TOPIC 5: ANCIENT GREEK SCIENCE - 500 YEARS LATER

1. TOPIC 5:ANCIENT GREEK SCIENCE - 500 YEARS LATER Development of the first real scientific paradigms

2. The Greek World After Aristotle • Aristotle died in (322 BC). • By that time, Alexander the Great had conquered the Greek and Persian World to form one great empire. • Upon his death (323 BC), the empire broke up into several kingdoms under the control of Alexander’s generals or relatives. • The Greek City States were all under the power of regional kings, yet they maintained local autonomy so that democratic institutions that fostered an interest in philosophy and education did not die. • This Helenistic world with kindred rulers broke down many of the societal and political barriers that were present before. This permitted many of the Greek philosophical ideas to be spread to the Middle East and Egypt.

3. Greek Centers of Learning • Lyceum of Athens - still the foremost teaching academy for more than 200 years after Aristotle. Later heads of the Lyceum included Strato and Theophrastus. • Alexandria also became an important center because of two non-teaching institutions: • Library of Alexandria • Museum of Alexandria • The Library and Museum were places where scholars worked and sometimes lived together (some common meals). But, these were not primarily teaching institutions, but rather the first real research institutions.

4. The Nature of Matter at the Time of Aristotle • All matter on Earth is composed of varying proportions of four elements - fire, water, air, earth. • All matter can be transmuted from one form to another. • The heavens (stars and planets) are composed of one element, aether, which is eternal and unchanging. • No void (space absent of matter); matter in the universe is continuous.

5. Strato (290 BC) on Air • Strato used actual experiments to deduce that air has substance (is matter). • ‘If an empty vessel is inverted and pressed into water, the air (in the vessel) does not allow the water to enter. But, if a hole is bored into the bottom of the vessel and the test repeated, the water will enter as the air escapes through the hole.’

6. Strato (290 BC) on the Void • Strato also addressed the issue of whether air is a continuous material or composed of small pieces (atoms?!) with intervening void (vacua). • He stated ‘Those who assert that there is no vacuum are satisfied with inventing many arguments for this and perhaps seeming plausible with their theory in the absense of sensible proof. If, however, by referring to the appearances and to what is accessible to sensation, it is shown that there is a continuous vacuum, but only one produced contrary to nature; that there is a natural vacuum, but one scattered in tiny quantities; and that bodies fill up these scattered vacua by compression; then those who put forward plausible arguments on these matters will no longer have any loop holes.’

7. Strato’sVacuum Experiments • Prepare a tightly sealed sphere of metal plate so as not to be easily crushed containing about 8 cotylae (~2 liters). Pierce a hole in it and let in a siphon (tube) that is sealed to the sphere with tin (weld). • The first experiment tests whether there are scattered vacua in air. If there were none, then air should be incompressible. But, if one blows air into the sphere through the siphon, one will introduce much breadth (air), in addition, without the air that is contained in the sphere escaping. This shows that air is compressible and must be a combination of air particles and small voids (vacua). • The second experiment shows that air can be evacuated from the globe. If one draws air out of the sphere by sucking on the siphon, a fair quantity will come out although no other substance takes its place in the sphere. So that by this means it is conclusively proved that a considerable accumulation of vacuum occurs in the sphere.

8. Theophrastus (~300 BC) on Fire • Theophrastus argued that fire is different from air, water, and earth and should not be considered one of the fundamental constituents of all matter. His reasons were as follows: • fire is different in that it can generate itself (not the others) • humans can make fire (not the others) • fire requires force to come into being • fire alone needs some other material as a substrate. • He stated that ‘Everything that burns is always as it were in a process of coming-to be and fire is a kind of movement, and it perishes as it comes to be, and as soon as what is combustible is lacking, it too itself perishes. Accordingly, it seems absurd to call this a primary element and as it were a principle, if it cannot exist without matter.”

9. Summary of Greek Thoughts on the Nature of Matter • Water, earth, air, and fire (despite views of Theophrastus) are fundamental materials. • Materials are composed of pieces (atoms). • A void exists. • Materials can be ‘transmuted’ from one material to another. • Stars are made of unchangeable aether.

10. The Nature of Motion at the Time of Aristotle • The Earth is fixed in the Universe and all planets, moons, and stars revolve around it. • Natural motion of all matter on Earth is up or down depending on the relative quantities of the four elements. Velocity of falling objects depends on their weight. • All other motion of inanimate objects is then unnatural or violent and requires a motive force (e.g., push or pull). Also, unnatural motion had a continuing source for the motion. For example, if one throws a rock, the rock keeps going because the air in front of the rock is displaced as the rock travels, and moves to behind the rock where it continues to push the rock along. • Motion of animate objects is driven by thought. • The stars, planets, Sun, and Moon are made of aether and have ‘natural’ circular motion.

11. Strato on Natural Motion • Strato was the first to describe clear evidence that downward motion is not a simple uniform process. Rather, objects move faster (accelerate) as they come closer to their natural place (Earth). • He stated that ‘If one observes water pouring down from a roof and falling a considerable distance, the flow at the top is seen to be continuous, but the water at the bottom falls to the ground in discontinuous parts. This would never happen unless the water traversed each successive space more swiftly.’

12. Strato on Falling Objects • Strato also stated that ‘ if one drops a stone or other weight from a height of about a finger’s breadth, the impact made on the ground will not be perceptible. But, if one drops the object from a height of one hundred feet or more, the impact on the ground will be a powerful one. Now, there is no other cause for this powerful impact. For the weight of the object does not increase, the object itself has not become greater, nor is it impelled by a greater (external) force, rather it moves more quickly.’

13. Philoponous (6th Century AD) on Unnatural Motion • Philoponus argued against the notion of air continuing to push an object as it moves through the air. • He stated ‘How is it then that the air, pushed by the arrow, does not move in the direction of the impressed impulse, but instead, turning about, as by some command, retraces its course? Furthermore, how is this air, in so turning about, not scattered into space, but instead impinges precisely on the notched end of the arrow and again pushes the arrow on and adheres to it? Such a view is completely implausible and is more like fiction.’ • Philoponous developed an alternative ‘impetus’ theory – the act of setting an object in motion impresses on that object a force or impetus that keeps it in motion. (This concept is also WRONG, but it took another 1000 years to figure that out.)

14. Summary of Greek Thoughts on the Nature of Motion • The Earth is at rest. • Motion on Earth is natural, unnatural, or animated. • Natural motion is not uniform in speed, but has acceleration toward the Earth. • Unnatural motion is due to transfer of motive force (impetus) to object in motion. • Motion of astronomical objects is circular with constant uniform speed.

15. Astronomy at the Time of Aristotle • The Earth is a sphere at rest in the center on the Universe. • The Sun, Moon, Five planets, and stars revolve around the Earth in circular paths. • Eudoxus of Knidus (~365 BC) developed a conceptual theory of an Earth-centered universe with all planets and stars embedded in a series of concentric spheres surrounding the Earth. Each sphere rotated about an axis embedded in a sphere that was farther way from the Earth.

16. Eratosthenes (~225 BC) was first to Measure the Size of the Earth • He used simple geometry to measure the angle of the Sun’s rays at zenith (noon) from two locations a known distance apart (Alexandria and Aswan, Egypt). • He determined that the Earth’s circumference is 252,000 stades; that is ~36,000 km to ~41,000 km depending on which estimate of stade length one uses) was within 10% of our current measurement (40,009 km).

17. Eratosthenes’ Method for Calculating the Size of the Earth

18. Aristarchus (275 BC) the ‘Heretic’ • Aristarchus believed the Sun was at the center of the Universe and that the Earth revolved around it in an orbit much smaller than the radius of the sphere holding the stars. • He also thought that the Earth rotated on its own axis every day rather than that the sphere with stars rotated. • This revolutionary theory was not appreciated by other astronomers for two basic reasons. • First, why do we see no evidence of Earth’s rotation? The planetary surface must be moving at speeds of more than 1000 km/hr if the planet rotates. • Second, why do astronomers see no difference in angles between pairs of stars (parallax) as Earth revolves around Sun (and thus changes position with respect to the star sphere which is centered on the Sun.)

19. Nature of Parallax

20. Other Detailed Astronomical Observations • Stars all move together at the same speed in motion about the North Star • Stars always remain the same size • Planets and Sun move at different speeds at different times • Planet/Sun sizes change with seasons • Seasons are not the same length • Planets have retrograde motion

21. Retrograde Motion

22. Apollonius (~210 BC) and the Geometry of Solar/Planetary Motion • Apollonius (~210 BC) was able to explain Solar/planetary motions by arguing that they are either eccentric or composed of two different circular motions. (The observational results are the same.) The idea of eccentric motion is that the Sun revolves in a circular orbit around some point in space, but the Earth is not exactly at the center. Rather it is ‘eccentric’ and lies a small distance from the center (but still at rest). • Apollonius stated that an equivalent idea is to say that the Earth is at the center of the Sun’s circular orbit (or sphere) which he termed the deferent circle. But that, the Sun also revolves in a second circular orbit (or sphere), which he termed the epicycle circle, around a point on the deferent circle. The advantage of this view is that the Earth stays at the center of the Universe and that all motions are still circular.

24. Ptolemy (150 AD) • Developed the most detailed catalogue of star and planetary motions • Made detailed calculations of the geometry of each planet’s deferent and epicycle motions • Developed the final Geocentric model of the Universe that was in use for the next 1300 years.

25. A Summary of Greek Thoughts on Astronomy • Earth at rest at the center of the Universe • Sun, Moon, five planets, and all stars together revolve around Earth on 8 concentric spheres in uniform circular motion. • Each sphere (deferent) rotates on an axis embedded in the next outer sphere. • Astronomical objects also revolve around smaller epicycle spheres embedded in the deferent spheres.

27. QUESTIONS? • Always Ask Questions • Best way to learn! • Slows me down!