Quantum mechanics must be one of the most successful theories in science. Developed at the start of the twentieth century, it has been used to calculate with incredible precision how light and matter behave – how electrical currents pass through silicon transistors in computer circuits, say, or the shapes of molecules and how they absorb light. Much of today’s information technology relies on quantum theory, as do some aspects of chemical processing, molecular biology, the discovery of new materials, and much more.
Yet the weird thing is that no one actually understands quantum theory. The quote popularly attributed to physicist Richard Feynman is probably apocryphal, but still true: if you think you understand quantum mechanics, then you don’t. That point was proved by a poll among 33 leading thinkers at a conference in Austria in 2011. This group of physicists, mathematicians and philosophers was given 16 multiple-choice questions about the meaning of the theory, and their answers displayed little consensus.
That’s because quantum theory poses all sorts of strange questions that stretch the limits of our imagination – forcing us, for example, to conceive of objects like electrons that can, in different circumstances, be either waves or particles.
One of the most controversial issues concerns the role of measurements. We’re used to thinking that the world exists in a definite state, and that we can discover what that state is by making measurements and observations. But quantum theory (“quantum mechanics” is often regarded as a synonym, although strictly that refers to the mathematical methods developed to study quantum objects) suggests that, at least for tiny objects such as atoms and electrons, there may be no unique state before an observation is made: the object exists simultaneously in several states, called a superposition. Before measurement, all we can say is that there is a certain probability that the object is in state A, or B, or so on. Only during the measurement is a “choice” made about which of these possible states the object will possess: in quantum-speak, the superposition is “collapsed by measurement”. It’s not that, before measuring, we don’t know which of these options is true – the fact is that the choice has not yet been made.
This is probably the most unsettling of all the conundrums posed by quantum theory. It disturbed Albert Einstein so much that he refused to accept it all his life. Einstein was one of the first scientists to embrace the quantum world: in 1905 he proposed that light is not a continuous wave but comes in “packets”, or quanta, of energy, called photons, which are in effect “particles of light”. Yet as his contemporaries, such as Niels Bohr, Werner Heisenberg and Erwin Schrodinger, devised a mathematical description of the quantum world in which certainties were replaced by probabilities, Einstein protested that the world could not really be so fuzzy. As he famously put it, “God does not play dice.” (Bohr’s response is less famous, but deserves to be better known: “Einstein, stop telling God what to do.”)
Wonderful, wonderful Copenhagen
Schrodinger figured out an equation that, he said, expressed all we can know about a quantum system. This knowledge is encapsulated in a so-called wavefunction, a mathematical expression from which we can deduce, for example, the chances of a quantum particle being here or there, or being in this or that state. Measurement “collapses” the wavefunction so as to give a definite result. But Heisenberg showed that we can’t answer every question about a quantum system exactly. This is Heisenberg’s uncertainty principle: the more precisely you determine an electron’s momentum (as measured by mass multiplied by velocity), the less you can know about its position in space, and vice versa. In other words, there are some pairs of properties for which an increasingly accurate measurement of one of them renders the other ever fuzzier.