Page 18 - Class-11-Physics-Part-1_Neat

4 PHYSICS

Physics is exciting in many ways. To some people
Hypothesis, axioms and models
the excitement comes from the elegance and
universality of its basic theories, from the fact that One should not think that everything can be proved
a few basic concepts and laws can explain with physics and mathematics. All physics, and also
phenomena covering a large range of magnitude mathematics, is based on assumptions, each of
of physical quantities. To some others, the challenge which is variously called a hypothesis or axiom or
in carrying out imaginative new experiments to postulate, etc.
unlock the secrets of nature, to verify or refute For example, the universal law of gravitation
theories, is thrilling. Applied physics is equally proposed by Newton is an assumption or hypothesis,
demanding. Application and exploitation of which he proposed out of his ingenuity. Before him,
physical laws to make useful devices is the most there were several observations, experiments and
interesting and exciting part and requires great data, on the motion of planets around the sun,
motion of the moon around the earth, pendulums,
ingenuity and persistence of effort. bodies falling towards the earth etc. Each of these
What lies behind the phenomenal progress required a separate explanation, which was more
of physics in the last few centuries? Great or less qualitative. What the universal law of
progress usually accompanies changes in our gravitation says is that, if we assume that any two
basic perceptions. First, it was realised that for bodies in the universe attract each other with a
scientific progress, only qualitative thinking, force proportional to the product of their masses
though no doubt important, is not enough. and inversely proportional to the square of the
Quantitative measurement is central to the distance between them, then we can explain all
growth of science, especially physics, because these observations in one stroke. It not only explains
the laws of nature happen to be expressible in these phenomena, it also allows us to predict the
precise mathematical equations. The second results of future experiments.
most important insight was that the basic laws A hypothesis is a supposition without assuming
of physics are universal — the same laws apply that it is true. It would not be fair to ask anybody
in widely different contexts. Lastly, the strategy to prove the universal law of gravitation, because
it cannot be proved. It can be verified and
of approximation turned out to be very substantiated by experiments and observations.
successful. Most observed phenomena in daily
An axiom is a self-evident truth while a model
life are rather complicated manifestations of the is a theory proposed to explain observed
basic laws. Scientists recognised the importance
phenomena. But you need not worry at this stage
of extracting the essential features of a about the nuances in using these words. For
phenomenon from its less significant aspects. example, next year you will learn about Bohr’s model
It is not practical to take into account all the of hydrogen atom, in which Bohr assumed that an
complexities of a phenomenon in one go. A good electron in the hydrogen atom follows certain rules
strategy is to focus first on the essential features, (postutates). Why did he do that? There was a large
discover the basic principles and then introduce amount of spectroscopic data before him which no
corrections to build a more refined theory of the other theory could explain. So Bohr said that if we
phenomenon. For example, a stone and a feather assume that an atom behaves in such a manner,
dropped from the same height do not reach the we can explain all these things at once.
ground at the same time. The reason is that the Einstein’s special theory of relativity is also
based on two postulates, the constancy of the speed
essential aspect of the phenomenon, namely free
fall under gravity, is complicated by the of electromagnetic radiation and the validity of
physical laws in all inertial frame of reference. It
presence of air resistance. To get the law of free would not be wise to ask somebody to prove that
fall under gravity, it is better to create a the speed of light in vacuum is constant,
situation wherein the air resistance is independent of the source or observer.
negligible. We can, for example, let the stone and In mathematics too, we need axioms and
the feather fall through a long evacuated tube. hypotheses at every stage. Euclid’s statement that
In that case, the two objects will fall almost at parallel lines never meet, is a hypothesis. This means
the same rate, giving the basic law that that if we assume this statement, we can explain
acceleration due to gravity is independent of the several properties of straight lines and two or three
mass of the object. With the basic law thus dimensional figures made out of them. But if you
found, we can go back to the feather, introduce don’t assume it, you are free to use a different axiom
corrections due to air resistance, modify the and get a new geometry, as has indeed happened in
the past few centuries and decades.
existing theory and try to build a more realistic

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