Chapter One origins

By its very nature, the scientific method depends on the application of specific tests to bits and pieces of the natural world, and explaining, by virtue of these tests, how the natural world will react in a given situation. Hypotheses and theories are devised to explain the outcomes. The tests must be repeatable -- that is, other researches at other sites must be able to replicate the experiments (tests) with similar results. If replication is impossible, or if other outcomes are observed, the hypotheses and theories are discarded and replaced with new ones. Figure 1.4 shows the process.

Can these methods be applied to speculations about the natural world that are not subject to test or observation? By definition, they cannot.


5. What is the nature of “truth” in science? Can anything be proven absolutely true?
Nothing is ever proven absolutely true by the scientific method. Hypotheses and theories may change as our knowledge and powers of observation change; thus all scientific understanding is tentative. The conclusions about the natural world that we reach by the process of science may not always be popular or immediately embraced, but if those conclusions consistently match observations, they may be considered true.

6. What if, at the moment you shake the keys, the wires under the hood are jostled by a breeze and fall back into place? What if the car starts when you try it again? Can you see how superstition might arise?
For an interesting perspective on this question, find some back issues of Skeptical Inquirer magazine. Also, author Michael Shermer has written extensively on the issue of irrational superstition and the alarming rise of illogical thought in American society.


7. Outline the main points in the condensation theory of star and planet formation.

The life of a star begins when a diffuse area of a spinning nebula begins to shrink and heat up under the influence of its own weak gravity. Gradually, the cloudlike sphere flattens and condenses at the center into a knot of gases called a protostar. The fusion process begins – the star “turns on.” Much of the outer material eventually became planets, the smaller bodies that orbit a star and do not shine by their own light.

After fusion reactions begin, the star becomes stable— neither shrinking nor expanding, and burning its hydrogen fuel at a steady rate. Over a long and productive life, the star converts a large percentage of its hydrogen to atoms as heavy as carbon or oxygen. This stable phase does not last forever, though. The life history and death of a star depend on its initial mass. When a medium-mass star (like our sun) begins to consume carbon and oxygen atoms, its energy output slowly rises and its body swells to a stage aptly named red giant by astronomers. The dying giant slowly pulsates, incinerating its planets and throwing off concentric shells of light gas enriched with these heavy elements. But most of the harvest of carbon and oxygen is forever trapped in the cooling ember at the star’s heart.

After a long and productive life, an average star converts a large percentage of its hydrogen to atoms as heavy as carbon or oxygen. When a medium-mass star begins to consume these heavier atoms, its energy output slowly rises and its body swells to a stage aptly named red giant by astronomers. The dying giant slowly pulsates, throwing off concentric shells of light gas enriched with these heavy elements. But most of the harvest of carbon and oxygen is forever trapped in the cooling ember at the star’s heart.

The dying phase of a massive star is more interesting. The end begins when its depleted core collapses in on itself. This rapid compression causes the star’s internal temperature to soar. When the infalling material can no longer be compressed, the energy of the inward fall is converted to cataclysmic expansion called a supernova. The explosive release of energy in a supernova is so sudden that the star is blown to bits, and its shattered mass is accelerated outward at nearly the speed of light. The explosion lasts only about 10 seconds, but in that short time the nuclear forces holding apart individual atomic nuclei are overcome—and atoms heavier than iron are formed. The gold of your rings, the mercury in a thermometer, and the uranium in nuclear power plants were all created during such a brief and stupendous flash. The atoms produced by a star through millions of years of orderly fusion, and the heavy atoms generated in a few moments of unimaginable chaos, are sprayed into space.

Every chemical element heavier than hydrogen—most of the atoms that make up the planets, the oceans, and living creatures—was manufactured by the stars.

Radiometric dating (described in an appendix) is a powerful technique based on the discovery that unstable, naturally radioactive elements lose particles from their nuclei and ultimately change into new stable elements. The radioactive decay occurs at a predictable rate, and measuring the ratio of radioactive to stable atoms in a sample provides its age. Using radiometric dating, researchers have identified small zircon grains from western Australian sandstone that are 4.2 billion years old. The zircons were probably eroded from nearby continental rocks and deposited by rivers. (Older crust is now unidentifiable, having been altered and converted into other rocks by geological processes.)

Observing the rates of mountain-building and erosion also provides clues. If we assume the processes we observe occur now at rates similar to rates in the past, we can extrapolate suggestions of age. Even the rate at which heat leaks from within Earth can provide data.

One of the most interesting methods of age-dating searches for cosmic-ray traces in metallic meteorites that have fallen from space. It is likely these objects are remnants of the cores of failed planets, or from material ejected from Earth that ended up in our moon. The density of the traces suggests how long it has been since the objects formed.

The moon itself has provided clues. Its angular momentum and orbital shape can tell of its early formation and subsequent movements, and samples brought back by Apollo astronauts have confirmed an age similar to (but slightly younger than) Earth.

12. How was the moon formed?

About 30 million years after its initial formation, a planetary body somewhat larger than Mars smashed into the young Earth and broke apart. The metallic core fell into Earth’s core and joined with it, while the rocky mantle was ejected to form a ring of debris around Earth. The debris began condensing soon after and became our moon. The newly formed moon, still glowing from heat generated by the kinetic energy of infalling objects, is depicted in Figure 1.13.

These heavy rains may have lasted about 20 million years. Large amounts of water vapor and other gases continued to escape through volcanic vents during that time and for millions of years thereafter. The ocean grew deeper. Evidence suggests that Earth’s crust grew thicker as well, perhaps in part from chemical reaction with oceanic compounds. Although most of the ocean was in place about 4 billion years ago, ocean formation continues very slowly even today.

The volcanic venting of volatile substances including water vapor -- outgassing -- gave rise to the present ocean. As hot water vapor rose, it condensed into clouds in the cool upper atmosphere. Recent research suggests that millions of tiny icy comets colliding with the Earth may also have contributed to the accumulating mass of water vapor, this ocean-to-be.

Next, let's look at orbital characteristics. Our Earth is in a nearly circular orbit at just the right distance from the sun to allow liquid water to exist over most of the surface through most of the year.

Next, consider our planet's cargo of elements. We picked these up during the accretion phase. At our area of orbit there was an unusually large amount of water (or chemical materials that would led to the formation of water).

So, with a stable star, a pleasant circular orbit that is well placed, and suitable and abundant raw materials, we are a water planet. This marvelous combination is probably not found in many places in the galaxy.