The Conditions of Space and Time
The German philosopher Immanuel Kant (1724â1804) delved into the critical limits of knowledge in his revolutionary text, The Critique of Pure Reason (1781). He expounded the view that space and time do not exist independently of our consciousness.
Nevertheless, until Einstein, the dominant philosophy of physicists was inherited from Sir Isaac Newton (1643â1727).
Newtonâs Classical Laws of Physics
Newton was arguably the greatest of physicists and mathematicians. He contributed significantly to optics, formulated his three laws of motion, and developed differential and integral calculus independently of G.W. Leibniz (1646â1716). But, in terms of understanding Einsteinâs relativity, Newtonâs law of universal gravitation is the most crucial for us.
A famous but untrue story has Newton sitting under an apple tree when his great discovery of gravity literally hit him on the head.
The specific importance of Newtonâs law of universal gravitation is that it explains and unites several phenomena within a single theory. This quest for a single unifying theory would become the driving force of 20th- and 21st-century physics.
The Law of Gravity
Newtonâs law of universal gravitation states that the force of gravity (F) between two objects of masses m and M is given by âŚ
where r is the distance between the two objects and G is Newtonâs constant. G is very small since gravity is very weak.
There are at least two implications to this law of gravity âŚ
Newton took several things for granted in his theory. While the earth was no longer the centre of the universe â and had not been so in the eyes of many scientists since Nicolaus Copernicus (1473â1543) â it was assumed that space and time were fundamentally different things and that both were absolute, set in marble.
The idea of unifying the two, apparently different, concepts of space and time fell to Einstein, as weâll later discuss.
Maxwellâs Theory of Electromagnetism
Theoretical physics had made significant progress before Einstein. In particular, James Clerk Maxwell (1831â79) had unified magnetism with electricity to give electromagnetism.
By means of four equations, Maxwell explained all the different manifestations of electricity and magnetism â from the emission of light and electric currents to the earthâs magnetic field. Maxwellâs equations linked the electric and magnetic fields to each other and showed how each of their various manifestations arose as special cases of a general theory.
Simple magnetic fields can occur when there is no electric field (and vice versa).
But, in general, if the intensity of an electric field varies in time, it will generate magnetic fields ⌠and vice versa.
This happens in the case of light, which consists of oscillating electric and magnetic fields propagating through space and time â at the speed of light.
The unification that Maxwell achieved is thus similar conceptually to that of Newton when Newton realized that the force acting on the apple is the same as that holding the earth in orbit around the sun.
Problems in Classical Physics
A number of problems had been identified in this progressive story of physics. One of these concerned gravity itself. Newtonâs theory of gravity correctly predicted that planets should move in elliptical orbits.
Puzzle of the Atom
The atom was another major thorn in the flesh of physicists. The prevailing picture around the turn of the 20th century was that atoms are made up of a positively charged nucleus surrounded by negatively charged â and much less massive â electrons. The electrons must orbit the nuc...