As Helge Kragh writes, Planck was a reluctant revolutionary. Already established as professor of physics in Berlin, the 42-year-old Planck was not intent on rewriting the laws of physics. Thermodynamics, that most old-fashioned but important area of physics, was his first love. He had written his PhD thesis on the second law of thermodynamics and was, as Kragh explains, still obsessed by it.
It is difficult to imagine what physics was like at the time of Planck's lecture: Schrödinger was just 13 years old, Pauli was still in nappies, and Heisenberg and Dirac had not even been born. X-rays, radioactivity and the electron had just been discovered, but significant numbers of leading physicists did not believe in atoms and molecules. Today, as Gerhard Rempe describes, physicists can trap an atom in a cavity with just one photon.
Yet in 1900, by accident or design, Planck lit a torch that was taken forward by Einstein, who in 1905 proposed that light was quantized, and later by Bohr, who introduced his planetary model of the atom in 1913. Bohr also inspired a new generation of young physicists, including Wolfgang Pauli, Werner Heisenberg, Paul Dirac and Pascual Jordan, who were all in their early 20s when they made their names, and Louis de Broglie and Erwin Schrödinger, who were 31 and 36, respectively. Collectively they defined the quantum mechanics of wavefunctions and the uncertainty and exclusion principles that we know today. Quantum mechanics went on to become the most successful and the most accurate theory in science. The quantum prediction for the strength of the interaction between an electron and a magnetic field agrees with the experimental result to a precision of two parts in 1012 (D Kleppner and R Jackiw 2000 Science 289 893).
Quantum mechanics also laid the foundations for the entire communications and information industries. Some argue that these industries will soon hit the quantum buffers - the insulating layers in semiconductor devices will become so thin that quantum tunnelling will undermine the operation of the device. But these quantum effects can also be exploited in new kinds of devices. In the short term spintronics - in which the spin of the electron, rather than its charge, is manipulated - should have the biggest impact. In the longer term, quantum computers and other devices that exploit the ability of quantum particles to be in two or more quantum states at the same time could become important.
Within the quantum community, uncertainty still lingers about what the theory means, while the unification of quantum theory and the general theory of relativity is the biggest single outstanding challenge in fundamental physics (see Physics World December 1999 p21). The revolution that Max Planck so reluctantly started 100 years ago still has some distance to run.
Physics and wealth creation
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