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A community enterprise (1 of 5) - collaboration
Collaborations and division of labor are increasingly important today, as our scientific understanding, techniques, and technologies expand.

A community enterprise (2 of 5) - cumulative knowledge
The scientific community provides the cumulative knowledge base on which science is built.

A community enterprise (3 of 5) - evaluation
Scientists present their work to others for evaluation.

A community enterprise (4 of 5) - balancing bias
Because of the scientific community's diversity, individual biases are balanced out.

A community enterprise (5 of 5) - fraud
Though fraud is rare in science, it sometimes happens, but are identified through the scrutiny of the scientific community.

An explanation of the asteroid hypothesis
Luis Alvarez hypothesized that the asteroid had blown millions of tons of dust into the atmosphere, blotting out the sun, stopping photosynthesis and plant growth and hence, causing the global collapse of food webs and resulting in a major extinction.

Average ozone levels
The graph shows the average October ozone levels recorded by Farman's group at Halley Bay from 1957 through 1984.

Balanced reporting (1 of 2)
The public should be able to get information on all sides of an issue but that doesn't mean that all sides of the issue deserve equal weight.

Balanced reporting (2 of 2)
In untangling conflicting viewpoints in the media, it pays to investigate each person's area of expertise.

Behaving scientifically
Science is open to anyone and benefits tremendously from the expanding diversity of perspectives. However, science only works because the people involved with it behave "scientifically."

Building a scientific argument - cartoon 1
Scientific arguments involve three components: the idea (a hypothesis or theory), the expectations generated by that idea (frequently called predictions), and the actual observations relevant to those expectations (the evidence).

Building a scientific argument - cartoon 2
Building a scientific argument is hard work.

Building a scientific argument - different order
Though the structure of this argument is consistent (hypothesis, then expectation, then actual observation), its pieces may be assembled in different orders.

Building the argument for ozone depletion
Researchers needed to incorporate nitrogen dioxide into the hypothesis but they weren't sure how it would affect the expectations generated by the hypothesis. Would we still expect to see significant ozone depletion?

Building the argument for rapid ozone depletion
Rowland, Solomon, and Garcia's work suggested a modification to the original hypothesis: CFCs cause significant ozone depletion and they do it much more rapidly with the help of polar clouds.

CFC molecular structure (1 of 2)
The molecular structure of trichlorofluoromethane (CCl3F), a banned CFC once widely used as a refrigerant.

CFC molecular structure (2 of 2)
CFCs like trichlorofluoromethane (CCl3F) break down when exposed to solar radiation in the upper atmosphere, freeing up chlorine atoms.

Checking the code of conduct (1 of 6)
Checking the scientist's code of conduct: Step 1.

Checking the code of conduct (2 of 6)
Checking the scientist's code of conduct: Step 2.

Checking the code of conduct (3 of 6)
Checking the scientist's code of conduct: Step 3.

Checking the code of conduct (4 of 6)
Checking the scientist's code of conduct: Step 4.

Checking the code of conduct (5 of 6)
Checking the scientist's code of conduct: Step 5.

Checking the code of conduct (6 of 6)
Checking the scientist's code of conduct: Step 6.

Chicxulub crater
The map shows the location of the Chicxulub impact crater, the site of a massive asteroid impact.

Comparing ozone and chlorine monoxide levels
A plot of chlorine monoxide and ozone concentrations from data collected by an aircraft passing through the Antarctic ozone hole. Outside the hole (left side of graph), ozone levels are high and chlorine monoxide levels are low, while the reverse is true inside the hole (right side of graph) just as the Molina-Rowland hypothesis would lead us to expect.

Competing ideas (1 of 5)
Scientists are more likely to put their trust in ideas that generate more specific expectations (i.e., are more testable).

Competing ideas (2 of 5)
Scientists are more likely to put their trust in ideas that can be more broadly applied.

Competing ideas (3 of 5)
Scientists are more likely to put their trust in ideas that are more parsimonious.

Competing ideas (4 of 5)
Scientists are more likely to put their trust in ideas that are more consistent with well-established theories in neighboring fields.

Competing ideas (5 of 5)
Scientists are more likely to put their trust in ideas that generate more new ideas.

Complexity of the process (1 of 4)
There are many routes into the process.

Complexity of the process (2 of 4)
Scientific testing is at the heart of the process.

Complexity of the process (3 of 4)
The scientific community helps ensure science's accuracy.

Complexity of the process (4 of 4)
The process of science is intertwined with society.

Core of science
Testing ideas with evidence to build scientific arguments forms the core of science.

Dealing with data
Data become evidence only when they have been analyzed, interpreted, and ultimately shared with the scientific community.

Deuterium
A hydrogen atom has only a single proton in its nucleus, whereas deuterium, a rarer isotope of hydrogen, has a proton and a neutron.

Diversity of perspectives (1 of 3)
People from different backgrounds may approach the same question in different ways.

Diversity of perspectives (2 of 3)
A diverse community is better able to generate new research methods, explanations, and ideas.

Diversity of perspectives (3 of 3)
Diversity helps to balance bias.

DNA symmetry
This graphic demonstrates the symmetry of DNA using pencils.

DNA symmetry II
Watson and Crick realize that since DNA crystals could be flipped upside down and backwards, and still look the same.

Endosymbiosis and the tree of life
A cartoon of endosymbiosis in which distinct cell lineages joined together to become a single organism.

Endosymbiosis doesn't fit with established theory
Endosymbiosis did not seem to fit with accepted evolutionary theory.

Endosymbiosis vs. step-by-step
Two opposing hypotheses: endosymbiosis vs. step by step

Episodes of endosymbiosis
A cartoon illustrating Margulis' idea that early bacteria experienced several episodes of endosymbiosis.

Ernest Rutherford
Library of Congress photo of Ernest Rutherford.

Even theories change (1 of 4)
Science is always a work in progress and even theories change. For example, in the 1600s classical mechanics was the accepted explanation of the movement of objects both in space and on Earth.

Even theories change (2 of 4)
Then classical mechanics was one-upped by Albert Einstein's theory of special relativity because it explained more phenomena.

Even theories change (3 of 4)
Then special relativity was superseded by another theory, general relativity.

Even theories change (4 of 4)
General relativity seems up for a change. For example, it doesn't mesh with what we know about the interactions between extremely tiny particles. Will physicists develop a new theory that simultaneously helps us understand the interactions between the very large and the very small?

Examining the source (1 of 2)
When evaluating a media message about science, one of the first things to consider is where the information came from.

Examining the source (2 of 2)
An original piece of scientific research may be interpreted many times over before it reaches you.

Extinctions
Percentage of organisms that have gone extinct over the past 200 million years, based on the fossil record.

Global existence of the iridium anomaly
This world map shows some of the sites where an iridium anomaly at the KT boundary has been observed.

Growing hole in the ozone layer
These maps, generated from NASA satellite data, show the growing hole in the ozone layer over Antarctica for each October from 1979 to 1984.

Households items with CFCs
Examples of household items that used to contain CFCs.

How the base pairs match up
Given the correct forms for the bases, Watson was able to figure out how adenine-thymine and guanine-cytosine pairs matched up, and formed weak hydrogen bonds with one another. Watson and Crick originally suggested that there were two bonds between guanine and cytosine but later it was found that a third existed.

Hypothesis of ozone depletion
Molina and Rowland's over-arching hypothesis (that releasing CFCs into the atmosphere would cause significant ozone depletion) was based on many supporting hypotheses, backed by their own lines of evidence.

Ignaz Semmelweis
Photo of Ignaz Semmelweis.

Image B 51
Franklin and Gosling's image confirmed the idea that DNA was helical in shape and that its bases were stacked pancake-style.

Interpreting the evidence
Interpreting the evidence relating to an idea is not always clear-cut.

Iridium graph
A simplified graph showing iridium content across the KT boundary as measured at Gubbio, Italy.

Is the scientific community's confidence in the ideas accurately portrayed?
The tentative nature of science doesn't mean that scientific ideas are untrustworthy.

Limits of science (1 of 4)
Science doesn't make moral judgements.

Limits of science (2 of 4)
Science doesn't make aesthetic judgements.

Limits of science (3 of 4)
Science doesn't tell you how to use scientific knowledge.

Limits of science (4 of 4)
Science doesn't draw conclusions about supernatural explanations.

Magnetic flip-flops
As new seafloor forms, the igneous rock records the current state of the Earth's magnetic field. Sedimentary rock layers forming at the bottom of the ocean may also record these magnetic flip-flops as sediment layers slowly build up over time.

Making assumptions (1 of 2)
Much as we might like to avoid it, all scientific tests involve making assumptions many of them justified.

Making assumptions (2 of 2)
All tests involve assumptions, but most of these are assumptions can be verified separately.

Mathematical models (1 of 2)
Mathematical models of a species interaction can be both simple or complex.

Mathematical models (2 of 2)
Models are based on sets of hypotheses about how a system works.

Misconceptions about science
This cartoon portrays several misconceptions about science and scientists.

Misconduct - cartoon 1
Overlooking data is frowned upon!

Misconduct - cartoon 2
Serious and damaging cases of scientific misconduct are almost invariably found out.

Molecular components of DNA
The molecular components of DNA: phosphates, deoxyribose (a sugar), and the four nitrogenous bases, adenine, guanine, cytosine, and thymine.

Molecular structure of chlorine nitrate
In the presence of another molecule to serve as a catalyst (not shown), nitrogen dioxide (NO2) and chlorine monoxide (ClO), a byproduct of the breakup of ozone molecules by CFCs, react to form chlorine nitrate (ClONO2).

Multiple explanations for the extinction of dinosaurs
The evidence supports the idea that a giant asteroid struck Earth at the end of the Cretaceous, but additional explanations (volcanic activity, global climate change, sea level change, and tectonic movements) may also have played a role.

Niels Bohr
Photo of Niels Bohr.

Northern hemisphere ozone change
Change in the amount of ozone in the Northern Hemisphere over a 17-year period. Note the large decline in ozone during the winter months and how the winter decline is greatest at higher latitudes.

Nuclear vs. mitochondrial DNA
Unlike nuclear DNA (left), mitochondrial DNA is only inherited from the maternal lineage (right) a quirk which allowed Margulis to determine whether mitochondrial DNA encoded unique traits.

Ozone depletion - CFC phaseout
A graph showing world production of three major CFC types between 1970 and 1988. The dot marks the year (1977) that the US phaseout of CFCs was announced.

Ozone fluctuation
Ozone levels fluctuate so widely that it is difficult to detect subtle trends over a short-term period, as shown by these ozone measurements for the atmosphere over Switzerland taken between 1926 and 1975.

Ozone recovery
With the CFC ban in place, atmospheric levels of chlorine are beginning to decline.

Peer review
Peer review of a scientific study submitted for publication provides assurance that someone who knows what they're doing has double-checked it.

Products of deuterium fusion
According to nuclear theory, deuterium atoms fuse and forming helium-4 which has a lot of energy so much energy that it is unstable, so the atom discharges some of this energy by releasing a neutron, proton, or gamma ray.

Prokaryotes and eukaryotes
Comparing prokaryotic and eukaryotic cells.

Puzzle piece (1 of 10) - heredity
DNA is the component of chromosomes that carries genetic information.

Puzzle piece (10 of 10) - Evidence for the structure of DNA
Watson and Crick figured out how all the evidence fit together but the evidence came from many different members of the scientific community.

Puzzle piece (2 of 10) - DNA structure
DNA is composed of phosphates and sugars attached to nitrogenous bases, which are arranged in stacks and always have the same ratio.

Puzzle piece (3 of 10) - DNA structure
DNA has a regular structure arranged in a helix.

Puzzle piece (4 of 10) - DNA structure
Franklin and Gosling find that water molecules can cling to the DNA.

Puzzle piece (5 of 10) - DNA structure
Based on the ease with which DNA took up water, Franklin reasoned that the phosphates (which attract water) must be on the outside of the helix.

Puzzle piece (6 of 10) - DNA symmetry
DNA crystals look the same when turned upside down and backwards.

Puzzle piece (7 of 10) - DNA helix
Image B 51 also suggested the number of bases per twist and the diameter of the helix.

Puzzle piece (8 of 10) - DNA structure
The DNA puzzle after the addition of the stacked bases and helix diameter findings.

Puzzle piece (9 of 10) - DNA bases
Correct hydrogen placement revealed the shapes of DNA's bases

Real process of science (1 of 3)
In contrast to the linear steps of the simplified scientific method, the process of science is non-linear.

Real process of science (2 of 3)
Successive investigations of a topic often lead back to the same question, but at deeper and deeper levels.

Real process of science (3 of 3)
The process of science is not predetermined.

Replication
Scientists aim for their studies' findings to be replicable.

Revisiting Mendel
Gregor Mendel (left) showed that if you know the genotypes of the parents in a cross, you can predict the ratios of different offspring genotypes (right).

Roles of the scientific community (1 of 4)
One of the roles of members of the scientific community is to serve as fact checkers/critics.

Roles of the scientific community (2 of 4)
Members of the scientific community also serve as innovator/visionary by generating new ideas.

Roles of the scientific community (3 of 4)
Members of the scientific community may also serve as watchdog/whistleblower to eliminate bias and fraud by keeping watchful eye.

Roles of the scientific community (4 of 4)
Members of the scientific community may serve as cheerleader/taskmaster to motivate fellow scientists.

Rutherford and the atom (1 of 4)
Ernest Rutherford used alpha particles (helium atoms stripped of their electrons) to learn about the structure of the atom.

Rutherford and the atom (2 of 4)
By firing alpha particles through gold foil, Rutherford was able to test ideas about the interior of the atom.

Rutherford and the atom (3 of 4)
Most of the alpha particles passed through the gold foil without changing direction much as expected, but some came bouncing back in the opposite direction.

Rutherford and the atom (4 of 4)
Rutherford published a description of his idea, which was later modified by Niels Bohr.

Rutherford's argument (1 of 2)
In the early 1900s, Ernest Rutherford and his colleagues performed this experiment to test the hypothesis that an atom's mass and positive charge are spread diffusely throughout the atom and found that their expectations and actual observations did not match at all.

Rutherford's argument (2 of 2)
Rutherford constructed a new scientific argument.

Science and technology working together
Scientific knowledge allows us to build new technologies, which often leads to new ideas, which inspires new technologies and so on.

Science Checklist
This checklist provides a guide for what sorts of activities are encompassed by science.

Science is a community endeavor
Science simply works better when lots of different sorts of people participate in it.

Scientific arguments (1 of 2)
Taken together, the expectations generated by a scientific idea and the actual observations relevant to those expectations form what we'll call a scientific argument.

Scientific arguments (2 of 2)
Though the elements of a scientific argument are always related in the same logical way, those elements may be assembled in different orders.

Scientific community
Scientists sometimes work alone and sometimes work together, but communication within the scientific community is always important.

Scientific community
People from all over the world from all sorts of different cultures and backgrounds are a part of the scientific community.

Scientific culture (1 of 4) - scrutiny
In science, all ideas must stand up to rigorous scrutiny.

Scientific culture (2 of 4) - honesty
Scientists expect other scientists to act with honesty and integrity.

Scientific culture (3 of 4) - credit
Scientists give credit where credit is due.

Scientific culture (4 of 4) - ethics
Scientists adhere to ethical guidelines.

Scientific explanations (1 of 3) - hypotheses
Hypotheses are proposed explanations for a fairly narrow set of phenomena, usually based on prior experience, scientific background knowledge, preliminary observations, and logic.

Scientific explanations (2 of 3) - theories
Theories are broad explanations for a wide range of phenomena. They are concise, coherent, systematic, predictive, and broadly applicable.

Scientific explanations (3 of 3) - over-arching theories
Over-arching theories are particularly important and reflect broad understandings of a particular part of the natural world.

Scientific method
Science is complex and cannot be reduced to a single, prepackaged recipe.

Sources of scientific information (1 of 3)
Find sources with scientific expertise.

Sources of scientific information (2 of 3)
Avoid websites from groups that might have ulterior motives.

Sources of scientific information (3 of 3)
Look for sources that provide a list of citations.

Summing up scientific arguments
Scientific arguments are formed by figuring out what we would expect to observe if a particular idea were true and then checking those expectations against what we actually observe. A match between expectations and observations lends support to the idea, while a mismatch helps refute the idea.

Testable ideas
A scientific idea may require a lot of reasoning to work out an appropriate test, but to be scientific, an idea must be testable.

Testing an idea (1 of 3)
Psychologist Edward Tolman wanted to know how rats successfully navigate their surroundings for example, a maze containing a hidden reward. Would they build mental maps of the maze as they investigated it or would they learn to navigate the maze through stimulus-response?

Testing an idea (2 of 3)
Tolman and his colleagues trained rats in a maze which offered them many different tunnels to enter first. One of the tunnels twisted and turned but consistently led to the reward, and the rats quickly learned to go down that tunnel.

Testing an idea (3 of 3)
When the entrance to the reward tunnel was blocked, most of the rats picked a tunnel that led in the direction of the food, supporting the idea that rats navigate using something like a mental map.

Testing hypotheses (1 of 2)
The atoll Eniwetok (Anewetak) in the Marshall Islands an oceanic ring of exposed coral surrounding a central lagoon.

Testing hypotheses (2 of 2)
Two hypotheses which is the better explanation?

Testing scientific ideas
Scientific testing occurs in two logical steps: (1) if the idea is correct, what would we expect to see, and (2) does that expectation match what we actually observe? Ideas are supported when actual observations (i.e., results) match expected observations and are contradicted when they do not match.

Testing the endosymbiotic hypothesis
Testing the hypothesis: If some organelles evolved via endosymbiosis, we would expect to see a close genetic relationship between the organelles and bacteria.

Testing the hypothesis of ozone depletion (1 of 3)
Molina and Rowland needed to test their ideas with actual atmospheric evidence, but figuring out just what evidence to look for was tricky.

Testing the hypothesis of ozone depletion (2 of 3)
If CFCs are destroying the ozone layer, then we would expect/predict that more CFCs will be present at lower altitudes.

Testing the hypothesis of ozone depletion (3 of 3)
Chlorine monoxide is one of the products of ozone destruction. Since there is no other known source of chlorine monoxide, finding this chemical in the upper atmosphere would strongly support the idea that chlorine is destroying ozone.

Testing the idea of cold fusion (1 of 6)
Testing the hypothesis: If cold fusion is taking place, then we would expect to see heat generated.

Testing the idea of cold fusion (2 of 6)
Testing the hypothesis: If cold fusion is taking place, we would also expect to neutrons or helium-4 generated.

Testing the idea of cold fusion (3 of 6)
Pons' neutron results didn't agree with his heat measurements, with Jones' neutron results, or with established nuclear theory, which suggested no fusion should be occurring at all!

Testing the idea of cold fusion (4 of 6)
Testing the hypothesis: Could cold fusion really be taking place, or had Pons and Fleischmann made a mistake?

Testing the idea of cold fusion (5 of 6)
Testing the hypothesis: If cold fusion is taking place, we would expect elevated levels of helium-4 in the palladium rod.

Testing the idea of cold fusion (6 of 6)
Testing the hypothesis: If cold fusion is taking place, we would also expect to see many neutrons released.

Testing the supernova hypothesis
To further test the supernova hypothesis:If a supernova had occurred, it would have also released plutonium-244 as well as iridium. Original tests looked positive but replicating the analysis showed that there was no plutonium in the sample, contradicting the supernova hypothesis.

Testing the two hypotheses (1 of 4)
Comparing the evidence for two opposing hypotheses.

Testing the two hypotheses (2 of 4)
Examining the evidence for two hypotheses: the endosymbiotic or step-by-step origins of mitochondria, plastids, and tubule organelles.

Testing the two hypotheses (3 of 4)
New sequencing technologies leads to the evidence that both mitochondrial and plastid DNA are more similar to bacterial DNA.

Testing the two hypotheses (4 of 4)
A group of scientists showed that if you remove tubule organelles from a eukaryotic cell, they can grow back. This discovery argued against the idea that these organelles evolved via endosymbiosis.

The argument for plate tectonics (1 of 6)
Powerful scientific ideas generate many different expectations, not just one. As an example, let's return to the idea that the continents as we know them today were once joined together into a supercontinent and have been moving apart ever since. This idea generates many different expectations; we would expect to find corresponding fossils on now distant continents.

The argument for plate tectonics (2 of 6)
We would expect to find that the continents are shaped in ways that could have once fit together.

The argument for plate tectonics (3 of 6)
We would expect to find that rock layers and geological features on now distant continents match up where they were once joined.

The argument for plate tectonics (4 of 6)
We would expect to find that the evolutionary relationships among non-marine species reflect the ancient supercontinental break up.

The argument for plate tectonics (5 of 6)
We would expect to find direct evidence of ongoing tectonic movement through sensitive satellite measurements.

The argument for plate tectonics (6 of 6)
We would expect to find a plausible mechanism by which the continents could have moved.

The asteroid hypothesis
The team came up with a new idea: an asteroid impact which would explain the iridium and the lack of plutonium but not why the extinctions.

The endosymbiotic hypothesis
Margulis' endosymbiotic hypothesis.

The fusion cell (1 of 3)
Pons and Fleischmann put together what they called a "fusion cell."

The fusion cell (2 of 3)
A detailed diagram of Pons and Fleischmann's cold fusion cell from one of their published papers.

The fusion cell (3 of 3)
To really know how much heat is being produced by the fusion cell it is necessary to estimate how much heat is escaping from it.

The iridium anomaly
The results of the iridium analysis were surprising. The team found more than 30 times what they had expected.

The ozone layer
The ozone layer protects Earth from dangerous UV radiation.

The science toolkit
A set of questions for evaluating scientific messages.

The supernova hypothesis
An existing hypothesis proposed that a supernova at the end of the Cretaceous had caused the extinction of dinosaurs. Supernovas throw off heavy elements like iridium so the hypothesis seemed to fit with the team's discovery of high iridium levels.

Thymine and guanine
The visiting American chemist, Jerry Donohue, provided a key piece of evidence when he revealed that the forms given for thymine and guanine in most textbooks were wrong. Note the changes, indicated by the glowing hydrogens.

True scientific controversy (1 of 4)
Scientists disagreeing about a central hypothesis or theory.

True scientific controversy (1 of 4)
Scientists disagreeing about a less central aspect of a scientific idea.

True scientific controversy (3 of 4)
Disagreement within the scientific community or society at large over the appropriateness of a method used for scientific research.

True scientific controversy (4 of 4)
This is not a conflict over a scientific idea, but over how such ideas should be applied.

Two explanations for the shape of mitochondrial DNA
Two interpretations of the circular shape of the DNA in mitochondria.

Two hypotheses for the rate of deposition
Alvarez posed two hypotheses: either the clay was deposited over a few years and he would find no iridium, or over a few thousand years and the iridium would definitely be present.

Two sites showing the iridium anomaly
Gubbio, Italy and Stevns Klint, Denmark sites that confirmed the widespread presence of an iridium anomaly.

Types of misconduct
Some types of misconduct.

Untangling media messages
Media representations of science and science-related policy are essential for quickly communicating scientific messages to the broad public; however, some important parts of the scientific message can easily get lost or garbled in translation.

Updating evolutionary theory
Margulis didn't overthrow any of the core ideas of evolution, but she did force some of them to move over and make room for endosymbiosis.

Watson and Crick's first model of DNA
Watson and Crick's model erroneously placed the bases on the outside of the DNA molecule with the phosphates, bound by magnesium or calcium ions, inside.

X-ray diffraction
X-ray diffraction was a tool developed in the first half of the 20th century to infer atomic structure.

 


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