The cosmological multiverse and falsifiability in science

The cosmological “multiverse” model talks about regions far beyond the observable portion of our universe (set by the finite light-travel distance given the finite time since the Big Bang). Critics thus complain that it is “unfalsifiable”, and so not science. Indeed, philosopher Massimo Pigliucci states that instead: “… the notion of a multiverse should be classed as scientifically-informed metaphysics”.

Sean Carroll has recently posted an article defending the multiverse as scientific (arXiv paper; blog post). We’re discussing here the cosmological multiverse — the term “multiverse” is also used for concepts arising from string theory and from the many-worlds interpretation of quantum mechanics, but the arguments for and against those are rather different.

Applying falsifiability in science

Falsifiability. as famously espoused by Karl Popper, is accepted as a key aspect of science. When a theory is being developed, however, it can be unclear how the theory might be tested, and theoretical science must be given license to pursue ideas that cannot be tested within our current technological capabilities. String theory is an example of this, though ultimately it cannot be accepted as a physical explanation without experimental support.

Further, experimental science is fallible, and thus we do not immediately reject a theory when contradicted by one experimental result, rather the process involves the interplay between experiment and theory. As Arthur Eddington quipped: “No experiment should be believed until it has been confirmed by theory”.

Sean Carroll recently called for the concept of falsifiability to be “retired”, saying that:

The falsifiability criterion gestures toward something true and important about science, but it is a blunt instrument in a situation that calls for subtlety and precision.

Meanwhile, Leonard Susskind has remarked that:

Throughout my long experience as a scientist I have heard un-falsifiability hurled at so many important ideas that I am inclined to think that no idea can have great merit unless it has drawn this criticism.

Disagreeing (partially) with Sean Carroll about what is science

Sean Carroll talks a lot of sense on the nature of science, and in a recent post gave a definition of “science” that focuses on the methods and attitudes of science. He wrote:

Science consists of the following three-part process:
1. Think of every possible way the world could be. Label each way an “hypothesis”.
2. Look at how the world actually is. Call what you see “data” (or “evidence”).
3. Where possible, choose the hypothesis that provides the best fit to the data.

It’s a good operational definition, separating a scientific attitude from a pseudo-scientific or religious attitude, although the process needs to be iterative, with the hypothesis of step 3 then being tested against new data; and I would add in Feynman’s maxim: try hard to see whether you are fooling yourself, remembering that yourself is the easiest person to fool.

Later in the article, however, I start disagreeing with Carroll about what is and isn’t science. He says: