Model of Universe

Many students pick science stream for their high school, without really understanding what science really is.

To provide a perspective, let us discuss the model of universe during ancient Greek period (at the time of Aristotle). We call this model as:

Early Geocentric Model

As per early model, the universe is made up of 5 basic elements - Earth, Water, Air, Fire, Ether - in that order of rarity (rarity is inverse of density - more rare = less dense)

The arrangement is such that - earth is at the center, then water, then air, then fire. After fire all heavenly bodies are attached on their own celestial spheres. These celestial spheres are hollow concentric shells, enveloping the earth. 

The outer most celestial sphere comprises of starts visible in the night sky through the naked eye (without any telescopic equipment). These stars seem to move throughout the night as if they are rigidly attached to an invisible bowl that rotates around a fixed pole in the sky - called the North/South Celestial Pole.

Further inside of the outermost sphere, are the spheres of 7 planets - moon, sun, mercury, venus, mars, jupiter and saturn. At that time, universe was considered to be geocentric and all 7 astronomical bodies were visible and found to be moving with respect to the background of the stars. These bodies were called planets from the greek word "planetai" - means wanderer. Compared to the uniform regular motion of the stars, the motion of planets appeared to be more non-uniform and philosophers of those times were busy in providing an explanation for planetary motion that is rational and aesthetically harmonious, but should lead to such an appearance.

The motion of the 7 planets was explained by letting each planet be carried on a transparent sphere of its own, one within the other, all seven enclosed by the sphere of fixed stars, and the earth at the center of the whole scheme (see figure). The sphere of the moon separates the terrestrial region (composed of concentric shells of the four elements Earth, Water, Air, and Fire from the celestial region. Beyond the moon are the concentric spheres carrying Mercury, Venus, Sun, Mars, Jupiter, Saturn and the fixed stars.

Modified Geocentric Theories

There were some anomalies in the above model:

1. The sun, the moon, Venus, Mars and Jupiter at times seemed brighter or nearer and at other times farther away from the earth. A set of uniform rotations of the celestial bodies on spheres concentric with the earth could never allow them to change their distance from the earth

2. Retrograde motion: Some planets like mars moved in the reverse direction then in forward direction again. Such motion is called as retrograde motion. See figure

    As per modified geocentric theory, epicyclic motion of planets was suggested (epi is "on" in greek -> it means cycle (circle) upon cycle). See below figure

Point P is having 2 cycles as it is moving in 2 circles centered at points D and O. With different distances and and angular speeds, and further adding such circles, we could describe any periodic motion. 

All substances (matter) were believed to be made up of the 5 basic elements (in different proportions). There was a dominant element and the substance had most properties of the dominant element. e.g. oil has the dominant element of water, solids have the dominant element of earth etc. The dominant element would attract the element accordingly. e.g. dominant element of a feather was air and a cannon ball was earth. So, when left freely, a cannon ball drops down to earth and a feather rises up to air. The corollary thus was, a heavier object should touch ground faster, as it had more dominant earth element. 

CLASSICAL MODEL OF PHYSICAL UNIVERSE

From the ancient model of universe, we have come to agree upon a comparatively modern model of universe. By modern I mean around 17th century (1600 - 1699).

The helio-centric model (sun at the center) is -> 8 planets revolving around the sun (mercury, venus, earth, mars, jupiter, saturn, uranus, neptune). Planets have their own satellites revolving around them. The stars are not attached to a concentric enveloping sphere and there is practically infinite space and infinite stars.

All substances are made of atoms. Atoms have 3 basic sub atomic particles called - electron, proton and neutron. Neutron and proton reside in the nucleus of the atom and electrons are outside the nucleus. Atoms form molecules due to chemical reactions occurring due to electrons. The properties of substances are determined by their electronic configurations. There are 94 naturally occurring elements on earth (lithosphere, hydrosphere, atmosphere).

From biology perspective also, we developed theory of evolution, germ theory of disease etc.

Mathematics is also considered a branch of science, but it does not require a physical universe and can be studied independently. Physics, chemistry, biology, astronomy fall under the branch of natural science. The subjects of natural science, need a physical universe -> to study a phenomenon from a scientific perspective.

The method to study natural science can be summarized as below:

1. Consider an event occurring in physical world that requires scientific study - e.g. phases of the moon

2. Isolate the physical system as much as possible - observer (from earth) + moon

3. Carry out observations and note them. e.g. does the phase of the moon look different from Delhi and Mumbai, or is it same? what is the period? what is the pattern for waxing and waning?

4. Draw out theories that are consistent with other scientific observations. e.g. a theory like a demon is swallowing the moon and then regurgitating it, is inconsistent with other scientific observations. Do other celestial bodies like mercury, venus, mars etc. show phases (they do have phases)? Why does sun not show phases?

5. Tides are correlated with phases of the moon. During full moon and new moon, there is spring tide, during quarter moon there is neap tide. Is it that the periodic tides and phase (both having same time period) are having cause and effect relation? Or maybe, the cause of tides and cause of phases is same - and both are 2 different effects of the same cause.   

6. Measure the physical quantities that characterize the phenomenon. e.g. we know that the time period of phases of moon is same as time period of moon's revolution around the earth. Also, moon is reflecting sun's light. We use these numerical values to form a mathematical model.

This mathematical model should relate the physical quantities.

E.g. Using mathematics, we form causation relations:

Effect (Physical Quantity) = (Sensitivity Factor) * Cause (Physical Quantity)

OR

Cause (Physical Quantity) = (Capacity Factor) * Effect (Physical Quantity)