Friday, April 17, 2015

Three ways physicists understand the physical universe

"Ten years after Einstein completed his theories, Werner Heisenberg and Erwin Schrödinger invented quantum mechanics, describing the behavior of atoms and light-quanta in a radically different way. Experiments confirmed that quantum mechanics gives a true picture of atomic processes that Einstein’s theories could not explain. Niels Bohr worked out a philosophy, generally known as the Copenhagen interpretation, to explain quantum mechanics. I prefer to call it the dualistic philosophy, since it describes the universe as consisting of two layers. The first layer is the classical world of Einstein, with objects that are directly observable but no longer predictable. They have become unpredictable because they are driven by events in the second layer that we cannot see. The second layer is the quantum world, with states that are not directly observable but obey simple laws. For example, the laws of the second layer decree that every particle travels along every possible path with a probability that depends in a simple way on the path.

"The two layers are connected by probabilistic rules, so that the quantum state of an object tells us only the probabilities that it will do various things. The dualistic philosophy allows us to divide our knowledge of nature into facts and probabilities. Observation of the first layer gives us facts about what happened in the past, but only gives us probabilities about what may happen in the future. The future is uncertain because the processes in the second layer are unobservable. The power and the beauty of quantum mechanics arise from the fact that the physical laws in the second layer are precisely linear.

"All points in a linear theory are equal, and a linear space has perfect symmetry about any of its points. As a result of the linearity of the laws, the second layer possesses a wealth of marvelous symmetries that are only partially visible in the first layer. For example, in the first layer, symmetries between space and time are only partly visible. In daily life, we do not mix up inches with seconds or miles with days. In the second layer, as the result of Paul Dirac’s elegant equation describing the quantum behavior of the electron, the mixing of space with time in the electron’s movements would be clearly visible. But we do not live in the second layer, and so the mixing is hidden from us.

"The dualistic philosophy gives a natural frame for the new sciences of particle physics and relativistic cosmology that emerged in the twentieth century after Einstein and Bohr were dead. The new sciences are dominated by mathematical symmetries that are exact in the second layer and approximate in the first layer. The dualistic philosophy seems to me to represent accurately our present state of knowledge. It says that the classical world and the quantum world are both real, but the way they fit together is not yet completely understood. The dualistic philosophy is flexible enough to accept unexpected discoveries and conceptual revolutions.

"Now, eighty years after the dualistic philosophy was invented by Bohr, it is generally regarded by the younger generation of physicists as obsolete. The younger generation mostly rejects duality and accepts what I call the quantum-only philosophy. The quantum-only philosophy says that the classical world is an illusion and only the quantum world exists. The concept of a classical world arose because the effects of quantum mechanics are rapidly erased by a phenomenon known as decoherence. Decoherence hides the quantum world by destroying rapidly the waves arising from quantum effects. After the waves have disappeared, whatever is left obeys classical laws and looks like a classical world. According to the quantum-only philosophy, the marvelous harmony of Einstein’s classical universe is only an approximation, valid when quantum waves happen to be small enough to be neglected.

"To summarize the present situation, there are three ways to understand philosophically our observations of the physical universe. The classical philosophy of Einstein has everything in a single layer obeying classical laws, with quantum processes unexplained. The quantum-only philosophy has included everything in a single layer obeying quantum laws, with the astonishing solidity and uniqueness of the classical illusion unexplained. The dualistic philosophy gives reality impartially to the classical vision of Einstein and to the quantum vision of Bohr, with the details of the connection between the two layers unexplained. All three philosophies are tenable, and all three are incomplete. I prefer the dualistic philosophy because I give equal weight to the insights of Einstein and Bohr. I do not believe that the celestial harmonies discovered by Einstein are an accidental illusion."

Excerpt from Freeman Dyson, "Einstein as a Jew and a Philosopher" NYRB May 7, 2015. Accessed May 17, 2015.