Unlike Pauli, Einstein was not afraid of suggesting unobservable things. In 1905, the same year he published his theory of special relativity, he proposed the existence of the photon, the particle of light, to an unbelieving world. (He was not proven right about photons for nearly 20 years.) Mach’s ideas also inspired a vital movement in philosophy a generation later, known as logical positivism – broadly speaking, the idea that the only meaningful statements about the world were ones that could be directly verified through observation. Positivism originated in Vienna and elsewhere in the 1920s, and the brilliant ideas of the positivists played a major role in shaping philosophy from that time to the present day.
But what makes something ‘observable’? Are things that can be seen only with specialised implements observable? Some of the positivists said the answer was no, only the unvarnished data of our senses would suffice – so things seen in microscopes were therefore not truly real. But in that case, ‘we cannot observe physical things through opera glasses, or even through ordinary spectacles, and one begins to wonder about the status of what we see through an ordinary windowpane,’ the philosopher Grover Maxwell
wrote in 1962.
Furthermore, Maxwell pointed out that the definition of what was ‘unobservable in principle’ depends on our best scientific theories and full understanding of the world, and so moves over time. Before the invention of the telescope, for example, the idea of an instrument that could make distant objects appear closer seemed impossible; consequently, a planet too faint to be seen with the naked eye, such as Neptune, would have been deemed ‘unobservable in principle’. Yet Neptune is undoubtedly there – and we’ve not only seen it, we sent Voyager 2 there in 1989. Similarly, what we consider unobservable in principle today might become observable in the future with the advent of new physical theories and observational technologies. ‘It is theory, and thus science itself, which tells us what is or is not … observable,’ Maxwell wrote. ‘There are no
a priori or philosophical criteria for separating the observable from the unobservable.’