What's bad about this habit, May 2009

Abstract

After we came out of the church, we stood talking for some time together of Bishop Berkeley's ingenious sophistry to prove the nonexistence of matter, and that every thing in the universe is merely ideal. I observed that though we are satisfied his doctrine is not true, it is impossible to refute it. I never shall forget the alacrity with which Johnson answered, striking his foot with mighty force against a large stone, till he rebounded from it – “I refute it thus.” – James Boswell, The Life of Samuel Johnson There is nothing … more abstract than reality. – Giorgio Morandi, interview with Edouard Roditi A bad habit is something you do, without being fully aware of it, that makes life harder than it needs to be. It is a bad habit of physicists to take their most successful abstractions to be real properties of our world. Since the distinction between real and abstract is notoriously problematic, you might wonder what it means to wrongly confer reality on something abstract. I shall illustrate our habit of inappropriately reifying our successful abstractions with several examples. Perhaps the least controversial examples are provided by quantum mechanics. The quantum state may well be the most powerful abstraction we have ever found. Are quantum states real? In considering what that question might mean, recall that in the early days Erwin Schrodinger thought that the quantum state of a particle—in the form of its wavefunction—was as real a field as a classical electromagnetic field is real. He abandoned that view when he recognized that nonspreading wavepackets were a peculiarity of the harmonic oscillator, and that the wavefunction of N particles is a field only in a 3 N -dimensional space. But that does not prevent advocates of the de Broglie–Bohm “pilot wave” interpretation of quantum mechanics from taking the wavefunction of N particles to be a real field in 3 N -dimensional configuration space. They give that high-dimensional configuration space just as much physical reality as the rest of us ascribe to ordinary three-dimensional space. The reality of the wavefunction is manifest in its ability to control the motion of (real) particles, just as a classical electromagnetic field is able to control the motion of classical charged particles.