In 1911, at the first Solvay Conference in Brussels, Albert Einstein and Hendrik A. Lorentz discussed a simple problem that later led to a central research tool within what we now call the 'old quantum theory.' At the time, it was assumed that mechanical systems subject to the yet embryonic quantum laws could only make 'all or nothing jumps' between allowed states of different energy. What would happen, Lorentz asked, if one takes a pendulum with an allowed energy and shortens the length of the string by grasping it with two fingers? Einstein remarked that, even though the pendulum's period would decrease and its energy would increase, if the string is shortened very slowly ('adiabatically'), the product of the two quantities will remain constant: a pendulum whose frequency changes adiabatically does not undergo a quantum jump and the product of the period and frequency is quantized.
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