Quantum theory is reliable but fraught with paradox. Philip Ball asks if scientists will now find an object existing in two places at onceby Philip Ball / September 21, 2011 / Leave a comment
The 1927 Solvay conference on particle physics: back row, third from right, Werner Heisenberg, sixth from right, Erwin Schrödinger; middle row, from right, Niels Bohr, Max Born, Louis de Broglie and centre, Paul Dirac. Front row, second from left, Max Planck, next to him, Marie Curie, then Hendrik Lorentz and Albert Einstein. Of the 29 pictured, 18 won Nobel prizes, Curie in both physics and chemistry
Quantum mechanics is more than a hundred years old, but we still don’t understand it. In recent years, however, physicists have found a fresh enthusiasm for exploring the questions about quantum theory that were swept under the rug by its founders. Advances in experimental methods make it possible to test ideas about why objects on the scale of atoms follow different rules from those that govern objects on the everyday scale. In effect, this becomes an enquiry into the sense in which things exist at all.
In 1900 the German physicist Max Planck suggested that light—a form of electromagnetic waves—consists of tiny, indivisible packets of energy. These particles, called photons, are the “quanta” of light. Five years later Albert Einstein showed how this quantum hypothesis explained the way light kicks electrons out of metals—the photoelectric effect. It was for this, not the theory of relativity, that he won his Nobel prize.
The early pioneers of quantum theory quickly discovered that the seemingly innocuous idea that energy is grainy has bizarre implications. Objects can be in many places at once. Particles behave like waves and vice versa. The act of witnessing an event alters it. Perhaps the quantum world is constantly branching into multiple universes.
As long as you just accept these paradoxes, quantum theory works fine. Scientists routinely adopt the approach memorably described by Cornell physicist David Mermin, as “shut up and calculate.” They use quantum mechanics to calculate everything from the strength of metal alloys to the shapes of molecules. Routine application of the theory underpins the miniaturisation of electronics, medical MRI imaging and the development of solar cells, to name just a few burgeoning technologies.
Quantum mechanics is one of the most reliable theories in science: its prediction of how light interacts with matter is accurate to the eighth decimal place. But the question of how to interpret the theory—what it tells us about the physical universe—was never resolved by founders such as Niels Bohr, Werner Heisenberg and Erwin Schrödinger. Famously, Einstein…