In the wild world of string theory, quantum gravity, and multiple universes, should the standard scientific methodology still apply?

In the Dec. 15, 2014 issue of Nature, cosmologists George Ellis and Joe Silk made an impassioned and unusual plea. In an essay titled “Scientific Method: Defend the Integrity of Physics,” the two renowned scientists made the case that while several new developments in science — particularly string theory and the concept of a multiverse — seem to offer elegant solutions to otherwise intractable problems in physics, they nonetheless lack elements required for them to be truly scientific, most notably the capacity to be empirically tested and falsified.

“In our view,” they wrote, “the issue boils down to clarifying one question: what potential observational or experimental evidence is there that would persuade you that the theory is wrong and lead you to abandon it? If there is none, it is not a scientific theory.” Ellis and Silk end their commentary with a specific call for philosophers of science to work together with scientists to develop better tools for dealing with the issue.

Chris Smeenk, a philosopher of cosmology at Western University in Canada, remarked on just how rare an occurrence this was. “It’s unusual for scientists to think of philosophers as essential partners in this sort of discussion,” he says. While many sciences have benefited from collaboration with philosophers, such collaboration is especially valuable in cosmology, a field that confronts unusual constraints. When attempting to reconstruct the first moments of the universe—conditions that can’t be recreated in a lab or otherwise directly manipulated—some standard scientific practices become impossible. “Philosophers,” says Smeenk, “have a unique set of tools to contribute to scientific discussions taking place under such constraints. We’ve thought about some epistemological topics more deeply than have many physicists—or at least have thought about them from a different angle.”

MAPPING THE PHILOSOPHICAL LANDSCAPE

In order to complement the work philosophers of cosmology have already done on how scientific methodology can be applied to explanations of the universe’s first moments, Smeenk and several colleagues are undertaking a new project to help survey the field and explore several potentially fruitful areas of inquiry.

For the project’s first major segment, Smeenk and the project’s co-leader, University of California–Irvine philosopher of physics James Weatherall, are writing a book that will chart various major cosmological theories and the potential philosophical grounds for deciding whether or not they constitute acceptable science.

“The current lack of cohesion in the field means that people don’t see how their positions on different topics relate to one another,” Smeenk says. “Are anthropic explanations valid? Are multiverse theories scientific? These questions are often addressed in isolation, but the advantage of taking them on systematically is that we can show how they are connected to one another. Even if the overall view we develop is controversial, we hope it will lead people to see these as interconnected questions.”

SIMULATIONS AND MEANING

Cosmologists have only one universe and one set of physical realities to study, so computer-driven simulations have become a common tool within cosmology. These simulations provide a way of evaluating theories concerning, for example, why the observable universe developed in the way it did, or how features of alternate universes might have arisen from different initial conditions. But it is not always clear to those who work with these models how to determine if a simulation is correct, or even sufficient to yield scientifically valid results. Here again, philosophers — many of whom have worked on simulations in other contexts—are able to make important contributions. Smeenk and Weatherall will work with one of Weatherall’s colleagues, physicist and astronomer James Bullock, on the project’s second main task, which aims to evaluate the problems and promises of cosmological simulations.

“The ultimate goal is to develop something like a toolkit that will introduce novel techniques to determine the reliability of simulations,” Smeenk says. For instance, it remains an open question whether fine-tuning a simulation so its results match other observations might undermine the scientific validity of the simulation. Science tends to prefer simple solutions over complicated ones, but an overly fine-tuned simulation could be the modern-day equivalent of mechanical contortions that pre-Copernican astronomers utilized to bring the geocentric theory of planetary motion in line with increasingly accurate telescopic observations.

THE FUTURE OF SPACE-TIME

The third major thrust of the New Directions in Philosophy of Cosmology project focuses on the philosophical implications of recent work aimed at integrating general relativity and quantum theory. This sub-project will focus on three themes: physical geometry, quantum cosmology, and emergence,” and will have valuable assistance provided by Robert Brandenberger, a theoretical cosmologist at McGill University.

“In cases like the emergence of space-time, there are many different theoretical avenues that people are pursuing, so part of what’s driving the discussion is just being clear about the different concepts involved,” Smeenk says. “I think addressing underlying conceptual confusions is one way that philosophers can step in to resolve some issues in the field.”

AIMS AND MEANS

In addition to funding research by established philosophers of science, the project aims to lay the groundwork for the discipline’s future. “One of the other things that we hope to do is give younger scholars a venue to pursue their work in this area,” Smeenk says. The project will include small competitive awards for junior scholars, seed funding for pre-doctoral scholars, and an edited collection of writing by new voices in the field.

“I think it’s really challenging right now to find a career trajectory in these sorts of crossover fields,” Smeenk says. “This project won’t solve that problem, but we’re at least going to provide scholars who have inter-disciplinary competence a place where they can publish and engage others in the field.”

For his part, Smeenk says he is grateful when scientists of the stature of Ellis and Silk make public calls for philosophers to help address key challenges, although he also acknowledges that many working cosmologists are not directly affected by these particular debates.

“It’s amazing to consider the kind of evidence related to cosmological theory that scientists in this field have been able to gather in the last 50 years. In light of this our picture of the universe has gotten much more refined,” Smeenk says. “As a philosopher I am not equipped to contribute to this part of the field. But there are fundamental debates about the aims of cosmology and how we should assess theories within this unique science. These are topics that philosophers of science sink their teeth into and where they can add significant value.”

STILL CURIOUS?

Learn more about the projects key investigators Chris Smeenk and James Weatherall.

Read the entry on the Philosophy of Cosmology that Smeenk co-authored for the Stanford Encyclopedia of Philosophy.