Part B - The Quest for Understanding
B.1 The universe
B.1.1 Origin of the universe
Essential Idea:
Observations, initially through the naked eye and then using advancing technologies eg optical telescope, X ray telescopes, radio telescopes etc together with theoretical developments of a mathematical nature have led to our present knowledge of the universe.
Understanding the Nature of Science:
Some explanations for the origin of the universe cannot be tested scientifically; scientific claims must focus on the observational evidence and must be testable.
Different cultures have contributed to our scientific understanding of the universe.
Developments in theoretical knowledge such as relativity have led to refinements in our model of the universe.
Newton’s work on gravitation advanced the quantitative analysis of the universe.
Olber realised that the night sky being dark in certain directions did not fit with an infinite universe model (Olbers paradox).
Einstein’s theory of general relativity provides an explanation of Newton’s Law of gravitation.
Acceptance of a steady-state theory avoids questions about the origin of the universe.
The concept of the light year as a measurement of large distances.
Cosmic background radiation (CMBR) signals were serendipitously detected in 1964 by Penzias and Wilson and this refuted the steady state theory by providing evidence of the Big Bang.
The present theory of the origin of the universe is that there was a Big Bang about 13.8 billion years ago, which resulted in a spontaneous release of energy from which the universe has been created.
Computer modelling of galaxy formation from the initial fluctuations in the density of the early universe as shown by COBE and WMAP satellite data help explain the appearance of the universe.
Present day astronomical observations generate vast amounts of data and rely on rapid computer processing to convert it into useful information.
Current models of the universe require the presence of large quantities of “dark matter” and “dark energy”.
B.1.2 Galaxies
Essential Idea:
Initially most of the visible objects in the sky were thought of as being similar, but as a result of improved observational technology, gradually different groups, including non visible objects have been identified, and their origins deduced, showing the value of pattern recognition and classification in science.
Understanding the Nature of Science:
Improvements in observational technology have enabled more detailed information to be obtained regarding the large scale structure of the universe.
Stars are clustered together in galaxies and these galaxies are clustered into groups.
Hubble found evidence, through the “red shift”, for the recession of galaxies. This implied the universe might have had its origin at a singularity. Measuring the rate of expansion allowed the age of the universe to be estimated.
There is great uncertainty in the value of the current rate of expansion and this may also be changing over time. Recent measurements suggest that the rate of expansion is increasing.
B.1.3 Stars
Essential Idea:
Application of the basic laws of physics has enabled scientists to deduce the origin and evolution of stars.
Understanding the Nature of Science:
Current models postulate that stars are created when massive clouds of dust and gas coalesce under gravitational attraction, when the resulting extreme temperatures and pressures initiate the nuclear fusion of hydrogen.
Stars can be classified into different groups according to their appearance, interpreted more quantitatively in graphs of luminosity against temperature, called Hertzsprung-Russell (HR) diagrams.
Most stars evolve from “main sequence” through red giants, white dwarves and neutron stars to form black holes, when their supply of hydrogen runs out, though very massive stars undergo a catastrophic collapse known as a supernova.
Main sequence stars, similar to our Sun, use nuclear fusion to convert hydrogen to helium. Other types of stars convert helium into other light elements.
As their formation would be endothermic, elements heavier than iron could not have been produced in this manner and they are currently thought to result from extreme events such as supernovas.
Observations of the spectra of stars can be used to deduce their composition.
B.1.4 The solar system
Essential Idea:
The movement of the bodies that comprise the solar system has been systematically measured and recorded by many civilisations. The interpretation of detailed observations proving it to be heliocentric rather than geocentric was one of the major triumphs of scientific method.
Understanding the Nature of Science:
Our basic understanding of the solar system, its components and their motion was dependent on new technological developments that improved our observational capacity and mathematical modelling.
The changes in position of the planets over time compared to the stars was a challenge to the geocentric model.
Recent observations show that stars other than the Sun are also surrounded by planets.
Guidance:
A detailed knowledge and the underlying mathematical treatments of the Physics involved, such as relativity and planetary motion will not be expected. The emphasis will be on general concepts and on critical observations and their significance.
B.2 Nature of our planet
B.2.1 The origin of the Earth
Essential Idea:
The origin of the Earth and the way it has metamorphosed into its current state are not open to experimental investigation. Scientists have had to base their ideas on detailed studies of the Earth as it now exists, and rely on inferences and models that can explain their results.
Understanding the Nature of Science:
1 The age of the Earth
Models of the Earth’s formation are part of broader models that explain how the solar system was formed.
Quantitative data derived from radioactive emissions from meteorites found on the Earth and the moon leads us to infer that the Earth formed at least 4.6 billion years ago.
2 The structure of the Earth
The interior of the Earth is inaccessible to direct study. Advanced techniques in analyzing seismic waves, of the Earth’s magnetic field, and geomagnetic and gravity measurements made at the surface have helped scientist gather information about the composition and thickness of the Earth’s internal structure.
3 Rocks that compose the Earth
Rocks can be dated by both absolute and relative techniques and the study of rocks provides scientists with important information about Earth, including evidence of the Earth’s origin and changes that Earth has undergone over time.
Living organisms significantly affect these processes by taking in liquid water and releasing water vapour to the atmosphere. Aquatic living organisms also affect the levels of dissolved gases and the levels of dissolved solids.
B.2.2 Plate Tectonics
Essential Idea:
Sometimes support for a theory does not come just from a single piece of evidence, or field of research, but from the way in which the theory explains observations over a range of different sciences.
Understanding the Nature of Science:
1 Origins of the theory
Evidence for the concept of plate tectonics gradually built up over the years based on the complementary shape of parts of the present continents as well as similarities in rock formations and plant species, though much of the scientific community was initially sceptical.
The results of studies involving paleomagnetism in the 1960s, especially critical evidence concerning the magnetic orientation of rocks from either side of the mid-Atlantic ridge, lead to a widespread acceptance that continental plates move over time.
As a result of new technologies to measure distance and position very precisely, nowadays the movements of the tectonic plates can be directly observed.
2 The nature of continental masses
The Earth’s crust is comprised of a number of separate plates that gradually move relative to each other.
Over geological time the number and shape of the plates, as well as the positions of these, have undergone many changes.
Interactions of the plates are responsible for earthquakes and physical features such as volcanoes and mountain ranges.
B.2.3 Equilibrium
Essential Idea:
The Earth’s atmospheric composition is in a dynamic equilibrium and such complex systems are very difficult to model but changes to it have occurred in the past and will occur in the future. The extent to which humans contribute to this change is under scientific examination.
Understanding the Nature of Science:
The Earth is surrounded by an atmosphere, the composition of which has changed, and continues to change.
Radiation from the Sun, especially UV light, causes chemical reactions between components of the atmosphere, giving rise to an important ozone layer in the upper atmosphere.
Equilibrium exists between gases in the atmosphere and gases dissolved in water on the surface of the Earth, which hence acts as a reservoir of gases.
A detailed study of the atmosphere has enabled us to develop models that can predict the effects of changes in gas composition that may occur in the future.
Direct evidence of more recent changes in the Earth’s atmosphere (about the last half million years) come from ice core samples, especially from the Antarctic, which contain trapped air.
The total amount of water, and the ratio of water to ice, has caused sea levels to change over geological time. As these variations continue, and affect mankind, an understanding of the factors controlling this is important.
Most of the energy on Earth originates from the Sun. Solar radiation is absorbed by the Earth’s surface and transferred to the atmosphere. The Earth emits infrared radiation, some of which is absorbed and re-radiated by some gases in the atmosphere, creating a greenhouse effect.
The balance between absorbed and emitted radiation results in a surface temperature that makes life possible.
Variations in solar activity are hypothesized to affect global climate by 5%.
Fossils and sediments also provide information on how the Earth’s climate has changed over time?
Changes to the atmosphere that result from human activity have a significant effect on the climate.
There are several greenhouse gases, each with a different contribution to global warming.
B.3 Evolution
B.3.1 Ideas on the origin of life
Essential Idea:
Scientists have proposed hypotheses, models and theories to explain the diversity of life on Earth which has ontological and chronological characteristics. Evolution is a unifying principle of the life sciences with great explanatory power.
Understanding the Nature of Science:
The changes that have occurred over the 4 billion year history of life on Earth are thought to be the result of natural processes.
The origin of life on Earth is still unknown. A number of models and theories exist but the scientific community requires verifiable evidence.
Scientists developed a testable hypothesis to determine if the organic molecules necessary for life could be synthesized through the interactions of inorganic molecules.
Evidence supports the presence of organic molecules in space, based on examination of comets / meteorites.
Observations of primitive life forms near volcanoes and deep ocean hydrothermal vents suggest a possible location for the origin of single-celled organisms.
Scientists studying the composition of ancient rocks have deduced that there was no oxygen in the early Earth’s atmosphere.
Analysis and understanding of photosynthesis helped identify cyanobacteria as the source of oxygen.
A unifying hypothesis in the life sciences is the connection between these primitive prokaryotic cells and all of the diverse forms of life that followed.
B.3.2 Evolution and the theory of natural selection
Essential Idea:
Observations of the natural world have revealed verifiable broad concepts and general principles that explain the diversity and complexity of life on Earth.
Understanding the Nature of Science:
Darwin developed a theory in which natural selection provided a possible and verifiable mechanism for evolution. A similar theory was also independently developed by Alfred Wallace.
Charles Darwin’s publication, “On the origin of the species by means of natural selection” connected previously unrelated ideas into a coherent view of life.
The theory of natural selection is built on inferences based on observations. The theory explains how increased reproductive success of individuals with favourable heritable characteristics, can lead to change in the genetic composition of a population.
Although natural selection involves interactions between organisms and their environment evolution is measured by the changes in populations. Examples of natural selection, including multiple antibiotic resistances in bacteria and pesticide resistance in rodents have been studied to increase understanding of the relationship between selective environmental pressures and survival of different organisms.
Darwin’s ideas were not well accepted as they challenged the prevailing scientific thinking and tested long standing theological beliefs. However, within a short time he had convinced many scientists that biological diversity resulted from evolution because of his logical sequence of ideas based on verifiable evidence.
B.3.3 Evidence for evolution
Essential Idea:
Scientists rely on observations and evidence in many forms - from fossils to biochemical analysis of nucleic acids. Increased powers of instrumentation and advances in available techniques, combined with multi disciplinary cooperation have yielded an extensive body of evidence supporting evolution.
Understanding the Nature of Science:
Darwin’s evidence for evolution was based on geographical distribution of species and the fossil record. His observations of the homologous anatomical structures of different mammals supported his reasoning that all mammals descended from a common ancestor.
Continuing research and the development of improved technologies has led to new interpretations of the fossil record and a more complete picture of early animal evolution.
Although artificial selection does not apply to natural ecosystems, it does provide evidence that species can change over time with selective breeding.
Modern molecular biology supports evolution by comparing the DNA and proteins of current and ancestral species.
Guidance:
Include the different types of evidence required, an example of selective breeding (e.g. dogs), an example of a molecular clock.
B.3.4 Human evolution
Essential Idea:
Molecular data provided by advances in technology has increased the clarity of the evolutionary lineage of Homo sapiens deduced from incomplete fossil records.
Understanding the Nature of Science:
There is uncertainty about the ancestry of humans due, in part, to an incomplete fossil record and misinterpretation of existing remains.
Radioactive dating has established the existence of a bipedal ape in Africa 4.4 million years ago.
Development of tool making, hunting skills and language have contributed to survival and thus natural selection.
There are several models of early human migration, but evidence from mitochondrial DNA (mtDNA) and Y chromosome indicates the original source as the Rift valley of Central Africa.
A newly mapped Neanderthal genome provided evidence of some interbreeding between modern man and Neanderthal man leading to a reclassification of Neanderthal man as Homo sapiens neanderthalensis.
Modern humans have demonstrated rapid cultural evolution with minimal biological evolution.
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