How long is long enough when it comes to waiting for a new cosmology? How long will scientists, and everyone else, wait for an exciting but speculative theory to find some basis in experiment?
If you have never heard the term before, the multiverse is a universe of universes. It's the idea that beyond the universe we can see (via light that has been traveling towards us for the last 13.7 billion years), lie other entirely distinct "pocket universes," each with their own histories and even their own laws of physics.
The multiverse has become a popular topic in cosmological circles. In Sean's column he explains why he and other cosmologists find the multiverse so attractive and useful. I will let you read his cogent explanation on your own because today I want focus on the one fly in the multiverse's ointment: There is not a shred of evidence to support it.
Now don't get me wrong, I would love for the multiverse to exist. C'mon, how cool would that be? All these other possibilities existing out there are the stuff of science fiction, and I love science fiction. Furthermore, as Sean has argued forcefully, (and Marcelo has written on here) there are good reasons to adopt the multiverse perspective.
But the questions I want to take up today are simply these: What do you do with a theory that can only explain what you see by assuming the existence of a whole lot more you can't see? And how long do cosmologists, and the culture that supports them, wait for evidence of that unseen portion of the universe before everyone moves on to some other idea.
This debate about science turning from the universe we can observe to potential or implied universes we can't has heated up over the last decade. I dedicated more than a few pages to it in my recent book on cosmology and culture.
Superstring theory, with its seven "hidden" dimensions of space, was the catalyst for much of the early discussion. After the initial, heady rush toward strings (it is after all a beautiful idea), critics like Lee Smolin and Peter Woit saw all its "hidden" features to be a retreat from reality rather than a higher level of its description. String theorists have always claimed that some kind evidence for the idea would be forthcoming. Hopes were pinned on the confirmation of supersymmetry — a particle-physics theory that is the ground on which superstring theory is based. Recent news from the Large Hadron Collider on that front, however, has not been encouraging. The simplest versions of supersymmetry were definitely ruled out by the LHC (though there still remains lots to do).
Like string theory's seven unseen dimensions, the multiverse idea uses all the unseen universes to explain features of our own single Universe (it's worth noting that string theory in its current form likely implies the existence of a multiverse). If the multiverse is the cost of explaining the universe we do see, then, is that cost too high? Are we substituting work on possible universes at the expense of finding new explanations that rely solely on what we can see in this universe?
Many folks are hard at work looking for observational consequences of the multiverse idea. Perhaps, for example, we can see imprints of interactions between pocket Universes on some feature of our own. But how long does one wait for these projects and when do we call negative results complete? String theory, for example, is now its third decade and there remains no evidence for its existence as a working model for reality.
What will count as a definitive rejection of the multiverse? Theorists are clever folks and it's often possible to explain away negative results.
The history of relativity in the late 19th and early 20th centuries offer lessons in this regard. For decades physicists were sure that the universe was filled with an all-pervading aether that allowed light to propagate across "seemingly" empty space. Even as evidence against the aether was found, many scientists modified their aether theories to save the idea. It was not until Einstein found a way to completely redefine the problem that the aether concept was finally and fully abandoned.
The aether concept took many decades to develop and many decades to die. Will that be the fate of the multiverse's unseen universes and string theory's hidden dimensions? Or, are the heady visions of parallel worlds and invisible realities the first tentative imaginings of a vastly expanded set of possibilities? While it is true that only time will tell, how much time do these ideas get?
You can keep up with more of what Adam Frank is thinking on Facebook and Twitter. His new book is About Time: Cosmology and Culture at the Twilight of the Big Bang.