ACARA v9 CONTENT DESCRIPTION “explain how new evidence or different perspectives can lead to changes in scientific knowledge”
Builds on the idea that scientists use models to picture things too small to see. Here we look at how those models change: a model is the best explanation of the evidence so far, and when stronger evidence arrives, even a long-accepted model is revised.
A model is the best explanation so far
Scientific knowledge is not a fixed list of facts handed down once and for all. It is the best explanation that fits the evidence available right now. When new tools and new experiments reveal something the old explanation cannot account for, scientists build a better model. The story of the atom shows this clearly: over about a century the accepted picture changed several times, each time to fit fresh evidence.
The changing model of the atom
Add each new piece of evidence in turn and watch the accepted model of the atom be revised to fit it.
New evidence (1 of 4)
Early 1800s: John Dalton finds that elements combine in fixed whole-number ratios, as if matter were made of identical, indivisible units.
Accepted model: Atoms are tiny, solid, indivisible spheres, one kind for each element.
Add the next piece of evidence and watch whether the accepted model holds or has to change.
Changing your mind is how science works
It can seem worrying that scientists changed the model so many times. In fact this is the opposite of a weakness. The scientists who accepted the plum-pudding atom were reasoning carefully from the best evidence they had. When new evidence did not fit, they were willing to give up the old picture for one that explained more. Knowledge that can be revised by evidence is more trustworthy, not less, because it is always being tested.
A second case: continents that drift
The atom is not the only model that new evidence overturned. In 1912 Alfred Wegener proposed that the continents had once been joined and had slowly drifted apart. For decades most geologists rejected the idea, because Wegener could not say what force could move a continent. Then, in the 1950s and 1960s, new measurements of the ocean floor arrived. Sort the findings below into those that are real evidence for drifting continents and those that only sound related.
Did the evidence support drifting continents?
Wegener claimed the continents had moved apart. Decide which findings genuinely test that claim.
Claim: The continents were once joined and have slowly drifted to their present positions.
The coastlines of South America and Africa fit together like pieces of a jigsaw.
Matching bands of rock and the same fossils are found on coasts now separated by a wide ocean.
The sea floor is youngest at the mid-ocean ridge and older the further away you measure.
Many maps of the world are printed with the Atlantic Ocean in the centre.
Wegener was trained as an astronomer and a weather scientist, not a geologist.
Decide whether each statement is evidence for the claim, or not.
The measurement that settled it
The strongest new evidence was the age of the ocean floor. When ships measured the rock beneath the Atlantic, the sea floor turned out to be brand new at the mid-ocean ridge and steadily older with distance from it, on both sides. Read the data below as a table, then as a graph. The same numbers that look like a plain list become an unmistakable trend, showing the sea floor spreading outward and carrying the continents with it.
Age of the Atlantic sea floor with distance from the ridge
Switch between the table, the bar chart and the line graph. A pattern hidden in the list jumps out once it is drawn.
The rock is newest at the ridge and gets older with distance, in a steady, even climb. That spreading sea floor was the missing evidence that finally turned a rejected idea into accepted science.
Why this matters
Understanding that scientific knowledge is the best current explanation, open to revision, helps you read science news with a clear head. A new finding does not mean the old science was useless; it usually means the picture is being sharpened. Knowing how and why models change is the difference between trusting a process and expecting impossible certainty.
Quick self-check
1. Why did the accepted model of the atom change over time?
2. Rutherford fired tiny particles at thin gold foil. A few bounced almost straight back. This was evidence that...
3. When the plum-pudding model was replaced, the earlier scientists were...
4. What does the history of the atom show about scientific knowledge?
5. A student says, "Because the model changed before, the science must be unreliable." A better response is: