Seed Magazine has a video and transcript up of a discussion between cosmologist Paul Steinhardt and philosopher of science Peter Galison advertised as The physicist and the historian discuss the nature of truth as theoretical models of the universe become increasingly difficult to test.
Steinhardt is no fan of the anthropic landscape and makes a general attack on the idea of eternal inflation, explaining why he prefers his cyclic model:
The original idea — the way it’s often talked about in literature and textbooks, even the way we talk to students — is that inflation makes everything in the universe the same. What we’ve learned is that inflation actually divides the universe up into little sectors that are all different from one another. Some regions of space would be habitable like ours, but others would be inhabitable; still others would be habitable but would not have the same physical laws or the same distributions of matter that we see here…
Because you have an infinite number of everything, you have no rigorous mathematical or statistical way of computing a probability — it’s not even a sensible question to ask. So people are in the process of trying to regulate this infinity. For example, they try to invent a rule for deciding probability that makes what we see likely. But there’s no way of deciding why that rule instead of some other one. They simply keep trying until they’ve found the answer they wanted. Some people are going down that path and are prepared to declare victory if they find something they think works.
Others take a different path. They accept the infinity of infinities and the fact that they can’t find any measure for deciding whether our circumstance is more probable or not. They’ll be satisfied with the fact that at least some patches look like what we see, and will declare victory on that basis.
Personally, I don’t find either of these approaches acceptable, which is why I have developed an alternative picture in which the big bang is not the beginning. A big bang repeats at regular intervals of a trillion years or so, and the evolution of the universe is cyclic.
The two then get into a philosophy and history of science discussion, starting with Steinhardt’s:
We’ve been talking about an example in which you have a complex energy landscape and an infinite number of possibilities for the universe. But we have no real explanation as to why things are the way they are, because it could have been different.
So it has no power. And without real explanatory power, it’s not interesting to me. But I’d be interested to hear how this has played out in the history of science.
and Galison’s response:
We have that sort of split right now among the string theorists. One side says, “Look, what’s really scientific is to say there’s this infinite or very huge number of craters to be imagined in some landscape, each of which carries different physical particles and different physical laws and so on. And we happen to live in one of them.”
But the other says, “You’ve given up! You’ve given up the historical project of science. We went into string theory because we wanted to produce a theory that had one parameter, or very few movable parts. And now instead of a glider you’ve got a helicopter with 10,000 little pieces that have to move exactly the same way. If the slightest thing goes off, it falls to the ground in a heap of burning aluminum.”
It’s really an interesting moment in that way.
Steinhardt describes the current situation as follows:
I think it’s historic. There’s a certain community that feels, “This is an ‘aha’ moment. Science has to change. We have to accept that science has limits. There’s only a certain amount that we’ll be able to predict. Beyond that we’re going to accept that we live in some special corner of space in which seemingly universal laws — including Newton’s law of gravity — are just local environmental laws that aren’t really characteristic of the whole.”
Other groups say, “Hold it, this is failure. We either find ways of fixing the problems in those theories, or we scrap them and replace them with something else.”
The source of the problem here is not actually eternal inflation, but string theory. It is the fact that one needs to postulate a huge landscape in string theory in order to have something complicated and intractable enough to evade conflict with experiment that is the problem. Once one has this, and populates it with eternal inflation, then one has a pseudo-scientific framework with no explanatory or predictive power. Galison notices that string theorists are dividing up into those who follow this path, and those unhappy with it, but it is only Steinhardt who makes the obvious point that what’s going on here is just garden-variety scientific failure. The failure though is not attributable to the general idea of inflation, but rather to the string theory-based assumption that fundamental physical theory involves a hopelessly complicated set of possibilities for low-energy physics.