This week, a group of about 100 physicists and philosophers have gathered in Munich to reassess a question at the heart of science: “Why trust a theory?” In an opinion piece by George Ellis and Joe Silk, published in Nature earlier this year, the two physicists expressed worry about current developments in some areas of theoretical physics. In particular, they were concerned with untestable attempts to address the fundamental questions concerning space, time and matter. Their article gave rise to the idea of the current meeting.

Image credit: Ellis & Silk, Nature, via
Image credit: Ellis & Silk, Nature, via

An increasing number of physicists, Ellis and Silk observed, have become strongly convinced of the viability of theories that have no empirical confirmation. This trend is most pronounced in the quest for a theory of quantum gravity – notably string theory – and in cosmology where theories for the early universe give rise to a multiverse. Why, they ask, do scientists trust theories that have not been experimentally tested? Worse, in some cases, these theories cannot even been tested in principle. Is this still science?

Philosopher Richard Dawid, one of the organizers of the Munich meeting, has observed the same development and, in his book “String theory and the scientific method” argued that string theorists in particular use a method of “non-empirical theory confirmation.” This method is used during the development of a theory and is based on collecting indications which increase the physicists’ confidence that a theory describes nature. These indications are, for example, the amount (or absence of) alternative solutions to a problem, the degree by which a theory is connected to already confirmed theories, and the amount of unexpected insights that the theories give rise to.

While Dawid focused on string theory, non-empirical theory confirmation is used and has been used in theoretical physics for a long time. What was missing so far is a legitimate philosophical underpinning. Dawid’s arguments provided such an underpinning. String theorists, needless to say, were delighted to now have philosophical support for their procedures, but not everyone was pleased to see the scientific method being watered down. This is what prompted Ellis and Silk to “defend the integrity of physics.” The topic of the present workshop is the following question: under which circumstances, if any at all, is non-empirical theory confirmation a justified procedure?

It is a pressing and timely question. As physics has matured, experimental test of new, more fundamental theories have become increasingly difficult. Many existing theories are so difficult to test that they are widely believed to be untestable in the foreseeable future. The methods from the past are not working any more. “We are in a different era of science,” says Nobel Laureate David Gross.