Einstein and a century of Physics
IN 1905, Albert Einstein wrote three papers in Physics: one that founded the Special Theory of Relativity, the second that proposed that Light is made of Quanta and the third on Brownian Motion that enabled the Atomic Hypothesis to be verified experimentally. Thus Einstein made three fundamental discoveries in a single year.
Such a feat is unique in the annals of science. Hence 1905 is called the Year of Miracles or the Miraculous Year and 2005 is to be celebrated as the World Year of Physics. I will give an elementary account of these three contributions of Einstein and also their impact in the development Physics.
Classical Physics
To appreciate the import of Einstein's revolutionary contributions, one must start with Classical Physics, namely Physics as it existed up to 1900. All of classical physics at the fundamental level could be summarised by two great systems of dynamical laws, one was due to Newton and governed `Matter' and the other was due to Maxwell and governed `Field', the electromagnetic field.
Newton's law of motion for material bodies stated that mass times acceleration is the applied force and his universal law of gravitation stated that the gravitational force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of their distance of separation. With this system of laws, Newton could unify the dynamics of bodies in the Heavens with those on the Earth.
Maxwell's dynamical laws for the electromagnetic field were based on the intuitive picture of the electric and magnetic fields that Faraday had built out of the results of many famous experiments. Once Maxwell succeeded in constructing his complete and consistent system of mathematical equations dictating the dynamical behaviour of the electromagnetic field, he could predict that electromagnetic waves existed and that their speed would be 300,000 km per second. By that time the speed of light was already known to be the same number. So Maxwell could identify that light was also an electromagnetic wave and thus solved the puzzle of what light was.
In Classical Physics, there is a third dynamical system dealing with heat, called thermodynamics, but unlike the other two described above, this is not an independent dynamical system. Heat is nothing but molecular motion, and so thermodynamics is derivable from the other two. Thermodynamics is the same as statistical mechanics of a large collection of atoms or molecules. This was the great contribution of Boltzmann. However, existence of atoms and molecules was not a universally accepted hypothesis at the end of the 19th century. We shall now take up Einstein's 1905 discoveries one by one.
Brownian motion
In 1828, a botanist Robert Brown discovered that pollen grains from plants suspended in water showed zig-zag motions. Although these motions were attributed to some life force, it became clear that that was not true; in fact inanimate particles of matter also showed it. This Brownian motion is now known to be due to the impact of molecules of water on the suspended particle. These impacts will be on all sides and directions and so tend to cancel on the average but not completely. A residual force does remain and so the particle moves in that direction. This will be a random motion. Einstein gave the correct quantitative explanation of Brownian motion. He gave a formula for the Brownian displacement during an interval of time relating the displacement to the time interval and the number of molecules in a cc (cubic centimetre) of water.
Later Jean Perrin experimentally verified Einstein's formula and then used it to determine the number of molecules in a cc of water. This helped to establish the reality of atoms and molecules and this is the importance of Einstein's work on Brownian motion. Even the skeptics now had to accept that all matter is made of atoms.
Theory of Relativity
Sometimes it is said that Einstein proved that everything is relative and this is Relativity Theory. This is wrong. Einstein's Theory of Relativity is about relative motion. A momentous confrontation between Newton's dynamical laws for matter and Maxwell's dynamical laws for electromagnetic field developed towards the end of the 19th century.
This confrontation, which had to do with relative motion was resolved by Einstein in favour of Maxwell's laws. He had to change Newton's laws. This is the story of relativity.
Galileo and Newton had removed absolute motion or absolute rest from dynamics. We must remember that Newton's law of motion deals with acceleration, which is rate of change of speed. The absolute value of the speed of motion does not enter into it. Hence Newtonian dynamics retains its form in a moving platform as long as the platform moves with uniform speed.
How about Maxwell's dynamics? To answer this, let us think of a light wave travelling with the speed already mentioned, since light wave propagation is a part of Maxwellian dynamics. Again think of a moving platform such as a space vehicle and let us observe the light wave from this moving platform. If the platform is moving in the same direction as light, we would have expected that light as observed from the platform will move at a lower speed.
This was the expectation based on our pre-Einsteinian notions of relative motion and relative speed. But since the speed of light was calculated from Maxwell's laws to have the value already mentioned, it would seem that these laws do not retain their form in the moving platform. In other words, Maxwell's laws seemed to depend on absolute motion, in contrast to Newton's laws.
Next week: What did the experiments say?
(Based on a lecture given by the author at The Children's Club, Mylapore for Plus-Two students on January 2. Emai: graj@imsc.res.in)
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