Historical Episodes

1
Lecture 4

• Historical Episodes

2
Readings

• You are expected and advised to read the relevant
  chapter before the lecture.
• Purposes of lecture
• Go over and clarify material in the readings.
• Expand on material and link different parts of
  the material together.
• Discuss pertinent philosophical issues.

3
After this lecture you should

• Have a good understanding of certain important
  events in the history of science.
• Be better able to apply the six-step program to
  real cases.
• If youre lost. Stop me.

4
Theoretical Model
Represents
Real system
Constitutes
Set of statements (laws)
Fictional story
Similar to
Real world events
Constitutes (tells)
Set of statements
5
Newtonian physics paradigmatic scientific theory

• All we need are simple verbal expressions of the
  laws
• 1. If there is no force acting on a body, the
  momentum of the body will remain constant.
• 2. F m a
• 3. If one body exerts a force on a second, then
  the second exerts a force on the first that is
  equal in strength but opposite in direction
• (UG) Any two bodies exert attractive forces on
  each other that are directed along a line
  connecting them and are proportional to the
  product of their masses divided by the square of
  the distance between them.

6

• Any model in which these statements are true is a
  Newtonian model.
• A theoretical hypothesis says that some part of
  the world is approximately similar to a
  theoretical model of some sort.
• Examples
• The earth and the sun are (approximately) a
  two-particle Newtonian system.
• The earth and a satellite are (approximately) a
  two-particle Newtonian system.

7
Testing the Newtonian model

• Could the comet observed in 1682 fit a Newtonian
  model?
• Halley guessed that two previously recorded
  comets were the same as the one observed in 1682.
• Assuming that the earth and the comet could be
  represented by a Newtonian model, Halley could
  make a prediction for when the comet would return.

8

• Step 1. Real World. The real world object of
  interest is the comet observed in 1682.
• Step 2. Model. The model is a Newtonian model of
  two bodies in an elliptical orbit attracting one
  another by the force of gravity.
• Step 3. Prediction. The prediction was that the
  comet would return near the end of 1758.
• Step 4. Data. A comet with the requisite orbit
  did appear.
• Step 5. Negative evidence? No. The data agreed
  with the prediction.
• Step 6. Positive evidence? The only alternative
  was that another comet with the same orbit
  happened to appear at the right time. This is
  very unlikely. So the data strongly supports the
  Newtonian model.

9
The Copernican Revolution

• Philosophy of science is largely about how
  science ought to be done.
• But philosophers learn from how science is done.
• They seek to rationalize and explain the
  behaviour of scientists.
• Science is most exciting when revolutions take
  place the Copernican revolution was one of the
  biggest.

10
Two Models

• The Earth is at the centre of the universe
  (overhead).
• Planets are observed to change directions
  (overhead this is cool. And hard.)
• The Ptolemaic system already had ad hoc epicycles
  to explain the retrograde motion of the planets
  (overhead).
• So the Copernican system was simpler than the
  Ptolemiac system.

11
Why care about simplicity?

• Occams Razor Unnecessary entities should be
  cut out. Or the simplest explanation is usually
  correct.
• Fact Scientists prefer simpler theories.
• But why?
• 1. Instrumental reasons Simpler theories are
  easier to understand and work with.
• 2. Theoretical reasons. The world is simple.
• When I saw how beautiful the theory was, I knew
  it was true. Einstein?
• Everything should be made as simple as possible,
  but not simpler.
• Relativity is really hard.

12
The Crucial Experiment

• Venus is always seen near the sun (first
  overhead).
• Therefore, on the Ptolemaic system Venus revolves
  in a small orbit centred on a line connecting the
  Earth and the sun (last overhead).
• So Venus could never be fully illuminated.
• On the Copernican System, Venus would sometimes
  be fully eliminated.
• Galileo observed Venus change from a crescent to
  fully illuminated with his newly developed
  telescope.

13

• Step 1. Real World. The real world object of
  interest is the arrangement of the sun and
  planets
• Step 2. Model. There are two models a) the
  Ptolemaic model and b) the Copernican model
• Step 3. Prediction. The Ptolemaic model predicts
  that Venus can never be seen fully illuminated.
  The Copernican model predicts that Venus can be
  seen going through a complete set of phases.
• Step 4. Data. Venus was seen to go through a
  complete set of phases

14

• Step 5. Negative evidence? The data disagree with
  the Ptolemaic prediction, so the data provides
  evidence that the Ptolemaic model does not fit
  the world. The data agree with the Copernican
  prediction.
• Step 6. Positive evidence? How likely was it that
  a complete set of phases would be observed
  anyway? It would not be observed given the
  Ptolemaic model. It is not something that we
  would independently expect. In the absence of any
  alternative model that would predict the phases
  of Venus, the Copernican model is strongly
  supported.

15
Mendelian Genetics

• Background story
• A type of garden pea is either Tall or Short.
• Short plants bred together produce short plants.
• Tall plants seemed to come in two
• varieties
• Some pairs of Tall plants produced only Tall
  plants.
• Call these true-breeding plants.
• Some pairs of Tall plants produced a mixture.
  Call these hybrid plants.
• If you cross-fertilize the true breeding Tall
  plants with the Short plants, the result is all
  tall plants.
• These tall plants are hybrids. When
  self-fertilized, a mixture of Tall and Short
  plants are produced.

16
x
True-breeding tall plants
True-breeding short plants
x
Hybrids (all tall)
Hybrids (all tall)
17
Mendels Model (Theoretical)

• Suppose there are two things that determine which
  traits are exhibited.
• Call these things genes, and suppose that each
  plant has two genes.
• Suppose one type of gene (H) is associated with
  being Tall and another (h) with being Small.
• Suppose that on pollination, the seeds for the
  next generation get one gene from each parent.
• Moreover, assume the selection of a gene from
  either parent is the result of a random process.
• Finally, suppose that one of these genes is
  dominant, and the other recessive.

18
Mendels Model (Diagrammatic)
HH
hh
True- breeding tall
True- breeding tall
x
Hh
Hh
Hybrids (all tall)
x
HH
Hh
hH
hh
Short (true-breeding)
True-breeding tall
Hybrids (tall)
19
Testing Mendels ModelThe Backcross Experiment

• Fertilize the short plants with the hybrid plants.

1 2 3 4
hh
Hh
1 3 1 4 2 3 2 4
Hh Hh hh hh
Data Mendel reported 106 tall and 102 short
20
Analysis

• Step 1. Real Word. The real-world process is the
  inheritance of genes by offspring from sexually
  reproducing parent.
• Step 2. Model. The model is Mendels two-factor
  model satisfying the law of segregation.
• Step 3. Prediction. The prediction is that the
  ratio of tall plants to short plants in the
  backcross experiment should be 1 to 1.
• Step 4. Data. The data were 106 tall plants and
  102 short plants.
• Step 5. Negative evidence? No. The data are in
  close agreement.
• Step 6. Positive evidence? Was the prediction
  likely to agree with the data anyway? Giere The
  question requires more thought. It depends on
  what the alternative models might be.

21
The Rise and Fall of Phlogiston

• Phlogiston The fire stuff.
• Phlogiston is the stuff driven out of objects
  when they burn.
• Combustible materials contain phlogiston
• Cooling makes it less volatile.
• Smothering holds it in.
• A burning candle in an enclosed container goes
  out because the air gets saturated with
  phlogiston and the remaining phlogiston has
  nowhere else to go.

22
Lavoisiers Experiment
Suns rays
Lens
Glass jar
Ash forms on the surface of the mercury
Water
23
Analysis

• Step 1. Real World. The process of combustion. In
  particular, the combustion of mercury.
• Step 2. Model. The phlogiston model, in which
  phlogiston is given off by certain materials,
  including mercury, when appropriately heated.
• Step 3. Predictions. The phlogiston model
  predicts that a) the water level should fall and
  b) the weight of the mercury / ash should fall.
• Step 4. Data. The data are that 1) the water
  level went up and b) the weight of the mercury /
  ash combination weighed more than the original
  mercury alone.
• Step 5. Negative evidence? Yes. The data provide
  evidence that the phlogiston model fails to
  represent the combustion of mercury.

24
Assignment 2Due Thursday 25th September

• Exercise 3.2
• The Discovery of Neptune.
• p. 86
• Follow the model of the textbook examples Half a
  page is sufficient.
• Philosophical Grading You get credit for clarity
  and conciseness.
• If your Mum cant read it and understand it, its
  not clear enough.