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In this lesson, you will learn about how theories are used to explain and explore the natural world, and investigate some important scientific theories.
A single test does not prove a hypothesis, but it can support it. Generally a hypothesis must be tested many times and under different conditions before it is generally accepted. A theory is an explanation of the natural world that incorporates conclusions from a number of well-tested hypotheses. Theories are not static knowledge. As new experiments and observations expand the base of knowledge, they are incorporated into the theory. For example, chemists have refined the atomic theory to account for observations about how atoms interact. In the early 20th century, a model was developed, showing electrons moving in fixed orbits around the nucleus, which could explain what was known about how atoms behaved. This model was incorporated into the atomic theory. Later experiments were not consistent with that model and the theory was modified. According to the current atomic theory, the model represents electrons by a “cloud” of likely positions. If new observations are not consistent with this model, the theory will be modified again. This is how science works — dissent with modification.
In common use, the words theory and hypothesis are often used interchangeably, while in science they represent very different concepts. This difference in usage has sometimes led to the misconception that a theory is a guess or belief. In science a theory is supported by a large body of evidence based on many hypotheses and it is a tool for predicting experimental results, and understanding how the natural world works. While a theory can serve as an explanation of observed phenomena, it also serves as a starting point for new investigations into predicted phenomena. Some theories have generated many new hypotheses, expanding the reach of an entire field of science.
There are thousands or millions of theories in science, ranging from explanations of one particular phenomenon to theories that encompass entire fields of scientific understanding. Some of the most influential theories that have driven scientific research over the past few centuries include:
Although ancient Greek philosophers first introduced the concept of atoms, it was not a scientific theory because it was not supported by experimental data. In the early 1800’s John Dalton designed experiments to test the existence of atoms as the basic units of the elements. Dalton modeled the atom as a hard ball, the basic particle of matter. Building on his work, later researchers have developed new models. According to modern atomic theory, the atom is composed of smaller charged and uncharged particles, held together by forces of attraction. Dalton ‘s basic idea—that an atom is the smallest particle of an element—is still a key part of atomic theory. The atomic theory is the basis of all of our applications of chemistry, including all sorts of modern materials and pharmaceuticals.
One of the implications of Albert Einstein’s theory of relativity is that the universe is expanding. This concept combined with observations that distant galaxies are moving apart at increasing speeds forms the basis of the Big Bang Theory. According to this theory, the universe began about 14 billion years ago in a sudden expansion from an extremely hot, dense phase. In the 1960s, scientists discovered the Cosmic Microwave Background (CMB)—light waves that permeate the universe. The CMB was one of the predictions of the Big Bang Theory. Today, almost all cosmologists accept the Big Bang as the best theory of the beginning of the universe. Data from the Hubble Telescope and many other space and ground-based observatories support this explanation. The theory has been used to develop an understanding, not only of the broad universe, but also the basic particles that make up all matter.
Classical and quantum mechanics are concerned with the motions and forces that cause the movement of objects. Classical mechanics was first developed more than 300 years ago through the work of Newton , Kepler, and Galileo (and others), whereas quantum mechanics is much more recent. The main scientists responsible for our progress in quantum mechanics are Bohr, Planck, and Einstein (and others). The basis of classical mechanics on mathematical explanation was a decisive stage in the development and history of science. Not simply using math in rigorous ways, classical mechanics scientists also relied on experimentation for the first time and established the formulation of quantitative predictions, and how to test them through carefully designed experiments and measurements. The main tenants of classical mechanics deal with objects on a human scale and hold for 99.99% of our everyday activities. However, the revelation of quantum mechanics was a major breakthrough for our understanding of all motion – even at the subatomic scale. Quantum mechanics holds for energy, inertia, and the motion of all objects (including waves) and is most notable for the idea that light is a wave-particle duality. In a sense, quantum mechanics is more fundamental than classical mechanics because it provides precise explanations and predictions across all scales. Whereas there are some major holes in classical mechanics, quantum mechanics has yet to be refuted after more than a century of experimentation.
The theory of evolution began with the introduction of natural selection in a joint 1858 paper by Charles Darwin and Alfred Russel Wallace. They both independently, and later together, assembled and collated data from their observations of plants, animals, and fossils to build the theory of evolution by natural selection. Darwin and Wallace hypothesized that evolution occurs when a trait that increases an individual’s chance of successfully mating becomes more common from one generation to the next. Evolution affects population genetics and eventually leads to the emergence of new species because individual organisms acquire and pass on novel traits via inheritance. The key element of the theory is that differential survival and reproduction (based on competition for resources and/or mates) allows some organisms to pass on genetic variation to their progeny. Although the theory has been modified since Wallace and Darwin’s time, natural selection remains a key concept on which our current understanding of biology, genetics, and ecology are built. The Modern Synthesis of the early 20th century has incorporated the idea of a gene as the unit of inheritance and we now understand the molecular biology of DNA replication, population and community ecology, and genetics in much greater detail. The theory of evolution is the most powerful theory in the life sciences.
In the early 20th century, Alfred Wegener noted how South America and Africa seem to fit together. He also researched the similarity of fossils at the edges widely separated continents. He proposed a hypothesis of continental drift, in which all the continents had once formed a single land mass that he called Pangaea and are now moving away from one another. Wegener’s ideas were not widely accepted because he lacked an adequate explanation for the mechanism of continental drift. In the 1960s, scientist discovered evidence that supported Wegener’s hypothesis. Magnetic materials in the rocks of the sea floor in the deep ocean provided evidence that the crust of Earth is made of plates that move relative to one another. This observation, as well as the results of other experiments, was combined with Wegener’s continental drift in the theory of plate tectonics. Scientists have used this theory to study the prehistoric structure of the continents and to develop other theories about the internal structure and the history of the planet.
A scientific law is a statement of fact that explains a relationship in nature that is generally accepted to be true and universal. Scientific laws are based on numerous observations and tests, with no exceptions being found. Like mathematical postulates, scientific laws don’t require proofs. They are accepted because of the fact that they have always been observed to be true. Scientific laws include the law of gravity, conservation of mass, and the gas laws that describe the behavior of gases. In many cases, scientific laws can be expressed as mathematical formulas.
Which of the following is an established scientific theory?
The answer to this one is A — plate tectonics is a theory built on a number of tested hypotheses. Intelligent design is not a scientific theory because it is not falsifiable and therefore cannot be tested. Conservation of mass is a scientific law and the internal combustion engine is an example of technology.
Science is a process, not just a body of knowledge. In a broad sense, everyone is a scientist. A small child learns about the forces of gravity and friction, usually the hard way, by experimenting with toys. Learning by making a hypothesis, designing experiments to test them, collecting data, and revising the hypothesis is science. Anyone can do it because the scientific method is based on the way our brains work.
Science is a process for discovery and it doesn’t end once a question is answered. Each discovery leads to new questions. Each advance opens up new fields of research. Today’s frontiers in science are built on the hypotheses and theories that directed past research. A sampling of these frontiers includes:
In this lesson, we will review the importance of measurements to a scientific investigation, how to record measurements and convert measured values between different units.
Although mathematics is a branch of science, it is also a key tool for nearly all scientific investigations. Because conclusions rely on reproducible data, most scientific investigations involve measurement of one or more physical quantities. Some attributes that can be measured include length, mass, volume, time, temperature, and number of particles. Every measurement has two components: a unit that is associated with a physical quantity, such as length or mass, and the number of units. Both parts of a measurement must be included. For example, a length of 2 is not a meaningful measurement, while 2 millimeters, 2 kilometers, 2 inches, and 2 light-years are all meaningful measurements.
Over the course of history, people have developed many different systems of measurement. For example, one distance can be expressed as 2.00 meters, 6.56 feet, 0.4 rods, or 0.00124 mile. Sharing data and experimental results is much easier, however, if everyone uses the same units.
Scientists around the world have agreed to use a metric system of measurement, based on powers of ten, known as the International System of Units (SI). The system uses fundamental units for length, time, mass, temperature, electric current, amount of a substance, and luminosity.
| Name | Symbol | Quantity | Definition |
|---|---|---|---|
| kilogram | kg | Mass | This unit of mass was originally defined as the mass of one liter of water at 3.98˚ Celsius and standard atmospheric pressure at sea level. |
| second | s | Time | The second is based on an atomic clock of the transition between two levels of the ground state of the cesium 133 atom. |
| meter | m | Length | The unit of length was originally defined as the length of a pendulum with a half-period of one second. The meter is the length of the path traveled by light in a vacuum during a time interval of 1 / 299792458 of a second. |
| ampere | A | Electrical current | The unit of electrical current is the value of the current between two conductors that, if placed 1 meter apart in a vacuum, would produce a force between these conductors equal to 2×10−7 newtons per meter of length. The more commonly used definition is that one ampere of current (I) is equal to a flow of one coulomb of charge (Q) per second of time (t): I = Q / t. |
| kelvin | K | Thermodynamic temperature | The unit of absolute temperature is the fraction 1/273.16 of the triple point of water and where 0 is the absence of all motion. A temperature given in kelvins (K) is measured with respect to absolute zero, where molecular motion is considered to stop. It is also common to give a temperature relative to the Celsius temperature scale, with a reference temperature of 0 °C = 273.15 K, approximately the melting point of water under ordinary conditions. |
| mole | mol | Amount of substance | A number having a value of 6.022142 X 1023 and is also called Avogadro’s number. The number of atoms or molecules in 1 mole of a substance |
| candela | cd | Luminous intensity | The unit of luminous intensity is emitted by a monochromatic light source of wavelength 555 nm and that has a radiant intensity of 1/683 watt per steradian. |
Units that are larger or smaller than the base unit are designated in SI by prefixes that indicate their value in different powers of ten. The table below shows the SI prefixes and their values. Note that the base unit of mass (kg) is defined as 1000 grams.
| SI Prefixes | |
|---|---|
| Factor | Prefix Name |
| 1021 | zetta |
| 1018 | exa |
| 1015 | peta |
| 1012 | tera |
| 109 | giga |
| 106 | mega |
| 103 | kilo |
| 102 | hecto |
| 101 | deka |
| 100 | -meter-liter-gram- |
| 10-1 | deci |
| 10-2 | centi |
| 10-3 | milli |
| 10-6 | micro |
| 10-9 | nano |
| 10-12 | pico |
| 10-15 | femto |
| 10-18 | atto |
| 10-21 | zepto |
Other derived units, such as those for density, speed, and power are combinations of the fundamental units of the SI system. The table below lists a number of derived units and their relationship to the SI base units.
| Physical quantity | SI unit | in symbols |
|---|---|---|
| area | square meter | m2 |
| volume | cubic meter | m3 |
| speed, velocity | meter per second | m/s |
| acceleration | meter per second squared | m/s2 |
| momentum | newton second | N · s or kg · m/s |
| density, mass density | kilogram per cubic meter | kg/m3 |
| heat capacity, entropy | joule per kelvin | J/K or kg · m2 · s-2 · K-1 |
| molar heat capacity, molar entropy | joule per kelvin mole | J · K-1 · mol-1 or kg · m2 · s-2 · K-1 · mol-1 |
| specific heat capacity, specific entropy | joule per kilogram kelvin | J · K-1 · kg-1 or m2 · s-2 · K-1 |
| frequency | hertz | Hz or s-1 |
| force | newton | N or kg · m · s-2 |
| electric charge | coulomb | C or A · s |
| electric potential, electromotive force | volt | V or J/C or kg · m2 · s-3 · A-1 |
| electric capacitance | farad | F or C/V or kg-1 · m-2 · s4 · A2 |
| electrical resistance, impedance, reactance | ohm | Ω or V · A-1 or kg · m2 · s-3 · A-2 |
| energy, heat, work | joule | J or N · m or kg · m2 · s-2 |
| pressure | pascal | Pa or N/m2 or kg · m-1 · s-2 |
| power | watt | W or J/s or kg · m2 · s-3 |
| temperature | degree Celsius | °C or K – 273.16 |
Measured values can be treated just as regular numbers, added, or subtracted, but only if their units are the same. How do you work with measurements that use different units—for example (500 m + 2.3 km) or (6 ft – 1 m)? A conversion factor is a ratio of equivalent values that expresses a single quantity in two different units. For example, let’s look at 1 km = 1,000 m.
(500 m) (1 km/1000 m) = 0.5 km
After making this conversion, you can add the first example above
500 m + 2.3 km = 0.5 km + 2.3 km = 2.8 km
Note that the 2.3 km can alternatively be converted to 2,300 m, yielding a sum of 2,800 m.
Using units incorrectly can have drastic effects on results of an experiment. In 1999, NASA sent a probe, the Mars Climate Orbiter, to investigate the Martian atmosphere. After the rockets that were supposed to place the probe into an orbit around Mars fired, the probe was lost, apparently crashing on the surface. An investigation revealed that two teams of scientists and engineers had worked on the calculations needed to time the firing of the rockets. One team had used SI units and the other had used English units (feet and pounds). Oops.
Below are a number of sample conversion tables that are fairly straight forward, but not exhaustive. Take a little time to reacquaint yourself with the different units and understand the different and relative values (e.g., you should know that a kilogram is roughly twice as much a pound). Please note that we have not included all possible conversions.
| Conversion Table (English to SI) | |||
|---|---|---|---|
| Name of unit | Symbol | Definition | Relation to SI units |
| inch | in | 1/36 yd | 25.4 mm |
| foot | ft | 12 in | 0.3048 m |
| yard | yd | 3 ft | 0.9144 m |
| mile | mi | 1760 yd = 5280 ft | 1609.344 m |
| square inch | sq in | 1 in² | 6.45 ×10-4 m² |
| square foot | sq ft | 1 ft² | 9.29 ×10-2 m² |
| acre | ac | 4840 sq yd | 4046.856 m² |
| liter | L | 1000cm3 | |
| cubic inch | cu in | 1 in³ | 16.387 mL |
| fluid ounce (U.S.) | Fl oz | 1/128 gal (US) | 29.574 mL |
| gallon (U.S. fluid) | gal (US) | 231 cu in | 3.785 L |
| ounce (mass) | oz | 1/16 lb | 28.350 g |
| pound (mass) | lb | 7000 grains | 0.454 kg |
| kilometer per hour | km/h | 1 km/h | 2.78 ×10-1 m/s |
| foot per second | fps | 1 ft/s | 3.05 ×10-1 m/s |
| mile per hour | mph | 1 mi/h | 0.45 m/s |
| standard gravity | g | 9.81 m/s² | |
| joule (SI unit) | J | N·m = W·s = V·A | kg·m²/s² |
| calorie (thermochemical) | calth | 4.184 J | |
| British thermal unit (thermochemical) | BTUth | 1 lb/g × 1 calth × 1 °F/°C = 9.489 ÷ 9 kJ |
1.054 kJ |
| kilowatt-hour | kW·h | 1 kW × 1 h | 3.6 MJ |
Which of the following is equivalent to 60 miles per hour?
The correct answer is B: (60 mi/hr) (1.61 km/mi) (1000 m/km) (1 hr/3600 s) = 26.8 m/s.
In addition to measurements of dimensions and time, many scientific investigations measure temperature. How should you dress if the weather report calls for a high temperature of 35°? That depends. If the temperature is 35°F, you will want to bundle up; if it is 35°C, light clothes are in order. If the temperature is 35K, then you are probably on the Moon, not Earth. These values correspond to the three main temperature scales used in science. The illustration shows several equivalent temperatures, including the melting and boiling points of water on the Fahrenheit, Celsius, and Kelvin scales. A third point, absolute zero, is the theoretical coldest possible temperature. Absolute zero is the starting point of the Kelvin scale.
Although the Fahrenheit scale is commonly used in the United States, most of the world, including the entire scientific community, uses the Celsius scale. This scale establishes 0° as the freezing point of water and 100° as the boiling point of water, making much more sense than our relatively illogical 32°F for freezing and 212°F for boiling. The Kelvin scale is also used by scientists, particularly when they are working at very low temperatures. The standard unit of temperature in the SI system is the Kelvin (K), which is the same as 1°C. Note that the degree symbol is not used in the Kelvin scale. As with other units, conversion factors can be used to calculate equivalent temperatures in the three scales.
| Temperature Units | ||
|---|---|---|
| Name of unit | Symbol | Definition |
| kelvin | K | SI base unit |
| degree Celsius | °C | T [°C] = T [K] − 273.15 T [°C] = T [°F] / 1.8 – 32 |
| degree Fahrenheit | °F | T [°F] = T [K] × 1.8 − 459.67 T [°F] = 1.8 × T >[°C] + 32 |
Scientists have measured the mass of an electron with fantastic precision:
0.000000000000000000000000000000910939 kg.
Imagine using that number in a research paper several dozen times. Fortunately, there is an easier (and more concise) way to express small numbers and large numbers. Scientific notation expresses a large number as a number between 1 and 10 multiplied by a power of ten. For example, in scientific notation 500 is expressed as 5.0 x 102
500 = 5 · 100 = 5 · 10 2
For numbers smaller than 1 (1 x 100), the exponent of 10 is a negative number:
0.125 = (1.25) (1/10) = 1.25 · 10 -1
Therefore, the mass of an electron is 9.10939 · 10 -31 kg.