The pendulum problem was first formulated by Lord Rayleigh in 1902, although some mathematical aspects have been discussed before by Léon Lecornu in 1895 and Charles Bossut in 1778. Unaware of Rayleigh's work, at the first Solvay conference in 1911, Hendrik Lorentz proposed a question, How does a simple pendulum behave when the length of the suspending thread is gradually shortened?, in order to clarify the quantum theory at that time. To that Albert Einstein responded the next day by saying that both energy and frequency of the quantum pendulum changes such that their ratio is constant, so that the pendulum is in the same quantum state as the initial state. These two separate works formed the basis for the concept of adiabatic invariant, which found applications in various fields and old quantum theory. In 1958, Subrahmanyan Chandrasekhar took interest in the problem and studied it so that a renewed interest in the problem was set, subsequently to be studied by many other researchers like John Edensor Littlewood etc.