescope, Chile.

This introductory lecture places astronomy in the broader context of science. It discusses the nature of science, how science is distinguished from other modes of thought, the difference between science and technology, and some of the main results of science.

Astronomy has had a strong influence on other sciences and defines the limits of the scientific universe. We will illustrate some of the mind-boggling cosmic spatial and temporal scales revealed by astronomical research.

A. What is Science?

My definition

Science is the systematic understanding of empirical phenomena.
"Empirical" = experienced or observed
Science is both a body of knowledge and a way of thinking.
Scientific values & methods are relatively new in human history, starting ca. 1500 AD.

Characteristics

• Logical; rational

• Clear, orderly; choose the simplest alternative ("Occam's razor")

• Cumulative development
  New interpretations are required to be consistent with all established facts.
  Given all that we now know about nature, this is a tremendously challenging requirement!

Key feature

• Science is based on an empirical criterion of truth: ideas are tested by objective experiment/observation in the real world.
  • Evaluation of evidence must be rigorous, objective, and public.

  • Any valid scientific proposition is falsifiable by empirical tests. I.e. it is well enough defined that you can specify in advance those outcomes that would show the idea to be wrong.

  • The results of tests must be repeatable.

  • Confidence in ideas grows with the number of independent tests or validations. "Independence" implies verification not only by different people but also by different methods of inquiry or lines of evidence.

The scientific method in practice

Scientific methods were not invented or prescribed wholesale by some individual or group. Rather, they emerged over a period of several centuries as the set of analytical techniques that proved most successful in the struggle to understand nature.
The usual formalized statement of "The Scientific Method" (e.g. Hypothesis ===> Prediction ===> Experiment ===> Interpretation ===> Repeat) is misleading:
Scientists do not necessarily work from "hypotheses." They frequently simply explore the characteristics of new phenomena, without any particular intent to distinguish between interpretations but with the hope that they might prove interesting or important.
A number of sciences, including astronomy, deal with phenomena that cannot be manipulated by humans. Experiments are impossible in these cases. These sciences must rely instead on passive observations.
For astronomy, the only exception is that we can, with difficulty, sample and experiment with the surfaces and atmospheres of some of the other bodies in the solar system.

Science and "Truth"

Scientific ideas are not claimed to be absolute truths. They are always provisional, to a greater or lesser degree. They are regarded as approximations to the truth, subject to revision as new evidence emerges. They systematically improve with time, by discarding older, less successful interpretations.

Science and "Common Sense"

The characteristics of scientific thinking should sound familiar to you: this is really nothing more than slightly refined, standardized, and tough-minded "common sense." Scientific thinking deviates from "common sense" mainly in that it insists on validating ideas in the context of a vast store of existing knowledge that goes far beyond everyday experience.
The subjects and conclusions of science, however, often differ drastically from the "common-sense perspective" that we intuitively develop in our everyday lives. The familiar structures that surround us (e.g. trees, rivers, mountains) are utterly different from those that characterize spatial scales only a couple of orders of magnitude removed, either larger or smaller. And nature comprises an almost unimaginable range of physical scales, as illustrated at the right.
Humans directly experience only a minuscule range of the distances, timescales, masses, densities, velocities, and energy releases that prevail in the universe around us, and those are often completely non-intuitive for us.
In fact, most of science deals with phenomena that are "invisible" in everyday life even if they, like electrons and bacteria, are actually ubiquitous in the world around us. They have been recognized by scientists only gradually through systematic, painstaking observations, experiments, and measurements.
One of the difficulties in communicating science to others is leading the audience to appreciate this real but invisible landscape.

We will lay the groundwork for going beyond the "common-sense perspective" to a cosmic perspective in Section G below.

Science and mathematics

Mathematics is integral to science. In most physical sciences, including astronomy, mathematics is an essential technical ingredient in interpretation. Large swaths of modern biology and medical science, based on interpretation of the structures of huge nucleic acid and protein molecules, are likewise highly quantitative. Mathematics is needed in the design and analysis of experiments. For example, it is used to determine how the inevitable uncertainty in any measurement affects the reliability of scientific inferences, especially since these often depend on combining measurements of dozens of different quantities, each with its own limitations. But even where complicated mathematics is not required, the rules of logic, as elaborated by mathematics, are still essential.
It is actually surprising that mathematics, developed for the most part in the abstract, is capable of describing the real, empirical universe with exquisite accuracy. This mysterious "unreasonable effectiveness" of mathematics has been the subject of a number of thoughtful studies.
The symbiosis between science and mathematics is evident even in the historical context. The first society to develop a scientific outlook was ancient Greece, whose thinkers were simultaneously laying the foundations of mathematics. And Galileo, whose 17th century experiments were the breakthrough that ignited modern physics, believed that without mathematics "we are wandering in vain through a dark labyrinth" --- i.e. that the clarifying insights of mathematics are essential to understanding the universe.

B. Alternative Modes of Thought

It is worthwhile distinguishing science from some other important modes of thinking.

Revelation (religion)

Explanations are sought by appeal to a spiritual realm beyond the natural world ("super"-natural).
Prior to the development of science, important physical phenomena were usually interpreted as the product of deliberate and purposeful manipulation by supernatural agencies, with motives largely beyond human understanding. Knowledge of these agencies was thought to be possible only through intentional revelation by them.
By contrast, science deals only with the natural world. It seeks explanations of natural phenomena in terms of other natural phenomena.
Supernatural explanations are not acceptable to scientists because there is no way to test them.
Objective empirical tests are impossible for the supernatural world. Science cannot predict supernatural behavior or its intervention in our world since, by definition, the laws of nature don't apply.
However, science could in principle detect supernatural intervention in the natural world a posteriori (after the fact) because it would appear as a breakdown in the laws of nature. The fact that discontinuities in natural law have never been verified shows that direct intervention is rare at current levels of detection. Frequent intervention would produce an arbitrary and largely unintelligible world.

When revelation was the dominant means of trying to understand the world there might have been as many different interpretations of a given phenomenon as there were religious traditions. By applying empirical tests, science replaced this confusion with a unified, pan-cultural vision of the universe.
Dependence on revelation to interpret the world produces a hierarchy of privilege. Access to understanding, and the corresponding power, is normally accorded only to a few (priests, prophets, gurus). This is a central danger to this kind of thinking, which is not consistent with a democratic society.

Idealism

Idealism gives priority to the human mind and its constructs over evidence from the external world. It asserts that truthful conclusions about the natural world can be reached purely by analysis of ideas.
Idealism was first championed by ancient Greek philosophers, most famously Plato. Deductions about the physical world are made from abstract, a priori postulates. It is assumed that empirical evidence is corrupted and unreliable.
While this approach may be fine for mathematics (in which the Greeks excelled), it is badly flawed as a means of assessing the real world.

Authority

Ideas are tested by appeal to the pronouncements of an individual or small group. These cannot be questioned or superseded. Again, a non-democratic system.
The most familiar, and dangerous, variety of authoritative "truth" is found in dictatorships. But authority is also the basis of the case law system ("precedent") in American jurisprudence.
Social systems based on revelation almost always invoke authority as well.

Despite the public impression, science is not based on authority. Instead, it operates by consensus. Any scientist is entitled to question and retest any idea at any time and attempt to convince others to change their minds. Science is democratic.
Scientific "authorities" are respected not because of who they are but instead because of what they say. Their ideas are continuously subjected to new tests. The best idea prevails.
Young scientists often aspire to overthrow "authorities" like Einstein; they almost always fail because their ideas are not as good when tested against the evidence.

Pseudo-science

"Pseudo-scientific" investigations fall at the margins of science. They are often devoted to topics of some popular interest -- such as astrology or "unidentified flying objects" (UFOs) in the case of astronomy -- but which have either not yet been validated by mainstream science or have been rejected by it.
Advocates of pseudo-scientific investigations apply some, but not all, of the established methods of science. These fall short of the basic criteria listed above in important ways. They always involve a lack of adequate evidence and frequently contradict validated principles of science. But they have credibility for many people because they resemble science, and this opens a large arena for human folly.
We will discuss pseudo-science further, and the "UFO phenomenon" as an example, later in Study Guide 18.

C. Skepticism

"Cultivated skepticism" is a cornerstone of science.

All good scientists are skeptics. This means that they maintain an attitude of doubt or of suspended judgement about scientific ideas.

Scientists insist on the best possible evidence in support of new ideas. The adjective that best describes the scientific approach to evidence is "relentless."
Accepted ideas, no matter how well established, are constantly tested against new evidence, sifted for quality. All preconceptions are subject to scrutiny. History shows that truth only emerges through this kind of skeptical "scrubbing" of ideas and evidence. The ideas that survive this process are very robust.
Competition over ideas, often intense, is a hallmark of science in practice. There are strong incentives for scientists to discover new phenomena or interpretations that countervail the accepted wisdom. That is how young people make careers in science.
However, scientific doubt isn't frivolous; it must be based on a balanced evaluation of the quality of the empirical evidence. Good scientists are scrupulous in their treatment (positive or negative) of the evidence; their reputations suffer otherwise.
Don't confuse healthy scientific skepticism with the activist "skeptics" who have emerged over the last couple of decades to dispute the scientific consensus on evolution, global warming, vaccination against disease, tobacco-induced illness, and other politically or religiously sensitive subjects. These advocacy groups mostly do not care about the evidence or hold their judgement in suspension -- rather they have a committed belief in a contrary view or a financial stake in opposing the consensus. This kind of agenda-driven "skepticism" is now sometimes called denialism.

Skepticism based on facts is not an established element in many other modes of human thinking. In fact, most people are uncomfortable with skepticism and instead see virtue in conviction, certainty, and strong beliefs.

As Bertrand Russell remarked, and as we can see daily in news reports, this may be the "fundamental cause of trouble in the world today."

Errors occur in science, as in any human endeavor. Historically, there have been many more wrong ideas in science than right ones. Understanding the natural world is hard. Every scientist has a long list (usually private) of mistakes they have made. But because of skepticism and a reliance on real evidence, there is a strong self-correcting mechanism operating in science: the errors are identified and discarded.

"He believed in the primacy of doubt, not as a blemish upon our ability to know
but as the essence of knowing."
---- J. Gleick, writing about physicist Richard Feynman.

The Golden Gate Bridge, a premier example
of 20th century technology

D. Science vs. Technology

Although science and modern technology are often intertwined, they have different goals and value systems; and we need to clarify the distinctions between them. Science and technology are symbiotic but distinct:

• Science: The attempt to understand nature; to build a conceptual framework.
  Adjectives used: "unapplied," "pure" or "basic"
  Examples: principles of gravitation, electromagnetism, biochemistry, astronomy

• Technology: Application of basic concepts to societal needs.
  Examples: structural engineering, electrical generators, weapons, pharmacology, computers

All technology has a societal motivation, whether for ultimate good or ill, but the main motivation for "basic" science is simply curiosity and the desire to understand.

Job descriptions:

• Scientist: "be curious"

• Technologist: "be useful"

E. Results of Science

The basic result of science

Nature is understandable and operates according to a small set of universal principles, or "laws"
We may take this for granted today, but it is an astonishing fact. None of the early scientists who wrestled with trying to interpret nature could have predicted it. The notion that a rainbow, lightning, and the shape of a snowflake are all governed by exactly the same underlying principles would have sounded absurd to them.

Some other key results

• The laws of nature are the same everywhere in the observed universe
  
• All matter, including living systems, is organized hierarchically from a smaller set of subunits
  
• The structure of molecules & atoms is determined by electromagnetic force
  
• The Sun is a star
  
• The Earth is a planet
  
• Genetic information is transmitted by DNA molecules
  
• Most communicable diseases are caused by microorganisms
  
• The universe is very ancient but had a definite beginning about 14 billion years ago. Its structure and contents have changed systematically with time
  

Precursors

Very few of the important results of science had recognizable precedents in modes of thought prior to 1500 AD.
The main exceptions are Greek physics and astronomy, e.g. the models of the solar system by Ptolemy, ca. 130 AD, or the atomic theory of Democritus, ca. 420 BC. The Greeks made great progress. However, because of their inclination to idealism, they disdained empirical testing of scientific ideas, which circumscribed their ultimate accomplishments. And those were forgotten or actively suppressed in the Western world for nearly a thousand years starting about 400 AD. See Study Guide 6.

How well determined are scientific results?

Some scientific results are known with effective certainty (e.g. Earth is a planet; only one Sun in the solar system).
Aside from reaching reliable conclusions, scientists have been able to measure many important aspects of nature with amazing precision. Even in astronomy, where we cannot exert control over the things we are measuring, a number of quantities -- e.g. the distance between the Earth and the Sun -- have been determined to a precision of better than one part in ten billion.

But, as noted above, scientific results are subject to revision based on new evidence. Scientific ideas normally systematically improve by discarding older, less successful interpretations.
You can picture the progress of science as an inward spiral toward the truth as better evidence accumulates.

At any given time, some scientific results are much less well established than others, so there is a huge range of validity (say from 99.999% certain to less than 50% certain) throughout the scientific literature.
Among science that is less well established are many "interesting" results in areas like human psychology and the influence of lifestyle on health (especially diet). Unfortunately, these tend to get wide publicity (isn't it great to hear that chocolate is good for your heart?). And when contrary studies appear later, this diminishes confidence in science overall.
Difficulties in these areas arise because it isn't ethical to experiment on human subjects in ways that might produce more definitive conclusions. Large samples are often hard to gather, but small samples yield large experimental scatter and also prevent assessment of the effects of the huge range of personal characteristics. And in the absence of hard statistics, investigators are sometimes swayed by random variations in ambiguous data sets that seem to confirm their hypotheses and are reluctant to publish studies that contradict those. You shouldn't judge the reliability of basic scientific conclusions by the muddled impression emerging from the popular press.

Instead, here is a simple, practical measure of the validity of the most important scientific ideas:
If our scientific understanding of nature was seriously flawed, then most of our technology would simply not work. The validity of ideas like Newton's laws of motion or Maxwell's theory of electromagnetism or the germ theory of disease is tested continuously in every automobile, cell phone, and hospital in the world.
Keep this success in mind when evaluating debates over scientific results in the public arena. The same analytic procedures and standards of evidence that produced the technologies we depend on in our daily lives are behind the scientific consensus in areas like biological evolution, the value of vaccines, or global warming that are disputed by activist groups.

Influence on society

As a new mode of thinking about the natural world, science succeeded brilliantly. It demonstrated that humans can solve many problems that at first seemed intractable: e.g. questions like "what causes bubonic plague?" or "what are the stars?" or "how old is the Earth?"
Success in science has been a great source of optimism in human cultures. This was a major wellspring for the Enlightenment and the political philosophies that culminated in the founding of Western democracy.

Science provides a demonstrably reliable and powerful understanding of the physical and biological world. Over the past two centuries it has transformed human societies. It is now the basis of most of our technology, our wealth, and our collective and individual well-being. The impact of science on our society is discussed further in Study Guide 9.

The Moon and Venus at dusk

F. Astronomy as a Science

Astronomy is the study of the physical universe beyond the Earth's atmosphere.

It is the oldest science, nearly universally practiced in literate and pre-literate societies, and it has been a major stimulus to other scientific fields from Greek times to modern physics

Relevance to society

1. Astronomy investigates ultimate origins (an adjunct or alternative to religion)
   
2. Astronomy provides our basic global perspective of time and space, i.e. a cosmic context
   
3. Astronomy was fundamental to the historical development of scientific thinking and the formulation of the first generalized physical laws by Newton.
   
4. Study of the other planets and our cosmic environment is essential to assessing the viability of Earth's biosphere and prospects for long-term human survival.

History of societal influence

• All societies: practical time- and calendar-keeping, navigation
• 5000 BC - 1609 AD: awareness of the Solar System (planets, sun)
• 16th-17th centuries: astronomy leads in the demolishment of medieval thinking
• 1609: the newly-invented telescope reveals a vast stellar system beyond the planets, enormously expanding the horizon of human thought
• Late 1600's: astronomy leads Newton to his laws of motion, which initiate the "scientific revolution" and modern technology
• 19th century: speculation about extraterrestrial life
• 20th century: recognition of galaxies ("island universes"), the expanding & evolving universe, the Big Bang, exploration of the Moon and planets by the space program, the extraterrestrial cause of mass biological extinctions; discovery of other solar systems.

Starfield near the center of the Milky Way. How many stars can you count here?
Click for a full-resolution enlargement. (Image from the Hubble Space Telescope.)

G. ASTRONOMICAL DISTANCES AND AGES

Astronomical time- and distance-scales are tremendously larger than the "common sense" scales we encounter in everyday life. See the chart above for scales of 10⁷ meters and above. Unfortunately, this means that astronomical scales (not to mention atomic or molecular scales) are utterly non-intuitive for us.

It is important for astronomers to develop a good cosmic perspective that transcends the perceptual biases of everyday experience. But it is difficult for anyone to visualize the scales involved. Because the cosmic range is so enormous, scientists make regular use of scientific mathematical notation ("powers of ten"). For a review of this notation, see Supplement I (PDF file).

Example astronomical scale models

Simple scale models can provide a rough, if never completely adequate, impression of cosmic scales:

1. As an example of the contrast between human perceptions and physical reality consider the Earth's atmosphere. It seems enormous and all-encompassing, right? Atmospheric disturbances (rainstorms, tornados, hurricanes, droughts) easily destroy human structures and livelihoods.
   Can you propose a simple scale model for the atmosphere, using, for example, a basketball to represent the Earth?
   Click here for the answer.
   Despite our impression, the atmosphere represents only a tiny fraction (one part in 1600) of the Earth's diameter. See this image taken from a spacecraft, showing the atmosphere as a thin blue line on the horizon. (The dark ridge of snow-capped mountains running diagonally across the Earth's surface is the Himalayas, the world's highest.)
   In fact, you can actually walk out of Earth's atmosphere. Anyone who has been above 26,000 feet altitude (and there are 14 mountains this high) has been beyond the point where the air density has dropped a factor of 2.7---the official "one scale height" definition of the atmosphere's thickness.

2. A sample cosmic time scale:
   • The age of the Sun is about 5 billion years, a middling age for an older star. Suppose we wanted to use the run of all the letters in our textbook to represent this span of time. How many years would we have to assign to each letter?
     We discussed this in the introductory lecture, and by "rough order of magnitude estimation" we determined that there would be about 2000 years per letter(!) if the entire textbook represented the age of the Sun.
     All of recorded human history would fit within the first four letters of the text! The existence of Homo sapiens on Earth would extend only a couple of sentences! (Modern humans have existed only for about 200,000 years, or 0.004% of the age of the Sun.)
     Here is an image of the first two pages of our textbook with the beginning of this time-scale model superposed.
     The Earth is almost the same age as the Sun, so our scale model graphically illustrates the vast span of time over which the surface, atmosphere, and biosphere of our planet have been shaped into their present form.
     Your first homework assignment was simply to stare at random pages from the textbook and try to absorb, in the context of this scale model, the nearly unimaginable period over which the Earth has evolved.

   • The "dripping faucet effect": the very long time scales that characterize planetary evolution mean that small but persistent changes can eventually have major consequences. We will see many examples of these (e.g. polar precession, continental drift, biological evolution).

3. Finally, a healthy, fruit-based cosmic distance scale model:
   • Use an orange to represent the Sun. This is an appropriate choice not just because of the color but also because the mean mass density of the Sun is about the same as an orange (1 gram/cc).
     In this model, the Earth to Sun distance (defined to be one "Astronomical Unit") is 25 ft.
     [Useful mnemonic: Earth diam : Sun diam : Astron Unit = 1:100:10,000]

   • In this model, what is the scale distance to the next nearest star (Alpha Centauri)?
     An additional relevant piece of information that might help you think about this question is that the Sun is about 10 billion times brighter than the brightest stars as seen from Earth and that most of this difference is a distance effect.

   • Here's the surprising answer. It illustrates how incredibly thinly matter is spread on a cosmic scale by the standards of our everyday, "common-sense" world.

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Reading for this lecture:

Bennett textbook: Ch 1 and Secs 3.4, 3.5.
Study Guide 1
Optional reading: Alan Cromer: Uncommon Sense; Bertrand Russell: A History of Western Philosophy; Carl Sagan: The Demon Haunted World

Reading for next lecture:

Bennett textbook: Ch 1 and Secs 3.4, 3.5.
Study Guide 2
Supplement I (PDF file) Skim and then refer to this later as needed.
Optional: Cosmic History: A Brief Narrative
Optional: browse the material on the structure & evolution of the universe in the Bennett textbook Chs 22 and 23

Web Links:

Slides shown in lecture (pptx)
History of Science from Encyclopedia Britannica

Note: you must be logged on through UVa to access Britannica
Web Index for History & Philosophy of Science, Technology, & Medicine
Internet Encyclopedia of Philosophy
Field Guide to Critical Thinking (from Skeptical Inquirer Magazine)
Bad Astronomy (a site devoted to debunking astronomical misconceptions, fraud, and pseudoscience, run by UVa Astronomy grad Phil Plait)
Simanek's Pseudo Science & Related Links
TEDx Talk on Skepticism (nice 24-minute tour of faith, denialism, and the value of skeptical thinking in our lives by George Hrab)
Correspondence with a student on the practice of science (O'Connell)
More on the scale of the universe & powers of ten (Study Guide 2)
Science, Technology, and Society (Study Guide 9)
Size of our biosphere (Study Guide 12)
More discussion of UFO's & pseudoscience (Study Guide 18)

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Last modified October 2023 by rwo

Text copyright © 1998-2023 Robert W. O'Connell. All rights reserved. Twilight image of Moon and Venus over Mt. Shasta by Jane English. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.